Release and transformation of phosphorus in chemical looping combustion of sewage sludge

Abstract

Sewage sludge contains high content of phosphorus and represents an important secondary phosphorus source. Meanwhile, chemical looping combustion (CLC) is an innovative sludge treatment technology with low NO emission and potential to effectively recover phosphorus. A full understanding of the phosphorus transformation mechanism is vital for the development of CLC of sewage sludge. In this work, the release and transformation behavior of phosphorus in CLC of sewage sludge was studied in a batch fluidized bed reactor with hematite oxygen carrier. The increase of fuel reactor temperature had a marginal influence on phosphorus transformation. Approximately 66.81–69.30% of phosphorus in sewage sludge was concentrated in fly ash during CLC of sewage sludge. The fraction of phosphorus in flue gas was relatively constant, since nearly all of the organic phosphorus volatized. Moreover, the available phosphorus in sludge CLC fly ash was vastly superior to that in sludge combustion ash. The higher available phosphorus content could fulfill the quality parameters for a P-fertilizer. Considering the effect of phosphorus on the reactivity of hematite, the promotion by phosphorus catalyzing and weaken by deposition appeared to be in competition with each other. The reduction reactivity of hematite increased at lower phosphorus content, but it decreased at higher phosphorus content (>3%). Regarding the effect of sludge ash on hematite, the reactivity of hematite increased by adding proper content of sludge ash. Simultaneously, the reactivity of sludge ash as oxygen carrier in reducing period was tested. Despite the fact that sludge ash contained high fractions of Fe and Ca, the performance of sludge ash was unstable in CLC process. This indicated that it was infeasible of using sludge ash as oxygen carrier.