Low Temperature Carbonization

Low-temperature carbonization was originally developed to provide town gas for residential and street lighting and to manufacture a smokeless fuel for domestic and industrial heating. The by-product tars were economically important and were often essential feedstocks for the chemical industry or were refined to gasoline, heating oils, and lubricants [19]. Low-temperature carbonization evolved and was used extensively in industrialized European countries but was eventually abandoned after 1945 as oil and natural gas became widely available. These early processes used fixed- and moving-bed technology, operated in batch or continuous mode, and consisted of vertical or horizontal retorts with direct or indirect heating [1]. In the 1970s, interest in low-temperature carbonization was resurrected after the oil crisis. Most of the techniques utilized now, however, are different and mainly consist of using fluidized-bed or entrained-flow pyrolyzers. Some of the low-temperature carbonization technologies are the FMC Coke, COED, U.S. Steel Coke, Occidental Pyrolysis, Lurgi-Ruhrgas, Coalite,

Phurnacite, and Home Fire processes [2,35]. In addition, technologies such as the liquids-from-coal (LFC) process and the advanced coal conversion process (ACCP), are examples of commercial/near-commercial technologies to upgrade low-rank coals [36-38].

The preferred coals for low-temperature carbonization are typically lignites, subbituminous coal, or highly volatile bituminous coal, which, when pyrolyzed at temperatures between 1100 and 1300° F, yield a porous char with reactivities that are typically not much lower than those of their parent coals [19]. These reactive chars are easily ignited and are used as smokeless fuels or as feedstocks to gasification processes, are blended with coals to make coke-oven feed, or are used as a power plant fuel [19,22,37,38]. The tars that are produced during low-temperature carbonization are much different than those from high-temperature carbonization. High-temperature carbonization tends to produce mainly aromatic compounds, whereas those produced during low-temperature carbonization are predominately aliphatic compounds, hence the different end-use applications of the tar by-products. Gas yield and composition are also different during low-temperature carbonization, with gas yields being ~25% of those produced during high-temperature carbonization, but the gas contains more methane and less hydrogen, giving it a higher heating value [22].

Smokeless Fuel Commercial Processes

The primary application of low-temperature carbonization is to make smokeless fuels for use in homes and small industrial boilers in areas that have high population density and rely on coal as a fuel, particularly coal that has a high volatile-matter content. This is especially true in Great Britain, which has regulated smoke-controlled areas, and several commercial plants are producing smokeless fuels for open fires, room heaters, multifuel stoves, cookers, and independent boilers [39]. These fuels are marketed under names such as Coalite, Sunbrite, Phurnacite, Taybrite, and Home Fire.

Examples of two processes used in Great Britain are the Coalite and the Home Fire processes. The Coalite process uses moving-bed, vertical-retort technology and is a continuous process using indirect heating [1]. The Coalite works are located at Bolsover in Derbyshire and started operation in 1937. In the Coalite process, deep-mined British bituminous coal is carbonized in several batteries, each battery consisting of 40 metal retorts, assembled in two rows of 20, at a temperature of ~1200°F [40]. The coal charge to each retort, which is ~660 lb, remains in the retort for 4 hours, after which time a ram pushes the Coalite into a cooler. Typically, 1 metric ton of coal blend will produce 1100 to 1870 lb of smokeless coal or semi-coke, 5300 to 6350 standard cubic feet (scf) of gas, 18 to 20 gallons of coal oil, 3 to 5 gallons of light oil, and 45 to 48 gallons of aqueous liquor. The gas is recycled for on-site use to heat the batteries, generate steam, and general heating. Oils are distilled to yield pitch (for use as a boiler fuel), heavy oil (to produce creosotes and disinfectants), and middle and light oils (to produce phenols, cresols, and xylenols). The liquor contains dissolved chemicals—principally, ammonia, monohydric and dihydric phenols—and through extraction and fractionation a wide range of chemicals is produced, including catechol, resorcinol, and methyl resorcinol. Coalite is the leading manufacturer of smokeless fuel in the United Kingdom. The Home Fire process uses a blend of bituminous coals and fluidized-bed technology. At the Home Fire plant, located near Coventry, the coal is crushed to 1/4-in. particles, dried, and devolatilized for 20 minutes at 800°F in a fluidized-bed reactor [22,35]. The hot char is extruded into hexagonal briquettes, cooled, and quenched.

Low-Rank Coal Upgrading

The U.S. Department of Energy (DOE) cofunded two programs through their Clean Coal Technology (CCT) program (which is discussed in detail in Chapter 7, Future Power Generation) in which low-rank coals were upgraded using low-temperature pyrolysis/thermal upgrading technologies as part of the process. Western U.S. low-rank coals, primarily subbituminous coals and lignite, are generally low in sulfur, making them (specifically, the subbituminous coals) attractive as power plant fuels in place of high-sulfur eastern U.S. coals; however, disadvantages of the low-rank coals are high moisture content and low heating value. Consequently, two new processes—ENCOAL's liquids-from-coal process and Rosebud SynCoal Partnership's advanced coal conversion process—have been developed and successfully demonstrated, and are ready for commercialization.

The LFC technology uses a mild pyrolysis or mild gasification process to produce a low-sulfur, high-heating value fuel and a coal-derived liquid [3638,41]. In the process, coal that has undergone some coal drying is fed into a pyrolyzer that is operated near 1000° F to remove any remaining moisture and release volatile gases. The solid fuel—process-derived fuel (PDF)—is used as a boiler fuel. The pyrolysis gas stream is sent through a cyclone to remove entrained particles and then cooled to condense the desired hydrocarbons— coal-derived liquids (CDLs)—and stop any secondary reactions. The CDLs were utilized at seven industrial fuel users and one steel mill blast furnace during the 5-year demonstration; however, studies were performed on upgrading the CDLs to produce cresylic acids, petroleum refinery feedstock for producing transportation fuels, oxygenated liquids, and pitch. The process was demonstrated for approximately 5 years in a plant feeding 1000 short tons coal per day located near the Buckskin Mine (Triton Coal Company) in Gillette, Wyoming, and produced over 83,000 short tons of solid fuel product and 4.9 million gallons of liquid product. The process is considered commercial and is actively being marketed in the United States (in the Powder River Basin, Alaska's Beluga field, and the lignite fields of North Dakota and Texas) and abroad (in China, Indonesia, and Russia) [37,41,42]; five detailed commercial feasibility studies have been completed [42].

A large-scale commercial plant has been designed, with participation from Mitsubishi Heavy Industries, to utilize 15,000 metric tons coal feed. The plant, located near Gillette, Wyoming, has received the Industrial Siting Permit and an Air Quality Construction Permit, but is on hold due to lack of funding [42].

The ACCP process is an advanced thermal conversion process coupled with physical cleaning techniques to upgrade high-moisture, low-rank coals to produce a high-quality, low-sulfur fuel. In this process, coal is fed to a vibratory fluidized-bed reactor to remove surface moisture. It then flows to a second vibratory reactor, where coal is heated to 600°F to remove chemically bound water, carboxyl groups, volatile sulfur compounds, and a small amount of tar. In this process, the volatiles are not collected but are used in a process heater. The technology was demonstrated from 1993 to 2001 (longer than the planned 5-year period) in a 45-short ton/hour facility located adjacent to a unit train load-out facility at Western Energy Company's Rosebud coal mine near Colstrip, Montana, in which 2.8 million short tons of raw coal were processed. Nearly 1.9 million short tons of SynCoal were produced and shipped to various customers, including cement and lime kilns and utility boilers, and the product was used as a betonite additive in the foundry industry. Three different feedstocks were tested at the ACCP facility—two North Dakota lignites and a subbituminous coal—and the products were fired in a utility boiler located in North Dakota and three utility boilers located in Montana [43]. The technology is being marketed and promoted worldwide, and a project was actively pursued for Minnkota's Milton R. Young Power Station, which had test fired SynCoal produced from one of the North Dakota lignites; however, the project was suspended due to a lack of equity investors.

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