Study On Microscomic Mechanism Of Hemicellulose Gasification In High Temperature Steam

An efficient gasification technology of biomass is used as the background of this thesis for investigation of the hemicellulose mold compound xylan. Based on the quantum chemistry density functional theory(DFT), to research hemicellulose's micro reaction mechanism in the high temperature steam's gasification process and the TG technology is applied to analyze hemicellulose's gasification behavior under different water vapor concentration atmosphere(20%, 40%, 60%, 80%, 100%), so as to establish the kinetics model of hemicellulose gasification reaction in high temperature steam. The research results is of great significance for revealing the gasification mechanism of biomass in high temperature steam. The main researches in this thesis are as follows:(1) Based on the density functional theory(DFT), the Gaussian software is adopt to simulate and optimize the molecular structure of hemicellulose, and to calculate its vibration frequency so as to set up the steady state model of molecules with proper configuration parameters. Then, the researches verify the correctness of the speculation on the hemicellulose molecular structure model through comparing the hemicellulose molecular's structure group frequency and theoretical calculations which are measured by infrared spectrometer.(2) Eight reaction paths of the hemicellulose gasification in the high temperature steam(700K~1500K) are designed, and the reaction mechanism of hemicellulose gasification in high temperature steam is studied. Using quantum chemistry density functional theory(DFT),geometry optimization and vibration frequency of stagnation points(reactants, intermediates,transition state and product) in gasification reaction are investigated at B3 LYP / 6-31 g(d)level. The results show that there is minimum gibbs free energy change in the reaction using path 1 and path 2 at all temperature settings, which means the reactions following path 1 and path 2 are most easily happen with most generation of the 2-furfural(P1). There is least activation energy in the reaction using the path 4, which proves that it's the optimum reaction path.(3) Thermogravimetric study of hemicellulose gasification reaction is conducted at different water steam concentration atmosphere(20%, 40%, 60%, 80%, 100%), the influenceof different concentrations on hemicellulose gasification reaction is analyzed, and the kinetics characteristics of steam gasification under different concentrations is investigated as well. The studies show that the gasification of hemicellulose in high temperature steam can be divided into two stages, i.e., the volatile precipitation stage(190?~690?) and the carbocoal gasification stage(690?~770?). The activation energy that hemicellulose required increases with the rise of temperature and arbocoal gasification is harder to happen than volatile precipitation. With the increasing of water steam concentration, the activation energy of change shows a trend of increasing at first and then decreasing. When the water steam concentration increases from 20% to 40%, the activation energy that required increases from50.24 kJ/mol to 53.18 kJ/mol. While continuing to increase the water steam concentration to100%, the activation energy that required reduces to 38.55 kJ/mol.