Thermodynamic evaluation on chemical looping conversion of Cd- and Zn-contained phytoremediation plant with different CaO pathways

Abstract

With the development of phytoremediation for soil contamination, disposal of phytoremediation plant becomes a serious problem. Thermochemical conversion of phytoremediation plant can greatly reduce the volume and mass, meanwhile the clean and reusable utilization is realized. As one of the thermochemical conversion technologies, chemical looping (CL) offers a carbon negative way for clean utilization of biomass. In this technology, CaO has binary roles of heavy metal solidification and CO2 sorption for gasification enhancement. To assess the CaO pathway in CL of phytoremediation plant, two different CL processes are constructed and comparatively studied based on thermodynamic evaluation. The effects of different operating parameters on the products of gasifier (GR) and reduction reactor (RR) are compared and discussed. Results demonstrated that the CaO addition in GR is beneficial to the production of pure combustible gases. Increasing RR temperature can promote the chemical looping reactions in RR. Under lower temperature, CaO in RR can consume more CO2 leading to CO2 free environment. When it is higher than 850 °C, there is no effect of CaO in RR. Increasing the amount of OC in system can enhance the conversion of combustible gases. When αOC is higher than 0.3, the OC is reduced to a mixed state of Fe3O4 and FeO. When the CaO circulates only between GR and calciner, pure CO2 can be captured at the outlet of calciner. Existence of CaO is beneficial to retain Cd and Zn in solid phases. When the gasification temperature increases from 500 °C to 800 °C, the Cd(g) increases while Cd decreases in both CL1 and CL2. For a long lifetime of OC, CaO is suggested to circulates between GR and calciner.