Low-temperature carbonization

Low-temperature carbonization was mainly developed as a process to supply town gas for lighting purposes as well as to provide a smokeless (devolatilized) solid fuel for domestic consumption (Wilson and Clendenin, 1963 Seglin and Bresler, 1981). Low-temperature carbonization, though 

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Low Temperature Carbonization. Low temperature carbonization, when the process does not exceed 700°C, was mainly developed as a process to supply town gas for lighting purposes as well as to provide a smokeless (devolatilized) soHd fuel for domestic consumption (30). However, the process by-products (tars) were also found to be valuable insofar as they served as feedstocks (qv) for an emerging chemical industry and were also converted to gasolines, heating oils, and lubricants (see Gasoline and OTHER motor fuels Lubrication and lubricants) (31). 

The TOSCOAL Process. The Oil Shale Corp. (TOSCO) piloted the low temperature carbonization of Wyoming subbituminous coals over a two-year period in its 23 t/d pilot plant at Rocky Falls, Colorado (149). The principal objective was the upgrading of the heating value in order to reduce transportation costs on a heating value basis. Hence, the soHd char product from the process represented 50 wt % of the starting coal but had 80% of its heating value. 

Pyrolysis. Heating in the absence of oxygen releases moisture at low temperatures, carbon dioxide at temperatures >200° C, and a variety of gaseous products at very high temperatures. Acid washing of the raw coal is used to remove extractable cations, followed by treatment with selected cations. Yields of CO2, CO, CH, H2, and H2O depend on the amounts of inorganic species in the coal (42). 

Low Temperature Carbonization. The Lurgi Sptlgas process was developed to carbonize brown coal at relatively low temperatures to produce tars and oils (Fig. 5). A shaft furnace internally heated by process-derived fuel gas (Spblgas) is used. The product can range from a friable coke breeze to hard lump coal depending on the quality of the briquettes used in the feed. The briquettes, made in normal extmsion presses, break down into smaller sizes during carbonization. 

Liquid Fuels. Liquid fuels can be obtained as by-products of low temperature carbonization by pyrolysis, solvent refining, or extraction and gasification followed by catalytic conversion of either the coal or the products from the coal. A continuing iaterest ia Hquid fuels has produced activity ia each of these areas (44—46). However, because cmde oil prices have historically remained below the price at which synthetic fuels can be produced, commercialization awaits an economic reversal. 

The passing of the Clean Air Act in the United Kingdom in 1956 resulted in a revival of interest in low temperature carbonization to produce a very reactive coke suitable for open fires. In the CoaUte process, the coal is heated at 600—650°C for 4 h in small retorts each hoi ding 6—7 metric tons (5). The Rexco process employed large internally heated retorts in which charges of 34 metric tons were heated to 700—750°C for 6 h, but is no longer in operation in the United Kingdom (6). 

Low Temperature Carbonization. Lower temperature carbonization of lump coal at ca 700°C, primarily used for production of soHd smokeless fuel, gives a quantitatively and quaUtatively different yield of sofld, Hquid, and gaseous products than does the high temperature processes. 

Low- and Medium-Temperature Coke (773 to 1023 K or 932 to 1382°F.) Cokes of this type are no longer produced in the United States to a significant extent. However, there is some interest in low-temperature carbonization as a source of both hydrocarbon liquids and gases to supplement petroleum and natural-gas resources. 

The Fischer assay is an arbitrary but precise analytical tool for determining the yield of produces from low-temperature carbonization. A known weight of coal is heated at a controlled rate in the absence of air to 773 K (932°F), and the produces are collecled and weighed. Table 27-3 gives the approximate yields of products for various ranks of coal. 

Coal Char This type of char is the nonagglomerated, nonfusible residue from the thermal treatment of coal. Coal chars are obtained as a residue or a coproduct from low-temperature carbonization processes and from processes being developed to convert coal to hquid and gaseous fuels and to chemicals. Such chars have a substantial heating value. The net amount of char from a conversion process varies widely in some instances, it may represent between about 30... 

Schwel-. relating to low-temperature carbonization smoldering, -anlage,/. low-temperature carbonizing plant. 

Schwelgas, n. gas from low-temperature carbonization producer gas partially burned gas. 

Schwel-ofen, m. low-temperature carbonizing furnace, -raum, m. carbonizing space or chamber, -retorte, /. retort for low-temperature distillation, -teer, m. tar from low-temperature carbonization, -teerol, n. carbonization tar oil, esp. from lignite. 

Schwelung, /. low-temperature carbonization slow burning, smoldering. 

Schwel-vorgang, m. low-temperature process of carbonization, -wasser, n. an aqueous liquid from the low-temperature carbonizing process, containing ammonia, phenols, etc. foiil water, -werk, n. low-temperature carboniza tion plant. 

Tieftemperatur-teer, m. low-temperat ire tar. -vergasimg, -verkokung, /. low-temperature carbonization. 

Table 1.28 Some salient features of low-temperature carbonization (LTQand high-temperature carbonization (HTC).

The production of coke involves the heating of coal in the absence of air, called the carbonization or destructive distillation of coal. Carbonization, besides its main purpose of production of coke, also results in a coproduct called coke oven gas from which various liquid products such as tar, benzol, naphthalene, phenol, and anthracene are separated. There are two main types of carbonization based on the temperature to which the coal is heated in the absence of air. One type is low-temperature carbonization (LTC) the other is high-temperature carbonisation (HTC). Some features of LTC and HTC are listed in Table 1.28. The LTC Process is mainly carried out to manufacture domestic smokeless fuel. This presentation, however, concentrates on the HTC process by which metallurgical coke is produced. 

Gangwal, S. K., McMichael, W. J., Howe, G. B., Spivey, J. J., and Siiveston, P. L., Low-temperature carbon-based process for flue-gas cleanup. IChemE Symp. Series, No. 

Zheng X-C, Wu S-Fl, Wang S-P, Wang S-R, Zheng S-M, Huang W-P (2005) The preparation and catalytic behavior of copper-cerium oxide catalysts for low-temperature carbon monoxide oxidation. Appl Catal A 283(l-2) 217-223... 

Although it is stable at low temperatures, carbon suboxide will readily bum, and it polymerizes when heated. Pentacarbon dioxide, C502, has been prepared, but like C302 it has no important uses. 

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