Mechanism Researches Of Cellulose And Hemicellulose Pyrolysis And Their Products Regulation

Lignocellulosic biomass can be converted into bio-fuel and bio-chemicals efficiently via pyrolysis. It is difficult to improve the quality of bio oil and the selectivity of target chemicals due to lack of in-depth understanding of the biomass pyrolysis mechanism. Oxygen-rich cellulose and hemicellulose are the major components of lignocellulosic biomass. Their pyrolysis behaviors can largely determine the overall characteristics of biomass pyrolysis. Thus, it is the key for cellulose and hemicellulose regulation during the pyrolysis process. In this study, we have investigated the main chemical bond break, intermediate product formation and decomposition, pyrolysis products distribution and the target products regulation during pyrolysis process through several methods including pyrolysis process tracking, fast pyrolysis-spin trapping, and catalytic pyrolysis, etc. This study aims to provide theoretical and technical supports for the preparation of high quality liquid fuel and high value-added chemicals via biomass pyrolysis.The evolution characteristics of cellulose and xylan pyrolysis were investigated through tubular reactor platform. The results indicated that although the structure of cellulose was not fundamentally altered below 300 oC, the crystallinity was decreased. With the increase of temperature, β-1,4-glycosidic bond broke firstly, followed with the dominant pinacol dehydration reaction. Cellulose was decomposed and β-1,4-glycosidic bond was broken completely at 300-325 oC. With dehydration, decarboxylation and decarbonylation reaction, a large amount of anhydrosugars, furans, aldehydes, and ketones were released. The yield of levoglucosan reached 48.99%. During xylan thermal decomposition process, two peaks were obeserved in DTG curve at 235 and 280 oC, respectively. In the first phase of xylan pyrolysis, the side chain was broken preferentially; the glycosidic bond of the main chain was broken partially; and the degree of polymerization was decreased. With the increase of temperature(above 250 oC), the glycosidic bonds of xylan were completely broken. A large amount of acids, ketones, alcohols, esters, and furans were released, which the yield of acids was more than 30%. At the high temperature zone, benzene compounds and phenolic compounds were formed. CO2 and CO were the main gaseous products derived from cellulose and xylan pyrolysis. The yield of biochar derived from cellulose and xylan at 500 oC were 22.22% and 32.00%, respectively. Moreover, the biochar had a strong paramagnetism.Fast pyrolysis process of cellulose, hemicellulose and their model compounds was investigated. The results indicated that during glucose, xylose and cellobiose fast pyrolysis process, these compounds were converted to a molten state and formed active intermediates. The molecular weight of intermediates was 300-1500, which suggested that they were consisted of 3-9 sugar units with β-1,4-glycosidic bond. During the pyrolysis process, intermediates were boiling and would be further degraded to the lower molecular products. While the molten state of monosaccharides and disaccharides was not observed during cellulose and xylan fast pyrolysis process. PBN, NMP, and DMPO, as spin traps, were used for fast pyrolysis- spin trapping experiments, respectively. The results implied that free radical reaction mechanism was involved during cellulose and hemicellulose and their model compounds pyrolysis. Furthermore, the carbon centered free radicals may be produced. The intensity of spin adducts detected by EPR was higher at 800 oC pyrolysis than that at 500 oC, indicating that the generation of free radicals more abundant with the increase of pyrolysis temperature.The pyrolysis-in-suit experiment indicated that dehydration, decarboxylation and decarbonylation reaction were occurred during the main weight loss stage of cellulose, hemicellulose and their model compounds. Acids, ketones, furans and anhydrosugars were released. The respective peaks assigned to OH, C-H, C=O and C-O of the FTIR spectra were recorded. Moreover, with the temperature increased, there were the of the solid product carbonization and secondary reactions derived from volatile products were dominant and the intensity of the peaks was enhanced.Pyrolysis- in-suit EPR was carried out initially for free radical study in biomass pyrolysis. The results indicated that free radical reaction exactly occurred during cellulose, hemicellulose and their model compounds pyrolysis process. A large amount of free radicals were released in the main weight loss stage of their thermal decomposition process. With the temperature increased, the secondary reaction became dominant and the intensity of EPR signal gradually enhanced.The yield of bio-oil derived from cellulose and xylan fast pyrolysis at 600 oC under the nitrogen regime were 86.96% and 63.53%, respectively. Under the hydrogen-rich(H2/N2=1/9) atmosphere, the yield of bio-oil derived from cellulose and xylan pyrolysis were decreased. However, at the hydrogen-rich atmosphere, it could inhibit acids formation significantly. Anhydrosugars were the major molecular products of cellulose pyrolysis and the yield of levoglucosan reached 36.98 wt.%. While ketones were the major molecular products of xylan pyrolysis, the yield of 1-hydroxy-2-acetone was up to 2.13 wt.%. Under an inert atmosphere, catalyst HZSM-5, ZSM-5, and NCK-5 with 5-20% addition could inhibit acids formation derived from cellulose and xylan pyrolysis, while they were ineffective under hydrogen-rich atmosphere. What’s more, the three catalysts could promote the further transformation of cellulose dehydration effectively under the hydrogen-rich atmosphere.