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Combustion efficiency

The combustion efficiency ηf describes the yield of sensible heat generated during the combustion of a fuel. It is determined by calculating the thermal losses q_A in the flue gas in relation to the ambient temperature level. Unburned components of the fuel are not taken into account in oil and gas combustion, as they should not occur in relevant quantities in practice.

Information about net and gross calorific values

The combustion efficiency refers to the net calorific value of a fuel and is calculated by subtracting the flue losses from the maximum achievable 100 %.

Formula for calculating the combustion efficiency

$η_{f} = 100 % - q_{A}$

Excess air

Excess air is the ratio of the amount of air actually supplied to the amount of air stoichiometrically required.

$λ = \frac{m_{L}}{m_{L, st}}$

The simplified equation for converting the oxygen content of the flue gas only applies to a flue gas-air ratio of ~ 1.

$λ \approx \frac{21 %}{21 % - O_{2}}$

λ	Excess air
m_L	Actual heat
m_{L, st}	Stoichiometric heat
O₂	Oxygen content [vol%]

To calculate the flue loss, the CO₂ or O₂ content in the flue gas and the temperature differential between the flue gas temperature and the ambient temperature are determined. In addition, the maximum CO₂ content in the flue gas, which depends on the respective fuel, and the Siegert factor f, which depends on the measured O₂ content, are required.

Formula for calculating the flue loss

$q_{A} = \frac{f}{{CO}_{2, \max}} \cdot \frac{21 %}{21 % - O_{2}} \cdot (t_{A} - t_{L})$

q_A	Flue loss, based on the burner output and the net calorific value [%]
f	Siegert factor, linear dependence on the excess air λ
CO_2,max	Maximum CO₂ content in the dry flue gas [vol.%]
O₂	Measured oxygen content in the dry flue gas [vol.%]
t_A	Measured flue gas temperature [°C]
t_L	Supply and combustion air temperature in accordance with EN 12953 Part 11 constant 25 °C

If only the carbon dioxide content in the dry flue gas is measured, the following conversion applies:

Formula for calculating the residual oxygen content from the carbon dioxide content

$O_{2, r} = 21 % \cdot (1 - \frac{{CO}_{2}}{{CO}_{2, \max}})$

O_2,r	Calculated oxygen content in the dry flue gas [vol. %]
CO₂	Measured CO2 value in the dry flue gas [vol. %]
CO_2,max	Maximum CO₂ content in the dry flue gas [vol. %]

		Siegert factor	Siegert factor
Fuel	CO_2,max	f₁ = f (O₂ = 0 %)	f₂ = f (O₂ = 5 %)
Natural gas L	11.67 %	0.4792	0.4530
Natural gas H	11.94 %	0.4731	0.4469
EL fuel oil	15.31 %	0.4535	0.4342
Propane	13.69 %	0.4575	0.4352
Propane-butane	13.78 %	0.4570	0.4349
Butane	13.99 %	0.4563	0.4346
HVO	12.5 %	0.0541	0.0721
Hydrogen H₂	0 %	1.013	–

Siegert factors for different fuels

Calculation of the Siegert factor for any oxygen content in the dry flue gas O₂:

$f (O_{2}) = f_{1} + \frac{f_{2} - f_{1}}{5 % - 0 %} \cdot O_{2}$

Excess Air Oxygen Content Siegert Factor

	21 % (21 % – O₂)
	Natural gas L
	Natural gas H
	EL fuel oil
	SA fuel oil
	Natural gas L
	Natural gas H
	EL fuel oil
	SA fuel oil
	Propane
	Propane-butane
	Butane
	Natural gas GZ35
	Natural gas GZ41.5
	Natural gas GZ50
	Medium fuel oil HL Schwechat
	Medium fuel oil CLU 3

Correlation between oxygen content in dry flue gas, excess air and Siegert factor

Notes:

For excess air: Natural gas GZ 41.5 / 50, propane, butane, propane-butane are almost identical to natural gas L and are therefore not shown.
Medium fuel oil CLU 3 and medium fuel oil HL Schwechat are between the curves of fuel oil EL and SA, and are therefore not shown.

The combustion efficiency increases, starting from the full load until roughly 35 % partial load for a hot water boiler system. The excess air and therefore the CO₂ content measured in the dry flue gas increases only slightly while the flue gas temperature falls due to more efficient utilisation of the heating surface in the boiler. At a partial load of < 35 %, the greater amount of excess air that is necessary prevails and the combustion efficiency falls again.

Fig. “Efficiency with condensing flue gas heat exchanger”

Fig. “Flue gas temperature with condensing flue gas heat exchanger”

The combustion efficiency is determined, for example, during emission measurements carried out by a competent engineer or by trained customer service engineers. Heat losses due to radiation and conduction at the boiler surface are not taken into account here.

The following diagram shows the dependence of the combustion efficiency on the flue gas temperature for natural gas H at different excess air levels. The higher the flue gas temperature, the lower the efficiency.

The diagram also clearly shows that, especially at high flue gas temperatures, the efficiency gain is particularly high due to a lower excess air, i.e. low λ values.

Efficiency curve as a function of excess air λ without condensation, exemplified by natural gas H

	λ = 1 (O₂ = 0 %)
	λ = 1.1 (O₂ = 2.14 %)
	λ = 1.15 (O₂ = 3.09 %)
	λ = 1.2 (O₂ = 3.96 %)
	λ = 1.25 (O₂ = 4.77 %)
	λ = 1.3 (O₂ = 5.52 %)

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