Bio-FLASHCHAIN? theory for rapid devolatilization of biomass. 7. Interpreting AAEM catalysis in primary lignin devolatilization

摘要

? 2022 Elsevier B.V.This paper extends lig-FC, the reaction mechanism for the devolatilization of mineral-free lignins formulated in Part 6 of this series, to cover catalysis by alkali and alkaline earth metals (AAEMs) in parallel with the spontaneous reaction channels. Quantitative comparisons with data cover temperatures to 800 °C, heating rates from 10 °C/min to 8000 °C/s, contact times to 900 s, and AAEM loadings to 0.22 mol cation/mol lignin monomer. During lignin devolatilization, AAEMs catalyze charring via monomer decomposition and bimolecular recombination with different sensitivities, while the kinetics for all other chemical reaction processes are unaffected. Lig-FC accurately depicts the impacts of the AAEMs with a minor reduction in the activation energy for monomer decomposition, and minor-to-modest reductions in the energy for bimolecular recombination, depending on the AAEM cation, and correctly ranks the catalytic intensities as K > Na > Ca > Mg. Molecular weight distributions of lignin oils are unaffected by AAEM catalysis. Compared to AAEM catalysis of cellulose devolatilization, the catalysis of lignin devolatilization is much weaker in absolute terms. Catalytic impacts with lignins intensify for progressively faster heating rates whereas those with cellulose sharply diminish. This is because at fast heating rates two of three charring processes predominate with lignins whereas depolymerization predominates with cellulose.