A bench scale system incorporating a high temperature flow reactor has been utilized to study the reaction of metallic species and sorbent compounds. The reaction and interaction between a lead precursor and a vapor phase silica precursor is studied. The effect of chlorine and the effect of sorbent to metal feed ratio on lead speciation and size distribution is determined. The size distributions of the reacted compounds are measured by a scanning mobility particle sizer (SMPS) and an optical particle counter (OPC). The product powders are also collected and their compositions established by X-ray diffraction (XRD) or, if an amorphous powder is collected, by infrared and Raman spectroscopy. Experiments have been performed at 500, 750, 1000, and 1250 °C and for reactor residence times of 0.5−1.0 s. For example, at 750 °C, for a lead feed in conjunction with a vapor phase silica precursor compound, the mean particle size exiting the reactor is significantly larger than for a lead only feed or a silicon only feed. The composition of the resulting aerosol is determined, via infrared and Raman spectroscopy, to be lead silicate, PbSiO3. Higher temperature results are similar. At lower temperatures, silica particles are not formed in the reactor. Higher silica precursor feed rates result in particles large enough for capture by conventional pollution control equipment.

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A detailed characterization was performed of the particles produced under various Pb:Si molar feed ratios for a maximum flow reactor temperature of 1000 °C. The silica particles formed in the high temperature region coagulated and only partially coalesced to form large agglomerate structures of high specific surface area. For a lead-only feed, the resulting particles were hydrocerussite with small but detectable amounts of massicot. As the silica precursor was inlet in excess amounts (Pb:Si < 1:12, the crystalline lead compounds disappeared and amorphous lead-silica complexes dominated. The particle morphology also changed from cylindrical, polygonal, and spherical shapes to large agglomerate structures composed of several size modes of primary particles. At Pb:Si molar feed ratios of 1:12 and 1:29, the particles making up the chainlike agglomerate structure were primarily spherical, with larger lead silicate spherical particles (=0.5 |im) attached to the agglomerate. The lead was found to be distributed throughout the large agglomerate structures, implying easier capture of lead emissions in particulate control devices. IMPLICATIONS Sorbents generated in situ for toxic metals capture in combustors have the potential advantage over bulk solid sorbents of more effective sorbent utilization by providing more available surface area for condensation of the metal vapor phase compounds. Trace levels of sorbent precursors can be used to capture trace levels of toxic metal compounds. These sorbents consist of large agglomerate structures that are amenable to capture in conventional air pollution control devices. By reactively scavenging the toxic metal vapors in the high temperature region of the combustor, homogeneous nucleation of the metal compound vapors is suppressed, thereby preventing the formation of very fine metal oxide particles. INTRODUCTIONVolatile metals introduced into a combustion device may enter the gas phase at combustor temperatures, and then as the temperature decreases downstream of the combustor, undergo nucleation to form a submicrometer-sized aerosol that is not easily captured by conventional air pollution control equipment. 13 Various bulk solid sorbents used in sev-