An intrinsic kinetics model to predict complex ash effects (ash film, dilution, and vaporization) on pulverized coal char burnout in air (O2/N2) and oxy-fuel (O2/CO2) atmospheres

Abstract

During coal combustion, char chemical reaction is the slowest step, particularly in the last burnout stage, where the char consists of small amounts of carbon in a predominant ash framework. Existing kinetics models tend to deviate from experimental measurements of late char burnout due to the incomplete treatment of ash effects. Ash can improve pore evolution through vaporization, hinder oxygen transport by forming an ash film, and reduce active carbon sites and available surface per unit volume by penetrating into the char matrix. In this work, a sophisticated kinetics model, focusing on these three ash evolution mechanisms (ash vaporization, ash film, and ash dilution) during pulverized coal (PC) char combustion, is developed by integrating them into a thorough mechanistic picture. Further, a detailed comparison of the three distinct ash effects on PC char conversion during air (O2/N2) and oxy-fuel (O2/CO2) combustion is performed. For the modeled coal, the mass of ash vaporization is approximate 3 orders less than the mass of ash remaining, which participates in ash dilution and ash film formation, both in O2/N2 and O2/CO2 atmospheres. The influence of these phenomena on burnout time follows the order: ash dilution > ash film > ash vaporization. The influence of ash vaporization on burnout time is minor, but through interactions with the ash dilution and ash film forming processes it can have an impact at high extents of burnout, particularly in O2/CO2 atmospheres. In O2/N2 atmospheres the residual ash predominately exists as an ash film, whereas it mainly exists as diluted ash in the char matrix in O2/CO2 atmospheres. The residual ash particle is encased by a thick film when the ash film forming fraction is high (low ash dilution fraction). These results provide in-depth insights into the conversion of PC char and further utilization of the residual ash.