Combustion Possibility of Dry Sewage Sludge Used as Blended Fuel in Anthracite-Fired Power Plant

Abstract

The aim of this study is to predict the combustion possibility of dry sewage sludge used as a blended fuel in commercial anthracite-fired plants. The combustion characteristics of various sludge blends prepared on the basis of design anthracite coal were studied by thermogravimetric analysis (TGA) and drop-tube furnace (DTF). The TGA results showed that the volatiles components in dry sludge burned more rapidly those in anthracite. The sludge mainly comprised two organic fractions with different reactivities; the more reactive fraction decomposed and burned at a lower temperature than that of anthracite in the case of sludge blends with a blending ratio of less than 15 wt%, and the burning temperature were closer to those of anthracite. In sludge blends with a blending ratio of more than 20%, interactions were observed between the anthracite and sludge components. The Arrhenius kinetic parameters were calculated from the experimental results, by considering the process as a series of consecutive first-order reactions. Addition of low quantities of sludge (blending ratio <15%) hardly brought about any improvement in the coal activation energy. Activation energies for the overall combustion of anthracite and sludge were 75.380 and 17.410 kJ/mol, respectively. The activation energies for the combustion of 30% and 50% sludge blends were 20.951 and 15.140 kJ/mol, respectively, and they were inversely proportional to the volatile matter content and sludge blending ratio. The DTF test results showed that the volatile content of the sludge significantly improved the combustion efficiency of the sludge blends, whereas the unburned carbon in combustion by-product reduced. The conversion behaviors of the anthracite, sludge, and the sludge blends observed in DTF tests were similar to those reflected in TGA. DTF char studies showed that sludge blends were higher conversion rates than did anthracite coal, because of porous char formation; most of the sludge blends underwent complete combustion at residence time of 1 s and at 1,100°C as in the case of commercial coal-fired plant. In the case of the sludge blends with blending ratios of less than 30%, the high initial deformation temperature (IDT) of the sludge ashes was not expected to be associated with slagging and fouling in pulverized coal-fired systems.