energy conversion system

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ENERGY CONVERSION TECHNOLOGY

Wind Energy Conversion System

1.0 Introduction

Differential heating of the earth's surface by the sun causes the movement of large air masses

on the surface of the earth, i.e., the wind. Wind energy conversion systems convert the kinetic

energy of the wind into electricity or other forms of energy. Wind power generation has

experienced a tremendous growth in the past decade, and has been recognized as an

environmentally friendly and economically competitive means of electric power generation.

2.0 Structure of Wind Energy Conversion Systems

The major components of a typical wind energy conversion system include a wind turbine,

generator, interconnection apparatus and control systems, as shown in Figure 1. Wind

turbines can be classified into the vertical axis type and the horizontal axis type. Most modern

wind turbines use a horizontal axis configuration with two or three blades, operating either

down-wind or up-wind. The major components in the nacelle of a typical wind turbine are

illustrated in Figure 4. A wind turbine can be designed for a constant speed or variable speed

operation. Variable speed wind turbines can produce 8% to 15% more energy output as

compared to their constant speed counterparts, however, they necessitate power electronic

converters to provide a fixed frequency and fixed voltage power to their loads. Most turbine

manufacturers have opted for reduction gears between the low speed turbine rotor and the

high speed three-phase generators. Direct drive configuration, where a generator is coupled to

the rotor of a wind turbine directly, offers high reliability, low maintenance, and possibly low

cost for certain turbines. Several manufacturers have opted for the direct drive configuration

in the recent turbine designs. At the present time and in the near future, generators for wind

turbines will be synchronous generators, permanent magnet synchronous generators, and

induction generators, including the squirrel cage type and wound rotor type. For small to

medium power wind turbines, permanent magnet generators and squirrel cage induction

generators are often used because of their reliability and cost advantages. Induction

generators, permanent magnet synchronous generators and wound field synchronous

generators are currently used in various high power wind turbines. Interconnection apparatus

are devices to achieve power control, soft start and interconnection functions. Very often,

power electronic converters are used as such devices. Most modern turbine inverters are

forced commutated PWM inverters to provide a fixed voltage and fixed frequency output


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with a high power quality. Both voltage source voltage controlled inverters and voltage

source current controlled inverters have been applied in wind turbines. For certain high power

wind turbines, effective power control can be achieved with double PWM (pulse width

modulation) converters which provide a bi-directional power flow between the turbine

generator and the utility grid.

Figure 1: Structure of a typical wind energy system.

Figure 2: Major components inside the nacelle of a turbine.


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Table 1: Costs of Various Wind Power Plants

3.0 SOLAR ENERGY CONVERSION TECHNOLOGY

3.1 Solar Technology Overview

Figure 3: Solar energy conversion paths and Technologies

A wide variety of solar technologies have the potential to become a large component of the

future energy portfolio. Passive technologies are used for indoor lighting and heating of

buildings and water for domestic use. Also, various active technologies are used to convert

solar energy into various energy carriers for further utilization:


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Photovoltaic directly converts photon energy into electricity. These devices use inorganic

or organic semiconductor materials that absorb photons with energy greater than their

bandgap to promote energy carriers into their conduction band. Electron-hole pairs, or

excitons for organic semiconductors, are subsequently separated and charges are collected at

the electrodes for electricity generation.

Solar thermal technologies convert the energy of direct light into thermal energy using

concentrator devices. These systems reach temperatures of several hundred degrees with high

associated exergy. Electricity can then be produced using various strategies including thermal

engines (e.g. Stirling engines) and alternators, direct electron extraction from thermionic

devices, Seebeck effect in thermoelectric generators, conversion of IR light radiated by hot

bodies through thermophotovoltaic devices, and conversion of the kinetic energy of ionized

gases through magnetohydrodynamic converters.

Photosynthetic, photo (electro)chemical, thermal, and thermochemical processes are used to

convert solar energy into chemical energy for energy storage in the form of chemical fuels,

particularly hydrogen. Among the most significant processes for hydrogen production are

direct solar water splitting in photo electrochemical cells or various thermochemical cycles

such as the two-step water-splitting cycle using the Zn/ZnO redox system

3.2 Photon-to-Electric Energy Conversion

A solar cell, or photovoltaic cell (PV), is a device that converts light into electric current using the

photoelectric effect.Solar cells produce direct current (DC) power which fluctuates with thesunlight's intensity. For practical use this usually requires conversion to certain desired

voltages or alternating current (AC), through the use of inverters.

Multiple solar cells are

connected inside modules. Modules are wired together to form arrays, then tied to an inverter,

which produces power at the desired voltage, and for AC, the desired frequency/phase. Many

residential systems are connected to the grid wherever available, especially in developed

countries with large markets.[16]

In these grid-connected PV systems, use of energy storage is

optional. In certain applications such as satellites, lighthouses, or in developing countries,

batteries or additional power generators are often added as back-ups. Such stand-alone power

systems permit operations at night and at other times of limited sunlight.Photovoltaic devices allow the direct production of electricity from light absorption. The

active material in a photovoltaic system is a semiconductor capable of absorbing photons


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with energies equal to or greater than its bandgap. Upon photon absorption, an electron of the

valence band is promoted to the conduction band and is free to move through the bulk of the

semiconductor. In order for this free charge to be captured for current generation, decay to

the lower energy state, i.e. recombination with the hole in the valence band, has to be

prevented through charge separation.

In photovoltaic devices made of inorganic semiconductors, charge separation is driven by the

built-in electric field at the p-n junction. As a consequence, their efficiency is determined by

the ability of photon generated minority carriers to reach the p-n junction before recombining

with the majority carriers in the bulk of the material. Thus, bulk properties such as

crystallinity and chemical purity often control the device efficiency.

Figure 4: Schematic of a p-n junction and of an organic bilayer structure

In both inorganic and organic photovoltaic technologies, many strategies are under

investigation for achieving efficient light absorption, charge separation, transport, and

collection. The strategies are based on technologies that involve the use inorganic

semiconductor materials such as silicon (c-Si, pc-Si, or -Si), III-V compounds (e.g. GaAs,

InP), chalcogenides (e.g. CdTe, CIGS), and various organic-based thin films:

Additionally, advanced thin-film technologies, called 3rd

generation photovoltaics, are

considered as a promising route to increasing the efficiency and/or lowering the cost of

photovoltaics.


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3.3 Photon-to-Thermal-to-Electric Energy Conversion

This section involves solar thermal technologies that produce electricity through

concentration of solar energy for the production of heat and subsequent conversion into

electric current. There are a number of options available at different stages of development.

The most developed technologies are the parabolic dish, the parabolic trough, and the power

tower.

The parabolic dish is already commercially available. This system is modular and can be used

in single dish applications (with output power of the order of 25kWe) or grouped in dish

farms to create large multi-megawatt plants.

Parabolic troughs are a proven technology and will most likely be used for deployment of

solar energy in the near-term. Various large plants are currently in operation (California -

354MW) or in the planning process in the USA and in Europe.

Power towers, with low cost and efficient thermal storage, promise to offer dispatchable, high

capacity factor power plants in the future. Together with dish/engine systems, they offer the

opportunity to achieve higher solar-to-electric efficiencies and lower cost than parabolic

trough plants (see Table 2), but uncertainty remains as to whether these technologies can

achieve the necessary capital cost reductions.

Table 2: Characteristics of major solar thermal electric power systems

Parabolic troughs

Parabolic trough systems use single-axis tracking parabolic mirrors to focus sunlight on

thermally efficient receiver tubes that contain a heat transfer fluid (HTF). The receiver tubes

are usually metallic and embedded into an evacuated glass tube that reduces heat losses. A


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Power towers

In a power tower plant, hundreds of two-axis tracking heliostats are installed around a tower

where they focus sunlight with concentrations ranging from 100 to 10,000 suns. The absorber

is located on the top of the tower and can reach temperatures from 200 oC to 3000oC]. Hot air

or molten salt are usually used to transport the heat from the absorber to a steam generator

where superheated steam is produced to drive a turbine and an electrical generator. Power

towers are suited for large-output applications, in the 30 to 400MWe range, and need to be

large to be economical. Thermal storage can be easily integrated with this type of solar

systems, allowing the enhancement of the annual capacity factor from 25% to 65% and the

stabilization of the power output through fluctuations in solar intensity until the stored energy

is depleted.

Since the early 1980s, power towers were built in Russia, Italy, Spain, Japan, France, and the

USA, with power outputs ranging from 0.5MWe to 10MWe (Solar Two, Southern California)

and using various combinations of heat transfer fluids (steam, air, liquid sodium, molten

nitrate, molten nitrate salt) and storage media (water/steam, nitrate salt/water, sodium,

oil/rock, ceramic)

The efficiency of a solar-powered steam turbine electric generator used in the power towerconcept is a critical function of the temperature TR of the receiver, which is influenced not

only by the incident energy but also of several factors including the heliostat optical

performance, the mirror cleanliness, the accuracy of the tracking system, and wind effects.

Table 3 Properties of the principal HTFs for parabolic troughs and power towers


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The development of new heat transfer fluids (HTFs) is crucial for increasing the operating

temperature of a solar thermal plant, and hence the efficiency of the steam cycle. Stability at

high temperature, low flammability, low vapor pressure at high temperature, low corrosivity

in standard materials, low freezing point, high boiling point, and low cost are the main

required characteristics. Table 2 lists the operating temperature and the main characteristics

of some HTFs considered for parabolic troughs and power towers.

Dish-engine systems

Dish-engine systems can be used to generate electricity in the kilowatts range. A parabolic

concave mirror concentrates sunlight; the two-axis tracked mirror must follow the sun with a

high degree of accuracy in order to achieve high efficiencies. At the focus is a receiver

whichis heated up over 700C. The absorbed heat drives a thermal engine which converts the

heat into motive energy and drives a generator to produce electricity. If sufficient sunlight is

not available, combustion heat from either fossil fuels or biofuels can also drive the engine

and generate electricity. The solar-to-electric conversion efficiency of dishengine systems

can be as high as 30%, with large potential for low-cost deployment. For the moment, the

electricity generation costs of these systems are much higher than those for trough or tower

power plants, and only series production can achieve further significant cost reductions for

dishengine systems. A number of prototype dish-engine systems are currently operating in

Nevada, Arizona, Colorado, and Spain. High levels of performance have been established;

durability remains to be proven, although some systems have operated for more than 10,000

hours.

3.4 Photon-to-Chemical Energy Conversion

Photoconversion processes are used for producing a large variety of chemicals with clear

energetic and environmental advantages compared to conventional technical processes. This

section focuses on the synthesis of chemical fuels e.g. ammonia, methane, or hydrogen

since this application has the largest potential in terms of energy production. Moreover, it

could partially solve one of the principle shortcomings of conventional solar technologies,

which is the lack of capacity for energy storage. Among the large variety of identified

processes and technologies, we consider here three main categories of solar-to-chemical

conversion processes: photo(electro)chemical processes, thermochemical processes, and

photosynthetic processes in natural systems. Photochemical and photoelectrochemicalsystems use light-sensitive materials (in aqueous suspension or in the form of bulk electrodes,


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respectively) for absorbing photon energy and producing electrons with sufficient energy for

splitting water. In thermochemical technologies, concentrated solar flux is used to produce

the high-temperatures necessary to drive endothermic reactions such as syngas production

from natural gas, water thermal decomposition, and water splitting through high-temperature

chemical cycles.

4.0 Hydropower Technologies

Hydropower transforms the potential energy of a mass of water flowing in a river or stream

with a certain vertical fall (termed the head10). The potential annual power generation of a

hydropower project is proportional to the head and flow of water. Hydropower plants use a

relatively simple concept to convert the energy potential of the flowing water to turn a

turbine, which, in turn, provides the mechanical energy required to drive a generator and

produce electricity (Figure 6).

Figure 6: Typical low head hydropower plant with storage

The main components of a conventional hydropower plant are:

Dam: Most hydropower plants rely on a dam that holds back water, creating a large water

reservoir that can be used as storage. There may also be a de-silter to cope with sediment

build-up behind the dam.

Intake, penstock and surge chamber: Gates on the dam open and gravity conducts the

water through the penstock (a cavity or pipeline) to the turbine. There is sometimes a head


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race before the penstock. A surge chamber or tank is used to reduce surges in water pressure

that could potentially damage or lead to increased stresses on the turbine. The penstock

conveys water under pressure to the turbine and can be made of, or lined with, steel, iron,

plastics, concrete or wood. The penstock is sometimes created by tunnelling through rock,

where it may be lined or unlined.

Turbine: The water strikes the turbine blades and turns the turbine, which is attached to a

generator by a shaft. There is a range of configurations possible with the generator above or

next to the turbine.

Turbines are devices that convert the energy from falling water into rotating shaft power.

There are two main turbine categories: reactionary and impulse. Impulse turbines extract

the energy from the momentum of the flowing water, as opposed to the weight of the water.

Reaction turbines extract energy from the pressure of the water head. The most suitable and

efficient turbine for a hydropower project will depend on the site and hydropower scheme

design, with the key considerations being the head and flow rate. The Francis turbine is a

reactionary turbine and is the most widely used hydropower turbine in existence. Francis

turbines are highly efficient and can be used for a wide range of head and flow rates. The

Kaplan reactionary turbine was derived from the Francis turbine but allows efficienthydropower production at heads between 10 and 70 metres, much lower than for a Francis

turbine. Impulse turbines such as Pelton, Turgo and cross-flow (sometimes referred to as

Banki-Michell or Ossberger) are also available. The Pelton turbine is the most commonly

used turbine with high heads. Banki-Michell or Ossberger turbines have lower efficiencies

but are less dependent on discharge and have lower maintenance requirements.

Generators: As the turbine blades turn, the rotor inside the generator also turns and electric

current is produced as magnets rotate inside the fixed-coil generator to produce alternating

current (AC). There are two types of generators that can be used in small hydropower plants:

asynchronous (induction) and synchronous machines. Asynchronous generators are generally

used for micro-hydro projects.

Transformer: The transformer inside the powerhouse takes the AC voltage and converts it

into higher-voltage current for more efficient (lower losses) long-distance transport.

Transmission lines: Send the electricity generated to a grid-connection point, or to a largeindustrial consumer directly, where the electricity is converted back to a lower-voltage


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current and fed into the distribution network. In remote areas, new transmission lines can

represent a considerable planning hurdle and expense.

Outflow: Finally, the used water is carried out through pipelines, called tailraces, and re-

enters the river downstream. The outflow system may also include spillways which allow

the water to bypass the generation system and be spilled in times of flood or very high

inflows and reservoir levels.

5.0 GEOTHERMAL ENERGY

Geothermal is, simply, heat from the Earth. It is a clean, renewable resource that provides

energy in the United States and around the world. It is considered renewable because the

heat emanating from the interior of the Earthgeothermal energyis essentially limitlessand is constantly being regenerated. The Earths interior is expected to remain extremely hot

for billions of year to come, generating heat equivalent to 42 million megawatts of power.

If geothermal power plants are managed properly, they can produce electricity for decades or

more.

GEOTHERMAL FLUID

Geothermal fluida hot, sometimes salty, mineral-rich liquid and/or vaporis the carrier

medium that brings geothermal energy up through wells from the subsurface to the surface.

This hot water and/or steam is withdrawn from a deep underground reservoir and isolated

during production, flowing up wells and converting into electricity at a geothermal power

plant. Once used, the water and condensed steam is injected back into the geothermal

reservoir to be reheated. It is separated from groundwater by thickly encased pipes, making

the facility virtually free of water pollution.

A resource that uses an existing accumulation of hot water or steam is known as a

hydrothermal resource. While several other types of geothermal resources exist, all

producing geothermal plants in the United States use hydrothermal resources. Characteristics

of the geothermal fluid, including temperature, chemistry, and non-condensable gas content

(NCG), which can influence power plant design.

POWER PLANT BASICS

Like all conventional thermal power plants, a geothermal plant uses a heat source to expand a

liquid to vapor/steam. This high pressure vapor/steam is used to mechanically turn a turbine-


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generator. At a geothermal plant, fuel is geothermal water heated naturally in the earth, so no

burning of fuel is required. At many power plants, a steam turbine is used to convert the

thermal energy extracted from pressurized steam into useful mechanical energy. Mechanical

energy is then converted into electricity by the generator.

Geothermal plants rely upon one or a combination of three types of conversion technology

binary, steam, and flash to utilize the thermal energy from the hot subsurface fluids and

produce electricity.

CONVERSION TECHNOLOGIES

A conversion technology represents the entire process of turning hydrothermal resources into

electricity. Of the four available to developers, one of the fastest growing is the binary cycle,

which includes a Rankine cycle engine.

I. Steam

Dry steam plants have been operating for over one hundred yearslonger than any other

geothermal conversion technology, though these reservoirs are rare. In a dry steam plant like

those at The Geysers in California, steam produced directly from the geothermal reservoir

runs the turbines that power the generator. Dry steam systems are relatively simple, requiring

only steam and condensate injection piping and minimal steam cleaning devices. A dry

steam system requires a rock catcher to remove large solids, a centrifugal separator to remove

condensate and small solid particulates, condensate drains along the pipeline, and a final

scrubber to remove small particulates and dissolved solids. Today, steam plants make up a

little less than 40 percent of U.S. geothermal electricity production, all located at The Geysers

in California.

The basic cycle for steam plants remains similar to the structure that first operated in 1904 in

Larderello, Italy, pictured in the figure above. Even so, incremental technology

improvements continue to advance these systems. Figure 7 shows a dry steam plant.


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Figure 7: Dry Steam Power Plant Diagram

II. Flash

The most common type of power plant to date is a flash power plant, where a mixture of

liquid water and steam is produced from the wells. About 45 percent of geothermal

electricity production in the U.S. comes from flash technology. At a flash facility, hot liquid

water from deep in the earth is under pressure and thus kept from boiling. As this hot water

moves from deeper in the earth to shallower levels, it quickly loses pressure, boils and

flashes to steam. The steam is separated from the liquid in a surface vessel (steam

separator) and is used to turn the turbine, and the turbine powers a generator. Flash power

plants typically require resource temperatures in the range of 350 to 500oF (177

oC to 260

oC).

A number of technology options can be used with a flash system. Double flashing, the most

popular of these, is more expensive than a single flash, and could concentrate chemical

components if they exist in the geothermal water. Even considering potential drawbacks,

most geothermal developers agree that double flash is more effective than single flash

because a larger portion of the resource is used.


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Figure 8: Well Flowing Steam through a Silencer at Coso, a Double Flash Plant in California

Steam processing is an integral part of the gathering system for flash and steam plants. In

both cases, separators are used to isolate and purify geothermal steam before it flows to the

turbine. A flash system requires three or more stages of separation, including a primary flash

separator that isolates steam from geothermal liquid, drip pots along the steam line, and a

final polishing separator/scrubber. A steam wash process is often employed to further

enhance steam purity. All geothermal power plants require piping systems to transport water

or steam to complete the cycle of power generation and injection.


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Figure 9: Single Flash Steam Power Plant Schematic

Figure 10: Double Flash Steam Power Plant Schematic


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III. Binary

Technology developments during the 1980s have advanced lower temperature geothermal

electricity production. These plants, known as binary geothermal plants, today make use of

resource temperatures as low as 165oF, or 74oC (assuming certain parameters are in place)

and as high as 350oF (177

oC). Approximately 15 percent of all geothermal power plants

utilize binary conversion technology.

In the binary process, the geothermal fluid, which can be either hot water, steam, or a

mixture of the two, heats another liquid such as isopentane or isobutane (known as the

working fluid), that boils at a lower temperature than water. The two liquids are kept

completely separate through the use of a heat exchanger used to transfer heat energy from

the geothermal water to the working fluid. When heated, the working fluid vaporizes into

gas and (like steam) the force of the expanding gas turns the turbines that power the

generators.

Figure 10: Binary Power Plant Schematic


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6.0 Biomass Energy Conversion

6.1. Biomass conversion processes

The development of conversion technologies for the utilization of biomass resources for

energy is growing at a fast pace. Most developing countries find it hard to catch up because

the level of technology is beyond their manpower as well as their manufacturing and

technological capability. Added to this is the unavailability of local materials and parts for the

fabrication of these conversion units. Figure 1 shows the different methods for converting

biomass into convenient fuel. Biomass conversion into heat energy is still the most efficient

process but not all of energy requirement is in the form of heat. Biomass resources need to be

converted into chemical, electrical or mechanical energy in order to have widespread use.

These take the form of solid fuel like charcoal, liquid fuel like ethanol or gaseous fuel like

methane. These fuels can be used in a wide range of energy conversion devices to satisfy the

diverse energy needs. In general, conversion technologies for biomass utilization may either

be based on bio-chemical or thermo-chemical conversion processes. Each process will be

described separately.

Figure 11: Methods of using Biomass Energy

6.1 Bio-chemical conversion processes

The two most important biochemical conversion processes are the anaerobic digestion and

fermentation processes.


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6.1.1 Anaerobic digestion

Anaerobic digestion is the treatment of biomass with naturally occurring microorganisms in

the absence of air (oxygen) to produce a combustible gaseous fuel comprising primarily of

methane (CH4) and carbon dioxide (CO2) and traces of other gases such as nitrogen (N2) and

hydrogen sulphide (H2S). The gaseous mixtures is commonly termed biogas. Virtually all

nitrogen (N), phosphorus (P) and potassium (K) remain in the digested biomass. The entire

process takes place in three basic steps as shown in Figure 2. The first step is the conversion

of complex organic solids into soluble compounds by enzymatic hydrolysis. The soluble

organic material formed is then converted into mainly short-chain acids and alcohols during

the acidogenesis step. In the methanogenesis step, the products of the second step are

converted into gases by different species of strictly anaerobic bacteria. The percentage of

methane in the final mixture has been reported to vary between 50 to 80%. Atypical mixture

consists of 65% methane and 35% CO2 with traces of other gases. The methane producing

bacteria (called methanogenic bacteria) generally require a pH range for growth of 6.4 to 7.2.

The acid producing bacteria can withstand low pH. In doing their work, the acid producing

bacteria lower the pH and accumulate acids and salts of organic acids. If the methane-

forming organisms do not rapidly convert these products, the conditions become adverse to

methane formers. This is why the first type of reactors developed for conversion of biomasswastes into methane have long retention times seeking equilibrium between acid and methane

formers. Municipal wastes and livestock manures are the most suitable materials for

anaerobic digestion. In the US, numerous landfill facilities now recover methane and use it

for power generation. Aquatic biomass such as water hyacinth or micro-algae can be digested

and may become valuable sources of energy in the future. Anaerobic digestion of organic

wastes may constitute an effective device for pollution control with simultaneous energy

generation and nutrient conservation. A major advantage of anaerobic digestion is that it

utilizes biomass with high water contents of as high as 99%. Another advantage is the

availability of conversion systems in smaller units. Also the residue has fertilizer value and

can be used in crop production. The primary disadvantage of anaerobic digestion of diluted

wastes is the large quantity of sludge that must be disposed of after the digestion process

including the wastewater and the cost of biogas storage. In cold climates, a significant

fraction of the gas produced may be used to maintain the reactor operating temperature.

Otherwise, microorganisms that thrive on lower or moderate temperatures should be used.


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6.2 Thermo-chemical conversion processes

Biomass wastes can be easily converted into other forms of energy at high temperatures,

They break down to form smaller and less complex molecules both liquid and gaseous

including some solid products. Combustion represents a complete oxidation to carbon dioxide

(CO2) and water (H2O). By controlling the process using a combination of temperature,

pressures and various catalysts, and through limiting the oxygen supply, partial breakdown

can be achieved to yield a variety of useful fuels. The main thermo-chemical conversion

approaches are as follows: pyrolysis/charcoal production, gasification and combustion. The

advantages of thermo-chemical conversion processes include the following:

a. Rapid completion of reactions

b. Large volume reduction of biomass

c. Range of liquid, solid and gaseous products are produced

d. Some processes do not require additional heat to complete the process

6.2.1 Pyrolysis

Pyrolysis or destructive distillation is an irreversible chemical change caused by the action of

heat in the absence of oxygen. Pyrolysis of biomass leads to gases, liquids and solid residues.

The important components of pyrolysis gas in most cases are hydrogen, carbon monoxide,

carbon dioxide, methane and lesser quantities of other hydrocarbons (C2H4, C2H6, etc.). The

liquid consists of methanol, acetic acid, acetone, water and tar. The solid residue consists of

carbon and ash. Thus pyrolysis can be used to convert biomass into valuable chemicals and

industrial feedstock.

In a typical pyrolysis process the feed material goes through the following operations: (a)

primary shredding (b) drying the shredded material (c) removal of organics (d) further

shredding to fine size (e) pyrolysis (f) cooling of the products to condense the liquids and (g)

storage of the products.

Different types of pyrolytic reactors include vertical shaft reactors, horizontal beds. Among

these, the simplest and generally cheapest is the vertical shaft type. Fluidized bed reactors

are relatively a recent development. Figure 11 shows a rotary kiln pyrolysis reactor. The unit


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is cylindrical, slightly inclined and rotates slowly which causes the biomass to move through

the kiln to the discharge end.

Numerous technologies have now been developed for the production of bio-oil and char

using the pyrolysis process, Many of the reactors developed are improvements on the

traditional reactors used in rural areas of developing countries that include simple pit kilns

or drum type reactors. The energy efficiency of charcoal production using these methods is

only the order of 17-29% while theoretically, efficiencies as high as 40% could be achieved.

Fig. 12. Schematic of the rotary kiln pyrolysis reactor

6.2.2 Gasification

Gasification is the thermo-chemical process of converting biomass waste into a low medium

energy gas utilizing sub-stoichiometric amounts of oxidant (Coovattanachai,1991).

The simplest form of gasification is air gasification in which biomass is subjected to

partial combustion with a limited supply of air. Air gasifiers are simple, cheap and

reliable. Their chief drawback is that the gas produced is diluted with nitrogen and hence

has low calorific value. The gas produced is uneconomical to distribute; it must be used

on-site for process heat. In oxygen gasification, pure oxygen is used so that the gas

produced is of high energy content. The chief disadvantage of oxygen gasification is that


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it requires an oxygen plant and thus increases the total cost of gasification. The schematic

diagram of the processes occurring in a gasifier is shown in Figure 12 including the

temperature profile at each important step in the process.

Fig. 12. Schematic diagram of processes occurring in a gasifier and the temperature profile.

6.2.3 Biomass combustion

One of the most common methods of biomass conversion is by direct combustion or

burning. The simplest units include numerous cookstoves already developed in rural areas

of developing countries. Much improved and continuous flow designs include the Spreader-

Stoker system used in many refuse derived fuels (RDF) facility for converting solid wastes,

and the fluidized bed combustion units (similar to that shown in Figure 12). The number

component parts of this system is listed below:1. Refuse charging hopper

2. Refuse charging throat

3. Charging ram

4. Grates

5. Roller bearings

6. Hydraulic power cylinders and control valves

7. Vertical drop-off


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8. Overfire air jets

9. Combustion air

10. Automatic sifting removal system

In a spreader-stoker system, the fuel is introduced into the firebox above a grate. Smaller

particles will tend to burn in suspension and larger pieces will fall onto the grate. Most

units, if properly designed, can handle biomass with moisture content as high as 50-55%.

Moisture contained in the fuel is driven off partially when the fuel is in suspension and

partially on the grate. The feed system should provide an even thin layer of fuel on the

grate.

In a fluidized bed combustor (FBC), the fuel particle burns in a fluidized bed of inert particles

utilizing oxygen from the air. Advantages of fluidized bed combustion include: (1) high heat

transfer rate, (2) increased combustion intensity compared to conventional combustors and,

(3)absence of fouling and deposits on heat transfer surfaces.

The schematic diagram of a fluidized bed combustor is similar to that of a fluidized bed

gasifier. The only difference is the use of excess air for combustion processes and starved air

for gasification processes. So far FBC has been used mostly for coals. A number of wastes,

e.g. wastes from coal mining and municipal wastes, are also sometimes incinerated in

fluidized beds. It has been suggested that certain quick-maturing varieties of wood could be

combusted in fluidized beds for generation of steam. There is indeed a global search for

suitable varieties of wood for this purpose and FBC is likely to play an important role in

supplying energy requirements in certain countries in the future.


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Fig. 13. Schematic diagram of a reciprocating grate combustor (Courtesy of Detroit

Reciprogate Stocker).

Granular biomass fuels, e.g. paddy husk and chips of wood up to 2cm x 2cm x 2cm in size

have been successfully combusted in fluidized beds of sand particles. Conventional

combustion of paddy husk is slow and inefficient. Nearly complete combustion and high

combustion intensities of paddy husk can be achieved in a fluidized bed combustor. The same

combustor can also be used for burning wood. Combustion intensities up to about 500

Kg/hr-m2 have been achieved in fluidized bed combustors using biomass fuels. A number of

thermo-chemical conversion processes exist for converting biomass into liquid fuels. These

can be crudely divided into direct liquefaction and indirect liquefaction (in which the biomass

is gasified as a preliminary step) processes. While all these techniques are relatively

sophisticated and will generally be suitable for large scale conversion facilities, they do

represent an important energy option for the future because the heavy premium

that liquid fuels carry. The steam produced from heat of combustion of biomass may power a

steam turbine to produce electricity. However, because of the high ash contents of most

biomass resources, direct combustion of these biomass resources is not practical and efficient

due to slagging and fouling problems. Because of these problems, some biomass with high

ash are often mixed with low ash biomass such as coal, also termed co-firing.


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6.2.4 Biomass co-firing

Co-firing refers to mixing biomass and fossil fuels in conventional power plants. Significant

reductions in sulphur dioxide (SO2 an air pollutant released when coal is burned) emissions

are achieved using co-firing systems in power plants that use coal as input fuel. Small-scale

studies at Texas A&M University show that co-firing of manure with coal may also reduce

nitrogen oxides (NOx- contribute to air pollution) emissions from coal (Carlin, 2009).

Manure contains ammonia (NH3). Upon co-firing manure and coal, NH3 is released from

manure and combines with NOx to produce harmless N and water. Biomass co-firing has the

potential to cut emissions from coal powered plants without significantly increasing the cost

of infrastructure investments (Neville, 2011). Research shows that when implemented at

relatively low biomass-to-coal ratios, energy consumption, solid waste generation and

emissions are all reduced. However, mixing biomass and coal (especially manure) does create

some challenges that must be address.

There are three types of co-firing systems adopted around the world as follows:

a. Direct co-firing

b. Indirect co-firing , and

c. Separate biomass co-firing.

Direct co-firing is the simplest of the three and the most common option especially if the

biomass have very similar characteristics with coal. In this process, more than one type of

fuel is injected into the furnace at the same time. Indirect co-firing involves converting the

biomass into gaseous form before firing. The last type has a separate boiler for the co-fired

fuel. It was reported that the carbon life cycle and energy balance when co-firing 15%

biomass with coal is carbon neutral or better (Eisenstat, et al., 2009). In this research, carbon

emissions are reduced by 18%.

energy conversion system

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