Condensation behavior of heavy metal vapors upon flue gas cooling in oxy-fuel versus air combustion

Vaporization of Pb and Zn-loaded model compounds has been carried out in a lab-scale rotary kiln reactor to clarify their condensation behavior upon flue gas cooling in both air-firing and oxy-fuel combustion modes. The influence of flue gas impurities including HCl, SO2and H2O has been examined systematically. For the two metals existing separately in the reactor, namely in single mode, both preferentially condensed as chlorides in the presence of HCl, under air-firing and oxy-fuel conditions. SO2 and steam in flue gas promote chloride into sulfide at high temperature. It was noticed that the deposition propensities of Pb and Zn vapors under the oxy-fuel condition of the single mode were slightly enhanced over the air-firing condition. This is due to the lower Lewis number of oxy-fuel combustion that is in favor of the formation of highly polydispersed fine particles. However, this discrepancy was diminished in the case where both metals coexisted as in the mixture mode in the reaction system. In the mixture mode from 700 to 400 K, the condensation fraction of either Pb or Zn was confirmed higher than that in the single mode, due to an enhanced heterogeneous nucleation of Zn vapors on PbSO4 nuclei.