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1. Abstract
Biogas is a clean environment friendly fuel. Raw biogas contains about 55–65% methane (CH4), 30–45% carbon dioxide (CO2), traces of hydrogen sulfide (H2S) and fractions of water vapors. Presently, it can be used only at the place where it is produced. There is a great need to make biogas transportable. This can be done by compressing the gas in cylinders which is possible only after removing its CO2, H2S and water vapor components. Pilot level trials to compress the biogas have been carried out by a number of earlier investigators working on the subject. This paper reviews the efforts made to improve the quality of biogas by scrubbing CO2 and the results obtained. There is a lot of potential if biogas could be made viable as a transport vehicle fuel like CNG by compressing it and filling into cylinders after scrubbing and drying. Thus the need emerges for a unified approach for scrubbing, compressing and subsequent storage of biogas for wider applications.

2. Introduction
In recent years the interest in bio fuels has been increasing, motivated on the one hand by the need for reducing green house gas emissions and on the other hand by the desire to improve energy security by reducing our dependence on imported fossil fuels. During the last few weeks the desire was rocketed driven by the tremendous increase in oil prices.
Through directive 2003/30/EC the European commission has obliged all member states to develop and implement policies to promote the use of bio- fuels in the transport sector. Goals are set at 2 % replacement of the transport sector’s energy use by bio-fuels bye the end of 2005 and 5.75% replacement by the end of 2010. EU member states have the freedom to choose fuels and policy instruments that fit to their national situation. Sweden has chosen to promote – beside ethanol – the utilization of biogas. Switzerland, who is not the member of EU , has taken the same decision on a completely facilitative basis driven by two major associations involved in the gas business , the biomass association and the Natural Gas association .
Based on the experiences gained in these two countries, task 37 aims to promote the technology. The opportunities promote the technology are ample. The task has decided to take actions in the upcoming years in the following fields stimulate research in the area of gas upgrading and disseminate the respective results provide information on the technology to engineers and decision makers in the member countries maintain strong contacts with the gas industry to promote the introduction of upgraded biogas into the gas grid and to formulate opportunities where the expensive and energy intensive gas upgrading process can be minimized.

Maintain strong links to car manufacturers and the European natural gas association (ENGVA) to promote the sale of NGV’s driven on mixtures of biogas and natural gas. Maintain exchange of results with ongoing EU and US projects in the field of alternative fuel utilization and make their finding available to member countries.

3. Background
Upgraded biogas has the same properties as natural gas. It therefore can be used with the same engine and vehicle configuration as natural gas. Worldwide there are about 3.8 million natural gas fuelled vehicles, mainly in Argentina, brazil, Pakistan, Italy, India and the US(ENGVA,2004) and about 10,000 biogas driven cars and buses, demonstrating that the vehicle configuration is not a problem for use of biogas as vehicle fuel. This equals about 0.5% of the world vehicle stock. European car manufacturer start to have an increasing interest in NGV’s since a couple of years, was it primarily FIAT who offered a choice of models, so is now ford leading the space. Even Volkswagen who was long against NGV came up with a first model. Four engine companies provide a total of 10 different bus models and seven truck models. This is quite an improvement. Six years back truck engines still had to be imported from the USA.
Exploiting biogas for energy purposes has taken on considerable importance in the countries of the European Union over the past twelve years or so. The sector’s relevance in economic, energetic and financial terms is now a fact in 20 European countries representing a total production (of the 25 member E.U.) in region of 4.625 million toe (ton oil equivalent) in 2004.
Today biogas results from landfills mainly urban and industrial sewage treatment plants. Agricultural installations and collective co-digestion units come next in terms of importance. Like a years ago, the UK produced still the most biogas in 2004. Most of it derives from landfill sites in contrast to Germany, the follower up. In a short period of time Germany will double the UK thanks to its increasing amount of agricultural plants and the communal biogas plants constructed upfront the land fills to reduce the organic fraction in the waste according to the very stringent German law put in force in June 2005. Agricultural biogas installations (which exploit the animal farming and breeding waste besides an increasing amount of energy crop) represent a type of application that growing greatly and whose number has gone from 1500 in 2001 to more than 2000 in 2004. In total, there are more than 4000 biogas production sites. However, not all of these sites do necessarily vaporize their biogas in the form of final energy (heat, electricity, fuel or network gas). Almost all of the biogas for energy purposes is used to produce electricity. Alternatively there is a huge potential for renewable fuel. In densely populated areas the affect of lower traffic emission brings a higher ecological effect than electricity production. Only in remote agricultural areas where no gas grid is existing, it might be more sensible to produce electricity. Biogas upgrading units are too expensive for small scale systems.
If production of biogas fuel should be limiting in future, then a comparable gas can be produced with wood gasifiers. A first demonstration unit developed by the technical university Vienna, is operated in Gussing, Austria which is equipped for a partial stream with gas upgrading plant from Switzerland (PSI).

4. Raw biogas
Upgraded biogas can be used as a stand alone fuel. There are examples where it is distributed at pumping stations next to the place of production like in otelfingen, Switzerland. In other cases it is collected and transported by trucks to pumping stations usually in urban areas like in stockholm.In other places the biogas is transported over a special gas line to the city in page link oping, Sweden.
The predominant part of the biogas for fuel in Switzerland is upgraded and fed into the natural gas grid. The fuelling at the pumping station is virtual, i.e. you usually buy natural gas however, on a data base the provider keeps exactly track on how much biogas is introduced and how much has been used at their pumping stations. The data base which is controlled by a third party. In Switzerland it is the state who does the control because the biogas is tax free.
Fuel Quality
Biogas produced in AD-plants or landfill sites is primarily composed of methane(CH4) and carbon dioxide (C02) with smaller amounts of hydrogen sulphide (H2S)and ammonia (NH3).Trace amounts of hydrogen (H2), nitrogen (N2), carbon monoxide (CO), saturated or halogenated carbohydrates, oxygen, and siloxanes are occasionally present in the biogas. Usually, the mixed gas is saturated with water vapor and may contain dust particle. The characteristics of biogas are somewhere in between town gas and natural gas. The energy content is defined by the concentration of methane since there are no energy rich carbohydrates present. 10% of methane in dry gas corresponds roughly to1 kWh per m3. For biogas as a fuel, most of the impurities have to be removed. They may cause corrosion, deposits and wear of the equipment. Substances requiring attention are: hydrogen sulphide, water, CO2, Halogen compounds (chlorides, fluorides), Siloxanes aromatic compounds air (oxygen, nitrogen)
For Stoichiometric port injected Otto engines biogas must be upgraded to at least the quality of the G25 reference test fuel (85% methane, 14% nitrogen), as this is the minimum fuel quality for which the NGV’s are type approved . It is particularly important that the gas quality is maintained at a fix composition in order to prevent an increased NOx concentration. For the removal of most of the compounds a number of processes have been developed. For an effective use of biogas as vehicle fuel it has be enriched in methane. This is primarily achieved by carbon dioxide removal, which then enhances the energy value of the gas to give longer driving distances with a fixed gas storage volume. Removal of carbon dioxide also provides a consistent gas quality with respect to energy value. The latter is regarded to be of great importance from the vehicle manufacturers in order to reach low emissions of nitrogen oxide. At present four different methods are used commercially for removal of carbon dioxide from biogas either to reach vehicle fuel standard or to reach natural gas quality for injection to the natural gas grid.
These methods are:
• Water absorption,
• Polyethylene glycol absorption,
• Carbon molecular sieves,
• Membrane separation.
Hydrogen sulphide is always present in biogas, although concentrations vary with the feedstock. It has to be removed in order to avoid corrosion in compressors, gas storage tanks and engines. Hydrogen sulphide is extremely reactive with most metals and the reactivity is enhanced by concentration and pressure, the presence of water elevated temperatures. Due to the potential problems hydrogen sulphide can cause, it is recommended to remove it early in the process of biogas upgrading. Experiences have also shown that two of the most commonly used methods for hydrogen sulphide removal are internal to the digestion process:
1) Air/oxygen dosing to digester biogas and
2) Iron chloride dosing to digester slurry.
The most common commercial methods for hydrogen sulphide removal are:
• Air/oxygen dosing to digester biogas,
• Iron chloride dosing to digester slurry,
• Iron sponge,
• Iron oxide pallets,
• Activated carbon,
• Water scrubbing,
• NaOH scrubbing,
• Biological removal on a filter bed,
• Air stipping and recovery
Higher hydrocarbons as well as Halogenated hydrocarbons, particularly chloro-and fluoro – compounds are predominantly found in landfill gas. They cause corrosion in CHP engines, in the combustion chamber, at sparkplugs, valves, cylinder heads etc. for this reason CHP engine manufacturers claim maximum limits of Halogenated hydrocarbons in biogas. They can be removed by pressurized tube exchangers filled with specific activated carbon. Small molecules like CH4, CO2, N2 and O2 pass through while larger molecules are adsorbed. The size of the exchangers is designed to purify the gas during a period of more than 10 hours. Usually there are two parallel vessels. One is treating the gas while the other is desorbed. Regeneration is carried out by heating the activated carbon to 200°c, a temperature at which all the adsorbed compounds are evaporated and removed by a flow of inert gas.
Organic silicon compounds are occasionally present in biogas which can cause severe damage to CHP engines. During incineration they are oxidized to silicon oxide which deposits at spark plugs, valves and cylinder heads abrading the surfaces eventually causing serious damage. Particularly in Otto engines this might lead to major repairs. Because of the increased wearing of the combustion chambers caused by the silica deposits. Nowadays manufacturers of CHP engines claim maximum limits of siloxanes in biogas. It is known that the organic silicon compounds in biogas are in the form of linear and cyclic methyl siloxanes. These compounds are widely used in cosmetics and pharmaceutical products, and as anti-foaming agents in detergents. Siloxanes can be removed by absorption in a liquid medium, a mixture of hydro carbons with a special ability to absorb the silicon compounds. The absorbent is regenerated by heating and desorption.
The problem of all the existing technologies is the cost. First it is a question of economy of scale. Most of the (agricultural) biogas plants produce less than 100m3 is the absolute minimum. Best results are achieved at sizes 200m3/hr.
Another problem causing high prices is the fact that so far no real market exists, which would allow to optimize and mass produce upgrading plants. They are still tailor made to a large extent. Also there is a huge potential for new and simplified technologies. With the increasing interest there will more research money available.
It is the duty of the task to help stimulating and coordinating research and propagate new and innovative techniques .Upgrading of biogas to natural gas quality is also necessary for introduction of biogas into the natural gas distribution grid. There are still some fears of the national gas companies to accept biogas. Again Switzerland and Sweden have set the pace to improve the situation by giving a good example. About 6 months ago an international working group of gas industry (Marco gas) started to discus conditions to accept gas from third parties into the grid. They have established a first draft which is already an improvement as such however, the requirements are still too stringent. The task will work on this document and pass their recommendation to the national team participants and before all to our member Owe jonsson who participates in this working group.

Emission control
The emission benefits (CO, NOx, PM) of natural gas is generally accepted today. Upgraded biogas comparable to H- quality of natural gas achieves the same emission values. A number of emission tests in Switzerland [EMPA] have confirmed the low emissions. In 1998 two Swiss studies [BUWAL 1998] with different methods of environmental rating gave natural gas a 75% over all advantage over diesel and a 50% advantage over petrol. Human toxicity gave a 70 lower value; the ozone potential was reduced by 60 to 80% acid formation by more than 50%.
Parallel monitoring of comparable car engines fuelled with either petrol, diesel or natural gas in a town cycle (EU standard) demonstrated a reduced NOx emission for gas of 57% respectively, 88% when compared to petrol resp. diesel a 96% reduction of the ozone potential and virtually no emission of carcinogenic compounds, non methanogenic hydrocarbon was reduced by 73%.

5. Need of scrubbing of biogas
As the biogas is the mixture of 65% methane (CH4), 45% carbon dioxide, hydrogen sulphide and small amounts of water vapors. Due to the presence of CO2, combustion properties of biogas reduce. Because CO2 helps in combustion process.
Therefore it is necessary to remove the percentage of CO2 from biogas in order to produce pure methane gas having highest calorific value .The following information gives the calorific value of some fuels.

TYPE OF FUEL C.V (KJ/KG)
Rough biogas 5500-6000
Petrol 4600
Diesel 4500
L.P.G gas 4618
Scrubbed biogas 5500

From the above observation it is clear that C.V of scrubbed biogas that is pure methane having highest calorific value as compare to petrol, diesel & L.P.G gas . calorific value of rough biogas is very small.
Therefore it is very necessary to remove CO2 from biogas such a method is called of scrubbing of biogas. After production of scrubbed biogas, it can be used as an alternating fuel in place of petrol,diesel and L.P.G gas.

6. CO2 scrubbing from biogas
A variety of processes are being used for removing CO2 from natural gas in petrochemical industries. Several basic mechanisms are involved to achieve selective separation of gas constituents. These may include physical or chemical absorption, adsorption on a solid surface, membrane separation, cryogenic separation and chemical conversion.

6.1. Physical absorption
For biogas scrubbing physical/chemical absorption method is generally applied as they are effective even at low flow rates that the biogas plants are normally operating at. Also the method is less complicated, requires fewer infrastructures and is cost effective.
One of the easiest and cheapest method involves the use of pressurized water as an absorbent. The raw biogas is compressed and fed into a packed bed column from bottom; pressurized water is sprayed from the top. The absorption process is, thus a counter-current one. This dissolves CO2 as well as H2S in water, which are collected at the bottom of the tower. The water could be recycled to the first scrubbing towers. This perhaps is the simplest method for scrubbing biogas.
Bhattacharya developed one such water scrubbing system. The process provides 100% pure methane but is dependent on factors like dimensions of scrubbing tower, gas pressure, and composition of raw biogas, water flow rates and purity of water used.
Vijay developed a packed bed type scrubbing system using the locally available packing materials removing 30–40% more CO2 by volume compared with the scrubbing systems without a packed bed.
Khapre designed a continuous counter-current type scrubber with gas flow rate of 1.8 m3/h at 0.48 bar pressure and water in flow rate of 0.465 m3/h. It continuously reduced CO2 from 30% at inlet to 2% at outlet by volume.
Dubey tried three water scrubbers having diameters 150 mm (height: 1.5 m), 100 mm (height: 10 m) and 75 mm (height: 10 m) to absorb CO2 present (37–41%) in the biogas. He found that the CO2 absorption is influenced by the flow rates of gas and water than different diameters of scrubbers.
The G.B. Pant University of Agriculture and Technology, Pantnagar, India developed a 6 m high scrubbing tower, packed up to 2.5 m height with spherical plastic balls of 25 mm diameter. The raw biogas compressed at 5.88 bar pressure was passed at a flow rate of 2 m3/h while water was circulating through the tower. A maximum of 87.6% of the CO2 present could be removed from the raw biogas.
Water scrubbing method is popular for CO2 removal in sewage sludge based biogas plants in Sweden, France and USA. The results show that 5–10% CO2 remains in biogas after scrubbing.


6.2. Chemical absorption
Chemical absorption involves formation of reversible chemical bonds between the solute and the solvent. Regeneration of the solvent, therefore, involves breaking of these bonds and correspondingly, a relatively high energy input. Chemical solvents generally employ either aqueous solutions of amines, i.e. mono-, di- or tri-ethanolamine or aqueous solution of alkaline salts, i.e. sodium, potassium and calcium hydroxides.
Biswas reported that by bubbling biogas through 10% aqueous solution of mono-ethanolamine (MEA), the CO2 content of the biogas was reduced from 40 to 0.5–1.0% by volume. MEA solution can be completely regenerated by boiling for 5 min and thus can be used again.
Savery suggested that the three agents NaOH, KOH and Ca(OH)2 can be used in chemical scrubbing of biogas. The absorption of CO2 in alkaline solution is assisted by agitation. The turbulence in the liquid aids to diffusion of the molecule in the body of liquid and extends the contact time between the liquid and gas. Another factor governing the rate of absorption is concentration of the solution. The rate of absorption is most rapid with NaOH at normality's of 2.5–3.0.
6.3. Adsorption on a solid surface
Adsorption process involves the transfer of solute in the gas stream to the surface of a solid material, where they concentrate mainly as a result of physical or Vander wall forces. Commercial adsorbents are generally granular solids with a large surface area per unit volume. By a proper choice of adsorbent, the process can remove CO2, H2S, moisture and other impurities either selectively or simultaneously from biogas.
Gas purification can also be carried out using some form of silica, alumina, activated carbon or silicates, which are also known as molecular sieves.
Adsorption is generally accomplished at high temperature and pressure. It has good moisture removal capacities, simple in design and easy to operate. But it is a costly process with high pressure drops and high heat requirements.
Schomaker reported that CO2 could be removed from biogas by pressure swing adsorption which consists of at least three active carbon beds. One of the beds is fed with biogas under pressure (6 bar) CO2 is adsorbed. When there is saturation of CO2 in the adsorption bed, the process is shifted to the second bed. The saturated bed is depressurized to ambient pressure. The efficiency of this process is up to 98%.
Continuous monitoring of a small-scale installation (26 m3/h) in Sweden using pressure swing adsorption technique through carbon molecular sieves have given excellent results in terms of clean gas, energy efficiency and cost.
Pandey and Fabian used naturally occurring zeolite-Neopoliton Yellow Tuff (NYT) for adsorption. They found that the active component for CO2 adsorption is chabazite, which has adsorption capacity of 0.4 kg CO2 per kg of chabazite at 1.50 bar and 22 °C. During the adsorption process the H2S content is also reduced.
6.4. Membrane separation
The principle is that some components of the raw gas could be transported through a thin membrane (<1 mm) while others are retained. The transportation of each component is driven by the difference in partial pressure over the membrane and is highly dependent on the permeability of the component in the membrane material. For high methane purity, permeability must be high. Solid membrane constructed from acetate–cellulose polymer has permeability for CO2 and H2S up to 20 and 60 times, respectively, higher than CH4. However, a pressure of 25–40 bar is required for the process.
Wellinger and Lindberg described two basic systems of gas scrubbing with membranes: a high pressure gas separation with gas phases on the both sides of the membrane and a low pressure gas liquid absorption on separation where a liquid absorbs the molecule diffusing through the membrane. The high pressure gas separation membranes can last up to 3 years which is comparable to the life time of membranes used for natural gas purification—which last typically 2–5 years.
Rautenbach et al. designed a pilot plant for the removal of CO2 from biogas using membrane separation technique. He reported that Monsanto and acetate cellulose membranes are more permeable to CO2, O2 and H2S than CH4. The best separation occurred at 25 °C temperature and 5.50 bar pressure.
The gas flux across the membrane increases proportionally with the partial pressure difference. Thus, higher the pressure difference, the smaller is the membrane area required. However, maximum pressure which membrane can withstand must be taken into consideration.
6.5. Cryogenic separation
The cryogenic method of purification involves the separation of the gas mixtures by fractional condensations and distillations at low temperatures. The process has the advantage that it allows recovery of pure component in the form of a liquid, which can be transported conveniently. However, attempts to apply the cryogenic process for the removal of CO2 from digester gas by Los Angelus County sanitation have not proven successful. Rather complicated flow streams are involved and thermal efficiency is low. Capital cost and utility requirements are also high.
In a cryogenic method, crude biogas is compressed to approximately 80 bar. The compression is made in multiple stages with inter-cooling. The compressed gas is dried to avoid freezing during cooling process. The biogas is cooled with chillers and heat exchangers top −45 °C, condensed CO2 is removed in a separator. The CO2 is processed further to recover dissolved methane, which is recycled to the gas inlet. By this process more than 97% pure methane is obtained. No data is available on investment and operational cost.

6.6. Chemical conversion method
To attain extremely high purity in the product gas, chemical conversion method can be used. It reduces the undesirable gas concentrations to trace levels. Usually the chemical conversion process is used after bulk removal has been accomplished by other methods. One such chemical conversion process is methanation, in which CO2 and H2 are catalytically converted to methane and water. Chemical conversion process is extremely expensive and is not warranted in most biogas applications.
Due to highly exothermic nature of the methanation reactions, the removal of the heat from the methanator is a major concern in the process design. The requirement of the large amount of pure hydrogen also makes the process generally unsuitable.
7. Scrubbing of H2S
H2S is always present in biogas, although concentrations vary with the feedstock. It has to be removed in order to avoid corrosion in compressors, gas storage tanks and engines.
H2S is poisonous and corrosive as well as environmentally hazardous since it is converted to sulphur dioxide by combustion. It also contaminates upgrading process. H2S can be removed either in the digester, from the crude biogas or in upgrading process.
The most commonly used H2S removal process can be classified into two general categories namely: (1) dry oxidation process and (2) liquid phase oxidation process.
7.1. Dry oxidation process
It can be used for removal of H2S from gas streams by converting it either into sulfur or oxides of sulfur. This process is used where the sulfur content of gas is relatively low and high purities are required. Some of these methods are described below.
7.1.1. Introduction of air/oxygen into the biogas system
A small amount of oxygen (2–6%) is introduced in the biogas system by using air pump. As a result, sulfide in the biogas is oxidized into sulfur and H2S concentration is lowered.

2H2S+O2=2S+2H2O
This is a simple and low cost process. No special chemicals or equipments are required. Depending on the temperature, the reaction time and place where the air is added, the H2S concentration can be reduced by 95% to less than 50 ppm. However, care should be taken to
avoid overdosing of air, as biogas in air is explosive in the range of 6–12%, depending on the methane content.

7.1.2. Adsorption using iron oxide
H2S reacts with iron hydro-oxides or oxides to form iron sulfide. The biogas is passed through iron oxide pellets, to remove H2S. When the pellets are completely covered with sulfur, these are removed from the tube for regeneration of sulfur. It is a simple method but during regeneration a lot of heat is released. Also the dust packing contains a toxic component and the method is sensitive to high water content of biogas.
Wood chips covered with iron oxide have a somewhat larger surface to volume ratio than plain steel. Roughly 20 g of H2S can be bound per 100 g of iron oxide chips. The application of wood chips is very popular particularly in USA. It is a low cost product, however, particular care has to be taken that the temperature does not rise too high while regenerating the iron filter.
H2S can be adsorbed on activated carbon. The sulfur containing carbon can then either be replaced with fresh activated carbon or regenerated. It is a catalytic reaction and carbon acts as a catalyst.

7.2. Liquid phase oxidation process
This process is primarily used for the treatment of gases containing relatively low concentration of H2S. It may be either: (a) physical absorption process or (b) chemical absorption process.
In physical absorption process the H2S can be absorbed by the solvents. One of the solvent is water. But the consumption of water is very high for absorption of small amount of H2S. If some chemicals like NaOH are added to water, the absorption process is enhanced. It forms sodium sulfide or sodium hydrosulfide, which is not regenerated and poses problems of disposal.
Chemical absorption of H2S can take place with iron salt solutions like iron chloride. This method is extremely effective in reducing high H2S levels. The process is based on the formation of insoluble precipitates. FeCl3 can be added directly to the digester slurry. In small anaerobic digester system, this process is most suitable. All other methods of H2S removal are suitable and economically viable for large-scale digesters. By this method the final removal of H2S is about 10 ppm.


8. Biogas compression and storage
Biogas, containing mainly methane, could not be stored easily, as it does not liquefy under pressure at ambient temperature (critical temperature and pressure required are −82.5 °C and 47.5 bar, respectively).
Compressing the biogas reduces the storage requirements, concentrates energy content and increases pressure to the level required overcoming resistance to gas flow. Compression is better in the scrubbed biogas. Most commonly used biogas storage systems

Integrated units with facilities for scrubbing, compressing and storing have been developed in certain developed countries. For instance a water scrubber coupled with a gas compressor is being promoted for uniform use in New Zealand. Similarly, the biogas produced from poultry manure is being dried, scrubbed, compressed and stored at a pressure of 4 bar in 0.2 m3 steel tanks in Belgium [19].
Khapre conducted a study on scrubbing and compression of biogas and subsequently used it for domestic cooking. He found reduced requirement of scrubbed and compressed biogas (0.353 m3) than raw biogas (0.591 m3) for cooking a day's meal of a six member family. He stored the scrubbed and compressed biogas at a pressure of 7 bar in cylinder of 0.1 m3 capacity.
By purifying the biogas produced from the distillery wastes, scientists of Jadhavpur University, Kolkata, India claimed to have generated huge quantities of compressed methane, a gas with an immense potential and an alternative source of vehicle fuel. Experimenting with bulk distillery wastes, from alcohol manufacturing breweries, researchers produced the gas by bio-methanation of the effluents.

Similar results have also been reported from Netherlands, UK, Australia, New Zealand and USA. All these results indicate that biogas is one of the potential substitutes for present day fuels including CNG, petrol, diesel and LPG.
Nema and Bhuchner stressed on value addition to biogas by scrubbing and compressing, making it as good as the compressed natural gas (CNG). They reported the economic feasibility of producing energy from solid wastes of Delhi city. From 5000 tonnes wastes generated per day in Delhi, 100,000 Nm3/day biogas can be produced which is equivalent to 309.5 m3 CNG worth US $ 70,000 per day. Beside this, by adopting this technology 117 tonnes/day CO2 gas can be prevented from entering into the atmosphere.

9. Advantages of scrubbing of biogas

1) After scrubbing of biogas, it has highest calorific value.
2) Scrubbed biogas is used as an alternating fuel for power generation, to run the vehicles, for cooking purpose.
3) It is burned very smoothly and produces very less pollution.
4) It is very economical gas as it is produced from waste.
5) It is very easy to transport and storage.
6) In the process of scrubbing of biogas we get two products, methane gas and liquid CO2 .
7) Liquid CO2 is the very important refrigent used in refrigeration system which is very costly.
10. Conclusion
The aim of this study is to explore the potential of biogas production in India from animal waste and its prospects in wider perspective. Presently, biogas is mainly used for cooking purpose in India. To tap full potential of biogas, need emerges for its commercialization by making it transportable. Therefore biogas scrubbing and compression at high pressure for storage in cylinders are essential.
Different methods of scrubbing are reviewed and found that water scrubbing is simple, continuous and less expensive method for CO2 removal from biogas for Indian conditions. It simultaneously also removes H2S. After removal of CO2, biogas is enriched in methane and becomes equivalent to natural gas. It can be used for all such applications for which natural gas is being used viz. as a fuel for vehicles, CHP, electricity generation, etc.

11. References
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