Anaerobic digestion is the natural process of decomposition of organic matter in an oxygen starved environment. It occurs naturally in nature and releases methane gases into the atmosphere. When organic matter (such as slurry or grass silage) is put into anaerobic digester tanks it is possible to capture the methane gases produced and apply it to useful purposes such as meeting heating and energy needs. The remaining organic matter is called digestate and this can be used as an organic alternative to chemical fertiliser.
Biogas, is the “raw” gas produced through the anaerobic digestion process.
Biogas consists of around 50% to 60% methane with majority of the remaining gas being carbon dioxide.
Biomethane is biogas which has undergone post processing to clean the gas of impurities so that it is identical in terms of cleanliness and quality to natural gas used in Ireland today meaning it can be used directly in all applications where natural gas is currently the fuel of choice.
Renewable gas covers all gases of renewable and sustainable origin which can be injected into the gas network to replace natural gas. Biomethane is the most well established and matured of these but other potential sources do exist such as hydrogen and syngas. On this website renewable gas refers to biomethane.
How is renewable gas different from natural gas?
Natural gas is a finite resource of fossil fuel extracted from the ground.
Renewable gas is produced using a naturally occurring biological process and materials that can be regrown and replaced over a short time. It is sustainable, as greenhouse gas emissions which are created as the fuel is burned are offset by emissions which are avoided throughout the production of the gas. This makes biomethane low or zero carbon over its entire lifecycle.
From an end user’s perspective, no noticeable difference would exist in terms of application.
What is digestate?
Digestate is a by-product from anaerobic digestion. It is an excellent bio-fertiliser and organic soil improver that replaces carbon back into the soil. This helps to reduce the greenhouse gas, CO2, in the atmosphere, while regenerating the soil of grass or tillage lands. Digestate is more environmentally friendly than artificial fertilisers as it slowly releases nitrates and other nutrients into the soil, avoiding their run-off or leaching into water courses.