Grantee Research Project Results
1998 Progress Report: Optimization of In-Situ Capture by Sorbents of Toxic Metals in Combustion Processes
EPA Grant Number: R825389Title: Optimization of In-Situ Capture by Sorbents of Toxic Metals in Combustion Processes
Investigators: Wendt, Jost O.L.
Institution: University of Arizona
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999
Project Period Covered by this Report: October 1, 1997 through September 30, 1998
Project Amount: $293,068
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Air , Safer Chemicals
Objective:
The overall objectives of this research are to develop fundamental quantitative models able to predict the interaction of sorbents and toxic metals at high combustion temperatures, and to determine optimum sorbent composition for the most effective and efficient removal of multiple toxic metals (multimetals) with and without the presence of chlorine and sulfur.Progress Summary:
Progress during the past year has included the completion of multimetal screening tests, the rebuilding of the downflow furnace facility, the successful operation of the graphite furnace atomic absorption spectrometer, the manufacturing of a Berner low-pressure impactor (LPI), and the development of a parametric test matrix from which to determine multimetal capture mechanisms. Furnace Rebuild. Laboratory and small-scale pilot combustors, like the downflow furnace at the University of Arizona, generally use refractory or refractory-lined quarrels and walls. Subjected to constantly fluctuating extreme temperature conditions, this refractory material cracks and disintegrates. In July 1998, the downflow furnace developed cracks in the upper refractory section that were more substantial than could be repaired by patching. Consequently, the top half of the furnace was rebuilt. The new furnace features a number of improvements, including: (1) an anti-slagging refractory for greater durability, especially when coal is burned in the furnace; (2) a reinforced and more robust support structure for the upper part of the furnace to allow for a much thicker (and heavier) refractory section, which will have a longer life; (3) a three-port cap that allows visualization of the flame and atomizing jet through a sight glass; and (4) an improved burner plate and support. BLPI Manufacturing. The Berner LPI allows particle size segregation in the micron and submicron regions with enough resolution that meaningful particle size distributions may be obtained. We had borrowed an LPI from Physical Sciences Inc. (PSI) and found it to be an essential part of our experimental apparatus. Consequently, we built our own LPI in fall 1998. The impactor fundamentals were identical to that of the PSI LPI, and its performance was validated against it. However, an improvement over the PSI LPI was made in the impactor housing and closing mechanism design, which ensures that the impactor plates are compressed to the same extent each time, thus increasing its durability and ease of operation. Low Concentration Measurements Demonstrated. In July 1998, low in-furnace metal concentration experiments were performed. Cadmium was injected in the furnace to obtain concentrations of 1.0 and 0.5 ppm, with and without a 50/50 sorbent mixture of lime and kaolinite. Sampling was performed using the isokinetic sampling system and LPI described in the previous annual report. At such low concentrations of metal in the furnace, the number concentration of nuclei formed in the LPI from condensing metal vapor is too small to produce particles large enough (via coagulation) to be captured in the impactor. Hence, when low concentrations of cadmium were injected into the furnace without any sorbent, essentially no metal was captured in the impactor. However, when the mixed sorbent was injected into the furnace along with the low concentrations of metal, the cadmium was found to have been captured by the sorbent, and the recoveries of cadmium were comparable with those of higher concentration experiments, indicating that all of the cadmium was captured by the sorbent. The graphite furnace was used to measure the ppb metal concentrations on the filters of the LPI, while the flame atomic absorption spectrometer was used to measure the sorbent tracer concentrations. The low concentration experiment data also show that there was no sorbent particle size dependent capture of the metal, which indicates that no transport limitations existed, and that the reaction took place primarily on the surface of internal pore walls. Sorbent Mixture Experiments. Screening experiments in fall 1997 showed that cadmium could be captured by lime but was less effectively captured by kaolinite, and that lead was captured well by kaolinite but was less effectively captured by lime. For the multimetal system of lead and cadmium, kaolinite did not capture cadmium. The conclusion was that there are competition/inhibition mechanisms involved in the capture of lead and cadmium multimetals by kaolinite. Accordingly, in July 1998, experiments were performed to determine if cadmium and lead multimetals would be effectively captured by a mixture of lime and kaolinite. Results indicated that a mixture of lime and kaolinite can capture both lead and cadmium from a lead/cadmium multimetal system. Therefore, it is possible to effectively capture both lead and cadmium multimetals with a sorbent mixture. The next step is to optimize the mixture to completely capture both lead and cadmium as a function of metal concentration. Results of the mixture experiments, at relatively high concentrations (100 ppm in furnace), also indicate that reaction of the metal with the sorbent takes place on pore surfaces within the sorbent particles. Hence, transport limitations do not influence the interpretation of the data, and the mechanisms of capture may be compared directly with those at low metal concentrations. In summary, low concentrations of cadmium in the furnace (as low as 0.5 ppm) have been successfully run and low concentration (ppb) samples have been successfully measured by graphite furnace-atomic absorption spectrometer analysis. No diameter-dependent capture is observed for low- and high-metal concentration experiments. Therefore, the interpretation of capture mechanisms may be done without regard to transport limitations. Sorbent mixtures of lime and kaolinite have been shown to successfully capture both lead and cadmium from a multimetal system, thus setting the stage for sorbent mixture optimization.Future Activities:
Future activities include: (1) continuation of single metal/sorbent experiments to determine capture kinetics of individual metals by various sorbents; (2) continuation of experimental investigation of multimetal/sorbent interaction mechanisms (competition/inhibition/synergism); (3) obtaining data to construct a mixture design response surface for Cd, Pb, and Na multimetals with mixtures of lime and kaolinite, and development of a fundamental model to explain the mechanisms governing the response surface; (4) chlorine and SO2 doping into the furnace in the presence of multimetals and examination of the effect on metal capture by sorbents. A fundamental model will be used to predict this effect and thus, determine the mechanisms of multimetal interactions in the presence of chlorine and SO2; and (5) continuation of efforts to determine the reacted metal/sorbent product species using X-ray diffraction and raman microprobe spectroscopy.Journal Articles:
No journal articles submitted with this report: View all 10 publications for this projectSupplemental Keywords:
toxic metals, sorbents, lead, cadmium, incineration., RFA, Scientific Discipline, Air, Waste, Ecosystem Protection/Environmental Exposure & Risk, particulate matter, Ecology, Mathematics, Environmental Chemistry, Fate & Transport, Incineration/Combustion, Engineering, Engineering, Chemistry, & Physics, fate and transport, fate, particulates, quantitative models, atmospheric particles, Chlorine, toxic metals, sulfur, combustion process, chemical kinetics, kinetic models, sorbents, combustionProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.