Effect of Using Coke Dust as a Sorbent for Removing Mercury from Flue Gases on the Contents of Selected Ecotoxic Elements in Fly Ash

摘要

Passive methods, which are used for the purification of flue gases, can often be insufficient in the case of the emission of mercury and other ecotoxic elements into the environment. Therefore, it may often be necessary to introduce additional methods of reducing the emission of these pollutants, for example, the injection of powdered activated carbon or coke dust into flue gas ducts. The efficiency of coke dust has been confirmed by tests in the laboratory and a demo plant scale. In accordance with the proposed solution, coke dust is dosed before the electrostatic precipitator and then separated fully along with fly ashes. Before injection, coke dust contains negligible amounts of mercury (4.8-10.5 mu g(Hg)/kg, depending upon the size of the fraction), which is a value several times lower than the Hg content in sub-bituminous coals and lignites. However, it can also be a carrier of other ecotoxic elements, such as nickel and chromium. Their contents in coke dust are often much higher than in coal used for power production or in fly ashes. In the paper, the influence was determined of coke dust dosage as a sorbent for removing mercury from flue gases on the contents of arsenic, nickel, chromium, lead, copper, and zinc in fly ashes. The examined samples came from the demo plant for monitoring and abatement of mercury emission from coal combustion in pulverized coal boilers. The contents of particular elements in the tested samples were determined using atomic absorption spectrometry with flame atomization (FAAS, for Pb, Cu, Ni, Cr, and Zn) and with electrothermal atomization (GFAAS, for As).