Optimization of sub- and supercritical water gasification of rice husk enhanced with iron-doped alkaline-earth catalysts

Abstract

Biomass is a promising renewable energy source widely available worldwide, particularly in developing countries where clean and affordable energy is a major problem. Biomass gasification is an attractive technology that can transform biomasses into a more versatile fuel known as syngas, tar (bio-oil) and biochar. Syngas is a hydrogen-rich gas that could promote a clean and green energy and promote the agriculture sector. The utilisation of syngas would reduce dependence on fossil-based fuels and eventually reduce the carbon footprints. The gasification technology faces operational challenges; one of the problems is tar formation, slow char gasification reaction, and poor performance of catalysts. These challenges are influenced by inappropriate operating conditions and the precursors employed in catalyst synthesis. In this study, the optimisation of noncatalytic and catalytic gasification of rice husk is reported. The rice husk biomass was gasified under subcritical and supercritical water conditions in a batch autoclave reactor. The effect of temperature (350-500°C), residence time (30-120 minutes), and feed concentration (3-10 wt%) is optimised. Moreover, the effect of the addition of natural, calcined and Fe doped limestone and dolomite catalysts on the gasification yield is studied using a response surface methodology. The catalyst was prepared by a facile incipient wetness process using chlorine- and sulphur-free iron (III) ammonium citrate precursor. Optimisation of operating conditions suggests a quadratic model for gasification efficiency, gas volume, char yield, and gravimetric tar. The findings revealed that higher temperatures, longer residence times and lower feed concentrations favoured high gas yields. The lowest tar yield obtained was 2.98 wt%, while the highest gasification efficiency and gas volume attained was 64.27% and 423 mL/g, respectively. The findings of the catalyst characterisation revealed that the predominant reactive site of Fe/limestone catalyst is iron oxide, calcium ferrite, and calcium oxide. Under all conditions tested, the manufactured catalyst was highly active in promoting char gasification, gas volume and gasification efficiency. Tar yield was substantially promoted at low temperatures and high feed concentrations, but at high temperatures and low feed concentrations (500oC, 3 wt%), tar formation was suppressed by 22%, while char gasification was enormously enhanced by 65%. The maximum gas yield of 560 mL/g biomass was obtained under the catalytic conditions of 5%Fe/limestone, 15% catalyst loading, 500oC, 120 minutes, and 3 wt% feed concentrations. Therefore, these findings revealed that the rice husk's energy content could be harnessed using supercritical water gasification to obtain substantial fuel products.