Desulfurization and in situ tar reduction within catalytic methanation of biogenous synthesis gas

For the production of Raw-SNG (Substitute Natural Gas) from synthesis gas of allothermal biomass gasification beside the methanation reactions, also tar-cleaning and desulfurization must be considered. In this work the tar and sulphur containing synthesis gas of an allothermal bench-scale gasifier is used to investigate a process in which the three mentioned steps are done in a simple way, suitable for decentralized application. Therefore, for the step of desulfurization the use of metal-oxides in a simple fixed-bed adsorber is investigated. For the second and third step Nickel is not only used to catalyze the methanation-reaction, it is also used as catalyst for high-temperature hydrocarbon reforming. Both steps are carried out in one apparatus: In a polytropic reactor (reactor just cooled by thermal losses), the exothermic methanation-reaction leads to a temperature-rise in the reactor of up to around 500 degrees C. It is investigated, whether this rise in temperature is sufficient to achieve an in situ tar conversion. Due to strong chemisorptive bondings of sulphur components, remaining in the gas after the MeO-adsorber, as well as a result of the biomass-tar, an increased Ni-catalyst deactivation turns out to be disadvantageous. This work replies to the question whether this catalyst consumption results from remaining sulphur in the synthesis gas, or from carbon-depositions resulting from biomass-tar. Parameter-studies are done with the aim to reduce the mentioned catalyst-deactivation to an economically feasible level. In order to reach a synthesis gas to Raw-SNG-conversion near to the theoretic thermodynamic equilibrium, the overall reaction kinetics must be considered. It is determined which gas residence times are needed in order to reach the expected synthesis gas conversion. (c) 2013 Elsevier Ltd. All rights reserved.