1. Introduction
Low-power boilers firing wood pellets are increasingly used in home heating systems, single- and multi-family houses, and public buildings [1,2]. Their development has led to optimising combustion technology for wood biomass fuels thanks to increased energy and reduced toxic exhaust emissions [3,4]. Wood pellets are widely used and considered a carbon-neutral energy source [5]. Still, household biomass combustion remains a significant source of pollution in the global troposphere, with severe consequences for air quality, climate, and human health [6,7].
The combustion process and emissions of pollutants from boilers are influenced by factors such as the type and quality of fuel [8,9], kind of burner [10,11], and parameters related to combustion conditions such as air–fuel ratio (λ), combustion temperature, or mixing in the combustion chamber [12]. The combustion process must be adequately controlled, with an accurate airflow estimation for a given amount of fuel in current conditions [1]. Hence, fully automatic low-power boilers are becoming increasingly popular. The microprocessor system calculates the dose that initiates ignition, switches on the igniter, and detects the appearance of a flame. Then, it supervises the fuel combustion process, calculates the fuel dose for individual power ranges of the heating device, and supervises the cleaning of the burner and the heat exchanger. All these operations are performed by controlling individual peripheral devices, including the blower fan, fuel feed motors, igniter, etc. Therefore, an electric current is needed for the proper operation of low-power boilers and other electrical devices, which has three main characteristics: voltage, amperage, and frequency. The electrical voltage has the most significant influence on the operation of electric motors because, in most cases, the speed of electric motors is controlled by changing the electrical voltage.
Hence, ensuring a supply of high-quality electricity to recipients is crucial. The main parameters of the power supply system efficiency are power supply reliability (PSR) and power quality (PQ) [13]. Voltage stability of the power grid is a crucial aspect of the power system operation. It represents the ability of the system to maintain an appropriate voltage level in the power grid despite disruptions, changes in load, or unforeseen events [14]. The voltage stability of the electricity network significantly affects the operational stability of all the electrical equipment connected to it, especially appliances that lack protection against voltage fluctuations. The key factors influencing voltage stability in the grid are growing energy consumption and the growing number of recipients, which increases the electricity demand. Integrating wind farms, photovoltaic plants, and other renewable energy sources into the grid complicates the situation further. Energy production from these sources relies on changing weather conditions, such as wind and sunshine, leading to fluctuations in energy supply. Dynamic changes in the amount of energy supplied to the grid can result in voltage fluctuations, particularly in rural areas or at the ends of transmission lines [15,16,17]. This is particularly important as low-power boilers are installed in such areas.
The voltage in the electric network is not constant. The most significant voltage deviations occur outside urban areas. They depend on network load, distance from the transformer, the transmission network age and wear, and the number of photovoltaic panels installed. The range of permissible deviations in the 230 V low-voltage network is specified in the EN 60038:2012 standard [18]. Permissible deviations are currently ±10% of 230 V, i.e., from 207 to 253 V for 95% of the average values of 10 min periods from the weekly set, which means that during the week, 5% of the average 10 min values may exceed the above range. This does not change the fact that a constant voltage value in the 207–253 V range is permissible.
The blower fan supplying air to the combustion chamber of the burners is the device most susceptible to voltage changes because the rotational speed controls the amount of air. In contrast, the rotational speed depends on the voltage supplied to this device. To achieve higher efficiency and lower emissions, the combustion chamber must be supplied with the appropriate amount of air in the correct manner ratio [19]. Therefore, the boiler controller has a saved table of blower fan rotational speed values assigned to the appropriate value of the burner heating power, ignition, and extinguishing and cleaning processes. Therefore, voltage changes in the network can affect the operation of low-power boilers. This issue is essential because these boilers are often installed in areas where voltage changes are potentially the greatest.
Numerous research findings related to emissions from low-power biomass boilers are available in the literature. These studies were conducted on both laboratory and commercial boilers [20] or involved model tests [21]. In most instances, these studies were performed under controlled laboratory conditions, which do not necessarily reflect real-world scenarios. Consequently, some authors reference actual conditions [22,23]. Typically, research concentrates on the impact of wood pellet types [24,25,26], agro pellets [27,28], and waste biomass pellets [29,30,31] on emissions. Another area of investigation is the effect of load [32] and the transient states of operation (such as start-up and shut-down phases) of the boiler on emissions [33,34]. Modifying existing burner solutions to enhance combustion efficiency has also been explored [35,36]. Additionally, the literature reports the effect of air quantity and distribution on emissions. For instance, studies [37,38] investigated the impact of airflow, its distribution, and oxygen content on combustion properties and pollutant emissions. Other research [39] explored the effect of airflow position supplied to the burner at various excess air ratios on combustion characteristics and emissions in pellet boilers. Conversely, [40] focused on enhancing the combustion efficiency of a pellet boiler by utilising a PLC to control, for instance, the speed of the blower fan.
However, there is a lack of scientific reports on the emission characteristics associated with changes in the main voltage of domestic pellet boilers. A stable electric power supply, reliability, and power quality are the primary factors ensuring proper boiler operation. Consequently, the aim of this study was to evaluate the influence of the electricity grid voltage on the functioning of a low-power boiler. The scope of this work encompasses an analysis of the impact of voltage fluctuations within the range permitted by the EN 60038:2012 standard (230 V ±10%) on flue gas and dust emissions during boiler operation at full load. Furthermore, the effect of these variations on the boiler firing process was investigated. This has made it possible to identify the challenges of ensuring the quality of electric power supply, which is essential for minimising the negative environmental impact of biomass combustion.
2. Materials and Methods
2.1. Research Site
The test was carried out on an SKP MAX 25 (ZPH Krzaczek Sp. z o. o., Klikawa, Poland) automatic central heating boiler with a rated power of 25 kW. The boiler was of class 5 according to EN 303-5:2021 [41] and met the EU directive “Ecodesign” (2009/125/EC and (EU) 2015/1189). A diagram of the boiler is shown in Figure 1. The boiler was equipped with a pellet burner with a rotating combustion chamber (Figure 2). The burner was placed in the lower door of the boiler. The boiler combustion chamber was lined with 40 mm thick fireclay plates to isolate the direct contact of the fire with the boiler water jacket [42]. The exhaust gas outlet is located on the rear wall of the boiler. An electronic controller controls the combustion process. It controls all electrical equipment, including a ceramic igniter, a photoelement for flame detection, fuel feeders, and a blower fan with a Hall sensor, enabling the fan’s rotational speed to be read.
The blower fan used in the tested burner is the WPB 109-10 (MplusM, Obłaczkowo, Poland). It is a radial fan with forward-tilted blades, a maximum rotational speed of 2605 rpm, a maximum flow rate of 180 m3/h, and a maximum compression of 245 Pa.
The controller EcoMax 920P (Plum Ltd., Ignatki, Poland) of the tested boiler had typical blower fan operation ranges saved for individual burner power ranges and operating modes, e.g., cleaning. For the tested boiler and burner 25 kW, the minimum boiler operation power is 8 kW; for this power value, the blower fan speed is assigned 28%. For the average boiler and burner operation power of 16 kW, the blower fan speed is assigned 33%. For the maximum power of 25 kW, the blower fan speed is assigned 42%. The blower fan speed is 80% for the burner extinguishing mode, while for the cleaning mode, it is 100%. The burner controller is equipped with software enabling automatic smooth regulation of the fuel and air supplied to the furnace depending on the installation load, the temperature in the selected room, the temperature of the heating system, the outside temperature, and many others. During the test, this function was switched off. The boiler operated at a constant heat output of 25 kW, with the blower fan setting and fuel dose defined in the controller. The controller also determined the fuel feeding time and the interval between doses. The boiler’s microprocessor controller automatically calculated these parameters according to the following formula:
Q = q ⋅ LHV ⋅ 0.2778,(1)
whereQ—heating boiler power [kW];
q—dose of fuel fed at a time, calculated by the microprocessor controller [kg/h];
LHV—low heating value of fuel [MJ/kg].
For the set power of 25 kW using fuel with an LHV of 15.84 MJ/kg, the fuel dose was 5.66 kg/h.
The boiler was installed on a test stand (Figure 3) compliant with the EN 303-5:2021 standard in the laboratory of ZPH Krzaczek (Klikawa, Poland). The measurement stand has tubular heat exchangers, boiler water temperature sensors on the boiler supply and return, and an electromagnetic flow metre. The stand has a measuring section compliant with EN 303-5:2021 between the boiler flue and the chimney. Measuring probes connected to automatic data analysis devices are attached to the measuring section. A damper with an exhaust fan for regulating the chimney draft and an electronic micromanometer for continuous measurement is installed in the chimney. Thanks to this, the chimney draft can be stabilised to a specified value throughout the entire period of the test. The chimney draft was set to 15 Pa during the tests.
The combustion parameters (amount of supplied fuel and air) were selected so that the boiler achieved its nominal power of 25 kW and exhaust emissions were as low as possible and stable over time.
An exhaust gas analyser Testo model 320-2 LL (Testo SE & Co. KGaA Baden-Wurttemberg, Germany) was used to measure temperature (ambient and exhaust gas) and exhaust gas emissions (CO, O2, NOx). A Testo analyser model 380 (Testo SE and Co. KGaA Baden-Wurttemberg, Deutschland) was used to measure dust. The exhaust gases were sampled continuously at 1 s intervals.
2.2. Research Procedures
The measurements were carried out at an ambient temperature of 22.7 °C, relative humidity of 52%, and atmospheric pressure of 992 kPa, which were the same for each research stage.
Softwood pellets were used as fuel for the tests. According to the ENplus® ST 1001 [43] and ISO 17225-2:2021 standard [44], the pellets met the class A1 requirements. The fuel parameters obtained from the certified laboratory are shown in Table 1.
The supply voltage of the boiler was adjusted to the three ranges specified by the EN 60038:2012 standard, specifically 230 V ±10%:
230 V—the voltage defined as nominal (standard) in the EU;
207 V—the lowest voltage permitted by the standard;
253 V—the highest voltage permitted by the standard.
The boiler tests were carried out in three stages:
Determination of blower fan operating characteristics at three supply voltages;
Emission and exhaust temperature testing at three voltages;
Testing the ignition time at three voltages.
2.2.1. Blower Fan Testing
The blower fan speed was measured in 1% steps of the boiler controller settings in 19% ÷ 100% for three voltages: 230 V, 207 V, and 253 V. The burner controller did not allow the fan speed to be entered below 19%, the minimum value set by the controller manufacturer. The blower fan speed was read from a hallotron installed in the blower fan motor.
2.2.2. Exhaust and Dust Emission Testing
The combustion tests had exhaust emissions that lasted for 15 min and were conducted at a nominal boiler power of 25 kW. After firing up the boiler and obtaining the nominal boiler power, the amount of air was adjusted to obtain the best possible exhaust emission values (reference values). Then, the exhaust temperature, emission, and dust were recorded at the established parameters at a voltage of 230 V. In the next step, these parameters were recorded at a lowest voltage of 207 V. Then, the boiler operation was stabilised by changing the connection to 230 V. Then, the parameters were recorded at the highest voltage of 253 V.
As the EN 303-5:2021 standard, according to which the test was carried out, does not specify an exact value for the stabilisation time, it was set to 30 min during the experiment. No changes in the emission values of the individual compounds were recorded during this period. This satisfied the condition outlined in the EN 303-5:2021 standard for conducting another emission test once the emission values of the individual compounds had stabilised at a constant level.
2.2.3. Testing the Burner Ignition Process at Voltages 207, 230, and 253 V
The ignition time test was conducted for three defined voltages. Each time, the burner was extinguished, cooled to ambient temperature, and cleaned before the test. The ignition time and blower fan speed were recorded.
The boiler ignition process programmed in the controller was carried out according to the following scheme: an initial dose of fuel of 150 g was supplied, and the igniter and blower fan were switched on (ignition airflow 25%). For 4 min, the photoelement (photodiode) registered the amount of light in the combustion chamber, with an increase above 5% in time up to 4 min; the second stage of ignition took place: the blower fan speed was increased to 33% for 40 s, and the igniter was switched off. This process allowed for the complete ignition of the fuel dose supplied to the fireplace. If, for 4 min in the first stage, the amount of light in the combustion chamber was lower than 5%, the ignition process was repeated. The number of possible ignition repetitions was 2 times. If it was impossible to initiate the fire (with brightness above 5%) within three attempts, the controller triggered an alarm about unsuccessful ignition.
3. Results
3.1. Blower Fan Performance Characteristics
Figure 4 presents the results of the tests of the blower fan operation characteristics at three voltage values of 230 V, 207 V, and 253 V with the given controller settings. The graph shows the controller’s characteristic operating points: the minimum power of the 8 kW burner—28%, the average power of the burner—33%, the maximum power of the 25 kW burner—42%, the ignition blow-in—25%, and the post-ignition blow-in—33%.
The operation of the boiler at the highest voltage (253 V) caused an increase in the blower fan speed in relation to the speed in the nominal voltage, while the lowest voltage (207 V) caused a reduction in the speed. The most significant changes in voltage are visible in the boiler operation range of 8–25 kW (28–42% of the controller setting) and decrease with an increase in the controller setting. The speeds for these values are presented in Table 2. At the minimum power of the burner at a reduced voltage, the speed dropped by 33%; at an increased voltage, it increased by 34.6%; similarly, for average power, the drop was 43.3% and there was an increase of 32.7%, and for maximum power, it was 17.6% and there was an increase of 20.4%. It should be noted that the changes for the maximum setting of 100% for both voltages amounted to approx. 5%.
3.2. Emission Test Results
Table 3 presents the average temperature values of exhaust gases, carbon monoxide, carbon dioxide, nitrogen oxides, and oxygen emitted by the boiler during the combustion test. Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 graphically compare the results of the measurements of exhaust gas temperature and emissions for the individual voltages of 207 V, 230 V, and 253 V.
The voltage change caused visible changes in the exhaust gas temperature (Figure 5, Table 3). The voltage increase to 253 V caused the exhaust gas temperature to increase by 13.4% compared to the voltage of 230 V. This increase can be explained by the high pressure of the air supplied to the combustion chamber. The high pressure of the air supplied to the combustion chamber results in the acceleration of the combustion process and the release of more energy per unit of time. On the other hand, at a voltage of 207 V, a decrease in the exhaust gas temperature by 7.8% was observed. The reason for this decline was the decrease in the air pressure in the combustion chamber and the slowing down of the combustion process of a given fuel dose.
These findings are consistent with the existing literature. Sungur and Basar [39] showed through their research that a reduction in the excess air coefficient results in lower exhaust gas temperatures, a trend also observed by [40].
Analysing the time histories of changes in the CO2 concentration in exhaust gases (Figure 6), it can be seen that the voltage drop caused an increase in the CO2 concentration in exhaust gases. This increase was relatively high, as it amounted to 31% compared to a voltage of 230 V. At a voltage value of 253 V, a decrease in the CO2 concentration of 13.3% was observed. The observed changes in concentrations within the test range for voltages of 230 and 253 V are similar, but the observed changes are greater for a voltage of 207 V. This was because, at 230 V and 253 V, the air–fuel ratio λ was within the optimal operating range. At the lowest voltage (207 V), the average CO2 concentration was 14.3%, and the average coefficient λ was 1.4, which indicates a perfect match between the amount of air and the supplied fuel dose. However, at coefficient λ < 1.4, the combustion process of solid fuels becomes unstable. Sungur and Basar [39] obtained an analogous relationship in their research.
The analysis of CO changes during the test indicates higher concentrations at increased voltage and lower concentrations at decreased voltage. It should be noted that the averaged results showed that at reduced voltage, the decrease was 45% (Table 3), while increased voltage caused an increase in the CO concentration, amounting to 97%. In the case of voltages of 230 and 207 V, the recorded changes in concentrations over time were similar (Figure 7). Similar changes in the CO concentration resulting from the amount of supplied air can be found in [39,45]. In [25], the increase in CO was attributed to the increased amount of air inhibiting the oxidation reaction in the combustion chamber.
An analysis of the change in the CO concentration (Figure 7) shows two characteristic peaks at approximately 150 and 600 s at an increased voltage of 253 V. This is associated with the mode of operation of the rotary burner. A 5° rotation of the combustion chamber (Figure 2) is employed to rake the embers in the firebox, prevent sintering, and remove the solid parts of the spent fuel from the firebox. This rotation occurs every 450 s. The burner’s rotation during increased airflow caused the lighter parts of the glowing, unburned fuel to be ejected into the ash pan. The fuel particles afterburning without air resulted in the increase in CO emissions illustrated in the diagram. This phenomenon was not observed at 230 and 207 V due to the lower airflow velocity.
The recorded NOx concentration looked different. The lowest concentrations were recorded at a voltage of 230 V, while changes in voltage caused an increase in the NOx concentrations. For a voltage of 207 V, the increase in concentration was 11.7%, and for 253 V, it was 31.2%. The changes in concentrations shown in Figure 7 over time were similar.
Air flow and distribution affect the emission of nitrogen oxides NOx [46,47]. During the tests, the blower fan speed and the amount of air supplied to the combustion chamber increased with the increase in voltage, which resulted in higher NOx emissions. A similar trend in NOx emission changes with the amount of supply air was reported by [39]. Caposciutti et al. [45] showed that excess air in a biomass boiler was the main factor in the formation of CO and NOx.
The graph of oxygen content in exhaust gases (Figure 9) very well reflects the effect of voltage on the rotational speed of the blower fan in the burner. The amount of air supplied to the combustion chamber decreases with the decrease in supply voltage. The lower the voltage value, the less oxygen in the combustion chamber; large fluctuations in the amount of air in the exhaust gases in the range of 3.8 ÷ 7% indicate that combustion was unstable. Studies [40] show similar trends in O2 content changes with varying blower fan speeds.
3.3. Boiler Ignition Test Results
Table 4 presents the ignition times and blower fan speed results for individual supply voltage values.
As shown in Figure 3 and Table 4, the most significant differences in blower fan speeds depending on the supply voltage occurred for 19–40% of the controller setting. The operating settings during ignition were in this range. These differences had a visible effect on the boiler ignition process. During the operation of the device at a voltage of 253 V, problems were already visible at the ignition stage. The blower fan speeds were 1.6 times higher (1170 rpm) than at a voltage of 230 V (720 rpm). Such significant differences caused the airflow through the igniter to be too high and the heated air temperature to be too low to efficiently ignite the starting dose of the fuel. Only at the end of the second stage of ignition did the fuel ignite, which resulted in a long ignition time of 428 s. During ignition at a voltage of 207 V, the revolutions were about 31% lower than at 230 V; the lower revolutions caused the airflow through the igniter to be too low, which resulted in a long ignition time of 206 s. However, the ignition process was completed in the first ignition stage.
4. Discussion
To analyse the obtained results and refer to other works, the concentration values of individual components obtained in the tests were converted into CO, NOx, and dust emissions and related to 10% oxygen content. The results are presented in Table 5.
Changes in the voltage supplying the blower fan affected its rotational speed and the amount of air provided to the combustion chamber. Therefore, with the increase in voltage, the emission of the tested toxic exhaust components increased (Table 5). The emission of CO, NOx, and dust of 207 V and 230 V did not differ from the emission values described in the literature for automatic boilers fuelled with pellet fuel [48,49]. Thus, in most cases, CO emissions in steady-state conditions were below 1500 mg/m3, with the minimum values recorded for O2 concentration in flue gases of around 13%. The NOx emissions in steady-state boiler operation conditions correlated with the excess air coefficient λ and the nitrogen content in pellets [50]. During the combustion of pellets at full load in boilers with automatic ignition, the average dust emissions ranged from 13 to 18 mg/m3.
However, during the tests, for a rated voltage of 253 V, the CO and NOx emissions were twice as high as those described in [51]. The increase in exhaust gas temperature and excess air coefficient resulted in an increase in the CO concentration in exhaust gases and a decrease in the CO2 concentration (Table 5), an unfavourable phenomenon indicating that the combustion process was incomplete. CO is a flammable gas, and in the case of complete combustion, it is burnt to CO2. At the same time, CO2 may dissociate to CO at high temperatures. It should be emphasised, however, that even though the increase in the CO emissions was 145% and 459% of the lowest values, the detected emissions were much lower than the limit value (<500 mg/m3) specified in the standard in question and the Polish government programme “Clean Air” (Table 6). The highest CO emission value detected for a rated voltage of 257 V was less than 25% of the permissible value. The NOx emissions proved to be particularly problematic, as the recorded value exceeded the emission limit (<200 mg/m3) given in the cited guidelines (Table 6). At the same time, this was the only exceedance detected and amounted to 12.5% of the maximum permissible NOx emissions.
As already mentioned, the increase in the voltage supplying the blower fan, and consequently the increase in the blower fan rotational speed, caused an increase in the excess air coefficient λ and the exhaust gas temperature. The exhaust gas temperature reached a value below 150 °C during the tests. It should be remembered that no emission is strongly dependent on temperature for temperatures in a burner higher than 1000 °C [52]. In the conducted tests, the NOx emission increased with the increase in exhaust gas temperature; the lowest average NOx emission was recorded at 207 V, while the highest was at 257 V. In the work in [53], excess air was identified as the main parameter influencing NOx emissions, without a clear effect of the burner design and the height of the combustion chamber.
Deng et al. [54] studied the combustion of pellets in a test boiler with airflow control and found that CO, NO, and dust emissions were different at varying air flow rates. High air flow caused significantly higher CO emissions due to incomplete combustion, while lower airflow rates resulted in lower CO emissions. An increase in the amount of supplied air could have caused the reaction of nitrogen in the fuel with oxygen, resulting in NOx formation, which could explain the increase in NOx emissions with increased airflow. An increase in the airflow rate also caused an increase in dust emissions.
The air staging strategy is widely used to control NOx emissions; its application in small-capacity devices should be carefully considered. Air staging can increase CO and HC emissions. The operating parameters for NOx emission control are excess air ratio, primary and secondary air distribution, residence time, and temperature. Some authors conclude that the optimal primary air ratio is independent of the fuel used in a given technology, and the actual primary air ratio that minimises NOx emissions is a design feature of the boiler [55]. There are possibilities to reduce the emission of toxic exhaust components, but the use of primary measures does not allow for reducing the high NOx emissions for many solid biofuels [55].
With the increased rated voltage, an increase in the emissions of dust was also observed. All the recorded values were lower than the limit value of 20 mg/m3 given in the legal regulations (Table 6). Dust emissions increase with the increase in the blowing force [56]. This is caused by an increase in the amount of oxygen in the exhaust gases, which results in an unfavourable coefficient λ > 2 and, consequently, an unfavourable conversion of the emission values to 10% O2 [57]. The emission of toxic exhaust components with such a high excess air coefficient λ can be compared to the emission of exhaust gases during the lighting and extinguishing of pellet burners. However, in the case of ignition and extinguishing, this process lasts only a few dozen seconds and then stabilises. In the case of wood pellet heating systems, CO (63–95%) and TOC (48–93%) emissions are exceptionally high during the start-up and shut-down phases. On the other hand, NOx and particulate matter emissions dominate during continuous operation. Nevertheless, 30–40% of particulate matter emissions are generated during the start-up and shut-down phases of the boiler [58].
The changes in the boiler supply voltage affected the boiler start-up time. In the case of 253 V, this time was four times longer than in the case of 230 V. This was caused by the need for the controller to repeat the entire ignition process. With a reduced voltage, the start-up time was extended twice. It can be expected that the extended ignition time will affect the exhaust emissions. This study did not measure emissions during ignition because accurate emission measurements during periods of unstable boiler operation require appropriate devices that can measure low exhaust gas flow rates in these phases [59]. However, based on data from the literature, which indicate that boilers have higher start-up emissions [12,33], extended start-up times can be expected to contribute to greater emissions of harmful exhaust gas components.
Two solutions can be implemented to mitigate the impact of line voltage changes on the operation of a low-power boiler. One approach is to install voltage regulators on the blower fan supply line. The second approach is to employ a lambda probe that monitors the oxygen levels in the flue gas [60] and, based on these data, adjusts the blower fan speed. However, it must be considered that, to compete with other heating systems, pellet boilers need to maintain low production costs. Therefore, the air control system should utilise as few sensors as possible, and its control should not entail excessive computational complexity [61].
5. Conclusions
Improving the performance of biomass boilers is essential to increase their efficiency and reduce emissions. The mains voltage significantly influences the operation of components such as fans, which are crucial to the correct combustion process. The impact of the mains voltage on combustion emissions in low-power boilers is important from the point of view of the efficiency of the heating equipment and environmental protection. Given the complexity of combustion, particularly with solid fuels, experimental studies largely underpin the optimisation of this process. The research presented aimed to determine whether alterations in the mains voltage within the lower and upper limits of the EN 60038:2012 standard (230 V ±10%, i.e., 207 V–253 V) impact the operation of automatic low-power pellet boilers.
The test results demonstrated that variations in the mains voltage influenced the fan’s speed in supplying air to the combustion chamber. The boiler controller adjusts the blower fan settings using the recorded values for the nominal voltage (230 V). Therefore, changing the mains voltage affects the volume of air supplied to the burner, which in turn impacts the boiler’s emission factors. At the boiler’s maximum output (25 kW), the blower fan speed at 253 V increased by 17.6%, while at 207 V, it decreased by 20.4% (relative to the blower fan speed at 230 V). These fluctuations led to shifts in the excess air ratio (λ), which was 1.8 at 230 V, while it was 2.1 at increasing voltage and 1.4 at lower voltage.
Changes in the volume of air supplied to the burner also influenced the flue gas temperature. An increase in voltage from 230 V to 253 V raised the flue gas temperature from 131.6 °C to 149.2 °C (+13.4%), while at 207 V, the flue gas temperature decreased to 121.4 °C (−7.8%).
The observed changes in blower fan operation also affected exhaust emissions. During the test at 253 V, an increase in the emissions of the examined exhaust components in the exhaust gas was noted compared to the unit’s operation at 230 V. CO emissions rose from 127 mg/m3 to 225 mg/m3 (77.2%). For NOx, the increase was 31.2% (230 V—77 mg/m3, 253 V—101 mg/m3), while dust emissions grew by 12.5% (230 V—16 mg/m3, 253 V—18 mg/m3). When tested at 207 V, a decrease of 17.3% in CO (149 mg/m3), 11.7% in nitrogen oxides (86 mg/m3), and 18.8% in dust (13 mg/m3) was recorded.
The ignition time of the boiler at both 253 V and 207 V was longer compared to that at 230 V, which was 94 s. At 253 volts, the time was 428 s (a 4.5-fold increase), while at 207 volts, it was 206 s (approximately a 2-fold increase). At higher voltages, excessive airflow causes overcooling of the igniter, making it difficult to ignite the initial dose of fuel. Conversely, at a lower voltage, the longer ignition time resulted from insufficient airflow, which hindered the delivery of adequate thermal energy to ignite the fuel. Nonetheless, the ignition process was successful from the very first phase.
The results of these studies underscore the necessity of adapting boiler designs to shifting voltage conditions, which are occurring much more frequently, particularly in rural and peri-urban areas.
The results presented relate to boiler operation at full load and can only be interpreted within this range. In the presented studies, emissions during the boiler’s ignition and quenching phases, as well as operation at other load levels, have been overlooked. Under actual boiler operating conditions, variations in the mains voltage can lead to changes in emissions linked to boiler operation beyond the full load range. During the ‘summer’ period, characterised by using the boiler solely for domestic hot water preparation, frequent start-up and shut-down phases or minimum load operations occur. As demonstrated, voltage fluctuations significantly impact the boiler controller’s blower fan speed within these operating ranges. Consequently, long-term emission tests are necessary to validate the results presented. Future experiments should also explore different burner types, boiler outputs, or alternative pellets. Extending these studies will aid in identifying the most effective solutions to this issue.
Conceptualisation, J.G. and G.Z.; methodology, J.G.; software, J.G.; validation, J.G. and G.Z.; formal analysis, J.S.-B.; investigation, J.G.; resources, J.G.; data curation, J.G. and G.Z.; writing—original draft preparation, J.G., J.S.-B. and G.Z.; writing—review and editing, J.S.-B. and G.Z.; visualisation, J.G. and G.Z; supervision, G.Z.; project administration, G.Z.; funding acquisition, J.G. All authors have read and agreed to the published version of the manuscript.
The numerical data used to support the findings of this study are available from the corresponding author upon request.
The authors wish to thank the ManageStaff of the ZPH Krzaczek Sp. z o.o. in Klikawa (Stanisław Krzaczek, Paweł Iwaszko, Arkadiusz Berliński) for making this study possible by providing a boiler with a burner and a laboratory for testing heating devices.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Diagram of the tested central heating boiler: 1—combustion chamber, 2—water jacket, 3–combustion ash door, 4—inspection door, 5—cleaning door, 6—smoke tube, 7—flue tube, 8—burner, 9—flue, 10—water supply connector, 11—water return connector, 12—temperature sensor connector, 13—thermal insulation, 14—door screen, 15—fuel feeder, 16—fuel tank, 17—chamotte plates, 18—feeder gear motor, 19—flexible pipe, 20—safety valve connector, 21—water jacket of the combustion chamber, 22—blower fan.
Enlarge this image.Figure 2. Cross-section of the burner with a rotating combustion chamber: 1—blower fan, 2—rotating combustion chamber, 3—fuel feeder.
Enlarge this image.Figure 3. Test stand: 1—tested boiler, 2—weight scale, 3—tubular heat exchanger, 4—Volcano 75 kW cooler with fan, 5—flue gas measuring section, 6—damper with flue gas fan, 7—Testo 330-2 LL analyser, 8—Testo 380 analyser, red line—supply line (hot water), blue line—return line (cold water).
Characteristics of the fuel used in the tests.
| Parameter | Unit | Value |
|---|---|---|
| HHV | MJ/kg | 17.33 ± 0.22 |
| LHV | MJ/kg | 15.84 ± 0.22 |
| Moisture | % | 6.93 ± 0.22 |
| Ash | % | 0.24 ± 0.01 |
| Carbon | % | 44.22 ± 0.24 |
| Hydrogen | % | 6.04 ± 0.20 |
| Nitrogen | % | 0.22 ± 0.04 |
| Sulphur | % | 0.01 ± 0.001 |
| Chlorine | % | 0.017 ± 0.001 |
| Oxygen (calculated for 100%) | % | 42.332 |
Blower fan speeds depending on the controller settings for characteristic boiler operating points.
| Voltage | Minimum Burner Power | Average Burner Power | Maximum Burner Power | Maximum Fan Power |
|---|---|---|---|---|
| 207 V | 670 rpm | 906 rpm | 1242 rpm | 2200 rpm |
| 230 V | 1000 rpm | 1182 rpm | 1508 rpm | 2310 rpm |
| 253 V | 1346 rpm | 1568 rpm | 1816 rpm | 2426 rpm |
Average exhaust gas concentration at 207 V, 230 V, and 253 V during combustion tests.
| Parameter | Symbol | Unit | 207 V | 230 V | 253 V |
|---|---|---|---|---|---|
| Exhaust gas temperature | Tsp | °C | 121.4 | 131.6 | 149.2 |
| Carbon dioxide | CO2 | % | 14.8 | 11.3 | 9.8 |
| Carbon monoxide | CO | ppm | 25 | 46 | 91 |
| Nitrogen oxide | NOx | ppm | 86 | 77 | 101 |
| Air–fuel ratio | λ | – | 1.4 | 1.8 | 2.1 |
| Oxygen | O2 | % | 5.6 | 9.3 | 10.9 |
Ignition time and blower fan speed values during the ignition process for the tested voltage values.
| Voltage | Ignition Time | Blower Fan Speed for | Blower Fan Speed for Second Stage of Ignition (33%) |
|---|---|---|---|
| 207 V | 206 sec | 500 rpm | 906 rpm |
| 230 V | 94 sec | 720 rpm | 1182 rpm |
| 253 V | 428 sec | 1170 rpm | 1568 rpm |
Averaged CO2, CO, NOx, and dust emission results, converted to 10% oxygen for the tested supply voltages.
| Parameter | Symbol | Unit | 207 V | 230 V | 253 V |
|---|---|---|---|---|---|
| Carbon dioxide | CO2 | mg/m3 | 54 | 22 | 123 |
| Carbon monoxide | CO | mg/m3 | 149 | 127 | 225 |
| Nitrogen oxides | NOx | mg/m3 | 86 | 77 | 101 |
| Dust | - | mg/m3 | 13 | 16 | 18 |
Exhaust emission limits according to PN-EN 303-5:2021 and the “Clean Air” Polish government programme.
| Parameter | PN-EN 303-5:2021 Class 5 | “Clean Air” |
|---|---|---|
| CO | <500 mg/m3 | |
| OGC | <20 mg/m3 | |
| NOx | <200 mg/m3 | |
| Efficiency | >87% + log Q | >75% |
| Dust | <40 mg/m3 | <20 mg/m3 |
Q—heating boiler power [kW].
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