OCR Text |
Show 3 and toxic substances, such as dioxins in the gas are decomposed. In the pyrolysis furnace, low temperature pyrolysis in a reducing atmosphere is performed. Consequently, recovery of iron and aluminum, which are of high value as resources, is possible. Production of water-granulated, air-cooled, or slow-cooled slag from the slag discharged from the melting furnace is possible depending upon the cooling method. The exhaust gas from the melting furnace is passed through the air heat exchanger and/or the boiler for heat recovery, cooled in the gas cooler, and dedusted with a bag filter. Then, after temperature adjustment in the exhaust gas reheater, it passes through the N0X removal reactor and is discharged from the stack. The toxic heavy metals in the fly ash, collected in the exhaust gas line including the boiler, gas cooler, and bag house, are recovered as heavy metal sulfide by our own heavy metal separation and recovery technique, a hydrometallurgical process. The heavy metal sulfide can be returned to the mine. 2.2 Characteristics of pyrolysis furnace Various types of pyrolysis furnace, such as the rotary kiln type and the direct melting type which uses a shaft furnace, are available. The rotary kiln type is a furnace in which carbonization is carried out by indirect heating using high temperature air and, hence, is also applicable to waste with a relatively low heating value. However, it has some flaws, such as corrosion of the high temperature air heater and limitations in scaling up. The direct melting type is a simple furnace. However, it cannot use the heat of the pyrolytic gas in the melting process and, hence, charging with cokes, oxygen, and so on is necessary. On the other hand, Kobe Steel has employed the fluidized-bed type, which has been used successfully in municipal solid waste incinerators and sewage sludge incinerators and has good operating characteristics. This type of furnace thermally decomposes waste by partial combustion. All the waste charged in the furnace, except the incombustibles to be extracted from the bottom of the bed, flows into the directly connected melting furnace. Since pyrolysis proceeds at a low temperature and the atmosphere inside the furnace is reducing, the metallic materials in the incombustibles can be recovered from the bottom of the bed almost without being oxidized. These metallic materials are of high quality and easily recyclable. From a structural standpoint, this pyrolysis furnace has no moving parts and its scaling is the same as that of the fluidi zed-bed incinerator. |