Pyrolysis and Combustion Kinetics of Biomass-Related Compounds.

摘要

Detailed reaction pathways and kinetics have been developed for pyrolysis of biomass-related compounds such as beta-D-glucose (beta-D-glucopyranose), glycolaldehyde, glyceraldehyde, dihydroxyacetone, methanol, isopropanol, and propanal and for combustion of tetrahydrofuran (THF) and tetrahydropyran (THP). The purpose is to establish kinetics for reactor design and operations of bio-oil production and use.;Understanding glucose pyrolysis proves to be a key window into understanding cellulose pyrolysis. In the absence of ions from solvation or mineral salts, concerted reactions are shown to be dominant, notably including a new insight that molecular catalysis occurs through OH groups as in H2O, other glucose molecules, and most of the intermediates and products. Sanders et al. (J. Anal. Appl. Pyrolysis, 2003) had proposed a detailed but qualitative set of reaction steps for glucose pyrolysis. In the present work, transition states, elementary-reaction pathways, and rate coefficients are calculated for pyrolysis of beta-Dglucose (the monomer of cellulose) and related molecules, primarily using the high-level CBS-QB3 method. The result is quantitative, and elementary-reaction network that supports many features proposed by Sanders et al. while altering and adding others, including the competition between unimolecular reactions and OH-catalyzed reactions. Reactions for ring-opening and formation, retro-aldol condensation, keto-enol tautomerization, and dehydration are included. The dehydration reactions are focused on bicyclic ring formations that lead to levoglucosan and 1,6-beta-D-anhydrousglucofuranose. The bimolecular OH-assisted reactions are found to have lower activation energies compared to the unimolecular reactions. The levoglucosan-formation reactions should occur analogously for amorphous cellulose forming cello-n-san (e.g., cellotriosan) plus a shortened cellulose chain, a hypothesis supported by the very similar activation energies computed when alternate groups are substituted at the C1 glycosidic oxygen.;Second, to understand the role of different alcohol groups in glucose in the formation of levoglucosan, a parametric study has been performed by replacing alcohol groups at C1 and C6 with methyl and amine groups. The results show that the presence of lone-pair electrons in oxygen at C1 plays a key role in the substantial reduction in activation energy for levoglucosan formation. The effect of water molecules as a solvent and catalyst was tested using explicit, implicit (CPCM solvent model), and a combination of explicit and implicit water molecules. For unimolecular reaction, free energy of activation is lowest for glucose in implicit solvent with no explicit water molecules; for bimolecular reaction catalyzed by one water molecule and solvated by another water molecule in implicit solvent.;Third, pyrolysis experiments have been performed on small saccharides and alcohols such as glycolaldehyde, glyceraldehyde, dihydroxyacetone, methanol, and isopropanol. In these pyrolysis experiments, the products indicate that both concerted and free radical pathways may occur in the small-molecule pyrolyses.;Finally, elementary-reaction pathways are modeled and identified in THF and THP flames in order to understand the elementary chemistry of biomass-derived fuel combustion. This work's experiments and modeling of a fuel-rich THP/O2/Ar flame are published separately (Labbe et al., Proc. Comb. Inst., 2013). Modeling low-pressure THF/O2/Ar flat flames at different equivalence ratios by Kasper et al. (Z. Phys. Chem., 2011), the present uses rate coefficients from the literature or computes them using quantum chemistry, reaction theories, and analogy. Flat-flame modeling with CHEMKIN using the new set of kinetics, thermochemistry, and transport properties reveals that hydrogen abstraction from THF was the major fuel consumption pathway, proceeding by beta-scission reaction to form smaller species. Pollutants such as benzene and aldehydes were formed as intermediates in the combustion process. As in the experiment, polyaromatic hydrocarbons were not observed in the products at experimentally detectable levels, even at this fuel-rich condition.