Primary and secondary solid fuels are widely used in domestic and industrial purpose. Wood and coal constitute the important primary solid fuels. The important secondary solid fuels are charcoal & coke.
Wood
Wood is largely used as a domestic fuel ancient times. Wood contains 10-15% moisture in air dried condition and the calorific value is about 3500-4500 kcal/kg. it burns with long non-smoky flame and leaves behind little amount of ash. Wood charcoal is prepared by destructive distillation of wood. Wood charcoal is used for producing activated carbon which is used in industry.
Coal
Coal is a fossil fuel produced from vegetable matter under high temperature and pressure conditions over millions of years. It is chiefly composed of C, H, N, O and non-combustible inorganic matter. The plant materials are attacked by various bacteria and micro organisms and will be buried under sedimentary rocks. These deposits lose moisture and volatile matter due to gradual decomposition process in absence of oxygen. Final coal formation takes place in about 3 * 108 years.