Catalytic steam gasification of biomass over nickel based catalysts via reactive flash volatilisation

Abstract

Biomass gasification is a promising technology for delivering renewable energy. However, tar formation remains one of the main challenges. This research aims to develop in-situ nickel based catalysts for reactive flash volatilization (RFV) of biomass. RFV is a catalytic gasification process which uses high carbon space velocity and mass flow rate, with oxygen and steam as gasification agents, to produce tar free synthesis gas from non-volatile feedstock in a milliseconds residence time reactor. Ni, Pt-Ni, Ru-Ni, Re-Ni and Rh-Ni supported catalysts were investigated in this study. Effects of various operating parameters on the product selectivity (gas, tar and char) and syngas composition were evaluated. The operating parameters studied here include catalyst promoter, reaction temperature, carbon to oxygen feed ratio (C/O), carbon to steam feed ratio (C/S) and biomass ash content. Products were analysed using gas chromatography, total organic carbon (TOC) and CHNS/O analyses. Catalysts were characterised using Temperature Programmed Reduction, Temperature Programmed CO-Desorption, Transmission Electron Microscopy, nitrogen physisorption, X-ray Fluorescence and Powder X-ray Diffraction. Re-Ni, Rh-Ni and Ru-Ni, in that order, exhibited higher gasification efficiency in RFV of cellulose. This can be attributed to these catalysts’ higher reducibility and active metal surface area, and better coke resistance. Highest gasification efficiency was achieved at 750°C with C/O of 0.6 and C/S of 1.0, without any oxygen breakthrough. Kinetics study using a wire-mesh isothermal thermogravimetric analyser was conducted to investigate the role of catalyst in RFV. Result was modelled using a pseudo first order reaction model. Three distinct regimes of rate of mass loss were identified: pyrolytic decomposition, reforming and char gasification. Catalysts increased the rate of mass loss in reforming regime and improved the quality of synthesis gas. Rate of pyrolytic decomposition of lignocellose was found to be limited by devolatilisation of crystalline cellulose. Gasification efficiencies of pinewood and eucalyptus sawdust were found to be higher than cellulose under similar reaction conditions. Higher gas selectivity and lower char selectivity during pinewood and eucalyptus sawdust RFV was a result of more amorphous structure of lignocellulose compared to microcrystalline cellulose, and the catalytic effects of alkali and alkaline earth metals (AAEM) found in the lignocellulose ash. The catalytic effects of AAEM further reduced the coke deposition on the Ni catalysts, making the effect of noble metal promoter on the Ni catalysts less significant. Future studies may reduce the loading of noble metal to make RFV more economical.