final report on biomass gasifier

8/9/2019 Final Report on Biomass Gasifier

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Testing and Parametric Analysis of an Updraft Biomass Gasifier

MES CoE, Mechanical Engineering 2012-13

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Chapter 1. Introduction to Gasification

1.1 What is Gasification

Gasification is a process that converts carbonaceous materials, such as coal,

petroleum, biofuel, or biomass, into carbon monoxide and hydrogen by reacting the

raw material at high temperatures with a controlled amount of oxygen and/or steam.

The resulting gas mixture is called synthesis gas or syngas and is itself a fuel.

Gasification is a method for extracting energy from many different types of organic

materials.

Gasification is a thermal conversion process in which solid or liquid fuel is

converted into a gaseous fuel. Contrary to combustion, gasification produces a gas

that is combustible.

Gasification can be considered as combustion with a shortage of oxygen. The

process is generally operated at the point where just enough oxygen is added to the

process that the heat generated equals the energy that is required to volatilize the

feedstock.

The advantage of gasification is that using the syngas is potentially more

efficient than direct combustion of the original fuel because it can be combusted athigher temperatures or even in fuel cells, so that the thermodynamic upper limit to the

efficiency defined by Carnot's rule is higher or not applicable. Syngas may be burned

directly in internal combustion engines, used to produce methanol and hydrogen, or

converted via the Fischer-Tropsch process into synthetic fuel. Gasification can also

begin with materials that are not otherwise useful fuels, such as biomass or organic

waste. In addition, the high-temperature combustion refines out corrosive ash

elements such as chloride and potassium, allowing clean gas production fromotherwise problematic fuels.

Gasification of fossil fuels is currently widely used on industrial scales to

generate electricity. However, almost any type of organic material can be used as the

raw material for gasification, such as wood, biomass, or even plastic waste.


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Gasification relies on chemical processes at elevated temperatures >700C,

which distinguishes it from biological processes such as anaerobic digestion that

produce biogas.

1.2

Why Gasification

One of the most compelling challenges of the 21st Century is finding a way to

meet national and global energy needs while minimizing the impact on the

environment. Gasification can help meet those challenges.

Gasification is a time-tested, reliable, and flexible technology that converts

carbonaceous materials, biomass, municipal waste, scrap tires and plastics into a clean

high energy gas.

The Power Hearth produces a clean, particulate-free gas that can be used to

fuel industrial boilers or to power internal combustion or turbine engines with

generators to produce megawatts of electricity.

Gasification does not involve combustion, (or burning), but instead is a

thermal chemical process that uses high temperature in a controlled environment, with

limited oxygen, to convert carbon-based materials directly into a high energy

producer gas. The gasification process breaks these materials down to the molecularlevel, so impurities can be easily and inexpensively removed.

The high-temperature combustion refines out corrosive ash elements

allowing clean gas production from otherwise problematic fuel sources.

Gasification can recover the energy locked in biomass and municipal solid

waste - converting those materials into valuable products and eliminating the need for

incineration or landfill.

Gasification has been reliably used on a commercial scale for more than 50

years in the refining, fertilizer, and chemical industries, and for more than 35 years in

the electric power industry.

Gasification produces electricity with significantly reduced environmental

impact compared to traditional technologies.


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Gasification plants bring good jobs to a community construction jobs

needed to build a plant, and well-paying permanent jobs needed to run the plant.

Compared to the old coal-burning plants, gasification can capture carbon

dioxide much more efficiently and at a lower cost. This capture technology is being

successfully used at gasification plants in the U.S. and worldwide.

Gasification is an investment in our energy future.

Gasification is not incineration. Incineration is the burning of fuels in an

oxygen-rich environment, where the waste material combusts and produces heat and

carbon dioxide, along with a variety of other pollutants. Gasification is the conversion

of feedstocks into their simplest molecules - carbon monoxide, hydrogen and methane

forming a syngas or producer gas that can be used for generating electricity or

producing thermal heat.

1.3 Gasification v/s Combustion

Gasification is not an incineration or combustion process. Rather, it is a

conversion process that produces more valuable and useful products from

carbonaceous material. Following table compares the general features of gasification

and combustion technologies. Both gasification and combustion processes convertcarbonaceous material to gases. Gasification processes operate in the absence of

oxygen or with a limited amount of oxygen, while combustion processes operate with

excess oxygen. The objectives of combustion are to thermally destruct the feed

material and to generate heat. In contrast, the objective of gasification is to convert the

feed material into more valuable, environmentally friendly intermediate products that

can be used for a variety of purposes including chemical, fuel, and energy production.

Elements generally found in a carbonaceous material such as C, H, N, O, S, and Cl

are converted to a syngas consisting of CO, H2, H2O, CO, N2, CH4, H2S, HCl,

COS, HCN, elemental carbon, and traces of heavier hydrocarbon gases. The products

of combustion processes are CO2, H2O, SO2, NO, NO2, and HCl.


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Table 1.3: Comparison between Gasification and Combustion

FEATURES GASIFICATION COMBUSTION

Purpose

Creation of valuable,usable products from waste orlower value material

Generation of heat ordestruction of waste

Process Type Thermal and chemical

conversion using no or limitedoxygen

Complete combustion usingexcess oxygen (air)

Raw Gas

Composition

(before gas

cleanup)

H2, CO, H2S, NH3, and

particulates

CO2, H2O, SO2, NOx, and

particulates

Gas Cleanup Syngas cleanup at

atmospheric to highpressures depending on thegasifier design

Treated syngas used forchemical, fuels, or powergeneration

Recovers sulphur species inthe fuel as sulphur orsulphuric acid

Clean syngas primarily

consists of H2and CO

Flue gas cleanup atatmospheric pressure

Treated flue gas isdischarged to atmosphere

Any sulphur in the fuel isconverted to SO2that mustbe removed using flue gascleanup systems, generatinga waste that must belandfilled.

Clean flue gas primarily

consists of CO2 and H2O

Ash/char or slag

handling

Low temperatureprocesses produce a charthat can be sold as fuel.

High temperatureprocesses produce slag, anon-leachable, non-hazardous materialsuitable for use asconstruction materials.

Fine particulates are

recycled to gasifier. Insome cases fineparticulates may beprocessed to recovervaluable metals.

Bottom ash and flyash arecollected, treated, anddisposed as hazardous wastein most cases.

Temperature 1400F 2700F 1500F 1800F

Pressure Atmospheric to high Atmospheric


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1.4 Advantages of Gasification over Combustion

1.4.1 Environmental

Gasification has inherent advantages over combustion for emissions control.

Emission control is simpler in gasification than in combustion because the

produced syngas in gasification is at higher temperature and pressure than the

exhaust gases produced in combustion. These higher temperatures and

pressures allow for easier removal of sulfur and nitrous oxides (SOX, and

NOX), and trace contaminants such as mercury, arsenic, selenium, cadmium,

etc. Gasification systems can achieve almost an order of magnitude lower

criteria emissions levels than typical current U.S. permit levels and +95%

mercury removal with minimal cost increase. In addition, gasification systems

require less water than other technologies.

1.4.2 Carbon Capture Utilization and Storage

Similar to the removal of other contaminants, gasification lends itself to

efficient carbon dioxide (CO2) removal because of the high temperature and

pressure of the produced syngas. Studies show that in CO2removal

applications, integrated gasification combined cycle (IGCC) plants are more

efficient than other commercial technologies. Captured CO2is prevented from

entering the atmosphere through either utilization or storage. The two most

common options are carbon dioxide enhanced oil recovery (CO2EOR),

and carbon sequestration. CO2EOR is a highly practical utilization strategy, in

which CO2is injected underground into mature oilfields to sweep residual oil,

where CO2is stored underground in the process. Carbon

sequestration involves injecting the CO2 into a deep geologic formation for

permanent storage.

1.4.3 Feedstock Flexibility

Several gasifier designs have been developed to accommodate various grades

of coal in addition to wastes and various types of biomass. Gasifier can also

handle pet coke and other refinery products. The potential for using more than


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one feedstock in a single facility reduces project risk and may extend the

project lifespan.

1.4.4 Product Flexibility

Gasification can be coupled with advanced turbine technology to produce

electricity in an IGCC plant. Syngas produced by gasification can also be

further processed into liquid fuels (diesel, gasoline, jet fuel, etc.), hydrogen

and synthetic natural gas, or a range of fertilizers or other high-

value chemicals including anhydrous ammonia, ammonium sulfate, sulfur,

phenol, naphtha and CO2as mentioned above, among many others. Also, slag

produced from coal ash can be used in the production of building materials

such as cement.

1.4.5 High Efficiency

IGCC power plants offer efficiencies similar to or better than other coal power

plants. Additionally, in a carbon dioxide capture and sequestration (CCS)

scenario, an IGCC power plant is much more efficient than a pulverized coal

combustion power plant. This is mainly due to the decreased energy required

to remove CO2 from the process streams in gasification as compared with a

pulverized coal combustion system


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1.5 Gasification Methods

Fig.1.5.1: Different gasification Methods


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Chapter 2. Literature Review

In today`s scenario of depleting conventional fossil fuels, biomass provides an

alternate source of energy. Gasification is a chemical process that converts

carbonaceous materials like biomass into useful convenient gaseous fuels or chemical

feedstock. The product gas of gasification has a calorific value unlike that of complete

combustion process. The present study is going to be focused on parametric analysis

and study of the mathematical model to predict the effect of usage of various types of

fuels in gasification process and also the usage of oxygen as a gasifying agent.

Prof. M. K. Chopraand Shrikant U. Chaudhariin their paper Performance of

Biomass Gasifier Using Woodhave studied the basics of gasification using wood

as feedstock, the sensitive analysis of produced Syngas and the study of the

composition. (International Journal of Advanced Engineering Research and

Studies)

B.V.Babu and Pratik N. Sethhave discussed about the effect of oxygen and steam

enrichment on biomass gasification. Equilibrium model for a downdraft gasifier is

solved using Engineering equation Solver. The effect on calorific value of the

producer gas is studied in detail. (Modeling & simulation of biomass gasifier: effect

of Oxygen enrichment and steam to air ratio, Chemical Engineering Department

Pillani)

Gasification by Dr. Samy Sadaka, et.al have discussed various gasification

process, gasification zones, types of gasification, gasification agents and gasification

applications are stated. The chemical equations governing gasification are used to

analyse the gasification model. Effect of bed temperature, bed pressure, bed height,particle size, moisture content in fuel has been stated in detail. Parametric analysis of

gasification parameter is done.

Guidelines for safe and eco-friendly Biomass Gasification by Intelligent

Energy Europefunded by European commissionhas identified HSE (health safety

and environment) issues regarding gasification. Risk assessment has been done for

safer gasification process and gasifier manufacturing and thus certain guidelines have

been laid down.


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Handbook for Biomass Downdraft Gasifier for Engine System by Solar

Research Institute U.Sis a practical guide to gasifier systems to design, test, operate

and manufacturing of small downdraft gasifiers( upto200kW capacity). Fuel testing,

gas analysis, other methodology current development and future research scopes are

stated in great detail.

Development of a Small Downdraft Biomass Gasifier for Developing Countries

a report by M. A. Chawdhurya and K. Mahkamovb designed developed and tested

a small downdraft biomass gasifier at the DUBLIN UNIVERSITY( UK). They

found out that for composition, moisture content and consumption of biomass

feedstock (3.1 kg/hr for wood chips, 2.9 kg/hr for pellets), temperature inside the

reaction zone (950-1150

o

C), primary air flow rate (0.0015 m3/s), exit temperature ofthe producer gas (180-220 oC) was measured. The main constituents of syngas

included nitrogen (50-56%), carbon monoxide (19-22%), hydrogen (12-19%), carbon

dioxide (10-12%) and a small amount of methane (1-2%). These results were used in

Engineering Equation Solver (EES) software to obtain the lower calorific value of

syngas (4424-5007 kJ/m3) and cold gas efficiency (62.5-69.4%) of the gasifier, which

were found close to the calculated values. Again the thermal efficiency was calculated

as 90.1-92.4%.

Sirigudi Rahul Rao,he has developed process model of gasifier in which air is used

as gasifying agent and bioreactor in Aspen Plus software. Using the developed model

studied the performance of the gasifier by manipulating the process variables and

characterizing the effect on gas quality and composition.

Anil K. Rajvanshi, the Director of Nimbkar Agricultural Research Institute,

Phaltan, Maharashtra, India, in his chapter on Biomass Gasification in the book

Alternative Energy in Agriculture, has deeply studied the effect and use of

gasification in agricultural sector and has discussed the opportunities and challenges

faced by our country in the successful commercialization of the same.

Ola Maurstad, Howard Herzog et. all of The Norwegian University of Science

and Technology (NTNU) and Massachusetts Institute of Technology (MIT), in

their paper titled Impact of coal Quality and Gasifier Technology on IGCC


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performancehave given a brief review on the significant importance of quality of

coal and technology behind construction of gasifier when syngas is used in an IGCC.

Suresh Babu of Gas Technology Institute of USA, in his paper Biomass

Gasification for Hydrogen Production-Process Description and Research Needs

has discussed the possible application of Syngas in the production of hydrogen and

the significance of the process along with the usage of hydrogen in industry.

The paper titled, Thermodynamic Analysis of a Coal-Based Combined Cycle

Power Plant authored by P. K. Nag and D. Raha of Mechanical Engineering

Department, IIT Kharagpur, have given a brief outlook into the thermodynamic

aspect of a power plant run on coal gasification cycle. The paper deals with the

comparison of gasification cycle with Brayton and Rankine cycle.

After detailed study of the above, it was found that there is a lack of use of biomass

gasifiers on a large scale in the country. Our country, whose villages house an

abundant supply of biomass, will surely benefit from an installation of a biomass

gasifier in the rural area which would use the village biomass as feedstock. The

biomass gasifier can be combined into an integrated gasification combined cycle

(IGCC) to produce electricity for benefiting villages with electricity shortage. The

syngas produced after proper refinement could be used to run IC engines for vehicles.

The raw syngas can replace conventional chulha and LPG as cooking gas. Thus for

successful use of syngas in above situation, it is necessary to study the composition of

the syngas as well as the impact of different input factors on the heating value of

syngas and its composition. Hence a parametric or sensitivity analysis is necessary

along with successful trial and production of syngas.


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Chapter 3. Types of gasifiers

A variety of biomass gasifier types have been developed. They can be grouped

into four major classifications: fixed-bed updraft, fixed-bed downdraft, bubbling

fluidized-bed and circulating fluidized bed. Differentiation is based on the means of

supporting the biomass in the reactor vessel, the direction of flow of both the biomass

and oxidant, and the way heat is supplied to the reactor. Table 3 lists the most

commonly used configurations. These types are reviewed separately below:

Table 3.1: Comparison of different Gasifier types

Gasifier Type Flow Direction Support Heat Source

Fuel Oxidant

Updraft FixedBed Down Up Grate Combustion ofChar

DowndraftFixed Bed

Down Down Grate Partialcombustion ofvolatiles

BubblingFluidized Bed

Up Up None Partialcombustion ofvolatiles andchar

CirculatingFluidized Bed

Up Up None Partialcombustion of

volatiles andchar

3.1 Updraft Fixed Bed Gasification:-

Also known as counterflow gasification, the updraft configuration is the oldest

and simplest form of gasifier; it is still used for coal gasification. Biomass is

introduced at the top of the reactor, and a grate at the bottom of the reactor supports

the reacting bed. Air or oxygen and/or steam are introduced below the grate and

diffuse up through the bed of biomass and char. Complete combustion of char takesplace at the bottom of the bed, liberating CO2 and H2O. These hot gases (~1000

oC)

pass through the bed above, where they are reduced to H2 and CO and cooled to

750oC. Continuing up the reactor, the reducing gases (H2 and CO) pyrolyse the

descending dry biomass and finally dry the incoming wet biomass, leaving the reactor

at a low temperature (~500oC). Examples are the PUROX and the Sofresid/Caliqua

technologies.


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The advantages of updraft gasification are:

Simple, low cost process

Able to handle biomass with a high moisture and high inorganic content (e.g.

municipal solid waste)

Proven technology

The primary disadvantage of updraft gasification is that Syngas contains 10-

20% tar by weight, requiring extensive syngas cleanup before engine, turbine or

synthesis applications.

Fig 3.1: Updraft Biomass Gasifier


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3.2 Downdraft Fixed Bed Gasification

Also known as cocurrent-flow gasification, the downdraft gasifier has the

same mechanical configuration as the updraft gasifier except that the oxidant and

product gases flow down the reactor, in the same direction as the biomass. A major

difference is that this process can combust up to 99.9% of the tars formed. Low

moisture biomass (


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Fig 3.2: Downdraft Biomass gasifier

3.3 Bubbling Fluidized Bed

Most biomass gasifiers under development employ one of two types of

fluidized bed configurations: bubbling fluidized bed and circulating fluidized bed. A

bubbling fluidized bed consists of fine, inert particles of sand or alumina, which have

been selected for size, density, and thermal characteristics. As gas (oxygen, air orsteam) is forced through the inert particles, a point is reached when the frictional force

between the particles and the gas counterbalances the weight of the solids. At this gas

velocity (minimum fluidization), bubbling and channeling of gas through the media

occurs, such that the particles remain in the reactor and appear to be in a boiling

state. The fluidized particles tend to break up the biomass fed to the bed and ensure

good heat transfer throughout the reactor.

The advantages of bubbling fluidized-bed gasification are:

Yields a uniform product gas

Exhibits a nearly uniform temperature distribution throughout the reactor

Able to accept a wide range of fuel particle sizes, including fines

Provides high rates of heat transfer between inert material, fuel and gas

High conversion possible with low tar and unconverted carbon


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The disadvantages of bubbling fluidized-bed gasification is that large bubble

size may result in gas bypass through the bed

Fig 3.3: Bubbling Fluidized Bed Gasifier

3.4 Circulating Fluidized Bed

Circulating fluidized bed gasifiers operate at gas velocities higher than the

minimum fluidization point, resulting in entrainment of the particles in the gas stream.

The entrained particles in the gas exit the top of the reactor, are separated in a cyclone

and returned to the reactor.

The advantages of circulating fluidized-bed gasification are:

Suitable for rapid reactions

High heat transport rates possible due to high heat capacity of bed material

High conversion rates possible with low tar and unconverted carbon

The disadvantages of circulating fluidized-bed gasification are:

Temperature gradients occur in direction of solid flow

Size of fuel particles determine minimum transport velocity; high velocities

may result in equipment erosion


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Heat exchange less efficient than bubbling fluidized-bed. Most of the gasifier

technologies described in this report employ a bubbling fluidized-bed or circulating

fluidized-bed system.

Fig 3.4: Circular Fluidized Bed gasifier

3.5 Updraft Gasification

A gasification reactor provides a method to provide gas-solid reactions in

which a gas stream passes through a bed of particles. If the particles remain fixed in

their positions, the equipment is called a fixed-bed reactor. In fact, the particles are

usually allowed to move without detaching from each other and therefore the process

is better classified as moving bed. The particles will not detach from each other if the

gasification agent velocity is less than the fluidization velocity.

Fixed bed gasification can be of updraft, downdraft or cross draft type. Since

there is an interaction of air or oxygen and biomass in the gasifier, they are classified

according to the way air or oxygen is introduced to the system. Here, only updraft

gasification is discussed because this is the basis of the design of the reactor in the

project. Figure 2.1 shows a schematic view of a possible gasifier configuration using

this technique. The particles of biomass, for instance wood chips are fed at the top of

the reactor and slowly move to the bottom where the residual ash is withdrawn. The


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combustion and gasification agents normally air is injected through the distributor at

the bottom.

In their downward movement, the biomass particles undergo the following

main processes: drying, devolatilization, gasification, and combustion. During the

conversion in a gasifier, there is no sharp delimitation between these regions. For

instance, a descending particle may be going through devolatilization in its outer

layers while inner layers are drying. A simplified sequence of events occurring in the

updraft gasifier is described as follows starting from the top of the fuel bed:

Fig 3.5: Updraft Biomass Fixed Bed Gasifier

3.5.1 Drying

During this event, the temperature of the wood chips is increased and

the moisture in the wood is evaporated by heat exchange between the wood

and the hot gas stream that is coming from the combustion zone.

3.5.2 Devolatization

The temperature of the dry wood chips is increased further and the

volatile products are released from the wood chips thereby leaving char. For

all biomass, volatiles represent a significant portion of the fuel and in

gasifiers; devolatilization provides part of the produced gases. The release of

volatiles is driven by increase of temperature. As the wood chips slowly

descend, the hot gases produced in the gasification and combustion zones


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exchange energy with the colder solid. Three main fractions are produced

during pyrolysis of biomass - light gases (H2, CO, CO2, H2O, and CH4), tar

(composed of relatively heavy organic and inorganic molecules that escape the

solid matrix as gases and liquid in the form of vapour) and char, the remaining

solid residue.

3.5.3 Gasification (Reduction)

After drying and devolatilization, the char enters the gasification zone

where carbon reacts with steam, carbon dioxide, and hydrogen. Endothermic

reactions in this section produce carbon monoxide and hydrogen. The slightly

exothermic reaction of hydrogen with carbon produces methane. The carbon

monoxide produced also reacts with water to produce hydrogen and carbon

dioxide in the water gas shift reaction. Differentiation between the gasification

zone and combustion zone is based on the presence or absence of oxygen. The

reactions that take place in this region of the gasifier can be represented

3.5.4 Combustion

The remaining char is burned, using oxygen from air in the feed gas

and leaving an ash residue. From the point of view of energy generation and

consumption, if taken as irreversible, the combination of exothermic reactions

involves an energy input of 394 MJ/kmol of carbon (calculated at 298 K) and

is mainly responsible for the energy requirements of the process. This energy

is used to promote and sustain the gasification and pyrolysis reactions, which

are mostly endothermic. In typical updraft gasifiers the following processes

take place at temperatures indicated in table:

Table 3.5.4.1: Temperatures range of various zones

Process Temperatures

1. Drying >423 K

2. Pyrolysis 423-973 K

3. Reduction 1073-1473 K

4. Combustion 973-1773K

The gas exiting from the top of the reactor consists of products of drying,

devolatilization and gasification processes. It contains a significant amount of tar and


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moisture and is at low temperatures between 473 K and 623 K because of the high

heat exchange between the solid and gas phases. Updraft gasifiers are useful for

producing gases to be burned at temperatures of above 473 K. At higher temperatures,

the tars do not condense and can easily be burnt in combustors (e.g. burners for

cooking). The high tar level makes them difficult to cleanfor other applications where

clean gas is required for example in internal combustion engines.

The major advantages of this type of gasifier are its simplicity in design, high

degree of controllability, high charcoal burn-out and internal heat exchange leading to

low gas exit temperatures and high gasification efficiencies because of the high heat

exchange. Also, because of the internal heat exchange the fuel is dried in the top of

the gasifier and therefore fuels with high moisture content (up to 50 % wb) can beused. Furthermore this type of gasifier can even process relatively small sized fuel

particles and accepts some size variation in the fuel feedstock.

Major drawbacks are the high amounts of tar and pyrolysis products, because

the pyrolysis gas does not pass through the combustion zone of the reactor. This is of

minor importance if the gas is used for direct heat applications, in which the tars are

simply burnt when above condensation temperature.

Table 3.5.4.2: Comparison between different types of gasifier

Sr.

No.

Gasifier

Type

Advantage Disadvantages

1. Updraft Small pressure drop Good thermal efficiency Little tendency towards

slag formation

Great sensitivity to tarand moisture andmoisture content of fuel

Relatively long timerequired for start up of ICengine

Poor reaction capabilitywith heavy gas load

2. Downdraft Flexible adaptation of gasproduction to load

Low sensitivity to charcoaldust and tar content of fuel

Design tends to be tall Not feasible for very small

particle size of fuel

3. Crossdraft Short design height Very fast response time to

load

Flexible gas production

Very high sensitivity toslag formation

High pressure drop


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3.6 Factors affecting Gasification

Studies have shown that there are several factors influencing the

gasification of wood. These include the following:

3.6.1

Energy content of Fuel

Fuel with high energy content provides easier combustion to sustain

the endothermic gasification reactions because they can burn at higher

temperatures. Beech wood chips have an energy content of approximately 20

MJ/kg. This is typical for most biomass sources and has been proved to be

easy to gasify.

3.6.2 Fuel Moisture content

Since moisture is in effect water, a non-burnable component in the

biomass, it is important that the water content be kept to a minimum. All water

in the feed stock must be vaporized in the drying phase before combustion

otherwise there will be difficulty in sustaining combustion because the heat

released will be used to evaporate moisture. Wood with low moisture content

can therefore be more readily gasified than that with high moisture. Wood

with high moisture content should be dried first before it can be used as fuel

for the gasifier. The beech wood chips used in the experiments have been

factory dried to a moisture content of 10% prior to packaging. This makes it

suitable as a fuel for the gasifier .Updraft gasifiers are also capable of

operating with fuels that have moisture contents of up to 50%.

3.6.3 Size Distribution of the Fuel

Fuel should be of a form that will not lead to bridging within the

reactor. Bridging occurs when unscreened fuels do not flow freely axially

downwards in the gasifier. Therefore particle size is an important parameter in

biomass gasification because it determines the bed porosity and thus the fluid-

dynamic characteristics of the bed. On the other hand, fine grained fuels lead

to substantial pressure drops in fixed bed reactors. The experimental

wood

chips are approximately 10 x 10 x 2 mm and regular in shape. This size is not

fine grained when compared to the micron scale and thus no substantial

pressure drops occur in the reactor.


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3.6.4 Temperature of the Reactor

There is a need to properly insulate the reactor so that heat losses are

reduced. If heat losses are higher than the heat requirement of the endothermic

reactions, the gasification reactions will not occur. The reactor in the

laboratory has been insulated with 50 mm of alkaline earth silicate to keep

heat losses minimal.

Chapter 4. Mathematical modelling and Thermodynamic

Equilibrium Model

In this project, the non-stoichiometric equilibrium method is used as the base

for the mathematical modelling of the gasifier. The method is particularly suitable for

fuels like coal, biomass, the exact chemical formula of which is not clearly known. In

non-stoichiometric modeling, no knowledge of a particular reaction mechanism is

required to solve the problem. In a reacting system, a stable equilibrium condition is

reached when the Gibbs free energy of the system is at the minimum. So, this method

is based on minimizing the total Gibbs free energy. The only input needed is the

elemental composition of the feed, which is known from its ultimate analysis.

General gasification reaction can be represented as:-

CHaObNc+ dH2O + e (O2+ 3.76 N2) a1H2+a2CO+a3CO2+a4CH4+ a5N2+ a6H2O

The combustion reactions:

1.C + O2 CO (-111 MJ/Kmol)*

2.CO + O2 CO (-283 MJ/Kmol)*

3.H2+ O2 H2O (-242 MJ/Kmol)*

Other important gasification reactions include:

4.C + H2O CO + H2 (+141 MJ/Kmol)* the Water-Gas Reaction

5.C + CO2 2CO (+172 MJ/Kmol)* the Boudouard Reaction

6.C + 2H2 CH4 (-75 MJ/Kmol)* the Methanation Reaction

Combustion reactions will result in completion under normal gasification

operating conditions. Under the condition of high carbon conversion, the three


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heterogeneous reactions (reactions 4 to 6) can be reduced to two homogeneous gas

phase reactions of water-gas-shift and steam methane-reforming (reactions 7 and 8

below), which collectively play a key role in determining the final equilibrium syngas

composition.

7.CO + H2O CO2+ H2 (-41 MJ/Kmol)* Water-Gas-Shift Reaction

8. CH4 + H2O CO2+ 3H2 (+206 MJ/Kmol)* Steam-Methane-Reforming

Reaction

(*Enthalpy of reaction. Under the sub-stoichiometric reducing conditions of

gasification, most of the fuels Sulphur is converted to hydrogen sulphide (H2S) and,

to a lesser degree, carbonyl sulphide (COS). Nitrogen in the feed is converted to

nitrogen (N2), with some ammonia (NH3) and a small amount of hydrogen cyanide

(HCN). Chlorine in the feed is primarily converted to hydrogen chloride (HCl). In

general, the quantities of sulphur, nitrogen, and chloride in the fuel are sufficiently

small that they have a negligible effect on the main syngas components of H 2and CO.

Relative to the thermodynamic understanding of the gasification process; its kinetic

behaviour is more complex. Very little reliable kinetic information on coal

gasification reactions exists, partly because it is highly depended on the process

conditions and the nature of the coal feed, which can vary significantly with respect to

composition, mineral impurities, and reactivity. Certain impurities are known to have

catalytic activity on some of the gasification reactions. The kinetics of gasification is

as yet not as developed as is its thermodynamics. Homogeneous reactions occurring

in the gas phase can often be described by a simple equation, but heterogeneous

reactions are intrinsically more complicated.

Let A denote the air supply in kg dry air/kg dry fuel, F the amount of dry fuel

required to obtain one normal cubic meter of the gas (1 Nm3 of gas represents a

volume of 22.4 litre at NTP), and XCthe carbon content of the fuel (kg carbon/kg dry

fuel). Carbon is split between CO, CO2, and CH4. For 1 normal cubic meter of gas

produced, one can write the carbon molar balance between inflow and outflow

streams.


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4.1 Sample calculation of reaction balance from generalized equation

Assuming hydrogen balance;

Since during the reaction total moles of hydrogen will be same, the

summation of the number of moles of hydrogen on the right and left hand side

of the general equation will be same. Therefore moles of H2on left hand of the

equal to sign of the general equation will be the sum of hydrogen moles in

feedstock and those present in moisture. So the sum total is:

da 2/

Similarly the moles of H2 on right hand of the equal to sign of the

general equation will be the sum of hydrogen moles in constituents of product

gas i.e. in H2, CH4and H2O. So the sum total is:

641 *2 aaa

Equating both sides we get;

641 *22/ aaada

But while programming convenience we have considered a not as

the number of moles of atom of H2in feedstock but as the actual weight of H2

in feedstock (kg/kg of fuel). So to get moles of H2we will have to divide by

the molecular weight of each molecule. Molecular weight of hydrogen

molecule is two and that of water is eighteen. Thus the equation becomes

641 *2182

2/aaa

da

We now consider that we use 30 kg of feedstock to produce G Nm 3

of

syngas at NTP (1 bar pressure and 25

o

C temperature). As we know, 1Nm

3

of gas at NTP occupies 22.4 litres of volume the volume fraction for the each

constituent of product gas can be obtained by dividing it by 22.4. Finally the

above equation takes the form,

G

aaada

*4.22

*2

182

2/ 641

All the following reactions were obtained by elemental balancing by

following the above methodology.


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4.2 Elemental balancing

Carbon balance:

G

)VV(VX CHCOCOC

*4.2212

*3042

or

42**56 CHCOCOC VVVXG - - - (1)

Hydrogen balance:

Let S represent the total steam supplied as humidity associated with air

and added steam (kg steam/kg of dry fuel), and W represent the moisture

content of fuel (kg water/kg dry fuel).

We write the molar balance of H2as follows:

G

VVVWXS CHOHHH

*4.22

)*2(

1821830 422

or

422*2)**336()(**33.37 CHOHHH VVVXGWSG ...(2)

Oxygen balance:-

If Oa represents the mass fraction of oxygen in air and XO is the

oxygen content of the fuel (kg oxygen/kg dry fuel), hence the molar balance of

O2as follows:

G

VVVOaAWXS OHCOCOO

*4.22

)*5.0*5.0(

32

*

18321830 22

or,

OHCOCO

O

VVVOaAXGWSG

22*5.0*5.0

)*(**21)(**33.37

..(3)


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Nitrogen balance:

If XN is the nitrogen content of the fuel (kg nitrogen/kg dry fuel) and

Na is the mass fraction of nitrogen in air, the molar balance of N2gives:

G

VNaAX NN

*4.2228

*

2830 2

or

2)*(**24 NN VNaAXF (4)

The volume fractions of all constituents of the product gas add up to

1.0. We, therefore, also have:

122422

NOHCHHCOCO VVVVVV (5)

To estimate the values of the seven unknowns: VCO, VCO2, VH2, VH2O,

VN2, VCH4, and F total of seven equations are required. For this purpose,

besides the above five equations (Equation 1 to Equation 5), any two of the

equations can be assumed from the water-gas reaction, Boudouard reaction,

shift conversion and methanation to be in equilibrium. Working with Water-

gas reaction and Boudouard reaction was chosen. For the Boudouard reaction,

the equilibrium constant is:

2

2

COCOpb

PPK

where PCO is the partial pressure of CO, which is equal to volume

fraction of CO, (VCO* the pressure of the reactor, P)

22

*)(

*

)*( 22

CO

CO

CO

COpb

V

PV

PV

PVK (6)

Similarly, for the watergas reaction:

OHCOH

OH

COHpw

V

PVV

P

PPK

22

2

2 ***

(7)

Solving equations (1) to (7) equilibrium concentrations of gases are

found.

In this case seven equations can be solved simultaneously. MATLAB

is used for solving these simultaneous equations.


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LCV of the syngas can be calculated in two ways depending upon the

usage of syngas.

The first method deals with the usage where methane is separated from

syngas as it may react to give methanol. So while calculating the LCV of the

syngas it is assumed that the tar is completely removed, the gas is scrubbed

and the only constituents in the gas that contribute to its heating value are

hydrogen and carbon monoxide.

LCVCO= (282.99 kJ/Nm3) "LCV of CO"

LCVH2= (241.83 kJ/Nm3) "LCV of H2"

LCVCH4= (802.34 kJ/Nm3); "LCV of CH4"

So the LCV of the gas is calculated by the formulae;

LCVsyngas = (LCVCO*VCO) + (LCVH2*VH2)

The other method does take into account the heating value of methane

which is appreciable. So the heating value of syngas is given by;

LCVsyngas = (LCVCO*VCO) + (LCVH2*VH2) + (LCVCH4*VCH4)


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Chapter 5. Analysis of Fuels

5.1 Proximate Analysis

The proximate analysis of coal was developed as a simple means of

determining the distribution of products obtained when the coal sample isheated under specified conditions. As defined by ASTM D 121, proximate

analysis separates the products into four groups: (1) moisture, (2) volatile

matter, consisting of gases and vapors driven off during pyrolysis, (3) fixed

carbon, the nonvolatile fraction of coal, and (4) ash, the inorganic residue

remaining after combustion. Proximate analysis is the most often used analysis

for characterizing coals in connection with their utilization. The actual method

of analysis is described below:

5.1.1 Moisture

Known weight of coal heated in silica crucible at 105-110 C for 1 hour.

%M = (Loss in wt./Original Wt.)*100

5.1.2 Volatile Matter

Dry coal is heated at 950 C for 7 minutes in furnace

%V = (Loss in wt./Original wt.)*100

5.1.3 Ash

Dry coal heated in platinum crucible at 400-700 C then ignite for

hour at 700 C, weigh the burnt material and repeat process until weight

of burnt material remains constant

5.1.4 Fixed Carbon

%FC = 100 (%M + %V + %Ash)

5.1.5 Goutels Formula

GCV = 4.187 * (82 * %FC + a * %V * %M) kJ/kg


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5.2 Ultimate Analysis

The ultimate analysis indicates the various elemental chemical

constituents such as Carbon, Hydrogen, Oxygen, Sulphur, etc. It is useful in

determining the quantity of air required for combustion and the volume and

composition of the combustion gases. This information is required for the

calculation of flame temperature and the flue duct design etc.

5.2.1 Carbon and Hydrogen

22 COOC

OHOH 222 22

Absorbers used are:

Anhydrous magnesium perchlorate or calcium chloride for H2O

Soda lime & potassium hydroxide for CO2

%H = (2/18) * (H2O wt./Coal wt.)

%C = (12/44) * (CO2wt./Coal wt.)

5.2.2 Nitrogen (Kjehldahls Method)

Coal + conc.H2SO4 with Sodium

acidicNHalkaliheatSONHHN 3424 )(62

%N = (vol. of acid consumed * normality of NaOH * 1.4)/wt. of coal

5.2.3 Sulphur

Burn known weight of coal completely

10 ml distilled water in bomb pot

Collect washing of bomb pot

Add BaCl2

44223222 2 BaSOBaClSOHOHSOOSOOS

BaSO4 is precipitate. Weigh it.

%S = (Wt. of BaSO4* 32 * 100)/(Wt. of coal * 233)


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5.2.4 GCV Equation

GCV = [(33.8 * C) + 144 * (H (O/8))] + (9.375 * S)

Fig 5.2.1: Copy of Report of Proximate Analysis of Coal


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Table 5.2: Ultimate Analysis of Coal

Fig 5.2.2: Analysis of Rice Husk

Fig 5.2.3: Analysis of Wood Pellets

Element (% by wt)

1. Carbon 60.121

2. Oxygen 10.020

3. Hydrogen 1.358

4. Nitrogen 0


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Chapter 6. Manufacturing of a Gasifier

6.1 Materials of Construction

Gasifiers are usually constructed from commercially available

materials such as steel pipe, sheet, and plate. Although the metal temperatures

encountered in well-designed air gasifiers do not usually exceed the softening

point of mild steel, certain stainless steels or inconel may give the extra

temperature resistance necessary for critical areas such as the grate, hearth, or

nozzles.

Some of the mild-steel components may suffer chemical corrosion in

certain parts. Corrosion is likely to occur in areas where water condenses or

collects since gasifier water often contains organic acids. In these instances,

the steel should be replaced by corrosion-resistant materials such as copper,

brass, epoxy lined steel, or stainless steel as required.

6.2 Methods of Construction

A gasifier is built much like a water heater, and the same methods of

construction are used. The workshop should be equipped with tools for

performing tasks such as shearing sheet metal, rolling cylinders and cones,

drilling, riveting, grinding, painting, sawing, tube cutting, and pipe threading.

An oxyacetylene torch is valuable for cutting and welding tasks, but an

arc welder is preferred for mild-steel welding.All seals must be made gas-tight; threaded and welded fittings are preferred at all points, and exhaust-

pipe-type gaskets can be used if necessary. High-temperature, anti-sieze pipe

dope should be used on all pipe joints. High-temperature applications will

require ceramic fiber or asbestos gaskets. Silicone sealant is appropriate attemperatures below 300C and rubber or Viton "O" rings and gaskets will

perform excellently at room temperature. The system should be leak-tested

before the initial startup, as well as after modifications.

6.3 Sizing and Laying out of Pipes

When designing a gasifier, it is important to keep the pressure drop in

the system as small as possible. Because there are unavoidable pressure drops


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associated with the gasifier, the cyclone separator, and the cleanup system, it

is very important to use adequately sized pipe. On the other hand, gas

velocities within the pipes should be adequate so that entrained solids will be

conveyed to their proper point of removal, rather than deposited inside the

pipe.

When laying out pipe connections for a gasifier system, it is important

to allow access to various parts that may require cleaning or adjustment.

6.4 Instruments and Control

The gasifiers of the past were crude, inconvenient devices. Today's

gasifiers are evolving toward safer, automated processes that make use of a

wide range of present-day instruments and controls6.5 Temperature

Thermocouples (such as chromel-alumel type K) should be used to

measure various gasifier temperatures, especially below the grate, as a check

for normal or abnormal operation. Temperatures at the grate should not exceed

800C; higher temperatures indicate abnormal function. Consequently, the

signal from the thermocouple can be used by a control system.


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6.6 Design of Gasifier

Desired output = 20-22 kW

As per the literature review,

1KW == 3 lb biomass

20KW == 30 kg coal

Let,

L = total height of gasifier

di= internal diameter;

pi= design pressure;

t = thickness of cylinder

Density of coal = 550 kg /cubic meter

Density = Mass/Volume

Now,

Volume = 2(/4)(di)2(L);

Assume,

Di= 200 mm

Substituting values we get,

L = 970 mm

Syt= 200 N/mm2 (Cast Iron, Design Data Book)

f.o.s = 1.5

ti= 10 mm

di= 200 mm

= 0.3

Substituting these values in the equations given below:

,FOS

Sytall , 1

)1(

)21(

2

iall

ialli

p

pdt


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barpi 3.12

Operating pressure


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DIMENSION: Height=450mm, Diameter =340mm.

PROCESS: Sheet metal is rolled and longitudinally welded to form a

cylinder.

6.7.7

ASH BED

MATERIAL: Cast Iron.

DIMENSION: Height=300mm, top length=600mm,

bottom length= 440mm.

PROCESS: Cast iron sheets are welded to form a shell

of the frustum.

Fig. 6.7.1: Updraft Gasifier


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Fig 6.7.2: Solid model for experimental setup (All dimensions are in mm)

6.8 Dimensions

6.8.1 BLOWER


SPECIFICATION: 0.5hp, 12 cfm, 1500rpm.

6.8.2 STOPPER


SPECIFICATION: 0.5 inch (3 no), 0.75 inch (2 no).

6.8.3 NOZZLE

MATERIAL: Brass.

SPECIFICATION: 0.5 inch long, 5 mm outlet diameter.


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6.8.4 GASKET

MATERIAL: RUBBER.

SPECIFICATION: 5mm thick, can sustain temperature up to 200oC

6.8.5 BURNER

MATERIAL: Stainless steel.

SPECIFICATION: 1mm outlet opening.

6.8.6 DISTRIBUTOR

MATERIAL: Cast iron.

SPECIFICATION: Inlet -1 inch, Outlet - 0.5 inch.

6.8.7 NUTS AND BOLTS


SPECIFICATION: M10 (12 no).

6.8.8 AIR TUBES


SPECIFICATION: Diameter 1 inch, 2 nos.

6.8.9 STEAM PIPES


SPECIFICATION: 35 psi, Diameter 0.5 inch, can sustain

temperature upto150

o

C.

6.8.10 PIPES

MATERIAL: Pipes.

SPECIFICATION: Diameter 0.5 inch.


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Photo 1: Actual Experimental Model


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6.9 Procedure to operate the gasifier:-

6.9.1 Clean the gasifier and place it at a clean place.

6.9.2 Fill the gasifier water jacket till the 3/4thmark of the glass tube.

6.9.3

Porous grainy ash must be filled in the gasifier. The ash must

rise to about 3-4in from the bottom and just below the steam

and air ports.

6.9.4 Now in a separate panel burn some quantity of coal. The coal

must be heated till it becomes red hot. Normally 2-3 kg of coal

must be used for this purpose.

6.9.5 Pour this coal into the gasifier keeping the air vents open. Start

the blower.6.9.6 Now fill the ash collector with water 2 inches below the steam

and air ports.

6.9.7 Connect the steam and gas temperature sensors to the

indicators.

6.9.8 After about 10 min pour about 5-7 kg of feedstock in the

gasifier. Close the air vents and also close the top portion of the

gasifier.

6.9.9 Now after 30 min pour another 5-7 kg of coal into the gasifier

into the gasifier through the hopper.

6.9.10 10 min later add another 10-12 kg of coal such that it fills the

gasifier completely below the gas outlet port. Coal at this stage

must be added in batches of 2-3 kg.

6.9.11Now wait for about 3-4 hrs for the gasifier to be stable and gas

to be produced. Hold a matchstick in front of the gas outlet

port. If it burns it signals that gas is produced.

6.9.12 In between pocking with an iron rod must be done through the

pocking hole provided. This avoids even distribution of coal

and avoids blockage to gas flow. Gas flow if blocked causes an

increase in back pressure and this may push out with great

force the feedstock out of the gasifier from the ash collector.

Such a case occurred during one of the trial. The pocking rod


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can also be used to check the level of red hot coal in gasifier.

This is done by inserting the rod in the gasifier through the

pocking hole and keeping it in the gasifier for about 5-10 min.

Remove the rod and hold a thin paper edge against it at the

bottom. The paper begins to burn, now move the paper slowly

in vertical direction. At some distance the paper will stop

burning, mark the position. The distance of the mark on the rod

from its bottom is the height of the red hot coal zone.

6.9.13 Experienced operators can reduce down the stabilisation time

drastically. Also they can detect the production of gas from the

smell of CO.

6.9.14 Inexperienced operator may need more time and more than a

couple of trials to get the desired output.

6.10 Precautions while operating the gasifier:

6.10.1 The gasifier must not be compactly packed with coal as it leads

to build up of back pressure.

6.10.2 In case of power failure or shut down of gasifier the air

distribution valve must be closed and not the blower directly. Ifthis care is not taken the syngas may flow back into the blower

and catch fire damaging the blower. Such case was found to

occur during one of the trial on the experimental setup.

6.10.3 Asbestos hand gloves must be worn while operating hopper

when the gasifier is working.

6.10.4Not much of the gas must be inhaled to detect syngas from the

odour of CO since higher concentration of CO is harmful and

causes nausea.

6.10.5 The water level in the water jacket must not be allowed to fall

below the 1/4 thmark and gasifier must not be operated without

water in the water jacket.


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Photo 2: Actual Syngas flame after production


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Chapter 7. Syngas Testing Methods and Applications

To have proper control on combustion process, an idea about complete or

incomplete combustion of fuel is made by the analysis of flue gas. Thus,

(i)

if the gases contain considerable amount of carbon monoxide, it

indicates that incomplete combustion is occurring (i.e. considerable

wastage of fuel is taking flue).

(ii)

if the flue gases contain a considerable amount of oxygen, it indicates

the oxygen supply is in excess, though the combustion may be

complete.

The analysis of Syngas is primarily done by two methods Gas

chromatography and Orsat Gas Analyzer

7.1 Gas Chromatography

Gas chromatography - specifically gas-liquid chromatography -

involves a sample being vapourised and injected onto the head of the

chromatographic column. The sample is transported through the column by

the flow of inert, gaseous mobile phase. The column itself contains a liquid

stationary phase which is adsorbed onto the surface of an inert solid.

Fig 7.1.1: Schematic diagram of a gas chromatograph


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Instrumental Components are describes as follows:

7.1.1 Carrier Gas

The carrier gas must be chemically inert. Commonly used gases

include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas

is often dependant upon the type of detector which is used. The carrier gas

system also contains a molecular sieve to remove water and other impurities.

7.1.2 Sample Injection Port

For optimum column efficiency, the sample should not be too large,

and should be introduced onto the column as a "plug" of vapour - slow

injection of large samples causes band broadening and loss of resolution. The

most common injection method is where a microsyringe is used to inject

sample through a rubber septum into a flash vapouriser port at the head of the

column. The temperature of the sample port is usually about 50C higher than

the boiling point of the least volatile component of the sample. For packed

columns, sample size ranges from tenths of a microliter up to 20 microliters.

Capillary columns, on the other hand, need much less sample, typically around

10-3L. For capillary GC, split/splitless injection is used.

Fig 7.1.2:Spit/Spitless Injection System


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The injector can be used in one of two modes; split or splitless. The

injector contains a heated chamber containing a glass liner into which the

sample is injected through the septum. The carrier gas enters the chamber and

can leave by three routes (when the injector is in split mode). The sample

vapourises to form a mixture of carrier gas, vapourised solvent and vapourised

solutes. A proportion of this mixture passes onto the column, but most exits

through the split outlet. The septum purge outlet prevents septum bleed

components from entering the column.

7.1.3 Columns

There are two general types of column packedand capillary(also

known as open tubular). Packed columns contain a finely divided, inert, solid

support material (commonly based on diatomaceous earth) coated with liquid

stationary phase. Most packed columns are 1.5 - 10m in length and have an

internal diameter of 2 - 4mm.

Capillary columns have an internal diameter of a few tenths of a

millimeter. They can be one of two types; wall-coated open tubular(WCOT)

orsupport-coated open tubular(SCOT). Wall-coated columns consist of a

capillary tube whose walls are coated with liquid stationary phase. In support-

coated columns, the inner wall of the capillary is lined with a thin layer of

support material such as diatomaceous earth, onto which the stationary phase

has been adsorbed. SCOT columns are generally less efficient than WCOT

columns. Both types of capillary column are more efficient than packed

columns.

In 1979, a new type of WCOT column was devised - theFused Silica

Open Tubular(FSOT) column. These have much thinner walls than the glass

capillary columns, and are given strength by the polyimide coating. These

columns are flexible and can be wound into coils. They have the advantages of

physical strength, flexibility and low reactivity.


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Fig 7.1.3: Cross section of Fused Silica Open Tube Column

7.1.4 Column Temperature

For precise work, column temperature must be controlled to within

tenths of a degree. The optimum column temperature is dependant upon the

boiling point of the sample. As a rule of thumb, a temperature slightly above

the average boiling point of the sample results in an elution time of 2 - 30

minutes. Minimal temperatures give good resolution, but increase elution

times. If a sample has a wide boiling range, then temperature programming

can be useful. The column temperature is increased (either continuously or in

steps) as separation proceeds.

7.1.5 Detectors

There are many detectors which can be used in gas chromatography.Different detectors will give different types of selectivity. A non-

selectivedetector responds to all compounds except the carrier gas, aselective

detectorresponds to a range of compounds with a common physical or

chemical property and aspecific detectorresponds to a single chemical

compound. Detectors can also be grouped into concentration dependant

detectorsand mass flow dependant detectors. The signal from a concentration

dependant detector is related to the concentration of solute in the detector, and

does not usually destroy the sample Dilution of with make-up gas will lower

the detectors response. Mass flow dependant detectors usually destroy the

sample, and the signal is related to the rate at which solute molecules enter the

detector. The response of a mass flow dependant detector is unaffected by

make-up gas.


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Table 7.1.5: Comparison between different types of Detector

Detector Type

Support

gases Selectivity Detectability

Dynamic

range

Flameionization

(FID)Mass flow

Hydrogenand air

Most organic cpds. 100 pg 107

Thermalconductivity

(TCD)Concentration Reference Universal 1 ng 107

Electron

capture(ECD)

Concentration Make-up

Halides, nitrates,nitriles, peroxides,

anhydrides,organometallics

50 fg 105

Nitrogen-phosphorus

Mass flowHydrogen

and airNitrogen,

phosphorus10 pg 106

Flamephotometric

(FPD)Mass flow

Hydrogenand air

possiblyoxygen

Sulphur,phosphorus, tin,boron, arsenic,

germanium,selenium,

chromium

100 pg 103

Photo-ionization

(PID)Concentration Make-up

Aliphatics,aromatics, ketones,esters, aldehydes,

amines,heterocyclics,

organosulphurs,some

organometallics

2 pg 107

Hallelectrolyticconductivity

Mass flowHydrogen,

oxygen

Halide, nitrogen,nitrosamine,

sulphur

The effluent from the column is mixed with hydrogen and air, and

ignited. Organic compounds burning in the flame produce ions and electrons

which can conduct electricity through the flame. A large electrical potential is

applied at the burner tip, and a collector electrode is located above the flame.

The current resulting from the pyrolysis of any organic compounds is


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measured. FIDs are mass sensitive rather than concentration sensitive; this

gives the advantage that changes in mobile phase flow rate do not affect the

detector's response. The FID is a useful general detector for the analysis of

organic compounds; it has high sensitivity, a large linear response range, and

low noise. It is also robust and easy to use, but unfortunately, it destroys the

sample.

Fig 7.1.5: Flame Ionisation Detector

7.2 Orsat Gas Analyzer

An Orsat gas analyser is a piece of laboratory equipment used to

analyse a gas sample (typically fossil fuel flue gas) for its oxygen, carbon

monoxide and carbon dioxide content. Although largely replaced by

instrumental techniques, the Orsat remains a reliable method of measurement

and is relatively simple to use. It was patented before 1873 by Mr. H Orsat.

7.2.1 Construction

Consists of a water-jacketed measuring burette, connected in series

to a set of three absorption bulbs, each through a stop-cock.

The other end is provided with a three-way stop-cock, the free end of

which is further connected to a U-tube packed with glass wool (for avoiding

the incoming of any smoke particles, etc.)


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The graduated burette is surrounded by a water-jacket to keep the

temperature of the gas constant during the experiment.

The lower end of the burette is connected to a water reservoir by

means of a long rubber tubing.

The absorption bulbs are usually filled with glass tubes, so that the

surface area of contact between the gas and the solution is increased.

The absorption bulbs have solutions for the absorption of CO2, O2

and CO respectively.

First bulb has potassium hydroxide solution (250g KOH in 500mL

of boiled distilled water), and it absorbs only CO2.

Second bulb has a solution of alkaline pyrogallic acid (25g

pyrogallic acid+200g KOH in 500 mL of distilled water) and it can absorb

CO2 and O2.

Third bulb contains ammonical cuprous chloride (100g cuprous

chloride + 125 mL liquor ammonia+375 mL of water) and it can absorb CO2,

O2 and CO.

Hence, it is necessary that the flue gas is passed first through

potassium hydroxide bulb, where CO2 is absorbed, then through alkaline

pyrogallic acid bulb, when only O2 will be absorbed ( because CO2 has

already been removed) and finally through ammonical cuprous chloride bulb,

where only CO will be absorbed

7.2.2 Method of Analysis

The whole apparatus is thoroughly cleaned, stoppers greased and

then tested for air-tightness.

The absorption bulbs are filled with their respective solutions to level

just below their rubber connections.

Their stop-cocks are then closed. The jacket and levelling reservoir

are filled with water.


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The three-way stop-cock is opened to the atmosphere and reservoir is

raised, till the burette is completely filled with water and air is excluded from

the burette.

The three-way stop-cock is now connected to the flue gas supply and

the reservoir is lowered to draw in the gas, to be analysed, in the burette.

The sample gas mixed with some air is present in the apparatus. So

the three-way stop-cock is opened to the atmosphere, and the gas expelled out

by raising the reservoir.

This process of sucking and exhausting of gas is repeated 3-4 times,

so as to expel the air from the capillary connecting tubes, etc.

Finally, gas is sucked in the burette and the volume of the flue gas is

adjusted to 100 mL at atmospheric pressure.

For adjusting final volume, the three-way stop-cock is opened to

atmosphere and the reservoir is carefully raised, till the level ofwater in it is

the same as in the burette, which stands at 100 mL mark.

The three-way stop-cock is then closed.

The stopper of the absorption bulb, containing caustic potash

solution, is opened and all the gas is forced into this bulb by raising the water

reservoir.

The gas is again sent to the burette.

This process is repeated several times to ensure complete absorption

of CO2 [by KOH solution].

The unabsorbed gas is finally taken back to the burette, till the level

of solution in the CO2 absorption bulb stands at the constant mark and then,

its stop-cock is closed.

The levels of water in the burette and reservoir are equalised and the

volume of residual gas is noted.


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The decrease in volume-gives the volume of CO2 in 100 mL of the

flue gas sample.

The volumes of O2 and CO are similarly determined by passing the

remaining gas through alkaline pyrogallic acid bulb and ammonical cuprous

chloride bulb respectively.

The gas remaining in burette after absorption of CO2, O2 and CO is

taken as nitrogen.

Fig 7.2.2: Orsat Gas Analyzer Apparatus


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Photo 3: Collection of Syngas in collection tube

Photo 4: Inside of Gasifier furnace (after 3 batches)


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7.3 Electrical Power (IGCC)

Electrical power generation by integrated gasification combined cycle

(IGCC) power plant using coal or refinery bottoms as a feedstock has proven

to be economical. In addition, IGCC for municipal waste, and biomassfeedstocks are realizing some commercial applications, and could potentially

develop a large foothold in the market if certain drivers develop as expected,

including energy price forecasts and more stringent greenhouse gas

requirements.

7.4 Coal-to-Liquids

The synthesis gas (syngas) created by gasificationonce impurities

such as sulfur and mercury are removedcan be turned into liquid fuels and

chemicals via the Fisher-Tropsch process or other processes. Since impurities

are removed earlier in the process, these ultra-clean liquid fuels burn with

much fewer emissions than conventional diesel fuel. Environmental

considerations, national energy concerns, and global oil markets could play a

role in the development of these applications.

7.5 Coal-to-SNG (Synthetic Natural Gas) and Hydrogen

Syngas produced by gasification can also be used for the production of

synthetic natural gas (SNG) by a process called methanation. SNG is identical

to natural gas and is capable of the same applications. The future of the natural

gas market will play a large role in driving this application of gasification.

Syngas refinement using a water-gas shift process can be used to produce

hydrogen. This may become a significant gasification technology application

if hydrogen infrastructures and markets become established.

7.6 Coal-to-Chemicals

Gasification offers a means of converting coal to a variety of useful

products including fertilizers, ammonia, and the manufacture of plastics.


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7.7 Industrial Applications

The chemical and physical conversion characteristics of gasification

also allow for more specialized applications in a wide range of industries,

particularly in the production of electricity, chemical byproducts, and

hydrogen.

7.8 Distributed Generation/Biomass

Gasification of biomass holds potential for distributed power

generation and small-scale syngas production. Some groups are looking at

gasification of biomass for power, transportation fuels, and even cooking fuel

in remote locations.

7.9 Co-Generation

Gasification of multiple products by one plant has the potential to

change the way we view energy production. The ability to produce multiple

products allows plant management to optimize profits based on market

conditions and can improve plant efficiency, economics, and decrease overall

environmental impact versus multiple plants to each produce one product.

7.10

Integrated Gasification Fuel Cell (IGFC)

The Fuel Cells technology area, part of DOE's Advanced Energy

Systems R&D Program is working to develop and demonstrate high

efficiency, fuel flexible solid oxide fuel cells (SOFCs) and coal-based SOFC

power generation systems for large (greater than 100 MW) integrated

gasification fuel cell (IGFC) power plants. Fully integrated IGFC power plants

have the potential to achieve greater than 60 percent net efficiency, near-zero

air emissions (CO2capture greater than 99 precent) and minimal water

consumption.


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Chapter 8. Parametric Analysis and Results

8.1 Program Codes

Fig 8.1: Programming window for 1Nm3Synags


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Fig 8.2: Programming window for 30 kg Feedstock


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8.2 Effect of pressure on syngas composition

To observe the effect of pressure on product gas composition by

MATLAB model, pressure is varied up to 100 bar. There is increase in

methane and CO2 content in the synthesis gas with increasing pressure. The

yield of synthesis gas drops with pressure, whereas the heat content yields

increases (reflecting the higher methane content). Figure 15.1 (c) shows the

trend of LCV and operating pressure. The LCV of syngas increases with

increase in pressure.

Fig 8.2.1: Effect of pressure on syngas composition.


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Fig 8.2.2: Effect of pressure on syngas composition.

Fig 8.2.3: Effect of pressure on LCV of syngas.


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8.3 Effect of steam to fuel ratio on syngas composition

The hydrogen in syngas increases with an increase in steam to fuel

ratio but the carbon monoxide level drops with a rise in the steam to fuel ratio.

An increase in steam to fuel ratio in gasifier enhances the shift reaction in

which carbon monoxide converts into carbon dioxide with the presence of

steam. Therefore a rise in both hydrogen and carbon dioxide contents with the

expense of carbon monoxide.

Fig 8.3 Effect of Steam to fuel ratio on syngas composition.


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8.4 Effect of air to fuel ratio on syngas composition

The content of hydrogen, carbon monoxide, and methane in syngas

impacts on heating value of the syngas. At lower air to fuel ratio and up to

0.25, the syngas consists of high methane traces. The carbon monoxide

content in syngas is maximum at 0.20.25 air to fuel ratio.

Fig 8.4.1: Effect of air to fuel ratio on syngas composition

Fig 8.4.2: Effect of air to fuel ratio on syngas composition


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8.5 Effect of Oxygen Enrichment

Fig.15.4 (a) shows how the composition of gas changes with oxygen

fraction in the air for an oxygen factor of 0.3 to 0.8. Mostly all variations of

the molar fractions versus oxygen fractions are more or less linear. The mole

fraction of N decreases with increasing oxygen fraction as expected. The

composition of methane produced is very low. The percentage of hydrogen in

the fuel gas increases continuously with oxygen fraction for an increase of

oxygen fraction. It is interesting to know that carbon dioxide and water vapour

percentages are also increasing as nitrogen percentage is decreasing. In

producer gas, nitrogen, which is an inert, reduces and other component

fractions would increase as is evident from figure. Fig. 15.4 (b) shows a

significant increase in the calorific values of fuel gas by increasing the oxygen

fraction. Calorific value increases nonlinearly from 425 kJ/Nm3to 975 kJ/Nm3 for an

increment of oxygen fraction 0.25 to 0.7.This increment is due to increase in the

amount of CO and of H2.

Fig 8.5.1: Effect ofoxygen enrichment on Syngas composition.


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Fig 8.5.2: Effect ofoxygen enrichment on LCV.

8.6 Effect of air to fuel ratio on LCV of syngas

The content of hydrogen, carbon monoxide, and methane in syngas

impacts on heating value of the syngas. The LCV of syngas is high at

relatively low air to fuel ratios. The hydrogen content in biomass decreases

sharply with an increase in air to fuel ratio reducing the LCV. LCV of syngas

increases with increase in pressure and decrease in steam to fuel ratio. The use

of air with enriched oxygen also increases the heating value of the gas.


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Fig8.6 Effect of air to fuel ratio on LCV of syngas

8.7 Effect of steam to fuel ratio on LCV of syngas

Figure shows trend of LCV and steam to fuel ratio. With increase in

steam to fuel ratio the LCV of syngas decrease considerably from 4750kJ/Nm3

to 2125 kJ/Nm3 between steam to fuel ratio 0.2 to 1.8 due to increase in

hydrogen percentage. So, to get high heating value low value of steam to fuel

ratios are recommended.

Fig 8.7 Effect of steam to fuel ratio on LCV of syngas


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8.8 Result for Coal

Fig 8.8.1: Result for 1Nm3Syngas

Fig 8.8.2: Result for 30 kg coal


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8.9 Result for Rice Husk


Fig 8.9.2: Result for 30 kg rice husk


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8.10 Result for Wood Pellets


Fig 8.10.2: Result for 30 kg wood pellets


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Chapter 9. Future Prospective

9.1 Integrated Gasification Combined Cycle

An Integrated Gasification Combined Cycle (IGCC) technology allows coal

to be used to generate power as cleanly as natural gas.

IGCC technology has three basic components. In the gasification phase (1),

heat, pressure, pure oxygen and water are used to break coal down into its

component parts and convert it into a clean synthetic gas (syngas). The syngas is

cleaned before it can be converted into substitute natural gas (SNG) which

eventually fuels the power turbines. Remaining particulates are removed from the

syngas in the particulate scrubber (2). Carbon monoxide is converted to carbon

dioxide (CO2) by adding steam in shift vessel (3). The gasification process makes it

possible to capture most of the mercury (silver), sulfur (yellow)and carbon dioxide

(CO2) in the syngas (4). The captured CO2will be transported via pipeline for use

in enhanced oil recovery or storage in a saline geologic reservoir (5).

The IGCC plant then converts the syngas into substitute natural gas (SNG or

methane), through a process called methanation (6). The SNG, which is relatively

high in energy content, powers two gas turbines. Excess heat contained in the

exhaust from those turbines then heats water to power a steam turbine (7). Thishigh-efficiency approach is known as combined-cycle. The higher energy content

of the SNG (as compared with syngas) improves the efficiency of the power

production.


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Fig 9.1.1: Integrated Gasification Combined Cycle

Because the SNG is a clean fuel, nitrogen oxide (NOx) also can be

reduced considerably during and after combustion. The results are

substantially lower emissions compared to

final report on biomass gasifier

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