Roles of CO2 in carbon-negative syngas production from pyrolysis of cellulosic biomass (oat hull)

Abstract

The biological conversion of biomass into chemicals and biofuels is environmentally beneficial. Nevertheless, it is critical to minimize the inevitable carbon loss by microbial metabolisms such as respiration and cell reproduction. Pyrolysis has gained considerable attention because it enables to utilize all carbons in biomass as the form of syngas, biocrude, and biochar. In this study, oat hull (OH), one of the by-products generated during cereal processing, was selected as a model biomass. To impart the sustainability of pyrolysis process, carbon dioxide (CO2) was used as a partial oxidant. CO2 participated in homogeneous gas-phase reactions (HGRs) with volatile matters (VMs) derived from OH pyrolysis, resulting in the enhanced formation of carbon monoxide (CO). However, the HGRs induced by CO2 are evident only above 550 degrees C. To expedite the reaction kinetics of CO2, pyrolysis experiment was carried out in a reactor system equipped with external heating at 700 degrees C and loaded with Ni/Al2O3 catalysts (0.5, 2.0, and 5.0 wt% Ni). The use of Ni catalyst during OH pyrolysis enhanced the thermal cracking of the VMs into lighter molecular volatiles, accelerating the HGRs with CO2 under improved mass transfer. The production of CO-rich syngas increased proportionally to the Ni loading on the catalysts. Therefore, this study highlights mechanistic insights into the roles of CO2 in facilitating CO-rich syngas production, with enhancing carbon utilization efficiency during the OH pyrolysis.