The combustion of solid fuel has the following stages
- Preheating and drying of fuel.
- pyrogenous decomposition of fuel with evolution of volatiles and formation of Coke Residue (carbon).
Evolution of volatile on being heated to 200 degree centigrade or more solid fuels liberated gaseous components are H2, CO, CO2, CH4 etc.
With combustion of coal dust the main mass of volatiles has no time to evolve before ignition of dust particles i.e. volatiles burn essentially at the same time with the carbon
- Reaction of burning of carbon
C + O2 = CO2 +399253.5 KJ/K mole (exo)
2C + O2 = 2CO +246623.4 KJ/K mole
- Decomposition of carbon dioxide and water vapour by carbon
C + CO2 = 2CO - 162530.1 KJ/K mole (endo)
C + 2H2O = CO2 + 2H2 -65294.2 KJ/K mole (endo)
- Oxidation of carbon monoxide by oxygen
2CO + O2 = 2CO2 +571683.6 KJ/K mole (exo)
It is not definitely established yet weather CO or CO2 is formed first step. There are some reasons to support that CO and CO2 are formed simultaneously by the reaction 4C +3O2 = 2CO2 + 2CO
Before carbon burning oxygen penetrates deep into the crystal lattice of fuel. The rate of combustion of solid depends on rate of chemical reaction and the rate of oxygen diffusion.
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