Study On The Mechanism Of Biomass Pyrolysis And The Interactions Based On Its Components

Since the pyrolytic bio-oil from biomass can be used as the feedstock for producing high-grade liquid fuels, pyrolysis has attracted widespread attention in the recent years. However, biomass pyrolysis was a complex physical-chemical process, it is suggested that pyrolysis of biomass can be considered as the superposition of its major constituents (cellulose, hemicellulose and lignin). The biomass constituents are found to pyrolyze at different rates and by different pathways. Meanwhile, mutual interactions were existed among the biomass components during pyrolysis and thereby affected the biomass pyrolysis behaviors. So the elucidation of the pyrolytic behavior and the interactions of individual components of biomass during pyrolysis are essential to a better understanding of the intricate pyrolysis process of biomass and make it possible to produce high-grade liquid fuels by product manipulation.Firstly, model components of the major three constituents of biomass were pyrolyzed at temperatures ranging from200-950℃to investigate its degradation behavior and the evolution pathways of the products. The results indicated that cellulose served as the main contributor to biomass pyrolytic bio-oil (-69wt.%) and lignin was the main contributor for the mass of biomass chars(-50wt.%). Anhydro-saccharides, light oxygenates and phenols were the main compounds in pyrolytic bio-oil from cellulose, xylan and lignin, respectively. The formation of CO from polysaccharides concentrated on500-650℃, whereas H2and CH4was focused at650℃or higher temperatures. The structure evolution of chars during pyrolysis was investigated and found that chars underwent inter-and intra-molecular hydrogen bond and subsequent scission of structural bonds at temperature below500℃. With the increase of temperature, aromatic hydrocarbons with aliphatic branched chain emerged and condensed gradually to form large aromatic rings.The vapor phase and vapor-solid (raw or char) phase interactions and its impact on the formation of pyrolytic products were conducted on a vertical furnace. It was found that the vapor phase interactions could both enhance the formation of gaseous products but inhibit the formation of liquids. The ring-opening decomposition of levoglucosan has been restrained by the vapors from hemicellulose. Aromatization of small molecular was predominated at400℃, but the interactions yielded furans gradually at high temperature (700℃). The interactions between cellulose and lignin, hemicellulose and lignin could promote the formation of acids, ketones and furfural. The high electro-negativity of phenolic in lignin vapors formed phenol-saccharide polymers and resulted in the enhancement of cellulose levoglucosan formation.The acid compounds in volatile from hemicellulose resulted in the scission of glycoside bond, pyran ring of cellulose and branch aliphatic chains and oxygen-containing functional groups of lignin so as to increase the weight loss at280℃. The anhydro-saccharides in cellulose volatiles occurred re-polymerization to generated carbonized products at315℃. Xylan was more likely to decompose into low molecular liquid compounds as a result of the intensive vapor-solid phase interactions between xylan and lignin char and volatiles at650℃. Meanwhile, the chars could also accelerate the decarboylation and the demethoxylation of lignin but inhibited the cracking of carboxyl. The methyl radical tends to form alkylated phenols rather than phenol.Finally, natural corn cob was selected to study the original cross-linked structure of cellulose and hemicellulose and its impact on interactions. Meanwhile, peanut shells and walnut shells were delignified proportionally in order to study the interactions between the residue lignin and holocellulose. The pyrolysis of natural biomass obtained lower gas yield and higher liquid and solid yield in comparison with the synthesized samples. The contents of H2and CO2in pyrolytic gas were much lower whereas CH4and CO were higher in gas product. The melted phase of hemicellulose accelerated the opening of pyran ring and decarbonylation of carbonyls which is derived from dehydration of hydroxyl groups. As a result, the levoglucosan content in liquid decreased significantly. The inherent lignin in biomass inhibited the decarboxylation of hemicellulose but was favorable to the formation of cellulose levoglucosan.