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The following points highlight the eight main applications of biogas. The applications are: 1. Biogas as a Cooking Fuel and Some Common Indian Burner Designs 2. Burner Designs Commonly used in China 3. Use of Biogas as a Lighting Fuel 4. Utilisation of Biogas for Pumping Water and Miscellaneous other Applications 5. Biogas as a Fuel for Running IC Engines 6. Biogas as a Vehicle Fuel and Other Details.
Applications of Biogas
- Biogas as a Cooking Fuel and Some Common Indian Burner Designs
- Burner Designs Commonly used in China
- Use of Biogas as a Lighting Fuel
- Utilisation of Biogas for Pumping Water and Miscellaneous other Applications
- Biogas as a Fuel for Running IC Engines
- Biogas as a Vehicle Fuel
- Applications of Biogas for Power Generation
- Fuel Cell Linked Biogas Systems
Application # 1. Biogas as a Cooking Fuel and Some Common Indian Burner Designs:
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Biogas provides a clean and efficient fuel for cooking. Both biogas burner and lamp are structurally alike with identical nozzle, an air inlet and a mixing chamber in common. A burner has fire sieve element as an additional component. A nozzle is a hollow tube made of glass, metal, plastic or bamboo. Both in burner and lamp nozzle is of the size of a needle point with 0.5 to 0.7 mm. diameter whose other end is connected to the gas supply hose from the plant.
As biogas passes through the nozzle air is allowed to be drawn into the mixing chamber. For obtaining desired flame temperature, nozzle adjustment is done by trial and error. Biogas stoves normally operate at gas pressure of 75 to 90 mm (3-3.5 inch) water column. Brightness and combustibility of gas can be controlled by regulating gas pressure and air-fuel ratio which is generally maintained as 10 : 1. Combustibility of gas is maximum when flame is slightly yellow, a bit bright and burnished.
When the flame is turned blue and smokeless after air jet adjustment, it burns at a temperature of about 800°C. Cooking appliances are available in wide range both in single and double burner category with each burner consuming anywhere between 0.25 to 1.25 m3 of gas per hour. From 0.28 to 0.42 m3 (10 to 15 ft3) of biogas is needed for meeting the cooking needs of one person per day.
Biogas cannot be burnt on LPG/natural gas stoves as it tends to lift off due to slower flame speed factor. This problem can be somewhat overcome by reducing gas pressure but this result in higher gas consumption. Burning of biogas on country made crude stoves suffers from poor thermal efficiency of not more than 20 per cent and flame temperature seldom exceeding 500°C.
i. KVIC Design:
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KVIC designed burners are made of cast iron with injectors made of gun metal Figs. 11.1. These two types consume 6 ft3 of gas per hour which are adequate for 1-2 member and 5-6 member family, respectively. Initial pressure of gas in gasholder and burners vary from plant to plant.
While designing these burners initial pressure of gas in gasholder is assumed as 8.8 cms (3.5 inch) water gauge and in burner between 7.6 cms (3 inch) and 8.3 cms (3.25 inch) water gauge. These burners attain flame temperature of 800°C and ensure complete burning of gas and consequently foul smell of unburnt gas rarely exists.
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ii. Tin Burners:
These burners though not very efficient are made of tin to which gas is supplied from base through pipelines. Perforations are provided at upper portion of the tin to enable the gas to spurt out for burning. A number of small-sized stones are kept inside to facilitate improved air and gas mixing.
Tin burner can be embedded inside an earthen ‘Chulah’ to suit flat bottomed and round bottomed cooking vessels. The Indian Agricultural Research Institute (IARI) in past developed designs of tin burners which can be easily constructed using locally available materials (Fig. 11.3).
More details of two of such designs are as follows:
(i) This can be made from an empty cigarette tin by drilling 0.7 diameter hole from side at a height of 2 cm from the base and soldering 0.7 cm diameter and 7 cm long tube with 3 cm long portion inside and 4 cm projecting outside. The lid is perforated to have four holes of 2 mm diameter uniformly spaced on a pitch circle and with one additional hole in the centre.
A number of stones are kept inside to enable proper air and gas mixing. Biogas is made to pass through the tube inside the tin which finally emerges out through perforations and burns. The burner is embedded in an earthen vessel to make it convenient to use it for cooking.
(ii) The second version commonly known as the ‘Angithi’ burner can be made from boot polish tins or larger flat tins. A 0.7 cm dia metallic tube bent at right angles is soldered to the tin through a hole at its base. The tin cover is perforated with 2 mm dia. holes at a pitch of 2.5 cm on a uniform pitch circle diameter to ensure uniform heating from all around. This burner is fitted on a conventional iron ‘angithi’ which makes it easy to use it for cooking.
Application # 2. Burner Designs Commonly used in China
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In China a number of designs of biogas burners/stoves have been developed which can be constructed by using locally available materials like clay. Three commonly used designs are shown in Figs. 11.4, 11.5 and 11.6. For improving heat resistance properties, clay is used after mixing with salt water, burnt rice-husk or fine coal powder.
Design details of some commonly used Chinese clay burners are as follows:
i. Shower Head Burner:
This type of burner is made of potter’s clay which provides a stable flame. Diameter of the shower head varies from 4 to 8 cm and its base diameter is almost twice the upper diameter. Fifty to sixty holes each of 1.5 to 2 mm in diameter are made in three to four rows. The air inlet is made 10 mm long and 5 mm wide. Air-fuel ratio is controlled by adjusting the position of gas nozzle relative to air inlet (Fig. 11.7).
ii. Drum Burner:
This burner is considerably similar to the shower head burner with the added benefit that it can be made from any material easily available and not limited to clay only. It ensures efficient combustion as gas can be mixed properly in mixing chamber before burning. A metal gas pipe with nozzle is placed in gas and air inlet. A metal cover is fitted as an air control device (Fig. 11.8).
iii. Long Arm Burner:
This burner (Fig. 11.9) has an air and gas mixing tube at the top and is provided with a support. It is simple to construct and can be made from local materials. It provides strong and concentrated flame.
iv. Double Ringed Burner:
This burner which (Fig. 11.10) has gas and air inlet hole near upper surface. It has two annular air and gas mixing tubes at top having numerous holes through which air-fuel mixture emerges out and burns.
v. L-Shaped Burner:
This burner (Fig. 11.11) is a modified version of the shower head burner. It consists of a base shower head and a L-shaped gas inlet pipe. It also provides a strong and directed flame.
A stove of the following specifications is adequate for meeting the cooking needs of a family of 4 to 5 persons consuming 0.45 m3 (16 ft3) of gas per hour-
Jet size : 2.25 mm diameter
Area of jet: 3.98 mm2
Flame-port size : 6 mm diameter
Number of ports : 20
Total area of ports: 565 mm2
Ratio of jet area to flame port area : 1 : 1.42
Length of gas mixing pipe : 20 mm
Diameter of gas mixing pipe : 20 mm
Application # 3. Use of Biogas as a Lighting Fuel
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Biogas provides a clean fuel for lighting homes. Biogas lamp and burner are structurally alike both having a nozzle, an air inlet and a mixing chamber; but with additional mantle in case of lamp and fire-sieve plate in case of stove. In biogas lamp when gas is burnt, mantle of the lamp glows which causes lighting. Mantle in biogas lamp is similar to one used in a ‘Coleman’ or propane lamp. Mantle is normally made of Ramie fibre (which is also used in making glass cloth and liner) and is coated with thorium nitrate solution.
While burning, the Ramie fibre reduces to ash forming a layer of thorium dioxide which emits dazzling white light at high temperatures. Nozzle is of the size of a needle point having 0.5 to 0.7 mm. diameter. The other end of the nozzle is connected to gas supply hose which is linked to a biogas plant. Biogas emerges out from the nozzle at very high temperature forming low pressure area around it.
As biogas passes through the nozzle, air is allowed to be drawn into the mixing chamber to freely mix with it. Brightness of biogas lamp mainly depends on factors like gas pressure, and relative proportion of gas and air which is about 1:10, and thoroughness in mixing. For achieving bright intensity, nozzle need to be adjusted by trial and error. Low gas pressure causes poor light intensity, but higher pressure though it improves brightness tends to lower mantle-life.
Biogas lamps can be either suspended (hanging type) or put on table (standing type). As biogas is almost half as light as air and as hot air flows upwards, 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 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 designs. Indian lamps are designed with single or double mantles. Both single and double mantle types can be further categorised as internal or external types. An internal mantle lamp has a simple cover and is designed to produce 100 candle power which is equivalent to light intensity of a 60 watts electric bulb.
An external single mantle lamp has an outside protective cover to safeguard it from wind and is also capable to produce same light intensity of 100 candle power. These works at gas pressure of 70 to 85 mm (2.75 to 3.25 in) water gauge. A double mantle lamp can also be both internal and external type.
The internal type is fitted with a simple cover and can produce upto 100 candle power. The two mantle external type is equipped with a special cover to protect it from rain and wind and is also capable to produce 100 candle power. If despite higher gas pressure brightness of the lamp is found to be low, then this problem can be minimised by passing the gas through petrol or solvent oil kept in a container which leads to its enrichment.
As per the available norms, 4 litres of petrol or solvent oil is sufficient to last for 1500 hours of lamp usage. This is equivalent to saying that four litres of enriching agent can last for one complete year, if the lamp is lit daily for four hours.
A popular Chinese lamp consists of four major parts- an aluminium tube, a gas diffuser to which mantle is attached, a disc reflector with a glass globe, and a thin glass/plastic tube that acts as a nozzle. The aluminium tube is normally 10 cm long 1.5 cm in diameter with four air holes of 0.3 to 0.4 cm diameter located little below the top of the tube which is open.
The gas diffuser is held at the bottom of the aluminium tube to which the mantle is attached. The disc reflector is fitted at the bottom of the aluminium tube just above the diffuser. The glass or plastic tube is about 12 cm long and resembles a pipelle, and fits into the aluminium tube.
Whereas the upper end of the aluminium tube is open, its base holds the diffuser which is similar to one in the Indian type. In lamp made of clay, both tube and diffuser are integral and made of this material including reflector.
As for the starting sequence of the lamp, after opening it, clay nozzle (venturi) is fitted. This is then followed by opening of the mantle to form a hollow ball and subsequent joining to the venturi. After opening gas cock and regulator, mantle is lit to light the lamp. Lamp can be turned-off by operating the gas cock.
Regulator is adjusted for achieving requisite brightness. For preventing the possibility of any hazard, the lamp should be lit by bringing the match-stick close to the mantle either via the hole in the base of glass globe or via the reflector after opening it.
Application # 4. Utilisation of Biogas for Pumping Water and Miscellaneous other Applications
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Water-pumping by biogas-run IC engines is emerging as a very promising applications of biogas. R&D Division of Bharat Heavy Electricals Ltd. at Hyderabad has developed biogas-fuelled engines which on being coupled to a pumpset can be used for pumping water both for drinking water supply and irrigation purposes. One pumpset underwent field trials in past spanning over a period of five years and another pumpset gave trouble-free performance for a three year test period for supply of drinking water at Salojipally village in Medak district in Andhra Pradesh.
It was claimed that the cost of pumping 1000 litres of water upto ahead of 10 metres with this type of pumpset (as per the prices prevailing during 1988) was 3.5 paisa as against 9.1 paise by diesel pumpset of equal capacity which clearly indicated the cost-effectiveness of a biogas operated pumpset. Estimates of biogas plant capacity for varying water head and duration of pump operation are given in Table 11.6. Size of agricultural fields that can be irrigated with varying hours of pump operation are given in Table 11.7.
Following are some major technical specifications of BHEL designed biogas-fuelled pumpset:
V. Massey of Division of Agricultural Engineering, IARI, and New Delhi used two biogas-fuelled single cylinder two stroke petrol-cum-kerosene engines of 2.5 and 3 hp for chopping hay. First engine was run on petrol and then switched over on biogas by continuous injection of air-biogas mixture in suitable proportions.
Mixing of biogas with air was regulated with the help of a modified carburetor. Each engine was rigidly installed on a wooden table which was coupled to a 450 rpm chaff-cutter with the help of a canvas belt. The two 2.5 and 3.5 hp engines were reported to run at 90 and 80 per cent efficiency and utilised 49 and 52 ft3 of gas per hour at full load.
Biogas can also be directly utilised for operating some major button- making, plastic-moulding and toy-making machines. Ram Bux Singh reported norms of biogas consumption in operating various plastic moulding machines.
Application # 5. Biogas as a Fuel for Running IC Engines
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The idea of running internal combustion engines on gaseous fuels is not of recent origin. In fact the first gas engine was known to be tried as early as 1860. In 1868, Dr. Diesel, a legend in engineering, reportedly applied for a patent for a dual-fuel engine with one of the fuels being gaseous. These engines mostly utilised natural gas, producer gas, sewage gas and other gases generated by coal and chemical industries.
IC engines which can work both on liquid and gaseous fuels can be broadly divided into two categories, namely, compression ignition (CI) and spark ignition (SI) engines. In CI engines, gaseous fuel is normally burnt at atmospheric temperature and pressure and fuel is ignited by injecting a small quantity of liquid fuel, Injection for ignition of compressed air-fuel mixture is done while piston is at near top dead center. In SI engines, the gas is inducted through a gas carburetor or a mixing valve or by injection at low pressure depending upon the engine design.
Biogas provides a clean fuel for both SI (petrol) and CI (diesel) engines. Whereas biogas in diesel engines is used in combination with diesel petrol engines can be run on hundred per cent biogas. In diesel engines, temperature at the end of compression stroke is usually less than 700°C whereas the ignition temperature of methane and air mixture is 814°C. In view of this temperature difference, for igniting gas mixture, it is necessary to inject small quantity of diesel fuel before the end of compression stroke.
Use of biogas as an engine fuel offers several advantages. Biogas being a clean fuel causes clean combustion and reduced contamination of engine oil use of biogas also results in reduced deposits on piston and combustion chamber.
As gas needs are considerable, large capacity plants are needed to supply gas for running IC engines. It is to be noted that even a 5 hp engine for 8 hourly operation requires gas supply from a biogas plant of 500 ft3/day capacity. Community plants with a gas output of 50 m3 per day are generally used to drive biogas-fuelled electricity generating sets.
As per some rough estimates, nearly 575 million tonnes of cattle dung is generated per annum in the country. Assuming that about 75 per cent of it is available for biomethanation, an estimated 10,000 billion ft3 of gas can be available for use.
If two thirds of this gas is available for fuelling IC engines, with an assumed norm that 35 ft3 of gas can replace 1 lb of diesel oil, an estimated 9 million tonnes of diesel can be saved annually which is much more than the total diesel needed for operating irrigation pump-sets and multifarious agricultural activities at large.
Normal consumption rate of biogas for running IC engines is 0.45 to 0.54 m3/hour (16 to 19 ft3/hour) per hp or 0.60 to 0.70 m3/hour (21 to 25 ft3/hour) per-kw if used for power generation. Biogas pressure is found to vary from 25 to 100 mm (1 to 4 inches) water gauge. If an engine consumes 0.50 m3 of biogas per hp per hour, then quantity of gas needed for running a 10 hp engine for 10 hour operation per day becomes-
= (0.50 m3/hp/hour) x 10 hours x 10 hp
= 50 m3
Similarly, if an engine consumes gas at a rate of 0.65 m3/kw/hour, then quantity of gas needed for running a 10 kw generator for 10 hour operation each day becomes-
= (0.65 m3/kw/hour) x 10 hours x 10 kw
= 65 m3
Dual-fuel engines offer the flexibility of easy switch-over from dual-fuel to pure diesel operation and vice-versa. Important elements of a dual-fuel engine are air-filter, dual-fuel intake pipe, biogas choke and exhaust pipe.
While working out the economics of biogas use in dual-fuel operation, it is necessary to consider relative costs of biogas and diesel, extent of diesel replacement with biogas, degree of additional wear and consequent higher maintenance and lubricant needs, and comparative thermal efficiency of the two systems. Thermal efficiency is governed by dual-fuel mix, engine output and speed. For a given engine output and speed, thermal efficiency first increases with increasing gas proportion and which beyond a limit again starts declining.
Dual-fuel ratio for maximum efficiency is different for different engine loads at a given speed. It is to be seen that minimum fuel cost does not necessarily correspond to a gas-fuel ratio which causes peak efficiency. These rates are based on 8 hourly operation with 75 per cent plant efficiency with biogas consumption rate being 9 ft3/hp/hour having 50 per cent diesel replacement with biogas.
Although replacement of diesel can be as high as 80 per cent, maximum thermal efficiency is achieved when gas proportion in mixed fuel ranges between 30 to 60 per cent. Advancing the injection timing by 5° to 7° BTDC over the standard advance helps to improve thermal efficiency of dual-fuel based CI engine.
Conversion of diesel engine to run on biogas involves some technological changes. It can be achieved by installing an extra magneto and a spark plug to change the compression ignition system to spark ignition otto cycle. This modification enables diesel engine to work purely on biogas. Changeover is however difficult because in most CI engine designs no space is available for providing magneto and spark plug.
Conversion is particularly difficult when engine size is small. With sufficient supply of biogas a large multi-cylinder diesel engine can be modified with greater ease. A CI engine can also work on dual-fuel with biogas forming the main fuel supply. When adequate quantity of biogas is not available, it can be quickly changed over to diesel fuel.
A dual-fuel engine is first started with diesel fuel only. After it has attained normal running for some time, biogas choke is opened to admit gas into the combustion chamber. Biogas admission can be controlled by adjusting the choke. For stopping the dual-fuel engine, biogas choke should be stopped first followed by throttle. It should be endeavoured to keep the engine speed uniform.
If the engine does not pick up speed either due to engine overloading or due to higher intake of biogas, the engine should be allowed to run idle for some time. It can be again put on load after its running becomes normal. If during operation occasional sound or knocking is noticed, biogas choke should be adjusted, and if it is on account of small loads on engine it can be corrected by suitable increasing load on it. In a steady engine operation, dual-fuel achieves nearly 80 per cent saving in diesel consumption.
Dual-fuel operation is smoother when the compression ratio is kept low but it leads to low power output and gives rise to start-up problems. Diesel engines are generally set with an advance injection angle which makes them appropriate for running on dual-fuel as well as diesel alone.
Furthermore, biogas possesses reasonably good anti-detonation properties. In view of these factors while running on dual-fuel, original compression ratio and advance injection angle set for diesel operation should not be disturbed. This also facilitates quick reversal to diesel fuel if some difficulties in running with dual-fuel are encountered.
Petrol engines running on biogas can run solely on gas without mixing with petrol. A specific design of a carburetor for 10 hp petrol engine running on biogas is shown in Fig. 11.19. A plastic bag is provided near the gas inlet to facilitate easy sucking of gas.
Petrol engines running on biogas however cause some maintenance problems. Valves, plugs etc. remain more-clean and the sump oil needs less frequent replacement. However, engines working on biogas get heated up faster than petrol and accordingly latter need more effective cooling system.
For using biogas as a petrol substitute, small modifications in the fuel injection system are necessary. In a partially converted system, a gas tap is fitted to the inlet manifold of the engine. The engine is started initially on petrol and allowed to run until the running is smooth. This is followed by cutting off the petrol supply and slight opening of the gas tap. For minimising choking, air intake needs to be regulated. Opening of gas tap and adjustment of air supply is carried out simultaneously to achieve smooth operation.
After its initial start with petrol and engine warming up, SI engines can solely run on biogas. Biogas run SI engines are capable to achieve 85 per cent of maximum BHP. Brake thermal efficiency of engine when it is run on biogas is slightly higher. Specific gas consumption is close to 0.67 m3/bhp/hr and stoichiometric ratio as 11.2. Advancing the injection timing by 20° to 25° BTDC over the standard advance helps to improve thermal efficiency of SI engines.
Attempts to use biogas for running IC engines in India date back to forties when gas from a sewage treatment plant in Mumbai was used for this purpose. Later some experimental work in this area was done by the KVIC in Mumbai in sixties and early seventies. Studies were also carried out concurrently in this field at the Gobar Gas Research Station of the PRAD at Ajitmal (Etowah) under the technical supervision of Ram Bux Singh.
Investigations carried out established biogas potential as an engine fuel and identified major problems that need to be solved. Some experimental studies were also carried out at the Agricultural Engineering Division of the IARI, New Delhi. The Tamil Nadu Agricultural University conducted a trial for running a diesel engine with biogas. Biogas was injected through a nipple placed below the air cleaner.
The trial showed that biogas could replace diesel oil by over 50 per cent. In 1975-76 a progressive farmer-cum-entrepreneur from Panipat (Haryana) conducted a field trial for the use of biogas through inlet manifold in diesel engine with the help of a carburetor designed by him.
The Tractor Training and Testing Station, Budni in collaboration with the Tractor Training Centre, Hissar and the Indian Institute of Petroleum, Dehradun conducted similar trials on use of biogas as engine fuel. In 1976, Kirloskar Oil Engines designed and introduced biogas dual-fuel engines. The first 5 hp biogas engine capable to run at 1500 rpm was commercially released in June, 1977. From then onwards engines of higher capacity and rpm have been developed and used in the country.
IIP, Dehradun also carried out studies on use of sewage gas for dual-fuel operation. The studies were conducted at the experimental sewage gas plant of UP Jal Nigam at Dehradun. The plant has a capacity to produce 11.4 m3 (400 ft3) gas per day. Calorific value of sewage gas is marginally higher than biogas. Sewage gas consists of more methane as compared to biogas.
Hence for a given diesel fuel flow rate and power output, less quantity of sewage gas need be admitted as compared to biogas. Higher compression ratio was found to give better engine performance. Maximum thermal efficiency of the engine was found to occur when nearly 40-50 per cent of the total energy input was derived from gas. Exhaust smoke was considerably less with gas induction while carbon monoxide slightly more.
General engine cleanliness and lubricating oil performance were found to be more or less similar with and without sewage gas. Tractor Training and Testing Station at Budni (Madhya Pradesh) in association with the Tractor Training Centre, Hissar and the IIP, Dehradun carried out studies in seventies on dual-fuel operation of CI engines. Effect of varying dual-fuel proportions on engine speed and horse-power developed was studied.
Biogas for these experiments was obtained from a 150 ft3 capacity plant. No rough running of engine was reported for biogas share upto 76 per cent beyond which it occurred. Horse-power developed and engine speed declined with very high gas proportion in dual-fuel. This indicates that dual-fuel operation is more ideal at medium than high speeds.
Dr. (Mrs) P.P. Parikh, et al., of Dept. of Mechanical Engineering, IIT (Mumbai) carried out several studies in past on dual-fuel based IC engines. It was reported that light diesel oil (LDO) could substitute expensive high speed diesel (HSD). By keeping LDO at 20 per cent and raising biogas proportion gradually up, it was possible to raise the engine capacity to 120 per cent of its rated capacity. Smoke level in dual-fuel operation was less as compared to when LDO was used alone.
Though at part loads the efficiency of LDO-biogas operation was slightly lower than with LDO alone, at full loads as well as at loads 20 per cent higher, the efficiency was higher with dual-fuel operation. This finding has a great practical utility as one can use relatively less expensive LDO in place of expensive HSD for dual-fuel operation. For all proportions of LDO and gas, dual-fuel operation is beneficial for the engine as well as the user. Dual-fuel systems do not warrant any specialised training for operation.
G.L. Patankar, et al., at the GGRDC, KVIC, and Mumbai carried out several experimental studies in seventies in association with the VJTI, Mumbai on use of biogas in running IC engines. Experiments were carried out on a single cylinder, horizontal, four stroke, 600 rpm engine which was run on fuel-oil in normal course. The engine was modified to work on dual-fuel.
Rack in fuel injection pump was connected to the governor so that the engine could be started on fuel-oil, run on it by changing the lever position, and biogas by adjusting fuel-oil injection. Power developed and efficiency of the dual-fuel engine was comparable with that of solely fuel-oil.
The engine was later on run at Tulshishyam village of Junagarh district where it was used to generate electricity and run a flour-mill. Dual-fuel engine was however found to corrode very soon, due to carbon dioxide, hydrogen sulphide and moisture for which some satisfactory solution was needed.
Kirloskar Oil Engines Ltd at Khadki (Pune) took lead in past to manufacture dual-fuel engines utilising a mixed fuel of 80 per cent biogas and 20 per cent diesel. The company began with a 5 hp 1500 rpm engine which first rolled out in June, 1977. Subsequently, it developed engines of higher ratings upto 75 hp and beyond, with facilities for both water-cooled and air-cooled systems.
The engine can quickly alternate between dual-fuel and diesel according to the need and convenience. When running on dual-fuel, nozzle and fuel pump of the engine is used to create pilot diesel spray in combustion chamber to initiate combustion. A special carburetor is provided to ensure proper mixing of biogas and air before admission to the combustion chamber. For operating a 5 hp engine, 2.25 m3 of biogas is needed per hour.
The KOEL dual-fuel engine consumes 15ft3 of biogas per bhp hour. This implies that 75 ft3 of gas is needed to run a 5 bhp engine for one hour. The engine can be easily coupled to an irrigation pump set or used for operating farm machinery like crusher, thresher, flour-mill, power generating sets etc. R&D division of the company also analysed and compared the performance of diesel engines and dual-fuel engines.
Factors analysed covered density of exhaust smoke, temperature of exhaust gas, engine-deposits and condition of lubricating oil. Density of exhaust smoke was found to be less with dual-fuel than when diesel was used alone. Temperature of the exhaust gas was however found the same with no significant difference. As for engine-deposits, it was found that engine run on dual-fuel was cleaner than diesel alone. Lubricating oil with dual-fuel was cleaner as compared to diesel.
A typical dual-fuel engine requires 15 per cent diesel and balance proportion of fuel is obtained from biogas. If a normal 5 hp engine is run on only diesel, it requires 185 g of diesel per bhp hour, then for 8 hour daily operation, monthly (25 days) diesel requirement becomes-
If the engine is run on dual-fuel only then 33 litres of diesel will be needed resulting in a saving of 190 (223-33) litres of diesel per month.
K. Kasturirangan, et al., have carried out experimental studies concerning use of biogas as diesel engine fuel. The performance of a single cylinder dual-fuel engine with biogas and pilot diesel oil injection was studied under varying conditions. Tests were conducted at constant engine output with varying biogas diesel proportions. Tests were also conducted with variable output with fixed diesel oil flow rates. The peak output of the engine with dual-fuel was also analysed.
These tests confirmed that it is possible to operate normal type of diesel engines used for irrigation purposes on biogas with minor modifications. The engine develops the same power as with diesel oil. Thermal efficiency was satisfactory at high loads but rather unsatisfactory at part loads.
Throttling of air supply improved the efficiency to some extent. The engine could be operated over a wide range of gas to diesel ratios, depending on the availability of biogas and diesel oil, and offered the ease of switch-over to diesel oil when needed.
Several experimental studies were carried out in past by C.P. Kothandarman and S. Krishnamurthy at the PSG College of Technology, Coimbatore on dual-fuel based CI engine and biogas based SI (petrol) engine. In a CI engine, biogas reduced brake thermal efficiency throughout the operating range with higher reduction at lower outputs. When gas proportion was increased to 50 per cent, peak pressure increased by 10 per cent with only marginal increase in exhaust gas temperature.
At higher loads no vibration problems were encountered up to 50 per cent fuel replacement. Higher gas proportion beyond 50 per cent caused rough running (knocking) of the engine and high peak pressure. Biogas was found to reduce efficiency marginally at higher loads. At small loads dual-fuel caused less noise than diesel which was indicative of smoother combustion with biogas.
As for SI engine operation, the engine was first run on petrol at varying loads at the rated 1800 rpm and overall efficiency analysed. The same engine was then run purely on biogas. It was reported that for the same power developed, SI engine run on biogas registered an improvement in overall efficiency.
Z. Cao., made a detailed review of research on use of biogas in running single and multi-cylinder CI engines as dual-fuel involving study of compression ratio, selection of speed and thermal efficiency of engines. C.Q. Jiang, et al., carried out studies on using compressed biogas as part of dual-fuel in running CI engines.
They also studied problems of biogas compression, possibility as well as need of using compressed biogas, and economic viability of compressing biogas. According to B. Gehri, success of dual-fuel operation depends on using gas of fixed composition and in adequate supply at uniform pressure. For efficient running it is necessary to maintain dual-fuel engines by periodic replacement of filters, spark plugs and other vital parts.
According to D.J. Picken and H. Hassan biogas operated engines perform poorly on account of harmful effect of hydrogen sulphide on copper- based engine parts. Whereas many engine parts can be substituted by other materials or protected from biogas, small end bearings pose particular problems. On account of operational needs this bearing is more under high fatigue and stress, and cannot be easily replaced.
Moreover, it is also not convenient in smaller engines to protect it by flooding it with oil. Increased wear of cylinder bore can be attributed to high hydrogen sulphide content in biogas, its leakage into crank case and harmful impact on end bearing. For minimising this problem, copper small end bearings should be lubricated as far as possible.
Application # 6. Biogas as a Vehicle Fuel
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Compressed natural gas (CNG) as also biogas are commercially used as a vehicle fuel in New Zealand and Italy. Conversion kit for biogas-fuelled vehicle is virtually the same as for CNG which is marketed as an automotive fuel in many parts of the world. These kits permit changeover from gas to liquid fuel and vice-versa at a short notice. As methane is only half the weight of air, dispersal of the leaking gas is faster. This makes biogas a safer fuel than either LPG or petrol.
For use as a vehicle fuel, biogas is first compressed in a three or four stage compressor up to a pressure of 2500-3500 psi and stored in a high pressure gas storage cascade which helps to facilitate quick refueling of storage cylinders. The number of cylinders in cascade depends on digester capacity and number of cylinders provided on each vehicle.
For biogas from 45 m3 capacity digester, at least three or four 9.2 m3 capacity cylinders are required. A 9.2 m3 of gas contains only 13.9 litres of petrol equivalent. Compressor size should be such so that supply rate of compressed gas matches with gas generation rate. For a vehicle of up to 25 litres of liquid fuel capacity and for a distance range from 100 to 150 km, a cylinder of length 1 m, diameter 300 mm and weight 50 kg is sufficient.
In order to provide sufficient range, two cylinders are generally needed in a motor car and up to four in a truck. Gas can be filled in about a minute’s time with cylinder in place. Biogas needs to be used in compressed state for spontaneous flow otherwise space and volume requirements become excessive. Need for compression however adds to the costs which tends to offset the very purpose of using biogas as a vehicle fuel.
Being gaseous fuel, biogas provides excellent cold starting for vehicle as unlike petrol it need not first be vapourised. Once started the engine performs well instantly with no sluggish warm-up. This is because unlike petrol methane does not condense on cold intake manifolds cylinder heads and pistons. Exhaust emissions from biogas are much less as compared to petrol engines. Methane has a higher octane number as compared to petrol which causes slower combustion of the former.
This higher octane rating makes biogas suitable for CI engines which can lead to greater power output. In a SI engine, higher power output and high compression can be obtained by machining the cylinder head or by fitting higher compression pistons.
The permanent modification, however renders it unsuitable for any subsequent running on petrol. As biogas does not condense unlike petrol in cold areas it is free from problem of washing away expensive lubricating oil from top of piston ring.
Consequently, there is no dilution of the lubricating oil either and accordingly inter-fuel replacement interval can be made larger by as much as two times. Spark plug servicing is also less critical as there is no lead pollution from exhaust gases and no lead-fouling of spark plugs. A major disadvantage of methane-based vehicles is the limited range per cylinder of gas carried on a vehicle.
Biogas-based vehicles in limited way were used in UK in 1942 from the gas produced by Mogden Sewage Works, London. One litre of petrol or diesel fuel could be replaced by 0.75 m3 of methane. The fuel system of SI engine/petrol vehicle resembles a CNG vehicle. Each vehicle was fitted with one or two 31.5 litre methane cylinders equivalent to 9 litres of petrol. Gas was supplied to carburetor via a pressure relief valve and other devices.
The carburetor was in series between air filter and normal petrol carburetor. This enabled the throttle valve of the petrol carburetor to control the methane flow to the gas carburetor. As for the use of biogas in a diesel-fuelled vehicle as a dual-fuel, fuel pump maximum delivery was limited to 70 per cent of its normal maximum, and 30 per cent of the fuel was supplied by methane through a gas carburetor and main air inlet.
A linkage was fitted to connect the gas carburetor throttle valve to the diesel governor input so that the ratio of methane to diesel remained constant at all speeds and loads. The vehicle was fitted with three 31.5 litre methane cylinders. In other respects the entire system was the same as that of a petrol-driven vehicle. Initiated in 1983, as part of a methane utilisation project in Sao Paulo (Brazil) biogas from three sewage treatment plants was used as automobile fuel.
Sweetened gas following carbon dioxide removal with hot potassium carbonate solution was dried, compressed and bottled in 200 atmosphere cylinders, and used in running automobiles.
A half tonne weight pick-up truck fitted with a petrol engine was run on compressed biogas for several years starting from mid-seventies under the control of University of Manitoba, Winnipeg (Canada). Oxygen-type cylinders conforming to the Canadian Ministry of Transport Regulations were installed on the pick-up to carry the gas at a pressure of 2400 psi.
While using in uncompressed state, biogas consumption was 0.19 m3/km whereas the petrol consumption of the vehicle prior to changeover to biogas was 5.1 km/litre. For successful use of biogas as a vehicle-fuel, there should be adequate gas storage to cover a practical distance range. A comparative economic analysis of a vehicle capable to run on biogas and petrol indicated that frequent refueling was preferable to carrying multiple high pressure tanks.
Application # 7. Applications of Biogas for Power Generation
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Biogas can be used to produce electricity by coupling a dual-fuel engine to an asynchronous generator. Based on results of several studies carried out, 1 kwh of electricity can be generated from 0.75 m3 of gas which can light 25 electric bulbs of 40 w rating whereas 0.75 m3 of gas if directly burnt can light only 7 biogas lamps for one hour. Hence it is advantageous to first generate electricity and then light larger number of electric bulbs.
In China asynchronous generators of 3, 3.5 and 7.5 kw rating coupled to biogas-based IC engines are commonly used. However, electricity generation from biogas is economical only when gas is supplied by large community plants but it requires high initial capital investment. Decentralised power systems can be cost-effective especially when they minimise transmission and distribution costs.
For cost-effective system, unit cost of biogas-based electricity should be less than supplied by the electricity boards. Cost-effectiveness of biogas-based system improves with growing escalation in unit cost of conventional power.
Biogas is used to generate electric power as decentralised source of energy in several countries. Biogas-fuelled engines can be used for water pumping, crop processing and power generation. Economics of biogas-based decentralised energy systems is dependent on capital costs of the system and economic benefits derived from biogas as well as effluent produced.
A study carried out by D.B. Machin showed that cost-effectiveness of biogas-based power system was constrained by factors like inadequate sharing of information, lack of consensus on system design, low productivity of commonly used unheated digesters, and high cost of large digesters for providing sufficient gas at relatively low temperatures. T. Stahl, et al., in the context of an integrated energy farm project at the University of Missouri in USA developed a mathematical model to assess the feasibility of locating a biogas-fuelled engine/generator on a farm to produce electricity and space heating. O.P. Singhal emphasised that biogas-based power systems and biomass gasifiers are ideal considering that supply of conventional power to remote villages may still take many more years.
R&D Division (Hyderabad) of the state owned Bharat Heavy Electricals Ltd in past designed and developed biogas-based power systems. It successfully tested a 5 kw biogas power generator set. It estimated cost of biogas-based power as Rs 0.70 as against Rs 1.70 per unit with diesel generating set, which showed the cost-effectiveness of biogas-based power. Following are some major technical specifications of the BHEL designed biogas power system.
Type- Single cylinder 4 stroke, vertical, air-cooled engine directly coupled to 6 KVA generator.
Speed- 1500 rpm; speed control- by centrifugal governor.
Generator output- 5 kwh; 415 V; 3 phase.
Biogas consumption- 5 m3 per hour.
Starting- Self-starting DC motor and DC generator with cutouts for starting and charging battery.
C. La Torre described a large-sized plant in Italy which produces biogas and electricity from animal wastes on a farm with 35,000 hens and 1,000 pigs. Before fermentation, wastes are purified by aerated lagooning. Biogas produced by the plant is used without further treatment as fuel for boilers and to produce both electric power and hot water using a cogeneration unit.
While during winter energy needs of the farm matches the energy supply, during summer part of the gaseous energy becomes surplus which is compressed and used as fuel for tractors and farm vehicles. W. Tentcher provided technical details of the construction of a full-scale plug flow pilot biogas plant and design of an associated engine-generator set for power generation.
Red mud plastic (RMP) was used in constructing the gasholder. Wastes of about 800 pigs was used as feed material. The plant was operated at a maximum gas pressure of 10 cm water column. At ambient temperature and at 10 kw output, power generation was estimated as 1.4 kwh per m3 of biogas. H.H. Jawurek, et al., studied performance and combustion characteristics of a portable fixed ignition timing SI engine – alternator set fuelled with biogas.
Maximum power output at optimally adjusted air-fuel ratio declined with increase in carbon dioxide content of the gas which also caused harsh running of the engine. Low electric power output, poor combustion and harsh running can be reduced with a marginally higher share of diesel in dual-fuel comprising a mixture of diesel and biogas.
Biomass Energy Institute Inc., Manitoba in Canada evolved the following relationship for determining the number of animals needed for generating biogas-based electricity for a specified period in terms net fuel replaced with biogas.
Application # 8. Fuel Cell Linked Biogas Systems:
A fuel cell is an electro-chemical device which directly converts chemical energy of a conventional or non-conventional fuel into low voltage electricity. Fuel cells provide low voltage D.C. power which can be converted to A.C. supply using high efficiency inverters which can be put to variety of household and industrial applications. Fuel cells react with the interacting fuel (biogas) in presence of oxygen from air to produce D.C. power.
A single fuel-cell generates 100-200 watts, 1 volt D.C. electric power for each square foot of electrolytic cross-section. As voltage from individual fuel cell is low, a number of cells can be joined in parallel or series configurations to provide requisite voltage. In recent years, some attempts have been made to use biogas for generating electricity from fuel cells which can become a potential source of decentralised power supply in rural areas.
Being modular in nature, a decentralised fuel-cell biogas system provides necessary flexibility in accommodating a wide range of load factors in any given situation. Starting from a very small plant to meet requirement for a few households, it can be easily expanded in size module by module to meet added needs of a large number of households.
Furthermore, it provides a very high electrical conversion efficiency over a wide range of power levels. A mini plant providing 10 kw could operate at almost the same level of efficiency as a multi-mega-watt plant. Typical efficiencies of fuel cell based biogas system range between 35-40 per cent which can be higher than from several decentralised power systems.
The system provides higher efficiency over a wide range of load factors. This system is relatively inexpensive, more reliable with fewer breakdowns, and can have a life of more than 20 years. Moreover in a fuel-cell based biogas power system, resulting heat in the form of hot water or steam under pressure can be gainfully utilised for warming up digester contents for achieving higher gas yield.
Central Institute of Agricultural Engineering (CIAE) at Bhopal designed a biochemical fuel-cell which it named ‘gobar cell’. It consists of pair of electrodes and dung slurry to serve as electrolyte. As per CIAE design 60 g of dung is filled in a plastic cylindrical container of 42 mm diameter and 50 mm height. Two 6 mm dia and 50 mm long bars of carbon and zinc serve as pair of electrodes.
A group of these biochemical cells when combined together in modules form ‘gobar cell panels’ of varying voltage and current configuration. The ends of the electrodes are soldered to copper wire of 20 to 22 gauge specification. The panels are kept in wooden tray so as to provide operational ease without disturbing electrically soldered ends of the electrodes.
A unit of gobar cell utilising fresh dung can generate 0.8 volt of electromotive force and 12-15 milliampere of current. A panel of 32 cells when grouped in series- parallel configuration can operate a transistor receiver set of 3V, 50 mw ratings for three lays for daily five hourly operation. A 32-cells series-parallel configuration consist of eight parallel rows each containing four cells in series.
Apart from operating transistors, these cells can also be used to operate relay controlled mechanisms, pocket calculators, call-bells, hearing-aids etc. which require very small currents for continuous or intermittent use. When not in use they can hold charge for over a month. However, they discharge rapidly, when in use. Mixing of 10 per cent sodium chloride (common salt) practically doubles their charge retention capacity.