(757f) Secondary Reactions of Volatile Intermediates during Cellulose and Lignin Pyrolysis/Gasification

Secondary reactions of volatile intermediates during cellulose and lignin pyrolysis/gasification

Haruo Kawamoto
Graduate School of Energy Science, Kyoto University Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan kawamoto@energy.kyoto-u.ac.jp
Secondary reactions of the volatile products from pyrolysis of wood and its constituent polymers, that is, cellulose, hemicellulose and lignin, are quite important for understanding and control of the gasification and fast pyrolysis products formation. In this paper, the secondary reactions will be discussed at the molecular level especially focusing on the differences for cellulose and lignin. These wood constituent polymers decompose rapidly in the temperature range 300â??400 °C to form volatile intermediates which are further subject to the secondary reactions in the gas phase to form gaseous, tar and coke substances. These volatiles and their gas-phase decomposition products also decompose in the liquid phase after condensation on the reactor and pipe line wall with temperatures lower than their dew points. Such secondary decomposition behaviors were studied in details.
Pyrolysis studies at 800 °C with a Pyrex tube reactor having temperature gradient suggested that coke forming behaviors are different between cellulose and lignin; coke from cellulose pyrolysis was observed only in the liquid phase after condensation of the volatiles, whereas lignin-derived coke were obtained all over on the reactor wall. The latter coke formation was suggested to proceed in the gas phase. Furthermore, gas forming reactivity evaluated for the volatiles and coke/char materials from cellulose and lignin at 600 °C in a closed ampoule reactor indicated that only cellulose-derived volatiles have quite high gas forming reactivity compared with other intermediates. These secondary decomposition characteristics would be involved as the important steps for determining the product selectivity during gasification and fast pyrolysis of woody biomasses. We studied further to disclose the molecular reactions mechanisms for these conversions.
The coke and gas formation behaviors of cellulose-derived volatiles (major fraction: levoglucosan (1,6-anhydro-β-D-glucopyranose) were rationally explained by assuming that these reactivities are different in the gas and liquid phases. Most of these volatiles were stable against coke and heavy/refractory tar formation in the gas phase. On the other hand, in the liquid phase, these were quickly converted to coke and water along with the polymerization products. Thus, coke-forming dehydration and transglycosylation of carbohydrate and its pyrolysis products occurs only in the liquid phase. These carbohydrate-characteristic reactions were reasonably explained by proposing the acid-catalysis (hetelolysis) mechanisms arising from the intermolecular hydrogen bonding. In the gas phase, cellulose-derived volatiles were converted to gaseous products (CO, H2, CH4, etc.) selectively along with the fragmentation products (formaldehyde, glycolaldehyde, etc.) as their intermediates, probably through the homolysis of the weak bonds followed by radical reactions.
As for lignin, methoxyl group was suggested to be involved in the coke formation in the gas phase. Ortho-quinone methide intermediates formed via ipso-rearrangement of the methoxyl group, which occurs at temperatures > 450 °C, were proposed as the reactive intermediates. Thus, coke formation from lignin became extensive in this temperature range. During the reactions of the methoxyl group, catechol/pyrogallol and o-cresol/xylenol type products formed in the gas and liquid phases, and then these
intermediates were converted further. Demethoxylation products such as phenols were also formed at this stage. The catechol/pyrogallol type compounds were more easily converted to the gaseous products (especially CO) at the temperatures > 550 °C along with polyaromatic hydrocarbons (PAHs). On the other hand, the o-cresol/xylenol type compounds were relatively stable even at 600 °C and gradually converted to the demethylation products along with CH4, H2 and PAHs. These results were consistent with the observation that o-cresol, xylenol, phenol and PAHs were the final tar products in lignin pyrolysis. The same types of reactions were also suggested to proceed in the liquid/solid phase (as polymers). Interestingly, formation of PAHs was extensive only in the gas phase, not in the liquid/solid phase.