Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 111

Combustion efficiency

Thermal loss by
sensible heat of
flue gas

Thermal loss by incomplete
combustion

Due to insufficient mixing of
fuel with oxygen

Optimum excess
air ratio

Fig. 11 Combustion ef ciency versus excess air factor ( ʻ )

The AF is a property of each fuel and can be calculated from the ultimate
chemical composition of the fuel.
In Fig. 11 , the variation of combustion ef )is

shown. Unnecessary high excess air volumes besides the reduction of combustion
temperature increases the thermal loss with hot

ciency with excess air factor ( ʻ

ue gas stream exhausted to the

atmosphere through the stack.
If a burner is operated with stoichiometric air or a de ciency of air, some

combustible components of the fuel remain unburned and carbon monoxide (CO)
and hydrogen (H 2 ) will appear in the products of combustion. The unburned fuel
means a thermal loss due to its chemical energy content and therefore reduced
combustion ef

ciency. The soot, smoke, and carbon monoxide exhaust create
additional pollutant emissions and surface fouling. The surface fouling may lead to
plant damage. So, the combustion chamber should be fed with optimal amount of
excess air to prevent combustion problems associated with less excess air and to
avoid decrease of combustion ef ciency due to higher excess air.

Considering that the mixing of fuel with air is well performed, the amount of
excess air should be controlled. A combustion control device should be pro-
grammed to adjust the heat production to the heat demand (load control) and to
optimize the combustion process with respect to maximizing thermal ef

ciency and

minimizing pollutant emissions.