Biogas is an environmentally-friendly, renewable energy source. Biogas is not to be confused with ‘natural’ gas, which is a non-renewable source of power. Biogas is produced when organic matter, such as food or garden waste is broken down by microorganisms in the absence of oxygen, in a process called anaerobic digestion. For this to take place, the waste material needs to be enclosed in an environment where there is no oxygen.
Biogas is not a new discovery. The bacteria that break down organic material into biogas are some of the oldest multi-celled organisms on the planet. The anaerobic process of decomposition (or fermentation) of organic matter has been happening in nature for millions of years, even before fossil fuels, and continues to happen all around us in the natural world. Today’s industrial conversion of organic waste into energy in biogas plants is simply fast-forwarding nature’s ability to recycle its useful resources.
The first human use of biogas is thought to date back to 3,000BC in the Middle East, when the Assyrians used biogas to heat their bath!
Why is Biogas important?
Biogas is knowns as an environment-friendly energy sources, because it alleviates two major environmental problems:
- Biogas generation recovers waste materials that would otherwise pollute landfills, thereby preventing the release of methane gas every day.
- Biogas usage does not require fossil fuel extraction to produce energy.
Biogas production
Anaerobic Digestion (AD) is an ideal circular solution to create biogas from organic waste which can be converted into renewable heat and power. Biogas requires pre-treatment to reduce the H2S content, the treatment requires active carbon filters and/or scrubbers. In addition, the biogas requires cooling to 3-5 °C to reduce the water content. After pre-treatment the biogas can be used in a Combined Heat and Power (CHP) unit for the co-generation of electric and thermal energy, or it can be upgraded to natural gas.
Composition
Biogas consists mainly of methane and carbon dioxide. It can also include small amounts of hydrogen sulfide, siloxanes and some moisture. The relative quantities of these vary depending on the type of waste involved in the production of the resulting biogas. The table below shows the composition of biogas produced in an AD tunnel.
| Gas | Volume |
| CH4 | 50-55% |
| CO2 | 35-45% |
| N2 | 0-3% |
| O2 | 0-2% |
| H2S | 100-3.000 ppm (dependent on waste composition) |
| H2O | saturated |
How is biogas used?
This raw biogas produced in this digestion process is first treated to turn it into “clean biogas”. This involves removal of any sulphur & siloxanes – The resultant clean biogas has a multitude of uses:
Biogas for Combined Heat and Power (CHP) units
When biogas is used directly in a CHP unit, electric and thermal energy are available in similar amounts. The electrical energy can be used directly on site, to e.g. power the fans and other related equipment. This allows a site to be self-sufficient. The thermal energy can be used for:
- Heating the fermentation tank liquid to optimize the activity of mesophilic bacteria.
- Heating the percolate before irrigation to promote a quick rise of the temperature of a fresh batch of waste.
- Heating of the technical AD corridor, to reduce heat losses from process related equipment.
- Heating the air inlet from aerobic tunnels to optimize the drying process and reduce moisture content.
Upgrade to natural gas
Biogas can be upgraded to natural gas by separating the methane (CH4) from the other gases. As natural gas it can be injected into the local grid (dependent on local regulations), or it can be used as LNG for fueling trucks. In addition to methane, CO2 is also an important component in biogas, about 35-45% of the total volume. This can be extracted and used for:
- Flushing of the AD tunnels at the end of the process, before emptying the tunnels, to prevent entering LEL-zones.
- Nearby greenhouses or other local businesses to optimize plant growth.
- Parties that use it in liquified form, such as soft drink manufacturers.