Effects of CaO-Fe2O3-Fe3(PO4)2 in sewage sludge on combustion characteristics and kinetics of coal slime

Abstract

The co-combustion of sewage sludge (SS) and coal slime (CS) is a promising approach to realize the harmless disposal of these two solid wastes. Aiming at the effects of the minerals in SS on CS combustion during the co-combustion, the combustion characteristics and kinetics of CS under the action of CaO, Fe2O3, and Fe3(PO4)2 were investigated by a thermogravimetric analyzer. The effects of the single and the composite minerals on CS combustion were quantified, and the synergistic or inhibitory effects between the minerals were also analyzed. Furthermore, Flynn-Wall-Ozawa (FWO) method and the master plots method were used to calculate the combustion kinetics. The results revealed that the catalysis of the minerals in SS was an important factor for the interaction between SS and CS during the co-combustion. For the single minerals, Fe2O3 exhibited the best catalytic effect, which reduced the ignition temperature (Ti) and burnout temperature (Tb) by 8.2 °C and 42.2 °C, respectively, and increased the comprehensive combustion characteristic index (S) by 10.7%. For the composite minerals, Fe2O3 and CaO synergistically catalyzed CS combustion through a cascade chain catalytic mechanism, resulting in a 15.9% increase in S compared with CS combustion alone. However, the interaction between Fe2O3 and Fe3(PO4)2 at high temperatures produced dense products that inhibit the burnout process. The kinetic results showed that the minerals in SS led to a decrease in the apparent activation energy of CS during the combustion stage of the fixed carbon, and the order of decreasing degree was Fe2O3 + CaO > Fe2O3 > Fe2O3 + CaO + Fe3(PO4)2. In addition, the combustion of CS was a multi-step kinetic process, and the mechanism function changed from f(α)= (1-α)4 to f(α)= (1-α) as the combustion progressed. The composite mineral Fe2O3 + CaO participated in the combustion reaction of fixed carbon in CS, changing the reaction mechanism from F1 to F1.3.