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Here is a term paper on ‘Biogas’. Find paragraphs, long and short term papers on ‘Biogas’ especially written for school and college students.
Term Paper on Biogas
Term Paper Contents:
- Term Paper on the Introduction to Biogas
- Term Paper on the Properties and Characteristics of Biogas
- Term Paper on the Applications of Biogas
- Term Paper on the Biodegradation and Biodegradability of Substrate
- Term Paper on the Biochemical Oxygen Demand and Oxidative Reduction Potential
- Term Paper on the Important Properties of Biogas
- Term Paper on the Safety Aspects for Handling Biogas
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Term Paper # 1. Introduction to Biogas:
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Biogas can be produced by fermenting organic materials in absence of air (or oxygen) with the help of bacteria (micro-organisms) to break down materials to intermediates such as alcohols and fatty acids and finally to methane, carbon dioxide and water. This process is called anaerobic fermentation and was known to exist from quite long time back. Biogas has also been known as the swamp gas, sewer gas, fuel gas, marsh gas, wet gas, and in India more commonly as ‘gobar’ gas.
Natural gas is also produced by the action of anaerobic bacteria on plants that grew thousands of years ago. Biogas and natural gas are therefore very much akin to one another. The main fuel component of both is methane gas. However, over the years, pressure and temperature of underground rocks have converted part of the methane in natural gas to other gases such as ethane, propane, butane and the condensate.
In contrast, biogas is produced in a digester by anaerobic fermentation. A period of 15 days or so enables anaerobic bacteria to convert organic matter to biogas which however is too short for the conversion of methane to other gases like ethane, propane, butane. The enormous potential of the smallest living organisms such as bacteria, yeasts and fungi to transform organic wastes into valuable source of fuel and enriched fertiliser through a simple process of anaerobic fermentation has been widely recognised.
Anaerobic fermentation is a simple and low cost process which can be economically carried out in rural areas where organic wastes are generated aplenty which otherwise pollute environment and pose health hazards. Animal and human wastes are excellent feedstock for biomethanation which are available plentifully all over.
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Wastes in large quantities on renewable basis are also available from agricultural crops and residues, fruit and vegetable plants and municipal refuse. The potential for generating gaseous fuel and enriched fertiliser through biomethanation is enormous which can bring economic and environmental gain to a vast population.
Anaerobic fermentation is a biological process which takes place in the absence of air. A number of stages are involved in this process; initially organic material is hydrolysed by enzymes into simple sugars, alcohol peptides and amino acids. These are then converted to volatile fatty acids, hydrogen, carbon dioxide, water and some amount of methane. Methane forming bacteria then convert the fatty acids into methane, carbon dioxide, and water.
The process is temperature-dependent and slows down considerably below 30°C, the optimum being 35-38°C which is known as the mesophyllic range. Above the optimum temperature, the process again slows down between 40 and 45°C and then increases to a peak between 55 and 60°C, known as the thermophyllic range.
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The pH of digester contents also affects for fermentation as methane bacteria is less active below pH of 6 while the ideal pH is 7. Fermentation breaks down wastes at 35°C under controlled conditions that produces a mixture of methane and carbon dioxide in proportion of about 70 to 30 per cent.
The biogas produced can be used directly for heating purposes or used in an engine-driven generator to generate electricity. Anaerobic fermentation produces biogas on renewable basis, eliminates foul smell and reduces harmful bacteria of organic wastes, improves nitrogen and phosphate content of resulting sludge to yield highly enriched fertiliser, and reduces biological oxygen demand (BOD) and chemical oxygen demand (COD).
Term Paper # 2. Properties and Characteristics of Biogas
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Biogas which consists of 60-65 per cent methane, 35-40 per cent carbon dioxide and faces of hydrogen sulphide, ammonia and other impurities is an explosive and toxic gas. Methane which is the principal constituent of biogas, is colourless, odourless and tasteless.
Methane is also combustible in a concentration of 5 to 15 per cent in air and, therefore, potentially hazardous. Considering that biogas is warm when it leaves digester, it contains water vapour which condenses when exposed to colder temperatures. Condensation traps devised in gas pipelines are necessary to prevent water from blocking low points in pipelines.
Biogas after condensation of water vapour present therein or even without condensation, can be used as a fuel for cooking, lighting and several other applications. However, in stoves gas-burner-nozzle needs to be enlarged to compensate low heat value of biogas. Tests using biogas as exclusive fuel in a petrol engine showed that energy present in 200 ft3 (5.7 m3) of biogas is equal to energy present in 1 gallon of petrol.
Calorific value of biogas ranges from 500 to 700 BTUs per cubic foot (0.03 cubic metre). In comparison, natural gas has a heat value of 850 BTUs per cubic foot (0.03 cubic metre), petrol contains nearly 120,000 BTUs per gallon (3.8 litres), diesel oil comprises 133,000 BTUs per gallon and propane contains 92,000 BTUs per gallon. Biogas is difficult to store, compress or liquefy.
Biogas unlike Liquefied Petroleum Gas (LPG) cannot be converted to liquid state under normal temperatures. A temperature of – 260°F (- 162°C) is needed to liquefy methane at atmospheric pressure. Liquefaction of methane requires pressures of nearly 5000 psi (LPG liquefies at 160 psi) at a temperature of – 117°F (- 83°C) and is thus not practical. If biogas is compressed to 1000 psi, it needs about 1320 BTUs of energy to put 6350 BTU into a storage container.
As biogas cannot be easily liquefied economically, it is best suited for such applications as cooking, lighting, grain-drying or operating stationary internal combustion (IC) engines. Biogas is normally produced on continuous basis. Most applications are intermittent and hence some kind of storage facilities are required. Amount of gas storage is dependent on storage time and pressure.
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High demand applications such as grain drying are normally impractical following excessive storage capacity requirement. Although biogas can be used for variety of applications including crop-drying, refrigeration and running of diesel engines, so far it is mainly used as cooking and lighting fuel in most developing countries.
Methane content in biogas is affected by digester conditions. Biogas contains traces of hydrogen sulphide which is highly corrosive. This can be removed by passing the gas through steel wool or iron filings. Following condensation of water vapour present in biogas, it can be used as fuel for any standard application.
Biogas contains nearly forty per cent carbon dioxide which can be removed by bubbling it through lime water. Lime water is a mixture of 1.8 kg burnt lime in 1000 litres of water. 1000 litres of lime water removes 560 litres of carbon dioxide. For removing 35 per cent carbon dioxide in biogas, 625 litres of lime water is needed to purify 1 m3 of biogas. Lime water becomes milky following absorption of carbon dioxide.
Term Paper # 3. Applications of Biogas:
Biogas provides a clean fuel for cooking and lighting, biogas burner (Fig. 1.3) and lamp (Fig. 1.4) are structurally similar with their common items being a nozzle, an air inlet and a mixing chamber. The uncommon items are mantle in case of lamp and fire-stove plate in case of stove.
A schematic plan for using biogas as a cooking and lighting fuel is given in Fig. 1.5. Biogas lamp works on a simple principle that when gas is burnt, mantle of the lamp glows and emits light. Mantle in a biogas lamp is similar to one used in a ‘Coleman’ or propane lamp. Brightness of biogas lamp mainly depends on factors like gas pressure, gas/air ratio and extent of mixing.
Proper nozzle adjustment is necessary to achieve requisite light intensity. Low gas pressure causes poor light intensity but higher pressure though it improves brightness, lowers mantle life. Biogas lamps can be either suspended (hanging type) or table (standing) type. As biogas is almost half as light as air and as hot air rises upwards, the brightness of a standing biogas lamp is somewhat greater than that of a hanging type.
Biogas lamps are generally designed to produce 100 candle power and consume 0.11 to 0.15 m3 (4 to 5.5 ft3) biogas per hour. Several designs of biogas lamps are popular but most common ones are what have come to be known as the Indian, Chinese and Pakistani types.
Biogas stoves normally operate at gas pressure of 75 to 90 mm. (3 – 3.5 inch) water column. As biogas passes through the nozzle, air is allowed to be drawn in mixing chamber. Nozzle adjustment is generally necessary to achieve desired flame temperature. Brightness and combustibility of gas is regulated by controlling gas pressure and air fuel ratio which is generally maintained as 10:1.
Combustibility of gas is maximum when the flame appears to be slightly yellow, a bit bright and burnished. When the flame turns blue and smokeless after air jet adjustment, it burns at a temperature of about 800°C. For meeting cooking needs of an individual 0.28 to 0.42 m (10 to 15 ft3) of biogas is needed daily.
Cooking appliances in wide range are available both in single and double burner category with each burner consuming anywhere between 0.25 to 1.25 m3 of gas per hour. Biogas cannot be burnt on stoves designed for LPG/natural gas as it tends to lift off the stoves on account of its flame-speed factor being lower than these gases. This problem can be somewhat overcome by reducing gas pressure but this causes higher gas consumption and reduces burning efficiently.
Biogas provides reasonably efficient fuel for both spark-ignition (petrol) and compression- ignition (diesel) engines. Whereas diesel engines can run on biogas as a dual-fuel comprising both oil and gas, petrol engines can be made to run on cent per cent biogas. However in the latter case, it is initially necessary to use little petrol for start-up. While running IC engine on biogas, some modification in the fuel injection system is required.
In a partially converted system, a gas tap is fitted onto the inlet manifold of the engine. The engine is initially started on liquid fuel and a little gas supply, and later the proportion is reversed when engine develops desired speed. Opening of gas tap and adjustment of air supply is carried out simultaneously. In steady engine running, dual fuel helps to achieve about 80 per cent saving in diesel consumption. (Fig. 1.6).
Biogas can also be utilised for electricity generation. As per some studies carried out in the past, 1 kwh of electricity can be generated from 0.7 m3 of gas which can light about 15 electric bulbs of 60 watt rating for one hour. In contrast if 0.7 m3 of gas is directly burnt to light gas lamps, only 5 gas lamps of 100 candle power (CP) rating equivalent to 60 watt electric bulb can be lighted for one hour duration.
This shows that power-route is more efficient for lighting than direct gas burning. However, electricity generation from biogas is economical only from large sized plants and it requires high initial capital investment. Asynchronous generators of 3, 5.5 and 7.5 kwh rating are generally coupled with IC engines running on biogas.
Biogas can also be used as a vehicle fuel. It has been successfully used on a commercial scale in countries like Italy and New Zealand. Conversion kit suitable for biogas is virtually the same as used for compressed natural gas (CNG) which is marketed as an automotive fuel in many parts of the world. For use as a vehicle fuel, biogas is first compressed in a three or four stage compressor upto a pressure of 2500-3500 psi and stored in a high pressure gas storage cascade.
Cascade helps to facilitate quick refueling of storage cylinders. The number of cylinders in a cascade depends on the size of digester and the number of cylinders fitted to each vehicle. Being gaseous fuel, biogas provides excellent cold starting for vehicles because unlike petrol it does not first have to be vapourised.
Once started, the engine performs well immediately with no sluggish warm-up. This is because unlike petrol, methane does not condense on cold intake manifolds, cylinder heads and pistons. Biogas is carried in compressed state in cylinders which are fitted on to the vehicles. Each cylinder is about 1 m long, 0.3 m in diameter and weighs close to 50 kg. Two cylinders can be easily mounted on a biogas-driven car and four on a truck.
Term Paper # 4. Biodegradation and Biodegradability of Substrate:
Biodegradation involves destruction of a chemical compound by the action of living organism such as microbe. Degradation of biomass by micro-organisms under controlled conditions leads to the production of biogas. Whereas technology of biogas is sufficiently advanced, various physio-chemical processes leading to the biogas generation are still obscure and need to be studied further.
Biodegradation can be of different varieties. Biodegradation which completely changes the characteristic properties of a compound is called primary biodegradation in which case biodegraded compound no longer responds to analytical procedures prescribed for its detection.
Environmentally acceptable biodegradation refers to the minimum alteration of the original compound by micro-organisms necessary to remove its undesirable properties. Ultimate biodegradation refers to the complete conversion of a compound into inorganic end products and products associated with the microbes’ normal metabolic processes.
Degree of biodegradation of a chemical compound is called biodegradability. Biodegradability can be measured, among others, by following a procedure which involves incubating a solution of the compound under test with micro-organisms at a suitable pH and temperature.
This is followed by analysing the samples at specified times, and such experiments continue over a period of three to four months. For illustration purposes and understanding the concepts, biodegradability of four compounds A, B, C and D on the basis of experimental observations can follow similar pattern (Fig. 2.1).
Curves A and B indicate rapid biodegradation whereas C refers to a slow process. However, either of the curves A and B do not establish that ultimate biodegradation has taken place. Thus further tests on the products derived from A and B are necessary to establish the occurrence of ultimate biodegradation.
Neither can it be assumed that they will be biodegraded in all conditions. As for compound D, it cannot be concluded with definiteness that D is non-biodegradable. Flatness of curve D may be due to the absence of any microbes to biodegrade it or it could be due to unfavourable environmental conditions.
Curve C indicates that after slow start, compound C is around 60 per cent biodegradable. It is just possible that C is not a pure compound and that it is impurity which is responsible for its low biodegradation. This can be checked by adding more quantity of C and if biodegradation does not improve, it could be due to the presence of impurities or due to unfavourable environmental factors.
Biodegradability does not solely depend on physical and chemical composition of a compound or a substrate. It is just possible that though a substrate may be prone to biodegradation, yet following retarded growth of microbes, biogas yield may be low. Addition of certain stimulating materials including animal urine can accelerate biodegradation. Reduced growth of methane bacteria causes accumulation of fatty acids, drop in pH and low gas yield.
Methane bacteria have a generation time of over ten days. Studies for evaluating suitability of substrates for biogas yield should normally be continued for a period of five to six times the retention time. Several terms are used to describe different aspects of biodegradation of substrates. A stable process can be defined as one which lasts five times the retention period. A process which is not stable is called unstable process.
An unstable process can be improved by certain course-corrections. Addition of certain salts and alkaline matter is known to improve its biodegradability. Different growth patterns of various kinds of methane bacteria in relation to the substrate and other bacteria populations make it difficult to apply a general solution. Thus every organic substrate of unknown composition needs a long run test to establish its suitability for biodegradation.
Fermentation can be either wet or dry. In wet fermentation digester contents are mixed with excessive water so that proportion of dry matter is within 10 per cent. Animal wastes are particularly amenable for wet fermentation. On the other hand agricultural plant material following problems of mixing and consequent scum formation pose problems in wet fermentation.
For preventing scum formation on account of floating of plant materials, amount of water in digester is kept to a minimum which is just sufficient to keep the raw materials wet for active fermentation. This process is called dry fermentation wherein total solids range between 25 to 30 per cent with no free water.
Term Paper # 5. Biochemical Oxygen Demand and Oxidative Reduction Potential:
Oxygen in slurry is consumed by a quantity what is called biochemical oxygen demand (BOD) which is defined as the quantity of oxygen needed by the bacteria to oxidise an organic matter. The BOD of organic waste is determined by a standard laboratory procedure. Rate of reaction is proportional to balance oxygen left in organic matter.
The rate of reaction and BOD at time t (Lt) can be described by the following mathematical relationship:
k1 = Coefficient representing reaction velocity (value for a typical sewage is 0.4 per day and for industrial wastes from 0.15 to 0.75 per day at 20°C).
Methane forming bacteria are highly sensitive to oxygen. Presence of oxygen prevents them from functioning. Scientists in China often use a term Oxidative Reduction Potential (ORP) to describe the activity of aerobic and anaerobic bacteria which they call aerobiosis and anaerobiosis respectively.
Initially when a biogas plant is charged with feed environment within digester is aerobic following presence of air. Oxygen present in the beginning activates aerobic bacteria which in turn exhausts digester oxygen creating anaerobic environment.
Besides aerobic bacteria, facultative bacteria which can function both in aerobic and anaerobic environment also help to create anaerobic environment within the digester by reacting with oxygen. Efficiency of aerobic and facultative bacteria in reducing oxygen level is measured by the ORP.
Biogas yield, ORP, volatile acids and ammoniacal nitrogen are related to one another. For instance when ammoniacal nitrogen increases gas yield rises accompanied by corresponding decrease in volatile acid concentration and ORP.
Initially the ORP is usually high but starts declining as fermentation progresses and stabilises at a value around tenth day of fermentation which roughly corresponds to the period of changeover from acid forming to methane forming phase.
Term Paper # 6. Important Properties of Biogas:
In view of the fact that biogas is rarely available in its purest form, its composition and characteristics vary from source to source depending upon feed materials used. Accordingly, calorific value of biogas is also not fixed and varies within 500 to 700 BTU per cubic foot (0.03 cubic metre). As compared to this, natural gas has a value of 850 BTU per cubic foot (0.03 cubic metre) and petrol 120,000 BTU per gallon (3.8 litres).
In metric units, calorific value of biogas ranges between 4700 to 6000 kcal/m3. Biogas burns with a blue flame. It can be directly used in gas burning appliances used for heating, cooking, lighting and running internal combustion engines with a compression ratio of 8: 1 or even greater. Amount of air needed for combustion depends upon methane content of biogas. For example, for combustion of 1m3 of biogas with 60 per cent methane, 8 m of air is needed.
Flame speed factor of biogas is 11.1 which affects the manner in which the flame tends to lift off burners. Critical pressure of methane is 4710 KPA at – 82.3°C which makes its liquefaction easier. The high critical pressure of methane is the main factor which acts as hurdle in bottling of biogas and consequent portability like LPG.
Term Paper # 7. Safety Aspects for Handling Biogas
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Biogas is explosive gas and accordingly it needs to be handled with care. The most critical time to observe safety precautions is during periods of digester and gas pipeline repairs. Biogas in a concentration of 6 to 15 per cent with air forms an explosive mixture. While carrying out periodic repairs, it is necessary that all gas in gasholder be allowed to escape first to avoid presence of any air gas mixture inside.
In case of fixed dome biogas plant it can be achieved by allowing sufficient time for gas to escape. In a movable drum type plant this objective can be met by making gasholder to settle down to maximum possible depth over slurry. Soapy water may be applied on gas taps, gas cocks, joints and fittings for checking for possible leakages as indicated by bubble formation. Small leakages at joints can be repaired by applying bitumen coating.
After coating, affected joints can be rechecked for residual leakage. Fire-prone appliances should be prevented from coming in contact with leaking gas. Gas pipelines leading to flame in a burner or lamp be fitted with fire-arrester to prevent back-travelling of flame into plant. Flame traps in the form of wire mesh can be installed in gas delivery lines near gas-appliances.
In kitchen, vent can be provided near roof-top to enable the accumulated gas to move out which due to its being lighter than air tends to accumulate near ceiling. Plants should be periodically inspected for possible leakages to minimise involved risks and hazards. A primitive method of bringing fire close to the point of suspected leakage is highly risky and dangerous which is to be avoided. Inlet and outlet of biogas plant should be covered with strong lids to avoid falling of objects into them.
Areas where biogas may accumulate following leakage should have alarm facilities. Biogas should be prevented from discharging into air, closed and confined areas. This requires use of gas-tight lines, fittings and pressure relief valves vented to exterior of buildings and confined spaces. It is also advisable to have fire-fighting facilities available in areas where large number of plants exist in clusters. Necessary safety instructions prescribed by implementing agencies should be affixed near biogas plants.