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Show Simultaneous Reduction of PCDDs/PCDFs and Hg Emission from MSW Incinerator Eiichi SHIBUYA, Toru SATO, Katsumi YOSHIKUBO, Takuro NOMURA NKK Corporation, 2-1, Suehiro-cho, Tsurumi-ku, Yokohama, Japan 1. Introduction Toxic substances including Fly ash, HC1, SOx, NOx, Heavy metals and PCDDs/PCDFs are contained in the flue gas discharged from Municipal Solid Waste Incinerators. Recently, for the purpose of environmental protection, establishment of flue technology to strictly control emission of these toxic substances from MSW Incinerators has been strongly called for. - With respect to PCDDs/PCDFs, in particular, countermeasures were shown in the guideline issued by the Ministry-of Health and Welfare in December 1990. However, in some plants, with combustion control alone, it is difficult to reduce its emission to less than 0.5 ng/Nm3 which is the target value. it is indispensable, therefore, to install flue gas processing systems of some type or other. On the other hand, Hg which is typical of heavy metals originates in the waste dry cells, clinical thermometers and fluorescence lamps which are mixed in the Municipal Solid Waste. After volatilized in the incineration process, the Hg is borne in the flue gas normally at about 0.1 - 0.5 mg/Nm3 -Dry gas. Reduction of Hg by wet process has been effective so far, but in consideration of corrosion of the system or from the convenience of operation, development of a method of reducing Hg by drying process that could be well compared to wet process is earnestly desired. In the circumstances, we have tested two approaches to simultaneously reduce PCDDs/PCDFs and Hg contained in the flue gas from MSW Incinerators by a perfectly drying process and verified their performances. An outline of the test result is described below. 2. Test method 2.1 Outline The drying process designed to process flue gas that we tested is described below. 1) Injecting activated carbon powder into flue gas When activated carbon powder is injected into the flue gas at boiler outlet, PCDDs/PCDFs and Hg are adsorbed onto the said carbon powder. These substances are then collected in the succeeding dust collector together with the activated carbon powder to be disposed. 2) Passing flue gas into activated coke moving bed When the flue gas of dust collector outlet is made to pass through the moving bed filled with activated coke, PCDDs/PCDFs and Hg are adsorbed onto the activated cokes. 2.2 Test system and test method A schematic flow diagram of the pilot plant where we performed the tost is shown in Fig.l. The flue gas for the test was taken as follows; A part of the flue gas of the incinerator in full-scale operation was branched from the inlet of Acidic Gas Removing System (NKK LIMAR) and introduced to the test system, which consisted of mainly a spray type gas cooler, a device to blow in slaked lime and activated carbon, a bag filter and an activated cokes moving bed. The flow rates for the flue gas used for the activated carbon powder injection test were respectively 4200-4800 Nm 3/H. After adjusting the temperature of flue gas at the gas cooler outlet to 200 degC by spraying water within ·the gas cooler, activated carbon powder is injected into the flue gas duct at ~he bag filter inlet by a fan together with slaked lime. The activated carbon feed rate 0.03-0.65 g/Nm3 as against the slaked lime feed rate 2-3 g/ Nm3 • In the case of the activated coke moving bed test, instead of injecting activated carbon powder, a part of flue gas of which fly ash was removed when passing the bag filter was branched from the outlet of bag filter and fed into the moving bed from its bottom to make this flue gas counterflow contact with the activated coke in the moving bed. The flow rates for the flue gas used for I |