Coke production

There are few coking units in the world, and the majority of them is found in the United States, such that coke production is marginal. Different coking processes have been described, but only two have survived (see Chapter 10) ... 

The major portion of sait is found in residues as these streams serve as the bases for fuels, or as feeds for asphalt and petroleum coke production, the presence of salt in these products causes fouling of burners, the alteration of asphalt emulsions, and the deterioration of coke quality. Furthermore, calcium and magnesium chlorides begin to hydrolyze at 120°C. This hydrolysis occurs rapidly as the temperature increases (Figure 8.1) according to the reaction i. ... 

Most coal-tar chemicals are recovered from coproduct coke ovens. Since the primary product of the ovens is metallurgical coke, production of coal chemicals from this source is highly dependent on the level of activity in the steel industry. In past years most large coke producers operated thein own coproduct recovery processes. Because of the decline in the domestic steel industry, the recent trend is for independent refiners to coUect cmde coal tars and light oils from several producers and then separate the marketable products. 

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. 

The original process of heating coal in rounded heaps, the hearth process, remained the principal method of coke production for over a century, although an improved oven in the form of a beehive was developed in the Durham-Newcastie area of England in about 1759 (2,26,28). These processes lacked the capabiHty to collect the volatile products, both Hquids and gases. It was not until the mid-nineteenth century, with the introduction of indirectiy heated slot ovens, that it became possible to collect the Hquid and gaseous products for further use. 

When energy alternatives are available, a compromise between cost and quaHty is often realized. Blending of coals can be used to achieve more desirable quaHties. For example, lignite from the former Yugoslavia has been blended with, and even substituted for, the highly caking Rasa coal used for coke production in the iron (qv) and steel (qv) industries. 

The hydrocarbon feed rate to the reactor also affects the burning kinetics in the regenerator. Increasing the reactor feed rate increases the coke production rate, which in turn requires that the air rate to the regenerator increase. Because the regenerator bed level is generally held constant, the air residence time in the dense phase decreases. This decrease increases the O2 content in the dilute phase and increases afterbum (Fig. 5). 

Coke Production. Coking coals are mainly selected on the basis of the quaUty and amount of coke that they produce, although gas yield is also considered. About 65—70% of the coal charged is produced as coke. The gas quaUty depends on the coal rank and is a maximum, measured in energy in gas per mass of coal, for coals of about 89 wt % carbon on a dry, mineral matter-free basis, or 30% volatile matter. 

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). 

Some hydrogen cyanide is formed whenever hydrocarbons (qv) are burned in an environment that is deficient in air. Small concentrations are also found in the stratosphere and atmosphere. It is not clear whether most of this hydrogen cyanide comes from biological sources or from high temperature, low oxygen processes such as coke production, but no accumulation has been shown (3). 

Complexing extraction of pyridine bases from coal coking products with organic solvents 97KGS3. 

A number of indices relate metal activity to hydrogen and coke production. (These indices predate the use of metal passivation in the FCC process but are still reliable). The most commonly used index is 4 X Nickel + Vanadium. This indicates that nickel is four times as actiw as vanadium in producing hydrogen. Other indices [9] used are ... 

Heat to raise the coke products from the regenerator dense temperature to flue gas temperature... 

Nickel in the feed is deposited on the surface of the catalyst, promoting undesirable dehydrogenation and condensation reactions. These nonselective reactions increase gas and coke production at the expense of gasoline and other valuable liquid products. The deleterious effects of nickel poisoning can be reduced by the use of antimony passivation. 

Coke product (< 0.1 wt% of both lead and chlorine), which may be used as fuel in a cement kiln... 

The process needs input of lime and water next to the PVC waste. No energy input is needed since the organic condensate provides for the energy needed in the process. Energy needed for pretreatment can be up to 25-35 kWh/tonne. Downstream separation of the coke products needs another 30-40 kWh/toime. The process does not emit dioxins, metals or plasticisers. Due to internal recycling there are no aqueous waste streams. The reaction of lime with HCl forms some CO2. The coke product provides a calorific value. 

Figure 1.29 Coke production processes. (A) Beehive oven (B) by-product coke oven.

The presence of polycyclic aromatic hydrocarbons in the environment is of obvious concern and, apart from specific occupational environments, human exposure to these compounds derives from combustion products released into the atmosphere. Estimates of the total annual benzo[aJpyrene emissions in the United States range from 900 tons (19) to about 1300 tons (20). These totals are derived from heat and power generation (37-38%), open-refuse burning (42-46%), coke production (15-19%) and motor vehicle emissions (1-1.5%) (19,20). Since the vast majority of these emissions are from stationary sources, local levels of air pollution obviously vary. Benzo[aJpyrene levels of less than 1 pg/1,000 m correspond to clean air (20). At this level, it can be estimated that the average person would inhale about 0.02 pg of benzo[aJpyrene per day, and this could increase to 1.5 pg/day in polluted air (21). 

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