Characteristics of hematite and fly ash during chemical looping combustion of sewage sludge

• CLC is an attractive technology for processing sewage sludge.
• No char bypassed to air reactor during CLC of sewage sludge.
• The reactivity of hematite shows a slight decrease during 10 h operation.
• Phosphorus speciation in CLC fly ash is beneficial to agriculture.

Chemical looping combustion (CLC) of sewage sludge is an appropriate solution of how to manage the continuously increasing sewage sludge production and at the same time, how to rationally use renewable resource. This work attempted to investigate the combustion performance of sewage sludge in a continuous CLC unit based hematite oxygen carrier. Besides, the characteristics of hematite and fly ash during chemical looping combustion of sewage sludge were elaborated. Compared to either bituminous coal or anthracite, sewage sludge was a unique solid fuel in the term of high combustion efficiency even at 800 °C. Additionally, there were no char particles bypassed to air reactor during CLC of sewage sludge. During 10 h continuous operation, hematite shows a slightly decrease reactivity, indicating a good long-term reactivity of the oxygen carrier used. Although some ash particles deposited on the surface of hematite, no reaction between sludge ash and hematite were identified. The only phosphate identified incinerated sludge ash was Ca2P2O7. However, the form of phosphorus in fly ash based on CLC was CaH2P2O7 and CaHPO4. The presence of steam in flue gas and the reduction atmosphere can significantly accelerate the generation of CaH2P2O7 and CaHPO4. Then, possible options of utilizing sludge ash on agriculture were discussed.