UNIT IV
BIOGAS & GEOTHERMAL POWER GENERATING SYSTEMS
Bioconversion:
The term Bioconversion, also known as biotransformation refers to the use of live organisms
often microorganisms to carry out a chemical reaction that is more costly or not feasible non
biologically. These organisms convert a substance to a chemically modified form. An
example is the industrial production of cortisone. One step is the bioconversion of
Progesterone to 11-alpha-Hydroxyprogesterone by Rhizopus nigricans.
Another example of this is the conversion of organic materials, such as plant or animal waste,
into usable products or energy sources by biological processes or agents, such as certain
microorganisms, some Detritivores or enzymes.
The conversion of organic materials, such as plant or animal waste, into usable products or
energy sources by biological processes or agents, such as certain microorganisms.
The Bioconversion Science and Technology group performs multidisciplinary R&D for the
Department of Energy's (DOE) relevant applications of bioprocessing, especially with
biomass. Bioprocessing combines the disciplines of chemical engineering, microbiology and
biochemistry. The Group 's primary role is investigation of the use of microorganism,
microbial consortia and microbial enzymes in bioenergy research.
New cellulosic ethanol conversion processes have enabled the variety and volume of
feedstock that can be bioconverted to expand rapidly. Feedstock now includes materials
derived from plant or animal waste such as paper, auto-fluff, tires, fabric, construction
materials, municipal solid waste (MSW), sludge, sewage, etc.
Three different processes for bioconversion
1 - Enzymatic hydrolysis - a single source of feedstock, switchgrass for example, is mixed
with strong enzymes which convert a portion of cellulosic material into sugars which can
then be fermented into ethanol. Genencor and Novozymes are two companies that have
received United States government Department of Energy funding for research into reducing
the cost of cellulase, a key enzyme in the production cellulosic ethanol by this process.
2 - Synthesis gas fermentation - a blend of feedstock, not exceeding 30% water, is gasified in
a closed environment into a syngas containing mostly carbon monoxide and hydrogen. The
cooled syngas is then converted into usable products through exposure to bacteria or other
catalysts. BRI Energy, LLC is a company whose pilot plant in Fayetteville, Arkansas is
currently using synthesis gas fermentation to convert a variety of waste into ethanol. After
gasification, anaerobic bacteria (Clostridium ljungdahlii) are used to convert the syngas (CO,
CO2, and H2) into ethanol. The heat generated by gasification is also used to co-generate
excess electricity.
3 - C.O.R.S. and Grub Composting are sustainable technologies that employ organisms that
feed on organic matter to reduce and convert organic waste in to high quality feedstuff and oil
rich material for the biodiesel industry.[3] Organizations pioneering this novel approach to
waste management are EAWAG, ESR International, Prota Culture and BIOCONVERSION
that created the e-CORS system to meet large scale organic waste management needs and
environmental sustainability in both urban and livestock farming reality. This type of
engineered system introduces a substantial innovation represented by the automatic
modulation of the treatment, able to adapt conditions of the system to the biology of the
scavenger used, improving their performances and the power of this technology.
Biogas
Biogas typically refers to a mixture of different gases produced by the breakdown of organic
matter in the absence of oxygen. Biogas can be produced from raw materials such as
agricultural waste, manure, municipal waste, plant material, sewage, green waste or food
waste.
It is a renewable energy source and in many cases exerts a very small carbon footprint.
Biogas can be produced by anaerobic digestion with anaerobic bacteria, which digest material
inside a closed system, or fermentation of biodegradable materials.
Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of
hydrogen sulphide (H2S), moisture and siloxanes. The gases methane, hydrogen, and carbon
monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows
biogas to be used as a fuel; it can be used for any heating purpose, such as cooking. It can
also be used in a gas engine to convert the energy in the gas into electricity and heat.
Biogas can be compressed, the same way natural gas is compressed to CNG, and used to
power motor vehicles. In the UK, for example, biogas is estimated to have the potential to
replace around 17% of vehicle fuel. It qualifies for renewable energy subsidies in some parts
of the world. Biogas can be cleaned and upgraded to natural gas standards, when it becomes
bio methane.
Biogas
Biogas is formed by the anaerobic decomposition of putrescible organic material. Biogas
CHP (combined heat and power or cogeneration) is the utilisation of biogas, typically in a
biogas engine, for the production of electricity and useful heat, at high efficiency.
Clarke Energy is a distributor of GE Jenbacher biogas engines which are designed for robust
operation on difficult gases such as biologically-derived ones.
What is biogas?
Biogas is a gas that is formed by anaerobic microorganisms. These microbes feed off
carbohydrates and fats, producing methane and carbon dioxides as metabolic waste products.
This gas can be harnessed by man as a source of sustainable energy.
Biogas is considered to be a renewable fuel as it originates from organic material that has
been created from atmospheric carbon by plants grown within recent growing seasons.
Benefits of anaerobic digestion and biogas
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Production of renewable power through combined heat and power cogeneration
Disposal of problematic wastes
Diversion of waste from landfill
Production of a low-carbon fertiliser
Avoidance of landfill gas escape and reduction in carbon emissions
Biogas formation
Biogas creation is also called biomethanation. Biologically derived gases are produced as
metabolic products of two groups of microorganisms called bacteria and Archaea. These
microorganisms feed off carbohydrates, fats
and proteins, then through a complex series of reactions including hydrolysis, acetogenesis,
acidogenesis and methanogenesis produce biogas consisting mainly of carbon dioxide and
methane.
Biogas composition
Biogas consists primarily of methane (the source of energy within the fuel) and carbon
dioxide. It also may contain small amounts of nitrogen or hydrogen. Contaminants in the
biogas can include sulphur or siloxanes, but this will depend upon the digester feedstock.
The relative percentages of methane and carbon dioxide in the biogas are influenced by a
number of factors including:
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The ratio of carbohydrates, proteins and fats in the feedstock
The dilution factor in the digester (carbon dioxide can be absorbed by water)
Anaerobic digestion
Anaerobic digestion is the man-made process of harnessing the anaerobic fermentation of
wastes and other biodegradable materials. Anaerobic microbes can be harnessed to treat
problematic wastes, produce a fertiliser that can be used to replace high carbon emission
chemical fertilisers. It also is the process that results in the production of biogas, which can
be used to provide renewable power using biogas cogeneration systems.
Anaerobic digestion can occur at mesophilic (35-45˚C) or thermophilic temperatures (5060˚C). Both types of digestion typically require supplementary sources of heat to reach their
optimal temperature. This heat is most commonly provided by a biogas CHP unit, operating
on biogas and producing both electricity and heat for the process.
Often, biogas plants that treat wastes originating from animal material, will also require the
material to be treated at high temperature to eliminate any disease causing bacteria in the
slurry. These systems pasteurise the slurry, typically at 90C for one hour, to destroy
pathogens, and result in the provision of clean, high quality fertiliser.
Biogas engines
GE Jenbacher biogas engines are specifically designed to operate on different types of biogas.
These gas engines are linked to an alternator in order to produce electricity at high efficiency.
High efficiency electricity production enables the end user to maximise the electrical output
from the biogas and hence optimise the economic performance of the anaerobic digestion
plant.
Biogas engine electrical output
There are 4 ‘types’ of GE Jenbacher gas engines with different levels of power output and
electrical/thermal efficiency characteristics.
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249-330kWe – Type 2
499-1,065kWe – Type 3
844-1189kWe – Type 4
1,600-3,000kWe – Type 6
Biogas CHP
Biologically-derived gases can be utilised in biogas engines to generate renewable power
via cogeneration in the form of electricity and heat. The electricity can be used to power the
surrounding equipment or exported to the national grid.
Low grade heat from the cooling circuits of the gas engine, typically available as hot water on
a 70/90°C flow/return basis. For anaerobic digestion plants that are using a CHP engine, there
are two key types of heat:
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High grade heat as engine exhaust gas (typically ~450°C)
The low grade heat is typically used to heat the digester tanks to the optimum temperature for
the biological system. Mesophilic anaerobic digesters typically operate at 35-40°C.
Thermophilic anaerobic digesters typically operate at a higher temperature between 49-60°C
and hence have a higher heating requirement.
You can find out more about biogas CHP efficiency here.
High temperature exhaust gas heat can either be used directly into a drier, waste heat
boiler or organic rankine cycle unit. Alternatively it can be converted into hot water using a
shell and tube exhaust gas heat exchanger to supplement the heat from the engine cooling
systems.
Waste heat boilers produce steam typically at 8-15bar. Driers may be useful to reduce the
moisture content of the digestate to assist in reducing transportation costs. Organic rankine
cycle turbines are able to convert surplus waste heat into additional electrical output.
In the event that the local legislation requires for the destruction of pathogens in the digestate
(such as the European Animal By-Products Regulations) there may be the requirement to heat
treat the waste via pasteurisation or sterilisation. Here, surplus heat from the gas engine can
be used in the pasteurisation unit.
The heat from the CHP engine can also be used to drive an absorption chiller to give a source
of cooling, converting the system to a tri generation plant.
Minimum Flow Rate
The minimum gas flow rate to operate the smallest GE Jenbacher biogas engine at full load
(J208 @249kWe) is 127Nm3/hour at 50% methane.
Sectors
We have specific pages related to:
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Agriculture
Organic wastes
Distilleries
Mechanical biological treatment and anaerobic digestion of mixed wastes (MBT-AD)
Landfill gas
Sewage gas
Potential Contaminants
Biologically derived gases may include contaminants or impurities including water, hydrogen
sulphide and siloxanes. Please discuss your gas quality expectations with your local Clarke
Energy office. GE provides specific guidelines on fuel gas quality in technical instruction
documents.
Water
Biological gases contains water vapour due to the nature of the feedstock that produces the
gas. The quantity of water is linked to the temperature of the biological gas and the method
of production. Above certain limits the moisture content of the biogas becomes a combustion
challenge for the gas engines.
Water can be removed from the gas by using:
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Gas dehumidification (drying) units.
Ground tube dewatering
Hydrogen Sulphide
Hydrogen sulphide (H2S) is derived as a by-product of the anaerobic digestion process of
high sulphur feedstocks such as amino-acids and proteins. When burnt in a gas engine
hydrogen sulphide can condense with water to form sulphuric acid. Sulphuric acid is
corrosive to elements of gas engines and so must be limited to prevent adverse effects on the
CHP engine.
Processes for the removal of hydrogen sulphide include
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Activated carbon filters
Low level oxygen dosing into digester head space (typically <1%)
External biological scrubber towers
Ferric chloride dosing into the digester
Siloxanes
In some cases biogas contains siloxanes. Siloxanes are formed from the anaerobic
decomposition of materials commonly found in soaps and detergents. During the combustion
process of the gas that contains siloxanes, silicon is released and can combine with free
oxygen or various other elements in the combustion gas. Deposits are formed containing
mostly silica (SiO2) or silicates (SixOy). These white mineral deposits accumulate and must
be removed by chemical or mechanical means.
Siloxanes are often problematic in landfill gas and sewage gas plants due to contamination
that is often found associated with the organic wastes.
In source-segregated biodegradable waste and agricultural biogas plants, it is much less
common to find problems associated with siloxanes.
Production
Biogas is produced as landfill gas (LFG), which is produced by the breakdown of
biodegradable waste inside a landfill due to chemical reactions and microbes, or as digested
gas, produced inside an anaerobic digester. A biogas plant is the name often given to an
anaerobic digester that treats farm wastes or energy crops. It can be produced using anaerobic
digesters (air-tight tanks with different configurations). These plants can be fed with energy
crops such as maize silage or biodegradable wastes including sewage sludge and food waste.
During the process, the microorganisms transform biomass waste into biogas (mainly
methane and carbon dioxide) and digestate. The biogas is a renewable energy that can be
used for heating, electricity, and many other operations that use a reciprocating internal
combustion engine, such as GE Jenbacher or Caterpillar gas engines.[4] Other internal
combustion engines such as gas turbines are suitable for the conversion of biogas into both
electricity and heat. The digestate is the remaining organic matter that was not transformed
into biogas. It can be used as an agricultural fertiliser.
There are two key processes: mesophilic and thermophilic digestion which is dependent on
temperature. In experimental work at University of Alaska Fairbanks, a 1000-litre digester
using psychrophiles harvested from "mud from a frozen lake in Alaska" has produced 200–
300 liters of methane per day, about 20%–30% of the output from digesters in warmer
climates.
Composition
The composition of biogas varies depending upon the origin of the anaerobic digestion
process. Landfill gas typically has methane concentrations around 50%. Advanced waste
treatment technologies can produce biogas with 55%–75% methane, which for reactors with
free liquids can be increased to 80%-90% methane using in-situ gas purification techniques.
As produced, biogas contains water vapor. The fractional volume of water vapor is a function
of biogas temperature; correction of measured gas volume for water vapor content and
thermal expansion is easily done via simple mathematics which yields the standardized
volume of dry biogas.
Typical composition of biogas
Compound
Methane
Carbon dioxide
Nitrogen
Hydrogen
Hydrogen sulphide
Oxygen
Formula
CH4
CO2
N2
H2
H2S
O2
%
50–75
25–50
0–10
0–1
0–3
0–0
In some cases, biogas contains siloxanes. They are formed from the anaerobic decomposition
of materials commonly found in soaps and detergents. During combustion of biogas
containing siloxanes, silicon is released and can combine with free oxygen or other elements
in the combustion gas. Deposits are formed containing mostly silica (SiO2) or silicates (Si
xOy) and can contain calcium, sulfur, zinc, phosphorus. Such white mineral deposits
accumulate to a surface thickness of several millimeters and must be removed by chemical or
mechanical means.
Practical and cost-effective technologies to remove siloxanes and other biogas contaminants
are available.(broken link)
For 1000 kg (wet weight) of input to a typical biodigester, total solids may be 30% of the wet
weight while volatile suspended solids may be 90% of the total solids. Protein would be 20%
of the volatile solids, carbohydrates would be 70% of the volatile solids, and finally fats
would be 10% of the volatile solids.
Benefits
In North America, use of biogas would generate enough electricity to meet up to 3% of the
continent's electricity expenditure. In addition, biogas could potentially help reduce global
climate change. High levels of methane are produced when manure is stored under anaerobic
conditions. During storage and when manure has been applied to the land, nitrous oxide is
also produced as a byproduct of the denitrification process. Nitrous oxide (N2O) is 320 times
more aggressive than carbon dioxide and methane 21 times more than carbon dioxide.
By converting cow manure into methane biogas via anaerobic digestion, the millions of cattle
in the United States would be able to produce 100 billion kilowatt hours of electricity, enough
to power millions of homes across the United States. In fact, one cow can produce enough
manure in one day to generate 3 kilowatt hours of electricity; only 2.4 kilowatt hours of
electricity are needed to power a single 100-watt light bulb for one day. Furthermore, by
converting cattle manure into methane biogas instead of letting it decompose, global warming
gases could be reduced by 99 million metric tons or 4%.
Applications
Biogas can be used for electricity production on sewage works, in a CHP gas engine, where
the waste heat from the engine is conveniently used for heating the digester; cooking; space
heating; water heating; and process heating. If compressed, it can replace compressed natural
gas for use in vehicles, where it can fuel an internal combustion engine or fuel cells and is a
much more effective displacer of carbon dioxide than the normal use in on-site CHP plants.
Biogas Problems
The dangers of biogas are mostly similar to those of natural gas, but with an additional risk
from the toxicity of its hydrogen sulfide fraction. Biogas can be explosive when mixed one
part biogas to 8-20 parts air. When the tank is open for cleaning or repair work is being done
open flames, sparks, and smoking should be avoided. If light is needed a flashlight or sunlight
reflected off of a mirror should be used. Biogas leaks smell like rotten eggs (hydrogen
sulfide). If someone enters a biogas digester they should always have someone with them in
case they stop breathing due to low oxygen intake.
It is important that a biogas system never have negative pressure as this could cause an
explosion or kill the digesting bacteria. Negative gas pressure can occur if too much gas is
removed or leaked. Because of this biogas shouldn't be used at pressures below one column
inch of water, measured by a pressure gauge.
Frequent smell checks must be performed on a biogas system. If biogas is smelled anywhere
windows and doors should be opened immediately. If there is a fire the gas should be shut off
at the gate valve of the biogas system.
Advantages of Biogas
1. Renewable Source of Energy: To begin with, biogas is considered to be a renewable
source of energy. Since it often produced from materials that form sewage and waste
products, the only time it will be depleted is when we stop producing any waste.
2. Non-Polluting: It is also considered to be non-polluting in nature. The production of
biogas does not require oxygen, which means that resources are conserved by not using any
further fuel.
3. Reduces Landfills: It also uses up waste material found in landfills, dump sites and even
farms across the country, allowing for decreased soil and water pollution.
4. Cheaper Technology: Applications for biogas are increasing as the technology to utilize it
gets better. It can be used to produce electricity and for the purpose of heating as well.
Compressed Natural Gas (CNG) is biogas that has been compressed and can be used as a fuel
for vehicles. Production can be carried out through many small plants or one large plant.
5. Large number of Jobs: Either way, work opportunities are created for thousands of
people in these plants. These jobs are a blessing in rural areas, which are the targeted grounds
for the use of biogas. In fact, biogas can easily be decentralized, making it easier to access by
those living in remote areas or facing frequent power outages.
6. Little Capital Investment: Biogas are easy to set up and require little capital investment
on a small scale basis. In fact, many farms can become self sufficient by utilizing biogas
plants and the waste material produced by their livestock each day. A single cow can provide
enough waste material within a day to power a light bulb the entire day.
7. Reduces Greenhouse Effect: It also reduces the greenhouse effect by utilizing the gases
being produced in landfills as forms of energy. This is a major reason why the use of biogas
has started catching on. It recycles most forms of biodegradable waste and works on simple
forms of technology.
Disadvantages of Biogas
1. Little Technology Advancements: First of all, the current systems in place used to create
biogas are not as efficient as they get. Little new technology has been introduced for
streamlining the process and making it more cost effective. As a result, large scale industrial
production of biogas is still not on the energy map. Although it could solve the energy issues
being faced by countries all over the world, very few investors are willing to put in the startup
capital. It is also not the best idea to construct one biogas plant per house, which means that a
central system will have to be put into place.
2. Contain Impurities: Biogas contains a number of impurities even after refining processes
have been put into place. When compressed for use as fuel, these can become corrosive to the
metal parts of engines.
3. Not Attractive on Large Scale: The process of using biogas on a large scale is not
economically viable and it is very difficult to enhance the efficiency of biogas systems.
4. Unstable: It is also somewhat unstable, making it prone to explosions if the methane
comes in contact with oxygen and become flammable in nature.
Even with all of the disadvantages present, countries have started to apply the uses of biogas
in everyday life. Public transportation has been renewed and made efficient with the help of
CNG. Remote locations that are off the electric grid receive a steady supply of power from
these plants. The future use of biogas is bright, even with the problems it faces.
Anaerobic digestion definition
Anaerobic digestion is a biological process making it possible to degrade organic matter by
producing biogas which is a renewable energy source and a sludge used as fertilizer.
The production of biogas is carried out in the environment in a natural way (e.g. gas of
marshes - vegetable and animal matter decomposition where the formation of bubbles at
water surface can be observed).
In the absence of oxygen (anaerobic digestion), the organic matter is degraded partially by
the combined action of several types of micro-organisms. A succession of biological
reactions (see diagram) led to the formation of biogas and sludge.
The bacteria which carry out these reactions exist in natural state in the liquid manure and the
anaerobic ecosystems; it is not necessary to add more, they develop naturally in a medium
without oxygen.
Anaerobic Digestion Diagram
Hydrolysis
The organic macromolecules break up into simpler elements - solid waste thus is liquefied
and hydrolyzed in small soluble molecules (e.g. the cellulose is transformed into soluble
sugars such as glucose or cellobiose.
Acidogenesis
This process transforms these simple molecules into acids of weak molecular weight such as
lactic acid and volatile fatty-acids from 2 to 5 carbon atoms. In parallel are produced lowweight molecular alcohol, such as bicarbonate ethanol and molecular hydrogen.
Acetogenesis
The products resulting from fermentation require an additional transformation before being
able to produce methane. It is here that intervene the acetogenes reducing bacteria and the
sulfato-reducing bacteria, producing hydrogen sulphide (H2S)
Methanogenesis
The ultimate phase during which two types of methanogenes bacteria take over: the first ones
(acetogenes) reduce methane acetate, CH4 and bicarbonate. The second ones, reduce methane
bicarbonate.
Advantages of anaerobic digestion
Anaerobic digestion involves a considerable reduction in the organic load, thanks of the
biological reactions, and the polluting load of the digested sludge. It is thus, a complete
depollution. A correctly controlled anaerobic digestion leads to very high rates of
purification.
It also has other advantages:
Economic advantages:
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Additional income
Autonomy in heat in a context of increase in the cost of fossil energies
Diversification of outlets for crops
Reduction of manure purchase thanks to valorisation of digested sludge
Agronomic advantages
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Transformation of the liquid manure and the manure into a fertilizer, more easily assimilated
by the plants, with reduction in the odours and the disease-causing agents
Organic waste processing for competitive prices
Insect ellimination at the storage pit
Odours supression
Environmental advantages
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Biogas resulting by anaerobic digestion is a source of renewable energy because it replaces
fossil energy
Reduction of pollution due to nitrogen stripping (please refer to the “Sludge” heading)
Sustainable management of organic waste
Energy from Biomass
There are two distinct processes for obtaining energy from Bio mass, They are
1.Direct combustion
2.Indirect combustion
DIRECT COMBUSTION OF BIOMASS
Knowledge is Power
Indirect Combustion of Bio Mass
Knowledge is Power
Biomass to
Fuel
Conversions
Results:
Alcohol (Ethanol)
Biogas (Methane)
Syngas
Gasoline (Biocrude)
Diesel Fuel (Plant Oil)
Methods of Biomass to Energy Conversion
Direct Combustion
Pyrolysis: thermal decomposition into gas or liquid
Involves high temperatures (500-900°C), low oxygen
Biochemical processes:
Anaerobic digestion by methanogens
Controlled fermentation produces alcohols:
Ethanol (grain alcohol)
Methanol (wood alcohol)
Anaerobic Digester
Converts animal or plant waste into methane
Typical wastes:
Manure (feed lots,pig farms, poultry)
Olive oil mill waste
Potato processing waste
Big deal: Agricultural Science Depts
Why Biomass? Lower greenhouse emissions, cleaner fuel, less dependency on foreign oil
Biofuels – ethanol, biobutanol, biodiesel
Biopower – methane, syngas to produce electricity
Bioproducts – converting byproducts or the different building blocks
Biorefineries – similar to petroleum refineries – just feedstock goes in, end product comes out
Fossil fuels are mainly made up of hydrocarbons – which are hydrogens and carbon while
biomass is mainly carbohydrates – which are hydrogen, carbon, and oxygen which makes the
conversion slightly different.
Biochemical Conversion
 Plant matter – hemicellulose, cellulose, lignin
 Pretreatment
 Hydrolysis
 Sugar Fermentation
Aerobic Digestion
Aerobic digestion is a process in which bacteria use oxygen to convert organic material into
carbon dioxide. Products include nutrient-rich fertilizers and composts.
Anaerobic Digestion
Anaerobic Digestion is the decomposition of biomass by bacteria in the absence of oxygen.
Biogas, or methane, is the primary product produced.
Fermentation
Fermentation is a biological process in which enzymes produced by microorganisms cause
chemical reactions to occur. Products include ethanol, commercial levels of therapeutic and
research enzymes, antibiotics, and specialty chemicals.
Thermochemical Conversion
 Gasification, Pyrolysis, Direct Hydrothermal Liquefaction
 Carbon monoxide and Syngas (Hydrogen)
Gasification
Gasification is a thermochemical process in which biomass at high heat is turned directly
from a solid into a gaseous fuel called syngas (a mixture of carbon monoxide, hydrogen and
some methane).
Pyrolysis
Pyrolysis is the thermal degradation of organic components in biomass in the absence of
oxygen. Major products are oil, gas, and char.
The limitations for using biomass energy
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Either high technological level or catalytic combustion is needed.
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Large area is needed to grow plants for biomass energy use.
• When producing biomass fuel, large amount of waste will also produced.
The environmental problems are caused by biomass energy
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It will intensify air pollution.
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It may cause saltilization and decrease to total size of the arable land.
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The source of biomass can use fertilize soil, e.g., crop residues and animal manure.
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Cutting too many woods is a kind of deforestation can cause, soil erosion and natural
disasters