Coal

I

Introduction

Coal, a combustible organic rock composed primarily of carbon, hydrogen, and oxygen. Coal is burned to produce energy and is used to manufacture steel. It is also an important source of chemicals used to make medicine, fertilizers, pesticides, and other products. Coal comes from ancient plants buried over millions of years in Earth’s crust, its outermost layer. Coal, petroleum, natural gas, and oil shale are all known as fossil fuels because they come from the remains of ancient life buried deep in the crust.

Coal is rich in hydrocarbons (compounds made up of the elements hydrogen and carbon). All life forms contain hydrocarbons, and in general, material that contains hydrocarbons is called organic material. Coal originally formed from ancient plants that died, decomposed, and were buried under layers of sediment during the Carboniferous Period, about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden exerted increasing heat and weight on the organic matter. Over millions of years, these physical conditions caused coal to form from the carbon, hydrogen, oxygen, nitrogen, sulfur, and inorganic mineral compounds in the plant matter. The coal formed in layers known as seams.

Plant matter changes into coal in stages. In each successive stage, higher pressure and heat from the accumulating overburden increase the carbon content of the plant matter and drive out more of its moisture content. Scientists classify coal according to its fixed carbon content, or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.

II

Modern Uses of Coal

Eighty-six percent of the coal used in the United States is burned by electric power plants to produce electricity. When burned, coal generates energy in the form of heat. In a power plant that uses coal as fuel, this heat converts water into steam, which is pressurized to spin the shaft of a turbine. This spinning shaft drives a generator that converts the mechanical energy of the rotation into electric power (see Electric Motors and Generators; Steam Engine).

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Coal is also used in the steel industry. The steel industry uses coal by first heating it and converting it into coke, a hard substance consisting of nearly pure carbon. The coke is combined with iron ore and limestone. Then the mixture is heated to produce iron (see Iron and Steel Industry). Other industries use different coal gases given off during the coke-forming process to make fertilizers, solvents, medicine, pesticides, and other products.

Fuel companies convert coal into easily transportable gas or liquid fuels (see Synthetic Fuels). Coal-based vapor fuels are produced through the process of gasification. Gasification may be accomplished either at the site of the coalmine or in processing plants. In processing plants, the coal is heated in the presence of steam and oxygen to produce synthesis gas, a mixture of carbon monoxide, hydrogen, and methane used directly as fuel or refined into cleaner-burning gas.

On-site gasification is accomplished by controlled, incomplete burning of an underground coal bed while adding air and steam. To do this, workers ignite the coal bed, pump air and steam underground into the burning coal, and then pump the resulting gases from the ground. Once the gases are withdrawn, they may be burned to produce heat or generate electricity. Or they may be used in synthetic gases to produce chemicals or to help create liquid fuels.

Liquefaction processes convert coal into a liquid fuel that has a composition similar to that of crude petroleum. Coal can be liquefied either by direct or indirect processes. However, because coal is a hydrogen-deficient hydrocarbon, any process used to convert coal to liquid or other alternative fuels must add hydrogen. Four general methods are used for liquefaction: (1) pyrolysis and hydrocarbonization, in which coal is heated in the absence of air or in a stream of hydrogen; (2) solvent extraction, in which coal hydrocarbons are selectively dissolved and hydrogen is added to produce the desired liquids; (3) catalytic liquefaction, in which hydrogenation takes place in the presence of a catalyst; and (4) indirect liquefaction, in which carbon monoxide and hydrogen are combined in the presence of a catalyst.

III

Coal Formation

Coal is a sedimentary rock formed from plants that flourished millions of years ago when tropical swamps covered large areas of the world. Lush vegetation, such as early club mosses, horsetails, and enormous ferns, thrived in these swamps. Generations of this vegetation died and settled to the swamp bottom, and over time the organic material lost oxygen and hydrogen, leaving the material with a high percentage of carbon. Layers of mud and sand accumulated over the decomposed plant matter, compressing and hardening the organic material as the sediments deepened. Over millions of years, deepening sediment layers, known as overburden, exerted tremendous heat and pressure on the underlying plant matter, which eventually became coal.

Before decayed plant material forms coal, the plant material forms a dark brown, compact organic material known as peat. Although peat will burn when dried, it has a low carbon and high moisture content relative to coal. Most of coal’s heating value comes from carbon, whereas inorganic materials, such as moisture and minerals, detract from its heating value. For this reason, peat is a less efficient fuel source than coal. Over time, as layers of sediment accumulate over the peat, this organic material forms lignite, the lowest grade of coal. As the thickening geologic overburden gradually drives moisture from the coal and increases its fixed carbon content, coal evolves from lignite into successively higher-graded coals: subbituminous coal, bituminous coal, and anthracite. Anthracite, the highest rank of coal, has nearly twice the heating value of lignite (see Heat).

Coal formation began during the Carboniferous Period (known as the first coal age), which spanned 360 million to 290 million years ago. Coal formation continued throughout the Permian, Triassic, Jurassic, Cretaceous, and Tertiary Periods (known collectively as the second coal age), which spanned 290 million to 1.6 million years ago. Coals formed during the first coal age are older, so they are generally located deeper in Earth’s crust. The greater heat and pressures at these depths produce higher-grade coals such as anthracite and bituminous coals. Conversely, coals formed during the second coal age under less intense heat and pressure are generally located at shallower depths. Consequently, these coals tend to be lower-grade subbituminous and lignite coals.

IV

Components of Coal

Coal contains organic (carbon-containing) compounds transformed from ancient plant material. The original plant material was composed of cellulose, the reinforcing material in plant cell walls; lignin, the substance that cements plant cells together; tannins, a class of compounds in leaves and stems; and other organic compounds, such as fats and waxes. In addition to carbon, these organic compounds contain hydrogen, oxygen, nitrogen, and sulfur. After a plant dies and begins to decay on a swamp bottom, hydrogen and oxygen (and smaller amounts of other elements) gradually dissociate from the plant matter, increasing its relative carbon content.

Coal also contains inorganic components, known as ash. Ash includes minerals such as pyrite and marcasite formed from metals that accumulated in the living tissues of the ancient plants. Quartz, clay, and other minerals are also added to coal deposits by wind and groundwater. Ash lowers the fixed carbon content of coal, decreasing its heating value.

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