Catalytic steam reforming of indirect biomass gasification gas

In this study, the main objective was to investigate steam reforming of product gas derived from indirect gasification of biomass. This work is related to development of 50 - 200 MW size range production of SNG or hydrogen for applications such as refineries, SOFC and NG. The target process has investment and operating costs that are lower than in oxygen-blown gasification process. The emphasis of the work was on catalytic steam reforming of product gas derived from indirect gasification with two different bed materials, sand and dolomite. Indirect gasification leads to high concentrations of tar components and light hydrocarbons instead of targeted syngas components. The main problem in the steam reforming of hydrocarbon-laden gasification gas is the formation of coke which can lead to clogging, plugging of the reactor and catalyst deactivation. The experiments were made at laboratory-scale atmospheric plug-flow reactor with model gas. The gas compositions used in steam reforming experiments were obtained from the bench-scale gasification experiments. The two gas compositions will be further referred as sand and dolomite gas. Catalyst used was a commercial precious metal catalyst with particle size of 3x3 mm. With sand gas, the oven temperature was 912 °C and the catalyst outlet 876 °C and with dolomite gas, the temperatures were 900 °C and 862 °C, respectively. The main difference between sand and dolomite gases regarding reforming is the hydrocarbon concentration. The dolomite gas contains significantly less tar compounds and ethene than the sand gas. With sand gas the catalyst was coked after 15h, while the catalyst was successfully operated with dolomite gas for 500h. With dolomite gas, the tar and methane conversion stayed stable. The coke formed with sand gas was pyrolytic coke that forms in gas phase and deposits on the catalyst bed leading to encapsulation of catalyst particles, deactivation and finally to the blocking of the entire reactor. Based on the observations, sand gas has too high ethene and tar content leading to coke formation in the reformer and thus steam reforming of this type of gas is not technically feasible. However, the dolomite gas can be steam reformed without significant catalyst deactivation or coke formation. To prove the technical feasibility of the process based on steam gasification with dolomite and further steam reforming of the product gas, experiments in bench or pilot scale with real gasification gas are recommended for the future research.