Impact of complex reacting atmosphere on ash fusion characteristics and minerals conversion in coal combustion process

The surrounding atmosphere of coal particles changes continually during the combustion process in pulverized coal boilers. A part of noncombustible mineral composition in char will be melted under high temperature and then leads to the plugging of pores in char. The O₂ molecule used for combustion is hard to diffuse into the inside of char. However, the investigation on ash fusion properties of coal in real combustion atmosphere is still ambiguous. In the present study, the ash fusion temperatures (AFTs) of high Fe coal and low Fe coal were tested under complex atmospheres, which simulated the real condition during coal particles combustion. The ash samples were prepared at 1373 K in the typical conditions and then analyzed by X-ray diffraction to explain the conversion of minerals. In addition, the chemical equilibrium compositions calculations were carried out to obtain the transformation of atmosphere-sensitive Fe species with the atmosphere change. Results indicated that the AFTs reached the lowest point in a weak reducing atmosphere (N₂:CO:CO₂ = 80%:5%:15%). The Fe₂O₃ species existed in the ash that prepared in oxidizing atmosphere were reduced due to small quality of CO in weak reducing atmosphere and then formed the Fe²⁺ compounds with lower melting temperature. If the concentration of CO increases, the Fe²⁺ compounds would be reduced continually, which would convert into elemental Fe with relatively high melting temperature. Compared with the low iron coal, the AFTs of the high iron coal reduced more obviously, when oxidizing atmosphere turned into weak reducing atmosphere.