Numerical Simulation Of Single-particle Char Burnout And Char-nitrogen Conversion

Coal has always been the most important primary energy of China’s economic andsocial development and its efficient and clean utilization has far-reaching significancefor economic development and ecological environment. Ash agglomerating pressurizedfluidized bed gasification technology is an advantageous technology of gasificationclean combustion, but the gasified semi-char generated in the course of its operationcontains a large amount of residual carbon, which requiring high-temperaturecombustion or co-firing to improve the heat recovery and residual carbon burnout. Thegasified semi-char particles are characterized with poor reactivity, low volatile contentand high ash content. Therefore, combustion the numerical calculation combined hightemperature experiments to simulate semi-char particles burnout process will clarify andpromote our understanding of its high-temperature reaction characteristics and reactionmechanism.The char nitrogen experiences a slow conversion during the burnout of char, whichplays an important role of the formation of nitrogen oxides. However, char nitrogenconversion mechanism is still not clear currently, therefore, the employment of singleparticle combustion model for char nitrogen conversion during char combustion ishelpful to raise awareness of the char nitrogen conversion process, to reduce theconversion of char nitrogen to NOxand to provide guidance on the NOxcontrollingmeasures.Based on the computational fluid dynamics, a program is written to calculate thesingle-particle semi-char burnout and single-particle coal char nitrogen conversion. Thesimulation results are compared and analyzed with the experimental results andliterature researches to verify the reliability and usefulness of this model.For the semi-char burnout, the four-step two-phase surface shrinking core model isa more suitable description of the burn-off stage of the semi-char combustion bycomparing different solid combustion models and different kinetic reaction mechanisms,and this mechanism and model have some versatility to other coal chars burnoutprocedure. The temperature of the entire particle whose size is less than100μm tends tobe uniform and CO combustion is not significant within the pores. Under theexperimental conditions, the reaction of semi-char burnout lies in the transition zonecontrolled by both kinetic reaction and diffusion. With the growth of ambienttemperature, the reaction zone transfers from the kinetic-control regime to thediffusion-control regime. The oxygen concentration inside the particle increases withthe growth of conversion under the same temperature and oxygen concentrationconditions, and even reaches uniform throughout the particle when the temperature islow. With the increase of conversion, the specific surface area decreases and the pore radius increases, leading to the reduction of the diffusion resistance, therefore, thediffusion rate grows and even exceed the decreasing reaction rate, thereby emerging thephenomenon that the reaction transfers from the diffusion-control regime to thekinetic-control regime at the late stage of burnout. With the growth of the ambienttemperature and oxygen concentration and the reduction of the particle diameter, theburnout time decreases gradually.For the char nitrogen conversion of coal char, the conversion of char nitrogen toNO increases slightly with the growth of the environmental oxygen concentration butreduces linear monotonically with the increase of the initial environmental NOconcentration under the pulverized coal combustion conditions. The initialenvironmental NO concentration has a much larger impact for the char nitrogenconversion than the environmental oxygen concentration, therefore, the preciseparameters of NO reduction kinetics are very important for the char nitrogen conversionand the prediction of NO emissions. As the size of char particle increases, theconversion of char nitrogen to NO decreases. And the conversion of char nitrogen toNO increases firstly and then decreases with the growth of the temperature under thepulverized coal combustion conditions.