Molecular mechanisms for the gas-phase conversion of intermediates during cellulose gasification under nitrogen and oxygen/nitrogen

Abstract

Gas-phase conversions of volatile intermediates from cellulose (Avicel PH-101) were studied using a two-stage experimental setup and compared with those of levoglucosan (1, 6-anhydro-β-D-glucopyranose). Under N2 or 7% O2/N2 flow, vapors produced from the pyrolysis zone (500 °C) degraded in the secondary reaction zone at 400, 500, 600 or 900 °C (residence time: 0.8–1.4 s). The 69.3% (C-based) of levoglucosan was obtained at 400 °C under N2 flow along with 1, 6-anhydro-β-D-glucofuranose (8.3%, C-based), indicating that these anhydrosugars are the major volatile intermediates from cellulose pyrolysis. Levoglucosan and other volatiles started to fragment at 600 °C, and cellulose was completely gasified at 900 °C. Most gas/tar formations are explained by gas-phase reactions of levoglucosan reported previously, except for some minor reactions originating from the molten-phase pyrolysis, which produced benzene, furans and 1, 6-anhydro-β-D-glucofuranose. Synergetic effects of O2 and volatiles accelerated fragmentation, and cellulose gasification was completed at 600 °C, which reduced benzene and hydrocarbon gas productions. The molecular mechanisms including the action of O2 as a biradical are discussed. These lines of information provide insights into the development of tarfree clean gasification that maintains high efficiency.