coal

<body> <p>Coal is a combustible substance produced by the decay of plants, compacted and hardened by high temperatures and pressure in absence of oxygen. It is mainly composed of carbon (60/95%) and also by hydrogen, oxygen, nitrogen and sulphur. It can be found on the surface or in large underground deposits. Coal can be classified by its age or by its calorific value in four large categories: anthracite, bituminous coal, sub-bituminous coal, lignite and peat, the richest one being the oldest one and the one with the higher carbon percentage. Many countries and organizations use different classifications, so that, when in doubt regarding a compound, it is advisable to check its chemical composition, heat value, moisture, and coking properties. </p> </body>