In-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasification

Abstract

This paper aims to propose an effective tar conversion approach during biomass pyrolysis via in-situ dry reforming over rice husk (RH) char and char-supported Ni-Fe catalysts. Utilizing high pyrolysis temperature, tar from biomass pyrolysis could be removed effectively in the gasifier by mixing with the char-supported catalysts, simplifying the follow-up tar removal process. Under the optimized conditions, the conversion efficiencies of condensable tar can reach about 92.3% and 93% using Ni-Fe char (without calcination) and Ni char (with calcination), respectively. It is noteworthy that the condensable tar could be catalytically transformed into the non-condensable tar or small molecule gases resulting in the heating value increase of gaseous products to benefit of the power generation systems. Compared with the other catalysts preparation methods, Ni–Fe char exhibited more advantages of convenient and energy-saving. In the presence of catalysts, the concentration of CO2 (vol.%) was reduced slightly, while the CO concentration (vol.%) increased greatly because of dry reforming. Due to carbon loss, parts of RH char-supported catalysts (C-SiO2 catalysts) could be converted into SiO2-based catalysts because of high-content amorphous nano-sized SiO2 in RH char. In addition, partial metal oxides or ions via carbon (i.e., biochar) and gas (i.e., H2, CO) in-situ reduction were transformed into metallic states contributing to the enhancement of tar conversion. Therefore, RH char plays two significant roles during the process of biomass pyrolysis. On one hand, it works as an intermediate reductant to reduce the metal oxides and CO2; on the other hand, it can be considered as an adsorptive-support to adsorb metal ions and tar. After that, the char-supported catalysts could be used for tar conversion. In particular, since the metal catalysts still remain in the solid residues, the pyrolysis char could be regenerated via thermal regeneration using waste heat or gasified into syngas directly.