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Research on Co-gasification Mechanism of Biomass and Coal Based on Surface Physicochemical Structure Characteristics

Author: CheDeYong
Tutor: LiShaoHua
School: North China Electric Power University
Course: Thermal Power Engineering
Keywords: biomass coal co-gasification microstructure functional group TG-FTIR
CLC: TK6
Type: PhD thesis
Year: 2013
Downloads: 83
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Abstract


Biomass is an abundant and renewable source of energy. Co-gasification of biomass with coal could not only reduce the harmful gases emissions such as SOx and NOx, but also obtain the complete transformation of carbon and improve the efficiency of co-gasification. Therefore, the study of the co-gasification of biomass with coal process is one of the crucial areas of the clean utilization of coal. However, the mechanism of the synergistic effect in the co-gasification of biomass with coal process remains unclear. Further studies are necessary to understand the formation of gas products, dissociation mechanisms and the evolution of physical and chemical structure characteristics occurd on the surface of coke. In this paper, the physicochemical properties of biomass and coal and the change of microstructure in gasification process were analyzed. Based on these results, co-gasification characteristic of biomass and coal in fluidized bed was systematically studied in both theory and experiment, and from the effects of various factors such as biomass blending ratio and equivalence ratio(ER), the synergistic effect mechanism of co-gasification of biomass and coal was revealed. The following investigations are made in this thesis:The thermal analysis of co-gasification of biomass with coal was performed. The effects from heating rate, reaction atmosphere, mixing proprotion were studied on in details. The C-R and DAEM method were applied to theatrically analyze the co-gasification kinetics characteristic. A co-gasification reaction kinetics model was established, and parameters were acquired. The trend of activation energy in the critical reaction stage was analyzed. The reaction stage where co-gasification compensation effect happened was confirmed. The results showed that the co-gasification of biomass with coal process included four stages:drying, preheating, volatile precipitation and coke gasification. There was no significant synergistic effect in the volatile precipitation stage. However, a significant synergistic effect was observed in the coke gasification stage.This effect got the maximum, when mixing proprotion was50%. The pyrolysis stage of co-gasification can be described by a homogeneous reaction model. The reaction mechanism of the gasification matched the unreacted shrinking core model. The activation energy of coke gasification stage was significantly higher than that of the volatile precipitation stage. Based on thermal analysis experiments, a TG-FTIR was employed to study the gas products of co-gasification of biomass with coal. The effects of temperature-rising rate on the gas products were studied. By the analysis of the micro surface morphology/microstructure(SEM, EDX) and functional groups (FTIR) of gasification semicoke, a mechanism of formation of the flammable gas such as CO and CH4was proposed. The results showed the evolution of the surface morphology of semicoke in the gasification. Initially, the porous cannel structure formed. Then the poles collapsed and cannels were closed eventually. The main gas products of pine sawdust gasification were CO, CH4, H2O and organic matters such asalcohols, aldehydes, carboxylic acids, phenols and so on. In the co-gasification process, the formation of CH4was the results of the rupture and reformation of C-H bonds in the methyl, methylene and methoxy groups in lignite and pine sawdust, which throughout the process of volatiles releasing and coke gasification. CO is mostly released in volatile precipitation stage and coke gasification stage, which can be attributed to carboxyl, methoxy, carbonyl, ether bond rupture in volatile precipitation stage and the reduction of CO2in coke gasification. The gasification heating-rate affect the formation of the gas products. A lower gasification heating-rate might lead to more CO and CH4.The experimental investigation of co-gasification of biomass and coal was carried out on a self-designed fluidized bed reactor. The effects of biomass blending ratio, equivalence ratio(ER) as well as steam-to-fuel ratio(S/F) on co-gasification characteristic of biomass and coal were systematically studied, the above results provide a basis for selection and operation of co-gasification system of biomass and coal. The test results showed that when the blending ratio of pine sawdust increases, the volume concentration of CO incereased, the change of CH4was not notable, the heating value reached a maximum at a blending ratio of50%. The gasification efficiency reached a maximum at an ER of0.26. The results showed that the introduction of steam resulted in more hydrogen production. When the steam-to-fuel ratio(S/F) was0.28, the heating value, the carbon conversion ratio and the gasification efficiency all reached the maximum.Based on the bed test of the co-gasification of biomass with coal, the ASPEN PLUS was applied to simulate the co-gasification of biomass with coal in a fluidized bed gasifier by considering the hydrodynamic and reaction rate kinetics simultaneously, the external FORTRAN subroutines for hydrodynamics and kinetics nested in ASPEN PLUS simulate the gasification process, the model was developed to study the co-gasification of biomass with coal in fluidized bed gasifier was employed to studied the effects of equivalence ratio(ER), blending ratio as well as steam-to-fuel ratio(S/F) on the gas composition, heating value and gasification efficiency, and the results were compared with the experimental results. Above results show that the co-gasification of biomass with coal in fluidized bed gasifier could be simulated accurately with the model developed in this thesis.

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