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Increased efficiency at the burner system

Combustion air fan

An optimum fuel/air mixture is necessary for complete combustion. However, industrial boiler systems are often also operated at partial load. Both the fuel and air supply are reduced here.

The combustion air fan without speed regulation also runs at nominal speed in partial load ranges, as in this case the amount of air supplied to the combustion process is throttled purely by closing the air dampers. The fan consumes a high amount of electrical power, which is wasted due to throttling. If the air volume is mainly changed by modulating the speed of the fan, the power input in partial load ranges is much lower.

The reduction in power input is accompanied by a corresponding reduction in noise levels. All systems that are frequently operated at partial load for long periods of time should be equipped with variable-speed fans.

With a burner load profile with average utilisation, this can save around 65 % of electrical energy. This usually results in annual savings in the four-digit euro range, meaning that a variable-speed burner usually pays for itself within a year.

Example:
Boiler output	2,500 kW
Burner fan	10 kW
Electricity saving	Approx. 51,257 kWh/a (65.5 %)
Cost efficiency	Approx. €10,251/a (based on an electricity price of €0.20/kWh) as of 2022

Energy savings thanks to a variable-speed burner fan

Excess air

The ideal in combustion technology is stoichiometric combustion. This is the case when all fuel molecules react completely with the oxygen in the air, leaving no unburned fuel or oxygen behind.

If too little atmospheric oxygen is supplied during the combustion process, only an incomplete combustion reaction can take place. The negative consequence would be the formation of carbon monoxide, a highly toxic gas. If the amount of air is increased too much, all fuel molecules will have already reacted with the oxygen molecules. The remaining oxygen molecules form an unnecessary surplus. Since cold ambient air is usually used as combustion air, an unnecessarily high level of excess air merely heats up this cold combustion air, which is then released into the atmosphere together with the flue gases.

Optimal combustion air adjustment is therefore important for efficiency and safe, low-emission operation. Due to fluctuations in air pressure, air temperature and humidity on the one hand, and fluctuations in fuel quality on the other, which will continue to increase in Germany in the coming years as a result of the further liberalisation of the gas market, a certain level of excess air must be set as a safety measure in comparison to the theoretical optimum. The formation of toxic and explosive carbon monoxide must be prevented under all circumstances. These settings are normally made and checked when the boiler system is commissioned and during the six-monthly or annual maintenance work.

O2- and CO-regulation on the hot water boiler (simplified representation)

O₂ and CO regulation on the hot water boiler (simplified representation)

	Control
	O₂ test probe
	CO test probe
	Flue gas

Continuous measurement and control equipment is necessary in order to operate the systems closer to their optimum operating point, even under changing conditions. An O₂ regulation system essentially consists of an oxygen sensor installed in the flue gas flow and a control device. This continuously measures the residual oxygen content in the flue gas and transmits the signal to the burner control system, which readjusts the required air volume.

Combination electrodes (O₂ and CO) have been available for several years. In combination with a CO test, the excess air λ can be set even more precisely to the CO limit. By using O₂ and CO regulation of the excess air, the excess air of 3 – 4 vol.% oxygen in the flue gas, which is normally set at full load, can be reduced to 0.5 – 1.0 vol.% oxygen. At the same flue gas temperature, this corresponds to a reduction in flue loss of approximately 1 percentage point. CO regulation cannot be applied to oil fuel.

Residual oxygen content and excess air in O2 and CO regulation above the burner load

Residual oxygen content and excess air in O₂ and CO regulation above the burner load

	Without regulation
	With O₂ regulation
	With O₂ and CO regulation

Power adjustment

In existing systems, but also in new systems, hot water and heating boilers have far too much boiler output in most operating conditions in relation to the power actually required at any given time.

The reasons for this are:

Heating load curve over the course of the year. The maximum power is only required on very few days of the year
Reduction in demand in existing systems, e.g. through the elimination of heating systems or, above all, in local and district heating systems through better insulation of heat consumers
Subsequent utilisation of existing heat recovery potential
Oversizing in the planning of new systems
Excessive safety margins in the design of the actual heat energy demand, e.g. due to incorrect assessment of the simultaneity factors of the consumers, consideration of overly generous power reserves or an as yet unrealised expansion of consumers that has already been taken into account

The result is low or even very low heat consumption in relation to the boiler output. If the heat consumption falls below the minimum load of the combustion, this leads to a high number of burner start-ups and shutdowns. This causes pre-ventilation losses and temperature-induced stresses, which can be extreme, especially during long pre-ventilation periods.

The following measures can be taken to compensate for excessive boiler output:

Using burners with a wide control range
Fitting low-load controls, which delay immediate turning up once the burner has been started
Optimisation of the performance regulation, i.e. the control parameters for the burner switch-on and switch-off points
Adjustment of the burner output to the actual requirements. This then requires a corresponding burner modification or the installation of a burner with a smaller power range
Installation of heat cylinders
Selection of stepped outputs for multi-boiler systems

Temperature curve before and after adjusting the burner output in a high-pressure hot water system

Before correction (blue): Unprofitable boiler
Features: Two stage burner control; Burner switch-on interval approx. 7.5 minutes with 35 seconds pre-ventilation time each time; temperature-related alternating loads on the boiler are unnecessarily high here.
Pre-ventilation losses occur every time the burner is ignited.

After correction (purple): Cost-saving boiler
Features: Two-stage burner control, low-load operation during night-time hours; burner switch-on interval approx. 44 minutes with 35 seconds pre-ventilation time each time; temperature-related alternating loads on the boiler are reduced here to a level that does not cause fatigue. Heat loss during pre-purging significantly reduced to one-fifth.

Pre-purging

Before starting the burner, ensure that there are no flammable mixtures in the flue gas paths. In practice, this is achieved by pre-purging. Before the burner ignites the flame, the combustion air fan starts up and forces cold ambient air through the hot flue gas ducts, which are still at operating temperature. The cold air heats up in the process and extracts heat from the boiler. Adequate air change-over is required, which may represent a significant energy loss, especially with frequent burner starts.

In addition to the poor economic efficiency of frequent burner starts, these also reduce the service life. Ideally, 1–2 burner switch-on cycles per hour should be aimed for. If there are more than four burner switch-on cycles per hour, measures should be taken to reduce burner switching, such as adjusting the power.

Equation for estimating pre-purging losses

$Q_{v, Pre-purging} = 1,26 \cdot {\overset{\cdot}{Q}}_{F} \cdot Δ T \cdot t \cdot 10^{-7}$

$Q_{v, Pre-purging}$	Pre-purging loss of the system [kWh]
${\overset{\cdot}{Q}}_{F}$	Burner output of the system [kW]
$Δ T$	Temperature differential between the medium in the boiler and the ambient air drawn in [K]
$t$	Total of the opening and closing times of the actuator and the pre-purging time

Equation for calculating the temperature differential between the medium in the boiler and the ambient air drawn in

$Δ T = T_{K} - T_{L} = T_{s} (p_{m} = 13 bar) - T_{L}$

$ΔT$	Temperature differential between the medium in the boiler and the ambient air drawn in [K]
$T_{K}$	Temperatures of the medium in the boiler [K]
$T_{L}$	Temperatures of the ambient air drawn in [K]
$T_{s}$	Boiling point of the medium in the boiler at a specific pressure p_m [K]

Equation for calculating the total of the opening and closing times of the actuator and the pre-purging time

$t = t_{1} + t_{2} + t_{V}$

$t$	Total of the opening and closing times of the actuator and the pre-purging time [s]
$t_{1}$	Actuator opening time (approx. 30 – 60 s) [s]
$t_{2}$	Actuator closing time (approx. 30 – 60 s) [s]
$t_{V}$	Pre-purging time (≤ 120 s) [s]

Sample calculation for determining the total of the opening and closing times of the actuator and the pre-purging time

t

= [s] + [s] + [s] = 130 [s]

Sample calculation for determining the temperature differential between the medium in the boiler and the ambient air drawn in

Δ T

= [°C] − [°C] = 170 [K]

Sample calculation for estimating the burner output of the system

{\overset{\cdot}{Q}}_{F}

≈ [ kg h ] ⋅ 0,65 % ≈ 6700 [kW]

Sample calculation for estimating pre-purging losses

Q_{v, Pre-purging}

= 1.26 ⋅ 6700 [kW] ⋅ 170 [K] ⋅ 130 [s] ⋅ 10^–7 = 18.7 [kWh]

With an average of four burner starts per hour and an average boiler load of 20 %, this results in a heat loss of 6 % of the boiler heat output.

Extrapolated over a period of 4,000 hours per year, this results in a total heat loss of around 300 MWh/year, which equates to a loss of approximately €20,000 per year.

Sample calculation for estimating the annual pre-purging loss [MWh]
18.7 [kWh] ⋅ [ h a ] ⋅ [ 1 h ] = 299 [ MWh a ]

Sample calculation for estimating the annual pre-purging loss [€]
299 [ MWh a ] ⋅ [ € MWh ] = 20004 [ € a ]

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