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The Study of Pyrolysis Mechanism of Biomass

Author: LiHaoJie
Tutor: LiuChao
School: Chongqing University
Course: Power Engineering and Engineering Thermophysics
Keywords: Biomass Xylan Cellulose Pyrolysis The molecular simulation
CLC: TK6
Type: Master's thesis
Year: 2011
Downloads: 380
Quote: 2
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Abstract


With the growth of global population and the development of production, more energy will be consumed. As the reserve of conventional energy is limited and non-renewable, how to develop the renewable energy reasonably has become a new topic for human in 21st century. As the environmental and resources problem becomes prominent day by day and utilization technology of biomass resources becomes increasingly mature, biomass resources plays a more and more important role in energy and chemical raw materials. Biomass resources will eventually become one of the fundamental pillars of our sustainable development.Previous researches on biomass pyrolysis focus on the products of pyrolysis gas and liquid tar product, and whose objective is getting the largest yield by controlling reaction conditions. The main bond breaking and pyrolysis path of xylan through molecular dynamics and quantum chemistry methods has been studied by this paper. This kind of research method has broken the“black box”method which is widely used to study biomass pyrolysis before. The initial pyrolysis process of cellulose is studied by using laser experimental method and the existence of active cellulose has been affirmed. The main research results are as follows.The polymers builder of hyperchem and the semi-empirical method were used to build and optimize the molecular model of xylan, from which the characteristic parameter of xylan molecular were obtained and listed in this article. Based on Amber force field, pyrolytic decomposition of xylan molecular (DP 9) was simulated and analyzed under the condition of Periodic boundary conditions and temperature being from 300 to 1300K, in the size of Periodic Box of 15(A|·)×10(A|·)×45(A|·) . The simulation results showed that the glycosidic bond broke off at 550K. At the same time, the“-OH”on Pyran ring also broke off. When the temperature rose to 650K, the“C-O”bond of pyranose broke out of the chain, later the“C-C”bond broke down.This simulation got the main cracking groups and analysed the corresponding production to those cracking groups. Meanwhile, the mechanism of production such as 2- furfuraldehyde, HAA, CO2, CO was also analyzed.Eight paths of possible pyrolysis reactions were designed,and B3LYP/6-31++G(d, p) methods in Gaussian 03 package was used to study the pyrolysis reaction mechanism of xylan. The equilibrium geometries of the reactants, transition states, intermediate and products of every reaction path were optimized, and the transition states were checked by IRC method. The standard thermodynamic and kinetic parameters of every reaction path from 298 K to 1098 K were calculated. The result shows that: Path1 and Path 2 are exothermic reaction. Path3 converts endothermic reaction into exothermic reaction above 798 K, and other paths are endothermic reaction. When the room temperature below 798 K, the convert ratio of 2-furfuraldehyde (P1) is the highest, and above 898 K, the best path is Path4. According to the kinetic analysis, we get three better paths from all the paths of xylose molecule pyrolysis under the different bond breaking-way. The rate-determining step of path 2 is step 5, whose activation energy is 236.9kJ/mol. The rate-determining step of path6 is step7, whose activation energy is 363.9kJ/mol. The rate-determining step of path 7 or path 8 is step15, whose activation energy is 336.5kJ/mol. And, whatever thermodynamic analysis or dynamic analysis prior support Path 2 which produces 2-furfuraldehyde and water. It secondly support Path 6 which produces glycolaldehyde and methanal. At last, it supports Path 7 or Path 8 which produces glycolaldehyde, glycol and CO.The experiment device of laser pyrolysis was put up. The flash pyrolysis of cellulose was done on this device. Some yellow substances which may be dissolved in water were obtained from the experment. According to the analysis by mass spectrum, it is found that the main constituents of this kind of yellow substances are oligosaccharide whose polymerization degree is from two to eleven which is checked by simulate calculation. This result is the same as the active cellulose described in the literatures. According to the HPLC analysis, the yield of glucose, fructose and oligosaccharide are determined. The results show that glucose is not the only production decomposing from oligosaccharide and another production decomposing from oligosaccharide will compete against glucose at high-radiation power. The mechanism of the first pyrolysis of cellulose is analysed at last.

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