Coal, a combustible organic rock [1] composed primarily of carbon, hydrogen, and oxygen [2]. 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 [3], and other products. Coal comes from ancient plants buried over millions of years in Earth��s crust [4], its outermost layer [5]. Coal, petroleum, natural gas, and oil shale [6] are all known as fossil fuels [7] because they come from the remains of ancient life buried deep in the crust.
Coal is rich in hydrocarbons [8](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 [9] during the Carboniferous Period [10], about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden [11] 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 [12] 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 [13]. Scientists classify coal according to its fixed carbon content [14], or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.
NOTES TO THE TEXT
[1] organic rock�����
[2] carbon, hydrogen, and oxygen��̼�������
[3] pesticides��ũҩ
[4] Earth��s crust���ؿ�
[5] outermost layer�������ز�
[6] oil shale����ҳ��
[7] fossil fuels����ʯȼ��
[8] hydrocarbons��̼�⻯����
[9] layers of sediment ��������
[10] Carboniferous Period��ʯ̿��
[11] overburden�������Ҳ�
[12] inorganic mineral��������
[13] moisture content����ˮ��
[14] fixed carbon content���̶�̼����
