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Methane Digesters 

flameWith gas and fuel oil prices on the rise, many people are tapping into the alternative energy market. Quite a few farmers and municipalities are turning sewage, manure, or garbage into fuel. What sort of fuel is it and what is the process that produces it? Well, the best answer is, its perfectly natural. When you bury garbage in a landfill, bacterial action decomposes and forms volatile gases. Over time those gases leak into the atmosphere and contribte to the greenhouse effect. Landfills in particular are being challenged to become emission free, by trapping and burning the "landfill gas". Landfill gas, or biogas, as it is now called, is composed of approsimately 65% methane, 30% carbon dioxide, and 5% other minor gases.
The process by which biogas is produced is anaerobic digestion; meaning without oxygen. This is not the same process as "composting" vegetation in your backyard. Composting requires aerating the mixture by turning; thereby exposing it to air (oxygen). The byproduct of this reaction is soil and predominantly carbon dioxide gas. Anaerboic digestion requires that the reaction take place in a container that is sealed or isolated from the outside air. The product is a large amount of methane, a little carbon dioxide, and a high grade fertilizer.
A simple digester is just a big balloon full of crap! Once placed in a sealed container, oxygen loving bacteria within the orginal product begins to break down the manure into simpler compounds; these are typically ammonia compounds. At this point, a fair amount of CO2 is given off. Once all of the oxygen is used up, other critters, ammonia loving bacteria, take over. The ammonia loving bacteria work to liquefy the material. Eventually, fodder for the a ammonia loving bacteria is used up and the next process begins; the production of methane. For this process to begin and proceed in a timely manner, the reaction requires two things, a neutral or slightly basic pH and an optimum temperature. A well buffered digester will be at around 8 to 8.5 on the pH scale. A digester is pretty much self buffering once it is started, but you can speed things up by starting it off on the right foot. The optimum temperature is 95 degrees F to 110 degrees F. While you will produce gas at lower temperatures, it will take a much longer period of time. Landfills don't begin producing methane gas until many years after they are sealed up.
There are different types of digesters; the two major types being batch and continuous feed. Each is exactly as its name denotes. The simplest is the batch digester and it can literally be as simple as putting some cow manure in a barrel and sealing it up. However, without adding some heat, there will be very little perceptible activity. Add a little heat in the form of a drum heater, as shown in the picture above, and voila - instant fuel. A little tubing and a few valves, and you have a simple digester. We have heard of running a small gasoline engine from a barrel typ type batch digester. However, it is very limited in the amount of methane it can produce. Typical life of a batch digester is around 45 to 60 days. At that point, you would have to dump the fully digested material and refill the container with fresh manure.
The most practical type of digester is the continuous feed digester, since it will provide a continuous output of gas instead of a large volume all at once. A small quantity of manure is fed into one end of the digester at regular intervals. With each feeding, the previous "slugs" are forced along the lenght of the digester until it comes to the outlet at the other end; sort of like a big instestine!
TAP Point:
When you plan for your digester, plan for a constant supply of manure and a way of disposing of the effluent. The continuous feed digester requires regular feeding and will produce lots of high quality fertilzer and lots of gas!
Most of the examples we see today are on the large corporate farms with sophisticated controls and people to tend the daily needs of the digester. What about the small farm? Well, a digester can be designed to be as large or as small as you like. Design it to suit your needs and budget.
How can I use the methane that I produce?
Methane can be burned to provide both heat and cold. Used in an absorption chiller, the heat produced by burning methane can be converted to cooling for buildings or processes. This is actually a very efficient system since for every BTU produced, you get a BTU back in cooling. No so with electric production. If you choose to produce electricity, the system efficiency is very low. This is mainly due to the fact that internal combustion engines are exremely inefficient, only about 25% so. Diesel engines are higher, around 35%. However, if you opt for a conventional engine generator set, then you are going to end up with a converted gasoline engine.
Using the gas directly for water heating is another very efficient use of the fuel. New hot water tanks are around 95% efficient. If you start off with a small digester, then plan for several independent usage methods. That way if demand is low, you can use all of the gas that you produce. If you have to store it by compression, then you will use a lot of teh gas you produce just to compress the rest. This will discussed in more detail later.
Another way of utilizing the fuel you make is to heat water to make steam and drive a turbine connected to a generator. Most back yard mechanics are not going to tackle this and industrial units are hideously expensive. Still, there are some interesting new micro-turbines being marketed today that would be perfect for small installation.
Each of these alternatives requires a constant supply of gas, but will not be able to tolerate oversupply. Correct sizing of the components is critical to maintain operation and to get the most from your unit. It is impossbile to accurately determine the rate at which gas will be produced because of all the variables - manure content, temperature, amount of water. If you have control over at least some of these parameters, you should be able to predict the flow of gas within a reasonable tolerance. However, you need to plan for a reserve that can act as a flywheel or capacitor and provide even flow to your engine/generator. 

As we discussed in Part 1 of this series on Methane Digesters, the proces of producing methane from animal manure is called anaerobic digestion (without air or oxygen). Anaerobic digestion occurs when organic material decomposes biologically in the absence of oxygen. The process must take place in the complete absence of oxygen and so the digester must be leak tight and should maintain a positive pressure inside of the digester to keep air out. If the vessel is to be pressurized then it must be able to withstand a certain amount of pressure.
While there are many different organic compounds that can be digested to produce Biogas, some are better than others. Straight cow, pig, or chicken manure will work very well and produce lots of usable fuel. The amount of biogas that is produced with each material is shown in the chart below:
Cow.........................................3-5 cu ft/lb of solids
Pig............................................6-8 cu ft/lb of solids
Chicken....................................6-13 cu ft/lb of solids
Use this information in conjunction with the TAP Methane Calculator to determine the output of a digester for various feed stock. This will help you properly size the digester and the componenets that will be used in conjunction with it so that things do not get out of hand. For given volume of feedstock, the calculator will determine:
1. The amount of gas that will be produced.
2. Energy potential of the gas.
3. Size of engine/generator set that will consume 100% of the gas.
4. Cost of the TAP methane generator system.
5. Estimated cost of the engine/generator set.
6. Value of electricity produced if the gas is consumed in an engine/generator set.
7. Value of propane if the gas produced is used to replace propane in a heating system.
8. Value of gasoline if the gas produced is used in an automobile.
Proper sizing of components will allow for efficient use of materials to prevent a "runaway" system.
As we stated previously, the process that makes methane out of manure is anaerobic digestion. It is not just one simple process, but a series of complex processes acting in concert with one another. At each step in the process, compounds are produced that are used in the next step of the process. Once started, if the proper care is taken, the digester will be "self buffering;" which means all of the processes within the digester will reach a state of equilibrium with one another. A well buffered digester will be able to correct small upsets and achieve equilibrium again with no intervention from the operator. The best practice is "patience". Rather than try to correct a poorly producing or stuck digester, the best practice is to leave it along and let if correct itself. Keep it warm and feed it regularly and it will pump out a continuous stream of gas.
When the digester is first loaded and sealed, there will be a lot of oxygen trapped in the digester housing. Before the process of methane production can start, the oxygen must go. This will be taken care of by the oxygen loving bacteria that reside wihtin the maure. The process is fermentation. If there are no leaks, then all of the oxygen will be consumed within the period of about 2 weeks. During that time, large quantities of CO2 (carbon dioxide) will be produced. This can be vented off or extracted and compressed for other uses. CO2 is a greenhouse gas and there may be credits or financial incentives given for trapping it.
During the initial phase of the anaerobic process, proteins, fats, and starches are broken down into simpler compounds by acid producing bacteria. These bacteria multiply rapidly and excrete the enzymes that break down and liquefy the material. One of the acids that will be produced is acetic acid. Most of the methane that will be produced will come from the conversion of acetic acid.
At this point the methane producing bacteria can develop. This phase of the process moves slowly and it can take some time before large quantities of methane are produced. Ina continuous feed digester, a balance is struck between the acid producing bateria and the acid consuming/methane producing bacteria. This balance is maintained by regular feeding of fresh material, in the right quantity. Over or underfeeding can slow down the process or bring it to a halt.
The digester must be kept at a constant temperature in the range of 90 degrees F to 110 degrees F. This is usually accomplished by circulating heated water through pipes running through the interior of the digester. The heat can come from a boiler fired by the methane being produced, solar collectors, or heat from an internal combustion engine. A stationary engine consuming the gas produced to generate electricity is the most common method. While not the most efficient use of gas, the internal combustion engine produces large mounts of waste heat that can be channeled back into the digester. This is a good example of an energy system where the individual components compliment and support each other. Additional information about applying engines with digesters to produce electricity will be discussed in future articles.
In a continuous feed digester, as material is added at the inlet, material is forced out of the outlet. The feed and drain can be arranged so that the material comes inot the digester below the level of the contained fluid to maintain a seal. I advocate using a mixing chamber on the inlet and pumping the material into the digester with a sludge pump. The incoming material should be the consistency of molasses. The bacteria will liquefy the material once digestion starts so do not add a lot of water. Too much water will increase the CO2 output. The incoming material should only be wet enough to pump or pour it.
The effluent coming off of the other end of the digester is a high quality fertilizer. It does not have the high odor like raw dung. SInce it is already broken down into simpler compounds it can be used directly by plants. Driend effluent can be bagged and sold for fertilizer or used as bedding for animals.
Biogas is approximately 65% methane, 32% CO2, and some other trace gases (nitrogen, hydrogen, carbon monoxide, oxygen, and hydrogen sulfide). The energy content is 600 BTU per cu ft. Compared to other gaseous fuels:

Biogas..................................................600 BTU/cu ft
Natural gas.........................................1050 BTU/cu ft
Propane..............................................2400 BTU/cu ft
Biogas can be used directly in gas range, gas furnance, gas hot water tank, gas space heaters, gasoline or diesel engines. Obviously the energy value needs to be considered. It would be difficult at best to use biogas as a substitute for gasoline in an unmodified engine. The low energy value would reduce the power and pre-ignition would occur because of the mismatch in the required compression ratio. Still, its not that it can't be used - it just won't have the same performance. A better alternative is to use it as a supplement in a diesel engine. This can be accomplished be feeding the gas right into the air intake of the engine. The governer will back off to reduce the amount of diesel fuel compensate automatically. A duel fuel alternative is a very good way to ulitize the gas you produce with very little added effort. TAP Energy will be experimenting with a simplified conversion kit for diesel engines.
bunsen burnerUsing the biogas directly for heating or cooking is the most efficient method of capitalizing on your new energy source. An example can be seen in the Bunsen burner pictured to the left. In a gas range, very little difference may be seen over natural gas and no modification is necessary. In a hot water tank, the heat cycle time will increase by 30%, but no modification is required to substitute biogas for natural gas. The trace of hydrogen sulfide inherent in biogas makes it easy to detect a leak. Use the same precations you would use if you were working with natural gas.

anerobic digester process



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