Influence of O2 on structural evolution of biochar in oxidative pyrolysis of cellulose

Abstract

Oxidative pyrolysis could mitigate energy requirement in endothermic pyrolysis, but evolution of pyrolytic products might be impacted in oxidative atomosphere. This was investigated herein at 300–600 °C in a carrier gas of 11 vol% of oxygen with cellulose as a research object for the pyrolysis, as cellulose-derived aliphatic structures are prone to oxidization. The results indicated that O2 reacted with organics on cellulose/biochar even from 300 °C, decreasing biochar production while enhancing bio-oil formation. Oxidation of the organics on the biochar increased CO/CC ratio but did not increase oxygen content, as formation of the oxygen-containing aliphatic structures and their elimination via cracking took place in parallel, which in overall formed the biochar of higher C/O and C/H ratios, but not necessarily higher thermal stability. The abundance of the thermally unstable aliphatic structures was responsible for this, but it was also the reason for the improved comprehensive combustion characteristic index and combustion kinetics. Additionally, pyrolysis at 600 °C in presence of O2 formed the biochar with a specific surface area of 426.2 m2/g (mainly micropores), which was significantly higher than that of counterpart in nitrogen (28.9 m2/g). The in-situ IR characterization of the pyrolysis suggested that O2 favored formation of oxygen-containing species such as -OH, CO of varied forms, *CO2, olefinic CC but not aromatic CC.