Formation and Control of Fine Potassium-Enriched Particulates during Coal Combustion

Fine potassium-enriched particulates generated from coal combustion are harmful to the environment and human health. To study the formation and control mechanisms of fine potassium-enriched particulates during coal combustion, raw lignite, potassium-rich coal obtained from raw lignite, and potassium-rich coal mixed with kaolin were combusted in an electrically heated drop-tube furnace. The effects of the combustion temperature and atmosphere were considered. The particulate matter generated is subject to size distribution, concentration, elemental composition and morphology analysis. Thermodynamic calculations were also performed to simulate the vaporization behavior of potassium compounds in coal under various conditions. As a result, when the combustion temperature was increased from 900 to 1300 °C under O2/N2 conditions, the transformation of potassium from potassium-rich coal to PM1 decreased, because the formation mechanism of PM1 from volatile potassium had changed. When the combustion temperature was 900 or 1100 °C, K2SO4 was the main potassium compound vaporized and condensed to form PM1, with an average size of 0.5 μm. When the combustion temperature increased to 1300 °C, in addition to a small amount of K2SO4, gaseous KOH becomes the main potassium compound vaporized, most of which may react with aluminosilicates to form coarse particulates. The remaining vaporized K2SO4 and other sulfates/oxides condensed to form PM1, with an average size of 0.2−0.3 μm. As the combustion temperature was increased, volatile potassium transferred to coarse particulates in increasing amounts. Kaolin could effectively capture the vapor of potassium compounds during the potassium-rich coal combustion. When the potassium-rich coal mixed with kaolin was combusted at 900, 1100, and 1300 °C, the maximum capture efficiency was obtained at 1100 °C, as the result of combined effects of chemical adsorption increasing and physical activity decreasing with an increasing combustion temperature.