| Coal is the major energy source in China. There are many problems in coal utilization such as its low efficiency of energy utilization and serious environmental pollution, among which, reducing emission of carbon dioxide and other greenhouse gas is the problem demanding prompt solution. Large-scale coal gasification technology has become one of the effective ways to utilize coal efficiently and cleanly. On account of more than half of coal with high ash fusion temperature and high ash in China, the coal utilization is limited in entrained flow gasifier application. Instead, fluidized bed can solve the problem of high ash fusion temperature coal gasification effectively. For the purpose of developing large-size pressurized fluidized bed coal gasification to fit high ash fusion temperature coal, the pressurized fluidized bed gasification combined cycle power generation system and the carbon-dioxide recycle type pressurized fluidized bed gasification application system have been proposed in the paper, and the model simulation of pressurized fluidized bed coal gasification which is a key technology of the above systems has been studied to find the optimum operating conditions and design parameters. Also the coal char gasification reaction characteristics have been studied to provide necessary kinetic data for the mathematical models of high ash fusion temperature coal gasification.The mathematical models of pressurized fluidized bed coal gasification were built up, including the solid particle model, gas-side model, hydrodynamic model of bubbles and the enthalpy balance model. Based on the models, the effects of following factors on the carbon conversion rate, gas production rate and cold gas efficiency were analyzed, such as the coal feed rate, the oxygen mass and the steam flow, etc., and subsequently the optimum range of coal feed rate was determined.As the mathematical models have been solved in air atmosphere, results show as below. The carbon conversion rate is kept at a level higher than99%in the initial stage, and for gasifier with same bed area, their handling capacity may be increased by raising the operation pressure, and the gas production rate per kilogram of coal will be increased by34%when the operation pressure is raised by40%. High coal feed rate and high reaction pressure are beneficial to the generation of combustible gas (CO+H2+CH4). For an operation pressure of1.5MPa, the optimum range of coal rate is from1.0kg/(m2·s) to1.5kg/(m2·s). For an operation pressure of2.1MPa, the optimum range of coal rate is from2.0kg/(m2·s) to2.5kg/(m2-s). The cold gas efficiency may get up to77%when the oxygen-carbon ratio is in0.45-0.55. And the heating value of generated gas is in direct proportion to the steam-air ratio.As the mathematical models have been solved in carbon dioxide atmosphere, results show as below. In the carbon dioxide recycle system, more than70%(CO+H2) forming gas can be obtained. The gasification scale under CO2atmosphere is less than under air atmosphere by about2%. For an operation pressure of1.5MPa, the optimum range of coal rate is from1.3kg/(m2-s) to1.8kg/(m2-s). The cold gas efficiency may get up to76%when the oxygen-carbon ratio is0.5. The gasification temperature is in direct proportion to the oxygen-carbon ratio, and gasification temperature can be effectively regulated through controlling the ratio of oxygen-carbon ratio. The cold coal gas efficiency will get up a maximum value and the gas heating value will gradually increase with the increase of steam ratio. The operating range of reaction under carbon dioxide atmosphere is greater than under air atmosphere when gasification temperature is from1073K to1273K.The coal char-CO2/H2O gasification reaction characteristics were studied with isothermal thermogravimetry method at1073-1273K range. The results show that the gasification reaction rate of chars with high ash fusion temperature is lower than that of chars with the low ash fusion temperature. The gasification temperature must be higher than1273K to achieve high reactivity of high ash fusion temperature coal. The reaction rate increases with rising gasification temperature. The char-CO2/H2O gasification reaction rate changes like peak shape with increasing reaction time, low ash fusion temperature coal gasification reaction rate declines after reaching a peak with a speed faster than high ash fusion temperature coal. The reactivity of char-H2O is higher than that of char-CO2. |
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