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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.
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.
===Refining the gas===
The gas can be further refined for use in gensets. The methane in the gas may also be seperated off to increase the potency of the gas.
====Seperating off carbon dioxide====
[[Carbon dioxide]] (CO<sub>2</sub>) 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===
Carbon dioxide can also be scrubbed out.<ref>Why is scrubbing desirable?</ref>
The ways of dealing with 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 CO<sub>2</sub> with sulfur - e.g. see [[Biogas CO2 scrubbing project]]
* In future, separation technologies such as a plastic molecular sponge may become available.<ref>[http://www.wired.com/science/planetearth/news/2007/10/bio_plastic Plastic 'Sponge' Could Help Biofuels Scrub CO2 From the Environment]</ref>
====Scrubbing out hydrogen sulfide====
{{Main|Biogas H2S Scrubbing}}
[[Hydrogen sulfide]]{{w|Hydrogen sulfide}} (H<sub>2</sub>S, 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.
====Seperating out methane from the biodigester====
?


===Production using biodigesters===
===Production using biodigesters===
Line 56: Line 81:
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.
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. 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.<ref>[http://journeytoforever.org/biofuel_library/MethaneDigesters/MD4.html Even methane gas only being used with stationary heat engines]</ref> Vehicle engines typically better use (compressed) methane, or [[Comparison of alternative ICE fuels|yet another type of fuel]].
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 economicallu intresting to buy a genset to generate electricity with it.
 
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.<ref>[http://journeytoforever.org/biofuel_library/MethaneDigesters/MD4.html Even methane gas only being used with stationary heat engines]</ref> Vehicle engines typically better use (compressed) methane, or [[Comparison of alternative ICE fuels|yet another type of fuel]].


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Table 1: some biogas equivalents (Source: adapted from Kristoferson, 1991.)
Table 1: some biogas equivalents (Source: adapted from Kristoferson, 1991.)
===Use in IC engines to produce electricity===
Most homes use portable gensets (which are IC engines combined with an alternator or dynamo) in the range of 400 watts – 5000 watts (1,4 to 8 HP). The gensets used for running on biogas are the same ones as those used for running onpropane gas or natural gas. They are simple gasoline (not diesel) engines. To determine the size of the genset 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 genset running for 10hrs. Gensets consume about 0.5-0.9 m³ of biogas to generate each KWhr. So, if you have a 2KW genset, 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 genset (HP), and may create problems (ie underproduction or overproduction depending on the appliances used in the home at a same time).
The portable genset 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.
==Costs==
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==
==Adoption in developing countries==
Line 81: Line 117:
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.
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.


==Scrubbing out unwanted gases==
==Notes==
===Carbon dioxide===
Biomass gasification is a distinctly different process. ''See [[Biomass gasification]].''
[[Carbon dioxide]] (CO<sub>2</sub>) is present in biogas. This reduces its performance as a fuel.<ref>Is that the reason scrubbing is desirable?{{sp}} That's just my best guess. --[[User:Chriswaterguy|Chriswaterguy]]</ref>


The ways of dealing with this are:
==See also==
 
* [[Biodigester]] designs
* 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 CO<sub>2</sub> with sulfur - e.g. see [[Biogas CO2 scrubbing project]]
* In future, separation technologies such as a plastic molecular sponge may become available.<ref>[http://www.wired.com/science/planetearth/news/2007/10/bio_plastic Plastic 'Sponge' Could Help Biofuels Scrub CO2 From the Environment]</ref>
 
===Hydrogen sulfide===
{{Main|Biogas H2S Scrubbing}}
[[Hydrogen sulfide]]{{w|Hydrogen sulfide}} (H<sub>2</sub>S, 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.
 
==Biomass gasification==
Biomass gasification is a distinctly different process. ''See [[Biomass gasification]].''


==References and resources==
==References and resources==
Line 108: Line 130:
* Fulford, David, [http://developmentbookshop.com/product_info.php?products_id=126 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.
* Fulford, David, [http://developmentbookshop.com/product_info.php?products_id=126 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, [http://completebiogas.com/ The Complete Biogas Handbook], revised 2007. A very extensive work. There are several chapters available for download.
* House, David, [http://completebiogas.com/ The Complete Biogas Handbook], revised 2007. A very extensive work. There are several chapters available for download.
==See also==
* [[Biodigester]] designs


== External links ==
== External links ==

Revision as of 10:33, 6 August 2012

Template:Lang

Template:Content from Biogas is a an artificially produced gas created from manure, plant material, dead animals (ie as found in kitchen refuse, ...).[1]. As the gas is man-made, it is differentiated from natural gas. The gas is produced trough the biological breakdown of the organic matter using microorganisms in the absence of oxygen.

Composition

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

Biogas is produced by means of a process known as anaerobic digestion. It is a process whereby 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 man-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.

The source materials for making biogas

Although, in theory work it is possible to only use vegetation, most biodigesters are operated on a mixture of manure and vegetation. 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” (material that is digestible to the bacteria and which becomes available for gas production have been gathered) will yield biogas that is more potent. 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.

Mixing the source materials

When combining the source materials, attention must be payed that the 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 seperate 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.

Refining the gas

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

Seperating off carbon dioxide

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

Carbon dioxide can also be scrubbed out.[4]

The ways of dealing with 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

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.

Seperating out methane from the biodigester

?

Production using biodigesters

Figure 1. Chinese fixed dome biodigester
Figure 2. Indian floating cover biodigester

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

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 aswell 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

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

  • the production of biogas or pure methane for use as a fuel.
  • the waste is reduced to slurry which has a high nutrient content which makes an ideal fertiliser; in some cases this fertiliser is the main product from the digester and the biogas is merely a by-product.
  • 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 economicallu intresting to buy a genset to generate electricity with it.

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.

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

Table 1: some biogas equivalents (Source: adapted from Kristoferson, 1991.)

Use in IC engines to produce electricity

Most homes use portable gensets (which are IC engines combined with an alternator or dynamo) in the range of 400 watts – 5000 watts (1,4 to 8 HP). The gensets used for running on biogas are the same ones as those used for running onpropane gas or natural gas. They are simple gasoline (not diesel) engines. To determine the size of the genset 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 genset running for 10hrs. Gensets consume about 0.5-0.9 m³ of biogas to generate each KWhr. So, if you have a 2KW genset, 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 genset (HP), and may create problems (ie underproduction or overproduction depending on the appliances used in the home at a same time).

The portable genset 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.

Costs

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

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.

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

Biomass gasification is a distinctly different process. See Biomass gasification.

See also

References and resources

Template:Reflist

  • 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.

External links

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