Study On The Mechanism Of Coal Char Oxyfuel Combustion In Reaction Kinetics-controlled Regime

Oxyfuel?O₂/CO₂ or O₂/H₂O?combustion technology that reduces CO₂ emissions is receiving considerable attention.It is a potential technology option for retrofitting existing coal-fired power plants,studying the intrinsic reactivity differences between coal char conversion in oxyfuel and air atmospheres is helpful in revealing the mechanism of oxy-coal combustion,and it is of great significance to the industrial application of oxyfuel technology.This thesis studied the mechanism of coal char oxyfuel combustion in reaction kinetics-controlled regime,the main contents include as following:Firstly,the reactivity differences of two different ranks coal char?DT bituminous and NCP anthracite coal char?conversion in O₂/CO₂ and O₂/N₂ atmospheres were investigated by non-isothermal TG-DSC method.The entire process of coal char conversion can be divided into a chemisorption stage during the initial period and a combustion stage later according to the TG-DSC curve.In the chemisorption stage,CO₂ may compete with O₂ for the common active sites and is preferentially adsorbed on the char particle surface,resulting in a lower burning rate in O₂/CO₂ atmospheres compared with that in O₂/N₂ atmospheres at the same O₂ concentration.According to the differences in activation energies,the char reaction can be further divided into a multi-step chemisorption oxidation stage and a one-step C-O₂ burning stage.Experimental results show that:compared with TG method,DSC method is more suitable for describing the reaction mechanism of the multi-step stage.Furthermore,the char-O₂/CO₂ chemisorption partially shares common active sites with an interaction mechanism.In addition,specific surface activation rate?SSAR?combined with the nonlinear analysis of variable activation energy is suitable for distinguishing the intrinsic reactivity of different rank coal chars.Non-isothermal TG method was used to study the intrinsic reactivity differences of coal char conversion in O₂/H₂O,O₂/CO₂,and O₂/N₂ atmospheres.Experimental research shows:replacing N₂ in O₂/N₂ atmospheres by CO₂,inhibits the intrinsic reaction rate,for C?CO?produced from the CO₂ chemisorption reaction CO₂+C_f?C?CO?+C?O?is likely to occupy some active sites for O₂ particle,however,there has little influence on the activation energy.C?OH?and C?H?produced from the H₂O chemisorption reaction H₂O+C_f?C?OH?+C?H?can activate the surface structure of coal char which leads to a more uniform and greater reactivity of coal char conversion in O₂/H₂O conditions compared with the case in O₂/CO₂ or O₂/N₂ conditions at the same O₂ concentration,it lowers the activation energy and increases the intrinsic reaction rate.In addition,according to the surface reaction mechanism,it is reasonable to conclude that the variable activation energy method is suitable for distinguishing the intrinsic reactivity of coal char combustion under different combustion conditions.The surface activation characteristics of coal char conversion in O₂/CO₂ and O₂/N₂atmospheres were studied by isothermal TG-DSC method to compare the reactivity differences of same structure char at a constant reaction temperature.Experimental research shows:ACM model is suitable for describing the surface activation function?SAF?of coal char combustion in the conversion ratio range of 0–0.99,and the predictions agree well with the experimental data under O₂/CO₂ and O₂/N₂ conditions.Two reaction kinetics-controlled regimes with different reaction mechanism are existing during the process of the two coal char combustions at the temperature less than 900°C;and in the second reaction kinetics-controlled regime?600–900°C?,the influence of coal char rank on the reactivity is negligible.In the first reaction kinetics-controlled regime?450–600°C?,the intrinsic reaction rates and the the heat flows per unit char sample of the two coal chars are less in O₂/CO₂ atmospheres than those in O₂/N₂ atmospheres at the same O₂ concentration and char conversion ratio,due to the larger heat capacity and less thermal diffusivity of O₂/CO₂ mixture compared with those of O₂/N₂ mixture at the same bulk flow rate result in the decrease of the heat release rate and the reaction temperature of coal char.In addition,C?CO?produced from the CO₂ chemisorption reaction weaken the contact of O₂ with char surface,and results in the decrease of the value of F?X?for the high rank NCP char during the oxyfuel combustion process;However,it has little impact on the value of F?X?for the low rank DT char.Therefore,the change of F?X?is affected by coal rank.The activation energies for NCP and DT char burning in all O₂/CO₂ and O₂/N₂ atmospheres are 178–179and 163–167 kJ/mol,respectively;and the partial pressure orders of the two chars are0.71–0.75 and 0.76–0.84,respectively.Though general model?GM?was able to describe the oxyfuel combustion in non-isothermal conditions,it can not predict the SAF in isothermal experiments.It is reasonable to use the intrinsic SAF obtained by isothermal experiments to fit the non-isothermal data,which reduces the influence caused by the changing of activation energy with the reaction temperature.The molded results show that the obtained activation energies are significantly less than that obtained by GM model,and agree well with those from isothermal experiments.Though changing the SAF leads to a changed value of activation energy,the trends of activation energies with char conversion ratio remain unchanged.At last a most suitable intrinsic surface activation kinetics model for coal char combustion in O₂/CO₂ atmospheres was proposed based on the intrinsic SAF and kinetics parameters obtained above.Finally,isothermal TG method was used to study the kinetic characteristics of char-CO₂ gasification in reaction kinetics-controlled regime.Experimental research shows:the activation energies for NCP and DT char-CO₂ gasification are 243 and 218 kJ/mol,respectively;while the partial pressure orders of the two coal chars are 0.46 and 0.48,respectively.The gasification of NCP and DT chars obey a different mechanism,their reaction orders are 0.8 and 1,respectively.The ratio of char-CO₂ gasification intrinsic reaction rate to char-O₂ combustion is in the rang of 0.66–10.00×10^-5?800°C,1 atm?,confirms that the contribution of CO₂ gasification to the burning rate of coal char oxyfuel combustion is negligible in reaction kinetics-controlled regime.