Transformation and migration of cadmium during chemical-looping combustion/gasification of municipal solid waste

Abstract

The transformation and migration of cadmium during chemical-looping combustion (CLC) and gasification (CLG) of the synthetic municipal solid waste (SMSW) was investigated in a tubular furnace. Both copper-based and iron-based oxygen carriers (OCs) can make elemental cadmium (Cd (g)) into cadmium oxide (CdO) at 850 °C, but their complete reduction products do not have this ability. Cadmium was distributed mainly in the bed material during CLC, but distributed mainly in the fly ash during CLG. After 2 min of reaction, approximately 90% of the cadmium in the bed material distributed in the OC particles, indicating that large amount of cadmium would volatilize from the SMSW particles and migrate to the OC particles at the initial stage of CLC and CLG. As the reducing gas generating later may restore CdO to Cd (g) again during CLG, the cadmium migrating to the OC particles was detached gradually. A two-step-transformation and migration mechanism for cadmium during CLC and CLG was proposed. After five successive cycles of the CLC process, the concentration of cadmium in the OC particles increased linearly from 7.1 mg/kg to 33.3 mg/kg with an average capture efficiency of 85%, while after just one cycle of the CLG process, the high concentration of the cadmium in the OC particles can be removed efficiently. Overall, the cadmium in MSW can be captured by CLC, while be removed by CLG. Therefore, the coupling of CLC and CLG processes may provide an innovative and efficient control, even recycling technology for the cadmium in MSW.