Pyrolysis Study Of Wood Biomass By Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry

In consideration of the depletion of fossil fuels and serious environmental issues, the effective utilization of biomass resource is attracting great attention, because it is carbon-renewable and CO2neutral. China is rich in forest resources. However, direct combustion, a common utilization method of wood resources with low efficiency will cause severe environmental pollution. Biomass pyrolysis is a thermal conversion technology used for the production of biofuels, which would be a promising method of biomass utilization in the future. The common methods of studying biomass pyrolysis products mainly include thermogravimetry (TG), infrared spectroscopy, and chromatography. However, these methods cannot perform the rapid on-line analysis during biomass pyrolysis. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV PIMS) is a rapid detection method. In this work, SVUV PI technique in combination with high-performance commercial mass spectrometer was applied for the pyrolysis study of poplar, pine and their three main components, i.e., cellulose, hemicellulose and lignin.In the1st chapter, the importance of biomass and the necessity of studying biomass pyrolysis has been introduced based on energy and environmental issues. The research methods used in this thesis are briefly outlined.The2nd chapter shows the elemental analysis of poplar and pine, and then demonstrates the structural characterization of cellulose by infrared spectroscopy and cross polarization/magic angle spinning13C nuclear magnetic resonance (CP/MAS I3C NMR). The structural characterization of hemicellulose was investigated by infrared spectroscopy,1H nuclear magnetic resonance (1H-NMR), and13C nuclear magnetic resonance (13C-NMR). The structural characterization of lignin was studied by infrared spectroscopy,1H-NMR,13C-NMR and distortionless enhancement by polarization transfer (DEPT) technique. The results show that cellulose mainly consist ofβ cellulose and amorphous cellulose. Hemicellulose is linked by P-D-1,4-xylose units. It can be confirmed that4-O-methyl-D-glucuronic acid and arabinofuranosyl is linked to carbon atoms in the main chain of xylan. Lignin residue is also found in hemicellulose. Lignin is built by three cinnamyl alcohols, namely p-coumaryl, coniferyl, and sinapyl alcohol. The linkages among them mainly consist of β-O-4’ bond, β-5’, β-β’,5-5’, and β-1’units. Lignin also contains hemicellulose residue, where xylan is the dominant component.In the3rd chapter, the thermal decomposition characteristics of cellulose, hemicellulose and lignin at different heating rates are investigated by TG, which shows that the starting pyrolysis temperature of hemicellulose is lower than that of cellulose. The temperature range during lignin pyrolysis is wider than those of cellulose and hemicellulose. The pyrolysis of hemicellulose and lignin can be divided into two steps. According to the pyrolysis products study of three main components at low pressure by SVUV PIMS as well as GCMS, the thermal decomposition pathways during early stage of cellulose include transglycosylation and glycosidic rupture. However, during the late stage of cellulose pyrolysis, transglycosylation pathway dominates. At the early stage of hemicellulose pyrolysis, the formation of4-hydroxy-5,6-dihydro-(2H)-pyran-2-one through dehydration is the main pathway. As the reaction time increases, the formation of furfural can be observed gradually. During the early evolution of lignin pyrolysis, a series of species with a double bond in conjugation with the aromatic ring appear earlier than others in lignin, which is probably due to the insufficiency of hydrogen. The main pathways during the pyrolysis of three main components are discussed in this chapter. The solid residues of pyrolysis at500℃from three components are analyzed by infrared spectroscopy, and the results show that none of them is pyrolyzed completely.The4th chapter introduces the thermal decomposition characteristics of poplar and pine at different heating rates by TG, which shows that the temperature ranges during poplar and pine pyrolysis are nearly the same. The trends of the maximum weight loss as well as the maximum weight loss rate of poplar are consistent with those of pine, as heating rate increases. According to the pyrolysis products study of poplar and pine at different reaction pressure by SVUV PIMS as well as GCMS, the pyrolysis products of them are nearly the same, except for the pyrolysis products of lignin. Lignin in poplar is predominantly composed of guaiacyl and syringyl subunits, which shows that poplar is characteristic of hardwood. However, lignin in pine is rich in guaiacyl subunit, which demonstrates that pine is typical of softwood. The pyrolysis mechanisms of cellulose and hemicellulose in poplar are different from those of pure cellulose and hemicellulose, which is probably affected by other components in poplar. The same results during pine pyrolysis are also confirmed in this work. The reaction pressure affects the molecular weight distribution of pyrolysis products, where more high molecular weight species can be detected at higher pressure. The reaction pressure also affects pyrolysis speed of three main components in poplar and pine, especially for the appearance order of lignin pyrolysis products with and without double bond. The solid residues of pyrolysis at500℃from poplar and pine are analyzed by infrared spectroscopy, and the results show that lignin in poplar is easier to be pyrolyzed than lignin in pine.