What is biogas?
Biogas is a mixture of CH4 (40-80% and CO2 (60-20%) produced by the breakdown of organic matter in the absence of oxygen (anaerobic). Depending on the sauce of organics, landfill gas, digester gas or sewage gas, are also used.
In New Zealand, there are three major categories of industrial scale biogas production. First is the landfill gas, which is collected from several landfill sites at Auckland, Wellington etc. It is often collected by gas wells and used for power generation. Second is the sewage gas generated from the sludge at the sewage treatment plant. For small plants, the sewage gas is used mainly to maintain the digester temperature. For big plants, like Mangere, it is cleaned and used for power generation. The third category is biogas from industrial wastes, such as dairy and abattoir wastes.
CH4 is the main component of biogas and natural gas. CH4 in biogas is a carbon neutral energy source, therefore it is where value of biogas resides. Corresponding to the CH4 concentration, the energy content of 1 Nm3 biogas is around 6 kWh. In the biomethane production, CO2 creates value as well as it is separated and purified for industrial or agriculture use.
Besides CH4 and CO2, biogas often contains other impurities. First is water vapor. Biogas is often saturated with water vapor when it is produced. As it cools down along the gas system, condensate will gradually build up in the pipeline and cause blockage. Second is H2S, a poisonous and corrosive gas at low concentration. It is produced as protein is broken down, or from other sulphur rich minerals, such as gib board, sulphuric acid etc. For landfill gas, siloxane, N2, O2, CO and non-methane volatile organic compounds are commonly found as the landfill is not an enclosed environment.
A typical biogas composition is summarised in the table blow: