Transformation of mineral and emission of particulate matters during co-combustion of coal with sewage sludge

Co-combustion of coal with sewage sludge was carried out in laboratory-scaled drop tube furnace to understand the interaction between different fuels. The combustion conditions were selected as follows: the raw material feeding rate was 0.2–0.3 g/min, temperature was 1200 °C, the atmosphere of 10% O2 and N2 being balance was used to guarantee an air ratio of 1.5, and the residence time varied from 0.6 to 2.4 s. The coal/sewage sludge is kept at 50:50 (wt% to wt%), four fuel pairs were selected with respect to the mineral association within individual fuel. The results showed the obvious interaction between coal and sewage sludge during their co-combustion. For the carbon conversion, the devolatilization of mixing fuel occurred quickly; the combustion of both char and evolved volatile progressed almost completely. As a result, the unburnt carbon was almost zero in the fly ash. In addition, the evolution of both mineral and PM varied with the association of minerals in raw fuels. For both coal and sewage sludge rich in included minerals, they combusted separately in the furnace, less interaction occurred accordingly. Conversely, for both them rich in excluded minerals, the minerals reacted with each other to form much agglomeration, and therefore, the particle size of the fly ash was increased, while the amount of PM was decreased, which changed as the coarse fly ash particles. Finally, for the case of coal rich in excluded mineral and sludge rich in included mineral, their co-combustion led to the interaction of their minerals. As a result, more the fine particles were formed, which in part changed into PM. For the vaporized trace elements, they were adsorbed by the melt CaPO4/Al–Si in the ash and accordingly, their contents in the particulate matter were reduced whereas their particle size distribution shifted to the large value.