Air to fuel ratio should be monitored and controlled to minimize unburnts. Presence of 1% unburnts represents 2.5% excess fuel consumption.
During combustion heat is released when carbon combines with oxygen to form carbon dioxide. Theoretically if the right amount of air is supplied, fuel will consume the entire oxygen in air and this ratio of air to fuel is called the stoiciometric ratio for combustion. However, practically some air in excess of this ratio is always provided to ensure complete combustion.
At times due to improper damper settings or excessive fuel supply in the boilers, the quantity of air available for consumption reduces, leading to an air to fuel ratio lower that the stoichiometric ratio. Excess fuel loading is quite common in manual solid fuel fired boilers.
With less combustion air fuel does not burn completely leading to higher quantity of unburnts in the flue gas. Higher the quantity of unburnts higher is the fuel consumption. Unburnts can be easily identified with the color of the flue gas which, in this case, will be black in color. Typically presence of 1% unburnt leads to a 2.5% excess in the fuel consumed to deliver the required energy.
Fig : Boiler loss can be plotted as a function of excess air. (Top) The minimum point of the total loss curve of a boiler is where optimized operation is maintained. (Bottom) Most efficient operation of the boiler occurs when the amount of excess air in the stack balances the loss in unburned fuel.