Mechanism Investigation On The Physiochemical Effects Of Carbon Dioxide On Coal Particle Combustion Characteristics In O₂/CO₂ Environments

With the growing severity of environment problem,the technologies of pollution emission reduction become more and more important.Oxy-fuel combustion is regarded as one of the most promising technologies for CO₂capture.Oxy-coal combustion is greatly different from conventional pulverized coal combustion in air environment because of the different physicochemical properties of CO₂.Therefore,it is important to investigate the coal particle combustion characteristics to elucidate the underlying mechanism of coal combustion in O₂/CO₂ environments.The objective of the current study is to present a numerical method to quantitatively separate the different physicochemical effects of CO₂.The different effects of CO₂ on the char temperature and combustion rate in the comparable O₂/CO₂,O₂/CO₂/Ar and O₂/N₂ environments are quantitatively isolated and the results are analyzed to elucidate the separate effects of CO₂ on the underlying mechanism of coal combustion in O₂/CO₂ environments.Firstly,the ignition and combustion behaviors of coal particle for Zhundong coal are investigation in a laminar entrained flow reactor in different O₂/N₂ and O₂/CO₂ environments with different oxygen mole concentrations by employing a high speed camera and a fiber optic spectrometer.The ignition time and burnout time in 21%O₂/79%CO₂environment are lengthened than those in 21%O₂/79%N₂ environment.The average optical intensity of coal particles in 21%O₂/79%CO₂ environment are obviously lower than those in 21%O₂/79%N₂environment.The particle temperatures 21%O₂/79%CO₂ environment are lower by 380K than those in 21%O₂/79%N₂ environment.The emissivity of coal particle decreases with the burnout of coal particle.Secondly,the effects of CO₂ physicochemical properties on the char particle temperature and combustion rate in O₂/CO₂ environments are investigated by using a continuous film model with the different ambient temperatures from 1200K to 1600K.The results show that the predicted particle temperatures 21%O₂/79%CO₂ environment are lower by 489K than those in 21%O₂/79%N₂ environment at an ambient temperature of 1400 K.The char combustion rate increases by 15%if molar heat capacity of CO₂ is artificially set to be equal to that of N₂.At the same time,the char combustion rate increases by 27%if oxygen diffusivity in CO₂ environment is artificially set to be equal to that in N₂ environment.The char combustion rate decreases by 38%if char gasification reaction with CO₂ is artificially closed.A method is proposed to separate different physicochemical effects of carbon dioxide by adjusting the char particle temperature in O₂/CO₂ environment back to that in O₂/N₂ environment through replacing an appropriate portion of CO₂ with argon.And the effects of CO₂ on bituminous coal char combustion in O₂/CO₂environments are quantitatively isolated with specially designed combustion environments and the relative contributions of the oxygen concentration,thermal,and chemical effects on the char combustion rate are 82.1%,11.2%,and 6.7%,respectively,at an ambient gas temperature of 1200 K for bituminous coal char of 91?m.Char combustion in O₂/CO₂ environments with low O₂ concentration at a high ambient temperature are investigated to indicate the effects of CO₂ and H₂O on char combustion rate by using a modified continuous film model considering the char-H₂O gasification reaction.The presence of H₂O increases the char particle temperature in O₂/CO₂ environments due to lower specific heat capacity of H₂O compared with that of CO₂.And it decreases the char particle temperature in O₂/N₂ environments because of the stronger endothermic char-H₂O reaction compared with the char-CO₂ reaction.The combustion rates in O₂/CO₂ and O₂/N₂ environments both increase with the presence of H₂O.The combustion characteristics of coal particle are investigated in different O₂/CO₂ and O₂/N₂environments by using a modified continuous film model of coal combustion.The coal particle temperature in O₂/CO₂ environment are lower than that in O₂/N₂ environment.And the ignition time and burnout time of coal particle in O₂/CO₂ environment are larger than those in O₂/N₂ environment.The increases of coal particle temperatures in O₂/CO₂ environments are also lower than those in O₂/N₂environments with the increase of oxygen mole concentrations.Lastly,the computational fluid dynamics modeling on pulverized coal MILD combustion is conducted to simulate the International Flame Research Foundation furnace No.1 with two turbulence-chemistry interaction models.The internal flue gas recirculation ratio in the furnace is as high as over 5.4due to strong turbulent mixing and entrainment of hot flue gases in the furnace due to high momentum secondary air jets.Pulverized coal MILD combustion regime is depicted with the calculated Damk?hler number and Karlovitz number on the basis of different regimes of turbulent non-premixed flames.The numerical results indicate the micro-characteristics of pulverized coal MILD combustion and confirm that pulverized coal MILD combustion is in slow chemistry regime(Da_?(27)10,Ka>>1)and disperses throughout the whole furnace volume.Pulverized coal MILD combustion is a unique combustion mode with strong turbulent mixing and entrainment,high internal flue gas recirculation ratio and slow reaction rate under low local oxygen concentration with respect to the traditional flame combustion mode.