| The volume reduction,harmlessness and resource utilization of municipal solid waste?MSW?can be realized well by chemical-looping combustion?CLC?and chemical-looping gasification?CLG?of MSW.However,the concentration of heavy metals in municipal solid waste is relatively high,and the emission control of heavy metals is one of the challenges for CLC and CLG of MSW.By the combination of thermodynamic simulation and experiments,the migration and transformation of cadmium and lead during CLC and CLG of MSW were deeply studied in this paper.The main findings of this paper were as follows:?1?HSC chemistry 6.0 software was used to analyze the effects of three oxygen carrier?OC?active components?CuO,Fe2O3,CaSO4?on the valence distribution of elemental cadmium?Cd?and lead?Pb?.valence state of cadmium can be transformed into cadmium oxide?CdO?by CuO,Fe2O3 and CaSO4.Under the influence of CuO,Pb can be transformed into lead oxide?PbO?,while under the influence of Fe2O3 and CaSO4,Pb still existed mainly.Among the three types of oxygen carriers,only the copper-based oxygen carrier can simultaneously promote the conversion of elemental cadmium and lead to their oxidation states.?2?The influence of temperature?750/850/950??,oxygen to carbon ratio?O/C??0.5/1/2/3/4?and residence time?2/5/15 min?on the distribution of cadmium among bed material,fly ash and flue gas during CLC and CLG of MSW were studied on a tubular furnace.During CLC process,cadmium was mainly distributed in bed material,and Cu4Al6 had a significant inhibitory effect on the volatilization of cadmium,making the distribution of cadmium in bed material more than 94%.Under the experimental conditions,the effects of the O/C,temperature and residence time on the distribution of cadmium were hardly obvious.During CLG process,the distribution of cadmium existed mainly in fly ash,which was similar to that of the traditional gasification process.During the CLG process,the distribution of cadmium in fly ash is 6.38%at 750?,and the cadmium distribution in fly ash is over 95%after 850?,demonstrating that the effect of the temperature on the distribution of cadmium in fly ash was obvious.And the distribution in fly ash increased significantly with the extension of residence time,increasing from 44%to 87.7%,but the effect of oxygen to carbon ratio on cadmium distribution was not obvious.A two-step-transformation and migration mechanism of cadmium during the CLC process was proposed.Step 1:the cadmium was transformed into Cd?g?,desorbing from solid fuel particles and Step 2:Cd?g?was oxidized by the OC particles into CdO,and the CdO may adhere to the surface of the OC particles.Due to the opposite transformation of cadmium by both particles,cadmium would migrate from the SMSW particles to OC particles and be trapped in the bed material during CLC.?3?The effects of the temperature,O/C and residence time on the distribution of lead among the bed material,fly ash and flue gas during CLC and CLG of MSW were investigated.Lead was mainly distributed in the waste residue in the bed material during CLC and CLG of MSW,and the temperature,oxygen-carbon ratio and residence time did not significantly affect the distribution of lead.The large amount of lead present in the primer was caused by the hindrance of the diffusion of lead by the oxygen carrier particles.?4?An efficient and novel cadmium recovery control method was proposed by coupling the characteristic of the cadmium migration during CLC and CLG of MSW.During five successive cycles of the CLC process,the concentration of cadmium in the OC particles increases linearly with the increased cycles,from 7.1 mg/kg to 33.3 mg/kg with a highly and stably average capture efficiency of 85%.With a highly cadmium removal efficiency of 78%after one cycle of the CLG process,the mass ratio of 3%was more suitable for the efficient removal of cadmium from the OC particles by CLG.The enrichment of cadmium in the oxygen carrier can achieve by multiple CLC processes with a high and stable capture efficiency,while the high concentration of cadmium in the oxygen carrier can be removed efficiently just by one cycle of CLG process. |
PDF Full Text Request