Sewage sludge combustion in a CLC process using nickel-based oxygen carrier

• CLC is a promising technology for sewage sludge treatment.
• Sewage sludge shows a superior CLC conversion even at lower temperature.
• No interaction between oxygen carrier and sludge ash is identified.
• The soluble phosphates in fly ash based on CLC are beneficial to P recovery.

Sewage sludge is not only a type of solid waste but also considered as a renewable source of energy. Chemical looping combustion (CLC) is an attractive alternative technology for conversion of sewage sludge to energy with simultaneous destruction of pathogenic bacteria and reduction of volume. This work firstly attempted to investigate the CLC performance of de-watered sewage sludge in a batch fluidized bed reactor (FB) with Ni-based oxygen carrier. Different variables affecting the process were analyzed, such as the presence of Ni-based oxygen carrier and the temperature of reduction period. Besides, the CLC performance of bituminous coal and the characteristic of fine ash based on air combustion (AC) were for comparison. The presence of nickel-based oxygen carrier (Ni-based OC) improved carbon conversion and fuel conversion rate. Additionally, when compared to bituminous coal, higher values of both carbon conversion and fuel conversion rate of sludge were obtained at all temperature tested (700–900 °C). Nevertheless, lower CO2 capture efficiency was detected (around 75–84%) when sewage sludge was used. Increasing of CO2 concentration can be achieved by re-circulating exhaust gases to the fuel reactor or incorporating a secondary fuel reactor. Furthermore, although the reaction atmosphere had a minor influence on the chemical composition in fly ash, it had a significant effect on the crystalline structure. CaHPO4, CaH2P2O7, Fe3O4, Ni and NiAl2O4 could be detected in fly ash based on CLC, which were not detectable based on AC. Moreover, during 20 redox cycles’ operation, no sintering/agglomeration problems were detected, attributing to low reaction temperature and the high enrichment of CaO in sludge ash. No products of interaction between oxygen carriers and sludge ash were identified.