|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
Because biogas is not used at exactly the same rate at which it is produced, it must be stored somewhere. The gas must be efficiently transported from digester to storage tank to use if it is to be used in a simple, low-cost manner.
What should the gas be stored in and how big should the storage tank be? There is no one answer. Each answer depends on what the gas will be used for, how often it will be used, at what rate it will be used, local costs of locally available building materials and so on.
Using the digester as a combination digester and gas storage tank is not considered here because even though it can reduce construction costs, it also reduces the gas production rate.
Much of the literature on the subject of biogas storage tanks says that the storage capacity should be equal to one day's gas production rate.
Another possible guideline to use is that the gas storage tank(s) should be able to store enough gas so that:
· Sludge is not forced out of the overflow pipe to make space for the gas in the digester.
· Gas never bubbles out of a storage tank's water seal because there is no more room for gas.
· The storage tank is never so empty of biogas that the gas pressure drops below one and one-half inches of pressure.
I have tried two different types of biogas storage tanks: the floating tank method and the truck inner tube method. At the demonstration model scale, the two methods cost approximately the same, for the same amount of storage space. A large 12 x 2400 size truck inner tube can hold a half hour of biogas used for cooking. Car inner tubes can only hold five to ten minutes worth of biogas used for cooking purposes. To use an inner tube for gas storage, unscrew the pressure valve and clamp a ¼ inch plastic gas pipe onto the nipple.
Floating gas tanks and inner tube storage methods both have problems.
· If the floating tank is not kept straight by a center guide post and/or outside tracks, the gas (because it is under pressure) will force the gas tank to tilt and biogas will escape out of the bottom of the tank.
· The inner tube does not maintain gas pressure very well, and a stable gas pressure is very important. A large board must be placed on top of the inner tube and that one board must cover the whole inner tube. When concrete hollow blocks were placed on the board, the all important gas pressure was maintained, but only when the inner tube was relatively full.
· An inner tube or plastic bag cannot maintain gas pressure as well as a floating gas tank can. When the gas is used, the inner tube collapses under the weight of the hollow blocks, but the space inside the tube does not disappear as completely as it does when gas is used from a floating tank.
When pressure of the gas drops in a biogas system, the gas flame gets smaller and weaker, it can go out; then unburned biogas will escape, risking the danger of fire and explosion. It is best to use biogas between 7.5 cm/3.0 inches and 15 cm/6.0 inches of water pressure. Above that level and the biogas bacteria begin to find it hard to produce biogas; below that level and it becomes hard to maintain a flame. Adding up all the pluses and minuses, the separate floating gas storage tank is the method for gas storage recommended in this book.
Floating Tank Method
The floating tank method of gas storage consists of two parts: an open-top tank filled with water and another slightly smaller tank, with large holes in its bottom, floating in the water (see Diagrams 11 and 12). The floating tank, where the gas is stored, has a variable capacity to compensate for the differences between the rate of gas production and use. This variable capacity keeps the pressure inside the system constant. Biogas from the digester fills the gas tank, displacing water and pushing the gas tank up. When the biogas is used, water replaces the consumed gas and the gas tank falls.
The dimensions of the gas holder should be such that, when it is floating, no part should touch the water tank it is floating in. Guide posts should be provided so that the gas tank can move up and down without tilting or getting stuck.
· Guide posts and wheels can be placed in the space between the floating tank and the water tank, or a post or pipe can run through the middle of the floating tank.
· To prevent the gas tank from floating too high and letting gas escape, crossbars are located at a height such that when the floating tank is full of gas, its bottom remains eight inches below the tim of the water tank.
· The height of the cross bar above the top of the water tank should be eight inches less than the combined heights of the floating gas tank and the water tank. In this way, the pressure in the biogas system cannot go over 20 cm (8.0 inches) without the excess gas bubbling out of the water seal.
When the floating tank is not pressing up against the crossbar, the pressure in the system is only that which is necessary to raise the tank.
· If that pressure is less than four or five inches, put a concrete hollow block or two on top of the gas tank.
· Experience with the demonstration model system has shown that if the floating gas tank has room to tilt, the pressure of the gas inside it will force the bottom of the tank to move to one side or the other and biogas will escape in great big bubbles.
· In trying to keep the gas tank from tilting, do not stop it from being able to move up and down freely. If it happens while the gas is being used, the pressure will drop fast and the flame will go out. If the floating gas tank gets stuck when the gas is not in use, the pressure will build up very fast, forcing slurry out of the digester.
Biogas storage tanks can be made out of corrugated galvanized iron (GI sheets), concrete, fiberglass, plastic, etc. The following construction ideas were adapted from Practical Building of Methane Power Plants for Rural Energy Independence. These plans can be adapted to the building materials of your choice. The four guide poles can be replaced by one pole which runs through the center of the gas tank in small capacity tanks and perhaps in large tanks too, and the gas pipe coming in from the bottom can be replaced by one attached to the top of the gas tank, as long as it is attached in such a way that the flexible plastic pipe never even starts to fold.
The biogas storage tanks described here were made inexpensively from corrugated galvanized iron water tanks. All fittings for the gas pipes, guide poles, and bracing were standard plumbing items with the exception of the pulley wheels (rollers) which were mounted on brackets. The base of the brackets were wide enough to be bolted onto the tops of two adjoining corrugations in the tank. The concave roller fitted loosely around a one-inch diameter guide pipe.
An important feature of a gas tank is that it should move up and down freely, without too much friction. If more than one gas tank is used, each can have weights placed on it in succession so that the first to fill with gas will be the lightest. As the first one reaches its capacity, the upward movement will be stopped by the top of the gas tank pressing up against the crossbar. Gas will then flow to the next heavier tank and so on until the last tank is full, at which time any excess biogas will bubble out through the water seal.
Concrete hollow blocks make ideal weights to increase the gas pressure to 15 cm/6.0 inches of water pressure. At that pressure biogas will easily flow through the gas pipes for engine and household purposes.
Two tanks were made at a water tank factory:
1) The gas holder was 8.0 feet 9.0 inches in
diameter and 6.0 feet in height and made of 24 gauge, common-use corrugated
galvanized iron. One end, the top end when used as a gas holder, was 22 gauge.
The extra strength prevented the top from ballooning out when in use.
The bottom end floated constantly in water.
1.5) Since the bottom of the gas holder moved through the water when gas entered or exited, large holes were made to reduce friction. The whole bottom end was not removed in order to provide some lateral (sideways) strength and keep the rollers positioned firmly in relation to the guide pipes. The biogas pipe came up from the bottom through the largest of these openings.
2) The water tank was 9.0 feet, 8.0 inches in diameter, open at both ends, and 6.0 feet in height.
DIAGRAM 11: GAS STORAGE TANK DESIGN
-The height of the cross bar above the top of the water tank must be eight inches shorter than the combined height of the water tank and the floating gas tank. This will keep the gas pressure from getting dangerously high. When the pressure goes ever eight inches, the extra gas will escape from the bottom of the floating gas tank. If this happens often, gas is being wasted; build a second gas storage tank. Gas tank capacity should be about the same as digester capacity.
WARNING: A 58 gallon oil drum, cut down to be a gas holder, can only hold about 0.2 cubic meters of biogas.
The screw clamps need to be tightened very tightly to the plastic pipes and wrenches can tighten them tighter than screwdrivers can.
DIAGRAM 12: LARGE CAPACITY GAS STORAGE TANK DESIGN (Fry, 1974)
Construction Steps (see Diagram 12):
1) The floating gas tank was carefully measured and eight rollers were bolted on; first the four top rollers were placed one-quarter of the distance from each other around the circumference of the tank. The other four were bolted at the bottom and directly in line with the top ones. The tank was then positioned on level ground exactly where it was to be installed but raised off the ground for convenience in digging (explained later).
2) The four guide pipes were cut and threaded at both ends. The length was calculated as follows:
A) Since the tanks were each 6.0 feet the combined height had to be less than 12 feet.
B) 6.0 inches was deducted from each guide pipe. This 6.0 inches represented the upper limit of the gas pressure in the whole biogas system.
C) If the gas tank was to be any but the last holder in the series, a further 2.0 inches were removed. If the guide pipes were for the only gas tank, or the last of a series, the pipe length was 11 feet, 6.0 inches. Four more pieces of pipe, each 14 inches long, were then cut off the guide pipes and threaded at one end. The other end of the pipe was cut lengthwise for two inches in two cuts at right angles to each other. Each of the four pieces were then splayed (spread) out to form a footing for holding firmly in concrete. A coupling, sometimes called a socket, was then screwed firmly onto the short pieces and loosely to the long guide pipes.
3) The four guide pipes were then placed against the rollers and held in place by a piece of wire drawn around all four pipes and the tank. The wire was placed midway between the two sets of rollers and slight tension was applied. The tension was enough to bend the pipes inward by only 1/8 to 1/4 of an inch. Later on when construction was finished, this slight misalignment would provide enough play so that the rollers would not press to hard against the guide pipes.
4) Making sure that the gas holder was completely vertical, so that gas could not accidentally escape, the four guide pipes with their couplings and short lengths screwed on were placed on the ground. A small hole was dug around each splayed foot to a depth of 6.0 inches below ground level and a little concrete was poured into and around each.
Because there is always the chance that lightning might strike, it is strongly recommended, at this point in the construction, to drive a metal stake deep into the ground near one of the feet and bind the two together with wire to make a good electrical contact. This should also be done with digesters that are made of metal. L. John Fry did this on his farm and one of the tanks was indeed struck by lightning. Because the gas tank was grounded, no damage, explosions, or fire occurred.
5) With the floating gas tank in position, the gas pipe was then installed. The size of the gas pipe had to be proportional to the size of both the pipe used for daily sludge removal and the loading pipe of the digester, for the simple reason that when the digester is loaded, biogas is pushed into the gas holder very rapidly.
If sludge is removed through a 3.0 inch outlet, gas to replace it has to flow back to the digester. Since gas flows more easily than liquids, a pipe size of 2.0 inches for the gas pipe proved to be large enough. The 2.0 inch pipe was set in the concrete base of the gas holder with a 90 degree bend leading up to the same height as the top of the water tank. The pipe consisted of a threaded length leading in from outside and laid in a shallow, 2.0 inch deep ditch.
A slow bend (not an elbow joint) and a vertical length of pipe was used to bring the pipe level with the top of the water tank, a distance of about 6.0 feet, 4.0 inches from ground level. The open end of the pipe was kept from damaging the gas holder and also from shutting off the gas flow when the gas holder was in the down position by welding a small metal plate at right angles to it, one-half inch above the pipe opening.
The final positioning of the gas pipe consisted of laying it in the shallow trench so that the vertical portion passed freely through the largest of the openings cut in the bottom of the gas tank, without being in line to touch the tank at any point of its travel up and down. The gas pipe in the trench was checked with a spirit level to make sure that it was slightly off the level, so that any water condensation would lead away from the tank, to an outside condensation trap. Later, a little concrete was laid around the gas pipe to keep it in position until the concrete base was poured to hold it firmly in position.
6) When the concrete was set, the guide pipes were unscrewed at the couplings. This was done very gently to avoid moving the concrete. The guide pipes were marked for their respective final positions and laid on the ground. The gas holder tank was removed so it and the water tank could be painted. The outside walls of the tanks were painted to delay corrosion (the process of rusting). The inside walls of the tanks were painted with asphalt emulsion, otherwise the biogas would have stripped the galvanizing from the metal, causing corrosion from the inside.
One tank was not treated on the inside and was in use for over four years before repairs had to be made. The galvanizing had been completely removed by the gas, leaving bright steel which yellowed with rust within minutes of being exposed to air. This was further reason for coating the inside of the tanks, as well as the outside. Asphalt emulsion was used because of its relative cheapness and because no harmful fumes were let off while painting inside. A rule to remember is that corrosion is greatest at the line where water and air meet.
7) With the site clear except for the four short pieces of guide pipe with couplings and the gas pipe, the water tank was tested for fit by placing it over the four short guide pipe pieces. Boxing for concrete 6.0 inches deep was then made on all four sides, and the water tank was removed.
8) Concrete was then poured into the box 6.0 inches deep. Only the four couplings and the gas pipe were visible above the surface of the concrete. The outer (water) tank was immediately placed in position while the concrete was still wet and vibrated so that it sunk two inches into the concrete. A true vertical position was checked with a spirit level and the tank was propped with wood supports as necessary to hold the position while the concrete dried.
9) Final assembly: After a week to allow the concrete to slowly dry, three of the guide pipes were screwed into their respective couplings. The water tank was filled to the top. Planks of wood were laid across the top of the water tank and the gas tank was then placed on the planks in its designated position. The fourth guide pipe was then screwed into its coupling.
The four guide pipes were linked and cross-braced using ordinary pipe fittings and one-half inch pipe or railing fittings as used in pipe scaffolding. To ensure strength where high winds could damage the gas holder, guy wires went from the guide pipe structure to concrete hollow blocks a few feet from the base of the water tank. The gate valve on the 2.0 inch pipe was closed. Then by levering at a number of points, the planks were removed and the gas tank lowered to float in the water.
The gas holder was then left floating in the water tank for 24 hours to check for leaks. The next day, when the gate valve was opened, the air escaped and the gas holder slowly sank to its lowest point. The water tank was then topped off with water.
10) When first put into use as a gas holder, biogas displaced the water as it filled the gas tank, causing water to overflow the sides of the water tank until the gas tank began to float. After that, no more water overflowed and the gas tank rose and fell according to the flow of gas. More water does not need to be added to the water tank except to replace water that evaporates.
After four years of operation, sludge poured out from the overflow pipes of both digesters when the biogas went to one particular gas holder. The cause of this problem was that gas could not flow into the gas holder. The gas flow was switched to another gas holder and the pipes to the faulty one were taken apart. The metal gas pipes were found to be completely blocked with a black fur. It could be removed easily in the straight portion of pipe by rodding out, but this pipe had an elbow joint which prevented further cleaning and a plumber's snake drain cleaner had to be used to finish the job.
Caution: In the dismantling of metal biogas pipes--the rapid oxidation of iron sulfide deposits within the pipes may create heat or flame. In this respect, plastics pipes are safer.
The following paragraphs from the Guidebook on Biogas Development describe the use of concrete in making biogas digesters and biogas storage tanks.
Ferrocement is a composite material made of thin wire mesh layers inside a high quality cement mixture. It is a reinforced concrete, but unlike ordinary reinforced concrete, it has a high resistance to cracking. Ferrocement can be cast into sections as thin as 1.0 cm/½ inch. It is suitable for precast products because of its relatively low weight/high strength.
Ferrocement gas holders and digesters are:
· cheaper than mild steel gas holders and digesters of the same capacity,
· have lower thermal conductivity (will stay warmer longer),
· have a high resistance to corrosion (rust), and
· can be made in rural areas using local moderate-skill labor.
For manufacturing gas holders and digesters on a large scale, process equipment can be used to cast cylindrical ferrocement units. In this case, better control of the thickness and a higher degree of compactness may be achieved.
In bamboo-cement gas holders and digesters, the wire mesh is replaced by bamboo mesh, which is cheaper, lighter, and usually more available in many rural areas. Suitable coating should be applied to the inside and outside surfaces of both ferro- and bamboo-cement gas holders and digesters to improve their ability to be gaslight. It has been reported from India that bituminous (asphalt or tar) surface coatings have given satisfactory results. Ferro- and bamboo-cement gas holders and digesters should be tested for leaks before they are used and occasionally during their operation.
Where there are existing ferrocement skills, this technology can be tried. Otherwise it is best to learn the art first on structures where wall thickness, weight, and airtightness are less important. There is a complete set of instructions on the art of making ferrocement concrete structures in the Appendix.
In addition to methane and carbon dioxide, there can also be a lot of water vapor (steam) mixed with the biogas. This water vapor can be a problem. If enough of it collects as water in a low point of a gas pipe, the flow of gas will be stopped. Water vapor can rust metal pipes and it lowers the temperature of the biogas flame (the flame may look reddish). If at all possible, trap the water before it does any harm by using what are called water or condensation traps (water condensation is the changing of steam into water). There are several ways to do this, including four designs shown in Diagram 13.
For steam to condense into water, it has to become colder. A water trap that the sun shines on may work, but it will not work as well as it could. If the water trap is shaded from the sun, it will work much better. It is best to have more than one water trap in a biogas system. Two good places for a water trap are just outside of the digester and Just after the gas storage tank.
DIAGRAM 13: FOUR WATER CONDENSATION TRAPS
danger: some gas will escape with the water & if the valve is left open, all the biogas in the system will escape
Adapted from: Practical Building of Methane Power Plants for Rural Energy Independence.
The gas pressure gauge in Diagram 14 is a simple, homemade instrument for measuring how much pressure the gas is under within the biogas system. The gauge does not measure how much gas is in the gas storage tank and the rest of the system; it measures how tightly packed the biogas is. If the gauge reads one inch or less, one can safely guess that there is not much gas in the system. If the gauge reads seven inches or more, one can safely guess that there is a lot of gas in the system. In the appendix there is a description of a method for figuring gas volume, but it takes time and involves measurements and a math formula.
There is a real need for pressure gauge readings. Four to six inches are the best pressures at which to use biogas. Above eight inches the gas production rate goes down; if this happens often, a bigger gas storage tank is needed.
When the gas pressure drops below one inch, all use of biogas should be stopped before the gas stops burning on its own. When biogas stops burning because the pressure is too low, the gas will continue to come out of the pipe, and the danger of fire and explosion becomes real. A pressure gauge should be next to each and every use of biogas such as stoves and engines.
DIAGRAM 14: GAS PRESSURE GAUGE
The safety valve bottle on top of the pressure gauge is optional. The pipe must be left open if the safety valve is not included. If the bottle is used and if the pressure goes over ten inches, the gauge water will be saved.
To find out how much pressure the gas in the digester and storage tank is under, add the level of the colored water in both sides together. That combined number is for the gas pressure. For example: two inches on left + two inches on right = four inches gas pressure.
Why choose plastic (PVC: poly vinyl chloride) plastic pipes over metal pipes? Why choose clear plastic pipes over plastic pipes that cannot be seen through? Why choose expensive large diameter pipes over inexpensive small diameter pipes?
1) Plastics pipes are cheaper than metal pipes, they do not rust, and they can make turns without needing separate joint pieces (as long as the turns are gradual).
2) Clear plastic pipes can be checked for condensed water that could block the pipes.
3) PVC is a kind of plastic that is not as likely to leak as other kinds of plastic. All plastics eventually become hard, brittle, and crack; but some types age faster than others.
4) Small diameter pipes resist gas flow more than large diameter pipes do-this resistance lowers the all important gas pressure. If the price can be paid, use one or two inch diameter pipe, especially for long distances. Try not to use less than 3/4-inch pipe.
Plastic pipes have disadvantages, too.
1) If they touch something very hot, like a hot cooking pot, a hole can be melted very quickly.
2) Rats can bite holes in the pipes.
3) Unless clamped very tightly to metal pipes, there will be a gas leak.
4) If a plastic pipe is bent too sharply, it will develop a fold which the biogas will have a hard time getting past. The fold will decrease gas pressure and will be a point where a leak can easily develop.
Sometimes when PVC plastic pipes are bought, there are already folds in the pipe, and Just straightening the pipe does not get rid of the fold. There is an easy solution. Wrap a piece of cardboard around the spot and tie it very tightly with string, forcing the pipe to be round again. After a few months the cardboard can come off and the plastic pipe should stay round.
If the plastic pipe does not fit tightly over a metal pipe, wrap plastic tape around the metal several overlapping times, then push the plastic pipe on it (it should never go on too easily) and clamp the plastic pipe tightly to the metal using the type of clamp shown in Diagram 10. When the digester is in operation, check all Joints for gas leaks by smelling them. Sounds funny, but it works. The less than one percent of biogas which is hydrogen sulfide smells like rotten eggs.
A tool that makes a good temporary clamp for plastic gas pipes is the locking doctor's clamp called a forcep. One or two forceps can be used to seal a gas pipe while changes are being made, without damaging the plastic pipe. The locking type of adjustable vise-grip pliers were found to be very useful for working on metal pipes and gate valves.
Hot, direct sunlight and sharp changes in temperature shorten the useful life of plastic pipes. Soft PVC plastic, depending on conditions, could last three to ten years before becoming brittle and leaking. It will last longer if it is covered or in shade than it will in direct sunlight. A new kind of PVC plastic called red mud plastic is now being sold. It does not age as fast as ordinary plastic does, and it is supposed to last 20 to 30 years. Until red mud plastic is available, hide plastic pipes from the sun as much as possible. One way to do this might be to run plastic pipes through bamboo. There is more information on red mud plastic in the Facts and Figures section of the Appendix.