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Carbon-neutral gases as energy sources of the future

As carbon-neutral gases, green hydrogen, synthetic methane and blue hydrogen are energy carriers of the future. Their use can help to reduce greenhouse gas emissions in the building, transport and industrial sectors in an economically efficient manner. This means that these "green" gases play an important role in meeting climate protection targets.

Both hydrogen and synthetic methane will be crucial elements of the energy transition and will play a prominent role in the energy mix of the future. Hydrogen can serve as a clean energy source, e.g. for fuel cells. In the future, this technology could be used, for example, in locomotives, which are currently still around 40 percent diesel-powered in Germany. And alongside natural gas-powered vehicles, hydrogen and synthetic methane offer a useful supplement to electric cars, especially if used to power long-haul heavy goods vehicles. Synthetic methane could be used as a surrogate for natural gas and gradually make it greener, in other words turn it into a climate-neutral energy source in the long term.

Green hydrogen

Green hydrogen is a renewable gas which is produced by electrolysis in so-called power-to-gas plants. The source of the hydrogen is water, which is split into hydrogen and oxygen using electrical energy. If the electrical energy comes predominantly from renewable sources, the hydrogen is referred to as green according to Section 3 (10c) of the German Energy Industry Act (EnWG). Since the conversion process does not produce any CO2, the emission factor of green hydrogen is 0 g CO2/kWh. At present, there are three different technological variants of the electrolysis process, each with a different degree of technical maturity: alkaline water electrolysis, proton exchange membrane (PEM) electrolysis and high-temperature electrolysis. So far, alkaline water electrolysis and proton exchange membrane electrolysis are well established as low-temperature electrolysis and are used commercially. Alkaline water electrolysis is the more mature technology, which has been used for years in various applications. High-temperature electrolysis is still in the development phase.

The gas infrastructure and numerous end user applications, especially in the heating market, are already "H2-ready" up to a concentration of 10 vol.% (DVGW G 260/G262). In future, it will be possible to raise the hydrogen level in the gas grid to as much as 20 vol.%. The German Gas and Water Industry Association (DVGW) estimates that renewable or decarbonised hydrogen concentrations will reach 50 vol.% in the coming years. However, there are still some restrictions, especially in industry, but also in the transport sector or where gas is used to drive turbines.

Synthetic methane

The hydrogen produced by power-to-gas plants (electrolysers) can be converted into synthetic methane in an additional step by adding carbon dioxide (Sabatier process). Biogas can serve as a CO2 source, while other sources can be industrial processes and sewage treatment plants. The required CO2 could also be extracted from the air in the future (Direct Air Capture). The production of synthetic methane is carbon-neutral, and another option alongside the Sabatier process is biological methanisation, although this is not yet available on a large scale.

Since synthetic methane has the same combustion properties as fossil natural gas, it can be fed into the natural gas network without any quantity restrictions. In addition, there are no restrictions for its use in end user appliances.

Blue hydrogen

So far, hydrogen has been produced industrially, primarily by steam reforming from fossil natural gas (methane). The resulting product is referred to as grey hydrogen. If the CO2 produced during steam reforming is separated from the waste gas stream by means of Carbon Capture and Storage (CCS) and stored in geological structures and can be shown not to enter the atmosphere, the hydrogen produced is referred to as blue (greenhouse-gas neutral) hydrogen. Blue hydrogen is therefore a decarbonised gas.

An alternative to steam reforming in combination with CCS that is still under development is carbon-free hydrogen obtained from methane by pyrolysis. In the pyrolysis process, the carbon is removed as a solid and can thus be stored relatively easily or be used as a raw material, so that the emission factor is also close to zero. However, the process has not yet been established on the market and is only used on a research scale.

The gas infrastructure and numerous end user applications, especially in the heating market, are already "H2-ready" up to a concentration of 10 vol.% (DVGW G 260/G262). In future, it will be possible to raise the hydrogen level in the gas grid to as much as 20 vol.%. The German Gas and Water Industry Association (DVGW) estimates that renewable or decarbonised hydrogen concentrations will reach 50 vol.% in the coming years. However, there are still some restrictions, especially in industry, but also in the transport sector or where gas is used to drive turbines.

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