Metal nickel nanoparticles in situ generated in rice husk char for catalytic reformation of tar and syngas from biomass pyrolytic gasification

Abstract

This paper aims to propose a novel catalytic pyrolytic gasification technology for the in situ conversion of tar and syngas, accompanied by the silica-based nickel nanoparticles generated in situ and the highly dispersed rice husk char (RHC), namely RHC Ni. Partially oxidized nickel oxides (i.e., NiO) in the carbon matrix of biochar can be carbothermally reduced to metallic nickel (Ni⁰) nanoparticles by reducing gases (e.g., CO) or carbon atoms during biomass pyrolysis. Moreover, due to its strong reducibility, the addition of sodium borohydride (NaBH₄) can significantly promote the generation of Ni⁰ by the reduction of NiO, improving the biochar's catalytic activity. An ultra-low tar yield can be achieved by pyrolysis of RH Ni and RH Ni–B at 750 °C, in terms of the high tar conversion efficiencies of 96.9% and 98.6%, respectively, compared with the pyrolysis of raw RH. It is noteworthy that the condensable tar could be catalytically reformed into the small molecules of non-condensable tar or gases, which contributes to improving the syngas fuel characteristics in the favor of power generation systems, corresponding to the lower heating value (LHV) of syngas increasing from 10.25 to 11.32 MJ m⁻³. In addition, the increase of the polymolecular Ni⁰ was most possibly caused by the disproportionation reaction and strong reducibility of NaBH₄. In addition, the produced RHC Ni showed a good performance for the catalytic conversion of tar (conversion efficiency, 96.5%) through co-pyrolysis with biomass. After deactivation, the waste RHC Ni might be easily regenerated via thermal treatment or directly catalytically gasified into the applicable syngas, accompanied by the production of the silica-based nickel nanoparticles.