Ignition and combustion of single coal and char particles: A quantitative differential approach

Abstract

A quantitative differential technique for studying the coal particle combustion process and particularly the ignition step was developed. The approach is based on the continuous and simultaneous analysis of the carbon monoxide and carbon dioxide produced when a captive single coal particle is burnt after injection into an isothermal flow reactor swept with a preheated oxygen—nitrogen mixture. To check the results obtained with the new approach, the intensity of the light generated during the particle combustion was also registered. A microcomputer controlled and executed the data collection process in the millisecond time frame. Experiments were performed burning single particles from the 850–1000μm sieve fraction of a Wyoming subbituminous coal in air, at five gas temperature levels ranging from 928 to 1283 K. The gas product curves show the ignition mechanism is defined by the relative rates of volatiles evolution from the particle and oxygen diffusion to the particle. Initial peaks in the rate of formation of carbon monoxide or carbon dioxide allow the determination of the pyrolysis time during combustion. The occurrence of heterogeneous and homogeneous ignition for particles of the same coal was detected using both techniques: gas analysis and light intensity. A transition was detected in the ignition mechanism, from heterogeneous to homogeneous, when the reactor temperature was increased. Even though the light intensity technique is simpler, the results demonstrate that the approach developed in this work has more sensitivity, particularly in the transition zone. An additional advantage of the gas analysis approach is that the total carbon in the original particle and its variation with time can be calculated by integration of the gas product curves. This information provides a method for estimating the fraction of carbon released from the particle as volatile matter during the ignition. After ignition, the variation in the mass of carbon with time can be used to test different combustion mechanisms. As expected, the results show that at the higher temperatures used, the particle burn-off proceeds under external diffusion control. The total combustion times, as obtained from the gas product curves, show good correlation with the results obtained from light intensity measurements.