Experimental and theoretical analyses on ignition and surface temperature of dispersed coal particles in O2/N2 and O2/CO2 ambients

An experimental and theoretical investigation is conducted on the combustion of pulverized coal particle streams in either conventional or oxy-fuel conditions. The laboratory setup consisted of a Hencken flat-flame burner operated at the same temperatures of 1200, 1500 and 1800 K in N2 and CO2 environments. The visible light detection technique and the calibrated three-color pyrometry are separately used to characterize the ignition delay time and the luminous char surface temperature. First, experimental results indicate that the ignition delay time is relatively longer in CO2 environment in heterogeneous mode or even heterogeneous-controlled joint mode. The ignition delay is obviously enhanced in O2/CO2 ambient when the volatile-flame controlled joint mode is prevalent. The model predictions of particle temperature history further reveal that the extra gas flame radiation of CO2 and the declined difference between the thermal conductivity are two main reasons. Then, during char combustion, the measurement shows that the char surface temperature is lower in an O2/CO2 environment than in an O2/N2 environment. It is noted that the temperature difference between N2 and CO2 environments enlarges with the increasing ambient temperature. In contrast to conventional O2/N2 conditions, the lower O2 diffusivity in CO2, the endothermic carbon reaction with CO2 and the higher heat capacity of CO2, which are all temperature dependent, synergistically contribute to the enlargement of char surface temperature differences at high ambient temperature.