Biogas .A Report

Report by-1. Siddharth Krishna (Roll no. - 39)2. Rekha Kumari (Roll no. -22 )

Group Number - 22Branch - EEE BDate of Submission - 19th October 2012

INDEX

Biogas (An introduction) Why Biogas?? Natural cycle Biogas vs. Natural Gas Raw Biogas Biogas upgrading Biogas composition Biogas production Biogas digesters Investment saving Odour reduction Biogas scrubbing Biogas compression and storage Biogas sources Advantages Disadvantages Biogas as Energy Biogas fuels Fuel quality Injection of biogas in the natural gas network Transportation of biogas Biogas usage in industry Applications Developments around the world The final thing References

Biogas (An Introduction) -

Biogastypically refers to agasproduced by breakdown oforganic matterin the absence ofoxygen. Organic waste such as dead plant and animal material, animal feces, and kitchen waste can be converted into agaseousfuel called biogas. Biogas originates from biogenic material and is a type ofbio fuel. Biogasis a clean & environment friendly fuel. Rawbiogascontains about 5565% methane (CH4), 3045% carbon dioxide (CO2), traces ofhydrogen sulfide(H2S) and fractions of water vapors. Biogasis highly flammable and is produced through the anaerobic (without oxygen) decomposition of organic materials from plants and animals.Itis similar in most respect toNaturalgas(obtained fromfossil fuel)used for heating and cooking at homes and industries. Presently, it can be used only at the place where it is produced. There is a great need to makebiogastransportable. This can be done bycompressingthe gas in cylinders which is possible only after removing its CO2, H2S and water vapor components. Pilot level trials to compress thebiogashave been carried out by a number of earlier investigators working on the subject. This paper reviews the efforts made to improve the quality ofbiogasby scrubbing CO2 and the results obtained. There is a lot of potential ifbiogascould be made viable as a transport vehiclefuel like CNG bycompressingit and filling into cylinders after scrubbing and drying. Thus the need emerges for a unified approach for scrubbing,compressingand subsequent storage ofbiogasfor widerapplications.

Why Biogas??

India has 300 million cattle .Biogas technology may have the potential to short-circuit the 'energy transition' as Leach (1987) describes the transformation from biomass to 'modern' fuels.

The enormous potential of biogas, estimated at 17,000 MW The gas is useful as a fuel substitute Biogas systems also provide a residue organic waste, after anaerobic digestion that has superior nutrient qualities over the usual organic fertilizer. Functions as a waste disposal system, particularly for human waste. Prevent potential sources of environmental contamination. Provision of power for a rural-based industry Biogas may also provide the user with income generating opportunities By providing an alternative source of fuel, biogas can replace the traditional biomass based fuels, What is more certain, is the impact on rural womens' lives. A clean and particulate-free source of energy also reduces the likelihood of chronic diseases Significant reductions in emissions associated with the combustion of bio fuels, the use of biogas systems in an agrarian community can increase agricultural productivity

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Natural cycle

Biogas optimizes farm economy. The biogas plant is a sustainable system in which all the components form a natural cycle. The biogas is used for energy emitting CO2 and H2O, the minerals are going back into the soil and the organic waste will improve the structure of the soil.

Biogas vs. Natural Gas

Raw biogas, however, has much lower strength than natural gas because its methane content is lower than that of natural gas. Natural gas has between 90 to 99 % methane content where most raw biogas will have about 60 to 80% methane. To make biogas compete with natural gas, the impurities (gases such as CO2, H2S etc) should be removed. CO2 removal is termed biogas upgrade while the removal of H2S and other gases is often termed biogas cleaning.Therefore, after undergoing some purification, biogas can be usedjust the same way we usenatural gasto produce heating and cooking at homes and offices.

According to its composition, biogas presents characteristics interesting to compare with natural gas and propane. Biogas is a gas appreciably lighter than air, it produces twice as less calories by combustion with equal volume of natural gas.

Upgradedbiogashas 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 10,000biogasdriven cars and buses, demonstrating that the vehicle configuration is not a problem for use ofbiogasas vehicle fuel. This equals about 0.5% of the world vehicle stock.

Rawbiogas

Upgraded biogas can be used as a standalone 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 link Oping, Sweden.The predominant part of thebiogasfor 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 muchbiogasis 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 thebiogasis tax free.

Biogas upgradingRaw biogas produced from digestion is roughly 60% methane and 29%CO2with trace elements ofH2S, and is not high quality enough to be used as fuel gas for machinery. The corrosive nature ofH2Salone is enough to destroy the internals of a plant. The solution is the use of biogas upgrading or purification processes whereby contaminants in the raw biogas stream are absorbed or scrubbed, leaving more methane per unit volume of gas. There are four main methods of biogas upgrading, these include water washing, pressure swing absorption, selexol absorption, andamine gas treating. The most prevalent method is water washing where high pressure gas flows into a column where the carbon dioxide and other trace elements are scrubbed by cascading water running counter-flow to the gas. This arrangement could deliver 98% methane with manufacturers guaranteeing maximum 2% methane loss in the system. It takes roughly between 3-6% of the total energy output in gas to run a biogas upgrading system.

Biogas Composition

Typical composition of biogas

CompoundChemical%

MethaneCH45075

Carbon dioxideCO22550

NitrogenN2010

HydrogenH201

Hydrogen sulphideH2S03

OxygenO200

The composition of biogas varies depending upon the origin of theanaerobic digestionprocess.Landfill gastypically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 5575% methane,which for reactors with free liquids can be increased to 80-90% methane using in-situ gas purification techniques.As-produced, biogas also contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for both water vapor content and thermal expansion is easily done via a simple mathematic algorithmwhich yields the standardized volume of dry biogas.In some cases, biogas containssiloxanes. These siloxanes are formed from theanaerobic decompositionof materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes,siliconis released and can combine with free oxygen or various other elements in thecombustion gas. Deposits are formed containing mostlysilica(SiO2) orsilicates(SixOy) and can also containcalcium,sulfur,zinc,phosphorus. Suchwhite mineraldeposits 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 currently available.

Biogas ProductionBiogas is produced by theanaerobic digestionorfermentationof biodegradable materials such asbiomass,manure,sewage,municipal waste, green waste,plant material, and crops. Biogas comprises primarilymethane(CH4) andcarbon dioxide(CO2) and may have small amount of hydrogen sulphide(H2S), moisture andsiloxanes.Biogas is practically produced aslandfill gas(LFG) ordigestedgas. Abio gas plantis the name often given to an anaerobic digester that treats farm wastes or energy crops. Bio gas can be produced using anaerobic digesters. These plants can be fed with energy crops such as maize silage or biodegradable wastesincluding sewage sludge and food waste. During the process, as an air-tight tank transforms biomass waste into methane producing renewable energy that can be used for heating, electricity, and many other operations that use any variation of an internal combustion engine, such asGE Backbenchergas engines.There are two key processes:MesolithicandThermophilicdigestion.In experimental work atUniversity of Alaska Fairbanks, a 1000-litre digested usingpsychotropicsharvested from "mud from a frozen lake in Alaska" has produced 200300liters of methane per day, about 2030% of the output from dig esters in warmer climates. Landfill gas is produced by wet organic waste decomposing under anaerobic conditions in a landfill. The waste is covered and mechanically compressed by the weight of the material that is deposited from above. This material prevents oxygen exposure thus allowing anaerobic microbes to thrive. This gas builds up and is slowly released into the atmosphere if the landfill site has not been engineered to capture the gas. Landfill gas is hazardous for three key reasons. Landfill gas becomes explosive when it escapes from the landfill and mixes with oxygen. The lower explosive limit is 5% methane and the upper explosive limit is 15% methane.The methane contained within biogas is 20 times more potent as agreenhouse gasthan is carbon dioxide. Therefore, uncontained landfill gas, which escapes into the atmosphere, may significantly contribute to the effects ofglobal warming. In addition, landfill gas impact in global warming,volatile organic compounds(VOCs) contained within landfill gas contribute to the formation ofphotochemical smog.

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.

Related Terms:

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.AcidogenesisThis 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 low-weight molecular alcohol, such as bicarbonate ethanol and molecular hydrogen.AcetogenesisThe 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)MethanogenesisThe 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.

Biogas Digesters Types of biogas digestersVarious types of biogas digesters have been developed including floating drum, fixed dome, and plastic bag models. The most reliable design is that of the fixed dome type, made of masonry and/or concrete, largely underground, and installed in the yard of the family.

What are biogas digesters about?A biogas digester takes animal waste and turns it into a useful fuel (methane) for cooking, lighting, and heating. Biogas originates from the anaerobic digestion of organic material. The daily operation of a biodigester mainly consists on feeding the plant with a mixture of organic waste (dung) and water. Through the digestion process, the waste not turned into biogas is sanitized through: predation by anaerobic microbes; and the absence of oxygen. This sanitized waste effluent can be used as an organic fertilizer when diluted.

Some of the key expected benefits of an anaerobic digester are: Odor control Renewable energy production Pathogen reduction Greenhouse gas reduction Reduction in total oxygen demand of the treated manure (total oxygen demand is a measure of potential impact on aquatic systems)Investments savingNew enterprises can have considerable investment savings due to the possibility to avoid building new gas pipeline, electricity line, auxiliary generators and waste storage facilities. Thanks to the short digestion period the volume of waste lagoons can be reduced twice. Investment cost savings can reach about 30-40% from biogas plant price.Weed Seed DestructionWe did a simple germination test of the digested manure to test for presence of weed seeds and no weeds were detected.Seed germination of samples run through the CARE digester, and results will be available in 2010Greenhouse Gas ReductionsBurning methane has resulted in a reduction in greenhouse gases. In the first 10 operating months, it was estimated that the equivalent of approximately 680 tons of carbon dioxide were mitigated.25EmissionsAnaerobic digestion, besides methane and carbon dioxide, also produces small amounts of hydrogen sulfide (toxic to humans in certain situations26), nitrogen, ammonia and other trace gases. After combustion, this results in emissions of sulfur dioxide (SO2) and small amounts of nitrogen oxides (NOx) and particulate matter (PM). It should be noted that hydrogen sulfide would be emitted without the digester, and that by burning the biogas, hydrogen sulfide is converted into sulfur dioxide, which is less toxic to humans. Odour ReductionThe reduction in odor from the digester is very noticeable. Near the pond where the digested manure is stored, there is only a slight odor. Farms that injected the digested manure on their fields several times in the spring of 2000. Neighbors have not reported noticing a smell, where as when the Farms would apply raw manure neighbors would notice the smell for several days, although no complaints were made.. Payback of 5 years on investment is possible A good time to install a digester is when changing or expanding operations Electric utility cooperation is important Active management is crucial for stable digester and engine operation Digester design and engineering expertise is key There are barriers to financing digester systems Cooperative agency participation reduces the barriers to a projects success Manure collection method and collection frequency are importantAnaerobic digesters biologically treat manure and produce a stable effluent with slightly different chemical characteristics than raw manure. In the process, a biogas composed primarily of methane is produced, captured, and the gas is then combusted in an engine, boiler or flare. Manure treatment reduces total oxygen demand, odors and pathogens

Biogas Scrubbing

Need of scrubbing of biogasAs 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 FUELC.V (KJ/KG)Rough biogas 5500-6000Petrol 4600Diesel 4500L.P.G gas 4618Scrubbed 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.

CO2 scrubbing from biogasA 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.

1. Physical absorptionFor 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 3040% 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 0.465m3/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 (3741%) 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 2m3/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 510% CO2 remains in biogas after scrubbing.

2. Chemical absorptionChemical 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 0.51.0% 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 2.53.0.

3. Adsorption on a solid surfaceAdsorption 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 (6bar) 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.

4. Membrane separation

The principle is that some components of the raw gas could be transported through a thin membrane (