Coal

Types of Coal

There are four main classifications of coal. Each type of coal is formed as a result of the exposure to assorted combinations of temperature and pressure during formation. For instance, coal that was exposed to low temperatures and pressures is usually moister, softer, and has lower energy content. The four coal classifications include lignite, subbituminous, bituminous, and anthracite.

The first type of coal, lignite, is soft with a high moisture content and low thermal value as compared with other types of coal. Lignite is usually light brown to brown-black in color and resembles deteriorating and compressed wood. In the United States, most lignite deposits are found in the western states; specifically, North Dakota is the state that produces the largest amount of lignite. This type of coal is relatively inexpensive and, thus, highly desirable, from an economic standpoint, to fuel electric power plants.

The next classification of coal is subbituminous. This type of coal is the intermediate between lignite and bituminous coal. It has a moisture content of between 20 and 30 percent. This is higher than bituminous coal and anthracite; however it is lower than [Page 259]lignite. Subbituminous is desirable from a pollution standpoint because it has a low relative sulfur and carbon content, but the thermal value of subbituminous coal is low. However, many power plants prefer to burn subbituminous coal, as it provides cleaner energy generation. Subbituminous coal is found primarily in Alaska, Montana, and Wyoming and is recognized by its unpolished black color.

Pollution Controls

Bituminous coal is the third type of coal. Though it is harder than lignite or subbituminous coal, bituminous coal is often referred to as soft coal. This is due to the high sulfur content. Because bituminous coal has such a high amount of sulfur present, electricity generators that choose to burn this type of coal must equip their plants with sophisticated pollution-control systems. These mechanisms are called scrubbers because they scrub the pollution from the air by binding reactive toxins with inert, calcium-based substances located in the air stacks of power plants. The best available technology allows scrubbers to remove 98 percent of all sulfur emissions and 99 percent of all particulate matter from smoke stacks. Bituminous coal is dark black in color and may have dull-black bands striated throughout. This type of coal is typically found in Appalachian Mountain regions, the Great Lakes, the Mississippi Valley, and north and central Texas.

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