Understanding cellulose primary and secondary pyrolysis reactions to enhance the production of anhydrosaccharides and to better predict the composition of carbonaceous residues
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Pyrolysis is a promising method to convert biomass into an oil that can be refined into valuable fuels and chemicals as well as a carbonaceous residue that can be converted into adsorbents. In pyrolysis, cellulose, the most abundant biomass constituent, is thermally converted into mono- and oligo-anhydrosugars (chiefly levoglucosan and cellobiosan) in high yields with small quantities of a carbonaceous "bio-char" residue. However, the reaction mechanisms for cellulose pyrolysis have not been well investigated.The major goal of this dissertation is to advance our knowledge on the pyrolysis mechanisms responsible for the formation of anhydrosugars and carbonaceous residues under slow and fast pyrolysis. Our hypothesis is that under fast heating rate conditions, cellulose forms a very reactive liquid intermediate of cellulose primary products (levoglucosan, cellobiosan) which, if not evaporated fast enough, will undergo cross-linking reactions. Cross-linked product formation is a critical step for the formation of carbonaceous residues under slow heating rate conditions. It was found that cellulose crystallinity can affect the formation rate of this liquid intermediate, which is promoted by amorphous state. Our research shows that this liquid state is a temperature controlled step that enhances dehydration reactions and cross-linking reactions if persistent. We also determined that secondary reactions in the liquid phase derive not from levoglucosan remaining in the liquid phase, but from oligo-anhydrosugars like cellobiosan.Extended studies found sulfuric acid inducing dehydration and cross-linking reactions during pyrolysis of cellulose and levoglucosan. In pyrolysis of cellobiosan, sulfuric acid increased the yield of levoglucosan. The concentration of sulfuric acid at which the yield of levoglucosan from the pyrolysis of Douglas fir wood is maximized near the yield was found from cellulose. The changes occurred during slow heating of cellulose were studied using 13C-NMR, FTIR, IEC and SEM. A new reaction mechanism that includes the formation of cross-linked sugars was developed to explain the evolution of carbonaceous residues from cellulose pyrolysis at slow heating rates. The results in this dissertation provide fundamental insights into cellulose thermal reactions for developing new strategies to enhance yield of anhydrosugars or valuable chemicals and to better predict the composition of carbonaceous residues formed.