Theoretical And Experimental Investigation Of Coal Pyrolysis At High-temperature And Coal-char Combustion/gasification In Air/oxy Atmosphere

For a long time to come,coal is still the most important energy source in China.The research on the clean and efficient has important strategic significance for China's energy security and sustainable development.The coal-based multi-generation technology coupling the oxy-fuel combustion is a new type of coal clean utilization technology;it cannot only extract high value components?such as volatile and tar?in coal,but also reduce or even avoid the emissions of pollutants and CO2.In this thesis,the characteristics of volatile release during coal pyrolysis and the methods to adjust the components of volatile during coal decomposition are investigated in detail,meanwhile,the combustion and gasification characteristics of semi-coke which is the products during coal pyrolysis is also explored.The contents of this thesis are organized as follows:?1?coal pyrolysis in fixed bed reactor;?2?coal pyrolysis in drop-tube furnace;?2?the ignition characteristics of semi-coke;?3?the evolution of semi-coke's morphology and chemical structure during combustion and gasification;?4?oxy combustion of semi-coke and the development of semi-coke combustion model;?5?CFD modeling of pressurized oxy combustion.During exploration of coal pyrolysis in fixed bed,the volatile release characteristics at high-temperature and moderate heating rate and high pressure are clarified.When the pyrolysis temperature increased from 900^oC to 1100^oC at one atm,the coal weight loss?daf?increased from 33.6%to 36.6%.The increase of pyrolysis pressure would lead the significant decrease of coal weight loss within 10 atm,but the pyrolysis pressure above 10 atm has slight influence.The species of volatile are mainly composed by H2 and CO,also with slight CH4 and CO2,and there is hardly C3H8 and O2.When pyrolysis temperature increased,the yields of CO and H2 increased but the yields of CO2,CH4,C3H8 decreased.The yields of all species in volatile increased with pyrolysis pressure within five atm and then decreased.During exploration of coal pyrolysis in drop tube furnace,the volatile release characteristics at high-temperature and rapid heating are clarified,and the properties of semi-coke produced at different pyrolysis temperature are characterized,and the intrinsic oxidation and gasification of semi-coke were investigated with thermogravimetric?TG?.Long residence time,high pyrolysis temperature,CO2atmosphere,low particle concentration and small amount of O2 addition would increase the yields of volatile and change components of volatile.The instantaneous activation profile and intrinsic activation energy of semi-coke during oxidation and gasification was obtained using Friedman method or AIC method.Moreover,the rate profile and the peak in rate profile during semi-coke conversion in fixed temperature was explained in detail,and the operation of Gas switch in TG test and variation of particle temperature would change or even conceal the true position of peak of rate profile.In addition,the semi-coke conversion at low temperature was well simulated with intrinsic reaction model coupling random pore model and intrinsic activation energy?maximal deviation less than 5%?During exploration of semi-coke ignition characteristics,TG ignition test at low heating rate and Hencken ignition tests at rapid heating rate were conducted,and transient ignition model coupling gas reaction within particle boundary layer was developed.The semi-coke ignition mode at low heating rate is heterogeneous ignition,and it is hardly influenced by heating rate and oxygen concentration,but ignition temperature varies with above factors.Ignition mode in Hencken?semi-coke jet flame?is also heterogeneous ignition,and its ignition delay is affected by temperature,atmosphere,oxygen concentration,and particle diameter.Transient ignition model also shows that there is no volatile flame around the particle boundary layer,and the semi-coke does not exist a critical temperature in some conditions?small particle diameter et.al?according to Semenov theory.The heating time in transient ignition model agrees well with the experimental ignition delay time.The evolution of semi-coke's morphology and chemical structure was explored in different atmospheres and pressures.During semi-coke conversion,the smooth external surface became rough,and the particle seemed to be looser,and fragments started to occur,and the inner pore was exposed,but the particle skeleton was almost preserved,and the particle diameter hardly changed,and there was no ash accumulation around particle.The surface area and pore volume of meso-pore during gasification?CO2atmosphere?is significantly lower than those in oxidation?O2 atmosphere?,and the pore diameter in gasification is lower than that in oxidation,while the pressure would impose a complicate effect the on the variation of surface area,pore volume,and pore diameter in different atmospheres.Moreover,CO2 atmosphere and high pressure would moderate the graphitization during semi-coke conversion.An intrinsic semi-coke combustion model,which accounts for the intrinsic reaction mechanism,intraparticle diffusion,thermal deactivation,ash inhibition and several gas-solid reactions,was well developed,and this model was successfully used to predict the single particle conversion process and coal combustion in drop tube furnace.Moreover,a submodel that can directly calculate the particle diameter when several reactions are included was developed?DCCD model?,and this submodel can replace the traditional method-power law,and a new ash inhibition submodel was built.The CO2 gasification in oxy-coal combustion can increase the burnout by 3.4%at most and decrease the particle temperature in single particle conversion but slightly increase the particle temperature in particle cloud combustion.However,the effect of CO2gasification on carbon consumption is more pronounced than the effect of CO2gasification on particle temperature decrease.The burnout predicted by above intrinsic reaction model agreed well with experimental value within 3%deviation,and the temperature deviation was within 50 K.The effect of pressure,type of flue gas recirculation,fuel type,on the characteristics of pressurized oxy-fuel combustion was investigated in detail with revised intrinsic char combustion model and new WSGG model.The results show that the pressure and the type of flue gas recirculation has little effect on the profile of velocity and temperature,but high pressure and wet flue gas recirculation can increase the overall temperature and decrease the conversion time in each stage of particle conversion,and the carbon consumption due to gasification is also increased.Different fuel types?coal,CWS,semi-coke,semi-coke slurry?have effect on velocity and temperature profile,but the temperature with slurry injection decreased by at most 100 K compared to that with dry fuel,and the case with semi-coke sulrry injection has the lowest temperature.The evaporation time increased to 10^-2 s and carbon gasification proportion decreased significantly when slurry fuel was injected.