High-temperature sequestration of elemental mercury by noncarbon based sorbents

This work is concerned with sequestration of elemental Hg at high temperatures (900-1100 degrees C) on a sorbent that is mineral based, rather than carbon based. This sorbent consists of an intimate mixture of CaO, CaCO3, and Al2O3-2SiO(2), and is manufactured in industrially relevant quantities (metric tons) from residues produced in paper recycling processes. In contrast to activated carbon (AC), this noncarbon based sorbent has special advantages in that, it can actually enhance fly ash utilization for cement manufacture, rather than diminish it, as is the case for AC. Disperse phase experiments have been conducted, using an externally heated quartz tube reactor, with sorbent feeding rates ranging from 1 to 6 g/h. Preliminary results indicate that Hg removal efficiency is sensitive to sorbent feed rates and to furnace temperature. The Hg removal percentage increased with both these variables. Two mechanisms conic into play: an in-flight Hg sorption mechanism, and an Hg sorption mechanism related to sorbent deposits on the reactor wall. A maximum total (in-flight plus deposit-related) Hg removal efficiency of 83-90% was obtained at temperatures of 900-1100 degrees C. There was negligible sorption by either mechanism at temperatures below 600 degrees C. Results for the in-flight mechanism alone showed a maximum sorption efficiency at similar to 900 degrees C, whereas that on the reactor surface increased monotonically with temperature. This suggests that sorbent deactivation can occur in-flight at high temperatures, which is in agreement with other fixed bed results obtained in this laboratory. Deactivation was not apparent for the sorbent-related substance formed on the reactor wall. Raw and spent sorbents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectrophotometer (SEM-EDS) to identify the sorbent mineral transitions that seem to activate the process. The in-flight mechanisms appear to involve (1) activation of the sorbent, caused most probably by an internal solid solid reaction, followed by (2) Hg sorption, and (3) possible deactivation, if the temperatures are too high for longer period. Reactor surface mechanisms still remain to be elucidated. (C) 2009 Curtin University of Technology and John Wiley & Sons, Ltd.