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allowable temperature for the COG in the heat exchanger is about 840°F [450°C]; at lower temperatures, tar condenses and leads to soot formation on the heat exchanger surface.64 Cooling to 840°F [450°C] enables only about onethird of the sensible heat to be recovered. It is unlikely that integrated steel mills in the United States would pursue new technologies for heat recovery from crude coke oven gas. Not only will the industry face cost barriers with heat recovery from dirty exhaust streams, but the byproduct coke making process may become irrelevant in future years. It is likely that the industry will move away from the byproduct process to the nonrecovery process due to environmental considerations.65 In the non recovery process, the COG gas will be burned within the process, and a waste heat boiler used to recover the sensible heat in the offgases. Another source of sensible heat loss in coke ovens is the waste gases from the combustion of recycled (clean) COG. The recycled COG is used as a fuel in the heating flue, which is adjacent to the oven chamber. Combustion of the COG generates hot exhaust gases which leave the oven flue and pass through a regenerator to transfer heat to incoming combustion air and/or fuel.66, 67 Waste gases leave the regenerator at temperatures averaging around 400°F [200°C].68 In some cases, mainly outside the United States, the heat content of the waste gases are further recovered by use of a heat pipe69 or for preheating coal charge and reducing its moisture content. In this case, the temperature of the exhaust gases drops about 110°F [60°C].70 The waste heat loss from coke oven waste gas in the United States is estimated at about 11 TBtu/yr (Table 13). 4.3.1.2 Blast Furnace The major unit in integrated steel mills is the blast furnace, which converts iron ore (iron oxide, FeO) into pig iron (Fe). Raw materials are charged from the top, including ironcontaining materials (lump iron ore, sinter, or pellets), additives (flux), and coke, while hot air and supplemental fuels are injected into the bottom of the furnace. The burden moves down through the blast furnace and meets a rising current of hot gases. The hot air entering the furnace is provided by several auxiliary hot blast stoves (also known as furnace cowpers). In the blast stove, fuels such as blast furnace gas (BFG) and COG are combusted. The heat from the combustion exhausts is transferred to a checkerwork regenerator. When the regenerator reaches an appropriate temperature, the flow of air is reversed and cold air is forced through the regenerator, which transfers heat to the cold air. The heated air is then injected into the furnace. The system operates according to the same principles as a regenerator used for heat recovery; however in this case, the regenerator is not a “waste heat” recovery device, but rather the mechanism for transferring heat from the stove to the hot blast. Sources of offgas waste heat in blast furnaces include both the exhaust gases from the hot blast stove and the BFG leaving the blast furnace. Sensible heat loss from BFG in the United States is estimated at about 5 TBtu/yr. BFG consists of approximately 2028% CO, 1–5% H2, inert compounds (5055% N2, 1725% CO2), as well as dust, sulfur, cyanide compounds, and other contaminants.71 Older blast furnaces had high exhaust temperatures around 900°F [400°C].72 New furnaces have been designed for more efficient heat transfer; consequently, hot gases are in the lowtemperature range.73 Several plants recover blast furnace gas for use as a fuel in blast air heating, hot mill reheating furnaces, coke oven heating, power production, and steam generation. Since its heat content is only 80 to 90 Btu/scf,74 it is often mixed with other fuels such as natural gas or COG. As with COG, BFG must be cleaned before it can be used as a fuel, and the sensible heat contained in the gas is rarely recovered. In some cases, blast furnaces operate at a sufficiently high pressure (2.5 atm or higher) to economically use a top pressure recovery turbine (TRT) to recover the “pressure energy” of the BFG. The gas must be cleaned before entering the TRT, which is generally accomplished via wet cleaning, with the result that sensible heat of the offgas is lost. An alternative to wetcleaning technology is drycleaning, in which the temperature of the gas entering the TRT can be raised to about (250°F, [120°C]).75 Drytype TRT technology is already commercial; however, being significantly more 39 PDF Image | Waste Heat to Energy Tech Opportunities in US Industry
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