Interactions between semi-volatile toxic metals and sorbents in combustors

Abstract

The emission of semi-volatile toxic metals from boilers and incinerators is a major concern. One potential method to control semi-volatile metal emissions is high temperature sorbent injection. In this work, binary combinations of two toxic metals and three sorbents--kaolinite, hydrated lime, and a paper waste-derived sorbent (PWDS), were examined qualitatively and quantitatively. PWDS is a novel material produced from the waste sludge of the paper recycling process. An aerosol size-fractionation technique was developed to discriminate between metal vapor and metal that is either condensed on or reacted with particles in the furnace. A rapid-quench sampling system forces homogeneous nucleation to occur in the presence of other particles. Small nuclei generated from metal vapor are easily distinguished from metal reacted or condensed to larger particles. The aerosol size-fractionation technique was used to explore metal partitioning in the combustor. The longitudinal evolution of two mixtures, lead/cadmium, and cadmium/nickel, were examined. Based on this study, condensation and coagulation control aerosol processes in the furnace. Screening and parametric tests were performed to investigate the effect of injection temperature, sorbent/metal molar ratio and chlorine concentration on the capture of toxic metals by sorbents. In general, kaolinite and PWDS reacted rapidly with lead and achieved ∼100% lead sorption in a short time while cadmium was absorbed rapidly by lime only. The lead/lime reaction appeared to be slow compared to the other sorbents. In the presence of chlorine, the amount of lead captured by kaolinite and PWDS was significantly reduced. However, the small amount of lead absorbed by the lime seemed unaffected by chlorine. The reaction between lead and kaolinite was studied in detail. Lead capture increases with increasing sorbent feedrates and decreases with increasing chlorine concentration. The lead/kaolinite reaction appeared to be inhibited by higher temperatures. A simple global reaction model was proposed where the lead/kaolinite reaction product inhibits further reactions. Kinetic parameters were estimated from experimental results. The model was then modified to account for the effect of chlorine, the results of the model predict super-equilibrium concentrations of lead chloride vapor.