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).
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. 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.
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.
3. Electricity generation.
4. Domestic fuel for burners in kitchen.
PRESENT FUELS FOR INTERNAL COMBUSTION ENGINE :-
8. Producer gas.
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
b. Constituents of Organic Waste ?
The organic waste contains many constituents such as cellulose, Hemicelluloses, lignin, proteins, and starch, water-soluble, fats
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.
1. Continuous gas output.
2. Minimum space requirement
3. Suitable for individual family
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 ?
Carbon monoxide (CO2) 25-35%
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
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).
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.
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.
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%