Boiling point of levoglucosan and devolatilization temperatures in cellulose pyrolysis measured at different heating area temperatures

摘要

Fast heating conditions are known to improve the yields of volatile products in cellulose pyrolysis. The mechanism, however, is not yet well understood. For better understanding of the cellulose fast pyrolysis, the devolatilization temperature was directly measured during the pyrolysis of cellulose. An in situ measurement method employing a very fine thermocouple was first developed. A sample in a ceramic boat was pushed into the heating area, preheated at 430-700 ℃ under the flow of N_2 (150 mL/min). Employing this method, the boiling point (385 ℃) of levoglucosan, the major volatile product derived from cellulose fast pyrolysis, was measured directly for the first time, and was found to reduce to 345 and 292 ℃ under reduced pressures of 0.5 and 0.1 atm, respectively. High heating area temperatures increased the selectivity of levoglucosan evaporation by shortening the period required for the completion of evaporation. With these results in mind, pyrolysis of cellulose powder (Ceolus FD-F20) was studied using a similar procedure. The results obtained under the reduced pressures suggested that the depolymeriza-tion of cellulose, not evaporation of the volatiles products, is the rate-determining step for the pyrolytic devolatilization of cellulose. Interestingly, the depolymerization temperature varied depending on the heating area temperature as 360-385 ℃ (for 430-500 ℃) and 400-450 ℃ (for 500-700 ℃) (the values in the parentheses: heating area temperatures). The char morphology also changed; the pyrolysis conditions provided powder-like and film-like char materials, respectively. According to these results, the effects of heating area temperature on the cellulose pyrolysis mechanism are discussed, focusing on the behavior of cellulose crystallites.