The Chemical And Physical Effects Of CO₂ On The Ignition Of The Coal Particle In O₂/CO₂ Atmospheres

Oxy-fuel combustion technology is deemed one of the most feasible and the easiest way to achieve the application on a industrial scale to the reduction of CO₂ emissions in fossil fuel combustion process and has been comprehensively investigated. Many researchers have founded that the ignition of pulverized coal delayed in O₂/CO₂ atmospheres compared to in O₂/N2 atmospheres with identical mole fraction of O₂ and attributed the ignition delay to the higher volume specific heat of CO₂ compared with N2.However, besides the specific heat, the physical properties?radiation characteristics and mass diffusivity?of CO₂ has great difference compared with N2, while the chemical properties of CO₂ are more active and can participate in the reaction.The chemical and physical effects of CO₂ may influence the coal ignition mechnism in O₂/CO₂ atmospheres,while the researchers haven't detailly studied them.This paper establishes the mathematical model of coal ignition, and the ignition model is verified with the experiments. Using the ignition model, the paper mainly analyzes the chemical and physical effects?specific heat,radiation characteristics and mass diffusivity? of CO₂ on both coal homogeneous and heterogeneous ignitions in O₂/CO₂ atmospheres by numerical simulation.High-speed camera is used to record the ignition of pulverized coal?bituminous coal?in O₂/N2 and O₂/CO₂ atmospheres. The experimental temperature is set at 1373 K, while the mole fraction of O₂ is 21%,30% and 40%. The ignition images show that the pulverized coal occurs homogeneous ignition on the experimental conditions.This paper establishes the ignition models which are based on the judgement of ignition mechnism, and loads the UDF?user defined function? to numerically simulate the coal ignition. The simulation analysis may draw the same ignition mechnism and ignition positions with the experiments.The artificial materials X, Y, Z and W are introduced to isolate the effects of chemical properties, specific heat, radiation characteristic and mass diffusivity of CO₂ on the homogeneous and heterogeneous ignition of the coal particle in O₂/CO₂ atmospheres,respectively. The results show that chemical properties of CO₂ and the higher specific heat mainly cause the significant ignition delay on homogeneous ignition in O₂/CO₂ atmospheres compared with O₂/N2 atmospheres. In the process of volatiles ignition, the high concentration of CO₂ results in the promotion of the reaction CO₂+H<=>OH+CO,which is an endothermic reaction in the CO₂ case while an exothermic reaction in the N2 and X cases. Thus, the chemical properties of CO₂ significantly influence the homogeneous ignition delay of pulverized coal at O₂/CO₂ atmospheres for the reason that high concentration of CO₂ results in the sharp decrease of volatiles heat release of chemical reaction. The physical effects of CO₂ play dominant roles in the ignition delay in the heterogeneous ignition?Jovanovic's experiments? of pulverized coal in O₂/CO₂ atmospheres. The rank of the physical effects importance is: specific heat> thermal radiation property> mass diffusion coefficient. The chemical properties of CO₂ have no significant influence of the heterogeneous ignition delay of pulverized coal at O₂/CO₂ atmospheres.