| Fast pyrolysis technology could transform the abundant biomass resources into easy storage, easy transport and high energy density bio-oil. However, some drawbacks of crude bio-oil limit its high quality application. The studies focused on the biomass pyrolysis mechanism could provide theoretical guidance for the process design and products upgrading. Since the structure of biomass is extremely complex, to obtain the pyrolysis mechanism of whole biomass, it is feasible and efficient to research the individual pyrolysis behavior of the three major components in biomass, namely cellulose, hemicellulose and lignin. Cellulose presents ordered structure and can be easily acquired, and the cellulose components in different biomass contain the similar structure. Thus the pyrolysis mechanism of cellulose has been widely studied, and the relevant primary theoretical system has been established. However, the structure of other two major components, hemicellulose and lignin, are complicated and branched, and their pyrolysis studies are relatively inadequate. In order to refine the mechanism model of biomass pyrolysis based on the components distribution, this study focuses on the pyrolysis characteristics of the two complicated components. Based on this, in order to improve the quality of crude pyrolytic products, the influence mechanism of zeolite catalysis and torrefaction pretreatment on the biomass pyrolysis behavior was studied in detail.For hemicellulose, the minimally damaged isolation method based on dimethyl sulfoxide extraction was developed and applied to obtain six hemicellulose samples from two hardwoods, two softwoods and two straws, respectively. Through neutral monosaccharide composition analysis, molecular weight distribution analysis and 1H/13C/2D-HSQC NMR analysis, it was proved that all the isolated hemicellulose samples well preserved the high polymerization degree and thermally unstable side braches of natural hemicellulose. Hardwood, softwood and straw hemicellulose contained 4-O-methyl-D-glucurono-D-xylan, D-galacto-D-gluco-D-mannan and L-arabino-4-O-methyl-D-glucurono-D-xylan, respectively, as the major polysaccharide. A distributed activation energy model with two Gaussian functions (DG-DAEM) was introduced to realize the high accuracy fitness for the macroscopic pyrolysis kinetics of hemicellulose. In the parallel degradation reaction pathway, pyrolysis of softwood hemicellulose contained higher mean activation energy than that of hardwood and straw hemicellulose. The major pyrolytic products of softwood hemicellulose were six-carbon compounds, mainly including anhydro-sugars and 5-hydroxymethyl furfural. While pyrolysis of hardwood and straw hemicellulose yielded five-carbon compounds as the majority, and acids and furfural were the most typical products. As for lignin, the other complex component in biomass, various chemical and physical methods were used to isolate alkali lignin, klason lignin, organosolv lignin and milled wood lignin (MWL) from the same biomass, respectively. The structural characterization by using FTIR and NMR indicated that MWL well preserved the original lignin structure. However, the other lignin samples isolated by using alkali/acid treatment or high temperature organic solvent extraction suffered significant structural damage, such as alkali metal residue, aromatic condensation and the cleavage of aryl ethers, propyl branches and methoxyls. The pyrolytic kinetics for lignins were simulated by DG-DAEM method. It was found that the parallel degradation reaction route in MWL pyrolysis contained low activation energy and made the largest contribution to the devolatilization among the pyrolysis of four lignins. MWL pyrolysis could yield monophenols at low temperature due to its well-preserved aryl ethers.Based on the experimental analysis, the combination of the density functional theory (DFT) and transition state theory (TST) in quantum chemistry was introduced to theoretically study the decomposition pathway and molecular kinetics for pyrolysis of biomass model compounds. It was indicated that the formation of furfural was the most favorable reaction route during pentoses pyrolysis. The elevated pyrolysis temperature could largely enhance the competitiveness of the reaction pathway leading to the hydroxyacetone production. While pyrolysis of hexoses preferred to yield 5-hydroxymethyl furfural as the final product due to the high energy barrier for the dissociation of hydroxymethyl at C5 position. The glycosidic bond and sugar ring in xylan were easy to be cleaved during pyrolysis. The O-acetyl linked at C2 position of xylose residue was more thermally unstable than that at C3 position. Moreover, a synthetic β-O-4 dimer was used as the representative of lignin. Its most favorable pyrolysis routes at different reaction stages were studied in detail by using both experimental analysis and theoretical calculation. By comparing the reaction rate constants, it was found that the homolytic Cβ-0 breakage and the concerted retro-ene fragmentation were the most favorable free-radical reaction and pericyclic reaction, respectively. During the evolution of intermediate products, the cleavage of aryl ethers and the isomerization of enols were the most competitive pathways, whereas the intramolecular cyclization and methoxyl dissociation exhibited high energy barriers and low reaction rates. It was also found the homolytic demethylation of methoxyl in guaiacol to form catechol was much more favorable than demethoxylation (including homolysis and concerted reaction pathways) to form phenol.The above studies established the correlation between the complex functional group distribution in biomass components and the resulting pyrolysis behavior. The influence mechanism of two methods, zeolite catalysis and torrefaction pretreatment, on the biomass pyrolysis behavior was further investigated. The evolution behavior of typical volatiles under two different catalytic pyrolysis processes by which the catalyst (HZSM-5) comes into contact with the substrate (biomass based monosaccharides), namely, mixed with monosaccharide (in-situ) or layered above monosaccharide (ex-situ), were studied and compared in detail. It was concluded that the in-situ catalysis largely improved dehydration reaction under low temperature, enhancing the production of acids, anhydro-sugars and furans, while the ex-situ catalysis promoted the transformation of acids and furans via dehydration, decarbonylation and decarboxylation. Both two catalysis processes improved the formation of aromatics. Pentoses entered into the zeolite pores more easily than hexoses due to their smaller molecular size, hence pentose pyrolysis was drastically affected by in-situ catalysis. As for the studies on torrefaction pretreatment, it was found the content of hemicellulose in biomass decreased significantly after torrefaction. The structural characterization by using 2D-PCIS and 13C CP/MAS NMR illustrated that during torrefaction, the aryl ethers in lignin were cleaved, and carbohydrate components were depolymerized via the breakage of glycosidic bond; the O-acetyl side branches in hemicellulose were largely dissociated; the crystalline region in cellulose was damaged. The pyrolysis kinetic characteristics for raw and torrefied biomass were studied by using a proposed distributed activation energy model with three combined Gaussian functions. The mean activation energies for different parallel reaction pathways changed a little after torrefaction, while the contributions of these pathways to the pyrolytic devolatilization changed much more significantly. Since the biomass structure became more uniform after torrefaction, the distribution of activation energy for pyrolysis of torrefied biomass became narrower. Torrefaction decreased the production of acids, furans and anhydro-sugars in the pyrolysis products. The propyl side branches in lignin were dissociated during high temperature torrefaction, resulting in lower yields of the phenols containing propyl branches and higher production of the phenols without side branches. |
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