why do different type of coal have different composition
Answers
It is because of different composition found in coal they classified it into different types, based on carbon content(indicates calorific value), ash content(indicates waste generation), caking properties(used in steel making) and so on.
Coal is formed by the decay of plant matter.This decayed plant matter is called as peat and for this peat to become coal,it must be buried under sediments.Continued burial and the addition of heat and time causes the complex hydrocarbons in the peat to breakdown and alter in many ways.One of the gaseous product released during transformation of peat is methane and the deposit becomes more and more carbon rich with time as the elements within it disperse.
On this basis,we have different ranks of coal-peat,lignite,bituminous and anthracite
Overall,you need time heat and pressure for coal formation to take place.