Reaction and Transport within single pyrolysing wood particles - Modelling and experimental validation with in-situ measurements
Final Report Abstract
The main conclusions of this work (with support from literature) regarding the influence of heterogeneous secondary reactions and inorganic species on the pyrolysis mechanism of lignocellulosic biomass are: • Inorganic species in the presence of intra-particle transport limitations catalyze cellulose decomposition, shifting its devolatilization rate maximum to lower temperatures. The thermochemistry of the process changed to a more exothermic regime and an increase of char and gas yields due to a preference for the fragmentation reaction pathway is observed. Charring reactions were preferred when both inorganics and transport limitation phenomena are present, reducing CO and CH4. CH4 and PACs were released at temperatures above 400 °C following similar release patterns suggesting that both may be produced through the same reaction mechanism within the solid matrix. There was an inhibition of PACs release upon addition of K, suggesting a competition between PACs and char formation. • For xylan, both pretreatments (washing and doping) affected the devolatilization rate at the kinetic level by catalyzing both the decomposition of side-chain units and products resulting from depolymerization and fragmentation of xylose units. At the particle level, however, these effects were minimized by the strong exothermic behavior governing the decomposition mechanism. Like cellulose, the reaction pathway associated to the formation of CH4 was accompanied by the release of PACs. • Lignin devolatilization occurred after the formation of a liquid intermediate which bubbled and swelled throughout the devolatilization stage. Inorganic species favored the cleavage of functional groups, promoting decarboxylation, and H2O was expected to be enhanced through dehydroxylation and dehydration. At high temperatures CO formation was inhibited due to the preference for deoxygenation through CO2 and H2O release. The first release stage of CH4 decreased potentially due to the smaller amount of transferable hydrogen upon addition of K. No significant effects on the devolatilization rate and thermochemistry were observed. • Increasing K content caused a shift towards lower temperatures of the devolatilization rate curve in the slow pyrolysis of wood. When intra-particle resistances were enhanced, there was a promotion of exothermic reactions, mainly because of K on cellulose decomposition, that led to a narrowing and overlap of the devolatilization of the three main macrocomponents. Char and CO2 yields were enhanced when both transport limitation phenomena and inorganics were present. The emission of fluorescence-emitting species (mainly PACs) resembled the CH4 release rate curve, suggesting similar formation mechanism, as observed for cellulose and xylan. Its formation during the main devolatilization stage was mostly related to lignin decomposition, while, at high temperatures, it was associated to further reactions in the solid matrix of cellulose and hemicellulose. The inhibition of PACs release upon addition of K was attributed to the same mechanism as for cellulose, i.e., competition between PACs and char formation. The next step in this work will be to use the obtained experimental results and qualitative understanding on the synergistic impact of heterogeneous secondary reactions and inorganic species on the pyrolysis mechanism of lignocellulosic biomass to advance in the available pyrolysis kinetic schemes. As basis the pyrolysis kinetic scheme developed by Ranzi and his group at the Politecnico de Milano will be used. To aid in this work, the CFD model of the PLRC and the porous particle will be employed.
Publications
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Characterization of woody biomass pyrolysis based on its macrocomponents, Proceedings 30. Deutscher Flammentag, Hannover, 28-29 Sep 2021
H. Almuina-Villar, F. Behrendt, A. Dieguez-Alonso
