The term ‘Green gas’ refers to methane, as a component of biogas. Biogas is produced by anaerobic digestion of biomass – a microbiologic process in which micro-organisms convert digestible biomass into a gas, consisting of methane and CO2. The remaining biomass – called digestate – can be applied as a green fertilizer. The reaction takes place in lukewarm water. For heating the water, part of the biogas is combusted.
For the ETM it is assumed that the digested biomass is a mixture of animal manure and cultivated maize, a common combination of raw materials in the Netherlands and surrounding countries. The maize is ensilaged after harvest to guarantee whole year availability of maize for biogas production. The used manure would normally have been stored in a manure silo or manure basement, in which it would be digested spontaneously which produces methane. The methane would in that case be emitted to the atmosphere, giving significant greenhouse gas emissions.
Biogas production is mainly a local and small scale activity, quite different by nature compared with the large capacity power plant considered in the ETM. In order to be able to utilize biogas in centralized large scale power plants the gas will have to be injected into the national gas grid and transported via this grid to a large scale power station. But for injection into the gas grid the biogas will have to have a composition and pressure similar to that of the natural gas normally transported via this grid. So the biogas is purified and methane is isolated (by scrubbing under pressure with water), dried and compressed. The electricity required for these processes is extracted from the power grid.
Considering the whole chain, ‘green gas’ has a far lower (± 85% lower) greenhouse gas chain emissions than fossil natural gas, but is not a completely climate neutral fuel. Use of nitrogen fertilizer in maize cultivation and use of electricity for biogas pretreatment are associated with significant greenhouse gas emissions totaling approximately 45% of the greenhouse gas emissions related to natural gas use. However, the avoidance of methane emissions from manure storage largely compensates these chain emissions.
The maximum amount of green gas that can be produced in the Netherlands is limited to approximately 40 PJ per year because of a limited amount of manure and limited availability of land for maize cultivation for biogas production.
The numbers and references used to quantify the fuel chain emissions for green gas can be found on our 'Reference Manager'. The Fuel Chain Emissions documentation can be downloaded from here (will open in a new tab).