Understanding the thermochemical conversion of biomass to overcome biomass recalcitrance

Thermochemical conversion technologies are promising pathways for producing environmentally benign, sustainable biofuels and value-added chemicals from biomass. However, reaction pathways and chemistry behind these technologies such as pyrolysis and solvolysis of biomass are very complex. Contributing to the complexity are the many factors that could affect the reaction mechanisms. This research focuses on an external effect on thermal decomposition and internal reaction chemistry to provide an insight into the biomass decomposition for better performance. First, the effect of low concentration of oxygen in sweep gas during biomass pyrolysis in fluidized bed was investigated for practical purpose. It was found that the partial oxidative pyrolysis can increase the yield of pyrolytic sugars. A continuation of the study was performed to produce sugar-rich bio-oil from the biomass passivation of alkali and alkaline earth metals. Partial oxidative pyrolysis of passivated biomass produced approximately 21 wt% hydrolyzable sugars in bio-oil. Additionally, partial oxidative pyrolysis also prevented clogging within the reactor by reducing char agglomerations ensuring continuous operation. Second, solvolytic conversion of lignin was studied using a micro reactor in the presence of a hydrogen donor solvent. The results showed that hydrogen donor solvents were effective in converting lignin into alkylphenols. It was found that a hydrogen donor solvent could suppress repolymerization reactions by stabilizing the primary products to alkyl-substituted phenols. Pyrolysis mechanisms of lignin were further studied using methoxy substituted α-O-4 dimeric model compounds. Pyrolysis of aryl-ether linkage primarily involved homolytic cleavage. It was discovered that methoxy group substitution on the aromatic ring increases the reactivity toward C - O homolysis. Additionally, free radicals in the condensed phase of the pyrolysis products were detected by electron paramagnetic resonance spectroscopy, providing information on the presence of oxygen-centered phenoxy and carbon-centered benzyl radicals. Furthermore, methoxy group substitution was revealed to promote oligomerization reactions to form large molecular weight compounds. Lastly, a quantitative investigation of free radicals in bio-oil and their potential role in condensed-phase polymerization was conducted. It was confirmed that both lignin and cellulose pyrolysis involve homolytic cleavage generating free radicals. Lignin bio-oil fractions contained a significant amount of radicals, which were found to be stable species due to highly delocalized in a pi system