Gas evolution and the mechanism of cellulose pyrolysis

The real time evolution kinetics of formaldehyde, hydroxyacetaldehyde, CO and CO2 during the pyrolysis of cellulose, Whatman 41, were studied in a fast evolved gas-FTIR apparatus (EGA). The samples were subjected to rapid exponential temperature increases ranging from 400 to 800 degreesC within about one minute. A total of ten compounds were simultaneously detected in the gas phase by FTIR. Four of these: formaldehyde, hydroxyacetaldehyde, CO, and CO2, were studied in detail as a function of time. The yields of formaldehyde, hydroxyacetaldehyde and CO were found to approximately double with heating rate over the range of the experimental temperature profiles while that of CO2 decreased slightly. The kinetics of formaldehyde and CO formation were analyzed in terms of two competing first order reactions. The rate constants for the formation of formaldehyde and CO were found to have activation energies of 47 kcal/mole each while the competing reactions had activation energies of 35 kcal/mole in both cases. The case of hydroxyacetaldehyde was found to be more complex, with the same initial reactions as were found for formaldehyde and CO but requiring a third reaction step subsequent to the 47 kcal/mole reaction. The kinetics for CO2, were consistent with a single first order reaction with an activation energy of 35 kcal/mole. The results indicate that the formation reactions of formaldehyde, hydroxyacetaldehyde, CO and CO2 exhibit identical rate limiting steps that involve the major pyrolytic pathways of cellulose. (C) 2001 Elsevier Science Ltd. All rights reserved.