Coal utilization technologies in the U.S. industrial sector today fall into two main categories—coke plants that produce high-purity carbon for use in steel making, and combustion technologies that use coal to provide heat and power for industrial process operations (Table 6.1). In addition, a small amount of coal is currently used to produce substitute natural gas and high-value products such as chemicals and liquid fuels. These latter applications are projected to grow significantly over the next several decades in some of the scenarios discussed in Chapter 2.
The production of coke from coal is a centuries-old technology in which low-ash “metallurgical” coal is heated in an oven to drive off volatile matter, leaving a high-purity carbon product that is used in blast furnaces to produce iron for steel making. Modern coke plants consist of a battery of long, narrow, brick-lined rectangular ovens into which coal is fed. The volatile gases driven off by heating are collected, cleaned, and used as fuel. The hot coke product is pushed out of the oven into a rail car, quenched with water to cool it, and then shipped for use in steel making.
Most of the coal currently used at industrial facilities is burned in boilers to generate steam, just as is done in power plants. In many cases, electricity also is generated for on-site use, often in combined heat and power (CHP) systems (known also as co-generation) that yield high overall efficiencies, on the order of 80 percent or more. Industrial boilers are generally smaller than modern power plant boilers but use many of the same environmental control technologies found at larger facilities.
The recent rise in world oil prices, as well as domestic natural gas prices, has stimulated renewed interest in the production of gaseous and liquid fuels from coal. Coal liquefaction technology has long been used to produce high-quality transportation fuels, most notably in South Africa, which boasts the largest commercial facility in the world (the Sasol Group). Substitute natural gas (SNG1) also can be produced from coal, and one commercial plant has been operating in the United States since the 1980s (see Box 6.1). In both cases, coal gasification is a key technology. By adjusting the ratio of carbon monoxide and hydrogen in the syngas product, either gaseous or liquid products can be manufactured with the proper choice of catalysts and operating conditions (NRC, 1995, 2000, 2003a). However, the overall thermal efficiency of these processes is relatively low, on the order of 50 percent. Thus, total CO2 emissions for a coal-to-liquids plant, including CO2 from the conversion process and from combustion of the liquid fuel, are roughly twice that of diesel fuel produced from petroleum.