Figure 1. Chinese fixed dome biodigester

Biogas is a mixture of about 50-75% methane, 25-50% C02, 0-10% Nitrogen and 0-1% Hydrogen. The gas is created from anaerobically decaying organic mater, such as manure and plant material.[1] As the gas is man-made, it is differentiated from natural gas which is formed far beneath the Earth's surface and is mainly composed of methane.

Composition[edit | edit source]

The gas is composed of

  • methane: 54 – 70%
  • carbon dioxide: 27 – 45%
  • nitrogen: 0.5 – 3%
  • hydrogen: 1 – 10%
  • carbon monoxide: 0.1%
  • oxygen: 0.1%
  • hydrogen sulfide: traces[2]

Production[edit | edit source]

Biogas is produced by means of a process known as anaerobic digestion, where organic matter is broken down by microbiological activity and, as the name suggests, it is a process which takes place in the absence of air. It is a phenomenon that occurs naturally at the bottom of ponds and marshes and gives rise to marsh gas or methane, which is a combustible gas.

There are two common human-made technologies for obtaining biogas, the first (which is more widespread) is the fermentation of human and/or animal waste in specially designed digesters. The second is a more recently developed technology for capturing methane from municipal waste landfill sites. The scale of simple biogas plants can vary from a small household system to large commercial plants of several thousand cubic metres. Large commercial biodigesters (ie fed with animal feces from animal husbandry farms) are typically a lot more efficient than domestic biodigesters, as manure from cows (and similar animals) contains a lot more volatile solids than from humans. Also, there is the issue of the small quantity of feces and plant matter available in domestic systems, meaning that a lot less gas is produced making it a less obvious method for certain purposes as producing electricity.

The source materials for making biogas[edit | edit source]

Although, in theory it is possible to only use vegetation, most biodigesters are operated on a mixture of manure and vegetation. Starting digestion for vegetation is much more difficult compared to manure which readily digests. This, as by including manure, there is a continuous fresh input of the microbiological organisms, which is needed for the operation of the biodigester.[3] The microorganisms used in most biodigesters are thus the same as those found in the manure used. Besides plant material ie from gardens; grains and deoiled cakes (ie Jatropha, Pongamia,...) are sometimes added.

The type of manure used is also of great importance. Especially manure that has a large amount of "volatile solids" (VS) (material that is digestible to the bacteria and which becomes available for gas production have been gathered), in relation to the amount of total manure, (VS) will yield biogas. Cow manure is very high in volatile solids, horse, pig, human and chicken manure is far rich in volatile solids. The exact number (and quantity of produced manure/day are:

  • cow: drops an average of 52 lbs. of feces a day, of which about 10 pounds are solids, the rest being water. Of the 10 pounds of solids, 80% or 8 lbs. are volatile and can be turned into gas.
  • horse: produces an average of 36 pounds of feces a day, of which 5.5 lbs. are volatile solids.
  • pig: produces 7.5 lbs. per day of which 0.4 pounds are volatile solids.
  • human: produces 0.5 pounds of feces a day of which 0.13 pounds is volatile solid.
  • chickens produce 0.3 pounds a day which 0.06 pounds is a volatile solid.

Household kitchen waste has the potential of producing enough biogas to cook three meals a day for a family of four and still have enough left over to heat some water for washing up. (See the link to ARTI below.)

Mixing the source materials[edit | edit source]

With the wet digestion process, when combining the source materials, attention must be payed that a suitable carbon/nitrogen (C/N) ratio is attained (similar to regular composting). The process wants one part nitrogen to every 30 parts of carbon. Manure is nitrogen rich, averaging about 15 parts carbon for each part nitrogen, so all the studies show that gas production is substantially increased by including some carbon material along with the manure. The nitrogen proportion may be even higher in animal waste if urine is included with the feces. It is thus advisable to separate the urine from the feces and only use the latter.

To illustrate, straight chicken manure will produce only five cubic feet of gas for each pound of manure, but chicken manure mixed with paper pulp will produce eight cubic feet of gas for each pound of manure used. Cow manure will produce only 1.5 cubic feet of gas per pound, but cow manure mixed with grass clippings will produce 4.5 cubic feet of gas per pound of manure.

Another method exists called dry digestion. This process uses no manure at all and is thus more suitable for certain applications in which no manure needs to be processed. See anaerobic digestion.

Refining the gas[edit | edit source]

The gas can be further refined for use in gensets. The methane in the gas may also be separated off to increase the potency of the gas.

Separating off carbon dioxide[edit | edit source]

Carbon dioxide (CO2) is present in biogas. This reduces its performance as a fuel. The specific gravity of methane is about 0.55 in relation to the weight of air, so it rises, as does hydrogen. Carbon dioxide on the other hand is twice the weight of air. Within a vertical gas container, if the gases are allowed to settle, they will naturally separate themselves, the flammable gases rise to the top. This fact suggests that a good design should have a petcock at the bottom of a vertical gas holder. Use it to bleed off the accumulated carbon dioxide.

Scrubbing out carbon dioxide[edit | edit source]

Carbon dioxide can also be scrubbed out.[4]

The ways of doing this are:

  • Accept the lower performance - it will still do the job, and you'll save a lot of hassle. This may be the best option for very small applications.
  • Scrub the CO2 with sulfur - e.g. see Biogas CO2 scrubbing project
  • In future, separation technologies such as a plastic molecular sponge may become available.[5]

Scrubbing out hydrogen sulfide[edit | edit source]

Hydrogen sulfideW (H2S, also called "rotten egg gas") is a common product of anaerobic digestion. It causes an unpleasant odor, and in high enough concentrations can be highly poisonous. (Note that it numbs the sense of smell long before it becomes fatal - so if you can smell it, it's not deadly yet.)

Hydrogen sulfide is corrosive and renders some steels brittle, meaning that if there is any significant quantity, it is important to remove it before the gas passes through any equipment, especially iron or steel equipment. (What about other materials? It's only very weakly acidic, so it's seems to not be the acidity that causes the problem with steel.

Production using biodigesters[edit | edit source]

Figure 1. Chinese fixed dome biodigester
Figure 2. Indian floating cover biodigester
A small-scale, DIY, digester

Two popular simple designs of digester have been developed; the fixed dome biodigester (ie Chinese fixed dome digester) and the floating drum biodigester (ie Indian floating cover biogas digester). The mentioned examples are shown in figures 1 & 2. The digestion process is the same in both digesters but the gas collection method is different in each. In the floating cover type, the water sealed cover of the digester is capable of rising as gas is produced and acting as a storage chamber, whereas the fixed dome type has a lower gas storage capacity and requires good sealing if gas leakage is to be prevented. Both have been designed for use with animal waste or dung.

The waste is fed into the digester via the inlet pipe and undergoes digestion in the digestion chamber. The product of the process is a combination of methane and carbon dioxide, typically in the ratio of 6:4. Digestion time ranges from a couple of weeks to a couple of months depending on the feedstock and the digestion temperature. The residual slurry is removed at the outlet and can be used as a fertilizer.

Insulation[edit | edit source]

The temperature of the biogas production process is quite critical. Methane producing bacteria operate most efficiently at temperatures between 95°F and 100°F (or about 35°C). In colder climates heat may have to be added to the chamber to encourage the bacteria to carry out their function. In places where the temperature drops below 5°C (40°F) in winter, about 20% of the gas generated will be needed for heating the digestor and maintaining the digesting material. As such, proper insulation is needed.

The insulation would need to be applied entirely around the tank as well as below it (between the tank and the ground). This, as the temperature of the ground several feet below the surface remains relatively constant at 50°F – 55°F, hence acting as a heat sink deriving warmth from the tank. It is possible to dig the tank into the ground (to reduce cooling from the wind), yet it is best to keep a space open between the soil and the tank itself.

Uses[edit | edit source]

The digestion of animal and human waste to biogas has several uses:

  • the production of biogas or pure methane for use as a cooking fuel.
  • the waste is reduced to slurry which has a high nutrient content which makes an ideal fertilizer; in some cases this fertilizer is the main product from the digester and the biogas is merely a by-product. It is referred to as Biol.
  • during the digestion process bacteria in the manure are killed, which is a great benefit to environmental health.

Biogas is a well-established fuel for cooking and lighting in a number of countries, whilst a major motivating factor in the development of liquid biofuels has been the drive to replace petroleum fuels.

Small-scale biogas digesters usually provide fuel for domestic lighting and cooking. These are small units, and generate just a few cubic meters of biogas every day, providing enough cooking gas for a whole family. When biodigesters are used that generate more than a few m³ of biogas, it is economically intresting to buy a genset to generate electricity with it, though is not nearly as an efficient use as cooking fuel.

It can be used as a fuel for running heat engines as well, however it is a lot less potent than comparable gases (ie pure methane), it is generally only used with stationary heat engines.[6] Vehicle engines typically better use (compressed) methane, or yet another type of fuel. Biogas can be used in diesel engines by introducing it into the air which is injected to the engine for combustion, this allows for less diesel gas use.

Table 1: some biogas equivalents (Source: adapted from Kristoferson, 1991.)
Application 1 m3 biogas equivalent
Lighting equal to 60 -100 watt bulb for 6 hours
Cooking can cook 3 meals for a family of 5 - 6
Fuel replacement 0.7 kg of petrol
Shaft power can run a one horse power motor for 2 hours
Electricity generation can generate 1.25 kilowatt hours of electricity

Use in IC engines to produce electricity[edit | edit source]

Most homes use portable gen-sets (which are IC engines combined with an alternator or dynamo) in the range of 400 watts – 5000 watts (1,4 to 8 HP). The gen-sets used for running on biogas are the same ones as those used for running on propane gas or natural gas.[7] They are simple gasoline (not diesel) engines. To determine the size of the gen-set you need, you first need to examine your current electricity bills. Find out the daily power consumption. If you are consuming 20KWhr per day, then you need a 5KW genset running for 4 hours or a 2KW gen-set running for 10hrs. Gen-sets consume about 0.5-0.9 m³ of biogas to generate each KWhr. So, if you have a 2KW gen-set, it will consume about 1.2m³ of biogas each hour. So, if you run it for 10 hours then you will need 12m³ of biogas – the amount of gas generated by the fresh manure of about 20 cows. Most systems use a battery pack to store the power generated by the genset. Not using a battery at all is possible, but would require exact dimensioning of the gen-set (HP), and may create problems (ie underproduction or overproduction depending on the appliances used in the home at a same time).

The portable gen-set should always be started on gasoline, and then run for a minute on gasoline before switching over to biogas. Once it has warmed up, the engine will run entirely on biogas. The gas consumption of gasoline engines is about 60% that of diesel dual-fuel engines, so you get more power out of biogas using gasoline engines. The genset should be switched to gasoline mode while shutting down as this helps in flushing the engine of biogas.

Advantages and Disadvantages[edit | edit source]

Advantages [verification needed]

  • Reduces indoor air pollution and deforestation
  • Less time spent collecting wood as well as cooking if using gas for cooking
  • Very little operation and maintenance necessary
  • Reduces emissions contributing to climate change

Disadvantages [verification needed]

  • May not produce enough energy
  • Can only replace gas not other types of energy
  • Can be hard to store
  • Hard to get good results in colder climates

Costs[edit | edit source]

Biogas plants will cost between $150-200/m³ depending on their size and material of construction. Government grants and loans are available in most places for construction. The genset will cost between $700-1500. Other components you will need are monitoring equipment, like a gas meter, pressure meter, flowmeter, KWhr meter, current meter, and a voltage meter. A pH meter and thermometer will also be required to monitor the digestion in biogas plant.

Adoption in developing countries[edit | edit source]

Some countries have initiated large-scale biogas programmes, Tanzania being an example. The Tanzanian model is based on integrated resource recovery from municipal and industrial waste for grid-based electricity and fertiliser production.

Small-scale biogas production in rural areas is now a well-established technology, particularly in countries such as China and India. At the end of 1993, about five and a quarter million farmer households had biogas digesters, with an annual production of approximately 1.2 billion cubic metres of methane, as well as 3500 kW installed capacity of biogas fueled electricity plant. In India, the development and dissemination of gasification technology has been widespread and is used to meet a variety of rural energy needs - for example, irrigation pumping and village electrification.

Kenya relies on imported petroleum to meet 75% of its commercial energy needs. In 1980, in an effort to reduce this high level of dependence on an externally controlled fuel source, the Kenyan government set up the Special Energy Programme (SEP). One aspect of the programme was the introduction and dissemination of biogas plant technology. After a poor start working with educational institutions, the programme turned to local artisans and commercial outlets working in the private sector. Hands-on training was given to masons and plumbers and private traders were encouraged to manufacture and stock appliances such as cookers and lights. By 1995, the number of plants installed in Kenya was estimated to be 880.

Notes[edit | edit source]

Biomass gasification is a distinctly different process. See Biomass gasification. Bio Diesel is also a distinctly different process.

See also[edit | edit source]

References[edit | edit source]

Bibliography[edit | edit source]

  • Anonymous (Office of the Leading Group for the Propagation of Marshgas), A Chinese Biogas Manual, 1981. A classic work on biogas production in China, showing construction of small-scale, underground digesters.
  • Gunnerson C. G. and Stuckey D. C., Anaerobic Digestion - Principles and Practices for Biogas Systems. World Bank Technical Paper No 49, The World Bank, 1986. A good overview.
  • Gitonga, Stephen, Biogas Promotion in Kenya. Intermediate Technology Kenya, 1997.
  • Fulford, David, Running a Biogas Programme: A Handbook, Practical Action Publications, 1988 (being updated). Offering good information about the management of regional or country-wide biogas programs, and good technical information about the design of burners for biogas.
  • House, David, The Complete Biogas Handbook, revised 2007. A very extensive work. There are several chapters available for download.
  • Ravindranath, N. H. and Hall, D. O., Biomass, Energy and the Environment: A Developing Country Perspective from India. Oxford University Press, 1995.
  • Karekezi, S. and Ranja, T., Renewable Energy Technologies in Africa. AFREPEN, 1997.
  • Kristoferson L. A., and Bokalders V., Renewable Energy Technologies - their application in developing countries. ITDG Publishing, 1991.
  • Johansen, T.B. et al, Renewable Energy Sources for Fuels and Electricity. Island Press, Washington D.C., 1993.
  • Gunnerson C. G. and Stuckey D. C., Anaerobic Digestion - Principles and Practices for Biogas Systems. World Bank Technical Paper No 49, The World Bank, 1986.
  • Gitonga, Stephen, Biogas Promotion in Kenya. Intermediate Technology Kenya, 1997.
  • Stassen, H.E., Small-scale biomass gasifiers for heat and power: a global review. World Bank technical paper no. 296, Energy Series 1995.
  • Quaak, P., Knoef, H. and Stassen, H.E., Energy from biomass: a review of combustion and gasification technologies. World Bank technical paper no. 422, Energy Series 1999.
  • Anderson, T., Doig, A., Rees, D. and Khennas, S., Rural Energy Services: A handbook for sustainable energy development. ITDG Publishing, 1999.

External links[edit | edit source]

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Keywords biogas, fuel, energy
Authors Chris Watkins, KVDP, Matt Byrne
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Created April 2, 2007 by Chris Watkins
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