Effects of leaching and additives on the formation of deposits on the heating surface during high-Na/Ca Zhundong coal combustion

Abstract

Zhundong coal, high-quality coal with large reserves in China, faces great challenges on its large-scale utilization in utility boilers due to the ash-related problems caused by high alkali and alkali earth metals (AAEMs). In this research, to simulate ash deposition behavior on heating surfaces in high-temperature flue gas zone (panel superheater) and that in medium-temperature flue gas zone (superheater or reheater), deposition experiments were carried out during Zhundong coal combustion in a drop tube furnace (DTF) at the deposition temperature of 1200 °C and 900 °C. Meanwhile, water leaching, Na2CO3 additive and kaolin additive were employed to elucidate the formation mechanism of ash deposition and alleviate deposition and slagging. Results showed that leaching reduced the contents of troublesome elements such as AAEMs and S and increased the relative contents of Si and Al, thus alleviating ash deposition. Compared with raw coal, the deposition rate of leached coal decreased from 5.82 mg h−1 cm−2 to 0.88 mg h−1 cm−2 at the deposition temperature of 900 °C. The primary reason for the decreased deposition rate might be that the dominant minerals in leached coal ash were refractory SiO2, CaO and Ca2SiO4. The addition of Na2CO3 would aggravate the slagging and fouling on the deposition surface significantly. The gaseous Na2O decomposed from Na2CO3 reacted with SO2 and SiO2 in the flue gas to form low-melting Na2SO4 and Na2Si3O7, which were easy to cause severe slagging on the deposition surface. The inhibition effect of kaolin addition on the formation of deposition and slagging was similar to that of leaching. After the addition of kaolin, the dominant compounds of fly ash altering to high-melting minerals such as CaSiO3, CaSO4, Ca2MgSi2O7 and NaCaAlSi2O7, resulting in a lower deposition rate than raw coal. Besides, when deposition temperature increased from 900 °C to 1200 °C the slagging degree of deposition surfaces was aggravated, while the deposition rate of four fuels followed different change trends. Specifically, the deposition rate of raw coal and leached coal decreased, whereas that of the case with Na additive and kaolin additive presented an increasing trend. At last, the role of water leaching and additives on the formation of ash deposition during Zhundong coal combustion was clarified.