Control of trace metal emissions during coal combustion. Technical progress report, April 1, 1996--June 30, 1996

摘要

Emissions of toxic trace metals in the form of metal fumes or submicron particulates from a coal-fired combustion source have received greater environmental and regulatory concern over the past years. Current practice of controlling these emissions is to collect them at the cold-end of the process by air-pollution control devices (APCDs) such as electrostatic precipitators and baghouses. However, trace metal fumes may not always be effectively collected by these devices because the formed fumes are extremely small. The proposed research is to explore the opportunities for improved control of toxic trace metal emissions, alternatively, at the hot-end of the coal combustion process, i.e., in the combustion chamber. The technology proposed is to prevent the metal fumes from forming during the process, which would effectively eliminate the metal emission problems. Specifically, the technology is to employ suitable sorbents to (1) reduce the amount of metal volatilization during combustion and (2) capture volatilized metal vapors. The objectives of the project are to demonstrate the technology and to characterize the metal capture process during coal combustion in a fluidized bed combustor. The observed experimental results indicated that metal capture by sorbents can be as high as 91% depending on the metal species and sorbent involved. All three sorbents tested, i.e., bauxite, zeolite and lime, were observed to be capable of capturing lead and cadmium in a various degree. Zeolite and lime were able to capture chromium. Results from thermodynamic equilibrium simulations suggested the formation of metal-sorbent compounds such as Pb(sub 2)SiO(sub 4)(s), CdAl(sub 2)O(sub 4)(s) and CdSiO(sub 3)(s) under the combustion conditions. Additional experiments are being carried out to provide more statistically representative results for better understanding the metal capture process.