Excerpt: Practical Guide Industrial Flue Gas Analysis. Emissions and process measurement guide.

The air ratio can be determined from the concentrations of the flue gas components CO, CO₂ and O₂, the correlations are shown in the so-called combustion chart (see image right). When there is ideal mixing of fuel and air, any CO₂ content is related to a specific CO content (in the range λ<1) or to a specific O₂ content (in the range λ>1). The CO2 value on its own is not clear due to the curve profile running beyond a maximum, which means that an additional test is required to establish whether the gas also contains CO or O₂ in addition to the CO2. For operation with excess air (i.e. normal scenario), a definitive measurement of O₂ is now generally preferred. The curve progressions are fuel-specific, i.e. each fuel has its own diagram and a specific value for CO₂ max. The connections between these numerous diagrams is often summarized in an easy-to-handle nomogramm (“fire triangle”, not illustrated here.) This can be applied to any type of fuel.

The following two formula apply roughly to the theoretical calculation of the air ratio from the CO2 or O₂ readings:CO₂ max:

  • Fuel-specific maximum CO₂ value. If required, this value can be determined by Testo as a service.
  • CO₂ and O₂: Measured (or calculated) values in the flue gas

Energy balance of a combustion plant

In stationary operating mode, the sum of all the energies supplied to the plant must be equal to the sum of the energies delivered by the plant; see this table :


Supplied energies Discharged energies
Net calorific value and tangible fuel energy Tangible heat and chemically bound energy of flue gases (flue gas loss)
Tangible heat of combustion air Tangible heat and net calorific value of fuel residues in ash and slag
Thermal equivalent of the mechanical energy converted in the plant Surface losses as a result of heat conduction
Heat brought in through the product Heat dissipated with the product Convection losses as a result of furnace leaks
The main contribution to the loss is the flue gas loss. It is a function of the difference between the flue gas temperature and combustion air temperature, the O₂ or CO2 concentration in the flue gas and fuel-specific factors. In condensing boilers, this flue gas loss is reduced in two ways – via utilization of the condensation heat and via the resultant lower flue gas temperature. The flue gas loss can be calculated using the following formulae:

FT: Flue gas temperature
AT: Combustion air temperature
A2, B: Fuel-specific factors (see table)
21: Oxygen content in the air
O₂: Measured O₂ concentration
KK: Variable which shows the variable
qA as a minus value if the dewpoint is undershot. Required for measurement on condensing systems.

For solid fuels, factors A2 and B equal zero. In that case, using the factor f, the formula is simplified to create the so-called Siegert formula:

FT: Flue gas temperature
AT: Combustion air temperature
CO₂: Measured CO₂ concentration

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Practical guide contents

1. The combustion process

1.1 Energy and combustion
1.2 Combustion plants
1.3 Fuels
1.4 Combustion air, air ratio
1.5 Flue gas (exhaust gas) and its composition
1.6 Gross calorific value, net calorific value, efficiency
1.7 Dewpoint, condensate

2. Gas analysis for industrial flue gases

2.1 Combustion optimization
2.2 Process control
2.3 Emission control

3. Gas analysis technology

3.1 Terminology used in gas analysis technology
3.2 Gas analyzers

4. Industrial gas analysis applications

4.1 Power generation
4.2 Waste disposal
4.3 Non-metallic minerals industry
4.4 Metal/ore industry
4.5 Chemical industry
4.6 Others

5. Testo gas analysis technology

5.1 About the company
5.2 Typical instrument features
5.3 Overview of gas analyzers
5.4 Overview of accessories

Practical Guide for Industrial Flue Gas Analysis

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