Study On Reaction Mechanism And Its Performance Of Coal And Biomass Co-gasification With Steam

Biomass as a renewable clean energy resource has several advantages, such as low sulfur content, CO2neutral and so on. Thus, it has attracted considerable attentions from many researchers. However, direct utilization (eg.combustion) or indirect utilization (eg. gasification, liquefaction etc.) of biomass as a fuel still exist many problems, such as low volume heating value and poor quality of its products. Co-gasification of coal and biomass is one of the technologies to solve these problems. Co-gasification, as one of the co-conversion technologies, is believed to be the most promising technologies for the efficient utilization of coal and biomass. During the process of co-gasification, interactions between coal and biomass may occur, resulting in a very complex reaction process. Thus, this research focuses on the mechanism and the synergistic effect of the co-gasification of coal and biomass. This thesis can be categorized into several parts:The pyrolysis of three kinds of coals (lignite, bituminous coal and lean coal) and three kinds of biomass (sawdust, rice straw and corn straw) and their blends were studied by TGA. The formative products were monitored online by using FTIR contacted to the TGA. The kinetic mechanism and synergistic effect between coal and biomass during the pyrolysis were investigated. The results showed that the temperature at both starting weight loss and maximum weight loss peak of the blends pyrolysis were lower than those of the single coal pyrolysis. Biomass might promote the pyrolysis of coal. The promotion was more significant at higher temperature. The synergistic effect was most significant for the blend pyrolysis with the mass ratio in a certain range. The formations of CO, CO2and CH4gaseous products were affected obviously by blending coal and biomass. The kinetic parameters were calculated by DAEM method. The calculation results showed that the activation energy was not only simply to deepen monotonically increasing with reaction, but also was a function with conversion changing. And the synergistic effect was proved by variety characteristics of reaction activation energy when the coal to biomass blends.The steam gasification of three kinds of coals (lignite, bituminous coal and lean coal) and three kinds of biomass (corn straw, rice straw and sawdust) and their blends were studied by TGA. The kinetic of the gasification and synergistic effect between coal and biomass were investigated. The study found that the synergistic effect was most significant for blend of lignite and sawdust in a certain range. Kinetic calculation showed that the activation energy of lignite was decreased by blending sawdust. The formations of CO were affected obviously by blending coal and biomass.Using nitrogen isothermal adsorption/stripping, and Fourier infrared and Raman spectroscopy, the structures of the chars from single coal and biomass and their blends were analyzed to investigate the evolution of the physical and chemical structure of char during the co-gasification. The study found a random growth pattern during the pyrolysis stage of char pore; the gasifying agent firstly reacts with the active site around the area of mesopore and micropore to form mesopore. After the active site has been completely consumed, the gasifying agent begins to erode carbon skeleton, and the mesopore converges again until the carbon skeleton collapses. The main reason for the increase of active site during co-gasification was the introduction of hydroxyl groups from biomass. Based on the analysis of the physicochemical properties of the chars during co-gasification, the temperature is found to be one of the most significant parameters on controlling the evolution of the char structures. This results that the co-gasification process can be mainly divided into three stages:a) the stage when the poly condensation reaction is faster than micromolecule gasification reaction at temperatures under600℃; b) the stage when the micromolecule gasification dominates at temperatures between600℃-900℃; c) the stage when macromolecule carbon skeleton is gasified directly at temperatures above900℃.Finally, steam co-gasifications of coal, biomass and their blends were carried out on our self-design bench-scale gasification rig. The results showed that with the increase of sawdust mixed ratio, CO2and H2increases, CO, CH4reduces, and the carbon conversion, the gasification efficiency and char gasification activity firstly increases with sawdust mixing ratio and then decreases, reaching a maximum value with the mass ratio in a certain range, at the same time the highest co-gasification synergistic effect between sawdust and lignite occurs. The types of coal and biomass also influence the co-gasification rate obviously. The most obvious co-gasification synergistic effect was found between the lowest-rank coal lignite and sawdust, showing the highest gas production and carbon conversion rate. By using the ASPEN PLUS software, the coal and biomass gasification reaction model on this bench-scale reactor was built, showing good consistency between modeling results and experimental results.In this study, the synergistic effect between coal and biomass during co-pyrolysis and co-gasification, the chemical and physical properties of the char prepared at different pyrolysis condition and the changing of the char property during the gasification, and the gasification of single coal, biomass and their blends conducted on a bench-scale reactor were investigated. A large amount of fundamental and valuable data of the coal, biomass and their blends gasification were obtained and the mechanism of the co-gasification was studied. It is believed that the results obtained by this research can provide theoretical guidance for practical applications of coal and biomass co-gasification.