Production of Blast Furnace Coke

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). 

selected coals are blended and pulverized. Some coal tar is added for appropriate bulk density control. The blended coal is charged into the coking chambers by a charging car, which moves on the roof of the coke oven battery in a longitudinal direction, and the chambers are filled from the top in a specific sequence via charging holes. The brick walls of each coking chamber are heated up to about 1100 °C and, in the course of time, the coal charge is heated and transformed by pyrolysis 

After a coking time of about 20 h, a pusher machine, which travels on rails alongside on one side of the battery, removes the door of the respective coking chamber while a so-called coke guide car simultaneously opens the door on the reverse side of the chamber. The hot coke is then pushed out of the oven by the coke guide car into a coke quenching car for conveyance to the quench tower where the hot coke is cooled by wet quenching with water. The cooled coke is then finally transported to a blast furnace. 

Because of the transient nature of coke production, these processes take place within the chamber at different horizontal positions, which feature different temperatures. For example, the center of the chamber (midplane) may still not be completely dry and thus still stays at 100 °C, whereas the coal attached to the hot wall may already be transformed into coke. The plastic layers move from each wall towards the center of the chamber, trapping the liberated gas. Once the plastic layers have met the midplane, all of the coal has been carbonized. 

The coking process is complicated not only because of its transient nature it is hard to simulate. Beside transient heat transfer, the following aspects have to be considered for an accurate description of the process  

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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... 

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. 

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]. 

Pig Iron Basic raw material for steelmaking pig iron is the product of blast furnaces, and it is made by the reaction of iron ore and coke in the presence of limestone (it averages about 5 percent carbon, 2.5 percent phosphorus, and less than 1 percent of other impurities) also known as cast iron. 

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 ... 

Carbon refractories. Carbon blocks used in the construction of blast-furnace hearths are made from dense coke possibly mixed with anthracite. Suitable grain-size fractions (up to 5—15 mm) are mixed with tar to a plastic mass which is shaped at elevated temperature by pressing or ramming. The products are fired without air at about 1400 °C in saggers covered with fine-grained coke. The carbonized tar produces a firm bond between the grains. Granular mixes arc also used in certain applications. 

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]. 

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. 

Carbon Refractories. These refractories, consisting almost entirely of carbon, are made from a mixture of graded coke, or anthracite, pitch and tar the shaped blocks are fired (packed in coke). The fired product has an apparent porosity of 20-25% crushing strength 50-70 MNm-2 R.u.L. (350 kPa), 1700°C thermal expansion (0-1000 C), 0.65%. The principal use is in the lining of blast furnaces, particularly in the hearth and bosh (cf. plumbago). 

Burning coal generates about 40% of the electricity in the world. Coke is produced by pyrolysis of coal - heating in the absence of aii Coke contains 90-95% carbon, the rest being ash. It was once very much used as a fuel for heating houses but has now been replaced by oil, gas and electricity. In the blast furnaces used for the production of raw iron, coke is indispensable. It works both as a means of reduction and as a heat generator. The high content of carbon in the raw iron also comes from the coke. 

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. 

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. 

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. 

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