Coal is dark brown combustible rock generated when land plants or aquatic plants are heated and pressured after they are deposited and buried in the water in the geologic age. It has presumably been taken a long period of time in forming thick coal layers in broad areas. And these layers seemed to have been developed in sedimentary basins where the ground slowly sank. Scientists believe that coal had been slowly buried under the ground through repeated processes of sinking of plants on the ground, coverage of earth and sand, and then flourishing of plants.
More than one third of coal is believed to have been generated in the Paleozoic Era (before 500 million ~200 million years) that represents most coal reserves in Europe, North America and Asia. Rest of coal is believed to have been created in the Mesozoic Era and Cenozoic Era. The largest quantity of coal was generated in the Carboniferous period of the Paleozoic Era (late Paleozoic Era). Major plants turned into coal include ferns, equisetum hyemales, lepidendrales and cordaites, and they were 20~30 meters tall.
Coal came from plants, but shapes and features of those plants are not remained. As they were heated and pressed until turning into coal, their features are changed and volume is drastically minimized.
One can imagine that coal layers must be very thick if they are converted by original size of plants. So a large number of trees must have been repeatedly heaped after being fallen down in these regions. But if trees and leaves are exposed to air even though they are heaped, elements for basis of coal will disappear as they will be decomposed soon.
Therefore, trees or leaves should be buried under earth and sand before they are decomposed in order to turn into coal. Accordingly, coal is generated near lake, marsh and coast where earth and sand are easily piled up, with well-developed forest.
When they are covered by earth or sands in the water, stems and leaves of trees are not decayed because oxidation or activities of oxidizing bacteria and insects are slowed. And plants are gradually buried deeper as earth and sands are stacked. Then, oxygen and hydrogen in plants combine into water and distills. The residue is peat that contains much carbon. This is the first stage of carbonization, and it is called the peat carbonization. After that, moisture or other volatile components are distilled due to weight of thick earth and sands, pressure caused by diastrophism or volcanic and hot spring actions. This is the carbonization in the latter stage.
Through these processes, plants turn into peat or lignite, bituminous coal, and then eventually into anthracite. In the course of changing woods into anthracite, component of carbon gradually increases while that of hydrogen and oxygen steadily decreases. And the coal that contains more carbon is called the coal with progressed carbonization. Coal looks black because it contains much carbon. Color, gloss and splitting shape are physical properties of coal. As carbonization is progressed, color of plants gradually becomes darker from the color of old trees. When the surface of initially-baked pottery is rubbed by stone or mineral, one can identify the color of its powder. This is called streak color. This color becomes darker as it is carbonized. And it grows glossier when carbonization is progressed.
When soft coal with inconsistent quality is hit to split, it cracks with uneven surfaces. If its quality is uniformed, cracked surfaces look like those of shelled clams. The more the quality of coal is uniformed by progress of carbonization, it easily splits to smaller pieces. Weight of coal becomes heavier as carbonization is progressed. But the coal that contains much incombustible component, or ash, may be heavy even though its carbonization is not progressed.
Ignition temperature of wood is 470-degree C while that of coal is 280-degree C. As shown in appearance, the more carbonization is progressed, its ignition temperature is higher. The heating value of coal means calorie generated when 1 kilograms of coal is completely burned, and it is indicated as ‘kcal’. And 1 kcal is indicated as 1 coal calorie. The coal calorie increases as carbonization is progressed.
Chemical properties of coal include following things. Oxygen sharply reduces and hydrogen decreases to some extent as carbonization is progressed. And most carbon remains, resulting in sudden increase of its ratio. When you cast coal into fire, some coal melts down and coagulates as it burns depending on types of coal.
This is the coking property, and it is an important feature in making cokes. They call it the coal with strong coking property when it turns into hard cokes without generating much gas when it coagulates. And they call it the coal with weak coking property when it turns into soft cokes generating relatively much gas. Only bituminous coal and sub-bituminous coal have such coking property.
If one closely examines the surface of coal, one can find that glossy parts and non-glossy parts form striped patterns. Glossy part, which is called bright coal, is the carbonization of stems of trees, and some of such coals still retain cells of original plants. Non-glossy part, which is called dull coal, is the carbonization of small branches, leaves and decayed trees. Some of such coals retain pollens, spores or seeds. Sometimes, one can find parts that look like charcoal, which is called carbon charcoal coal . This is believed to be natural charcoal generated due to forest fire.
Plants are carbonized as they are pressed and destructively distilled by earth pressure and terrestrial heat while they are not decayed as vegetable materials are blocked from air. This result came as plants are completely sunk in water after being withered. No study has been made on conditions of alteration of properties, but it is generally known that the temperature between several dozens and 200-degrees C in several thousands of atmospheric pressure is ideal for carbonization.
Major component of coal is carbon, and it contains hydrogen and oxygen. In addition, it contains nitrogen, sulfur and inorganic substances. Inorganic substances rarely came from original plants, and most of them are brought by ground water after sedimentation. Molecule of coal is a kind a macromolecule with comprehensive aromatic ring as monomer, and it turns from anthracite to graphite as carbonization progresses. As it is linked by carbon chains, monomer has the structure of enveloping low molecular sieve.
The first record on coal is the sentence in the book written by Theophrastos, a Greek scientist, in BC315 stating, “There exists combustible rock among rocks, and it can be used for melting metals.” The Chronicles of the Three States in Korea first mentioned that East Toham Mountain in Mojiak was burned in 31st year of King Jinpyeong (609 A.D.), and this place is presumed to be the lignite region in Yeongil County, Gyeongsangbuk Province.