Pyrolysis during the initial stages of pulverized-coal combustion

Abstract

Mathematical modeling of the time-temperature history of coal particles in an entrained flow reactor revealed a nonisothermal pyrolysis process. Temperatures and residence times of particles in the reactor were strongly dependent on radial dispersion. Convective heat transfer was the dominant mode of heating the coal particles. Hence, the thermal diffusivity of the cold primary gas had an important influence on the heating rate. Predicted gas temperatures were lower in the presence of coal particles and predicted particle temperatures were lower when pyrolysis endothermicity was taken into account. The weight loss data and evidence from char morphologies indicated a coupling of heat transfer to the rate of pyrolysis. Increasing the heating rate produced a substantial increase in the weight loss rate. Furthermore, oxygen chemisorption produced a significant weight gain, particularly at short residence times. The observed variation of the weight loss rate with maceral composition is considered to be related to thermoplastic behavior. A mechanism for cenosphere formation by the softened coal under the rapid heating and dilute phase conditions is described.