Burning of coal is represented as ${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_{2( \mathrm{g})}\to {\mathrm{CO}}_{2( \mathrm{g})}$. The rate of this reaction is increased by:
 

FACTORS AFFECTING THE RATE OF REACTION:
Surface area of the reactants: In heterogeneous reactions, more powdered is the form of reactants, more is the velocity.
So, the rate of reaction will increase by powdering the lumps of coal as surface area increases in that case.