Gas- and coke/soot-forming reactivities of cellulose-derived tar components under nitrogen and oxygen/nitrogen

• Coke/soot- and gas-forming reactivities of cellulose pyrolysis products were found.
• Gas-forming reactivity was inversely correlated with BDE of weakest bond.
• Homolysis followed by fragmentation was proposed as gas-formation mechanism.
• Oxygen enhanced gas formation at 500 °C, but inhibited it at 600 °C.
• Oxygen promoted soot formation from furans.

Eight types of cellulose-derived primary volatile product, i.e., levoglucosan, glycolaldehyde, furfural, 5-hydroxymethylfurfural (5-HMF), hydroxyacetone, acetic acid, formic acid, and MeOH, were pyrolyzed separately under N2 and O2/N2 at 500–600 °C for 10 min in a closed ampoule, to clarify their gas- and coke/soot-formation behaviors. Under N2, levoglucosan and glycolaldehyde gave coke through liquid-phase reactions after they melted, whereas coke formation from furfural, 5-HMF, and acetic acid was observed all over the reactor wall. The latter type of coke was formed through gas-phase reactions after vaporization of the starting compound and its pyrolysis products. The compositions of the product gases were found to be related to the functional groups of the primary volatile products, and their formations were reasonably explained by homolytic cleavage of the weakest bonds, followed by radical reactions. The addition of O2 (0.3 M equiv with respect to the stoichiometric amount required for combustion) greatly altered the reactivities of furfural, 5-HMF, and acetic acid, although the effects were not large for the other components; the addition of O2 significantly increased the gas yields from the furans and acetic acid at 500 °C, but greatly reduced the gas yields at 600 °C. Soot formation from the furans was also significantly enhanced by the presence of O2. These unexpected results can be explained by the proposed pyrolysis pathways, in which O2 acts as a biradical for H abstraction or radical-coupling reactions.