Natural gas is the cleanest fossil fuel and has an important role to play in the transition to a low carbon economy. It can help to balance the electricity system through flexible electricity production with natural gas or by converting excess power to gas in case of a surplus. Natural gas in turn can also be substituted by a renewable gas which is often referred to as green gas. An introduction to this topic is provided below, additional resources include:

• For a more extensive list of papers, see our publication database
• A selection of "top green gas papers"
• A wiki on various aspects of green gas

An introduction to renewable gas

Renewable gas can be produced through anaerobic digestion of biomass, gasification of biomass or through power to gas. Currently by far the largest amount of renewable gas is produced through anaerobic digestion.

Through anaerobic digestion, a renewable gas can be produced from any source of easily accessible organic material such as manure, sewage sludge, organic material in landfills and other ‘biological’ waste. Biomass is accessible if it has a low lignin content.

Animal manure is normally digested on farms where the animal manure is available. Since manure has a low energy content co-substrates (also known as energy crops) are used to increase the amount of energy that can be produced from a given production installation.

In sewage treatment facilities the incoming water is first treated aerobically, a process which results in a large amount of sewage sludge. This sewage sludge is then anaerobically digested. This is mainly done in order to reduce the amount of sewage sludge, at the same time this process results in a renewable gas which can in turn be used in the water treatment process.

A third method is the collection of landfill gas. On landfills organic material decomposes automatically, resulting in a gas which is normally referred to as landfill gas. This gas can be collected in order to prevent methane emissions. The process also results in a valuable by-product; renewable gas.
The renewable gas produced in these processes is normally referred to as biogas. Biogas does not have the same properties as natural gas as it naturally occurs around the world. The properties are dependent on both the production method and the organic material that is used to produce the gas. Biogas typically contains around 60% methane and 40% carbon dioxide. This means that biogas has a lower energy content than common types of natural gas.
The energy content can be increased by removing carbon dioxide. Biogas also contains a number of unwanted elements such as H2S, NH3, siloxane and excess water which have to be removed before the gas can be safely fed into the public natural gas network. The gas in turn is compressed to the right pressure so that it can be injected into the natural gas grid. At this point, the gas is normally referred to as green gas. Green gas that is fed into the public natural gas network proportionally displaces natural gas.

The gas can also be upgraded to pure methane, in this case it is normally called bio-methane.
In 2011 the total EU renewable gas production amounted to over 12 bcm (in L gas equivalent). Even though this is only a small contribution to EU total energy supply (0,6%) , the production volume has rapidly increased. Between 2006 and 2011 the total EU production volume has doubled .

The EU’s three largest producers combined (Germany, United Kingdom and Italy) produce more than 78% of total EU renewable gas production.

Next to anaerobic digestion of biomass there are two ‘new’ promising technologies through which renewable gas could be produced; gasification and power to gas. Both do not amount to a serious amount of green gas production yet, but do significantly increase the green gas production potential.

Gasification is a technology in which biomass is exposed to extreme temperatures (around 1000°C) and high pressure to partially combust the biomass. This process results in a gas which is called syngas. If this is upgraded to natural gas quality it can also substitute natural gas. The advantage of this technology is that it can also convert biomass with a high lignin content. This material is available in larger quantities and the material can more easily be transported.

A final method to produce renewable gas is through power to gas technology. Excess renewable electricity can be transformed into hydrogen through the electrolysis process. The hydrogen can in turn be converted into methane by adding carbon dioxide through the methanation process. Power to gas is a way to store excess (renewable) electricity.