Direct modification of solid residues during co-firing of coal sludge and coal ☆

Abstract

In order to utilize the solid residues simultaneously with clean and high-combustion efficiency when firing high sulfur and ash coal sludge generated in coal washing process in China, the mineral composition and usability of combustion solid products were investigated when injecting compound additives into series of coals or coals blended with sludges. In this study, a single burner furnace was used for combustion test and the solid residues were analyzed by means of X-ray powder diffraction method. The influences of temperature, residence time, and compound additives on mineral characteristics of the residues have been discussed. The results indicate that, the compositions of solid products are principally free oxides and a small amount of gehlenite, dicalcium silicate, etc. below 1000 °C; when the firing temperature increases above 1100 °C, the major minerals are gehlenite and dicalcium silicate. Also it can be found that several seconds is not enough for mineral complete reaction, and suitable calcium oxide content and particles fineness are recommended. The result of combustion test shows that there is no evident effect on ignition and combustion when the additive/coal (a/c) ratio is below 30% by weight; and the normal stable combustion with ignition delay can be achieved when the a/c ratio is ranging between 30 and 50%; but the a/c ratio exceeding 50% will damage combustion and even result in fire extinction. Industrial test results indicated that the solid residues was composed of silicate mineral rich in belite by adopting compound additives, but the comprehensive reaction mechanism needs further studies as well as the effect of additives on combustion should be concerned.