Journal of chemical technology and biotechnology vol:78 issue:2-3 pages:213-219
Under certain operating conditions, polychlorinated dioxins and furans occur in the flue gases of thermal processing plants. Current EU Directives limit the concentration of these emissions to less than 0.1 ng toxicity equivalents (TEQ) per standard cubic metre of dry gas at 11vol% O-2. These Directives have been included in the emission regulations of most of the EU countries. Dioxins and furans are not particular to incinerators; they can also originate from metallurgical and other thermal processes when these are operated under unfavourable conditions. Adsorption methods employing non-flammable carbonaceous adsorbents (activated carbon, lignite) are particularly well suited for removing PCDD/Fs. For adsorption, which requires the sorbent to be brought in contact with the gas to be cleaned, various processes are currently available, ie classical adsorption in moving/fixed bed filters or the technique of adsorption in an entrained dust cloud with a downstream fabric filter or electrostatic precipitator to remove the spent sorbent from the waste gas (entrained process). The paper will firstly assess both adsorption concepts; it will then use results obtained from operational tests in incineration plants for municipal and industrial waste to describe the efficiency of the dioxin-sorption in an entrained bed (pneumatic transport of pulverized adsorbent) and a fixed/moving bed (granular adsorbent). Variations in the dosing rate and the use of different adsorbents, as well as the operating temperature, identify the selection criteria for determining the process-engineering optimum for flue gas cleaning. The paper will demonstrate that the PCDDIF emissions can be reduced to below 0.1 ng TEQ Nm(-3) and will identify the operating conditions required to achieve this standard. (C) 2003 Society of Chemical Industry.