1. Introduction

1.1 Brief – P D Desai

The aim of this report was to design a pyrolysis/gasification process to produce approximately 4.5 MW of electricity with a feed of 5 tonnes per hour of waste wood. With the objective being to produce maximum output along with high efficiency whilst remaining carbon neutral and creating minimum waste.

1.2 Background Information – P D Desai

The UK is the largest producer of oil and second largest producer of natural gas in the EU. Due to a number of reasons, not the least being the dwindling North Sea resources, it has started to import relatively large amounts of oil and gas. (Energy Information Administration, 2010).

In 2008, energy related carbon dioxide emissions peaked at 572 million metric tonnes in the UK . (Energy Information Administration, 2010). With the growing concern on Carbon Dioxide emissions alternative sources of energy must be explored.

There are a number of technologies that are being explored and have been explored by the UK Government.

They are:

Incineration Gasification Pyrolysis Hydrolysis Anaerobic Digestion Fermentation & in –vessel composting Cryogenics Autoclaving Co-firing Producing refuse derived fuel

Compiled from (Biffaward, 2004), ATT for East riding government.

The main sections of which will be focused on are pyrolysis and gasification.

Gasification is the heating of wood at roughly 1100 -1500 ⁰C with less than stoichiometric oxygen. Oxygen can either be pure or air can be used. Steam may also be used to ensure complete gasification and for increasing the efficiency of the process. They are mostly exothermic reactions when air/oxygen is used. Gasification produces syn gas.

Pyrolysis is the heating of carbonaceous material in the absence of any Oxygen to cause a thermal breakdown of substances into lighter hydrocarbons – Carbon Monoxide, Carbon Dioxide, Hydrogen, Bio – oil, etc. This process takes place between 400 -700⁰C. This is an endothermic process.

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As per the UNFCC - United Framework Convention on Climate Change, the definition of biomass is

‘A non-fossilized and biodegradable organic material originating from plants, animals and micro-organisms. This shall also include products, by-products, residues and waste from agriculture, forestry and related industries as well as the non-fossilized and biodegradable organic fractions of industrial and municipal wastes. Biomass also includes gases and liquids recovered from the decomposition of non-fossilized and biodegradable organic material.’ (UNFCCC, 2005).

A detailed document has been provided in the appendix.

Figure 1.2.1. The current use of biomass for energy production (IEA, 2009)

It can be seen from figure 1.2.1 that currently biomass contribution to total energy usage in the UK is minimal.

The aim of this report was to check the feasibility of a pyrolysis/gasification process to produce approximately 4.5 MW of electricity with a feed of 5 tonnes per hour of waste wood mixed with biomass. The main objective is to produce maximum output along with high efficiency whilst maintaining absolute minimum emissions and waste.

The method adopted by the group was the pyrolytic gasification route. It is the combination of pyrolysis and gasification technologies to produce syngas. The conditions are maintained to get the maximum output of electricity. As it is waste wood, it is already considered to be carbon neutral as there is no net increase in the carbon dioxide content. Therefore any energy produced using this particular source can be considered as an effective way of offsetting the carbon dioxide emissions thereby achieving carbon neutrality.

There are a couple of other reasons why thermal treatments must be used. Firstly, the volume of the fuel can be reduced thereby increasing the energy density of the fuel and making it easier and more economically viable to store it and transport it.

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Secondly the SynGas thus produced from the thermal treatment can be burnt in any normal energy generating device inter alia gas engines, turbines, boilers etc.

Syn gas has the potential to be the source of fuel for the future and even for the present can be used to power a medium sized power plant. (Van der Drift et al,2004 ).

1.3 Location of project – K Asiamah

In the past, almost all residual municipal waste in the United Kingdom has been sent untreated to a landfill. This is not a very sustainable option, as space for landfill is already running low. Mass-burn incineration has been considered as an alternative, but environmentalists have long opposed this because it destroys natural resources, adds to climate change from particulate matter emitted and undermines recycling. Recent developments have led processes able to extract heat from incineration of biomass and use it for useful purposes, like electric power generation to help meet the UK’s growing energy demand.

We have been proposed a specification to design a 4.5 MW biomass fuelled power plant, with the aim of producing carbon neutral electricity. From our design brief, the power plant will be based in the United Kingdom; however a specific plant site will have to be researched.

Arguably the key factor effecting the selection of the location is the availability of feedstock – the waste wood. As the fuel source is inexpensive, it is vital that to keep running costs low minimal transport of the feed should occur. Another major factor is the close proximity of a town or industrial development, this increases feasibility as there is demand for electricity in the area and it may be possible to create a CHP (combined heat and power) system if the plant is successful.

As the UK has a relatively similar climate throughout there is less emphasis on factors such as humidity and precipitation levels. Temperature however may need to be considered as some locations, particularly in northern Scotland for example, drop far below zero in winter months, whilst other locations will have much milder temperatures throughout the year.

The following elaborates on the factors mentioned above, and eventually leads to possible situations of our electric power production system, based in the United Kingdom.

Fuel supply

Site selection will be based on the availability of feedstock, which in this case will primarily be wood. Essentially, we intend for our power plant to be located where our feedstock is relatively abundant (i.e. sites in the United Kingdom with lots of trees), preferably near forests.

Location based on this factor is mainly for economic reasons as it reduces cost of delivery equipment and transportation costs to the site, which in turn will ultimately reduce our emissions from transportation vehicles as less distance is travelled to the site.

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Water supply

Having an easy access to a supply of fresh water maybe also a useful for cooling requirements, some gas clean up operations also require the use of water, however due to the size of this plant this is a much less important factor.

Possible locations

From the factors discussed previously, three possible situations in the United Kingdom for our electric power plant were considered. All of the locations researched complied with legislations governing environmental considerations and were close to a water source (i.e. to some degree had a coastal line).

For these reasons, the main factor considered in this discussion detailed below was the availability of feedstock, which in this case will be the abundance of forests.

ENGLAND – SUSSEX

Fig.1.3.1. Location of Sussex in England (knowledgerush.com, 2010)

Sussex resides in the south of England, bounded on the north by Surrey; east by Kent; west by Hampshire and south by the English Channel. Sussex boasts of four

major forests, Ashdown forest, Dalington forest, St. Leonard’s and Worth forest, making it abundant in our feedstock (i.e. wood).

However, majority of Sussex is dominated by industries of agriculture, iron working, clay working and service. This means that the abundance of land is rather difficult in this region due to competition implying a high amount of capital will have to be invested in order to acquire land to build the plant.

WALES – POWYS

Powys Figure 1.3.2 Location of Powys in Wales (knowledgerush.com, 2010)

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Sussex

Powys is a preserved county is Wales. It is bounded to the north by Gwynedd, Denbighshire and Wrexham; to the west by Ceredigion and Carmarthenshire; to the east by Shropshire and Herefordshire and to the south by Rhondda Cynon Taf, Merthyr Tydfil, Caerphilly, Blaeuau Gwent, Monmouthshire and Neath Port Talbot.

A major advantage of Powys as a location of our power plant is that it boasts of several lakes, reservoirs and waterfalls, giving ease to water access. However, being that Powys is mainly a tourist attraction; our plant cannot be situated here because of strict rules on the land, due to preservation regulations. Also, there are only a few forests in this location (i.e. Clun forest and Hafren forest), meaning feedstock might not be readily available.

SCOTLAND – DUMFRIES AND GALLOWAY

Fig.1.3.3. Location of Dumfries and Galloway in Scotland (knowledgerush.com, 2010)

Dumfries and Galloway

One of 32 council areas of Scotland, Dumfries and Galloway lies to the north of Solway Firth and to the east of the Irish Sea. It basically has 9 forest sites, which is the most in the United Kingdom. These include forest of Ave, Carsphaim forest, Dalbeattie forest, Dundeugh forest, fleet forest, Laurieston forest, Mabie forest and Penninghame forest.

This location, having the most forest in the United Kingdom, provides us with ease in obtaining wood feedstock. Also, this location has numerous well established railways, roads and ports hence providing easy and readily available means of transportation of feedstock if needed. This is mainly based around the town of Dumfries which would be an ideal location for the plant if this location is chosen.

Location chosen

After the research of the various locations, Dumfries and Galloway in Scotland was selected as the site to locate our power plant because of it having numerous forests and the town of Dumfries, this will be the exact location due to the availability of a workforce, potential for CHP is also present, land costs should also be relatively low.

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(Googlemaps, 2010)

Figure 1.3.4. The location of Dumfries within Dumfries & Galloway

Regional Population 148,000, (dumfries-and-galloway.co.uk, 2010)Town Population 31,600 in Dumfries – biggest town in region, (dumfries-and-galloway.co.uk, 2010)Local Economy Based primarily on agriculture and forestry, (dumfries-and-galloway.co.uk, 2010)Unemployment rate 4% in 2009, (hnm.org.uk, 2009) – national figureTemperature Range Extremes of -13°C, usually between -5°C and 20°C annually. (metoffice.gov.uk, 2010)Annual rainfall Jan and Dec over 120mm a month; average around 80mm a month (metoffice.gov.uk,

2010)Table 1.3.1. Key facts about Dumfries

As can be seen from the statistics the location chosen is viable. The town is not too large and a nearby power plant incorporating CHP could have great benefits for the towns infrastructure. It would also provide jobs to help lower the employment rate. As can be seen a large amount of waste wood may be available due to the nature of the main economic activity, this will be the ideal fuel source for power plant. Design conditions however must account for the range of temperatures, with specific care taken to ensure no problems at temperatures below zero degrees Celsius. The annual rainfall rate is also rather high, so it is essential that a covered storage area is available to prevent feedstock becoming saturated and therefore increasing the drying requirements

1.4. Feed stock Information – KB Rasedin

This process uses biomass, specifically wood as a fuel source. The reason wood is being used is due to several factors. One of the factors is the wood chips being used; don’t cost anything, as it is agricultural waste wood.

Furthermore, wood is in abundance in comparison to coal and oil and is renewable. Thus the supply of the waste wood is expected to be available all the times in the year and in future years to come. Besides that, by using wood waste as the fuel to generate electricity, our reliance to fossil fuel could be reduced. There is consistent pressure on governments due to greenhouse gases emissions constantly increasing. Figure 1.4.1 shows an example of how the fossil fuel such as oil supply nowadays has decreasing.

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Figure 1.4.1. Exploration, Discovery and Consumption of oil (How and When were the Limits of Oil Reserves Discovered, http://www.theglobaleducationproject.org/earth/energy-supply.php)

Furthermore another reason wood waste is being used is because it is exempt from the climate change levy and carbon tax. The climate change levy is basically a tax imposed on the commercial, industrial and public sector energy usage (Hoval, 2006). Levy rates are 0.15p/kWh for gas, 0.15 p/kWh for coal, 0.07 p/kWh for LPG and 0.43 p/kWh for electricity (Biomass Energy Centre, 2008). Meanwhile carbon tax is the tax imposed based on the carbon content of the fuel (Todd Litman,2010). Industries using only wood are not obliged to pay these fees as wood is a renewable energy resource that is carbon neutral.

Other reason wood is being used to generate electricity for this power plant is because of the abundance of waste wood produced by consumers and industries in UK. Based on the studies done by Environmental Agencies (Wrap,2010) it was found that approximately 10.6 million tonnes of waste wood was produced in 2005. Thus imagine now in 2010 how much waste wood is produced in the UK by taking into account the increasing number of population and wood based industries. Table 1.4.2 shows the estimation of wood waste production done by the Environmental Agency in the UK. The abundance of waste wood produced has created a problem in disposing these wastes. Landfill option is not a sustainable option since it would only destroy the potential of the land to be developed. Therefore there is a need to utilize this waste rather than dumping it because there is energy value content in this waste.

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Stream ERM/DTI (2006/2007)

WRAP & MEL (2005)

TRADA(2002)

BRE/Hurley (2004)

Municipal 1.1 1.1 2.5 0.8Industrial/Commercial 3.5 4.5 1.8 3.3Construction/Demolition/Remodelling 2.9 5.0 0.9 3.3Total 7.5 10.6 5.2 7.4

Table 1.4.1. Range of Estimates in Most Comprehensive studies Published on Wood Waste in the UK, all numbers in million tonnes (Wood Waste Market in the UK, http://www.wrap.org.uk)

The process using any type of waste wood available, however one of the most readily available to the plant will come from furniture. Tables 1.4.2 and 1.4.3 show the production of wood waste that

came from furniture based on the studies done by Environmental Agencies in the UK.

Waste stream England Wales Scotland NI UKHousehold

collection round (including kerbside recycling and non

CA bring

34 0.7 3.6 1.1 39

Bulky collections 230 11.5 43 6.9 291Civic Amenity 236 6.1 17.7 11.1 271

Non household collection round

Waste

0 0 0 0 0

Total furniture in MSW

500 18 64 19 602

Table 1.4.2. Furniture quantities in municipal waste in the UK ‘000 tonnes- 2003/4 estimates

Waste Type Tonnage Area CoveredFurniture manufacture 531 UKPanelboard manufacture 1107 UKWood Products for construction 201 Wales and EnglandWood Packaging 40 UKOther industrial wood wastes 2552 Wales and EnglandRailway Sleeps 26 UKUtility Poles 24 UKTotal 4481

Table 1.4.3. Estimate of Waste Arising from both Commercial and Industry(www.wrap.org.uk, 2005)

Source: WRAP ‘Review of wood waste arisings and management in the UK (June 2005), Waste Wood as Biomass Fuel- Market Information Report, http://www.defra.gov.uk/environment/waste/topics/documents/wastewood-biomass.pdf

The analysis of the fuel source is important as it determines the quality of wood the plant is likely to receive and as a result the amount of syn gas able to potentially harvest. Two types of analyses were done which were, the proximate and ultimate analysis. The proximate analysis gave

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the moisture content, volatile content, ash content and high heating values. Meanwhile the ultimate analysis gave the percentage of a component in the wood sample such as carbon, hydrogen and nitrogen as well as minor components such as sulphur and nitrogen. As the fuel used for the process is likely to come from waste furniture, there was no exact or certain value for proximate and ultimate analysis since there were many types of woods that have been used for furniture manufacturing. Thus, the data was gathered on several types of woods to find an average value for the analysis. These are values were presented on Table 1.4.3. The assumptions made for the wood used was that the chemical content such as shellac and methyl alcohol which usually used to give protection and shining layers on the wood surface was neglected in the process.

Type Proximate Analysis Component Composition (% dry wt)Moisture Ash Carbon Hydrogen Oxygen Nitrogen

Yew 8 0.45 46.4 5.9 45.26 0.085Beech 18.6 1 49.36 6.01 42.69 0.91Oak

(white)35 1.97 49.64 5.92 41.16 1.29

Birch 30 0.81 50.2 6.2 41.62 1.15Poplar 26 1.86 49.37 6.21 41.6 0.96Pine 14 5.3 54.9 5.8 39 0.2

Oak (red) 37 2.9 52.6 5.7 41.5 0.1Fir 47 0.72 48.52 5.81 45.4 0.25

Eucalyptus 12 0.75 48.5 5.89 45.1 0.28Maple 12 1.4 50.6 6 42 0.3

Redwood sequoia

20 0.3 53.5 5.9 40.2 0.1

Spruce 15 0.77 51.06 5.75 42.21 0.11Cherry Tree

- 1.35 49.52 5.81 45.78 0.31

Douglas Fir 25 0.98 52.3 6.3 41.3 0.1Average 23.04615Table 1.4.4. Proximate and Ultimate Analysis of Woods (Tillman,1978 and chestofbooks.com)

The average moisture content of the waste wood feed was found to be 23.05%, which for simple calculations is taken to be 20%. The moisture content is needed in order to calculate the mass of the syngas once its moisture content has been reduced to 6%. This will also be required in order to calculate the dryer specifications. Low moisture content gives a higher calorific value and reduce the residence time during pyrolysis process. This can be achieved through a combination of the dryer and pulveriser. Hence the proximate and ultimate analysis for woods at 6% moisture content are presented in Table 1.4.5.

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Type Proximate Analysis Component Composition (% dry wt)Moisture Ash Carbon Hydrogen Oxygen Nitrogen

Yew 6 0.42 43.61 5.54 42.54 0.079Beech 6 0.94 46.39 5.64 40.12 0.85

Oak (white) 6 1.85 46.66 5.56 38.69 1.21Birch 6 0.76 47.18 5.82 39.12 1.08

Poplar 6 1.75 46.40 5.83 39.10 0.90Pine 6 4.98 51.60 5.45 36.66 0.18

Oak (red) 6 2.73 49.44 5.35 39.01 0.09Fir 6 0.67 45.60 5.46 42.67 0.23

Eucalyptus 6 0.70 45.59 5.53 42.39 0.26Maple 6 1.31 47.56 5.64 39.48 0.28

Redwood sequoia

6 0.28 50.29 5.54 37.78 0.09

Spruce 6 0.72 47.99 5.40 39.67 0.10Cherry Tree

6 1.26 46.54 5.42 43.03 0.29

Douglas Fir 6 0.92 49.16 5.92 38.82 0.09Average 6 1.38 47.43 5.58 39.93 0.41

Table 1.4.5. Proximate and Ultimate Analysis of Woods at 6% Moisture Content

Carbon and Hydrogen composition is the most significant in this process since it determines the quantity of syngas produced through pyrolysis and gasification process.

Based on Table 1.4.4 and 1.4.5 it was found that the mode of carbon content on dry weight percentage at 6% moisture content was between 46 to 48 percent. Meanwhile for hydrogen the mode was observed to be at 5.4 to 5.6 percent. These values were used for further calculation of mass balance and energy balance.

2. General Process

2.1 Background Information – P D Desai

Pyrolysis is the degradation of organic matter usually carbonaceous, in the absence of oxygen to produce bio oils and non-condensable and condensable vapours. It usually starts at 400 ⁰C. (Fichtner, 2004)

There was a point of contention as some papers actually claimed that pyrolysis in fact didn’t produce any syngas. One of the more prominent ones was by Purdy,K.,R. ,et al. Papers and documents that did agree with syn gas production via pyrolysis were by Bridgwater(1994) and Biffaward, 2004 .

Gasification is the conversion of the said carbonaceous matter into a mixture of Carbon Monoxide & Hydrogen along with other compounds by heating the carbonaceous matter with less than

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stoichiometric amounts of Oxygen/Air and Steam. The temperature is usually higher, roughly around 1100 ⁰C.

The mixture of Carbon monoxide and Hydrogen is called Synthesis or Syn Gas.

It has been found that the syn gas produced has a CV of 4-7 MJ/Nm3 when Air is used and a CV of 10-18 MJ/Nm3 when Oxygen is used. (Bridgwater, 1994). The reduction in the CV when air is used is due to the Nitrogen content of air diluting the end product gas.

Biomass on pyrolysis can produce different products depending on the temperature, pressure, and the other process conditions.

Usually it produces flammable gases – including Hydrogen and Carbon Monoxide , non-flammable gases, pyroligionous liquor , water , Char , Tar and Ash.

The Non Condensable gases are the syngases mixed with a bit of methane and carbon monoxide.

The liquid contains a lot of chemicals including Tar, acetone, methanol, acetic acid, etc.

These are the common substances that are produced in the chamber and there is a large difference on the ratios of the substances produced. They depend predominantly on the process conditions and the quality of the biomass and its moisture content. Therefore it is highly advisable to pre-treat the biomass before actual pyrolysis or gasification.

Pre-treatment usually involves pulverising it and drying it.

It is pulverised to a suitable size using a suitable pulveriser. This makes it easier to use and less cumbersome for transport and feeding it in to the hopper and the reactor, etc.

Drying is also important as it helps in reduction of the volume and the bulk size and also helps in increasing the energy density of the biomass. As the calorific value rises significantly it is worth the expense of energy. In fact it is feasible for the exhaust gases of the engine to heat up the biomass and dry it. As the exhausts leave, they can be passed through the heat exchanger and then into the dryer.

The amount of water, volatiles & ash varies with the different types of feed and different process conditions.

There are a number of companies that use these technologies albeit with different feed materials. These were taken from the Defra article ‘Advanced Thermal Treatment of Municipal Solid Waste, 2007.’

The locations, the feed in tonnes per annum and the primary technology used are mentioned below

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ManufacturerPrimary

TechnologyCountry

Capacity in tonnes per annum

Feed

Compact Power Tube Pyrolysis UK 8000 Clinical Waste

EnergosGrate

GasificationNorway 10000 Industrial and Paper waste

EnergosGrate

GasificationNorway 34000 MSW

EnergosGrate

GasificationNorway 36000 MSW & Industrial Waste

EnergosGrate

GasificationNorway 70000 MSW & Industrial Waste

EnergosGrate

GasificationNorway 37000 MSW

EnergosGrate

GasificationGermany 37000 MSW & Industrial Waste

EnergosGrate

GasificationGermany 80000

MSW & Commercial, Industrial Waste

EnergosGrate

GasificationSweden 80000

MSW, Commercial & Industrial Waste

Foster WheelerFast Ablative

PyrolysisFinland 80000 Mix waste

Mitsui BabcockRotary Kiln Pyrolysis

Japan 80000 MSW

Mitsui BabcockRotary Kiln Pyrolysis

Japan 150000 MSW

Mitsui BabcockRotary Kiln Pyrolysis

Japan 50000 MSW

Mitsui BabcockRotary Kiln Pyrolysis

Japan 95000 MSW

Mitsui BabcockRotary Kiln Pyrolysis

Japan 75000 MSW

Mitsui BabcockRotary Kiln Pyrolysis

Japan 60000 MSW

Thermoselect Tube Pyrolysis Germany 225000Domestic & industrial

waste

Thermoselect Tube Pyrolysis Japan 100000Domestic & industrial

wasteThermoselect Tube Pyrolysis Japan 50000 Industrial waste

Techtrade/Wastegen

Rotary Kiln Pyrolysis

Germany 350000 RDF

Techtrade/Wastegen

Rotary Kiln Pyrolysis

Germany 100000Domestic & industrial

wasteTable 1.3.1. Advanced Thermal Treatment Facilities

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Pyrolysis produces the highest amount of pyroligineous liquid if the heating rate is high and the temperature is relatively low temperature, 500 ⁰C.

Char is produced in greater quantity if the heating rate is slow and the temperature is low as well.

At high temperatures, gas production is pretty high. The heating rate is not really a factor when high temperatures are taken. (Bridgwater,A.V. , Hatt , et al )

Just after the dryer, it enters the pyrolyser. The pyrolysing unit is usually between 350 - 800⁰C, and there are different ways that the products of pyrolysis are determined. However despite the amount of scientific literature on the subject, it has been observed from reviewed literature that the pyrolysis process is sensitive to the process conditions and as a result, a large amount of uncertainty is attributed to the scale up process, necessitating pilot plants where feasible.

After the pyrolysis, the remaining amount of bio-oils, tar and char is sent to the gasification unit. The gasifier will be at a temperature around 1100 ⁰C. The gasifier makes sure that most of the input feed is completely reduced. Just a little bit is left and that is usually ash residue. The ash can then be sequestered easily. The gasification ensures thermal cracking of the tars produced and thereby helping in the reduction of emissions.

After the gasifier the gas is passed though the gas clean up, which can either consist of an ESP - electrostatic precipitator, a hydrocyclone or a scrubber – wet or dry or a Venturi Scrubber.

Then the gas can be burnt either in an engine or a turbine. Heat recovery is better when one deals with turbines as the gases come out at higher temperatures but the efficiency is roughly around 30 % while the engine has an efficiency of 40 -47% but the gases come out at a lower temperature.

The gases coming out from these can then be used in the pre-drying of the feed.

The ash that has been formed can be easily biosequestered. (Department of Energy, USA , 2008)

3. Research into the Main technologies

3.1 Drying and Pre treatment – E Azulay Tchitchi

The moisture content (MC) of wood varies considerably under different conditions. Before wood can be used, care should be taken to ensure it is at the desired moisture content for the application since wood will shrink and or swell if moisture is removed or added to it with associated variations in its mechanical properties. Furthermore, moisture content affects process efficiency since wet wood would consume approximately 40% of the heat for drying during the process.

This problem can be overcome by drying the wood prior the process via choosing an appropriate dryer that would best suit the moisture content that is to be achieved. Drying is most opted to minimize transportation costs, process delays and inefficiency. In addition there are a wide of dryers available depending on condition and amount of feed, process temperatures and residence time. For instance, pneumatic dryers, rotary either indirect or direct fired, spray and fluidized dryers. For the

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purpose of this design the wood was dried to 6% from an initial 20% (equilibrium MC), in order to provide improved efficiency, improved heat transfer rates and reduced process residence times

In addition, it is advisable to have a pulveriser or grinding system prior the drying, as this considerably increases the wood drying rate as well as allowing a more uniform drying rate across the wood as improving wood handling in the process. The type and capacity of the grinder will depend on the size reduction planned. Furthermore, an incorporated system of metals such as magnetic separation and/or screening will be used for the removal of heavy ferrous material containments that may be present in wood feedstock.

Moisture Content (MC)

Wood from trees is highly wet; the moisture content can vary from a 20% up to 65% MC depending on factors such as: type of tree, climate, season of the year, storage and which section of the tree is being used. Woods containing a MC higher than 70% are known for their low energetic value and therefore cannot withstand burning processes and are said to be swollen. A standard value for the MC is usually taken to be 40% as a basis on forest area estimation (Huhtinen M.,2005). Shrinkage and swelling of wood will only occur in the internal (bound) water content- below its fibre saturation point- any free water present in the wood will not have such effect in its dimensions (Brown, T).

Empirically, wood shrinkage does increase in a directly proportional manner as it being dried. This does not seem to be the case with wood strength which also tends to increase with drying rate but not in a direct manner (Hoadley, R.B, 2000).

Moisture content can be defined on in two distinctive ways:

Dry Basis (Bridgewater, A.V. et al, 2002)

X= mA

ms

kg of moisturekgof drymaterial

(Strumillo et al:1930)

Wet Basis (Bridgewater, A.V. et al, 2002)

X’=mA

m=

mA

mA+mSkg of moisturekg of wet material

(Strumillo et al:1930)

As mentioned above, wood will shrink or swallow as it loses or adsorbs water (either in liquid or vapour form) respectively. If these two mechanisms are allowed to occur naturally and with no control, a poor wood structure and quality can be expected. The solution to swelling is to induce moisture scarcity in the wood structure by drying it, either naturally or by physical means. To what extent the wood is dried depends entirely on its future usage and to which conditions, physical or chemical, it will be exposed to (U.S. Department of Agriculture, 1973).

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From analysis of Table 1.4.4 it can be seen that wood will have an approximately 50% to 20% MC when fresh or if stored for a long time, respectively. This allowed us to set the initial MC of our feedstock to 20%, assuming that the wood would be stored for a long time before usage. It was also noted that wood with low moisture content will have a higher calorific value compared with the higher moisture content wood. This was taken as a basis of our final moisture content decision, taken to be 6%. Moreover, the low MC will fairly provide us with a more effective process compared if feedstock was wet and an ease handle of any by-products that will eventually be formed in the process, i.e. char and tar.

Grinding and Pre-Treatment (Krook, J. et al., 2005)

Grinding is a high energy mechanical process that utilises various factors, including impact, abrasion, pressure, shear force and other factors in order to reduce the size of particles. It alone accounts nearly to 20-25% of the total expenditures in machinery processes in major developed countries. Grinding mechanisms are mainly distinguished by the wide range of processes that are available, including hammer, eccentric and ball mills. Grinders are normally electric power driven and the required amount of energy will depend on grounding rate, type of wood and so on.

There are several reasons why grinding should be considered:

Material being used is to hard and/or larger than the required specifications Separation of mechanically bonded containments

Additionally, through research it was found that for most feedstock (wood) the concentration of ferrous materials contamination is high leading to pollution problems as a result of chemical emissions to the atmosphere or water or problems relating to process build-up or clogging. One preventative measure for low concentrations of solid contaminants is fit the plant with magnetic separators in order to screen the processed wood before it enters the main process. Fitting these processes after the grinding stage helps to separate metallic and wood particle which enables a higher separation efficiency.

Drying (Reeb, J. E.,1997),(Strumillo, C. Et al., 1930)

Drying wood is part of a process of cutting transportation expenses, better value and preservation as well as enriching the material for its final use, as well as improving material handling and wood flow ability should be expected from the drying stage. Wood is usually dried to equilibrium moisture content, that is, a moisture content value that will not allow any more moisture transfer to occur with its surroundings, these values will vary depending on the type of wood and location. At this value the moisture content within the wood is the same. However, an average value would be 12% MC in humid weathers and even lower in dryer environments. There are some problems associated in drying wood: as it is dried wood shrinks that can eventually cause defects in the wood structure, also drying wood requires a considerable amount of heat input in order to dry it to the required moisture content.

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3.2 Pyrolysis – S McCord

Before it is possible to consider how to design a process flow sheet, it is important that the technologies employed through the process are fully understood. Therefore it was seen as vital for the group to research the chemical, kinetic and physical properties of the main processes used – pyrolysis and gasification. This section contains a summary of what the group found out, the information gathered was used to make informed decisions on the types of reactor that were the most plausible for our ideal operating conditions – those that maximised the production of the fuel gas.

Pyrolysis mechanism

This is the central process of the initial flow sheet developed. Pyrolysis can simply be defined as the thermal decomposition of wood in an atmosphere absent of oxygen (Fichtner, 2004). One current major use of pyrolysis is that of turning solid carbon based fuels (including biomass and coal) into gaseous or liquid products to be used as fuel sources. Due to the specifications set out in the brief this section will focus on the pyrolysis of biomass – mainly wood based biomass.

Figure 3.2.1. The reaction mechanism of the pyrolysis of wood - Adapted from (Zajec, 2009)

The diagram above is based on the Broido-Shafizadeh schematic for the pyrolysis of cellulose, one of the main components of wood. It shows that although initial reactions may require heat to be added, secondary liquid cracking reactions are actually exothermic, so in higher temperature operations, the temperature may have to be regulated to prevent the reaction “running away” and destroying the equipment and causing injury to work personnel. It can be seen that for the pyrolysis of biomass the main products are:

A gaseous fuel, this has a composition of primarily Hydrogen, Carbon Dioxide, Carbon Monoxide and some short length hydrocarbons. The composition of the gas produced varies

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greatly with the final temperature of the reaction. For this process, this is the primary product needed from pyrolysis.

A set of liquids – bio-oil and tar; bio-oil can be used to produce many useful products using the Fischer-Tropsch process (Zajec, 2009). Conversely tar production needs to be minimised as it has no benefits and can cause major problems within the process equipment. Tar can also be hazardous to health and difficult to destroy after its formation.Bio oil doesn’t mix with hydrocarbon fuels, has a heating value of 18 MJ/kg and can be used in engines and turbines. Bio oil has a major problem: it is highly acidic (pH -2.5) and therefore corrosive. Another design problem associated with bio-oils is its complex and varied chemical composition, with over 300 different chemicals occurring in different quantities and different samples. (Bridgwater and IchemE, 1999)

A solid by-product – bio-char (charcoal), the char remaining has high carbon content and can also be used as a fuel due to its high calorific value, around 30 MJ/kg (Zajec, 2009).

The products of pyrolysis vary greatly on the conditions under which pyrolysis is undertaken. This variance is what makes designing pyrolysis processes difficult. The main factors that should be considered when designing a pyrolysis reactor are discussed below.

Operating temperature

The operating temperature of the pyrolysis chamber has arguably the biggest impact on the products that are derived. Lower operating temperatures lead to the increased production of liquid products, with maximum liquid yield being achieved around 500-550°C (Bridgwater,A.V. , Hatt,B.W, ,et al,1979). Above this temperature the amount of liquid product begins to subside with gaseous products becoming more abundant. This is due to the increase in secondary reactions taking place, these reactions are effectually the thermo-cracking of the bio-oil (and some tars), resulting in higher yields of hydrogen and carbon monoxide. The graphs below substantiates what is being said here,

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Figure 3.2.2 A graph showing the varying yields from pyrolysis depending upon the operation temperature (Bridgwater,A.V, Hatt,B.W, ,et al,1979)

The operating temperature also greatly effects the composition of the gas produced and its calorific value. The ideal gas produced would consist of entirely carbon monoxide and hydrogen, this is known as a syngas (bridgwater,A.V, 1994).

As the temperature increases above 550°C, not only does the volume of gas increase, so does its composition due to the water gas shift. This is explained further in the gasification section of this report. It results in a shift from a high CO2 content towards a higher CO content, thus giving a more combustible gas.

Heating rate

The rate at which the temperature of the feed is increased in the reactor, usually given in °C/min, is another factor that affects the pyrolysis products. Higher heating rates are usually linked to shorter residence times and are used to maximise liquid production (Bridgwater, 1999). Lower heating rates are used to maximise either bio-char or gas products – low heating value gas, the preference to either of these is linked to the operating temperature of the kiln (Bridgwater, 1999).

Residence time

Although this is often dependant on the two conditions stated above, it is plausible to set a residence time and affix one of the other two conditions instead. Different residence times affect the yield of each product by quite a bit. Temperatures around 400°C and residence times of 1-5 s. (Bridgwater, 1999). The longer the residence time, higher temperature leads to gas production, as greater thermal cracking is achieved.

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Number of pyrolysis stages

A major problem with pyrolysis is how to limit the production of tar. As previously mentioned many tars are difficult to handle and process, due to their high viscosities among other physical properties, it is therefore necessary to try to develop technologies which can minimise tar production. Tar reduction has to be top priority, it is highly carcinogenic and difficult to handle. It is also difficult to break down at that temperature. One way in which this is possible is to use staged pyrolysis. This allows a more gradual increase to the final temperature of

Another way, which has been followed in the second flow sheet, is to use conditions that favour lesser tar production or use gasification to crank up the temperature by using oxygen and steam, ensuring complete breakdown of the tars produced.

The characteristics above can be applied in different dynamics that result in a range of possible pyrolysis mechanisms. These pyrolysis methods are widely recognised and some are very established processes. They include:

Fast pyrolysis Slow pyrolysis – Gasification. Flash pyrolysis Carbonisation – the making of charcoal.

Fast pyrolysis

Fast pyrolysis is typically operated at temperatures between 400°C and 550°C (Kent-Riegel, 2007). The residence time is short; anything between around one to five seconds and a high heating rate of 100-1000°C/second (Kent-Riegel, 2007). Fast pyrolysis is a widely used and new technology, with the aim of producing primarily bio-oil. If the required conditions are achieved as much as 75% by weight of the feedstock can be converted into bio-oil (Bridgwater,A.V., 1994).

Slow pyrolysis – Gasification.

Conversely, slow pyrolysis has longer residence times with much slower heating rates. Residence times can vary from around 4-6 minutes to around 6 hours (Kent-Riegel, 2007), with heating rates much lower than those of fast pyrolysis. Slow pyrolysis is usually operated at a higher temperatures to increase the amount of gaseous products produced (Kent-Riegel, 2007).

Flash pyrolysis

Flash pyrolysis is similar in type to fast pyrolysis; accept residence time is usually less than a second with heating rates as high as 10000°C/second (Bridgwater,A.V., 1999). This type of pyrolysis produces a high amount gas or liquid (mostly liquid) but is not currently widely used in industry.

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Carbonisation

The production of charcoal is a type of slow pyrolysis reaction. Residence times are usually a couple of days with an operating temperature of around 350-450°C and therefore heating rates tend to be low, around 1-10°C/minute (Kent-Riegel, 2007). Obviously the main aim is to produce as much solid product as possible. This is in itself a large industry but has no use in this application due to the long residence times required.

Table 3.2.1 shows a summary of the typical products from the thermal decomposition of wood using a range of the methods discussed above:

Table 3.2.1. A table showing the varying state composition of pyrolysis products in different methods of pyrolysis (Kent-Riegel, 2007)

After researching different types of pyrolysis it is now important to consider different pyrolysis reactors, so that an informed decision can be made. Not only do process conditions determine the type of products one gets but the reactors play an important role in that determination as well. In the following section, reactors shall be looked into in greater detail.

3.3 Pyrolytic Reactors – K Asiamah

One of the main limitations with respect to pyrolysis reactor design is how to achieve the high heating rates required for pyrolysis processes in order to produce high liquid yields. Fluidised beds exhibit high rates of heat transfer; however, separation of the fluidising material/fines from the syngas produced is problematic.

Pyrolytic reactors can be classified depending on the way heat is supplied to the biomass (i.e. by direct heat transfer from inert material; e.g. sand or indirect heat transfer through the reactor walls), but the most suitable and common method of classification is based on the way solids move through the reactor. This is sub divided into four types (WIDE University, 2007):

Type A: No solid movement through the reactor during pyrolysis. (e.g. Batch reactors) Type B: Moving bed (e.g. Shaft furnaces)

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Type C: Movement caused by mechanical forces (e.g. Rotary kilns and Rotating Screw reactors)

Type D: Movement caused by fluid flow (e.g. Fluidised bed, Spouted bed and Entrained bed reactors)

According to (Malkow, T. 2004):

Fluidised Bed Reactors Entrained Flow Reactors Auger Screw Reactors Rotary Kilns & Rotating cones

are common pyrolysers. The fluidised bed reactors and entrained flow reactors are technically gasification systems and as such have been described in the gasification section. (Bridgwater,A.V. ,et al ,2002, 1999, and Maniatis ,K. , 2001, and Van der Drift, 2004 ) – say that the above reactors are predominantly classified as gasification reactors.

Auger Screw

Auger screw reactors attempt to improve the heat transfer properties of the system by using relatively small diameter screws and by using a heat carrier medium, usually sand (Difficult to separate) or small iron balls (Easy to separate) that are preheated and fed in the feedstock to the screw reactor, thereby helping to raise the temperature of the fuel rapidly to the pyrolysis temperature. However, auger screw reactors are generally limited to low throughput rates - The largest example of a auger screw pyrolysis reactor used in industry was a 200kg/h unit constructed by Renewable Oil International (Bain, R. L. (2004) "An Introduction to Biomass Thermochemical Conversion" National Bioenergy Center (NREL) http://www.nrel.gov/) and limited evidence of wide-scale use of such reactors can been found in industry. Furthermore, another limiting factor is the amount of heat carrier medium required: The Iowa State University "Alternative Pyrolyzer Design: Auger Reactor" (http://www.cset.iastate.edu) indicates “a heat carrier feed rate 20 times the biomass feed rate” which would be impractical on a large scale.

Rotating Cone

It follows the principle of centrifugation, and this drives the hot sand and biomass up the heated cone. The vapours then condense and the char is burnt while the hot sand is recirculated. The major problem in this reactor is the separation of the bio oil and sand particles. The tar separation is also very difficult. The tar also ends up sticking to the wall of the cone. (NREL – National Renewable Energy Laboratory )

The construction is very simple and that is the reactors greatest advantage.

Vacuum Moving Bed

This technology was developed by Pyrovac in Canada. It is a moving bed system with a negative pressure so as to make sure that there is a greater amount of gas produced. This makes it easier for the production of gas but increases the complexity of the process by a large amount. It isn’t used a lot in the industry. (NREL – National Renewable Energy Laboratory )

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Rotary Kiln

This is a rotating kiln at a very high temperature. Feed enters the kiln from one side and it is so designed that by the time the feed reaches the other end it has pyrolysed and most of the gas has been produced. The rotation ensures greater mixing as well as greater heat transfer. One can add Catalysts for tar cracking in the unit without affecting it. (Fichtner, 2004) .

There are two types of kilns, indirectly fired ones and directly fired ones. For greater efficiency, it is advisable to use an indirectly fired one rather than a directly fired one. (Yearham et al , 2002).

The tumbling action of the products within the rotary tube results in high degrees of temperature uniformity and gas-solid contact resulting in a more homogeneous product, increased production rate and reduced processing time. Agitation also ultimately leads to higher efficiencies.

The main commercial and technical risks related to rotary kilns involve poor overall thermal efficiency and unpredictability in kiln performance.

Heated Tube Pyrolysers

The tube is heated externally. Movement is achieved either by using a screw or by ramming it in. This reactor is fired by using the waste heat from the higher temperature gasification

process and works out to be quite an efficient way for pyrolysis.

One drawback is the residence time involved and the wear and tear of the screw. The screw can be replaced and it can be added to the cost estimate of the plant later on.

3.4 Gasification – P D Desai

Figure 3.4.1. The a basic flow sheet showing the process of gasification

Gasification is a process where solid or liquid carbonaceous matter is converted into combustible gas. The combustible gas is mainly hydrogen and carbon monoxide along with tiny amounts of methane, ethane, ethene & other hydrocarbons. There is also a slight production of tar and char. The process also includes the addition of air or oxygen and/or steam as the gasification agent. These ensure the partial oxidation of the feed.

Some classify gasification into two sorts of gasification.

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Carbonaceous matter- Solid/Liquid

Air/Oxygen &/or Steam

Combustible Gas

800-1500oC

It can be Direct and Indirect gasification. As per Belgiorno,V. , et al , 2003 ,

Figure 3.4.2. A flow sheet showing the passage taken through direct and indirect gasification

The combustible gas contains Carbon dioxide, Carbon monoxide, Hydrogen – due to steam, Methane, Water Vapour, trace amounts of higher hydrocarbons- ethane, propane, inert gases present in the gasification agent, like nitrogen if air is used, various contaminants such as small char particles, ash and tars including tar forming compounds and condensable organic compounds. (Bridgwater, 1994) & (Purdy et al,1985).

Gasification also achieves in increasing the energy density of the fuel whereby most of its energy is retained in its gaseous form. It is advantageous because gases are easier to clean, produce less emissions, easier to transport etc. (Swaaij, 1983)

The process has a shorter residence time, easy start up and there are other contributing factors that increase its appeal.

Biomass, when fired with steam produces a gas rich in hydrogen but there is still a problem with the tar production. (Hofbauer,H. et al,2000).

To circumvent this problem, oxygen has also been added to the process with the addition of Oxygen, not only does the Calorific value of the syngas produced go up, but also the tar production is kept to a minimum. The temperature achieved is also higher. Steam ensures the complete gasification of the incoming mixture of bio oils, tar and char also known as bio-slurry.

The calorific value of the Syn gas when air is used as the gasification medium is 4-7 MJ/m3.

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Carbonaceous Material

Using a gasifying agent with Oxygen

present Eg. Air / Oxygen

Direct Gasification , mostly Autothermal

Gas, Tar & Char

Using an Oxygen Free Gasifying agent

Eg. Steam

Indirect Gasification, external heat is required for this

process.

Gas, Tar & Char

The calorific value of the gas is around 10 - 18 MJ /m3 if oxygen is used as the gasifying medium. These are the higher heating values. (Bridgwater ,A.V., et al ,1999)

Type of Gasifier Gasifying Medium

Gas composition % V/V HHV - MJ/m3

Gas Quality H2 CO CO2 CH4 N2 Tars Ash

Fluid Bed Air 9 14 20 7 50 5.4 Fair PoorUpdraft Air 11 24 9 3 53 5.5 Poor Good

Downdraft Air 17 21 13 1 48 5.7 Good FairDowndraft Oxygen 32 48 15 2 3 10.4 Good Good

Table 3.4.1. Gasifier product characteristics – (Bridgwater ,A.V., et al , 2002)

Table 3.4.1 gives a rough idea of the values of the gases produced. Using different reactors and gasifying mediums.

The high temperature of the gasification chamber ensures thermal cracking of the tar present. It is carried out a temperature of 900 - 1500° C. Sometimes it can go as high as 2500° C . In this particularly route, the temperature hoping to be achieved is around 1100 ° C. This means that it says an energy intensive process. Many of the higher hydrocarbons start cracking thermally at this temperature.

Figure 3.4.3. An updraft gasifier (Goswami. Y, 1986)

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Syn Gas Produced

Drying – 160 ⁰C

Pyrolysis – 600 ⁰C

Reduction - 800⁰C

Oxidation – 1200 ⁰C

Ash pitGasifying agent – Steam, Oxygen or Air

Feed Entering

Gasification occurs in a sequence of steps,

Drying: this happens at a temperature roughly about 160 ° C. This volatilises the free bound moisture and most of the bonded moisture as well.

Pyrolysis: this starts at a temperature of about 600-700 ° C. This is the zone where the major production of syn gas takes place. Most of the compounds are broken down in to simpler components. There are condensable gases formed - bio oil vapours, tar vapours, etc. and the char is unaffected. There is some char production as well.

Actual gasification consists of two parts.

Reduction: this is where the reduction of higher hydrocarbons takes place including tars, oil vapours etc in to CO and H2. The temperature reaches the gasification temperature.

Oxidation: this is the zone where the temperature is the highest. Partial oxidation takes place in this zone. Most of the higher hydrocarbons are broken down into lighter gases. Char gasifies into Carbon Monoxide, Carbon Dioxide, etc and because there is steam present, Hydrogen is also produced. This is via the Water gas Shift reaction.

Reaction:

(Illinich. O, 2005)

Whatever else is left behind is usually ash and that can be sequestered effectively.

Any Carbon dioxide produced is acceptable as it is already considered to be carbon neutral seeing that the plant/tree has used it up during its life cycle.

3.5 Gasifiers – P D Desai and K Asiamah

When choosing a gasifier several criteria need to be taken into account in order to select the most efficient and self-sustaining gasifier as well as being cost effective. The main factors to be considered will be:

Cost: the cost involved in purchasing the equipment. Not too much capital should be invested in designing the equipment as the operating costs will also be higher.

Efficiency: This is essentially how efficient the gasifier equipment operates. This will determine the amount of combustible gas produced after gasification hence how much combustible gas can be supplied to fire-up equipment in the plant.

Other design factors to be considered will be scaling up and whether equipment can be retro-fitted and also biomass properties as this will affect the gasifier efficiency.

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Also to be noted is that the combustible gas evolved from gasification would be having quite a few emissions like tar vapours, POPs – persistent organic pollutants. Therefore cleanup would be necessary. The reactor would be chosen primarily on this basis.

The major gasifying equipment used in power plants include: (McKendry, P. 2002)

Updraft gasifier Downdraft gasifier Entrained flow bed gasifier Fluidised bed gasifier – Atmospheric and pressurised circulating and bubbling Cyclonic gasifiers

Of the total number of gasifiers used in industry, 75 % were downdraft, 20% were fluidised bed, 2.5 % were updraft & the rest were various other designs. (Maniatis,K. , 2001)

Figure 3.5.1: Technology development & strategic planning for power (Maniatis, 2001)

Figuew 3.5.1 represents the market attractiveness and the strength of technology being used and its acceptance. This helps in forming some idea about the gasifier to be used.

Atm. CFB stands for Atmospheric Circulating fluidised bed gasifier.

Atm. BFB. is Atmospheric Bubbling Fluidised Bed Gasifier

Press. stands for pressurised.

Several gasifiers have been investigated, taking into account the criteria specified, in order to find the best gasifier for the process.

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Updraft Gasifier – Counter Current

This type of gasifier operates counter-currently with the char (and other by-products) moving down a vertical shaft and contacted by an upward moving product gas stream. Upper size of a single unit is around 10 MWe hence larger plant capacities require multiple units

This technology is simple with a high thermal efficiency and high carbon conversion, particularly proven for fuels that are relatively uniform in size. However, the product gas is very low in temperatures and very dirty. Average energy conversion is about 60-70%. (McKendry. P, 2002)

The updraft gasifiers produce a lot of tar and because of that, their market attractiveness is really low.

Oxidant

Fig 3.5.2. Diagram of an updraft gasifier (Bridgwater,A.V, 2002)

Downdraft Gasifier

It is a Co-current Operation. Char moves slowly down a vertical shaft and air introduced reacts at the throat that supports the gasifying biomass.

A relatively clean gas is produced after gasification with extremely low tar production and high carbon conversion. However, this is only the case when the biomass has a low moisture content (<20%). There is an 80 % conversion for this process. It is an attractive option for small scale plants. It can take upto 2 MW. It is really cheap to build as well. Scale up is very tricky particularly because of the Throat but a couple of them can be used. (Bridgwater, A.V, 2002 & Maniatis , K. , 2002)Figure 3.5.3 gives an idea of the downdraft gasifier.

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FLUID BED

Biomass

ASH

Product gas

BIOMASS

PRODUCT GAS/ASH

Fig 3.5.3. Diagram of a downdraft gasifier (Bridgwater,A.V.,2002)

Fluidised Bed Gasifier

The char will be introduced into a fluidised bed of sand/silica from above or directly onto the bed. The fluidising medium is usually air, with the bed usually operating in turbulence.

This technology is favoured by high reaction rates and good temperature control. It is generally used for medium to large scale plants. Fluidising the particles increases the cost of the system as they have to be of a certain size and pulverised. Pulverising is energy intensive therefore the costs go up. Tar separation is very difficult and operation is very complex leading to a limited turn down capability.(Bridgwater,A.V., 2002). Flying char loss makes it a low heat efficiency gasifier (around 60%) (McKendry, P. (2002). Also highly turbulent bed gives high heat transfer coefficients which in the long-run could save cost as smaller heat exchangers will be required to give similar throughput.

Fluidised bed gasifier equipment is rather expensive compared to the other gasifying technologies. There are a number of fluidised bed gasifiers used and a brief explanation is provided for the following.

An important example is the Atmospheric Circulating fluidised bed gasifier used by several companies such as Lurgi, Foster wheeler, TPS, Battelle, Austrian energy, etc. (Maniatis,2001).

It is a large scale gasifier, and can function upto 100 MW th and can even go higher than that if required. The Scale up is relatively simple. Due to its nature, small scale and medium scale application is difficult and would result in high costs as well. A circulating Fluid bed Reactor is shown below.

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OxidantOxidant

Throat

Fig 3.5.4 Circulating Fluid Bed Gasifier : (Bridgwater,2002)

Atmospheric Bubbling fluidised bed gasifiers have been operated with a large variety of small and medium scale pilot plants. They can go upto 25 MWth , and therefore would be worthwhile to look into them for the design. The gasifier diameter is larger for the same capacity as compared to the Circulating Fluid Bed Gasifier. Their market attractiveness is high and so is the technology strength as can be seen from the graph above. A number of companies use and promote these gasifiers like Carbona and Dinamec. (Maniatis, 2002)

The diagram below shows what a bubbling fluidised bed reactor looks like. The diagram has been taken from Bridgwater,A.V, 2002

Fig 3.5.5. Bubbling Fluid Reactor (Bridgwater,2002)

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These gasifiers can also be pressurised and thus result in simpler handling of the syn gas and make it easier to transport it as it is already in liquid form. The advantage is negated by the complexity of the technology involved. Foster Wheeler promotes these reactors along with Carbona. (Maniatis, 2002).

Entrained Flow Gasifiers

This gasifier operates at high temperatures in co-current mode. Biomass is introduced into the gasifier with steam and burned with air. Part of the combustible gas produced can be used to fire up the burner via a slip gas stream.

This technology reaches the highest efficiency from biomass. Average energy conversion ranges from 80-84% (Kovac, R.J. & O’Neil, D.J, 1989), but this can further be increased by torrefaction but at an additional cost. It is a relatively new technology and therefore is not as well understood.

There is a cost reduction on cleaning up since a reasonably clean gas is produced. The size required for this technology is very small and therefore pulverising costs go high up.

3.6 Gas Clean Up – S McCord

After the pyrolysis and gasification processes it is necessary to “clean” the gas before it can be sent to downstream equipment. This is because the processes used can create a range of pollutants that can have various negative effects to both the process machinery and the environment/population. Some of the pollutants that are most likely to be formed and have caused issues in other biomass fuelled plants are included below. They have been split into two broad groups.

1 – Pollutants created from the process

These are formed during either pyrolysis or gasification and are primarily from the chemical processes associated with each technology. Solid particulates will be an issue due to the size of the wood chips after grinding and de-volatilisation, the particles will be very small and it will be possible for these to become entrained within the gas flow. Other non-gaseous organic compounds that could become entrained within the gas flow include the bio-oils and tar causing the same problems as associated with solid particulates. There are also a range of gaseous pollutants that can be formed during the high-temperature thermal cracking stages of bio-oil and tar, these include benzene, toluene and other polycyclic aromatic hydrocarbons (PAHs) ,PNA, Un-burnt hydrocarbons, etc.

2 – Pollutants that occur from the quality of feedstock

These pollutants are much harder to prevent during the pyrolysis/gasification processes. They may include chlorine, sulphur and nitrogen compounds that are created from the release of these

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elements during the thermal degradation of wood. They are usually only produced in smaller quantities but must be considered nevertheless. They consist of metal compounds, hydrogen sulphide, SOx, NOx , COx, VOCs etc .

Now that the main potential pollutants have been identified it is important to define why they can have such negative effects. There are two main reasons for why gas clean up needs to be undertaken:

1 – Prevention of damage to equipment

After the gas has been produced using by pyrolysis or gasification it is going to be sent to a combustion stage, this can either be an engine or a turbine for electricity generation, or into a burner unit for heating the process. However pollutants in the solid or liquid phase (especially highly viscous tars) can block injector or carburettor systems (Bridgwater,A.V., et al , 2002) and can erode gas turbine blades (Bridgwater,A.V., et al , 2002). This can lead to disastrous situations including the destruction of expensive and high-tech equipment. It is therefore vital to ensure that all particulates and tars are removed before entrance into the combustion equipment

2 – Prevention of damage to environment and the population

This applies to all the pollutants previously mentioned, as all of them can have extremely negative effects to both people and the environment. Particulates can cause breathing difficulties for plant workers and those living nearby, whilst tars and bio-oils could end up deposited into aquatic ecosystems – poisoning wildlife and leaching into soils to spread further damage. Nitrogen, sulphur and chlorine compounds lead to a range of well documented hazards, with arguably the most publicised being the formation of acid rain. Many other organic compounds (including benzene and some PAH’s) and dioxins can create illnesses for people living near to the plant, aswell as lowering local air quality if left un-monitored and not prevented.

It is for these reasons that gas clean up systems are a vital part of this project. The group decided to research a range of technologies widely used and established before deciding upon which systems were the best for the scale and required cleaning conditions of this plant. The group looked at four main clean up technologies:

Cyclones (including hydro-cyclones) Electro-static precipitators (ESP’s) Wet gas scrubbing technology Dry gas scrubbing technology

The following section is a summary of the technologies considered.

Cyclone technology

Cyclones are a relatively simple technology, that is widely used due to its reliability and its low running and capital cost. Cyclones remove particulates from gas by using a pressure drop system to

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slow the gas flow down, (Ditria,J.C., Hoyack, M.E. , 1994). The pressure drop is created by compressing the gas to increase its velocity; this velocity is reduced through the cyclone causing particles to drop out of suspension (Perry, 2008).

Higher the pressure drop, better is the separation possible.(Ditria,J.C., Hoyack, M.E. , 1994).

Advantages of using a cyclone system include (University of Dartmouth, 2007):

Low capital and maintenance costs The ability to operate at high gas temperatures.

However one downside to the technology is the need to overcome a pressure drop, which requires an inclusion of a compressor generating a higher capital and operating cost. The technology is only viable for particles bigger than 5μm (University of Dartmouth, 2007) which should be feasible for this process.

Electrostatic Precipitators (ESP’s)

ESP systems are used in many power generating plants to clean flue gases of particulates, they use highly charged electric plates to remove solids from the gas.

This can provide up to 99.9% efficiency in removal of solid particulates (Hamworthy Combustion, 2010). However due to the tendency for these systems to be very expensive they are often only used on much larger power plants. There could also be issues with tar becoming charged and effectively slagging the plates, this is not usually an issue in coal plants due to the fact that less tar is produced and that ESP units are usually located after the combustion stages.

They also require much more space than cyclone units and are very energy intensive, requiring a high power rating per m2, however this may be less of an issue due to the generation of electricity on site.

Wet scrubbing units

There are a wide range of wet scrubbing units to consider, these include Venturi scrubbers and absorption systems (wet scrubbers and fluidised wet scrubbers) (forbesgroup.co.uk, 2006). They all work on the same principle, using a contacting liquid solvent (usually water but sometimes organic/oil based solvents such as kerosene) to remove pollutants such as tars and solid particulates. They operate using a pressure drop to force gas through a chamber containing liquid, usually with a high surface area, high efficiencies can be achieved at relatively low operating costs (forbesgroup.co.uk, 2006).

Design Advantages:

There are a number advantages like its high efficiency on gaseous pollutants , de-dusting capabilities, ability to clean severely dirty gases including reducing corrosiveness of the gas if this is an issue. (Hamworthy Combustion, 2010)

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The major downfall of these systems is what to do with the dirty solvent, as the pollutant is just removed into a different phase. The dirty solvent can also then become a corrosive liquid with the potential to damage the unit (museumstuff.com, 2010) This becomes an even more important point if your solvent is relatively expensive (i.e. not water) as it may be costly to replace solvent regularly.

Dry scrubbing units

Some types of dry scrubbing units include:

Bag filters Semi-dry and dry scrubbing

Bag filters act as a sieve, they trap particles over a specified diameter whilst still allowing gas to flow through. Although bag filters are highly efficient, there are many negatives which must be considered before using them in gas clean up operations. There is a large pressure drop associated with this type of process and the filters need changing intermittently thus creating a high operating cost for this process.

Semi-dry scrubbing systems use a complex system involving the addition of powdered solids through a jet nozzle to create a large contact area with the dirty gas, Acid gases react with the solids to form salts which are cleaned away leaving a much cleaner gas (Dupont, 2008). Typical solids used include alkali compounds (usually calcium or sodium based) and removal of up to 99% of HCl and up to 70% of SO2 (swensontechnology.com, 2009) can be achieved. Semi –dry and dry vary on the way the solid is delivered, with the difference being the use of transport liquids (volatile solvents). A major issue is the need to replenish the solid reactant regularly.

3.7 Energy Generation – R Hodgkinson

The main considerations identified surrounding the choice of energy conversion, based on the design of the power plant, include:

Thermal (Electrical) efficiency Thermal output (CHP and heat integration) Emissions Operation on low calorific value fuel (Syngas)

Various technologies exist for the conversion of stored chemical energy into electrical power. The following technologies, including some which are normally disregarded, have been reviewed with relevance to the plant design:

Steam turbines

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Steam engines Gas turbines Gas engines Diesel engines (With bio-oil) Stirling cycle engines Fuel cells

Steam turbines

Steam turbines are widely used for large-scale power generation due to their simplicity, however, most simple Rankine cycle based systems suffer from relatively low overall conversion efficiencies compared to internal combustion technologies. As a result of this, steam turbines are increasingly being combined with gas turbines as energy recovery systems in integrated combined cycle (IGCC) plants in order to improve the overall plant efficiency. Such systems are viable on large scale plants where the levels of additional energy recovery from such systems is appreciable, but with smaller scale plants in the order of scale of that under consideration, combined cycle designs would add appreciable levels of complexity and cost to the process, noting that combined cycle systems from Siemens are typically >200MW, (http://www.energy.siemens.com 2010) and thus it is felt that on this basis waste heat from the prime mover should be recycled in the plant or sold as CHP as much as possible before further secondary heat recovery stages are considered in order to keep plant complexity and capital cost to a minimum.

Steam engine (Brief review of current technology and applications)

Steam engines are included here, despite the fact that “Traditional” steam engines have been phased out, since the technology is rarely considered now and a review of its applications is considered of interest. It is worthwhile to note that interest in steam engines has recently been rekindled since modern steam engine designs can be used to replace internal combustion engines for transport applications where variable load is required; non-explosive combustion gives rise to reduced emissions (http://www.dlm-ag.ch/en, 2010) and the ability to run on a wide variety of fuels is desirable. On a side note with relevance to biomass and transport, if projects such as the 5AT (http://www.5at.co.uk, 2010) were suitably adapted, direct firing with biomass may be possible, avoiding the complex conversion processes to bio-fuels. A few small scale prototype engines are also currently in development work such as the quasiturbine (http://quasiturbine.promci.qc.ca, 2010).

However, for larger scale applications and the purposes of this design project where continuous load and speed are required, steam turbines at the current level of technological development are more efficient and require considerably less maintenance than reciprocating steam engines.

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Gas turbines

Gas turbines are attractive in power generation applications due their high Power/Size and Power/Weight ratios, as well as the reduced number of moving parts giving rise to reduced maintenance requirements. However, gas turbines have only acceptable efficiency figures typically between about 25% and 30% (http://mysolar.cat.com, 2010) and the fan blades can be damaged by solid particles, either entering in the air intake or within the fuel, turbines directly fired with pulverised coal were briefly tried for locomotives but quickly scrapped, which indicates that careful syngas cleanup will be required if a gas turbine was used, as well as careful intake air filtration and sighting away from wood processing to minimise any potential damage or filter clogging with wood dust or chips. By virtue of their design, almost all of the thermal energy of the fuel not converted to mechanical (Electrical) energy is lost as heat in the exhaust stream of the engine, with temperatures ranging between about 450°C and 550°C. Gas turbines are predominantly designed to be fuelled on natural gas with a heating value of around 35MJ/m³; however due to the uptake of IGCC plants it is presumed that the gas turbines will run in a similar manner with the appropriate modifications on syngas and pyrolysis gas: “The syngas can also be used to fuel a dedicated gas engine. A syngas from a very well run gasifier, or further processed for example by reforming, may be suitable for use in a gas turbine.” (UK Government DEFRA, 2007)

Gas engines

Although rather larger than gas turbines and with considerably more moving parts for a given power output, and thus greater maintenance requirements, gas engines currently offer energy efficiencies of above 40% (http://www.mwm.net/modules/home, 2010). It should be noted that whilst gas engines feature more moving parts, gas turbines typically require greater levels of precision and tolerances in their construction due to the high temperatures and stresses placed on the engine components compared to gas engines. Surprisingly, gas engine data sheets indicate that the exhaust gas temperatures are also comparable to that of gas turbines albeit with significantly lower flow rates; most of the waste heat in a reciprocating IC engine is absorbed into the cooling water or circulating lubricating oil at around 80°C-90°C. UHC and NOx emissions are problems with gas engines, but can be addressed with technologies such as catalytic reduction (Wit. J, 2005). In the same manner as gas turbines, gas engines are usually designed to operate on natural gas, however numerous manufacturers including Caterpillar (http://www.uk.cat.com, 2010) offer gas engines specifically designed to operate on low-CV gas in the range of 10.8 to 25.6 MJ/Nm³.

Diesel engines

Many pyrolysis plants intended to generate electricity have been designed or built using pyrolysis to produce bio-oil, which is then burnt in a diesel engine. These designs feature high thermal to electrical efficiency in a similar manner to gas engines and the ability modify or use widely-available diesel engines. However, the use of bio-oils in IC combustion is plagued with problems relating to the composition of the bio-oil and associated pollutant products; and as a result such conversion systems are currently considered to have too high a risk of economic failure for this project due to the risk of the plant not meeting current and future emissions regulation limitations (Blowes, J. H, 2003) and the design or modification of engines to run on bio-oil currently being in the R&D phase.

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An example of the severe effects of a plant not meeting environmental regulations can be seen in the Isle of Wight Energos plant, though this is a gasification waste disposal facility, the legislative effects are the same, which is currently shut down due to breeching dioxin emissions limits (Say No to Green Lane Incinerator Campaign, 2010)

Stirling cycle engine (Brief review of current technology and applications)

Stirling cycle engines are currently used as the prime generators in small scale power generation systems such as solar concentrators and micro-CHP such as the Baxi EcoGen (http://www.baxi.co.uk, 2010); and for larger scale systems Stirling cycle engines have been proposed for thermal recovery on power generation and chemical processes due to Stirling cycle engines being able to run from small temperature differences. For example, RGP Systems (http://www.rgpsystems.com, 2010) proposes Stirling cycle engine energy recovery systems operating with supply temperatures of 250°C and 100°C with appreciable out power options ranging from 250kW to 1MW, however development is in the early stages as a 10kW prototype was scheduled for completion in Q1 2009; thereby making this a fundamentally untested and risky technology as far as new large scale industrial systems are concerned (http://peswiki.com, 2010)

Currently the primary technological problems facing Stirling engine development is often the degradation of components and materials at high temperatures; however when ran as energy recovery systems the low temperatures involved reduces such problems. Their compact size and self contained construction enables the possibility of retrofitting Stirling cycle engines to existing plants; however it should be noted that transferring thermal energy to the Stirling cycle engine is a major consideration since the systems supplied by RGP Systems are designed to use condensing steam from a “Reciprocating steam expander” – Condensing steam has excellent heat transfer properties, and thus is ideally suited for heating the hot end of a Stirling cycle engine. For the purposes of the plant design, however, the use of a Stirling cycle engine would be limited to the use with a steam based thermal cycle due to the poor heat transfer properties that are likely to result from the use of a Stirling cycle engine with a gas engine or turbine exhaust; and due to the additional resulting level of complexity it is again thought that a single stage energy conversion process should be used which converts as much of the thermal energy released from the fuel to electrical energy, with as much of the waste heat produced being recycled in the process where possible or used as CHP.

Fuel cells

Fuel cells are an exciting area of technological development due to their ability to operate with no moving parts (Minimal maintenance), high efficiency operation (>50%), high temperature operation (For SOFC fuel cells – Therefore feasible CHP), and the concentrated output of CO2 that results that can readily be captured and sequestered. Apart from the widely-discussed hydrogen fuel cells, research is currently ongoing, primarily with regards to the use of SOFC (Solid-oxide fuel cells) with synthesis gas under the European BioCellus project - One of the primary areas of interest of this research is “The SOFCs and/or the gas cleaning unit have to operate under high impurities concentrations (dust, tars, sulphur compounds, alkali, heavy metals...)” (Saule. M, 2010).

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Aside from CHP possibilities with high temperature fuel cells, possibilities also exist to integrate fuel cell technology with gas turbines to produce high-efficiency (Up to ~70%) electrical power with small scale systems (http://www.energy.siemens.com, 2010)

Although at this point the results of the research are promising, research is still required into the long term operation of fuel cells with syngas in industrial projects, and as a result whilst fuel cell technology is a highly attractive option, the current lack of large scale experience with this technology makes the use of fuel cells economically risky; in much the same manner as that described for Stirling cycle engine technologies. Within the near future however, when more experience is obtained n fuel cell operation, fuel cells hold the capability to radically transform electrical power generation when operated in conjunction with gasifier technology.

Process generation efficiency improvements

With regards to the generation of power from a fuel source, the following considerations can be applied to improving the yield of useful energy from a given amount of energy stored within a fuel:

Ensure complete combustion Reduce waste heat generation (Improve process efficiency) Reuse waste heat on site (Heat integration) Reuse waste heat offsite (CHP)

In almost all cases, modern power generation systems obtain relatively high combustion percentages such that factors including UHC (Unburnt hydrocarbons) are classified as a pollutant rather than as a form of wasted energy per se. Despite the marked improvements in thermal energy conversion in recent years, the efficiencies of thermal conversion processes are fundamentally limited by thermodynamics depending on the temperature differentials across which such heat engines operate, and in practice such processes are currently limited to, at best, just under 50% conversion efficiency for single stage, high efficiency IC engines – Such limitations can be bypassed by converting stored chemical energy directly into electricity via technologies such as fuel cells, but as previously mentioned fuel cells are currently unsuited for use with synthesis and pyrolysis gasses due to the current concern over the effects of contaminants within the syngas on the fuel cell operation. As far as conventional thermal technologies are therefore concerned, a large amount of heat is rejected from the process which represents lost generating capacity and thus lost economical and environmental benefit, and as a result the design of new energy generating installations places an increasingly large emphasis on the reuse of this waste heat. The following summary of important parameters for energy recovery applies to the use of gas engines and gas turbines and includes information presented earlier:

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Parameter Gas turbine Gas engine

Size and weight Compact Substantial

Moving parts Few Many

Efficiency 25-30% Numerous above 40%

Thermal output High temperature exhaust gas at 450-550°C

Some in high temperature exhaust ~400-500°C) but

most heat in cooling system (80-90°C)

Output power range Typically 1MW upwards Low to ~10MW

Table 3.7.1. Comparisons between gas turbine and gas engine

It should be noted that the specifications of a generating set are usually defined for particular operating parameters (Such as fuel CV and composition), and as a result it is reasonable to expect that the performance of such a generating set to varying depending on the fuel supply to the system; as a result the above figures are approximate and intended for comparison purposes only at this stage.

The obvious choice from the above data for maximum electricity production would be a gas engine, since the electrical efficiency of gas engine generator sets is generally somewhat higher than that of gas turbines; and the waste heat from the engine is captured primarily in the circulating cooling water and lubricating oils and can readily be used for off-site CHP applications via suitable heat exchangers. However, many stages in the process of producing syngas require significant thermal energy inputs, including but not limited to the drying (150°C), pyrolysis (~450°C) and gasification (~1200°C) stages under start-up and/or steady state conditions. Although the flue gas from a gas engine is at a high temperature, the flowrate is relatively low and it may be found that heat integration in the process has more energy reuse potential than can be extracted from the exhaust flue of a gas engine. Furthermore, whilst CHP is an attractive concept, CHP applications are limited due to the requirement to site a CHP plant near to an urban or industrial complex that can make use of the waste heat with all the usual considerations of fuel/waste transport, plant construction etc as well as the considerable costs of installing a hot water distribution network, which may not always be viable.

On the other hand, whilst gas turbines have generally lower efficiencies than gas engines, as previously stated almost all the waste heat produced is contained within the exhaust, making it readily usable for heating or at least preheating other areas of the process to the high temperatures required.

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Burners operating on syngas could also be used to heat process stages, in conjunction with a gas engine generator set, however this is a highly inefficient use of syngas considering the large amounts of thermal energy that are rejected into the environment from the electrical generation process and should generally be avoided except for process start-up requirements.

Aside from heat integration via the production of electricity, once raised to the appropriate operating temperature the gasification process is largely self-sustaining and produces high temperature syngas as a result of the gasification process. This gas must be cooled to allow effective clean-up and use within the power generation apparatus, and as a result there is considerable potential to re-use this thermal energy to heat other parts of the process. This is particularly the case with the plant design of group 2, since due to the low pyrolysis temperature used the secondary, exothermic reactions of pyrolysis do not occur, resulting in a net input of energy required to the pyrolysis process for at least the heating of the wood feedstock, which could be taken via the cooling of the gasifier syngas output. Alternatively, the cooling of the syngas can also be used to preheat the steam and air/O2 feeds to the gasifier.

Fuel Gasifier High temperature fuel cell Gas turbine Steam Turbine Stirling cycle engine CHP

4. Process Design 1

4.1. Process Design 1 description and flow sheet – S McCord

As mentioned previously due to the vast amount of technologies available for the process and the different process routes that can be followed, the group made the decision to create two separate processes. The first process system involved an emphasis on pyrolysis to produce combustible gas to feed to the gas engine. The gasification stage of the process is secondary; it is used to gasify the remaining bio-oil, tar and bio-char, this gas is then going to be used to power the process.

On the following page is a simple schematic diagram showing the main features of the proposed plant. There are several important features that can be seen here, these will be discussed within this section and a reason highlighted for why each of them has been chosen.

The process uses pyrolysis to limit the flow rate to the gasifier, as this is done at the highest temperature and is the most energy intensive component of the system. Lower flow rates at a higher inlet temperature into the gasifier reduces the energy required to gasify the remaining bio-char and bio-oil.

As can be seen there have been major developments from the initial process flow sheet. Below is the reasoning behind the decisions made within the design by the group.

3 staged pyrolysis and gasification

The most striking feature of the design is the introduction of the 3 staged pyrolysis system. The reasoning behind this has been discussed in the previous Pyrolysis – An overview section. The staged

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pyrolysis helps create more uniform conditions in each kiln – it creates conditions for greater temperature uniformity and a more controlled heating rate to be achieved (Hornung, 2008). This greater control of the heating rate allows for a lower yield of tar as it creates conditions that favour the formation of shorter chain hydrocarbons (Hornung, 2008). This is because a higher heating rate that is more varied results in the cellulose and lignin bonds breaking to form longer chained hydrocarbons that have high viscosity and high combustion temperatures – some tars may not even crack no matter what temperature is achieved.

Research has shown a number of disagreements about the maximum temperature of pyrolysis. Most researchers argue that wood pyrolysis has a maximum temperature of 550°C, and that anything above this temperature is actually gasification. This is because at 550°C the maximum yield of liquids is reached, and that at temperatures above this the amount of bio-oil produced is reduced as the amount of gas produced increases. The reason for this shift in composition is because at high temperatures, above 550°C to around 800°C thermal cracking of the hydrocarbons present in the liquid phase occur, increasing the production of gaseous product (Zajec, 2009)

The choice of the rotary kiln pyrolysis kiln

The choice of the rotary kiln will be discussed in the next section, Rotary kiln reactors for pyrolysis.

Gasification

The choice of the gasifier was made by the group following research conducted earlier.

The type of gasifier chosen was a downdraft gasifier, the main advantages of this type of gasifier have been discussed earlier, but are recapped here:

Low cost High efficiency, up to around 80% Low production of tar, especially in comparison to updraft gasifier units.

Gas clean up

The following system has been designed for the stage entitled “syngas clean up”:

Figure 4.1.1. The gas clean up system used for process design 1

As can be seen from the diagram there are two main stages for gas treatment – a cyclone and a wet scrubber system. There are also two primary stages for preparation of gas entering the clean up equipment.

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The first stage is a simple heat exchanger, this is to cool down the gas, as gas from the pyrolysis stage and gasification stage enter the clean up at temperatures of approximately 750°C and 1200°C respectively. This cooling of the gas is primarily due to the fact that the gas engine cannot receive fuel gas at temperatures above approximately 150°C. The clean up system will also lower the temperature so it is not necessary to achieve this temperature on outlet of the heat exchanger.

The second stage of preparation for gas purification is compression. This is because both of the cleaning systems require a pressure gradient to function, the gas is pressurised to increase inlet velocity. The exact exit pressure from the compressor is yet to be decided and will depend greatly on the system requirements for the later stages, this will be determined in the individual report.

The primary clean up stage will be a cyclone system. This will use a pressure drop (gases are compressed to increase velocity, the loss of velocity then creates a pressure drop) within a cylinder to remove particulate matter from the gas – the larger the pressure drop the more efficient the cyclone (Perry, 2008).

The second stage is the more substantial gas treatment stage, where tars will be removed (and some other gaseous pollutants) as well as the remaining solid particulates. Wet scrubbers use a solvent, usually water but organic molecules (usually kerosene) can be used, to remove pollutants from the gas using mass transfer, condensation and molecular diffusion. This process however requires solvents to be replaced or clean, water is usually replaced whilst more expensive organic solvents are usually cleaned, as the pollutants are only moved into a different phase and still require safe depositation elsewhere.

After the second clean up stage it is important that the pressure is returned to atmospheric levels to prevent damage to the gas engine, as it can only accept fuel at this pressure without a significant increase in cost and a foray into less well explored technology.

It has not been decided whether to use a separate gas clean up system for the pyrolysis gas and the gasifier gas yet, there are both positives and negatives for using a singular gas clean up system. The most obvious positive is the reduced capital and operating costs. However having separate systems effectively provides a back-up system if one needs maintenance, it may also be useful to use separate systems as the type and amount of pollutants may vary depending on the process that has been used.

Flue gas heat integration

As previously mentioned in the drying section, it may be possible to provide the heat required for the dryer from the flue gas. Gas engine flue temperatures are relatively high at 450°C (mwm.net, 2010) and this waste heat energy can be used to dry the wood to approximately 6% moisture content. It may be possible to then use any remaining heat to preheat some of the air required for combustion, but care must be taken not to remove too much heat as the flue gas still needs to exit through the stack without condensing.

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Start up system

Although the process may be self-sustaining after start up, it is important that the plant is designed with an effective start-up system, this will only be used after commissioning and after periods of maintenance on plant (this could be planned or emergency related). Unfortunately the group have struggled to find a carbon neutral method for start up, therefore an alternate method has been proposed. During start up only the dryer and gasifier will be active, along with the gas clean up system. The gasifier and dryer will be powered by natural gas in pilot burners; all the feed will be sent to the gasifier to undergo gasification. It is highly likely that the efficiency achieved here will not be as high as when the process is fully running. The gas produced here will then all be sent to the burners used to power the rotary kilns. As these all reach operating temperature the normal process will begin, as the process moves away from start up to continuous production the pilot burner for the gasifier will be turned to standby and be replaced by burner 7, powered by gas produced from the process. As flue gas begins to become available for heat integration and to power the drying process the final pilot burner can be turned to standby, with the process then generating enough heat to power itself whilst still meeting the requirement of producing 4.5MW of electricity.

Flow control system

It was decided that although this is only a general schematic of the plant, and that instrumentation is not usually included, it would be necessary to include a main feature of this plant – a flow controller just after the pyrolysis stage. This has been included because of the large variance in pyrolysis efficiency (with respect to gas) considered. As can be seen in the mass and energy balances later in this section if the pyrolysis stage only generates 30% or less syngas (percentage based on weight) then the 4.5MW of electricity cannot be generated using this gas alone. The flow controller will be used to identify these conditions if they occur; measuring the volume of gas produced, and if needed it will divert some of the gas produced in the gasifier away from the burners and into the gas engine. This system can then also be operated in reverse; if the efficiency of pyrolysis is too high, for example a 70 or 80% conversion to gas, then some of this gas can be diverted away from the engine and into the burners, to ensure the process always meets its own heating requirements as well as its electricity generation requirements.

4.2. Choice of pyrolysis reactor for process 1 – K Asiamah

Rotary kiln reactors for pyrolysis.

For our plant design, it has been suggested for Rotary Kiln Reactors to be used because:

Most suited for slow pyrolysis Capable of handling solid waste of different shapes/sizes and feed preparation costs reduced Capable of being heated indirectly hence the quality of gas produced is improved as

combustion gases completely isolated

It was also suggested that employing multiple rotary kilns, which is essentially a staging process with each consecutive rotary kiln reactor at a higher temperature, will improve the quality of syngas produced hence reduce costs on clean up. This is because in heating up the biomass step by step (i.e.

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staging), the gradient at the heat source is lowered and hence better quality of gases is produced as the biomass is not dried to defragmentation.

Solid waste of various shapes, sizes and heating values can be fed into the rotary kiln either in batches or continuously. Operating temperatures range up to 1000⁰C, with a power rating in the range of 500kW (Malkow, T. ,2004), making it suitable for use in slow pyrolysis and gasification.

The basic design consists of a cylindrical vessel, slightly inclined to the horizontal and rotated about its axis. Biomass fed in at the upper end of the cylinder gradually moves down towards the lower end due to the action of rotation of the kiln, which in turn provides a degree of mixing and stirring.

The solid residence time is an important design factor and requires selection of diameter, length, speed, slope and internal design. Two basic design concepts of rotary kilns exist, the difference being how heat is supplied to the reactor:

Directly Fired Rotary Kilns

In directly fired rotary kilns, heating takes place inside the kiln hence biomass being processed comes into contact with hot gases produced from burning combustible gases before entry. Heat transfer in these kilns is usually by convection (between the inner most solid surface and hot gases flowing over the bed) and radiation. The rotary kiln is most typically a carbon steel cylinder lined internally with refractory. In a direct-fired kiln the hot gases of combustion are in contact only with the refractory lining and product; thereby protecting the steel structure of the vessel from high temperature stresses.

Hot gas for heat transfer

Fig 4.2.1. Diagram of directly fired rotary kiln: (Feeco International, 2010)

Indirectly Fired Rotary Kilns

The indirectly fired rotary kilns are closely parallel to the direct fired rotary kilns in operations. Heating takes place outside these rotary kilns meaning the biomass being processed within these kilns is not in contact with combustion gases. Since products of combustion are completely isolated from the product being processed, two heat transfer mechanisms take place. Heat transfer is by conduction between the solids and the wetted portion of the hot shell. In most applications the shell is heated to fairly high temperatures, so radiation between shell and solids also prevails. Also, as these units are heated from the outside, the materials of construction must be capable of withstanding higher temperatures.

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Product gas

Feed Stock

Bio Oil/ Bio Char

Product gas

Feedstock Product gas

Bio-char/Bio-oil

Combustion gases

Burner

FIG 2: Diagram of indirectly fired rotary kiln (Feeco International, 2010)

Which sort of kiln should be used?

Both rotary kilns can be used to provide thermal decomposition to the biomass supplied. Generally, the directly fired rotary kiln is the most common as it’s more efficient in operation and offers lower capital cost. Also, a large amount of gases from the directly fired rotary kilns goes through an emission control system, hence emission of particulate matter (i.e. carbon dioxide) are reduced.

However, even though the gases coming from the indirectly fired units going through an emission control system is very small, products and combustion gases do not come in contact hence the quality of gases (i.e. syngas) produced is not affected as the only gases which need to be in direct contact with the material are any gases that may have evolved in the unit.

Pyrolysis reactions need to be performed in inert atmosphere. This can be achieved in directly fired rotary kilns but at significant additional costs of oxygen detection, explosion relief doors and automated extinguishing systems.

However, the indirectly fired units are heated from the outside and can have an internal environment relatively free of oxygen through well designed sealing mechanisms at no additional costs. Indirectly fired units are also more favourable when separating highly volatile products.

From the above deductions, the Indirectly Fired Rotary Kiln was selected as it best suited our design brief, sufficiently high in efficiency and lower in cost.

4.3 Mass balances for Process Design 1 – N Driver

The original feed has been specified as 5tonne/hour, which enters the dryer in the form of pellets. 5tonnes/hour equates to 1.389kg/sec of wet feed (presumed to be at 20% water) entering the dryer. The dryer aims to reduce the feed of the water from 20% to 6% and so reduces the total mass of the wood as well. Taking 14% of the mass of wet wood entering the feed gives the mass of dry would

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which will enter the pyrolysis kilns. The mass of dry wood entering the pyrolysis reactors is 1.194kg/sec.

The process flow sheet planned is relatively unknown and as a result the plant needs to be able to cope with large ranges of efficiency. As a result the mass balances have been carried out over ranges of 20-85% efficiency within the pyrolysis kiln. The mass of gas produced is merely a percentage of the original dry mass of wood entering the pyrolysis kiln.

Table 4.3.1. The mass of synthetic gas produced with varying efficiency of the pyrolysis kiln

As can be seen from this calculation alone, the varying efficiencies can produce a large difference in the mass of gas produced. These mass values need to be converted into volumes in order to gather how much energy will be generated from the amounts of syngas produced from the pyrolysis kiln. In order to calculate the density, the composition of syngas at the highest temperature used had to be found, in the case of this process flow sheet the highest temperature was 750°C.

% composition of syngas componentsTemperature

(oC)CO2 CO H2 CH4 C3H8

700 16.7 44.3 15.5 16.1 7.4800 9.1 50.2 20.8 14.2 5.7

Table 4.3.2. Syngas composition at temperatures 700 and 800oC

Using Table 4.3.2 the syngas composition at 750oC can be calculated roughly, assuming there is a linear correlation between the composition of syngas at 700oC and 800cC. Assuming this the syngas composition at 750oC is an average between 700 and 800cC of the percentage values for each compound. This gives a syngas with the following compositions;

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Efficiency (%) Mass of gas produced (kg/sec)20 0.23925 0.29930 0.35835 0.41840 0.47845 0.53850 0.59755 0.65760 0.71765 0.77670 0.83675 0.89680 0.95685 1.015

Table 4.3.3. The composition of syngas at 750oC

In order to get the density using Table 4.3.3, the known molecular masses of the individual compounds were used along with the knowledge that 22.4 moles of gas occupies 1m3.

Mass of CO2 in the syngas (kg per m3);

= [(% composition of CO2 in syngas at 750oC/100) x (MM (g/mol) of CO2 x 22.4)]/1000

= [(12.9/100) x (44 x 22.4)]/1000

= 0.1271

All of the other components’ masses per m3 were calculated using the same method with a different respective composition and density. A total of all the values taken will give us an accurate representative of the density of syngas at 750oC.

Table 4.3.4. The mass of each component per m3 of syngas

Totalling all the masses of each component per m3 found in Table 4.3.4. The density of syngas is found to be 0.5505kg/m 3 .

Knowing the mass of syngas produced depending on the efficiency of the reaction and the density of the syngas produced, the volume of gas produced from the pyrolysis kiln stage can be established. The energy this gas produces when burnt in a gas engine operating at 100% efficiency, can also be found due to the knowledge that the higher heating value of synthetic gas is between 13-15MJ/m3, an average of 14 will be taken for the calculations to follow.

Volume of gas produced at 20% efficiency (m3)

= (mass of gas produced at 20% efficiency) / (density of syngas)

= 0.239/0.5505

= 0.4340

Energy produced at 20% efficiency (MJ)

= (volume of gas produced at efficiency of 20%) x (calorific value of syngas)

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% composition of syngas componentsTemperature

(oC)CO2 CO H2 CH4 C3H8

750 12.9 47.25 18.15 15.15 6.55

Relative mass of components (kg/m3)Temperature

(oC)CO2 CO H2 CH4 C3H8

750 0.1271 0.2964 0.0081 0.0543 0.0646

= 0.4340 x 14

= 6.08

Efficiency (%) Mass of gas produced (kg/sec)

Volume of gas produced (m3)

Energy produced when burnt in gas

engine (MW)20 0.239 0.43 6.125 0.299 0.54 7.630 0.358 0.65 9.135 0.418 0.76 10.640 0.478 0.87 12.245 0.538 0.98 13.750 0.597 1.08 15.255 0.657 1.19 16.760 0.717 1.30 18.265 0.776 1.41 19.870 0.836 1.52 21.375 0.896 1.63 22.880 0.956 1.74 24.385 1.015 1.84 25.8

Table 4.3.5. The volume of gas and hence the energy produced when burnt in a gas engine, 100% efficient, in relation to efficiency of the pyrolysis kilns

These energies produced are assuming 100% gas engine efficiency; however from previous research it is known that a gas engine’s efficiency is more likely to be around 40%. In order to receive a relatively accurate energy production from the syngas produced by the pyrolysis kiln, the values in the energy produced column in Table 4.3.5 need to be multiplied by 0.4, therefore taking the gas engine efficiency into account.

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Table 4.3.6. The energy generated from syngas produced in the pyrolysis stage and burnt in a 40% efficient gas engine

Efficiency (%) Energy produced in a 40% efficient gas engine (MW)

20 2.425 3.030 3.735 4.340 4.945 5.550 6.155 6.760 7.265 7.970 8.575 9.180 9.785 10.3

Table 4.3.6 shows how easily the aims of generating 4.5MW for the national grid are being met from the pyrolysis stage only. If the efficiency of the reaction lies below 40% then syngas will have to be taken from the gasification stage in order to meet the demand required. The values of energy being generated with efficiencies higher than 40% is more than required and any extra can be used to power the process, in turn making it sustainable. However, the additional power that isn’t used in the gas turbine isn’t therefore reduced to 40% of its value, meaning there is even more energy available than suggested by table 4.3.6.

Any solids/liquids that have not been pyrolysed are transported to the gasifier. The main assumption from the gasifier is that the syngas composition is the same as that produced from the pyrolysis phase of the reaction. As this flow sheet uses a downdraft gasifier the efficiency of the gasifier is taken as 60%, found whilst researching the gasifier.

To mass of solids/liquids being fed to the gasifier depends on the efficiency of the pyrolysis stage of the process and so the gasifier mass balance needs to be carried out at these ranges of efficiencies. The volume of gas produced will be calculated through the same methods as used for the pyrolysis stage; however the values will be multiplied by a factor of 0.82 in order to take the gasifier efficiency into account.

Pyrolysis stage efficiency (%)

Mass of solids/liquids fed to gasifier (kg/sec)

Volume of gas produced (m3)

Energy available to power the process

(MW)20 0.956 1.04 14.625 0.896 0.98 13.730 0.836 0.91 12.835 0.776 0.85 11.840 0.717 0.78 10.945 0.657 0.72 10.050 0.597 0.65 9.155 0.538 0.59 8.260 0.478 0.52 7.365 0.418 0.46 6.470 0.358 0.39 5.575 0.299 0.33 4.680 0.239 0.26 3.685 0.179 0.20 2.7

Table 4.3.7. The volume of gas produced from the gasifier depending on the pyrolysis stage efficiency and hence the energy available for the process

Although the main aim of the gas produced from the gasifier is to power the process, it may be the case at any time that the pyrolysis kiln is not functioning efficiently enough in order to produce the 4.5MW required in order to meet the needs for the national grid. As a result some of the energy available in table 4.3.7 will have to be redirected to be burnt in the gas engine and as a result the energy produced will be 60% less than shown in the table, due to the gasifier efficiency.

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In order to really know how much energy will be available for the process depending on the efficiency a new set of equations has to be carried out.

If the values of gas produced from the pyrolysis stage (before the gas engine efficiency is taken into account) and the gasifier stage for each respective efficiency and then the value of (4.5/0.4) is taken from each of the energy columns then all national grid needs should have been met and any remaining syngas can be used to power the rest of the process.

Available energy at 20% efficiency; = (6.1 + 14.6) – (4.5/0.4) = 14.75MJ

Pyrolysis stage efficiency (%)

Energy produced from the pyrolysis stage

before fed through the gas engine (MW)

Engine produced from the gasification stage

(MW)

Energy available to power the process

(MW)

20 6.1 14.6 9.525 7.6 13.7 10.130 9.1 12.8 10.735 10.6 11.8 11.240 12.2 10.9 11.945 13.7 10.0 12.550 15.2 9.1 13.155 16.7 8.2 13.760 18.2 7.3 14.365 19.8 6.4 15.070 21.3 5.5 15.675 22.8 4.6 16.280 24.3 3.6 16.785 25.8 2.7 17.3

Table 4.3.8. The energy available to power the process depending on the efficiencies of the pyrolysis stage

Table 4.3.8 shows that the more efficient the pyrolysis stage reaction is, the more energy available for powering the process. So any method in which the pyrolysis stage can be made more efficient will be valued in helping to keep the process sustainable. The hypothesis is that 9.5MW, in theory the lowest amount of energy available for the process, should be enough in order to power all equipment within the process. This hypothesis cannot be confirmed however until the energy balances have been carried out.

Due to the inefficiency of the gasifier some solid and liquid waste, mainly ash will be produced as waste, 40% of the initial mass entering the gasifier in theory. As the pyrolysis efficiency decreases the mass entering the gasifier increases and as a result so does the mass of the waste.

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4.4. Energy balances for Process Design 1 – N Driver

To confirm the hypothesis taken from the mass balances, energy balances have to be carried out around the equipment involved in the Process flow sheet for group 1. The energy balances are a method in which a rough value can be found for each of the main equipments required energy in order to operate.

The main issue needing to be taken into account is the varying specific heats of wood. The specific heat of woods will vary with different batches received. In order to take this into account the energy balances are carried out over a range of specific heats, ranging from 1.88-2.72kJ/kg.K.

As the energy balances are just in order to prove that this is a viably sustainable process the calculations are very simple. The method being used in order to find the energy required value are as follows;

E=m x CP x ΔT

The value taken from the above equation is then multiplied by a factor of 1.5, in order to take into account the efficiency of the heating process. This is a rough estimation and takes into account the energy lost through the equipment walls and the slightly inefficient burners.

In order to distinguish between the value before and after the efficiency factor of 1.5 the value that has not included the efficiency factor will be called the ‘theoretical energy required’, whereas that after the efficiency factor has been included will be called the ‘actual energy required’.

Dryer

The dryer is the first piece of equipment to be evaluated. The dryer is heated to 150oC in order to dry the wood to 6% moisture content. The outside temperature for the UK is taken as 10oC for this

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Table 4.3.9 The mass of waste per second depending on the efficiency of the pyrolysis stage

Pyrolysis stage efficiency (%)

Mass of solids/liquids fed to gasifier (kg/sec)

Mass of waste (kg/sec)

20 0.956 0.38225 0.896 0.35830 0.836 0.33435 0.776 0.31140 0.717 0.28745 0.657 0.26350 0.597 0.23955 0.538 0.21560 0.478 0.19165 0.418 0.16770 0.358 0.14375 0.299 0.11980 0.239 0.09685 0.179 0.072

balance. In order to reach 150oC, the temperature will have to be raised by 140oC. The mass flow entering the dryer is the mass of wet feed, which is a value of 1.39kg/sec.

Specific heat of the wood

(kJ/kg.K)1.88 2.09 2.30 2.51 2.72

Theoretical Energy

required (kW)365 405 445 490 530

Actual Energy required (kW) 550 610 670 730 795

Table 4.4.1. The energy requirements for the dryer

Depending on the specific heat of the wood the energy required, in order to generate a temperature increase in the dryer of 140oC, ranges between 550kW to 795kW.

Rotary Kilns

It is presumed that not all of the wood will be heated to 150oC and so the input temperature into the first rotary kiln is presumed to be 120oC. The mass flow rate into rotary kiln 1 has been reduced by 14%, due to the water which has been removed. The mass flow rate entering kiln 1 is 1.19kg/sec. The wood will be heated to 350oC in order to pyrolyse it, resulting in a temperature increase of 230oC.

Specific heat of the wood

(kJ/kg.K)1.88 2.09 2.30 2.51 2.72

Theoretical Energy

required (kW)515 575 630 690 745

Actual Energy required (kW) 775 860 945 1035 1140

Table 4.4.2. The energy requirements for rotary kiln 1

Rotary kiln 1 can have energy requirements from 775kW to 1140kW and will inevitably have the highest energy requirements of all the kilns as not only does it have the highest temperature rise to cope with but is also pyrolysing the largest mass of wood.

As all of the wood should have reached 350oC by the time it is fed into the next rotary kiln the input temperature of rotary kiln 2 is taken to be 350oC. In rotary kiln 2 the temperature is raised by 200oC to 550oC, continuing the pyrolysis process. The mass flow rate entering rotary kiln 2 is taken as 0.95kg/sec and it was found that 20% of the wood has been converted into syngas at 350oC, leaving80% of the dry wood flow rate. (Bridgwater,A.V, Hatt,B.W, ,et al,1979)

Rotary Kiln 3 operates very similarly to rotary kiln 2, as it also has a temperature increase of 200oC, from 550oC (the outlet temperature of rotary kiln 2) to 750oC. However more of the wood has

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pyrolysed into syngas and so the mass flow rate has decreased even more. The mass flow rate into rotary kiln 3 is 0.86kg/sec as another 8% of the original mass has been converted into syngas

Specific heat of the wood

(kJ/kg.K)1.88 2.09 2.30 2.51 2.72

Theoretical Energy

required (kW)360 400 440 480 520

Actual Energy required (kW) 540 600 660 720 780

Table 4.4.3. The energy requirements for rotary kiln 2

Specific heat of the wood

(kJ/kg.K)1.88 2.09 2.30 2.51 2.72

Theoretical Energy

required (kW)325 360 395 430 470

Actual Energy required (kW) 485 540 595 645 700

Table 4.4.4. The energy requirements for rotary kiln 3

The energy requirements for rotary kiln 2 and 3 are similar at 540kW – 780kw and 485kW – 700kW respectively. As the mass of wood decreases the further it travels through the staged pyrolysis process so do the energy requirements required to heat it to the required temperature.

The remaining solids/liquids which have not been converted into syngas during the process of travelling through the rotary kilns are sent to the gasifier. The mass of these solids/liquids depends on the pyrolysis efficiency and although the rotary kiln energy requirements depend slightly on the pyrolysis efficiency for the mass balances it was assumed the efficiency did not affect the process. However, the gasifier’s energy requirements will vary significantly with the varying efficiency of the pyrolysis stage before it and so in order to satisfy this, the energy balances will have to be carried out over a range of efficiencies.

Whereas the specific heat of the wood has been used for the rotary kiln energies it is mainly char and bio-oil which will be entering the gasifier and so the specific heat of a 2:1 mixture in favour of char will be taken. The specific heat of char is 1.01kJ/kg.K and the specific heat of bio-oil is 1.42kJ/kg.K. Using a 2:1 mixture it the specific heat is average specific heat is found to be 1.15kJ/kg.K. (Perry, 1997)

The gasifier will have an input temperature the same as the outlet temperature of rotary kiln 3, 750oC, and will be used to raise the temperature of char and bio-oil to 1200oC. The total temperature increase in the gasifier is 450oC.

Efficiency of

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the pyrolysis stage (%)

40 50 60 70 80

Mass of solids/liquids

(kg/sec)0.72 0.60 0.48 0.36 0.24

Theoretical Energy

required (kW)370 310 245 185 125

Actual Energy required (kW) 555 460 370 280 185

The energy requirements for the gasifier varying from 555kW at pyrolysis efficiencies of 40% to as little as 185kW at efficiencies of 80%.

In order to be able to make a strong judgement as to whether this process is viable the total energy required for all the equipment will need to be taken. In order to prepare for the worst scenario that could occur in the process the largest energy values shall be taken from each respective piece of equipment.

Table 4.4.6. Maximum energy requirements for the process

In the worst case scenario, when the process is using the most amount of energy possible, the energy requirements are 4 MW.

In Table 4.3.8 the worst case scenario would leave 9.5MW available to power the process, once the syngas has been used to generate 4.5MW of electricity. In the worst case scenario all the processes needs would be met and 5.5MW would be in excess.

4.5. Cost Estimation for Process Design 1 – N Driver

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Equipment Maximum Energy required (kW)

Dryer 795

Rotary Kiln 1 1140

Rotary Kiln 2 780

Rotary Kiln 3 700

Gasifier 555

TOTAL 3970

Accurate cost estimation at this stage in the process is unlikely and for the time put in would still only yield slightly accurate results. However a cost estimate is essential to see whether the plant is viable. A viable plant needs to have the following.

- A net profit on the electricity they sell compared with the operational costs- A reasonable payback time (the time it takes for the project to break even)

The cost estimation carried out will be a preliminary estimation and are usually found to be 30% accurate (Sinnott .R.K, 1999). The cost estimation will be broken down into 3 sections; fixed cost, the one off payment to get the plant ready for start up, working cost, the additional investment required above the fixed cost, and finally the operational costs.

Fixed Cost

The fixed cost will be calculated using previous references to equipment prices and then scaling up or down and taking into account inflation. The cost estimation of the equipment will then be used to find the indirect cost estimation using a series of basic relationships.

The first cost estimation was carried out on the rotary kilns. In order to carry out a cost estimation the size needs to be found first. A rough size estimation using the following formula:

Size = residence time x mass flow rate

Rotary Kilns

The residence time was taken to be 4 hours in order for syngas to properly be produced at the low temperatures the rotary kiln performs at. However as rotary kiln 3 is technically a gasification stage the residence time was taken to be 30minutes and so in turn will be of a much smaller size. The mass flow rate of the feed was found to be 1.19kg/s for rotary kiln 1, 0.96kg/s for rotary kiln 2 and 0.86kg/s for rotary kiln 3.

SizesRotary Kiln 1 25m3

Rotary Kiln 2 20m3

Rotary Kiln 3 1.5m3

Table 4.5.1. Rotary Kiln sizes

Once the sizes had been found, a value for a currently operating rotary kiln had to be found. The rotary kiln cost quoted is from Perry’s Chemical Engineering, 1997, and gave a value of $488800 for a rotary kiln of 112m3, including burners. In order to scale the costing

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down to the rotary kilns being used in the process the values have to be plugged into the following equation.

C2=C1( S 2S 1 )

0.6

(Sinnott .R.K, 1999)

Where C1 and C2 are the cost of each piece of equipment and S1 and S2 is the size of each piece. The 6/10th rule is applied as well, presuming that this is the ratio of cost to scale up, for every time you double the unit in size the cost rises 6/10th’s.

The costing nearly always comes from a reference in the past and so in order to gain the pricing in 2010 a cost index value has to be taken, where the cost index relation based on a source of 1997 is 151.2 [UK and International Plant Cost Indices, 2009].

Taking the current exchange rate to be 0.62 US dollars to the pound, the overall cost of rotary kilns can be found.

The fixed capital cost can be found for each by taking into account the following relations:

Typical factors for estimation of project fixed capital costProcess Type

Fluids Fluids - Solids SolidsPCE Major Equipment

cost1.00 1.00 1.00

f1 Erection 0.40 0.45 0.50f2 Piping 0.70 0.45 0.20f3 Instrumentation 0.20 0.15 0.10f4 Electrical 0.15 0.10 0.05f5 Process buildings 0.15 0.10 0.05f6 Utilities 0.50 0.45 0.25f7 Storage 0.15 0.20 0.25f8 Site development 0.05 0.05 0.05f9 Ancillary buildings 0.15 0.20 0.30

3.40 3.15 2.80f10 Design and

Engineering0.30 0.25 0.20

f11 Contractor’s fee 0.05 0.05 0.05f12 Contingency 0.10 0.10 0.10

1.45 1.40 1.35Table 4.5.2. Typical Factors for cost estimation (Sinnott.R.K, 1999)

The process involves both fluids and solids and so the values will be taken from the respective column.

In order to calculate the fixed capital cost:

Fixed capital cost of equipment = Equipment Cost x (1+0.45+0.45+0.15+0.10+0.10+0.45)

Equipment Cost (£) Fixed Capital Cost

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Rotary Kiln 1 190000 515000Rotary Kiln 2 165000 445000Rotary Kiln 3 35000 94500

Table 4.5.3. Fixed capital cost of Rotary Kilns

Any equipments costs are found using exactly the same method.

Downdraft Gasifier

In 1988 a down draft gasifier was $200/kWh (Reed. T, 1988). Referring back at table 4.4.5 the process operates a downdraft gasifier at 550kWh. In 1988 this would have led the equipment cost of our gasifier being $110000.

The cost indices between 2010 and 1958-1959 is 532.3 (Plant cost Indices 2010) and the cost indices between 1988 and 1958-1959 is 342.5 (Baasel, W., 1988). This gives a cost index of 189.8.

Dryer

Drying area = 5000 x (1/670) x ( 1/ 3600) x 10 x (1/0.003) = 6.9 m2 = 75 feet2

Figure 4.5.1. Graph showing the purchase cost of dryers depending on the drying area (Loh. H. P, 2002)

As can be seen from figure 4.5.1 the purchase price of a direct contact rotary dryer would cost roughly $17000 for the drying size required. These values were taken in 2002.

The cost index between 2002 and 2010 is 136.7 (Plant cost Indices 2010).

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Gas Clean Up

The gas clean up is split into 2 pieces of equipment, a cyclone and a wet scrubber. The wet scrubber fixed costs are relatively small in comparison to its operating costs.

The cost for a wet scrubber in 1997 was $13000 per m3/sec (United States Environmental Protection Agency, 1997). The process operates at roughly 1m3/sec through the clean up. The cost index between these two years has already been calculated as 151.2.

The cost of a cyclone separator in 2008 was $10000 per m3/sec (Controlling emissions from wood boilers, 2008). The cost index between 2008 and 2010 is 98.0 (Plant cost Indices 2010)

Heat Exchanger

In the process flow sheet and gas clean up combined there are 2 heat exchangers. In 1998 a heat exchanger similar to the shell and tube ones used in this process were found to cost $14400 (Perera. C, 1998). The cost index between 1998 and 2010 was 142.3 (Plant Cost Indices 2010).

Gas Engine

The gas engine has been found to cost $35000 in 1997 (Perry. R, 1997). The cost index of 1997 to 2010 has already been found to be 151.2.

Equipment Cost (£) Fixed Capital CostRotary Kiln 1 190000 515000Rotary Kiln 2 165000 445000Rotary Kiln 3 35000 95000

Down-draft gasifier 130000 350000Dryer 14500 40000

Gas Clean Up 20000 50000Heat Exchangers 25000 70000

Gas Engine 35000 90000TOTAL 1655000

Table 4.5.4. The fixed capital cost of each piece of equipment, hence the total fixed capital cost

Assuming the working capital is roughly 15% of the initial cost gives a working capital of £250,000. This gives an overall initial cost estimation of £1,905,000.

Operational costs

The operational costs can be estimated as fractions of the overall initial cost. (Sinnott, 1999)

Miscellaneous materials – 10% of overall initial cost Maintenance – 10% Labour – 15% of operating costs Supervisor - £35000 Plant overhead – 15% of overall operating costs Capital charges, taxes and insurance – 15%

Taking all this into account the overall operating cost can be found to be roughly £950000.

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The electricity generated will be sold to National Grid. The current tariff applied by National Grid for every unit of electricity sold was approximately £122 per megawatt hour (The Times, 2010).

The energy produced by the plant in one year can be found below.

Electricity produced per hour = 4.5 MWh

Electricity produce per year = 4.5 x 24 x 365 = 39420 MWh

For this amount of electricity the gross profit generated would be £4.8million/year, giving a net profit of 3.85million/year.

Using this cost estimation the plant would have a payback time of 1-2 years, an extremely short time in industry. However, these are extremely rough estimations and so values found could vary by up to 30% on the values calculated.

5. Process Design 2

5.1 Process Design 2 description and flow sheet – P D Desai

This process involves the predominant use of gasification technology.

Initial pre-treatment and drying is the same as the general process. The biomass is pulverised to roughly 3 mm size and then dried to achieve a moisture content of 6%. There are a lot of different pyrolysis reactors to choose from and at the moment we have not decided upon the reactor; although currently auger screw, heated tube and rotary kilns have been found in industrial use. This reactor will be specifically designed so that it can be used for the indirect firing of the pyrolysis chamber with the syn gas coming out from the gasifier. The temperature of the pyrolysis stage here is around 450 ⁰ C since at this temperature pyrolysis starts to take place.

There is a small conversion of approximately 20% of fuel to pyrolysis gas at 450 ⁰C (Bridgwater,A.V. , et al. ,1999.), which is then passed through a gas heat exchanger to lower the gas temperature, through the gas clean-up process and then sent over to the gas engine to be combusted. Once that has been achieved, the flue gas at 450 ⁰ C is sent to the dryer and can be utilised to provide at least a proportion of the dryer heating requirement.

The remaining 80 % of the feedstock exits the pyrolysis process as oils, tars & char which are then gasified in the gasifier at 1100 ⁰ C. The gasifier ensures near-complete utilisation and use of tar and produces most of the process energy with minimal residue; which aids in mitigating environmental problems resulting from toxic and carcinogenic aromatic compounds and complex disposal issues; any remaining ash can be trapped by an ash trap and can be sequestered or sold to the construction industry where it has been widely used and its disposal (Department of Energy, USA, 2008) has been well researched and documented.

Numerous gasifier designs were researched and a shortlist was compiled: Updraft, downdraft, cross draft, entrained flow and fluid bed gasifier; with the downdraft gasifier proving to be the most likely candidate due to the low tar production and widespread use in industry (Bridgwater,A.V. , et al ,

58

2002 & Maniatis,K. , 2002); albeit the design is limited to approximately 1-2MW and as a result a parallel pair of gasifiers is proposed. This shall be looked into greater detail for individual design. The others have their own shortcomings and advantages and they have been discussed in the reactors section by the author.

The syngas coming out of the gasifier is at 1100 ⁰ C and it is used to indirectly heat the pyrolyser; with heating backup or supplementary heating in order to increase production flowrate provided by syngas or natural gas burners (for startup) requirements . Modulation of the flow rate of the gasifier will also affect the pyrolysis process, and as a result careful system control and monitoring will be required to stabilise and optimise the process. Of the total number of gasifiers used in industry, 75 % were downdraft, 20% were fluidised bed, 2.5 % were updraft & the rest were various other designs. (Maniatis,K. , 2002)

The syn gas after transferring some of its heat energy to the pyrolysis process is then cooled along with the pyrolysis gas and fed to the gas engine/turbine. It should be noted the appreciable levels of thermal energy will be recovered in this exchanger, which could possibly be integrated with other areas of the process (Such as preheating gasifier O2 and steam feed streams). This has to be further explored and shall be looked into with greater detail by the author for the individual design.

To raise the calorific value of the Syngas produced to an acceptable level for use in a gas engine or turbine (Bridgwater,A.V, et al , 1999), a PSA system was added for the in situ production of Oxygen. A PSA is a Pressure Swing Adsorption unit.

The amount of oxygen to be produced has yet to be fully determined and shall be explained in greater detail in the individual report conducted by the author.

The main purpose of the oxygen is to provide the heat input to maintain the gasification operating temperature. If air was used, the resulting dilution from the nitrogen content lowers the CV of the syngas. This shall also be explored in greater detail in the individual report.

Monitoring the emissions of the process by using FTIRs and Raman spectroscopes as well as Gas chromatographs has been considered. They can be used as probes or a regular check can take place.

Also, it has been proposed that in cities having the CHP system already in place, one can use the immense amount of heat generated to heat homes as well. This can increase the efficiency of the process by quite a bit.

The process is open to a wide range of further improvements and analyses which will be discussed in the individual projects.

5.2 Mass balances for Process Design 2 – E Azulay Tchitchi

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A process with an input will have products of equal amount in terms of mass or moles equals to that in the feed. This is called mass balance (MB) and it is the basis of the Law of mass conservation that states the mass cannot be created or destroyed. This means that what goes into a system will eventually come out of it in whichever form.

For this part of the process, a mass balance was carried out for the dryer, reactors and burners regarding Flowsheet 2. The balance included flows in and out, mass fraction of each component (dryer) and the oxygen/fuel ratio for complete combustion.

By making assumptions such as that the drying process was a continuous process, and that no water was being condensed in the dryer, the mass balance for the dryer was calculated and the following values were obtained:

Knowing the initial feed of 5000kg/h. The initial moisture content (MC) was taken to be 20% and wood was to be dried to a 6% MC.

From this, wood at feed was at 4000kg/h and water at 1t/h The total product stream for the dried wood was found to be 4.3kg/h. Of that, 4000kg/h was

the wood flow.

From an initial feed of 4.3kg/h from the dryer, the Pyrolysis chamber mass balance was carried out using the same method as the dryer MB and values for the gas produced were obtained from 430 to 4300 kg/h over efficiencies raging from 10-100%, respectively. In addition, the gasifier had a feed of 3440 kg/h from the chamber (assuming 20% efficiency) and 1500 kg/h of oxygen, again values were calculated over the same range of efficiencies and it produced fairly a higher amount the gas from 494-4940kg/h.

Finally, the ratio of oxygen/natural gas for the burners was calculated and found to be 4kg of oxygen for each kg of NG.

Law of conservation of mass states that mass cannot be created nor destroyed. This law is the basis of mass/material balances which refers to the material load that flow throughout process operations. This means that, any material input to a system will be the exact amount in the output whether in the same or different state as the feed or as waste or main product.

Dryer

Assumptions:

Continuous and Steady State process No accumulation (condensation) in the dryer

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Degrees of Freedom Analysis

Unknown Variables – Independent Equations = DOF

Unknown Variables: 5

Independent Equations: 3

DOF: 5-3 = 2

Underspecified process - more information on the variables is needed.

Wood is dried to a 6% Moisture content from an initial 20%. Therefore, XMoisture3 = (20-6) wt% = 14%

Xair2 = (100-14) wt% = 86 %

Unknown Variables = 3

Independent Equations = 3

DOF: 3-3 =0 Problem can be solved

Overall balance

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F + G = P + C

Wood Balance

XWood1 * F= XWood2 * P

Water Balance

XWater1 * F = XWater2 * P + XWater3 * C

Wood Balance

(80/100) * 5 t/h = (94/100) * P

0.8 *5 t/h = 0.94 * P

4000/0.94 = P

P = 4.255 t/h ~ 4.3 t/h=4300kg/h

As the water fraction in the product stream is know

Moisture present in the P stream = XMoisture2 * P = (6/100) * 4.3 t/h = 0.3 t/h = 300kg/h

Water Balance

(20/100) * 5 t/h = [(6/100) * 4.3 t/h] + [(14/100) * C]

0.2 * 5 t/h = (0.06 * 4.3 t/h) + (0.14 * C)

Initially 0.8 *5 t/h = 4 t/h =4000kg/h of wood is fed. Therefore, (5-4) t/h = 1 t/h = 1000kg/h of water into the system.

If 0.3 kg/h is discharged in the P stream, therefore (1-0.3) kg/h = 0.7 t/h = 700kg/h of water in stream C.

Air Balance

The air balance will be affected by a number of factors:

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Thermal content of gas used to perform drying defines minimum gas flowrate required Maximum moisture content of drying gas at outlet conditions (Saturation point defines

minimum gas flowrate)

The influence of these factors will be investigated in the individual reports.

Pyrolysis Chamber & Gasifier

Initial Dryer Flowrate at 5tonnes/hr (20%)

Pyrolysis Feed at 4.3tonnes/hr (6%)

Density of Gas: 1.2Kg/m3

Volumetric Flow = MassFlowrateDensity

Following the same line of principles used for the dryer, the following mass balance was calculated over a range of pyrolysis and gasifier efficiencies:

Efficiency Mass Flowrate (Gas) kg/hr

Volumetric Flow (Gas) m3/hr

Volumetric Flow (Gas)m3/s

10 430 358 0.120 860 717 0.230 1290 1075 0.340 1720 1433 0.450 2150 1792 0.560 2580 2150 0.670 3010 2508 0.780 3440 2867 0.890 3870 3225 0.9

100 4300 3583 1.0Table 5.2.1 -Pyrolysis Chamber Mass Balance

Gasifier feed: 3.44t/hr (char & tar) + 1.5 t/h/hr (Oxygen) = 3.95 t/h

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Table 5.2.2. Gasifier Mass Balance Summary

Since Natural Gas is majorly composed of methane, other components will be neglected in these calculations.Atomic Weights (kg/ kmol):Carbon (C): 12Hydrogen (H):1Oxygen (O): 16

Molecular weight of Methane (CH4) = (1*12)+(1*4) = 16 kg/kmolMolecular weight of oxygen = 2*16 = 32 kg/ kmolReaction in the burner:

CH4 + 2O2 -> CO2 + 2H20It can be seen that two molecules of oxygen are used for each molecule of methane. Therefore, the oxygen/natural gas ratio = (2 * 32) / (1*16) = 4.This means that for 1 kg of Natural gas, 4 kg of oxygen need to be provided for complete combustion.

The amount of air needed is: 4*(100/23) =17.4 kg air for 1 kg of NGIn this case the air/NG ratio is 17.4

5.3 Energy balances for Process Design 2

Dryer

The energy balance carried out earlier in the report indicates that for a feed of wood chips at 5T/h, with a moisture reduction from 20% to 6% and raising the temperature of the wood chips from 15°C to 150°C that theoretical energy requirement of approximately 1MWt (With 20% losses through air circulation) will be required, producing 4.3T/h of dried wood output.

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Efficiency Mass Flowrate (Char and Tar) kg/hr

Volumetric Flow (Gas) m3/hr

Volumetric Flow (Gas)m3/s

100 4940 4117 1.190 4446 3705 1.080 3952 3293 0.970 3458 2882 0.860 2964 2470 0.750 2470 2058 0.640 1976 1647 0.530 1482 1235 0.320 988 823 0.210 494 412 0.1

Pyrolysis kiln

From the dryer output temperature of 150°C, the temperature of the dried wood must be increased during its residence time in the pyrolysis kiln to 450°C. Using the same average wood specific heat value as determined for the dryer, a mass flow rate of 4.3T/h (1.2kg/s) with a temperature rise of 300°C will require approximately 0.8MWt input.

Based on an assumed overall conversion to pyrolysis gas of 20%, 860kg/h of wood will be converted to pyrolysis gas, with:

An approximate density of 1.2kg/m³ assuming syngas and pyrolysis gas have similar physical properties (Cioni, M., )

An approximate CV of 14MJ/Nm³ (UK Government DEFRA, 2007) This corresponds to a total of 2.8MWt of chemical energy within the pyrolysis gas. Due to the endothermic heat input requirements for the pyrolysis process being small compared to the net energy input to raise the feedstock temperature to 450°C, it is approximated that the heat input required to sustain the pyrolysis process is negligible.

From the pyrolysis kilns, the remaining 80% of the feed, or 3440kg/h will be converted into pyrolysis char and bio-oil, which is fed to the gasifier. On the basis of 1% of the feed forming tar and ash, 99% of the feed, or 3406kg/h forms syngas. The CV of syngas varies considerably depending on process conditions and the chemical nature of the feed, from anywhere between 10MJ/Nm³ to 20MJ/Nm³ (Bridgwater, A.V. (1994) “Catalysis in thermal biomass conversion” Applied catalysis, pp26-27). In particular the syngas CV can be diluted by the use of air with a corresponding increase in flowrate, rather than oxygen as proposed with the use of the pressure swing adsorption plant); and as a result as an estimate of 18MJ/Nm³ has been used giving rise to a CV of 15MJ/kg, resulting in approximately 14.2MWt of chemical energy contained within the syngas. Due to the gasifier operating with an air/oxygen feed resulting in partial combustion within the gasifier, this process is considered to be self-sustaining.

From the PSA system, 36T/d (0.4kg/s) of oxygen will be required based on 25% of stoichiometric requirement for combustion. Typical commercial figures for PSA systems indicate that an electrical energy input of 0.6kWh/m³ (http://www.gen-sys.net) of oxygen will be required for the PSA process; based on an oxygen density of 1.4kg/m³, 0.17kWh/s will be required for the PSA process, or approximately 0.7MWe with an additional 10% losses.

Total thermal input: Dryer (0.8MWt) + Kiln (0.8MWt) = 1.6MWt

Total energy in gas produced: Pyrolysis (2.8MWt) + Gasifier (14.2MWt) = 17MWt

Compared to a maximum of 22MW based on a wood CV of 16MJ/kg at 15% moisture (Kaye and Laby) indicates approximately 77% efficiency excluding dryer and kiln requirements.

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Generation: (17MWt – 1.6MWt) x 40% efficiency = 6.2MWe

Export: 6.2MWe – 0.7MWe (For PSA) = 5.5MWe

Note that although these calculations indicate an electrical output in the region of the required 4.5MWe, the calculations at this stage are very much approximate and is intended as a rough preliminary feasibility study only. When the process stages are investigated further in the individual design sections the energy input/output for different process sections will be estimated with greater accuracy, as well as factors potentially included for fan/kiln electrical energy requirements. Furthermore, these estimates do not include efficiency improvements from heat integration (Such as the cooling of syngas), which is likely to make considerable improvements to the process efficiency and output. It should also be noted that application of CHP to the process will generate additional output and revenue, approximately 9.2MWt in this example.

Costing for Process Design 2 – K Rasedin

The following method for costing will be carried out in a similar fashion to the costing carried out for process design 1.

Design 2

The second design emphasized on the gasification technology as the major process. Thus the cost for gasifier would probably much higher than the pyrolysis reactor. There are seven major equipments used in the second design which are rotary drum dryer, auger screw reactor, gasifier, pressure swing adsorption for oxygen production, gas heat exchanger, electrostatic precipitator, hydrocyclone and gas engine. Example of calculation to find fixed capital cost, working cost and total investment required for the second power plant design is presented as followed. The rest of the values are presented in Table 5.4.1.

Rotary drum dryer

Flowrate of wood waste = 5000 kg/hr

Density of wood = 670 kg/m3 (http://www.simetric.co.uk/si_wood.htm )

Drying area = 5000 x (1/670) x ( 1/ 3600) x 10 x (1/0.003) = 6.9 m2 = 75 feet2

Based on the equipment match calculator (Dryer cost, 2003) the cost for rotary drum dryer with indirect heating and made from carbon steel was found to be $33000 at 2007.

The average exchange rate between American dollar and British pound sterling on 2007 was 0.491 GBP for 1 USD (xrates.com, 2010)

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Hence the cost of the dryer in pound on 2007 was found to be:

$33000 x 0.491 = £ 16203

The Chemical Engineering Plant Cost Index and inflation rate between 2007 and 2010, the capital cost of the dryer at 2010 could be calculated.

CEPCI of process machinery at March 2010 = 614

CEPCI at 2007 = 525.4

Inflation rate between 2007 to 2010 (Rate Inflation, 2010) = 8.83%

Cost of dryer an 2010 = £16203 + [£16203 x (614/525.4) x 0.0883] = £17874 (approximately £18000)

To find fixed capital cost, cost of dryer at 2010 multiplied with the associated factors in Table 4.5.2.

Fixed capital cost for dryer = £17874 x (1+0.45+0.45+0.15+0.10+0.10+0.45) = £48259 (approximately £48000)

Major Equipments

Capital cost, £ Fixed capital cost, £

Rotary drum dryer

18000 48000

Auger Screw reactor

250000 730000

Downdraft gasifier

250000 730000

Pressure Swing Asorption

200000 710000

Gas Heat Exchanger

15000 46000

Hydrocyclone 15500 42000ESP 180000 500000Gas Engine 32000 98000 Working capital Total Investment

for buildingTotal 2904000 435600 3339600

Table 5.4.1. List of Major Equipment Cost

With reference to the Table 5.4.1, it was found that the fixed capital cost for the biomass fuelled power plant that makes use of gasification as the major process was approximately £3 million. The working capital was calculated by taking into account of 15% of the fixed capital cost and the value came out to be about £0.4 million. Thus the total investment required to build the plant was estimated to be £3.40 million.

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Since it was found from our research and calculation that the cost of both designs was relatively similar, the group has decided to come out with single cost estimation for a power plant that use biomass and produce approximately 4.5 MW of electricity. The final costing assessments for the plants are shown as followed (Claverton Energy Research Group,2009).

Based on our calculation it was estimated that the cost for building the plant was approximately £3 million. Other than that the cost for the plant to operate was estimated to be about £12 million. That summation of plant building and operating cost gave the overall installation cost of the plant to be roughly at £15 million pound.

4.5 MW of electricity is produced for every 5 tonnes per hour of wood waste. The electricity generated was decided to be sold to National Grid to get revenue for the plant. The tariff applied by National Grid for every unit of electricity sold was approximately £122 per megawatt hour (The Times, 2010). Thus the revenue of the power plant for 1 year could be calculated as followed:

Electricity produced per hour = 4.5 MWh

Electricity produce per year = 4.5 x 24 x 365 = 39420 MWh

Revenue per year = 39420 x £122 = £4809240 (approximately £4.8 million)

The running cost of the plant was estimated to be 20% of the installation cost that was calculated before and it gave a value as below:

Running cost = (20/100) x £15000000 = £3000000

The installation cost could be assumed as the total investment required for the plant to be built and operated. This sum of investment would have to be paid back and the source of fund for paying back the sum of money came from the net profit gained from the plant. The net profit was calculated as followed:

Net Profit = Revenue – Running cost

= £4800000 - £3000000

= £1800000

The payback time for the installation cost was estimated based on the interest rate used. For example for the power plant designed in this project it was estimated that the interest rate is meant to be at 7% per year. With that amount of interest rate it was found that the payback time for the installation cost was approximately 15 years with an annual payment about £ 2.32 million per year.

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6. Safety and Environment

6.1 Plant Safety – E Azulay Tchitchi

One matter that cannot be overlooked is the safety in a chemical plant. In any chemical plant potential hazards exist which need to be considered for a safe working environment and prevention of major accidents and incidents that could potentially occur during the course of the process and plant lifetime. It should be noted that new hazards may appear during the transition from laboratory or pilot scale due to process variations and scale up.

Full understanding of chemicals present in the process including flammability and toxicology is a mandatory measure in chemical plants and although health safety is important, personnel should be aware that it is not the only imminent hazard in a chemical plant. Other hazards examples include, fire hazards, electrical hazards (since this is an electrical power plant), mechanical apparatus and thermal hazards. Summarising:

Implement appropriate fire safety precautions and detection systems Recognise gaseous and dust explosion hazards Ensure good ventilation around the plant, in particular if any sections of the process are

enclosed within a building Design materials should be carefully specified, with emphasis placed on making sure that

material withstand more than the conditions expected for the process. Appropriate electrical safety and ignition prevention measures should be used in the plant

design, as well as ensuring potential sources of ignition are separated away from sources of flammable gases present, i.e. hydrogen, tar, methane, natural gas/air, carbon monoxide.

Engineering controls for leakage detection A safe shutdown procedure plan Training on-site personnel

Safety Issues

Toxic and flammable chemicals could be easily used, handled and stored for later use as long their chemical and physical characteristics are fully acknowledged and any necessary precautions are promptly available to address any situations which may develop. The bigger the plant, the more imminent and severe are the potential hazards that would already exist for a small plant (Christian, A,

1957). Chemical plants safety goes beyond wearing gloves and goggles for a certain procedure (still important); a closer look into what could damage the plant in a small as well as in a much larger scale need to be investigated.

Furthermore, a list of potential, thermal and any other possible safety issues needs to be issued and made available to the emergency services in case of an emergency.

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Hazard Control Hierarchy

Hazards need to be recognized and dealt with via suitable controls so the maximum level of hazard prevention and elimination can be achieved. A fairly important process in hazard recognition is the cycle Hazard identification-Control-Assessment and needs to be implemented on a continuous basis and reviewed regularly. Although hazards control could eliminate some of the unavoidable hazards there is still some limitation to their control, elimination and reduction range. What needs to be considered is that different hazards will be more dangerous than others, and to cover this lack of control range a mechanism called hazard control hierarchy is used to ensure maximum hazard eradication. This mechanism arranges actions by their relevance order and those are:

1) Hazard elimination2) Hazard reduction3) Provide engineering controls 4) Using administrative controls 5) Wearing Personal Protective Equipment (PPP).

It is important that the arrangement is followed as it is shown when a hazard control is being implemented. In addition, it needs to be noted that controls 2 and 5 will not eradicate the hazards and therefore they will still pose danger. In case elimination, reduction and engineered controls do not prove themselves sufficient, administrative control needs to be considered and standard operating procedure steps and PPP do need to be provided.

Potential Hazards in Our Plant

Flammable & Explosive Hazards

HYDROGEN: Class I flammable gases with flashpoints of -253oC. Leakage could lead to rapid diffusion of hydrogen into air posing a fire hazard and the creation highly flammable zone in the plant. Highly Asphyxiant. (College of the Desert, 2001) (ISOC Technology, Material Safety Data Sheet: Hydrogen)

METHANE. Also a Class I flammable gas with a Flashpoint of -188oC. Asphyxiant: dizziness, nausea and unconsciousness. If mixed major oxidizers such as oxygen can form explosive and/or flammable mixtures (ISOC Technology, Material Safety Data Sheet: Methane)

NATURAL GAS/AIR MIXTURE: highly flammable if not used in a neutral mixture. NG can also be highly Asphyxiant if present in high concentrations. Exposure to fire may cause fire and cause containers to explode and/or rupture. (BOC GASES, 2001)

TAR: Flammable and toxic. Health and Fire Hazard Scale: 2. It can readily burn in the presence of air and high temperatures. Death threatening if swallowed and/or inhaled. Irritant.

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CARBON MONOXIDE: Flammable over wide range of air concentrations, flames could be ignited by a simple spark or heated surface. Poisonous and Asphyxiant: Can replace oxygen in the air due to its higher haemoglobin affinity. Death and serious central nervous system damage threatening.

OXYGEN: Oxidising agent – Vastly increases furosity of fires and explosions

Substance Auto Ignition Temperature/ oC

Hydrogen 585Methane 540

Natural Gas 580Tar -

Carbon Monoxide 609Table 6.1.1 – Flammable Substances Auto Ignition Temperatures (Engineeringtoolbox.com)

Electrical Hazards

Grid power outage (Automatic plant disconnection from grid to prevent near short-circuit damage)

Insulation failure Switchyard hazards Generator malfunction Plant startup and phasing of generator to grid

Thermal Hazards

High Temperature and high pressures present in the process (In particular with the gasifier) Gasifier steam supply poses particular hazards if blockage occurs High Energy Intake and Release Process: Could be divided into: energy intake/release of

main and by-side reactions.

Devices Hazards

Grinders, Rotary kilns and Gas engine – Moving machinery Exhaust gas – Considerable CO content, high temperature Gas engine – Crankcase explosion, exhaust system explosion Gas turbine – Blade failure Engine RPM and pressure and temperature monitoring of cooling/oil circuilation systems on

gas engine/generator

Other Chemical Hazards (Health Hazards)

SOx: Soot primary material which could deposit in lungs. Carcinogen NOx: Respiratory tract and eyes damaging. ASH: Dust, high levels of contaminants and heavy metal content.

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Engineering Controls

Use of control valves whenever possible to release over pressurized products and to reduce the pressure of reactors.

Integrated back-pressure electronic recognition for the engine. Exhaust systems should be fitted with safety valves and stack with vacuum valves as well for

the rest of the process. Engine monitoring in the case of a gas engine to detect misfiring Infrared chemical detectors in case of a chemical leakage.

Precautions

Provide personnel with PPE where appropriate. Full understanding of the thermal profile/properties of the system and any temperature

deviation that could occur in the process and ensure that pressure rises could be accepted into the system.

Equip vessels and/or reactors with material that will withstand the range of temperature and pressure in the process.

Implement Calorimeters, ideally Scanning calorimeters to analyse thermal decomposition: Pyrolysis and Gasification for this process, while measuring the heat evolved.

Appropriate environment - Well air-ventilated with fans to ensure ventilation as appropriate. Process devices need to be well grounded and all the lines bonded to avoid static charge

buildup. Gas engines need to be well ventilated at start and end of each run. It is also recommended

that it is kept ventilated at idle times as most of gases accumulate in the exhausts. Use of electrical tools, in particular those with commutators and welding equipment must be

constrained to areas of the plant with a minimal explosion risk. Provide the plant with extinguishers suitable for the chemical present in the process Emergency evacuation procedure Above all, it is most important to keep flammable substances away from ignition sources.

In case of Shutdown:

Pressure need to be released from all the valves involved in the process in a safe process (E.g., Via gas flare stack)

Purging of the exhaust system to reduce the risk of exhaust system explosion Any flammable gas present that cannot be flared should be mixed with an inert gas to

decrease its flammability below its low flammability limit. Oxygen concentrations need to me correctly measured inside the reactor. Nitrogen covers need to be available to minimize flammable gases being released from the

reactor. If possible efforts should be made to reduce the high temperatures present by using fans or

cooling mechanisms.

Besides acknowledging hazards, on-site technical personnel need to be trained and educated of current and future hazards. A descriptive list with some examples follows:

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Personnel Education and Training (Hazard Communication)

Inform personnel of any new regulations implemented. Sign and ensure that personnel have full understanding of chemicals being used and how to

detect any leakage. Inform personnel of location and availability of Material Safety Data Sheet and emergency

plan. Physical, health, safety and chemical procedure training. New hazards need to be considered and new safety measures enforced. 6 month or annual reassessment of the above measures should take place.

6.2 Environmental Considerations – S McCord

This section of the report will consider the environmental implications of building the plant. There are several important points to consider before sanctioning the build of such a plant, as some of these can have serious implications on the plant after the commissioning stage.

Using waste wood as a fuel

The use of waste wood as a fuel source has two major benefits that are worth discussing. The first of these is the main advantage of using wood over fossil fuels to generate electricity. Unlike fossil fuels, wood and biomass are known as carbon neutral fuel sources. This is because trees absorb carbon dioxide whilst growing, and that burning the waste wood after results in no net carbon emissions to the atmosphere as this is the same carbon absorbed by the trees whilst growing. However, the transport used to bring the wood to the location of the plant will be using fuels extracted from oil; thus preventing a totally carbon neutral process. To minimise this, the plant has been located close to some of its potential fuel sources to reduce the transportation emissions.

The second major benefit of the scheme is that it can help reduce the amount of waste sent to landfill every year. The UK is Europe’s leading landfill producer (lga.org.uk, 2007). With over 500,000 tonnes per year of agricultural and 1.8 million tonnes per year of waste wood (from industries and domestic use) (defra.org.uk), this figure could be reduced if this plant is successful, and more and larger plants working on the same principal are commissioned.

UK pollution laws.

As the plant is to be operated within the UK it must abide by the UK laws regarding, amongst other aspects, gaseous emissions of potentially hazardous substances. As discussed in the gas clean up section there are multiple undesired chemicals that may be produced through the process. These can either form due to the process, formation of benzene, tars and other dangerous organic compounds, or through the quality of the feed wood – the amount of nitrogen, sulphur and chlorine present in the feed. Any plant built will have to meet standards set by the following laws:

The pollution prevention and control act of 1999 The clean air act of 1993

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Source: (environmental-protection.org.uk, 2010)

Renewable energy strategy (July 2009): 15% of energy from renewable sources by 2020. U.K cleaner energy legislation (July 2008): To meet international climate change targets and

to improve the countries security of energy supply. Energy bill (January 2008): Emphasise the main legal points of the Energy White Paper

(2007), which addresses long term energy challenges (i.e. cut CO2, maintain reliable energy supply, and promote competitive market).

Climate change bill (November 2007): enforces specific targets to cut emissions by 60% before 2050, compared to 1990 levels).

Source: (New Energy Focus, 2010)

Below is a summary of some of the legal limits of a variety of pollutants that may arise from process. If it was not possible to find UK limits, limits from other relatively “green” EU countries were used.

Compound Legal limitCO2 400 kg CO2/MWh of energy produced.NOx 400ppm maximum WID limit, (defra.org.uk, 2010)SO2 200ppm maximum WID limit, (defra.org.uk, 2010)CO 100ppm maximum (defra.org.uk,2010) –Waste Incinerator Directive (WID) limitHCl 10mg/m3, European limit (ingvar.is, 2010)Dust 10mg/m3, European limit (ingvar.is, 2010)Benzene 1mg/m3, European limit (Air quality directive2008/50/EC)Unburnt HC’s 1500mg/m3, Danish limit (European Parliament*, 2001)PAH’s Plant to plant basis, always very low (European Parliament*, 2001)

Formaldehyde 60mg/m3, Danish limit (European Parliament*, 2001)

Table 6.2.1. UK and European Emission limits for specific pollutants

Other chemical emissions that must be monitored (and have legal emission limits of production from power generation) including methane and ammonia. One way of possibly ensuring that regulations are adhered to could be to use post-combustion clean up technology, however this will further increase the cost of the plant. The emissions of these chemicals must be limited to prevent potential wide scale damage to the population and the environment. Some negatives can include:

Acid rain from NOx, SOx HCl causing damage to structures, forests (could be a big problem due to location near forests) and other features with fragile ecosystems.

Benzene, some PAH’s and un-burnt hydrocarbons are dangerous carcinogens. Dust can cause breathing related illnesses such as asthma and bronchitis.

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Carbon Monoxide is toxic, leaks are hard to detect, high levels of emissions could be disastrous

If these limits are not met there is a chance of serious repercussions, these have included fines in the past, and have also resulted in the closure of a plant on the Isle of Wight for breaching agreed dioxin emission limits. (Gala, 2010).

The other plant related issue is what to do with any ash or carbonaceous waste. There are a few options including:

Bio-sequestration or landfill of the waste If a large amount of char is left and relatively clean – could be used for activated carbon Using the waste in the production of construction materials (e.g. cinderblocks) Other low grade uses including road surfacing

7. Conclusion

In designing this plant it has been discovered that the project can meet the energy requirements of 4.5MW whilst still managing to be self sustaining and carbon neutral.

Either pyrolysis or gasification can be used as the dominant process, however the output is roughly similar in either system.

Syngas has the potential of becoming a dominant fuel in the future. In process 1 the plant was found to make a net profit of £3.85million/year, meaning the

plant would receive payback on the build within 2 years. Using oxygen within the process has more hazards involved yet the gas clean up required is

much smaller and the gas has a higher calorific value. Syngas production is a process which can be replicated into almost any place in the world, as

long as the wood supply is consistently good. Major care needs to be taken in order to meet emission limits, however as long as these are

kept below government levels the plant offers few other negative issues. Either pyrolysis or gasification can be used as the dominant process, however the output is

roughly similar in either system

8. References

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1.2

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Bridgwater,A.V. , Hatt,B.W, ,et al ,Watt committee on energy, report, Conversion of Biomass to fuels,IchemE , 1979

UNFCCC , 2005, United Nations Annex 8 Clarifications on definition of biomass and consideration of changes in carbon pools due to a CDM project activity, http://cdm.unfccc.int/EB/020/eb20repan08 .pdf

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1.4

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http://chestofbooks.com/crafts/mechanics/How-To/Wood-Composition.html

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2.1

Bridgwater, A.V., 1994. Catalysis in thermal biomass conversion. Applied Catalysis A: General 116, 5–47.

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Online , accessed on the 12/11/2010

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Bridgwater, 1995 , The technical and economic feasibility of biomass gasification for power generation, Butterworth Heinemann, Fuel , Vol 74, pp631-653 , UK

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Van Swaaji. W. M. P. Laboratory 7500 AE Enschede –Gasification – The process and Technology of Chemical (The Netherlands) Reaction Engineering,Twente University of Technology,

Goswami. Y,CRC Press, 1986, Chapter (No. 4) in book “Alternative Energy in Agriculture”,Vol. II, pgs. 83-102.).

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3.2

Zajec, 2009 – Slow pyrolysis in a rotary kiln reactor: Optimization and Experiment. Publisher Icelandic Renewable Energy School. Page: 7,13

Pdf available at: http://www.res.is/static/files/ThesisReports/Luka_Zajec.pdf

The pages correspond to the page numbers on the paper, not to those of the pdf file

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78

Kent-Riegel, 2007 – Kent and Riegel’s handbook of industrial chemistry and biotechnology, 2007, published by Springer – Verlag. Pages: 1510

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3.4

Hydrogen-rich Gas from Biomass Steam Gasification

H.Hofbauer, R.Rauch, P.Foscolo, D.Matera

1st World Conference and Exhibition on Biomass for Energy and Industry, Sevilla June 2000

Bridgwater, 1995 , The technical and economic feasibility of biomass gasification for power generation, Butterworth Heinemann, Fuel , Vol 74, pp631-653 , UK

Water Gas Shift Reaction (WGSR) ,Wolfgang F. Ruettinger; Oleg Ilinich Encyclopedia of Chemical Processing, Published on 30 November 2005

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Goswami. Y,CRC Press, 1986, Chapter (No. 4) in book “Alternative Energy in Agriculture”,Vol. II, pgs. 83-102.).

Bridgwater, A.V., 1994. Catalysis in thermal biomass conversion. Applied Catalysis A: General 116, 5–47.

Belgiorno, V. et al ,(2003) , Energy from gasification of solid wastes, Pergamon

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3.5

79

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