Co-combustion of Municipal Sludge and Zhundong Coal: Ash Formation Characteristics

Zhundong coal (ZD), which has a high content of Na and low ash fusion temperature (AFT), causes a serious problem of slagging in boilers. P-rich municipal sludge has suitable properties for managing problems with alkali metals, but its insufficient utilization makes sludge management harder. Thus, co-combustion of municipal sludge and ZD may be a suitable approach to solve the problems. Different proportions of sludge were added to ZD at 900 degrees C in different contents of O-2 in a laboratory-scale tube furnace experimental system. The transformation of the minerals, the AFTs of residual ashes, and changes of the sodium retention ratio were investigated, respectively. The test results indicate that the ash fusibility of ZD and the sodium retention ratio can be improved because of the generated high-melting-point phosphates, such as Ca2.71Mg0.29(PO4)(2), Ca-5(PO4)(3)(OH), Ca9Fe(PO4)(7), and Ca9MgNa(PO4)(7). With increases in the proportion of sludge in blended samples, the AFTs increase, but the extent of the increase tends to be reduced. Modified sludge with a lower SiO2/Al2O3 ratio is more favorable for the improvement of the AFT. It is observed that 20% oxygen is the most suitable environment of those tested for co-combustion, avoiding the disadvantages of excessively high or low oxygen content.