Tar reforming in biomass gasification gas cleaning: Dissertation

Thermochemical conversion of biomass can be used to produce synthesis gas via gasification. This synthesis gas can be further upgraded to renewable fuels and chemicals provided that the gas is ultra clean. To achieve this, impurities, such as light hydrocarbons and tar compounds present in the gasification gas can be converted to syngas by reforming. The amount of tar in gasification gas can be reduced already in the gasifier by using catalytically active bed materials. Typical bed materials in fluidized bed gasification are sand, olivine, dolomite and MgO. The tar conversion activity of dolomite and MgO were found to be high at atmospheric pressure. However, the activity was lost when the pressure was increased to 10 bar. Gasification gas contains, in addition to tar, ethene, which may contribute to further tar formation in high temperature zones of the process, especially at elevated pressures. Ethene forms tar compounds by radical chain reactions. The tar formed by thermal reactions of ethene resembles the tar from high temperature fluidized bed gasification, which contains mainly secondary and tertiary tar compounds. Carbon formation on the reformer catalysts presents a challenge in biomass gasification gas cleaning. The presence of sulfur in the gas, mainly in the form of H₂S, also complicates reforming. Typical catalysts used in the reformer after the gasifier are precious metal and nickel catalysts. The heat for reforming can be brought either indirectly in the case of steam reforming or by adding oxygen to the feed for autothermal reforming. Nickel and precious metal catalyst activities were analysed in experiments of around 500 hours with several different gas compositions. Catalyst deactivation was higher with steam than autothermal reforming. The use of catalytically active bed materials to reduce tar concentration already in the gasifier is especially favourable for steam reforming as the catalyst deactivation rate was decreased by the lower hydrocarbon content of the gas. Benzene, a highly stable compound, is a typical residual compound in the gas after the reformer. Thus, the reformer could be designed based on the reforming kinetics of benzene, for example in the production of synthetic natural gas. For this purpose, qualitative analysis of the effect of the main gasification gas compounds (H₂, CO, CO₂, H₂O) on reforming kinetics were studied with a nickel catalyst. Benzene reforming can be described by first order kinetics if the parameters are estimated for the specific gas composition.