Reaction-induced temperature deviations during coal devolatilization in a heated grid

Abstract

To gain further insight into the primary devolatilization process, particularly with respect to the tar yields, a new control circuit was designed to permit any heating rate from 800 C/sec to 4 x 10/sup 3/ C/sec to any final temperature in the 300 to 1100/sup 0/C range. With this circuit, the effects of sample characteristics on local heating of the grid were examined to assess the influence of devolatilization on programmed heating rates. The results clearly demonstrate that the devolatilization process has a considerable influence on the time-temperature history of the local screen in immediate contact with the sample. With respect to the coal particles, the direct implication is that the temperature path is the resultant of several components: the resistive heating of the grid, the physical properties of the samples, the devolatilization properties of the sample. Once the range of devolatilization temperatures of a particular coal is achieved, the primary devolatilization process appears to dominate the temperature-time trajectory. Because the heat requirement of the primary devolatilization affects the temperature trajectory of the coal particle, a real time model of primary coal devolatilization must necessarily include a consideration of this requirement. In addition, the data appears to indicate that themore » heat requirement varies with the rank characteristics of the coal. For a transient process such as rapid coal devolatilization the absolute magnitude of the heat required to vaporize the volatile components need not be large to result in a significant deviation from a programmed heating rate or a calculated heating rate of a nonvolatile particle. The results indicate that the tar release is closely coupled in time to the devolatilization-induced temperature deviations during primary devolatilization and the onset of the tar release significantly precedes the slower light hydrocarbon gas evolution.« less