State of the Art and Perspectives of CO_2 Methanation Process Concepts for Power-to-Gas Applications

摘要

In a highly interconnected energy system power to gas might play an increasingly significant role. Renewable electric energy can be transformed into a storable and save chemical energy carrier via electrolysis and methanation. The existing gas grid infrastructure can be used for energy storage and transportation. Thermochemical methanation means the conversion of H_2 and CO_2 at about 300 - 550 °Cusually with nickel-based catalysts. Thermochemical methanation processes can be realized in several reactor concepts. Mainly, these can be subdivided by the nature of the support (e. g. honey combs or pellets), the temperature control (e. g. isothermal or adiabatic) or the phases involved in methanation (two phase vs. three phase reactions). Another approach is the biochemical conversion. A microorganism serves as biocatalyst. The bioprocess takes place in aqueous solutions at temperatures between 40 - 70 °C. As limiting step, the transfer of hydrogen from gas phase into the liquid phase is named in literature. Hence, there are several reactor concepts under development aiming to reduce these mass transfer limitations. So far the stirred fermenter is the mostly used reactor. Mainly due to the higher process temperature and the resulting higher reaction velocity, thermochemical methanation requires much lower reactor volumes for a certain feed gas flow than biochemical methanation. However, full CO_2-conversion in a single step thermochemical methanation reactor cannot be achieved due to thermodynamic equilibrium limitations. Therefore, especially for small scale applications biochemical methanation can be advantageous. Both thermochemical and biochemical methanation processes will be figured in the paper. The main characteristics of the processes with advantages and challenges will be discussed as well as the state of development.