Producing simulated tar-laden gasification gas by ethane pyrolysis

One of the key issues in biomass gasification is the presence of tars in the product gas. Tar causes problems in downstream equipment by fouling and coke formation and thus, it needs to be removed. Tar model compounds, like naphthalene, are typically used in lab-scale gas cleaning tests to represent tars. Tar is nevertheless a more complex mixture of aromatic compounds and it would be highly beneficial for lab and bench-scale gas cleaning studies if a more realistic tar mixture could be used in testing. More reliable results could be obtained in hot gas cleaning studies regarding e.g. catalyst long-term stability or coke formation on the catalyst. Pyrolysis of ethene was found to provide a solution for generating a tar mixture better resembling real biomass gasification tar. Thermal reactions in ethene pyrolysis produce aromatic compounds, light hydrocarbons and also soot. The concept was first tested in lab-scale in varying conditions with ethene concentrations between 1 - 5 vol-% in nitrogen. The next step was to combine the generation of tars with the production of the main gasification gas compounds. This was carried out in bench-scale HOTPURI reactor by steam reforming/partial oxidation of natural gas and simultaneous ethene pyrolysis. Natural gas, ethene, steam and oxygen were used as feed gases. The gas generated in the HOTPURI reactor contains the main gasification gas components (CO, H2, CO2, CH4, H2O and small amounts of C2-C3-hydrocarbons), benzene, a mixture of tars and also soot. This gas has been used for hot gas filtration studies in a bench-scale filter test rig. Selected results regarding the tar yield and composition from both laboratory tests and HOTPURI tests are presented. Tar and gas compositions from the HOTPURI reactor are compared to those typically obtained in biomass gasification. Furthermore, the suitability of the described concept for producing simulated gasification gas is discussed and operation experiences are highlighted.