Effects of alkali and alkaline earth metal species on the combustion characteristics and synergistic effects: Sewage sludge and its blend with coal.

A large amount of alkali and alkaline earth metal species (AAEMs) are contained in sewage sludge (SS) ash, which will affect the combustion characteristics and synergistic effects of the co-combustion of SS and coal. The main objective of this paper is to investigate the effects of K, Ca, Na and Mg on combustion characteristics and synergistic effects of the blend of SS and coal by TGA and single particle combustion methods. The ash-free sludge (AS), impregnated AS and the blends of AS and bituminous coal (BC) were prepared. A high speed camera was used to record the combustion process, which were processed to calculate the ignition delay time, burnout time and combustion temperature. The synergistic effect of co-combustion process was studied by comparing the experimental results with the theoretical calculation results. The synergistic effects main occur at 350-530 °C and the blends of AS and BC with 2 wt% Na has the strongest strength of synergistic effects. The K, Ca and Na are conducive to the shorter ignition delay time, burnout time, higher combustion temperature and stronger synergistic effects of the blends. However, Mg will increase the delay time and reduce the combustion temperature of char of the blends. There are optimal concentrations of AAEMs on the promotion of combustion and synergistic effects, and too high concentrations of AAEMs will have negative impacts. K and Na (alkali metals) have stronger promotion effects than Ca and Mg (alkaline earth metals) on combustion of volatiles and synergistic effects.