Unraveling the intrinsic mechanism behind the retention of arsenic in the co-gasification of coal and sewage sludge: Focus on the role of Ca and Fe compounds.

Minimizing the emission of arsenic (As) is one of the urgent problems during co-gasification of Shenmu coal (SM) and sewage sludge (SS). The intrinsic mechanism of As retention was obtained by analyzing the effect of different SM addition ratios on the As form transformation during co-gasification at 1000 °C under CO atmosphere. The results showed that the addition of SM effectively promoted the enrichment of As in the co-gasified residues. Especially, the best As retention rate of 65.71% was achieved with the 70 wt% addition ratio of SM. The addition of SM promoted the adsorption and chemical oxidation of As(III) to the less toxic As(V) through the coupling of Ca and Fe compounds in the co-gasified residues. XRD and XPS results indicated that FeO adsorbed AsO(g) after partial conversion to FeO by the Boudouard reaction, while part of AsO was oxidized to AsO by lattice oxygen. Finally, the generated AsO was successively trapped by CaO and FeO to form stable Ca(AsO) and FeAsO. HRTEM and TEM analysis comprehensively proved that As(III) was stabilized by the lattice cage of CaAlSiO. In conclusion, the co-oxidation of Ca and Fe compounds and lattice stabilization simultaneously played a crucial role in the retention of AsO(g) during co-gasification.