| | Uses | CO2 removal | H2S removal | Biogas as transportation fuel
Biogas consists of methane (CH4) and carbon-dioxide (CO2) along with some trace gases such as water vapour, hydrogen sulphide (H2S), nitrogen, hydrogen and oxygen.
Carbon dioxide and trace gases such as water vapour and H2S must be removed before the biogas can be used because:
- the hydrogen sulphide gas is corrosive
- water vapour may cause corrosion when combined with H2S on metal surfaces and reduce the heating value
Uses
Biogas is mostly used as a fuel in power generators and boilers. For these uses, the H2S content in biogas should be less than 200 parts per million (ppm) to ensure a long life for the power and heat generators.
Biogas can also be upgraded to pipeline natural gas quality for use as a renewable natural gas. This upgraded gas may be used for residential heating and as vehicle fuel.
When distributing the biogas using pipelines, Canadian oil and gas pipeline standards may become applicable. According to Canadian oil and gas pipeline standards, the H2S content shall not exceed 4.6 ppm at 0 C. The CO2 level should be lower than two per cent (CH4> 95 %).
Removing water vapour is easier than removing CO2 and H2S from biogas. A condensate trap at a proper location on the gas pipeline can remove water vapour as warm biogas cools by itself after leaving the digester.
Producing pipeline gas quality requires the use of advanced and expensive technologies. The cost of cleaning and producing pipeline quality gas (renewable natural gas) is $3 to 6/GJ and $6 to 12/GJ, respectively.
The following sections include brief information about these technologies as well as about using biogas as a transportation fuel.
CO2 Removal
Water scrubbing
Carbon dioxide is soluble in water. Water scrubbing uses the higher solubility of CO2 in water to separate the CO2 from biogas. This process is done under high pressure and removes H2S as well as CO2. The main disadvantage of this process is that it requires a large volume of water that must be purified and recycled.
Polyethylene glycol scrubbing
This process is similar to water scrubbing; however, it is more efficient. It also requires the regeneration of a large volume of polyethylene glycol.
Carbon molecular sieves
The carbon molecular sieve method uses differential adsorption characteristics to separate CH4 and CO2. This adsorption is carried out at high pressure and is also known as pressure swing adsorption. For this process to be successful, H2S should be removed before the adsorption process.
Membrane separation
There are two membrane separation techniques:
- high pressure gas separation
- gas-liquid adsorption
The high pressure separation process selectively separates H2S and CO2 from CH4. Usually, this separation is performed in three stages and produces 96 per cent pure CH4.
Gas liquid adsorption is a new development and uses microporous hydrophobic membranes as an interface between gas and liquids. The CO2 and H2S dissolve while the methane (in the gas) is collected for use.
H2S Removal
Biological desulphurization
Natural bacteria can convert H2S into elemental sulphur in the presence of oxygen and iron. This can be done by introducing a small amount (two to five per cent) of air into the head space of the digester. As a result, deposits of elemental sulphur will be formed in the digester. Even though this situation will reduce the H2S level, it will not lower it below that recommended for pipeline-quality gas.
This process may be optimized by a more sophisticated design where air is bubbled through the digester feed material.
It is critical that the introduction of the air be carefully controlled to avoid reducing the amount of biogas that is produced.
Iron/iron oxide reaction
Hydrogen sulphide reacts readily with either iron oxide or iron chloride to form insoluble iron sulphide.
The reaction can be exploited by adding the iron chloride to the digester feed material or passing the biogas through a bed of iron oxide-containing material. The iron oxide comes in different forms such as rusty steel wool, iron oxide pellets or wood pellets coated with iron oxide.
The iron oxide media needs to be replaced periodically. The regeneration process is highly exothermic and must be controlled to avoid problems.
Activated carbon
Activated carbon impregnated with potassium iodide can catalytically react with oxygen and H2S to form water and sulphur. The reaction is best achieved at 7 to 8 bar (unit of pressure) and 50 to 70 C. Activated carbon beds also need regeneration or replacement when saturated.
Scrubbing and membrane separation
As discussed in the section on CO2 removal, the CO2 and H2S can be scrubbed by water, polyethylene glycol solutions or separated using the membrane technique.
Biogas as Transportation Fuel
Using the upgraded biogas for automotives is similar to using natural gas. In Canada, there are about 20,000 natural gas vehicles in use. This transportation fuel can either be in the form of compressed natural gas (CNG) or liquefied natural gas (LNG).
When CNG is used as a transportation fuel it is generally referred to as natural gas for vehicles (NGV). Typical vehicles require modification to run on natural gas. The cost of modification is about $6,000 and depends on the size of the vehicle.
Certified natural gas compressors for refueling the vehicles (known as vehicle refueling appliances or VRAs) are commercially available. These allow refueling at home or work. Your local natural gas company may provide information on different brands of refueling equipment and equipment providers.
Advantages
- CNG burns cleanly
- CNG engines make less noise than do diesel engines
- nitrogen oxide emission is very low
- natural gas users can take advantage of on-site refuelling units located at their home or business
- CNG is less likely to cause contamination than is gasoline in the event of leak or spill
Disadvantages
- high cost ($3-6/GJ) to clean the biogas
- reduced driving range
- less cargo space
Summary
Traces of impurities are present in biogas. Removal of these impurities (such as water vapor, CO2 and H2S) is essential prior to using as fuel for various applications.
It is possible to upgrade biogas to pipeline-gas quality using the above discussed techniques. The upgraded biogas may be termed as renewable natural gas and has similar applications as natural gas.
References
NRCAN 2007. Natural Gas Applications.
IEA Bioenergy 2005. Biogas Production and Utilization
For additional information, check the following web pages:
Anaerobic Digesters Agdex 768-1
Anaerobic Digesters: Frequently Asked Questions, Agdex 768-2
Biogas Energy Potential in Alberta, Agdex 768-3
Integrating Biogas, Confined Feedlot Operations and Ethanol Production, Agdex768-4
Biogas Distribution – Rural Utilities Division of Alberta Agriculture and Rural Development .
Incentives for Biogas Production – Alberta Bioenergy Producer Credit Program
Prepared by
Mahendran Navaratnasamy – Agriculture Stewardship Division
Jim Jones – Bio- Industrial Development
Bruce Partington – Rural Utilities Division
Agriculture and Rural Development
Source: Agdex 768-5. June 2008. |
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