Biomass Pyrolysis Mechanism Based On The Multi-components

Fast pyrolysis is one of the frontier technologies in the field of biomass energy. However, the utilization in large scale is quite limited due to the complex composition of bio-oil and the unstable properties, which are attributed to the lack of thorough understanding of the related reaction mechanisms. Supported by the Basic Research Program of China and the National Natural Science Foundations, an integrated research of biomass pyrolysis mechanism based on multi-components is presented in this thesis.Biomass is mainly composed of cellulose, hemicellulose, lignin, minor amounts of other organics, as well as a few minerals. Based on Van Soest method, the components in ten wood species were quantified, and the fibre present in each sample was separated into three fractions:neutral detergent fibre (NDF), acid detergent fibre (ADF) and strong acid detergent fibre (SADF). Pyrolysis of the detergent fibres was carried out on a thermogravimetric (TG) analyzer coupled with a Fourier transform infrared (FTIR) spectrometry. After the removal of extractives and soluble minerals, pyrolysis of NDF shows the characteristics of the three main components. Hydrocarbons, aldehydes, ketones, acids, alcohols and others are generated due to the primary pyrolysis of hemicellulose and cellulose in single stages. Pyrolysis of SADF, viz. Klason lignin undergoes in three consecutive stages, corresponding to the evaporation of water, the formation of primary volatiles and the subsequent release of small molecular gases. Phenols and alcohols are the dominant volatiles released from pyrolysis of SADF.With the view of the controversy about cellulose pyrolysis theory, a detailed study of the formation and consequential evolution of active cellulose in cellulose pyrolysis was carried out. An intermediate product that was yellow, soluble and solid at ambient temperature and liable to decompose at even high temperatures was obtained in a high-radiation flash pyrolysis reactor. The yield tended to increase initially until achieving a steady state, and then increase again with the progress of reaction. The compositional analysis of the yellow product was performed on high performance liquid chromatography (HPLC). It was indicated that the product mainly consisted of oligosaccharides, glucose, levoglucosan, methylglyoxal, and so on. The compounds including oligosaccharides such as cellobiose and cellotriose, and monosaccharides such as glucose were regarded as active cellulose. The relative yield of active cellulose increased initially, followed by a decreasing trend. It was suggested that active cellulose was quite unstable at high temperature, and easily decomposed into saccharides with lower degree of polymerization (DP), even char, volatiles, and gaseous products. On this basis an improved mechanism was proposed to describe the reaction route of formation and consequent evolution of active cellulose during cellulose pyrolysis. Based on the widely applied B-S kinetic mechanism model, and combined with the experimental conditions of the high-radiation flash pyrolysis, two numerical models were established corresponding to the thermally thin and thermally thick cellulose samples, respectively. For the thermally thin scenario, the pyrolysis is affected by both the available heat flux and the sample amount. The maximum conversion rate of active cellulose lies in the range of 90-94% with quite transient lifetime. For the thermally thick scenario, the surface layer sample begins to pyrolysis firstly, and then the reaction spreads to the lower layers of the sample. With the radiant flux of 6.5×106 W·m-2, both the surface temperatures of cellulose and active cellulose have a steady-state value, as 736 K and 918 K, respectively. Active cellulose exists in some period with a steady yield, which is in good agreement with the experimental results.For the study of another main component of biomass—lignin, the milled wood lignins (MWL) were extracted from Manchurian ash (MA) and Mongolian Scots pine (MSP) using the Bjorkman procedure, which has little effect on the structure of lignin. The MWL of MA has both syringl and guaiacyl units, showing the characteristics of lignin in hardwood, while the MWL of MSP only has guaiacyl units. In addition, the former has higher content of methoxyl groups. The pyrolysis behavior of both MWLs was studied by TG-FTIR, and apparent diversities were found. The formation mechanisms of the main products were investigated. The fast pyrolysis of both MWLs was performed on a mechanism experimental system. Higher yield of bio-oil was achieved during the pyrolysis of MWL from MA, while more char product was obtained during the pyrolysis of MWL from MSP. The yield of alcohols is related to the content of methoxyl groups, while the contents of guaiacol and syringol could be used to distinguish the lignin species.Fifteen synthesized biomass samples were prepared by mixing the three main components. Their pyrolysis TG/DTG curves by experimental and calculation showed inneglectable interactions between the components. Based on the compositional analysis, typical proportions were selected and four synthesized samples were prepared accordingly. During the fast pyrolysis, the interactions between the components result in lower bio-oil yield and higher gas and char yield. Both cellulose and hemicellulose restrained the formation of hydrocarbons during the pyrolysis of lignin. Hemicellulose strongly promotes the formation of 2,5-diethoxy-tetrahydrofuran and depressed that of levoglucosan during cellulose pyrolysis. Cellulose promotes the formation of acetic acid and furfural during hemicellulose pyrolysis, while lignin intensively restrained their formation. The existence of cellulose or hemicellulose improves the yield of phenols during lignin pyrolysis. Due to the close-knit structure in crude biomass, these interactions seem more violent. Compared with the crude biomass, lower bio-oil yields were achieved during the pyrolysis of synthesized samples, and obvious differences exist in the composition of bio-oil. When predicting the pyrolysis product distribution (especially the bio-oil composition) for crude biomass with the separated components, the interactions between them should be considered.