From carbon dioxide to methane
The Power-to-Gas Method
By this method, excess renewable energy is used to produce methane from the carbon dioxide generated by industrial sources such as the cement industry or biogas plants. In this process, methane only acts as an energy carrier. Its chemically stored energy is converted into electricity or heat when these cannot be provided by renewable sources, for example in winter. The process occurs in two stages: the "power", i.e. the fluctuating renewable electricity, is converted into the chemical energy of hydrogen by dissociation of water (see the solar-to-gas technology). Hydrogen is an efficient energy carrier, but is not compatible with today's natural gas infrastructure. To produce “renewable natural gas” (‘synthetic natural gas’), carbon dioxide and hydrogen are fed into a reactor, where, under the appropriate pressure and at the suitable temperature, they react with the help of catalysts and are turned into methane. If the carbon dioxide originates from sources such as waste gases from the cement industry or biogas plants, tiny impurities such as sulphur dioxide can penetrate the reactor and reduce the service life of the catalysts. Either complex filtration procedures or chemical processes with resistant catalysts are used to solve this problem.
The so-called conversion rate is an important factor in the production of methane (CH4) from carbon dioxide (CO2) and hydrogen (H2). In a chemical reaction, this rate defines how much of the starting material can be transferred to the product. In the case of methanogenesis, the question is: how much carbon dioxide can effectively be converted into methane? Until now, this rate was equal to 90 percent, which sounds like a satisfactory rate but is not sufficient if the methane is to be fed into the existing gas pipelines for distribution. The smallest impurities would attack and corrode the pipelines.
In order to solve this problem, Andreas Borgschulte and his team have searched for a method allowing the conversion rate in the reactor to increase to 100 percent. They have succeeded in doing so by adding a sorbent to the system. This agent absorbs excess water during the reaction, thus enhancing the conversion. The newly developed process not only produces purer methane, but also makes for milder reaction conditions. For example, while conventional reactors need to be pressurized, this is no longer necessary in the new ones, as the processes occur at normal ambient pressure, saving energy and money. For the construction of a reactor, expensive steel can now be dispensed with and replaced with cheaper aluminium. The team has built and put into operation a 1 kW reactor that makes use of the new sorption catalysts and displays an improved conversion rate. The method has now reached the technical level rated "Technology Readiness Level C" by the Swiss Federal Office of Energy (SFOE), which means that industry can now begin using prototypes involving this new technology for its own purposes.