10 Fully Metered Combustion Control
04.09.2016 Mk8 MM Expansion Features Installation and Set-Up Guide Page 143
10 FULLY METERED COMBUSTION CONTROL
10.1 Overview
10.1.1 Introduction
The fuel-air mixture will determine the combustion performance; poor mixing of the fuel and air will
reduce the burner’s combustion performance, and in turn, decrease the combustion efficiency. Too fuel
rich a fuel-air ratio will result in incomplete combustion, leaving unburnt fuel in the combustion
products. Unburnt fuel will cause soot build-up or release harmful CO emissions. In the boiler room,
incomplete combustion wastes the fuel, so more fuel is required to meet the load demand, causing a
high fuel bill. On the contrast, too much air in the combustion process will waste the heat generated by
the fuel burning to heat the excess air; again, the fuel bills will increase. The fully metered system is
used in applications where it is not possible to measure the exhaust gases in the stack, or if the firing
rate is critical to system and controlled remotely.
10.1.2 Importance of Excess Air
In ideal stoichiometric combustion, all the fuel is mixed with the exact amount of air for it to be
converted fully to CO
2, H2O (N2), releasing heat from the reaction. In the practical world where ideal
and laboratory conditions do not always exist, it is necessary to add more air than that required in
stoichiometric combustion to ensure complete combustion. The equivalence ratio of the combustion is:
/
/
Or alternatively, if the volume is known:
/
/
Where n is the number of moles of gas, proportional to the corrected volume.
The excess air is the extra amount of air supplied over the amount of air required for complete
combustion, and can be determined from the equivalence ratio:
1
1
100%
This can be converted to the wet exhaust gas O
2 by:
21%
1
1
Higher excess air levels will give higher O
2 values in the exhaust gases. The optimum excess air will
depend on the fuel type, the combustion chamber design and the burner turndown. High performance
burners will operate 3% O
2 (dry) when firing on natural gas, which is equivalent to 15% excess air
going into the burner, whilst producing 0ppm of CO. The stoichiometric data in the lookup tables of the
software is gives the excess air based on the fuel and air flow rates, and the calorific value of the fuel.
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