|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
What are biogas systems good for? Mr. Alejandro Judan, a farmer-businessman in Nueva Ecija, Philippines, raises pigs in his backyard. Bothered by the smell and the flies attracted by the pig manure, he decided to put up a biogas system. It consists of a vertical continuous-fed digester and a single sludge aging pond. The biogas is used for cooking meals and ironing clothes. The sludge is used to fertilize rice fields and fish ponds.
Mr. Judan solved the pollution problem in his backyard, but his next-door neighbor also raises pigs. Mr. Judan plans included letting his neighbor direct the pen washings from his pig pens into the digester. With the extra biogas, Mr. Judan was able to install an engine and electric generator to run a water pump and light both his and the neighbor's house.
Mr. Jose Parayno is a retired engineer and his wife is a retired school teacher. They operate a general store and a piggery of about 300 pigs behind their store in Pangasinan, Philippines. Although the manure was hauled every day to their rice fields some five kilometers away, their neighbors still complained of the foul odor coming from the piggery. Hearing about how biogas controls pollution and improves sanitation, Mr. Parayno visited the biogas system at Maya Farms and decided then and there to have one built for his piggery.
With the help of Maya Farms, he built a 31 cubic meter continuous-fed biogas digester, and for sludge conditioning he built a 5.0 foot by 13 foot by 3.0 foot deep sludge aging pond. The overflow from the pond went into a nipa palm grove behind the piggery. After the biogas system went into operation, his neighbors stopped complaining. Now it is Mr. Parayno who complains because so many visitors come to see his biogas system every day. The biogas is used for cooking, ironing clothes, and lighting the piggery.
Mr. Parayno enjoys recounting what happened during the long dry summer of 1977 when the hydroelectric plant in Central Luzon could not generate enough power. He extended the biogas pipe to the store and transferred some of the mantle lamps from the piggery. When the electric power was shut off, as frequently happened, he had the only brightly lit store in the area. This brought in many customers. Mr. Parayno is now thinking about using the gas to run an engine and a 2.5 kva electric generator (Maramba, 1978).
In Diagram 3 there is a picture of the temperature of the different parts of a biogas flame. Food cooks and water boils just as fast, maybe even a little faster, when only the top part of the flame is touching the cooking pot. When the flame spreads out around the bottom of the pot, it only looks hotter. More gas is used but the food is not cooked any faster.
The biogas flame will not usually stay lit in ordinary gas stoves or other gas appliances until the hole on the gas jet is enlarged or in some cases removed. (The air intake may also have to be opened slightly.) This should provide a stable flame. Fortunately, gas jets are made of soft brass, so it is easy to enlarge the hole using sharp pointed scissors. Because biogas pressure is low, it is necessary to modify most existing equipment. A gas stove burner will work only after the needle-thin Jet is enlarged to 1.5 mm. Stoves, lamps, and engines using biogas work best at the low pressures of 3.0 to 6.0 column inches (8.0 to 15.0 cm) of water. In India at least two commercial companies make small cooking stoves specifically for biogas. Stores that import consumer goods might want to inquire from their suppliers about the availability and prices of ready made biogas stoves. But it is easy to make simple biogas stoves such as those in Diagram 15.
DIAGRAM 15 : HOMEMADE BIOGAS BURNER DESIGNS
For lighting purposes, biogas gives a soft, white light when burned in gas lamps. The light is not as bright as a kerosene lamp. The mantle is about as bright as a 40 to 60 watt electric light.
A cooperative or business should not try to distribute biogas to individual homes. It would be expensive and dangerous. The gas should fuel an engine plus generator and the electricity could be distributed, or the gas could be used to meet cooperative or business energy needs, and the profits could be distributed to shareholders or investors.
The same amount of biogas, for the same length of time will light three times as many electric lights as gas mantle lights of equal brightness.
Whatever appliance is used, it is important that an adequate volume of air is made available to burn with the gas. This air need not be pre-mixed. Biogas will burn on contact with air (and a match), but what is important is that air should not be restricted in any way. If it burns freely in air, the burnt gas will release only carbon dioxide and a little water vapor. If biogas is made to burn with an excess of air, the burnt gas may release sulfides, making the eyes itch and causing a sharp acrid smell.
L. John Fry reported two small failures with biogas. One involved a gas iron in which a flame burned to bring about heat. The flow of air into the iron was too much for the amount of gas, resulting in a foul smell. The other problem had to do with a gas refrigerator of the absorption type which was fueled with biogas for three years with a special burner. The problem here was the pilot light. The flame was small and delicate. A light wind could blow it out. Even the slamming of the kitchen door would blow out the flame. On the positive side everyone agrees that when biogas is used for cooking, the pots and pans remain spotless.
Biogas and Stationary Engines
Any internal combustion engine, except a two-stroke, can be adapted to run on biogas. On spark-ignition gasoline engines (hereafter referred to as gasoline engines), a biogas and air mixer is needed in advance of the carburetor near the choke. The biogas is introduced via a five mm diameter tube connected to the biogas supply through a control valve. The engine is started on gasoline and then switched over to biogas after the engine is running. The engine can be switched back to gasoline if there is a shortage of biogas. For smooth running of the engine, the biogas flow should be steady; this can be done on stationary engines by counterbalancing the gas cap. Sheaffer and Roland, a company that has developed biogas systems for use in the United States recommends using gasoline engines. They only use biogas for fuel, but they keep propane bottled gas as a backup in case there is a shortage of biogas. The company also recommends that engines that are run continuously have once a week oil and filter changes.
Because the use of biogas to run engines and the use of the excess engine heat to heat digesters are often the most important factors in making biogas systems profitable, what follows are three different reports on using biogas as an engine fuel.
L. John Fry's account of his use of biogas to run engines on a farm during a six year period is one of the more impressive ones to be found in biogas literature. The following section is adapted from his book, Practical Building of Methane Power Plants for Rural Energy Independence.
Methane (biogas without carbon dioxide) makes an excellent fuel for internal combustion engines because it:
1) has a very high octane rating,
2) leaves little or no carbon deposits in cylinders or on pistons,
3) greatly reduces the amount of sludge build-up in the oil, which means longer operating times between oil changes,
4) does not dilute the oil on the cylinder walls during engine start-up as liquid fuels do, and thus promotes longer engine life (as do numbers 2 & 3),
5) has no tetra-ethyl lead in it to foul spark plugs and pollute the air,
6) mixes better with air than gasoline, resulting in a better explosion in the cylinder,
7) results in less valve burning,
8) burns clean, with fewer pollutants than many other fuels.
There is a direct relationship between pressure and temperature. When pressure goes up, so does temperature; when pressure goes down, so does temperature. This is exactly what happens inside the cylinders of gasoline and diesel engines.
In gasoline engines a fuel-air mixture is let into the cylinder, the piston pushes up and compresses the mixture, the spark plug fires, there is an explosion, and the hot gases formed by the burning fuel expand and push the piston down. At the very bottom of the piston's travel, the cylinder space has its greatest volume. At the very top of the piston's travel, the cylinder space is as small as it can be. The ratio of the largest volume to the smallest volume is called the compression ratio. If the compression ratio is four to one, the fuel-air mixture will be compressed by a factor of four. Or, to look at it another way, the exploding gases will expand four times their original volume.
Now, as the process of compression and firing repeats and continues, the cylinder walls heat up, and this increases the temperature of the incoming fuel-gas mixture. As this mixture is compressed by the piston, it becomes hotter than it would in a cold engine and may reach its ignition temperature before the piston has finished compressing it. Boom, the fuel-air mixtures explodes too soon (predetonates). This is commonly called knock. It steals power from the engine because the piston must continue upward against the force of the explosion pushing it down. Obviously, the more the fuel-air mixture is compressed, the greater will be its tendency to predetonate, since greater compression will mean higher pressures and temperatures.
It would seem that what is wanted in an engine is a low compression ratio, right? Wrong. As was pointed out above, the compression ratio is also the expansion ratio, and the more the exploding hot gases are allowed to expand, the more they will fall in temperature. In essence, this means that the greater the expansion of these hot gases in the cylinder space, the more efficient the engine will because it will convert more of that heat into the motion of the piston. The trade-off is between the knocking of predetonation and thermal (heat) efficiency.
Spark engine fuels such as gasoline are rated by their octane number. The octane rating of a fuel is a measure of how well it avoids predetonation. Methane has an octane number of 120 or more. This means that it can easily be used in high compression engines, because it rarely predetonates.
Biogas, which is methane mixed with carbon dioxide, has a lower octane rating than methane (but still over 100). Carbon dioxide also acts to decrease methane's ability to detonate when it is ignited, so not as much power is available from the methane in unscrubbed biogas as is from pure methane, given equal volumes of methane. The fact is that anything except oxygen mixed with methane will dilute it, because not as much methane can get into the cylinder, and clearly this will further reduce the power available from each power stroke in the cylinder. Removing the carbon dioxide will increase the power available.
The trace of hydrogen sulfide that is in biogas should only be removed if it is present in amounts (by volume) greater than 0.1 percent. But then there would be no way to smell gas leaks--because of all the gases in biogas, only hydrogen sulfide has any smell (rotten eggs). Hydrogen sulfide troubles can be partly overcome by replacing the standard engine valves with heat resistant valves and changing the thermostat in the cooling system so that the water circulates at 65 degrees centigrade (150° F) rather than 50 degrees centigrade (120° F). Sometimes even these precautions are not taken and the engine runs just fine.
Tests show that by using the best fuel-air ratios and averaging outputs, 100 percent methane outperforms a 50 percent methane/50 percent carbon dioxide mixture by approximately 86 percent in the same engine, all other conditions being equal. Looked at another way, diluted methane (biogas) has to provide 1.86 times the energy input to provide the same energy output that pure methane can.
Using a gasoline engine designed for research which had a variable compression ratio (4:1 to 16:1), it was found that output peaked at a compression ration 15:1, a fuel-air ratio of 1:10 (10 percent methane to 90 percent air by volume), and with the timing set so that the engine fired 30 percent before top dead center.
Ordinary four-cycle, spark-ignition gasoline engines can be easily converted to run on biogas, but they tend not to have the high compression ratios which can be used with biogas. Very small engines such as motorcycle engines often require a fuel mix of gasoline and oil. These two-cycle spark-ignition engines are not very suitable for biogas, but they can be used. Lubrication may be a problem, because these engines get some of their piston lubrication from the oil in the fuel mixture, of which biogas has none. Operating a two-cycle engine as a dual fuel (biogas and oil) engine might be an experiment worth trying, especially if the capacity of the biogas digester is too small to provide enough gas for a larger engine.
Another common engine type is the diesel. Diesel engines do not have spark plugs. What happens in a diesel engine is that air is compressed and when the piston reaches the right place in the cylinder space, the diesel fuel is squirted (injected) into the cylinder and the heat which has been developed by compressing the air ignites the fuel-air mixture, causing an explosion without need of a spark.
Diesel fuels do not have octane ratings; they have cetane ratings. The kind of measurement is different for diesel because the qualities needed for diesel fuel are very different than the qualities needed for gasoline fuel. In gasoline engines the fuel should not burn until it is lit with a spark. In diesel engines the injected fuel should burn as soon as it enters the cylinder. That is why cetane numbers are all about how easily the fuel ignites on its own in the cylinder.
Diesel engines are usually noisier than gasoline engines because the fuel burns with a faster and bigger bang. The compression ratios they use are higher than those for gasoline engines. Because of greater stresses, diesel engines are built better than gasoline engines and they generally last longer.
Diesel engines get better mileage per liter (or gallon) of fuel, but they are also more expensive.
Biogas is a great fuel for diesel engines. It has a low cetane number, so it works well when introduced into the cylinder with the air, compressed, and mixed with a little diesel fuel in order to cause an explosion (6-13 percent of the usual amount). A diesel engine run in this manner is referred to as a dual-fuel engine and the injected diesel fuel as the pilot oil.
The following is a summary list of ideas about running gasoline and diesel engines on biogas:
1) Pure methane will outperform unscrubbed biogas in the same engine, because with biogas not as much space in the cylinder can be devoted to methane; some space must be given to carbon dioxide which "steals" heat from the combustion reaction.
2) Higher compression ratios such as those used in diesel engines increase the power available when operated as duel-fuel engines with biogas.
3) The timing is often advanced in biogas powered gasoline engines.
4) The efficiency of an engine is affected by the load. A free running engine may not appear to use as much biogas as an engine which is driving a water pump or electrical generator, but per horsepower hour delivered, an engine without a load will actually use more biogas.
5) Use a high compression ratio. Some gasoline engines can be converted to a higher ratio using a special head. But remember that among other differences, a higher compression ratio will create greater engine stresses which could shorten engine life. Diesel engines already have higher compression ratios, but their effective compression can be increased with turbo-chargers. Talk to local auto parts dealers for more information.
6) In high compression engines, the spark plug gap may need to be set smaller. Fry ran a 13 brake horsepower, diesel-converted-to-spark engine with a spark plug gas of .008 inchest/.20 mm, which had to be regapped every two weeks. (No one else has reported this change.)
7) If the biogas has a high but not extremely high percentage of hydrogen sulfide in it (between 0.05 and 0.10 percent), change the oil or check it for an acid pH often. Always change the oil when the pH turns acid and if it turns acid quickly and very often, think about hydrogen sulfide scrubbing.
8) Change the oil whenever it thickens by 30 percent. You can measure this by counting the number of drops coming out of a small hole in a can in a certain amount of time. Measure the drip rate of the oil when it is new, and measure the drip rate of the oil when used at the same temperature and for the same depth of oil above the drip hole. When the drops come out only 70 percent as fast as did a sample of the oil when it was new, it is time to change the oil. The crankcase can be modified by replacing the plug with a valve if this test is to be used often.
9) The best way to find out how to use biogas as an engine fuel is to run an engine on biogas and experiment. There is an automotive testing instrument called a dynamometer which can be used to help judge engine performance; it should be available from auto parts dealers (House, 1978).
L. John Fry used biogas to supplement diesel fuel in a three brake horsepower Petter engine driving a water pump and a three kw alternator which supplied electricity to his entire farm. At first the engine was started at four in the afternoon and shut down at bedtime so as to save fuel. However, the supply of no-cost gas meant such diesel fuels savings that eventually the engine was left on all night.
According to Mr. Fry, the conversion process for diesel engines is almost unbelievably simple. Biogas (methane plus carbon dioxide) flowed directly from the gas storage tank under pressure generated by the digestion process and kept constant by the weight of the gas storage tank at a pressure of four inches of water pressure. Two valves were placed along the gas pipe to the engine. The first valve was used for fine tuning of the gas flow and the second, in series with the first, was used for on-off. Just after the two valves, the pipe led straight into the air intake of the diesel engine.
The engine was started on diesel only and warmed up for a few minutes. Then after opening the on-off valve, the fine tuning valve was adjusted slowly. While adjusting the fine tuning valve, one could see the diesel pump governor returning to about the idling, no-load position. The engine was then running mainly on biogas. The normal black diesel exhaust smoke disappeared completely (white smoke replaced it if too much biogas was used); and the characteristic knocking of diesel engines vanished. The latter can be explained by the cushioning effect of the carbon dioxide content of the biogas (carbon dioxide has no fuel value). To stop the engine, the biogas on-off valve was closed and to start the engine again, only the on-off valve was opened after the engine had been started on diesel fuel. This three-brake horsepower diesel engine was run for two years on a mixture of biogas and diesel fuel.
When accurate measurements of diesel fuel consumption in a given time with and without biogas were made, the tests showed that on nearly full engine load the biogas supplied 87 percent of the energy and the diesel fuel supplied 13 percent. Any lesser quantity of diesel fuel would cause the diesel injector to dribble over a period of time and eventually stop operating properly. The injector supplied the "firing" of the engine and had to be maintained in good running order or the diesel and biogas mixture would not explode within the cylinder. When operating a diesel engine as a dual-fuel engine, approximately 20 percent of the power should come from diesel fuel in order to avoid injector trouble. As for gasoline engines, they should not be run on biogas unless the engine is designed to use fuel with an octane rating of approximately 100.
A 13-brake horsepower Crossley slow-speed diesel engine had very little corrosion due to the hydrogen sulfide in biogas. Engine wear was only very slight after six years of continuous operation using 170 cubic meters of biogas every (24 hour) day. The fact that combustion took place at high pressure and at a relatively high temperature could account for the hydrogen sulfide being burned along with the methane and remaining in the cylinder for so short a period of time as not to corrode the metal. However, the exhaust gas was comparatively hot at 535 degrees centigrade (1,000° F) and caused pitting of the exhaust valve which required maintenance a little more frequently than is normal with diesel engines.
This is how the 13-brake horsepower diesel engine was converted:
1) A magneto (generator) from a six cylinder bus was bought at a scrap yard. Five of the six cams were ground-off so that only one spark was given off per revolution. The magneto was linked to the engine crankshaft by a simple bicycle chain around a sprocket welded to the magneto and another to a pulley on the drive shaft. In six years of operation about eight bicycle chains were worn out.
2) The one spark plug could be screwed in either at the top of the combustion chamber opposite the piston or to one side. It was found that the side position ran best. The spark plug gas had to be set at 0.008 of an inch for the spark to leap under the high compression of 15:1. When the gap burned to 0.016 of an inch after about 15 days of continuous operation, the spark would not occur and the engine would stop. A routine was developed to reset the spark plug gap at two week intervals.
3) Biogas was piped in a one inch diameter plastic pipe through a gate valve to a "butterfly" valve taken from the base of a Ford carburetor. The gas then mixed with air at the air intake. The mixing of gas and air took place in bends of the manifold before the gas-air mixture entered the combustion chamber (cylinder) itself. The butterfly-operating lever was connected to the engine's speed-regulating governor.
Engine speed was 475 revolutions per minute (RPM). The engine was run with the conversion as above, but it was found to increase and decrease speed, to hunt, as it called. To prevent this, a shock absorber of the lever movement (Armstrong) type from a small English car was installed. It was bolted to a place on the engine and the lever to the governor control point. The engine than ran very smoothly and if a sudden extra load was imposed on it, the speed pick-up took a fraction of a second. Electric lights would dip for only a second.
The engine drove a five cm (two inch) shaft which ran the entire width of the engine room and right outside, through a hole in the wall, where a flywheel was mounted to keep the revolutions steady. From the shaft a combined alternator (six AC kw) and generator (six DC kw) was driven. The DC current was used to drive a water pump for irrigation 183 meters (200 yards) away. When the pump was in use, the AC load had to be reduced to prevent overloading the engine.
Belts could be shifted so that the shaft could drive a nearby water pump directly. A deep well pump was also kept permanently in action pumping water. The small pump used for circulating water through the engine and heating the biogas digester was also driven from a pulley attached to the engine shaft.
A 24 or 30 brake horsepower engine should be operated at 20 brake horsepower in order to increase engine endurance and lower maintenance costs. Operated in this way, the engine will need approximately 300 cubic meters of biogas per 24-hour day. A heated horizontal digester with a 175 to 200 cubic meter capacity should be big enough to always be able to supply the engine with enough biogas.
In most cases it is not practical to make a bottled gas out of biogas because the processing of compressing the gas would cost too much. For those situations where it might be practical, the book Practical Building of Methane Power Plants for Rural Energy Independence has some basic, introductory information on how to compress biogas.
The importance of the detailed description of the machinery run from the engine is that a schedule must be adhered to so as to spread the power available over a series of different kinds of work at different times of the day. In many situations, not all power demands can be met at one time. Although there are many advantages to the almost free power of biogas, it is usually impossible to draw on that power as from electric power lines in order to drive many motors at the same time. The size of the digester will effectively limit the amount of biogas available.
If biogas is used to run an engine, there are two things that can be done to increase biogas production from an average of one cubic meter to better than two cubic meters of biogas per cubic meter of digester space per day:
1) Build large: A horizontal, above ground digester, built and operated along the lines suggested in this book and with a digester capacity of at least 30 to 40 cubic meters, will be necessary. The larger the digester, the more efficient it is. Even small one and two horsepower engines can use 10 to 20 cubic meters of biogas every day. Depending on needs and costs, anywhere from a 20 cubic meter digester to a 60 cubic meter digester could be the smallest economically profitable size. It all depends on local needs, costs, and the efficiency of the biogas system.
2) Heat the digester: Much of the engine's energy output goes out the exhaust pipe or is removed by the radiator water (or air cooling system) as heat. Only some of the energy released by the explosions in the cylinders is used to move the pistons. If the heat from the engine's exhaust and cooling system is used to heat the slurry in the digester--keeping the slurry at a constant temperature of 35 degrees centigrade (96° F) all day and all night--the digester should be able to double the gas production rate.
According to L. John Fry, one of the most effective ways to heat a digester is to have an engine fueled by some or all of the biogas with a pump to circulate the radiator water through the digester. An engine has about the same efficiency as a hot water boiler (50 percent) in converting energy into heat. The useful energy from the engine (about 25 percent of the original) can be profitably used to generate electricity or power to an endless variety of machinery.
A limiting factor indigester heating is that the hot water circulating through pipes in the digester should not exceed 55 degrees centigrade (130° F), since above that temperature the slurry will cake to the pipes and prevent the transfer of heat. This problem can be solved by burying the pipes just below the surface of the digester floor so that the heat can radiate through the floor into the digester slurry. The rule is that one square foot (929 sq. cm.) of pipe outside surface is needed for every 100 cubic feet ( 2.8 cubic meters) of digester space. These are minimum requirements; more pipe can and should be added to ensure sufficient heat transfer. Fry believes that two separate sets of pipe should be installed and set firmly in the floor. Should one set fail for any reason, there would be no necessity to clean out the digester to make repairs.
The digesters on Fry's farm were heated in this way, but the pipes were not laid in the concrete floor. They were laid against the inside walls of the digester floor. As a result, the pipes became displaced, bent, and ripped when the digester was cleaned. Black plastic pipe was used at first but later switched to steel pipe which clogged up after a few years. Next Fry tried copper pipe which he does not recommend because of its high price. Finally, he used galvanized iron pipe, passing the water through a water softener to prevent clogging the pipes and engine cooling system with mineral deposits.
There have been reports that burying the hot water pipes in the digester floor is not a very effective way to heat the slurry. When the heating pipes are laid on the digester floor, not in it, the digester gets the heat it needs, and the risk of the slurry caking on the pipes is minimal. If the pipes are buried in the digester floor, they should definitely be placed just below the surface. If the pipes are laid on the floor, they should be fastened to the floor so that they cannot move. One builder of biogas systems recommends using one inch diameter pipes raised a few inches above the floor of the digester.
Fry built the engine room next to the digester. The heat from the radiator water and the exhaust gas was used to maintain the digester temperature at the ideal of 35 degrees centigrade (96° F). The engine's exhaust gas was discharged against an outside wall of the digester. Rocks were packed against this wall and then covered with dirt. A thin layer of cement plastering was then used to cover the dirt and keep the heat in.
The engine design called for a maximum cooling water temperature of 60 degrees centigrade (140° F). This is also the maximum temperature pipes can be used in a digester. If a higher temperature is used, the slurry will cake on the pipes and prevent the transfer of heat.
A small 3/4-inch pump was installed and driven directly by the engine. It was run at slow speed (so that it would last longer) to circulate water between the hot engine and the cool digester. In the circuit was a 760 liter (200 gallon) insulated water tank that was used to keep the lines full at all times. The tank was also used as a means of cooling the water in order to prevent the digester temperature from rising above 35 degrees centigrade (96° F). (This water tank was not designed as a heat exchanger, but many engine/digester water systems do use heat exchangers.)
A thermostat controlled bypass valve is a device that automatically controls temperature; such a device must be attached to the water cooling system to control the temperature of the water which is heating the digester. L. John Fry operated this engine and digester combination day and night for six years, except for rare stoppages for cleaning and repairs.
Simple solar heat collectors that cost less than US$ 100 (VITA design) can be adapted to work with engines as a second source of hot water. In biogas systems that do not have engines, solar collectors can be used as the only source of hot water for heating the slurry. Digesters can also be heated in climates with cold winters or long rainy seasons by building greenhouses around them. There is more information on solar collectors and greenhouses in the New Ideas section of the Appendix.
Heating the slurry before it goes into the digester will make it easier to maintain an even digester temperature. If the temperature increases to 40 degrees centigrade (104° F), the biogas production rate will begin to swing wildly, sometimes up, sometimes down. If the digester temperature does get too high or too low, bring it back to 35 degrees centigrade very slowly, at the rate of one degree centigrade per day. Any fast temperature change can easily kill biogas bacteria.
The Chinese have had a lot of experience with biogas fueled diesel engines. The primary purpose of the Chinese engines is to run irrigation pumps. This section, edited and adapted from VITA's biogas literature files, is an account of some of that experience.
Dual-fuel means that internal combustion engines can use diesel fuel oil and another fuel, in this case of biogas and diesel oil. In practice biogas is the main fuel. Only a small amount of diesel oil is needed to ignite the compressed mixture of biogas and air. In case the supply of biogas is not sufficient to meet energy needs, the engine can be transferred quickly back to 100 percent diesel oil by an extremely simple method without the engine's performance being affected. (Switch back to diesel fuel before all of the biogas is used. It is dangerous to totally empty a biogas system because air could get into the digester and cause an explosion or kill the biogas bacteria.)
When diesel engines are used as dual-fuel engines, some fear that the engine might run rough because of the high compression ratios. But many experiments and much experience has shown that biogas has a good anti-detonation combustion property. Combustion detonation caused by pre-ignition has never happened. The engine may run smoother if the compression ratio is lowered, but then its power rating will also be lower. It is best not to change original compression ratios.
Biogas will burn readily when mixed with a proper quantity of air (5.0 to 15 percent biogas). The temperature of spontaneous combustion of the mixture of biogas and air is 814 degrees centigrade and the temperature at the end of the compression stroke is usually not over 700 degrees centigrade, which itself is higher than the spontaneous ignition temperature of diesel fuel. So the advanced injection of a little diesel fuel just before the end of the compression stroke is used to ignite the biogas and to assure the normal running of dual-fuel engines.
To insure the smooth running of the engines when they are switched back to just diesel fuel and to maintain engine performance, the advanced injection angle should not be changed. As the biogas supply temporarily falling short of demand is a common occurrence in rural areas due to maintenance and so on, it is very important to maintain the engine's ability to run on just diesel fuel oil.
The way to convert the intake system of a dual-fuel diesel engine is to install an extra mixing device at the rear of the air filter. This mixer consists of a valve controlling the quantity of biogas and a three-way pipe. Its principle is shown below in Diagram 16.
Two kinds of mixers, direct intake and cross pipe, are used with the single cylinder water-cooled diesel engines which are used in the rural areas of China. The direct intake mixer consists of cast three-way pipe (welded parts can be used), rear and front intake pipes with biogas valves. There are flanges at both ends of the three-way pipe connecting the intake pipe and air filter, respectively.
The cross pipe mixer is basically the same as the direct intake mixers. The principal difference between them is that there are several small holes drilled in the cross pipe, from which biogas flows and mixes with the air.
The starting procedure for a dual-fuel engine is the same as with an unconverted engine.
1) First, make sure the biogas choke is closed; then start the engine with just diesel fuel.
2) After starting, move the diesel throttle to the suitable position when the engine is under a load.
3) After normal running has begun, open the biogas choke slowly to let in the gas; by so doing the diesel fuel supply will automatically reduce under the action of the speed governor, and the engine will continue to run steadily.
4) If the engine takes in too much biogas, the diesel fuel may be cut off and an intermittent noise will be heard from the engine. If this happens, close the biogas choke a little until the engine runs normally.
Adjusting engine speed is done the same way as with unconverted engines that only use diesel fuel. That is, adjust the throttle handle only. In response to varying engine speeds or loads, the biogas choke should be slowly opened or closed to ensure smooth running and better fuel consumption rates. If the cooling water temperature is lower than it should be and the engine is under a light load or just idling, the efficiency of the fuel savings will be reduced. To stop the engine, close the biogas choke first; then shut the throttle.
Frequently check the biogas choke, pipes, and flanges for gas leaks to reduce the chance of fire. Operate the biogas choke gently; do not open it wide or close it suddenly. In order to avoid the frequent adjusting of the biogas choke that interferes with normal running and lowers the efficiency of fuel savings, the engine speed must be kept as steady as possible. In case there is not enough biogas or for other reasons the diesel fuel has to be used alone, close the biogas choke and the engine will operate again as a diesel engine.
Possible Problems and Their Remedies
Problem: Insufficient speed after starting, the engine cannot take on load, emits white or black smoke from the exhaust pipe and sometimes stalls.
· Cause: Overload of the engine or intake of biogas began too soon.
· Remedy: First let the engine idle for a while; then put on a load only after the engine is running normally. If it is necessary to start under load, diesel fuel should be used until the engine is running normally; then a gradual switch to biogas can be made.
Problem: Insufficient engine speed after starting and failure to take on load.
· Cause: Improper positioning of the governor control (throttle) handle.
· Remedy: Start with diesel fuel in the same way as with a purely diesel engine and then put on the load. Next, place the throttle handle at the proper position according to load and keep it in position; then introduce the biogas gradually.
Problem: After the engine has switched to biogas, its speed decreases or even stalls.
· Cause: The introduction of biogas has been too much, too soon.
· Remedy: Open the biogas choke slowly and let the biogas percentage of the fuel increase slowly. If an intermittent noise or knocking is heard, close the choke a little until the sound stops.
Problem: Occasional noise and knocking occurs when the engine is running.
· Cause: When the load is reduced during running, the engine takes in too much biogas.
· Remedy: When this happens, gently close the biogas choke some or increase the load until the sounds disappear.
When there is enough biogas, the load level is steady, and the engine is operating properly, the efficiency of diesel fuel savings in dual-fuel engines is from 75 to 95 percent. The main factors which affect the efficiency of fuel savings are:
1) a sufficient supply of biogas,
2) a skilled operator, and
3) maintaining a steady load on the engine with few changes or stops.
Biogas can be used in dual-fuel diesel engines for extended working periods without any early engine wear-out or serious corrosion. The only problem that might develop is fine cracks around the cylinder combustion chamber due to the local overheating on multicylinder engines which are equipped with swirl combustion chambers or pre-ignition chambers.
Biogas can be used for lighting in rural areas. But biogas lamps have a high gas consumption rate, therefore, from the point of view of better energy utilization, it is more economical and convenient to use biogas to generate electricity for lighting. In such a way, one cubic meter of biogas can light twenty-five 60-watt lamps for one hour, while one cubic meter of biogas can only light seven gas mantle lamps for one hour.
In rural areas it is very practical to include small internal combustion engines and asynchronous generators in biogas systems. There are many advantages, such as: simple structure, safe, easy to operate and maintain, few problems, and low installation cost. In China, asynchronous electric generator units of 3.0, 5.5, and 7.5 kilowatts have been put into mass production especially for use with biogas fueled engines.
While L. John Fry never had serious problems caused by the hydrogen sulfide in biogas, other people--especially in hot, damp tropical climates--have had problems. Engines run on biogas have been damaged by corrosion caused by hydrogen sulfide.
A simple and effective solution to this problem is in use in the Philippines. Just before the biogas goes into the engine, it is passed through a sealed box of rusty nails. The rust absorbs the hydrogen sulfide, and the engine escapes unharmed.
Another method of removing the hydrogen sulfide would be to have the pipe diameter doubled from a meter (3.0 feet) just before the engine. Inside the enlarged pipe loosely packed balls of rusty iron wire would absorb the hydrogen sulfide. The pipe size would have to be enlarged to keep the "all important" gas pressure from dropping. If the box method is used, loosely packed rusty wire--if it is available--would allow more hydrogen sulfide to come in contact with more rust and be absorbed.
Whatever method is used, the rusty iron would have to be changed on a regular basis when its ability to absorb hydrogen sulfide was used up.
In his book, Practical Building of Methane Power Plants for Rural Energy Independence, L. John Fry makes the following suggestions. "As an alternative to the expense and technology required to scrub the gas of carbon dioxide, hydrogen sulfide and moisture, the gas could be burnt to create steam to drive a steam engine (see the information on rankine-cycle engines in the New Ideas section of the Appendix)....Efficiency would be high and waste heat could be returned to the digester....The traces of hydrogen sulfide can also be filtered out by a sponge of saw dust mixed with ferrous oxide or iron filings from a machine shop [they should be rusty, not oily]...when the filter is saturated with hydrogen sulfide care must be taken when opening the lid since this gas is explosive and iron sulfides are known to heat even to the paint of burning when concentrated and then exposed to oxygen in the air."