Chemical looping gasification of sewage sludge using copper slag modified by NiO as an oxygen carrier

Chemical looping gasification (CLG) provides a novel approach to dispose the sewage sludge. In order to improve the reactivity of the calcined copper slag, NiO modification is considered as one of the good solutions. The copper slag calcined at 1100 °C doped with 20 wt% NiO (Ni20-CS) was used as an oxygen carrier (OC) in sludge CLG in the work. The modification of NiO can evidently enhance the reactivity of copper slag to promote the sludge conversion, especially for sludge char conversion. The carbon conversion and valid gas yield (Vg) increase from 67.02% and 0.23 m3·kg−1 using the original OC to 78.34% and 0.29 m3·kg−1 using the Ni20-CS OC, respectively. The increase of equivalent coefficient (Ω) facilitates the sludge conversion and a suitable Ω value is determined at 0.47 to obtain the highest valid gas yield (0.29 m3·kg−1). A suitable steam content is assigned at 27.22% to obtain the maximum carbon conversion of 87.09%, where an acceptable LHV of 12.63 MJ·m−3 and Vg of 0.39 m3·kg−1 are obtained. Although the reactivity of Ni20-CS OC gradually decreases with the increase in cycle numbers because of the generation of NiFe2O4-δ species, the deposition of sludge ash containing many metallic elements is beneficial to the sludge conversion. As a result, the carbon conversion shows a slight uptrend with the increase of cycle numbers in sludge CLG. It indicates that the Ni20-CS sample is a good OC for sludge CLG.