Coking Processes

Early processes for the production of coke were similar to those employed for the production of wood charcoal. Bituminous coal was built up into piles and ignited in such a way that only the outside layers actually burned while the central portion was carbonized [2]. Piles, also called kilns, appeared for the first time in England in 1657 and spread from there to other European bituminous coal producing regions. Around 1850, half-open brick kilns (i.e., the Schaumburg kiln) were constructed from which circular mounds of coal emitted tar-containing volatilized gases directly into the atmosphere [2]. The next development was the closed beehive oven, which in its original form discharged the distillation and flue gases through a chimney at a greater height. Beehive ovens were used in England until the end of the 1800s; today, some of these ovens are still in operation in South Africa, South America, Australia, and the United States. The beehive oven is a simple domed brick structure into which coal can be charged through an opening at the top and then leveled through a side door to form on a bed ~2 feet thick [19]. Heat is supplied by burning the volatile matter released from the coal, and carbonization progresses from the top down through the charge. Approximately 5 to 6 short tons of coal can be charged, and a period of 48 to 76 hours is required for carbonization. Some beehive ovens are still in operation because of system improvements and the addition of waste heat boilers to recover heat from the combustion products. Similarly, the heat required for coking in the pile and the Schaumburg kiln is produced by partial combustion of the coal, which results in a substantial loss of material by combustion, with a coke yield in these ovens (including the beehive oven) being at most 55% of the coal. Flame ovens, in which the coal was coked in chambers heated from the outside, were developed in 1850 in Belgium and the Saar District in Germany [2]. The high-heating-value, volatilized gases were burned in flues in the walls of the ovens to produce coke yields of ~75%, with coking times of 48 hours.

The first coke ovens that produced satisfactory blast furnace or foundry coke as the main product, and tar, ammonia, and later benzene as byproducts, were built around 1856 and were known as by-product recovery ovens [2]. Modifications to the design has continued but the basic design of these ovens, essentially the modern coke oven, was completed by the 1940s [1]. The horizontal slot-type coke (by-product recovery) oven, in which higher temperatures can be attained and better control over coke quality can be exercised, has superceded other designs and is used for coking bituminous coal [19]. Modern slot-type coke ovens are comprised of chambers 50 to 55 feet long, 20 to 22 feet high, and ~18 inches wide. A number of these chambers (from 20 to 100) alternating with similar cells that accommodate heating flues serve as a battery. Coal, crushed to 80% minus 1/8 in. with a top size of 1 in., is loaded along the top of the ovens using a charging car on rails and is leveled by a retractable bar. Coking takes place in completely sealed ovens, and when carbonization is completed (after 15-20 hours) the oven doors are opened and a ram on one side pushes the red-hot coke into a quenching car or onto a quenching platform. Coke yield is about 75%. By-product gas and tar vapors are removed from the oven to collector mains for further processing or for use in the battery.

A block flow diagram of the recovery of by-products from a coke oven is shown in Figure 5-21 [22]. From a ton of coal, a modern by-product coke oven yields about 1500 lb of coke, 11,000 ft3 of gas, 8 to 10 gallons of light oil, and 25 lb of chemicals, mostly ammonium compounds. The by-product gas and tar vapors leaving the coke oven undergo a separation process to remove the tars from the gas. The gas then is treated to recover ammonia, as ammonium sulfate or phosphate, while the tars are fractionated by distillation into three oil cuts, which are designated as light, middle (or tar acid), or heavy oil. The gas, mainly a mixture of hydrogen and methane, has about one-half the heating value of natural gas and is used on-site as fuel in the flue chambers in the coke ovens or in the furnaces used for heat-treating finished steel [22].

The light-oil cut (boiling point, <430°F) from the distillation of the coal tar consists primarily of benzene (45-72%), toluene (11-19%), xylenes (3-8%), styrene (1-1.5%), and indene (1-1.5%); it is either processed into gasoline and aviation fuels or fractionated to provide solvents and feedstocks for chemical industries [19]. In either case, sulfur compounds, nitrogen bases, and undesirable unsaturates are removed. Middle oils are usually cut to

FIGURE 5-21. Simplified block diagram illustrating the by-products recovered and processing performed to produce useful chemicals. (Adapted from Schobert [22].)

boil between 430 and 710°F, and, after sequential extraction of tar acids, tar bases, and naphthalene, they are processed to meet specifications for diesel fuels, kerosene, or creosote. The tar acids are mostly comprised of phenols, while the compounds produced from the tar bases include pyridine, picolines, lutidines, anilines, quinoline, isoquinoline, and methylquinolines [19]. The temperature at which distillation of the heavy oils is performed depends on what type of pitch residue is desired but usually is between 840 and 1040°F [19]. The distillate is a rich source of hydrocarbons, mainly anthracene, phenanthrene, carbazole, acenaphthene, fluorine, and chrysene. The remaining heavy oils are marketed as fuel oils or blended with pitches to meet specifications for various grades of road tar. The residual coal tar pitches are complex mixtures of over 5000 compounds. They have economic importance because of their resistance to water and weathering and are used as briquetting binders and as binders in the preparation of carbon electrodes and other carbon artifacts.

0 0

Post a comment