Effects of alkali and alkaline earth metals on co-combustion of sewage sludge and coal slime: Combustion characteristics, interactions, and kinetics

Abstract

The co-combustion of sewage sludge (SS) and coal slime (CS) is a preferred method for their resource utilization, however, alkali and alkaline earth metals (AAEMs) in SS may affect the co-combustion process. In this work, the co-combustion behavior of AAEMs-rich SS and CS was investigated in terms of combustion characteristics, interactions, and combustion kinetics using a thermogravimetric analyzer. Further, the role of AAEMs in co-combustion was evaluated by loading Ca, K, Na, and Mg individually after pickling. The results revealed that co-combustion compensated for the limitations of the individual combustion processes, with SS reducing ignition and burnout temperatures and CS improving the comprehensive combustion characterization. Principal component analysis (PCA) showed that the effect of CS on co-combustion was more significant compared to SS. Significant synergies were observed in the weight loss phase of fixed carbon in the blends with 40%, 50%, and 60% CS ratios, where the peak temperature of fixed carbon combustion was reduced by 9.8 °C, 12.6 °C, and 13.1 °C, respectively, compared to the theoretical values. The effects of AAEMs on combustion were as follows: all AAEMs promoted the precipitation of volatiles except Ca, which showed inhibition of light volatiles; AAEMs had a significant catalytic effect on fixed carbon combustion. The improvement effect of AAEMs on the comprehensive combustion characteristics during co-combustion was Na > K > Mg > Ca. The catalytic effect of Na on fixed carbon was strongest at a loading of 5%, leading to a decrease in the apparent activation energy of fixed carbon combustion by 22.2 kJ/mol and a change in reactor order from n = 1 to n = 1.2 during co-combustion. This work provides a better understanding of the role of AAEMs in SS-CS co-combustion.