Blast furnace coke production

Melendi, S., Diez, M., Alvarez, R., and Barriocanal, C. (2011) Plastic wastes, lube oils and carbochemical products as secondary feedstocks for blast-furnace coke production. Fuel Processing Technology, 92 471 78. 

Up to the beginning of the 1960s PA was mainly produced from naphthalene, that is, on the basis of tar from coke making. In the 1970s, the demand for PA increased. Simultaneously, blast furnace coke production decreased due to a reduction of steel production and increased efficiency of the blast furnace process. In 1960, 750 kg coke was needed per tonne of pig iron compared to 500 kg since 1970 (Peters and Reinitzhuber, 1994, see Fig. 6.5.17). This led to o-xylene becoming an economically attractive alternative feedstock, and to a shift from coal to crude oil based PA synthesis. Today, more than 85% of PA production worldwide is based on o-xylene. 

Coking coal is cleaned so that the coke ash content is not over 10%. An upper limit of 1—2 wt % sulfur is recommended for blast furnace coke. A high sulfur content causes steel (qv) to be brittle and difficult to roU. Some coal seams have coking properties suitable for metallurgical coke, but the high sulfur prevents that appHcation. Small amounts of phosphoms also make steel brittle, thus low phosphoms coals are needed for coke production, especially if the iron (qv) ore contains phosphoms. 

Worldwide demand for blast furnace coke has decreased over the past decade. Although, as shown in Figure 1, blast furnace hot metal production (pig iron) increased by about 4% from 1980 to 1990, coke production decreased by about 2% over the same time period (3). This discrepancy of increased hot metal and decreased coke production is accounted for by steady improvement in the amounts of coke required to produce pig iron. Increased technical capabihties, although not universally implemented, have allowed for about a 10% decrease in coke rate, ie, coke consumed per pig iron produced, because of better specification of coke quaUty and improvements in blast furnace instmmentation, understanding, and operation methods (4). As more blast furnaces implement injection of coal into blast furnaces, additional reduction in coke rate is expected. In some countries that have aggressively adopted coal injection techniques, coke rates have been lowered by 25% (4). 

A U.S. Bureau of Mines suivev of 12 blast-furnace coke plants, whose capacity is 30 percent of the total production in the United States, provides an excellent picture of the acceptable chemical and physical properties of metallurgical coke. The ranges of properties are given in Table 27-2. 

Iron goes through a number of stages between ore and final steel product. In the first stage, iron ore is heated with limestone and coke (pure carbon) in a blast furnace. A blast furnace is a very large oven in which the temperature may reach 2,700°F (1,500°C). In the blast furnace, coke removes oxygen from iron ore ... 

The primary use of coke is a fuel reductant and support for other raw materials in iron-making blast furnaces. Coke is also used to synthesize calcium carbide and to manufacture graphite and electrodes, and coke-oven gas is used as a fuel. Coal tar, a by-product of the production of coke from coal, is used in the clinical treatment of skin disorders such as eczema, dermatitis, and psoriasis. 

Coking produces a blast furnace coke feed substantially free of sulfur. However, the gaseous product, coke oven gas, has a sulfur gas content of 900-1, lOOg/m (at 15°C, 1 atm) [31]. This is mainly hydrogen sulfide, which may be removed either by the vacuum carbonate or Stretford processes. The sulfur gas removal efficiency of the Koppers Company s vacuum carbonate process is about 90%, which produces sulfuric acid, whereas the Stretford process can achieve 99% containment to a sulfur product (Chaps. 3 and 9). The choice of desulfurization process depends on the efficiency required and the sulfur product desired. Condensible hydrocarbons such as benzene (and other aromatics) and phenols have always been recovered by condensation, etc. [34]. 

Preparation of blast furnace coke involves the heating of metallurgical coal to 1,000-1,100°C in the absence of air in a battery of refractory brick-lined coke ovens. This is referred to as the by-product coke plant from the association of by-product recovery with coke formation. The coal charge is heated until all of the volatile matter has been vaporized and pyrolysis is complete, a process which takes 16-24 hr. The residual lumps of coke, still hot, are then pushed out of the oven through a quenching shower of water and into a rail car for final shipment. About 700 kg of coke plus a number of volatile products are recovered from each tonne of metallurgical coal heated. More details on the coking process itself are available [40]. 

The FMC process (Figure 17.4) is a multistage process for the manufacture of coke briquettes from high-volatile coals (Coal Age, 1960). In the process, the comminuted coal is oxidized, carbonized at low temperature, and calcined. On cooling, the low-tanperature tar or an extraneous binder is used. Weakly caking coals, which are not suitable for the production of coke by the conventional process, can be converted by means of this process into a suitable blast furnace coke. 

Large-scale carbonization of hard coal is performed at temperatures between 1,000 and 1,200 °C. The production of blast-furnace coke takes 14 to 20 hours. Each ton of coal yields 750 kg of coke, 370 m coke-oven gas, 35 kg of crude tar, 11 kg benzole, 2.4 kg ammonia and 150 kg water. Figure 3.11 shows the quantitative flow chart for a coke plant with a daily coal throughput of 7,0001. The blastfurnace gas is supplied by the blast furnaces which are linked for energy supply (underfiring) to the coke ovens. 

Production of blast furnace coke (coking of coal) 1 Own estimation... 

Coal 3.2 4.8 > 1.5 (estimated for production of chemicals and blast furnace coke)... 

Today, coal production has reached 3.3 billion toe. The consumption of coal has changed considerably in recent decades. In 1950, coal was still by far the dominant fossil energy, for example, in Europe with a share of 83%. Today, the share of coal is only 21%. Industrial processes of coal utilization can be divided into three areas, pyrolysis to blast furnace coke and gaseous by-products, coal combustion for heat and electricity production, and coal gasification to syngas. 

In the following, the two main processes needed for steel production are examined in detail, namely, the production of blast furnace coke (Section 6.5.2) and the production of pig iron in the blast furnace (Section 6.5.3). 

Figure 6.5.6 Influence of temperature on parameters of heat transport during coking of coal for production of blast furnace coke (a) thermal conductivity of the heated brick walls, (b) thermal conductivity of coal/coke, (c) heat capacity of coal/coke, (d) bulk density of coal/coke (dashed lines mean values used for estimations given below data from Hess, 1986).

While pyrolysis of coal plays an important role in the production of blast furnace coke on a large industrial scale (approximately 600 Mt of coal/year), it is of minor importance for biomass today. Around 50 Mio.t charcoal are used especially in South America as blast furnace coke. However, modem iron smelting processes would allow for substitution of coke by secondary reduction media and fuels produced from biomass and organic waste. Nonmetallurgical production of carbon is estimated at 18 Mt/year, approximately 600 000 t/year being used as adsorbent with its diverse applications. The latter is prepared mainly from coke, charcoal, and coconut shell coke. 

In 1990, U.S. coke plants consumed 3.61 x 10 t of coal, or 4.4% of the total U.S. consumption of 8.12 x ICf t (6). Worldwide, roughly 400 coke oven batteries were in operation in 1988, consuming about 4.5 x 10 t of coal and producing 3.5 x 10 t metallurgical coke. Coke production is in a period of decline because of reduced demand for steel and increa sing use of technology for direct injection of coal into blast furnaces (7). The decline in coke production and trend away from recovery of coproducts is reflected in a 70—80% decline in volume of coal-tar chemicals since the 1970s. 

DRI, in peUet/lump or HBI form, can be added to the blast furnace burden to increase furnace productivity and reduce coke requirements. It can be used for short-term increases in blast furnace output when a faciUty is short of hot metal during times of high steel demand, or when one of several blast furnaces is down for a reline. It also can be justified if the increased output is sufficient to allow operation of fewer blast furnaces long-term. 

The carbonization by-products are usually refined, within the coke plant, into commodity chemicals such as elemental sulfur (qv), ammonium sulfate, benzene, toluene, xylene, and naphthalene (qv) (see also Ammonium compounds BTX processing). Subsequent processing of these chemicals produces a host of other chemicals and materials. The COG is a valuable heating fuel used mainly within steel (qv) plants for such purposes as firing blast furnace stoves, soaking furnaces for semifinished steel, annealing furnaces, and lime kilns as well as heating the coke ovens themselves. 

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