Mechanisms governing multi-species metal capture by kaolinite, hydrated lime, and novel sorbents in high-temperature combustion environments

Abstract

The objective of this work was to provide a solution to the problem of toxic metal emissions from high temperature combustion processes. Multi-metal as well as single-metal interactions were investigated to provide an understanding of mechanisms that exist in industrial furnaces, where multiple metals are present. Specifically, multiple toxic metals, lead and cadmium, and a common non-toxic metal, sodium, were investigated. Sodium capture by kaolinite was found to exhibit a negative activation energy (between 1100 and 1300°C) similar to that shown previously for lead, due to a catastrophic deactivating melt initiated by the metal oxide/kaolinite reaction product. In addition, an overall sodium/kaolinite reaction rate (soluble + insoluble) was determined. It was also discovered that a larger percentage of sodium/kaolinite reaction product was water soluble when formed at lower equivalence ratios than at high equivalence ratios. The majority of the initially formed sodium/kaolinite reaction products were probably insoluble sodium aluminosilicates. However, at high sorbent utilizations (low equivalence ratios), the meta-kaolinite structure probably broke down to form sodium silicates and aluminates, thus enabling at least twice as much sodium capture as the sodium aluminosilicate products. The cadmium/kaolinite reaction rate was highly activated between 1100°C and 1300°C, due to a self-enhancing melt, which occurred at the high but not low temperature condition. For the Cd/Pb multi-metal system, cadmium capture was enhanced by a melt initiated by the lead/kaolinite reaction product. Also, cadmium enhanced lead capture by reducing the extent of catastrophic melt caused by the lead/kaolinite product. The formation of an optimum eutectic melt accounts for the enhancement of total bimetal capture by kaolinite at high and low temperatures (1100°C to 1300°C). Sodium capture by kaolinite completely dominated over lead capture from a bimetal system. On the other hand, sodium was found to behave similar to lead in terms of enhancing cadmium capture by kaolinite at the low temperature condition. In addition to kaolinite, hydrated lime was found to be effective at capturing cadmium, and CDEM sorbent, composed of calcium carbonate, lime, and kaolinite, was effective at capturing cadmium and a mixture of cadmium and lead.