Ash formation characteristics of two Indonesian coals and the change of ash properties with particle size

Abstract

It is difficult to maintain enough high circulating ash rate in circulating fluidized bed (CFB) boilers if they burn the Indonesian coals due to their low ash content. Hence, the ash formation data for these coals is of significance to assessing mass balance in CFB boilers. Two kinds of Indonesian coal that were burned in the 550 MW supercritical CFB boilers in Korea, Kideco and KCH, were tested with static combustion and cold sieving method to study their ash formation characteristics. Results showed that the average primary ash particle size of these two coals is quite fine overall and nearly no gangue exists. Besides, their attrition rate constant Kaf is much higher than that of some other bituminous or lean coals, and with the increase of particle size, the Kaf decays exponentially for Kideco, while it initially increases and then decreases for KCH. In addition, the physical and chemical properties of these two Indonesian coals' ash obviously change in particle size, including density, pore structure, chemical composition, etc. The larger ash particles are composed of some hard materials, such as quartz (SiO2) and mullite (Al6Si2O13). While some other materials with lower hardness value, like hematite (Fe2O3) and gehlenite (Ca2Al2SiO7), become major mineral components of smaller and softer ash particles. These results indicate that the difference of chemical compositions results in the discrepancy of attrition capability for different size ash particles.