| Fast pyrolysis is one of the promising technologies, which can convert biomass into liquid fuel "bio-oil". However, the high-quality utilization of bio-oil, substituted for transportation fuels, is quite limited by its complex chemical composition. It is attributed to the lack of sufficient understanding of reaction mechanisms in biomass pyrolysis. Supported by the related national projects, a systematic research on pyrolysis mechanism of biomass and bio-oil fractions is presented in this thesis.Biomass is mainly consisted of cellulose, hemicellulose and lignin. Among the three components. the structure of cellulose is simple and nearly changeless with the feedstock, the structure of hemicellulose is complex due to the existence of various polysaccharides, and lignin is extremely complex with unclear understanding till now. Mechanism research on fast pyrolysis of cellulose was first investigated by using an analytical pyrolyzer coupled with Gas Chromatography and Mass Spectrometer (Py-GC/MS). The results showed that the main products were composed of pyrans such as levoglucosan and levoglucosenone, furans like furfural and5-hydroxymethyl-furfural. and small molecular chemicals with linear chain like acetaldehyde and1-hydroxy-2-propanone. Small molecular compounds were obtained from the direct decomposition of cellulose, not the secondary cracking of the initial products. Based on the depolymerization of cellulose to form active cellulose, pyrans and furans were produced by dehydration reaction, ring opening reaction and cyclization reaction etc. Moreover. the simulation began with D-glucopyranose monomer, based on the density functional methods, were carried out combined with the previous experimental results on cellulose pyrolysis. It was identified that the formation of levoglucosan was easier than its isomeride of3.4-altrosan. The formation of5-hydroxymethyl-furfural was attributed to the degradation of D-glucopyranose, which had a higher conversion than levoglucosan. In addition, furfural was produced by the cleavage of hydroxymethyl group in5-hydroxymethyl-furfural spontaneously. The composition of products was closely related to the structure of samples. The products from fast pyrolysis of xylan mainly included acetic acid, furfural, and cyclopentenone etc.. and the products of lignin were predominantly composed of phenols, such as alkylphenols. guaiacols and syringols, as well as some aromatic hydrocarbons like benzene and toluene.The selective pyrolysis of biomass and its components was also studied by using the thermogravimetric analyzer coupled with a Fourier transform infrared (TG-FTIR) spectrometry. All the three zeolite catalysts HZSM-5, H-β and USY had significant influence on the dehydration reaction, resulting in an increase of weight loss in the initial stage, and meanwhile affected the char formation process during the final stage. The addition of USY had the most obvious effect on the dehydration reaction, and the effect of HZSM-5and H-β focused on the inhibition of the char formation. Though the pyrolysis products were nearly the same, their release intensities changed after the addition of catalysts. It was observed that the yield of oxygenated compounds reduced compared with pure samples pyrolysis, but the yield of light gases increased such as CO, CO2and methane. The presence of USY enhanced the conversion of oxygenated compounds mainly to water and CO2, while the addition of HZSM-5and H-β promoted the secondary cracking of residual chars to form light gases CO. CO2and methane.In view of the sequent researches on classification and upgrading of bio-oil for high-quality applications, pyrolysis behaviors of bio-oil fractions were studied on the TG-FTIR analyzer. combined with the first separation of bio-oil into fractions by using the molecular distillation technique. The obtained light fractions were constituted of water and acidic chemical, which began to loss weight at very low temperature by evaporation or was decomposed into small molecular compounds (CO2. CO. acetic acid, alcohols etc.) before200℃. Heavy fractions contained more compounds of phenols and sugars with high boiling point temperature and no water, so its heating value was highest and the stability was best among the three fractions. The weight loss range of heavy fraction was wide and the yield of char was high up to30wt%. During pyrolysis, self-evaporation was dominant at lower temperature, and decomposition was leading at higher temperature to produce more stable chemicals. The pyrolysis behavior of middle fraction was between light fraction and heavy fraction with the most kinds of products. Its yield was lowest, so it was regarded as light fraction or heavy fraction selectively based on the experimental conditions. |
