Catalytic Co-gasification Of Biomass And Coal In External Circulating Radial-flow Moving Bed Gasification System

With the increasing consumption of fossil fuels and the severe environmental pollution, biomass as a renewable clean energy attracts greater attention around the world. Biomass gasification is one of the promising process among all biomass conversion processes. Co-gasification of biomass and coal can takes full advantages of the renewable energy at a reduced consumption of coal and as a result a reduced emission of pollutants. In this thesis, co-gasification of biomass and coal were carried out in an external circulating radial-flow moving bed (ECRMB) decoupled gasification system with solid heat carrier under different gasification condition. This gasification system consists of a solid-solid cocurrent and gas-solid countercurrent moving-bed pyrolyzer, a gas-solid crosscurrent radial flow moving-bed gasifier, and a riser-type combustor. The circulation ofbed material, whichacts as both catalyst and heat carrier, is achieved through the three reactors. The gas in combustor is separated with that in the other reactors. The required heat for pyrolysis and gasification reactions is provided by combustion of the pyrolysis char and coke on catalyst through the circulation of the heat carrier. The tar in product gas was reduced markedly by using radial flow moving-bed gasifier. The evolution of catalyst during the circulation in the gasification system and tar formation at different gasification condition was investigated.Firstly, co-pyrolysis characteristics of biomass and coal were conducted on a thermogravimetric analyser. The effects of biomass ratio and catalyts on the co-pyrolysis were investigated. The result showed that the pyrolysis characteristics of individual biomass and coal were maintainded, and there showed no synergetic effect during the co-pyrolysis due to different pyrolysis temperature region of the individual biomass and coal. The addition of olivine and olivine supported metal catalyst promoted the co-pyrolysis reaction and showed improved carbon conversion rates.Biomass and coal steam gasification were carried out with olivine as catalytic bed material on the ECRMB gasification system. The effects of feedstock type, gasifer temperature, circulating rate of bed materials, feeding rate of feedstock and S/C on gasification performance were investigated. The result showed that, feedstock with higher volatile content was more suitable for production of H2-rich gas. Tar destruction was promoted by the incraesing gasifier temperature. The gas yield, carbon conversion and syngas content increased with the increasing gasifier temperature. The increase of feedstock feeding rate increased the gas yield and the H2 concentration in the product gas and decreased the CO2 concentration. But it had unemarkable influnce on the tar yield. Gasifier temperature, S/C and coal feeding rate are the most important factors for coal gasification.Increasing temperature and S/C promoted tar cracking and steam reforming reactions.The gas yield and concentration of H2 were promoted by the increasing feedstock feeding rate and the decreasing bed material circulation rate.Co-gasification test were carried out with pine sawdust and bituminous coal as feedstock. The effect of different gasification condition (biomass ratio, pyrolyzer tempetarue, gasifier temperature, S/C) on co-gasification performance and synergetic effect between biomass and coal were investigated. The result showed that, the gas yield, carbon conversion, lower heating value (LHV) and chemical efficiency were increased with the increasing biomass ratio. Synergetic effect was found in terms of gas composition during co-gasification. Tar creacking and steam reforming reactions were promoted with the increase of S/C from 0 to 1.3, however, no remarkable changes on gas yield and gas composition had been shown during further increment of S/C. Pyrolysis temperature (500-700℃) and feedstock partical size (0.18-0.83mm) had little impact on product gas composition.The catalytic effect of olivine, olivine supported nickel (NiO/olivine), olivine supported iron (Fe2O3/olivine) catalysts on the performance of gasification process and the evolution of catalysts were investigated during the redox-cycling by means of XRD, TPR, SEM-EDX. The NiO/olivine showed high tar destruction and CH4 reforming catalytic effect during biomass gasification. The gas yield of 1.55Nm3/kg, H2 concentration of 54.3%, tar yield of0.96g/Nm5, catatlytic tar removal efficiency of 97.0% and water conversion of 14.5% were obtained at the gasification temperature of 800℃ with NiO/olivine. Catalytic activity of Fe2O3/olivine on tar and CH4 reforming was similar to that of olivine. Catalytic activity were improved and the consumption of H2 and CO reduced by decreasing catalyst circulation rate.Tar evolution and its composition under different gasification condition were studied during co-gasification of biomass and coal. It was found that, the phenols were the most dominant compound in the pyrolysis tar. The alkyl-substituted PAHs,3 and 4 ring PAHs and the sulfur containing compound in the tar decreased with the increasing biomass ratio. Naphthalene was the dominate tar component for the biomass or coal gasification as well as for co-gasification of biomass and coal. Its relative content has been raised with the increasing BR, gasifier temperatures, and decreased with increasing S/C. Compared to quartz sand, olivine showed higher catalytic activity for tar decomposition. Phenols and one-ring aromatics were totally decomposed at the gaifier temperature of 800℃. Formation of naphthalene was inhibited with the increasing S/C.The results verified that this new concept of biomass and coal gasification is feasible. The combination of ECRMB gasification system with effective catalyst is a good solution to decrease the tar yield and enhance the efficiency of the process.