7. Precipitation Characteristics of Alkali/Alkaline Earth Metal in High Alkali Coal

Serious problems of fouling and slagging occurred in combustion of high alkali coal. As the physical and chemical characteristics of coal ash may change at different ashing temperature (AT). The investigation on the precipitation characteristics of alkali/alkaline earth metal (AAEM) in high alkali coal is important for the safety and stable operation of the boilers. To understand the effect of AT on AAEM precipitation characteristics, four different coals were selected as samples. For each coal, three coal ashes were acquired at 200 °C/500 °C/815 °C respectively. X-ray fluorescence spectroscopy (XRF) and Inductively Coupled Plasma Optical Emission (ICP-OES) were applied for the elementary analysis of ash. The test results indicated that ash yields and alkali contents in coal ash decrease while alkaline earth metal content changes little as AT increases. The precipitation temperature of chlorine is lower than 500 °C, and chlorine is substantially completely precipitated before 815 °C. The precipitation characteristic of Na is closely related to the content of chlorine in the coal. Low-temperature ashing will cause carbon burnout problems for high ash coals while the relative error of alkali metal measurement is limited. The test results of ICP-OES on element content in same sample is lower than XRF. However, the elemental content changes with AT are similar between two measurement results. It is recommended to use the ICP-OES test method when measuring AAEM content due to its higher precision.