Effects of leaching and additives on the ash fusion characteristics of high-Na/Ca Zhundong coal

Abstract

Considering the intensive application of Zhundong coal and its serious slagging during combustion in pulverized coal fired boilers, the effects of water leaching and different dosages of additives (Na2CO3, CaO, SiO2, and kaolin) into the leached coal on ash fusion characteristics were studied using sintering tester, X-ray Fluorescence, and X Ray Powder Diffraction. After leaching, the contents of Na2O, K2O, CaO, MgO, SO3, and Cl decreased, whereas the contents of SiO2 and Fe2O3 increased. As a result, the slagging and fouling index of Basic/Acid ratio decreased from 1.6 in the raw coal ash to 1.1 in the leached coal ash, and alkali-induced slagging was reduced. However, the decreased ash fusion temperatures (AFTs) of the leached coal worsen the melt-induced slagging. Detailed analysis on the effects of ash compositions on the AFTs by additions of Na2CO3, CaO, SiO2, and kaolin additives into the leached coal showed that Na2CO3 and SiO2 improved the formation of low-melting Na2SO4 and Na2SiO3, and thus decreased the AFTs; However, overdosed Na2CO3 (for instance 9 wt%) improved the formation of relative high-melting Na4Al2Si2O9 and thus increased the AFTs. Overdosed SiO2 as a high-melting contaminant also increased the AFTs. Similar with the function of overdosed SiO2, CaO in the ash of leached coal was excess, and the AFTs increased with increased CaO dosage. Kaolin, which decomposed as high-melting mullite and cristobalite, caused an elevation of the AFTs. Therefore, water leaching decreased the AFTs of Zhundong coal and consequently worsen melt-induced slagging, lime and kaolin additives increased the AFTs effectively, but Na2CO3 and SiO2 additives showed ‘V’ shaped functions on the AFTs. These general results are useful for the pretreatment of Zhundong coal during combustion and alleviating slagging.