Novel sintered metal filter elements: Performance evaluation in biomass gasification conditions

Novel sintered metal filter elements: Performance evaluation in biomass gasification conditions Background Filtration is an essential step in biomass gasification processes. In synthesis gas applications, where full removal of gas contaminants is required, the filter unit is typically placed upstream the catalytic reformer in the purpose of shielding the reformer from dust deposits and preventing plugging of the catalytic layer. In state-of-the-art biomass gasification concepts, the raw product gas is cooled down to ca. 500 °C prior to filtration. This results in increased oxygen consumption in the reformer as the gas must be reheated after filtration to reach the targeted reforming temperatures of 850-950 °C. To improve the biomass-to-fuel conversion efficiency of biomass gasification-synthesis plants, the latest R&D efforts in this field have been targeted at operating the filter unit at gasifier outlet temperature (750-850 °C), which would eliminate the intermediate cooling and reheating steps. According to VTT’s estimate, the overall efficiency from biomass to Fischer-Tropsch liquids could increase from 53% to 58 %, if the filter temperature was raised from the conventional 550 °C level to 850 °C. Aim Filter elements that are currently available for hot gas filtration at temperatures up to 1000 °C are manufactured from metal alloys, high temperature insulation wool or fine ceramic granules. However, these state-of-the-art filter elements have certain restrictions, such as low ductility, high pressure drop or deficient corrosion resistance. With the aim to generate a metal-based filter material withstanding mechanical loads and corrosive conditions, GKN has developed innovative filter elements out of a modified Iron-Chromium-Aluminium powder (material No. 1.4767 mod.). The focus of this work was on evaluating the performance and corrosion resistance of these novel filters in biomass gasification conditions at temperatures up to 750 °C. Method The novel metal filter material was first tested at VTT’s laboratory to study its long-term corrosion resistance in the presence of H2S. Twelve one-meter-long filter elements were then implemented into VTT’s pilot-scale dual fluidised-bed gasification test facility and operated in steam gasification conditions at temperatures up to 750 °C. These full-scale elements were exposed to reducing conditions for over 300 hours. Various woody residues, straw and demolition wood was used as feedstock in the test campaigns. Particulate samples were retrieved downstream the filter unit to verify the particulate removal efficiency. One of the filter elements was removed after the testing period and inspected at GKN. Main results The novel filter material showed excellent results with respect to corrosion resistance in VTT’s lab-scale tests. No degradation or corrosion was observed in the filter sample after 1000 hours of exposure to high sulphur levels (3000-7000 ppm-vol). Moreover, the full-scale filter elements were successfully operated in pilot-scale gasification test campaigns. Stable operation could be achieved even at high filter temperatures (> 700 °C) and no filter breakages occurred. Particulate sampling showed that full removal of particulates was achieved. Finally, metallographic analyses were conducted at GKN for the used filter element. No residues of dust particles were found inside the filter pores indicating that the selected filter grade was suited for this application. Moreover, no corrosive products could be found on the surface and the cross-sectional investigation showed a close protecting oxide layer on top of the filter surface. The work described in this paper was carried out in research project BTL2030 and H2020 project COMSYN.FILTECH October 22-24, 2019 Cologne-German