Coking furnaces

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. 

Gross-koks, m. large-sized coke, (blast) furnace coke, -kraftwerk, n. large power station, gross-luckig, a. coarsely porous coarse-grained. 

Conversion of carbon to carbon monoxide. In the lower part of the furnace, coke burns to form carbon dioxide, C02. As the C02 rises through the solid mixture, it reacts further with the coke to form carbon monoxide, CO. The overall reaction is... 

T3. Tigerschoid, M., and Ilmoni, P. A., Proc. AIME Blast Furnace Coke Oven Raw Mater. 9, 18 (1950). 

Foundry Coke This coke must meet specifications not required of blast furnace coke. The volatile matter should not exceed 1.0 percent, the sulfur should not exceed 0.7 percent, the ash should not exceed 8.0 percent, and the size should exceed 100 mm (4 in). 

Shapiro, N., Gray, R. J., Eusner, G. R., Proc. Blast Furnace, Coke Oven, Raw Mater. Comm. 20 (1961). 

G.R. Hornke and R.E. Powers, Ore handling and beneficiation at Great Lakes Steel, Paper presented at AIME Blast Furnace Coke Oven, and Raw Materials Conference, St. Louis, Missouri, April 6,1959. 

G. R. Homke and R. E. Powers, paper presented at The yUME Blast Furnace Coke Oven and Baw Materials Conference, St. Louis, Mo., Apt. 6, 1959. 

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]. 

During the operation of a blast furnace, coke reacts with the oxygen in air to produce carbon monoxide, which, in turn, serves as the reducing agent for the iron ore. Assuming the formula of the iron ore to be Fc203, calculate the mass of air needed for each ton of iron produced. Assume air to be 21 % 2 by mass, and assume that the process is 93% efficient. 

Visbreaking is a relatively mild thermal (noncatalytic) cracking process that is used to reduce the viscosity of residua. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing overhead. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Coke formation can occur and the coke accumulates on visbreaker walls periodic decoking (cleaning) is necessary. 

Research and Bench-Scale Feasibility Studies. The reaction between carbon and sulfur dioxide at elevated temperatures is well known and has been used for numerous processes. For example, sulfur was produced at Trail, British Columbia from 1935 to 1943 by blowing sulfur dioxide and oxygen into the bottom of a coke-fired reduction furnace. Coke was charged at the top and ash was removed on a rotary grate at the bottom of the furnace. The hot zone of the furnace was kept at 1300°C to maintain rapid reaction rates and smooth operation. Sufficient sulfur dioxide was added to the gas to react with the carbon monoxide and carbon oxysulfide contained in the reduction furnace off-gas. Coal was con-... 

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