Gasification Characteristics Study Of Pyrolvzed Bituminous Coal Char

Coal consists of various contents, and different contents have different reaction characteristics. The staged conversion utilization coal polygeneration system can utilize the different contents of coal separately via combining the pyrolysis, combustion and gasification processes together, and has a high coal utilization efficiency. Therefore, it is expected to efficiently and clean utilize the diverse kinds of coal in China. Zhejiang University has proposed a staged conversion utilization coal polygeneration system based on coal pyrolysis and char gasification technologies. This system suits for the coal with high tar and volatiles content. It combines a bubbling fluidized bed pryolyzer and a circulating fluidized bed gasifier together. The volatiles are extracted from coal to produce tar and pyrolysis gas at first. The char is gasified with steam and oxygen to produce syngas. The tar is used to produce fuel oil via hydrogenation or extract other high value chemical products. The pryolysis gas and syngas are used to produce methanol or fuel oil via chemical synthesis. This system can reduce the energy consumption during coal conversion process, and can increase conversion efficiency. In this paper, the researches focus on char gasification mechanisms and technology based on this staged conversion utilization coal polygeneration system.Firstly, the process simulation of the staged conversion utilization coal polygeneration system based on coal pyrolysis and char gasification was established on Aspen Plus software. The system thermal efficiency, products yields and capital investment of this system were obtained from the simulation. For comparison, a conventional coal polygeneration system based on coal gasification and Fischer-Tropsch (F-T) synthesis process was also established on Aspen Plus. The simulation results show that the staged conversion utilization coal polygeneration system based on coal pyrolysis and char gasification with a coal feedrate of227t/h Yulin bituminous coal can produce about15t/h fuel oil,27t/h methanol and511700 kWh electricity, and has a thermal efficiency of51%. Compared with the conventional coal polygeneration system based on coal gasification and Fischer-Tropsch (F-T) synthesis process, the staged conversion utilization coal polygeneration system based on coal pyrolysis and char gasification has higher thermal efficiency and lower capital investment. This is mainly due to the less energy and water consumption during tar hydrogenation for fuel oil production and less oxygen consumption of char gasification. This system has good fuel adaptability, lower oxygen and water consumption, lower cost of fuel oil production, and higher system efficiency.Secondly, the reaction mechanisms of bituminous coal char gasification at atmospheric and elevated pressures were studied. The char samples were prepared in a bubbling fluidized bed, and the char gasification experiments were conducted in the mixture of H2O and CO2or in the mixture of H2O, CO2, H2and CO, using a modified PTGA system. The homogenous reaction model and Langmuir-Hinshelwood (L-H) model were used together to predict char gasification reaction rate. The effect of gasification pressure on char gasification was analyzed. The kinetic parameters of L-H model at atmospheric or elevated pressures were obtained from experiments, and the assumptions of separate or common active sites reaction mechanisms were validated in the experiments. Results indicate that the L-H model based on common active sites assumption is more accurate for char gasification in the mixture of H2O and CO2or in the mixture of H2O、CO2、H2and CO at atmospheric pressure. These means the common active sites reaction mechanism is right. However, neither the L-H model based on common active sites assumption nor the L-H model based on separate active sites assumption are accurate at elevated pressure. Therefore, a modified L-H model was proposed. This model added a modification factor to the L-H model based on common active sites assumption. The modified L-H model can easily and accurately predict char gasification reaction rates at elevated pressure no matter in the mixutre of H2O and CO2or in the mixutre of H2O、CO2、H2and CO. Besides, the research also found that the char-CO2reaction can inhibit the char-H2O reaction.Thirdly, the effects of pyrolysis temperature and atmosphere on char gasification reactivity were studied. The char samples are prepared in a bubbling fluidized bed under different temperatures in the atmosphere of N2and simulated pyrolysis gas. The SEM, Raman spectra, and FT-IR spectra were used to characterize the physical and chemical properties of char samples. The results show that the char samples become more porous as the pyrolysis temperature increases, and the graphite/disorder degree also becomes higher as the pyrolysis temperature increases. The pyrolysis gas atmosphere has obvious effects on carbon structure and char surface chemistry groups. The gasification experiments, using a modified PTGA system, indicate that the char samples prepared in pyrolysis gas atmosphere have lower H2O or CO2gasification reactivity than those prepared in N2atmosphere. This may be caused by the disproportionation reaction of CO and the loss of O-containing structures induced by H2.Fourthly, the char gasification experiments were conducted on a test bench of ciuculating fluidized bed gasifier, and the influence of gasification temperature, O2/Char mass ratio and HaO/Char mass ratio on char gasification characteristics were obtained. The experimental results show that using a circulating fluidized bed gasifier for char gasification is feasible. The combustible fraction, lower healing value, and flowrate of syngas, carbon conversion degree and cold gas efficiency continuously increase as the reaction temperature or O2/Char mass ratio increases. The combustible fraction and lower heating value of syngas, and the cold gas efficiency increase slightly at first and then decrease as the H2O/Char mass ratio increases. While the flowrate of syngas and carbon conversion degree decrease continuously as the H2O/Char mass ratio increases. In our experiments, the combustible fraction, flowrate, lower heating value of syngas gas, carbon conversion degree of char, and cold gas efficiency of gasifier can reach42%,7.7Nm3/h,5MJ/Nm3,94%, and45%, respectively.Finally, a pilot dual-fluidized bed for coal pyrolysis and char gasification with a2t/h coal feeding capacity was designed. This pilot dual-fluidized bed can produce about200kg/h tar,375Nm3/h pyrolysis gas, and3200Nm3/h syngas. This design will provide reference for the pilot plant construction and operation in the future.The researches of this paper help other scholars to deeply understand bituminous char gasification mechanisms, and provide technical support and references for pilot plant construction of the staged conversion utilization coal polygeneration system based on coal pyrolysis and char gasification in the future.