Comparison of different system layouts to generate a substitute of natural gas from biomass and electrolytic hydrogen

Abstract

The production of electrolytic hydrogen is considered among the best solution to mitigate the grid instability problems which arise from the widespread distribution of renewable energy sources, such as wind and solar. However, hydrogen is not easy to stock and distribute. Possible solutions are represented by its direct injection into the existing pipeline for natural gas distribution or its utilisation for the production of a substitute of natural gas. In this last case, which follows the so called approach of “power to gas”, a source of carbon is required. Preferably the carbon should come from biomass, since it can be considered “renewable carbon”.Starting from this idea, this study analyses two different approaches, depending on the grid power demand. In a first layout, biomass is gasified with electrolytic hydrogen to generate directly a methane rich syngas. After water condensation, the syngas is fed to a methanation process to convert almost completely carbon in methane.In the second layouts the biomass is gasified with electrolytic oxygen and the syngas is fed, together with other electrolytic oxygen, to a power unit, such as an internal combustion engine, a gas turbine or a high temperature fuel cells (SOFC). The exhaust gas from these power units is composed almost exclusively by carbon dioxide and water vapour. After water condensation, the carbon dioxide is fed together electrolytic hydrogen to a methanation process to obtain the substitute of natural gas.An overall best efficiency of roughly 74% is obtained when the plant is not connected to the grid. On the contrary, when electricity can be absorbed by the grid, best efficiency of 59.4% is reached utilising, as power unit, a SOFC fed at 6 bars.In all cases the input is low value energy (biomass and unstable electric power) and the output is high value energy constituted by a substitute of natural gas and stable electric power.