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Stack loss is calculated by applying a simple formula which relates it to other easily measurable parameters:
Qs = A2 + B Tf-Ta
CO
2
Where:
A2, B = factor that depends on the fuel used
Tf = flue gas temperature
Ta = combustion air temperature
CO
2
= % carbon dioxide in the flue gas
Thus in order to calculate the stack loss and hence the thermal efficiency of a plant, one must measure the two
temperatures (flue gas and air) and the level of carbon dioxide contained in the flue gas (% CO
2
). These
operations are performed automatically by the flue gas analyser during testing.
Let’s take a look at the gases produced by combustion that need to be kept under
control:
CO
2
: CARBON DIOXIDE
The maximum CO
2
values that can be obtained from perfect combustion (theoretical) for the different types of
fuels are:
In truth, the percentage of CO
2
that can be detected during analysis will always be lower than these limit values.
CO: CARBON MONOXIDE
Carbon monoxide (CO) is usually produced by bad combustion that is weak in oxygen: since CO is a highly
dangerous gas (it is fatal for man even in very low concentrations: exposure to 400 ppm for 3 hours is already
fatal), standard UNI 10389-1 has established a limit value beyond which the test results of the boiler plant are
deemed unsatisfactory. The percentage of gas considered by the standards, however, is not the value measured
directly in the flue gas, which is "diluted" with other combustion products, but is the value referred to the volume
of flue gas generated by perfect combustion, that is, where the oxygen is zero.
This limit is:
CO (referenced to 0% O
2
) = 1000 ppm = 0.1%
Fuel
% max CO
2
Methane
11,7
Propane
13,9
LPG
13,9
Butane
13,9
Diesel oil
15,1
Fuel oil
15,7
