Mechanism Research Of Hemicellulose Pvrolvsis Based On Py-GC/MS

As one of the technologies which could make use of biomass efficiently to produce high grade liquid fuels, biomass pyrolysis has got worldwide research attention. Due to the complexity and instability of bio-oil, widespread utilization of biomass pyrolysis technology has been restricted, which might attribute to the lack of thorough study of mechanism of biomass pyrolysis. As difficult extractive technique and varied species, well research of hemicellulose pyrolysis mechanism are still required. Supported by relevant research programs, a systematic research of hemicellulose properties and pyrolysis mechanism is presented in this thesis.Start with the nature of hemicellulose, microstructure of hemicellulose was known by elemental analysis and FTIR analysis. Then the thermogravimetric analyzer coupled with a Fourier transform infrared (TG-FTIR) spectrometry was applied to pyrolyze the four model compounds of hemicellulose(xylan, mannose, galactose, arabinose) contrastively. Pyrolysis behaviors of the four model compounds were compared based on the combination of pyrolysis process and microstructure. It is found that the pyrolysis procedure of the four model compounds had four phases: evaporation of free water, deprivation of chemical water, decomposition of main structure, formation of coke. The yield of coke was in the order of mannose<galactose<xylan<arabinose. Arabinose had a poor thermal stability, while xylan, mannose and galactose had a high thermal stability, of which xylan and galactose led to more small molecular products.Mechanism study of xylan fast pyrolysis was carried on an analytical pyrolyzer coupled with Gas Chromatography and Mass Spectrometer (Py-GC/MS). The results showed that xylan pyrolysis products were composed of aldehydes like furfural, acids like acetic acid, ketones such as1-hydroxy-2-propanone and furanone, and pyrans. Acetic acid and furfural were the the most typical products of xylan pyrolysis, and the production process of these two products were competitive. After the depolymerization of xylan to form unstable intermediates with straight chain, various products were formed by breakage and cyclization reaction. Based on the density functional methods, the simulation began with D-xylose monomer was carried out combined with the previous experimental results of xylan pyrolysis. It was identified that the formation of furfural was the most easy, and the formation of glycolaldehyde was difficult. The generation of1-hydroxy-2-propanone and furanone were between furfural and glycolaldehyde, and their yield were considerable.The catalytic pyrolysis of xylan was carried out in this thesis. Zeolite HZSM-5was selected as the catalyst to proceed the comparative pyrolysis of xylan and cellulose. The study found that the addition of HZSM-5to the xylan pyrolysis would promote decomposition of pyran to form aldehydes, and oxygenated compounds like ketones and acids to deoxidize to produce small molecular product CO2.HZSM-5could promote the fracture of the side chain of the5-(hydroxymethyl) furfural. Furanone and cyclopentenone were reduced, while1-hydroxy-2-propanone increased firstly and then decreased. Dehydration of pyrans was promoted strongly, followed by the decomposition reaction of the pyran ring. As for cellulose pyrolysis process, HZSM-5increased the small molecule products and aromatic hydrocarbons sharply, and plenty of CO2appeared. Sugar anhydrides products like levoglucosan were further dehydrated to form levoglucosenone. Meanwhile, furfural and5-(hydroxymethyl) furfural were inhibited, while the generation of furan was promoted.