Regulation of ash slagging behavior for sewage sludge by rice husk addition: Focusing on control mechanisms

Abstract

Combustion of sewage sludge (SS) is a promising alternative waste-to-energy process. However the mono-combustion of SS may induce severe ash slagging behavior, which will shorten the life of the combustion reactor. In this study, rice husk (RH) as a clean and high-energy density auxiliary was introduced to improve the slagging characteristics of SS and the underlying mechanism was revealed. The results indicated that RH addition can raise ash fusion temperature (AFT) of SS during co-combustion and the growth rate of AFT increased significantly under high RH dose. Slagging and fouling indices showed that slagging tendency of the co-combustion ashes could be neglected under 50% RH addition. Overall, co-processing of SS and RH is an ideal route to solve ash-related problems. The inhibition mechanism of RH addition to the slagging behavior of co-combusted SS was determined as follows: (1) Insufficient iron-bearing phases caused the formation of refractory co-combustion ash; (2) a reduced liquid phase, which acts as a glue to bond the ash grains, played a significant role in slagging control; (3) fiber-like structures, distributed as discrete particles to form a porous structure, reduced the cohesive bonds in ash agglomerates, which further inhibited ash slagging.