INTRODUCTION
A.
Relevance :-
The
economic of India depends to a large extent on the wheels of transport. The
specter of economy ruin due to depleted oil reserves has changed the interest
of scientist and research work towards alternative fuels for motor vehicle. Viable substitute for motor spirit are
gaseous hydrocarbons, hydrogen gas, alcohol & electricity that run on
hydrocarbon gas & electricity are still in the experimental stage. While
alcohol is used as a fuel chiefly in Brazil, it’s feasibility as motor fuel
depends on the successful cultivation & processing of sugarcane. Gaseous
hydrocarbons seem to be the best immediate option presently available. These
are mainly COMPRESSED NATURAL GAS (CNG) & LIQUIFIED PETROLIUM GAS (LPG).
LPG is being imported whereas CNG is available in abundance in India. Till
recently, technology to permit conversion of vehicles from petrol burners to
gas burners had to imported, but now due to the pioneering efforts of
departments of mechanical engineering at the INDIAN INSTITUTE OF TECHNOLOGY,
MUMBAI.
India
is largest cattle breeding country, there is abundance of raw material for
producing biogas. Also municipal sewage can be used for this purpose.
The use of methane separated from biogas as a fuel
will substantially reduce harmful engine emission and will help to keep the
environment clean. Biogas consists of approximately 55-60 % of methane. It is
economical and slurry can be used as organic manure.
One of the alternate technologies Sulabh propagates is the
biogas plant that utilises human excreta as its raw input. In the last 20 years, it has setup a hundred
such plants throughout India. The
plants’ twin outputs, similar to those of cattle biogas plants, are
nutrient-rich sludge and methane-rich biogas.
The sludge is used primarily as manure, and the biogas either as cooking
fuel or as street-lighting gas.
B.
The Technology :-
Biogas
dates as far back as the 16th century, when it was used for heating bath-water
in Persia. It has been used in India for
almost a hundred years (Sampat, 1995).
The Indian government introduced large-scale biogas production in 1981
through the National Project on Biogas Development. Biogas is produced by extracting chemical
energy from organic materials in a sealed container called a digester. 2 million biogas plants were in operation in
1995, and about 10 million rural Indians were benefiting from the electric
power and cooking
fuel the gas provided, and also from the rich agricultural
fertilizer the plant produces as a byproduct.
Central to the generation of biogas is the concept of
anaerobic digestion, also called biological gasification. It is a naturally occurring, microbial
process that converts organic matter to methane and carbon dioxide. The
chemical reaction takes place in the presence of methanogenic bacteria with
water an essential medium. The anaerobic
digestion process, as the name states, is one that functions without molecular
oxygen. Ideally, in a biogas plant there
should be no oxygen within the digester.
However, efforts to completely remove it will be prohibitively
expensive. Oxygen therefore exists in the digester, dissolved mainly in
water. Fortuitously, some microbes
within the digester are facultative anaerobes, i.e. they utilize oxygen and
lower the dissolved oxygen concentration to levels suitable for other anaerobic
microbes to perform their chemical reactions.
Oxygen removal from the digester is important for two main reasons. First, the presence of oxygen leads to the
creation of water, not methane. Second,
oxygen is a contaminant in biogas and also a potential safety hazard. Due to
presence of oxygen, calorific value of biogas becomes low.
First, cow
dung, the primary raw input for almost all operating biogas plants is
widespread and easily available. India
has more cattle than any other country (450 million head, 19% of the world
population).[i]
Second,
the cow is held in religious veneration and its products are considered
purifying agents. Hence, there is a
universal acceptance of even its dung, which otherwise would instinctively be
thought of as repulsive. Dung (or gobar in Hindi) is widely used in
India for house construction (as an infill material and external plaster), in
religious rituals, as composted fertilizer and as a cooking fuel (dung cakes).
Dung accounts for over 21 percent of total rural energy use in India, and as
much as 40 percent in certain states of the country.[ii]
Third, only 27% of rural India has access to electricity supplied by the
national grid (ostensibly, 84% of all villages are connected). Localized biogas plants obviate the
dependency on the grid by producing energy from a locally controlled and easily
accessible raw material.
C. Present Theory and Practices
i.
Biogas
cars: -
Koges,
Switzerland is developing a new fuel based on biogas which would be eco
friendly and cheaper than petrol. Wastage from kitchens and gardens are
collected, non-biodegradable matter removed and again put into fermentation
reactor. Here, in the anaerobic environment microorganisms transforms the
garbage into compost and biogas called
kompogas. Gas obtained from 100kg. Of waste can fuel a
medium sized car up to 100km. The engine runs more quietly on kompogas,
vibrates less and the exhaust is almost odourless. At the present 150 vehicles
are running on kompogas.
ii. In India,
some projects are undertaken in which diesel and biogas as dual fuel for diesel
pump.
iii. In Israel,
biogas is used as a fuel for loaded vehicles.
D. Need
Till
date, LPG, CNG has been used as fuel. But they have their own limitations. LPG
is explosive, CNG is expensive. Methane separated from biogas is equivalent to
CNG but economical than CNG. Now-a-days the whole world is facing energy
crisis. Available sources of liquid fuel will be depleted after few years. In
this situation biogas can serve as best alternative fuel.
E. Applications :-
1.
Fuels for internal combustion engine.
2.
Pump.
3.
Electricity generation.
4.
Domestic fuel for burners in kitchen.
PRESENT FUELS FOR INTERNAL COMBUSTION ENGINE :-
1.
Gasoline.
2.
Diesel.
3.
Alcohol
4.
LPG.
5.
CNG.
6.
Electricity.
7.
Solar.
8.
Producer gas.
9.
Hydrogen.
Present Fuels and Limitations:-
There
are so many fuels used in I.C. Engines, but they have certain physical and
chemical properties. In other words, fuels used in I.C. Engine re designed tom
satisfy performance requirements of engine system, in which they are used. The
limitations of fuels that are used presently are as follows
1.
Gasoline contains many impurities. It has low octane
number. All petroleum fuels oxidize slowly in presence of air. The oxidation of
unsaturated hydrocarbons result in formation of resinous materials called
gum
and reduces its lubricating quality and tends to form sludge and warmish on
piston and rings. It has less knock resistance as well as energy per unit mass.
It has less efficiency compared to other fuels. It has high cost.
2.
In alcohol, higher latent heat of vaporization reduced
charge temperatures before combustion. Alcohols suffer disadvantages of water
absorption, corrosive and lubricant incompatibility.
3.
In LPG, it reduces volumetric efficiency due to its
high heat of vaporization. The road sensitivity is very high. It is very
corrosive. Response to blending is very poor. It has higher cost of
transportation. It has higher cost for conversion kit, installation of
extensive.
4.
In electricity, they use in initially generated power
stations that use fossil fuel of nuclear power. There are other problems too.
The problem is with batteries in these vehicles. These batteries are quite heavy and life of
these is also low. Cost of replacing
these batteries is high.
PREPARATION OF BIO-GAS
I. Micro
Organisms And Mechanism Of Bio-Gas Production
a. Micro Organisms-
An organic
waste consist of many organisms but the organisms useful for biogas production
are
i.
Aerobic
ii.
Anaerobic
b. Constituents of Organic Waste –
The organic
waste contains many constituents such as cellulose, Hemicelluloses, lignin,
proteins, and starch, water-soluble, fats
Soluble etc.
c. Mechanism of biogas production: -
Stage 1 It involves
the decomposition of cellulose, hemicellulos
Lignin, starch, protein, fats etc. Into simpler organic compounds like
acids, alcohols and gases like CO2, H2, and NH3,
H2S etc. by aerobic and anaerobic
Micro-organisms.
Stage 2: - The anaerobic organism or
methane bacteria utilizes
Simple
carbon compounds available from first stage and produce methane.
This is biogas production.
II.Bio-gas plants: -
There are two types of plants-
i.
Daily fed or continuous type.
ii.
Batch fed or periodic type.
1). Daily fed or continuous type biogas plants: -
It consists of 5 m. deep underground tank of masonry construction. It is
known as digester or fermentation well. The inlet tank is
connected to digester by an inlet pipe and the outlet
tank
is connected to digester by the outlet pipe as shown in figure. The gas holder collects biogas
produced in digester. It can be taken
for use through gas outlet pipe.
The
organic waste such as cattle dung is mixed with water in 1: 1
Proportion
and poured in inlet tank everyday. This material is usually known as substrate. Substrate gets
collected in the fermentation well through the inlet pipe. The trapped air is
removed from the digester through the gas outlet and the gas holder is placed
in the position.
When
the plant is commissioned, an inoculation of the bacteria is brought from
existing biogas plant and is injected in the digester to accelerate the purpose
of decomposition to produce biogas at faster rate.
Size
of plant depends upon
1.
The required amount of gas daily and
2 Available quantity of cattle dung daily.
After
the digester is full of substrate within a week’s time, the digester start
coming out through the gas outlet pipe. It is displaced out. This gas can not
burn. Initially high CO2 contained in the gas makes it unsuitable
for use.
Within
4-8 weeks, the microorganism develops sufficiently and biogas is generated.
This stabilized gas burns continuously in the burner. The gas outlet is covered
with wire mesh to prevent a flame rushing into the digester.
The
plant should be exposed to the sunlight and shielded from the wind to
accelerate the growth of bacteria. The substrate should not be added till the
steady flame of gas is obtained at the burner. The plant is operated at low
pressure for proper burning of gas and proper fermentation.
The
used out substrate passing to outlet tank through the outlet pipe. The residual
slurry gets stored in this tank. The solid residue can be used for diluting the
dung. Sometimes mixture is used in the digester to help digestion. The digester
may be surrounded by water and heating coil to maintain temperature.
Advantages: -
1.
Continuous gas output.
2.
Minimum space requirement
3.
Suitable for individual family
Disadvantages:
-
1. Substrate of uniform quality is desirable
2. Daily attention is required
3. Daily feeding is
necessary
III.
Purification of Biogas:-
Biogas coming from tank contains
–
Composition:–
|
Methane(CH4)
|
50-68%
|
|
Carbon monoxide (CO2)
|
25-35%
|
|
Hydrogen(H2)
|
1-5%
|
|
Nitrogen (N2)
|
2-7%
|
|
Oxygen (O2)
|
0-.1%
|
|
Hydrogen Sulphide (H2S)
|
Rare
|
Out of these CO2 does not help in combustion
process but reduce the calorific value of biogas. H2S is in minor
quantity but it has corrosive action on combustion chamber and also reduces
calorific value of biogas. Also traces of moisture are to be remove for better
thermal efficiency. So harmful gradients are removed and use only methane as a
fuel.
Different Purification Processes:-
1) Removal
of H2S -
The gas coming out of
system is heated to 1500 C
and over ZnO bed, maintained at 1800
C leaving process gas free of H2S.
ZnO + H2S = ZnS + H2O.
ZnSO4
+ 2NaOH = Zn (OH) 2 + Na2SO4
2) Removal of CO2 –
CO2
is high corrosive when wet and it has no combustion value so its removal
is must to improve the biogas quality.
The processes to remove CO2
are as follows –
a) Caustic
solution, NAOH – 40%
NAOH + CO2 =
NAHCO3
b) Renfield
process – K2CO3 - 30 %
K2CO3
+ CO2 = 2KCO3
3) Removal
of NH3:-
The chemical
reaction is as:
NH3
+ HCL =NH4Cl
4) Removal of H2O:-
For the removal of moisture, we passed the gas from
above reaction, through the crystals
of white silica gel.
PROPERTIES OF BIO-GAS
In its pure state, it
is color less, odorless, tasteless. For safety reason, an odorant is added so
that any leak can be easily detected because of typical smell.
The composition of bio gas is never
constant. Methane is by far the largest component, its presence accounting for
about 95% of the total volume. Methane is a simple hydrocarbon, a substance
consisting of carbon & hydrogen. There are many of these compounds each has
its own carbon & hydrogen atoms joined together to for a particular
hydrocarbon gas as fuel gas. Methane is very light fuel gas. If we increase the
number of hydrogen & carbon atoms, we have got progressively heavier gases,
releasing more heat, therefore more energy, when ignited.
Specific
gravity of methane is .55 which is less than petrol & LPG. This means that
biogas will rise if escaping, thus dissipating from the site of a leak. This
important characteristic makes biogas safer than other fuels. It does not
contain any toxic component; therefore there is no health hazard in handling of
fuel.
The air to biogas (stoichiometric) ratio by
volume for complete combustion is 9.5:1 to 10:1.
Biogas has a very slow flame velocity, only .290 m/s. at its
highest. The range of flammability is 4 to 14% which can give good combustion
efficiency.
Biogas has very high octane number
approximately 130. By comparison, gasoline is 90 to 94 & alcohol 105 at
best. This means that a higher compression ratio engine can be used with biogas
than petrol. Hence, cylinder head of the engine is faced so that clearance
volume will be reduced & compression ratio can sufficiently increase. Thus
volumetric efficiency & power output are increased. Because of its high
octane value the detonation occur however high the compression may be. The
Boiling point of biogas is above 300 degree Celsius while the calorific value
is 35.390 MJ/m3
One
of the promising renewable energy sources is biogas, which is compound gas
consisting mainly of methane (CH4) and carbon dioxide (CO2).
It is normally formed with the decomposition of organic substances. Because of its low energy density, the gas is
generally stored in high-pressure gas bomb. To store it in a condition of high
density, it is also attempted to store methane in the form of clathrate. The
clathration of methane requires normally high pressure and low temperature. If
the clathration of biogas and methane could be achieved under the
normal pressure and temperature, this would make the gases a very useful energy
source. In this study, the clathration
of methane under the normal pressure and temperature was first attempted by
using Tetrahydrofuran (THF) as additive. Further, to realize the higher storage
density of methane, CO2 must be removed
beforehand because not only methane but also CO2 form clathrate. To
achieve CO2 removal, the possibility of absorption method using
Monoethanolamine (MEA) is experimentally investigated, aiming efficient biogas
utilization in final.
Advantages
of Biogas : -
1)
It is light fuel gas.
2)
It mixes easily with the air.
3)
It is highly knocked resistant.
4)
Due to uniform distribution thermal efficiency is
higher.
5)
Biogas has a high octane number.
6)
It reduces pollution.
7)
Higher compression ratio can be used with biogas.
8)
Plants capital cost is low.
9)
Domestic fuels for burners used in kitchen.
10)
No toxic to skin.
REPORT HIGHLIGHTS POTENTIAL
BENEFITS OF BIOGAS
A study
released at the NGV Conference highlights the benefits of using biogas as a
source of fuel for NGVs. Biogas consists primarily of methane and is
given off in places where decaying organic material is found. According
to the report, one of the primary benefits of capturing biogas generated at
landfill sites, sewage waste treatment plants, and animal feedlots would be a
substantial reduction in greenhouse gas emissions. The report also finds
that capturing and burning biogas would provide significant reductions in toxic
emissions and ozone forming pollutants, and lower particulate emissions in the
case of heavy-duty vehicles. In addition, the report finds that water
quality could be improved as a result of reduced waste runoff near sites where
biogas is captured and used in NGVs.
The
potential reductions of greenhouse gas emissions presented in this paper are
staggering. Much of this benefit is derived from capturing and burning
methane emissions that currently are released into the atmosphere. The report
indicates that an NGV using fuel derived from biogas that otherwise would have
been vented provides as much benefit as removing six petroleum-fueled vehicles
from the nation's highways. Stated differently, use of biogas in NGVs
would produce 600 percent less greenhouse gas emissions when compared with
using petroleum as a motor fuel.
Using
biogas that currently is flared instead of vented would provide about a 100
percent net reduction in greenhouse gas emissions when compared with burning
petroleum motor fuel in a similar vehicle. The study also finds that
utilization of available supplies of biogas could potentially reduce the motor
vehicle-related greenhouse gas emission by more than 340 million tons -- a 23
percent reduction in overall emissions of motor vehicle greenhouse gas
emissions.
The amount
of natural gas that potentially could be produced from decaying material around
the country is substantial. The report indicates that biogas could
displace about 6 billion gallons of motor fuel a year, accounting for nearly
four percent of all the gasoline and diesel currently used by motor
vehicles. The report indicates that some of this biogas can be produced
at prices competitive with conventional petroleum fuels. Much of the
fuel, however, is not economic at today's fuel prices, but could easily be made
economic if the right types of incentives or credits were provided.
BIOGAS in INTERNAL COMBUSTION ENGINE
1.
S. I. Engines
The only
adoption for a spark ignition engine is a gas (not gasoline!) carburetor to
work at the supply pressure (just like an LPG conversion, but an evaporator
would not be needed as the storage pressure is low). It is also a good idea to
scrub the H2S (as it causes corrosion) and to
derate the engine (unless you want to replace it each year if operating
continuously).
Modification of S.I. Engine -
S.I. engines can run completely on biogas,
however, the engines are required to be started on petrol at the beginning,
conversion of S.I. engine for the entry
of biogas, throttling of intake air & advancing the ignition timing.
Biogas can be admitted to S.I. engine through the intake manifold & air
flow control valve can be provided on the air cleaner pipe connecting air
cleaner & carburetor for throttling the intake air as shown in fig.
2.
C.I. Engine :-
Diesel engines also need a gas carburetor and scrubbing, but
require at
least 10% diesel via the injectors for ignition (and cooling). The initial
starting of diesel engine is done on pure diesel
Modification of C.I. Engine:–
C.I. engine can operate on dual fuel &
the necessary engine modification include
provision for the entry of biogas with intake air, provision of carburetor
& system to reduce diesel supply, advanced injection timing. The entry of biogas and mixing of gas with
intake air can be achieved by providing the mixing chamber below the air
cleaner which facilitate through mixing of biogas with air before entering into
the cylinder. The arrangement is shown in fig. is largely used in stationary
engine commercially available in India. The capacity of mixing chamber may be
kept equal to the engine displacement volume. The pilot injection of cycle is
required to be advanced for smooth and efficient running of engine on dual
fuel. The admittance of biogas into the engine at the initial stage increases
engine speed and therefore a suitable system reduces the diesel supply by
actuating the control rack needs to be incorporated.
There is a
wide range of thoughts on what treatments should these biogases be subjected to
before being used as fuel. Most operators simply remove the water present in
the biogas, leaving it to the engine manufacturers to design engines which will
cope with the impurities inevitably included in the biogas (significant
maintenance costs); other Operators are seriously evaluating maintenance costs
against initial investments in biogas clean up technologies such as has been
developed by Acrion Technologies (although Acrion's technologies are
mainly aimed at biogas contaminant removal and separation into methane
and carbon dioxide as feed stocks for a variety of commercial applications).
PRACTICAL DIFFICULTIES
To
use the biogas as a fuel in SI engine there are some practical difficulties. It
is not possible to compress the methane, separated from biogas by available
method, because the gas could be liquefied through chilling below -161 0C.
This process is
adapted by installing the units required when there use of methane separated
from biogas as a fuel. Since gas can not be compressed it requires large space
for storage.
PERFORMANCE
1.
In purification method, by reducing CO2 and
moisture along H2S impurities in biogas, the engine performance is
improved.
2.
Effect of spark timing :-
Biogas is
slow burning fuel. Hence in order to get optimum engine performance, spark
timing does not advance, and then combustion continues in major part of the
expansion stroke. This reduces effective work done. By advancing, spark timing
power is improved on low speed at partial throttle condition as well as high
speed at full throttle condition.
EXHAUST EMMISSIONS
The
exhaust emission contains three specific substances which contribute the air
pollution, hydrocarbon, carbon monoxide &oxides of nitrogen. Hydrocarbons
are the unburned fuel vapour coming out with the exhaust due to incomplete
combustion. Hydrocarbon also occurring in crankcase by fuel evaporation. The
emission of hydrocarbon is closely related to many design &operating
factors like induction system, combustion chamber design, air fuel ratio,
speed, load. Lean mixture lower
hydrocarbon emission.
Carbon
monoxide occurs only in engine exhaust. It is the product of incomplete
combustion due to insufficient amount of air in air- fuel mixture. Some amount
of CO is always present in the exhaust even at lean mixture. When the throttle
is closed to reduce air supply at the time of starting the vehicle, maximum
amount of CO is produced.
Oxides
of nitrogen are the combination of nitric oxide & nitrogen oxide
&availability of oxygen are the two main reasons for the formation of
oxides of nitrogen. The spark advance means lower peak combustion temperature.
It causes high NO concentration in the exhaust. With biogas, co emission levels
are low than that of gasoline.
Comparison
of Exhaust Emission :-
|
|
METHANE
Vs GASOLINE
|
|
Power
Reduction
|
11%
|
|
CO
Reduction
|
99%
|
|
HC
Reduction
|
99%
|
|
NO
Reduction
|
59%
|
|
ISFC
Increase
|
19%
|
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