Numerical investigation of physicochemical effects of CO2 on coal particle ignition in O2/CO2 ambience

Abstract

Coal combustion is a complex phenomenon involving multiscale chemical and physical processes. In this paper, the physical and chemical effects of CO2 on the ignition behaviors of dispersed coal particles are investigated based on a self-developed transient ignition model. Attention has been paid on the separate effects of CO2, namely the thermal conductivity, the species diffusion rate, the heat capacity, and the gasification reaction, on the ignition behaviors in oxy-fuel ambience. Differences of coal ignition delay time and ignition modes in air and oxy-fuel ambience are compared. The heterogeneous ignition (HI) is significantly delayed in CO2 ambience, with the most significant contribution from the lower thermal conductivity of CO2. The negative effects of slower species diffusion rates and the endothermic C-CO2 gasification reaction in CO2 are less important; however, they both become prominent with increasing ambient temperature up to 1800 K. By contrast, the gas phase homogeneous ignition (GI) is less sensitive to CO2 enrichment. The positive effects of slower species diffusion rates, together with the negligible effects of higher heat capacity and extra gasification reaction, impair the negative effect from lower thermal conductivity. In summary, the CO2 substitution makes the ignition mode move from the hetero-homogeneous joint regime towards the homogeneous dominant regime. Thus, coal particle ignition under high temperature (1500–1800 K) and low oxygen fraction (~0.1) transits to the homogeneous dominant case, yielding a reduced difference of ignition delay between air and oxy-fuel combustion. This feature has been observed for coal samples of different ranks from several independent macroscopic experiments, and can be well interpreted by the transition mechanism of ignition mode.