Process Study Of Two-stage Entrained Flow Process Coupled With Pyrolysis And Gasification

Entrained flow gasification technology is the mainstream coal gasification technology widely used in the modern coal chemical industry.Continuously improving the gasification efficiency of the entrained flow gasifier is a hot spot in the academia and technical circles.Therefore,the two-stage entrained flow process coupled with pyrolysis and gasification is proposed,in which the first-stage gasification chamber is dominated by char gasification,and the second-stage gasification chamber is dominated by pyrolysis of pulverized coal.The sensible heat required for the pyrolysis of coal comes from the high-temperature syngas produced from the first stage gasification chamber.In this thesis,the rapid hydropyrolysis characteristics of pulverized coal are studied,based on which,a comprehensive pyrolysis model is established.The particle residence time of the second stage chamber and the flow field characteristic of the two-stage entrained flow gasifier are studied through numerical simulation,and the effects of the gasifier structure on the gasification performance of the two-stage entrained flow gasifier are investigated.The mathematical model of the two-stage entrained flow gasification system is developed and the effects of operating parameters on the two-stage entrained flow gasification system are investigated.The main contents are given below.1.The rapid pyrolysis process of Shenfu bituminous coal in a dropper furnace experimental device under nitrogen and hydrogen atmosphere are studied.The conversion process of pyrolysis products are analyzed by combining gas chromatography,liquid chromatography,Raman spectroscopy and infrared spectroscopy.The experimental results show that at the same pyrolysis temperature,the yields of CH4 and H2O generated in the hydropyrolysis process are significantly higher than the nitrogen atmosphere pyrolysis process,and the yields of CO and CO2 are significantly lower than the nitrogen atmosphere pyrolysis process.The yield of CH4 generated during the hydropyrolysis process reaches a maximum of 72.83 ml/g(daf coal)at 900?.Compared with the pyrolysis process under a nitrogen atmosphere,the yield and quality of tar generated in the hydropyrolysis process are improved.When the pyrolysis temperature is 800?,the yield of tar obtained in the hydropyrolysis process is 11.77wt%,which is about 2%higher than that of the pyrolysis process under a nitrogen atmosphere.The yield of char obtained in the hydropyrolysis process is lower than that of nitrogen pyrolysis.Besides,the degree of graphitization of the hydropyrolysis char is relatively low,indicating that its gasification activity is relatively high.2.A comprehensive pyrolysis model is proposed,which mainly includes two sub-models.In the first submodel,the model parameters of the two-step devolatilization model are modified by the CPD model to predict the overall yield of volatiles.In the second sub-model,the detailed composition and yield of the volatiles are predicted by the multi-step pyrolysis kinetics modified model and nonlinear constraint optimization method.Under different pyrolysis final temperature and heating rate,a comprehensive pyrolysis model is used to predict the devolatilization process of 14 coal types.The prediction results are in good agreement with the literature values,thus verifying its applicability and accuracy.3.Based on the Opposed Multi-Burner(OMB)gasifier,two-stage entrained flow gasification composed of pyrolysis and gasification processes was proposed.A 3D numerical model of the two-stage entrained flow gasifier is established to study the particle residence time distribution(RTD).The realizable k-? model is used to model turbulent flow,and the stochastic trajectory model is employed to track particle motion.The simulation results show that increasing the diameter of the second-stage chamber can effectively increase the residence time of particles.The throat diameter mainly affects the residence time of particles with large particle size(>100?m).The residence time of large particles(>100?m)gradually increases as the throat diameter decreases.As the gas velocity at the inlet of the second-stage gasification chamber increases,the average residence time of small-sized particles and low-density particles increases first and then decreases,while the average residence time of large-sized particles and high-density particles decreases monotonously.4.A 3D numerical model of the two-stage entrained flow gasifier is established to obtain the velocity,temperature,and component concentration distribution in the two-stage entrained-flow gasifier.The realizable k-? model is used to model turbulent flow,and the stochastic trajectory model is employed to track particle motion.The comprehensive pyrolysis model is used to model the pyrolysis process of coal.The Random Pore Model(RPM)and the"effective factor" method are employed to model char gasification under high pressure and temperature.The jet of the second-stage chamber nozzle will have a blocking effect on the radial fluid from the first-stage chamber,resulting in the formation of a recirculation zone in the throat area,which increases particle residence time.The exit temperature of the second-stage chamber quickly dropped to 840? due to the pyrolysis process of pulverized coal.The heat and mass transfer process between gas-solid phases are intensified due to the high slip velocity of gas-solid phases in the impinging flow region in the first-stage gasifier.5.The mathematical model of the two-stage entrained flow gasification process with pyrolysis-gasification coupling is established to investigate the effects of operating parameters on the gasification system.When the exit temperature of the first stage gasifier increases from 1200? to 1600?,the exit gas temperature of 2nd gasifier increases from 761? to 830?,the CH4 mole fraction increased slightly,the tar yield decreased significantly,and the specific coal consumption and the specific oxygen consumption increased slightly.When the steam/coal ratio increases from 0.15kg/(kg coal)to 0.35kg/(kg coal),the exit gas temperature of 2nd gasifier increases from 801? to 817?,the CH4 mole fraction and the tar yield decreases slightly,the specific coal consumption and the specific oxygen consumption increased slightly.Compared with the single-stage pulverized coal entrained flow gasification system,the cold gas efficiency(wit tar)of the two-stage increases by about 5?8%,and HHV increase by about 3.3 MJ/Nm3.Secondary oxygen supply process(inside or outside the gasifier)can effectively reduce the tar content in the syngas to less than 300mg/(Nm3 syngas).Under the current process conditions,the oxygen/coal ratio of the two processes is 477Nm3/(t coal)and 484 Nm3/(t coal),the gas exit temperature is 939? and 997? respectively,the CH4 molar fraction is about 5?6%,and the cold gas efficiency is about 85%.