Biogas Production | Specialty Gases | Coregas NZ

Biogas production

Biogas digesters, sludge digesters and gas collectors from landfill gas can be used to produce methane which is supplied to the natural gas distribution grid.

Wastewater and organic matter is converted to biogas through anaerobic digestion. Typical unit operations are:

  1. Municipal wastewater: is sent to an aerobic wastewater treatment plant.
  2. Aerobic wastewater treatment: plant treats the wastewater and produces sewage sludge.
  3. Anaerobic digestion: sewage sludge is mixed and anaerobically digested over 2-3 weeks. Agricultural, food processing, industrial organic and municipal waste can be added to this process to boost biogas production.
  4. Biogas storage: raw biogas is stored in biogas storage holders.

  5. Electricity production: raw biogas is sent to the generator to produce electricity which is distributed to the electricity grid.
  6. Biogas purification and distribution: raw biogas is dehydrated and impurities removed to produce biomethane. Biomethane is upgraded and injected into the natural gas network or may undergo liquefaction to convert it into bioLNG or compression for use as bioCNG.
  7. BioLNG and bioCNG: local, green and renewable fuel source available for commercial use.

In some cases, a dedicated crop such as maize or grass will be grown to feed the biogas reactor. If so, it is introduced to the process at stage 3 above.

Anaerobic sludge digesters are at the heart of biogas production

Biomethane differs from natural gas in many ways. It is CO2-rich and methane-lean. It also contains no higher hydrocarbons. It is therefore not as rich in energy as natural gas. If the biogas has been produced using domestic waste water, it is also likely to contain traces of ammonia, siloxanes and halogenated hydrocarbons which must be removed prior to introduction into the natural gas distribution grid.

If the energy source for the anaerobic reactor was a crop such as grass the production of ammonia still occurs, but the siloxanes and halogenated hydrocarbons are unlikely to be present in the biogas.


ComponentBiogasNatural gas
Other hydrocarbonsTrace5-10%
Carbon dioxide15-50%5-10%
Halogenated hydrocarbonsTrace0

Ammonia will cause corrosion of process equipment when it mixes with any traces of water present in the biogas or natural gas. It also forms pollutant oxides of nitrogen during biogas combustion. Ammonia is soluble in water, so it can be removed by scrubbing with water.

The presence of siloxanes in biogas causes abrasive siloxane deposits on valves and gas compression equipment, which causes erosion and reduces their life significantly. Removal can be adsorbed from the biogas using silica gel or activated carbon. Activated carbon can also be used to remove halogenated hydrocarbons.

Monitoring of the above impurities and measurement of the biogas methane and CO2 content is critical to ensure a high quality product and reduce the risk of corrosion and potentially dangerous toxic gas leaks. An online gas chromatograph with TCD detector that can measure the biogas composition within a very short (two to four-minute) cycle time is often used. Helium 5.0 grade is the most common carrier gas selected for this application although Hydrogen 5.0 grade may also be used.

A stainless steel two-stage regulator should be specified for the carrier gas. Two stages of pressure regulation will be required for this continuous flow application and stainless steel is recommended to protect the regulator from the traces of hydrogen sulphide gas that are inevitable around waste water treatment facilities.

The calibration gas mixture for rapid response the GC-TCD will be a blend that is designed to match the biogas composition. If this is a process control application an ISO/IEC 17025 certified gas mixture will be sufficient. If there is also custody transfer of the biogas taking place then an ISO 17034-accredited certified reference material should be specified to ensure traceable and accurate invoicing.

A single-stage stainless steel regulator will be appropriate for the calibration gas mixture.