Biogas
Production of Biogas
Biogas is commonly produced by anaerobic digestion as part of the treatment of wet organic waste. This occurs in municipal wastewater and sewage treatment plants, industrial operations that have liquid wastes containing organic material, and on types of farms where animals are kept or held in a small area, such as pig or poultry farms.
Biogas is a mixture of mainly methane and carbon dioxide with very small amounts of hydrogen sulphide and other impurities. The methane content can range from 50% to 80% (on a volumetric basis).
The high amounts of carbon dioxide in biogas typically reduce the heating value to between 18 and 26 MJ/m 3 (GCV) compared with natural gas typically around 40 MJ/m 3 (GCV).
Sewage treatment plants are methane generators by the nature of the process. The gas can be used on site to produce electricity for local consumption or exported from the site. Plants at Christchurch and Auckland are good examples where both methane and natural gas supplies are used in generators at each site.
In many industrial and farming cases treatment of the waste to produce biogas is not economical in itself but is carried out for other reasons such as waste management. Also, small-scale generation of biogas is rarely economic because of the high labour requirements and dilute nature of the effluent being treated.
Biogas from anaerobic digestion can be used to produce heat for the digestion process itself, or for process heat and electricity in other parts of the plant. It can be upgraded to “natural gas” quality and fed into a local utility network. It can also be used directly as a fuel in a number of different types of plant such as reciprocating gas engines, mini-gas turbines, Stirling engines, and fuel cells or by direct combustion in boilers or other CHP heat plant.
Anaerobic digestion is a mature technology and is used worldwide, particularly for municipal waste water treatment. Here the scale of treatment can justify the costs of installing and operating the equipment needed. If the organic content of wet waste stream is too dilute, recovery of the energy content will be made more expensive. Excess moisture may cause handling problems for gasification processes.
Anaerobic digestion is essentially a continuous process so it requires a reliable continuous feed of material.
Publications on Biogas Production
Presentations from a Workshop held in 2004 in Christchurch are listed below:
- Opportunities for anaerobic digestion in New Zealand (Brian Cox, East Harbour Management Services
- Anaerobic digestion technology - today and tomorrow (J H Thiele,Waste Solutions)
- Biogas use in engines (D Wallace, AB Industries)
- Biogas, Solar and Ice Banks (I G Bywater, Engenius Solutions)
- Integrated Systems off-Grid Digesters (R Harlow, Integrated Energy Systems)
- High rate digesters for tight spots (J H Thiele, Waste Solutions)
- Potential for biogas recovery from anaerobic ponds in New Zealand (R Craggs, NIWA)
- Prediction of anaerobic pond biogas production using a computational fluid dynamic model (J Flemming, NIWA)
- Low costs, high rate digesters in the food industry (M C Board, Waste Solutions)
- The Sydney plant - integrated digestion of putrescible waste (C Hearn, Waste Solutions)
- Anaerobic digestion and carbon trading in SE Asia, challenges and opportunities (T Cohen, Waste Solutions)
- Experience with effluent treatment systems at Fonterra Tirau and other plant (H Archer, Beca)
Biogas Interest Group
The Bioenergy Association has established a Biogas Interest Group for discussion and shared activities. Membership of the Interest Group is by way of membership of BANZ. If you are a member then use your password to access the member's only area to participate in this group.
Events relating to Biogas
The Bioenergy Association is planning a workshop to be held in the Waikato in April 2008. Interested people should contact the Executive Officer on (04) 383 3359 or brian.cox@bioenergy.org.nz