Article Operational Testing of a Solid Boiler with Different

Norbert Érces * and László Kajtár

Department of Sociology and Communication, Faculty of Economic and Social Sciences, Budapest University of Technology and Economics, M˝uegyetemrkp 3, 1111 Budapest, Hungary; [email protected] * Correspondence: [email protected]

Abstract: In the course of our investigations, we burned high-quality logs as well as wood in a conventional, manually fed mixed-fired boiler, under different operating parameters. Based on the evaluation of the measurement results, there is a significant difference in terms of recoverable and monoxide emissions for the two fuels burned in the same device at different air supply parameters. Studies have shown that a constantly changing position of the draft control door has an adverse effect on carbon monoxide emissions as well as the energy produced. In the case of a constant draft door setting, the preset values that can be considered ideal for energy yield and CO emissions were determined for the two fuel types. The obtained results were compared with the requirements according to the MSZ EN 303-5 standard.

Keywords: ; CO emission; air pollutant; biomass boiler; flue gas

  1. Introduction Citation: Érces, N.; Kajtár, L. Operational Testing of a Solid Fuel Solid fuel boilers play a key role in pollution across Europe. Although good quality Boiler with Different Fuels. Energies wood burning can be considered as an environmentally conscious way of producing 2021, 14, 2966. https://doi.org/ heat, appropriate emission indicators can only be obtained by using a combination of 10.3390/en14102966 high-quality fuels burned in good quality boilers. As a result of the fragmentation of the economic and infrastructural development characteristics of each country, the use of Academic Editors: Vladislav modern equipment characterizes heating production based on a large number A. Sadykov, Jaroslaw Krzywanski of solid fuel boilers to a small extent. Outdoor causes about 400,000 premature and Rajender Gupta deaths a year, as well as an even higher number of serious illnesses across Europe [1,2]. One of the major emitters of air pollution is household energy consumption. The most Received: 25 March 2021 commonly used heating energy sources are gas burning, as well as wood burning. The Accepted: 10 May 2021 distribution of fuel use without district heating is shown in Table1. Published: 20 May 2021 From the 1990s to the present day, the combined use of gas and solid fuel is very common in the single-family zone. In addition to the above table, in proportions in Publisher’s Note: MDPI stays neutral Hungary, about 45% of dwellings use only natural gas, and 21% use solid fuel (wood, , with regard to jurisdictional claims in or a mixture thereof). A combination of gas heating and a solid fuel boiler is used in 15% of published maps and institutional affil- apartments [3]. iations. Households using solid fuel are highly concentrated in terms of territory, where it is worth mentioning that the distribution is strongly dependent on the socio–economic and infrastructural development of the given region. In 19 districts, more than 50% of the dwellings are exclusively heated by wood. In further 22 districts, 75% of the dwellings Copyright: © 2021 by the authors. are at least partly heated by wood. Although wood burning is a CO2-neutral burn with Licensee MDPI, Basel, Switzerland. renewable energy, it emits significant emissions under inappropriate conditions [2,4]. This article is an open access article For each solid fuel appliance, the standard MSZ EN 303-5 defines clear requirements distributed under the terms and in terms of efficiency and emissions (among other requirements), but the fulfillment of conditions of the Creative Commons these parameters is true when determined, specific laboratory conditions, professional Attribution (CC BY) license (https:// operation, and last but not least, strict fuel quality requirements are provided and met. creativecommons.org/licenses/by/ This follows from the socio–economic and infrastructural dependence mentioned above 4.0/).

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Energies 2021, 14, 2966 that the emissions from solid fuels mainly depend on the operating equipment and2 of the 10 quality of the fuel burned in the equipment. Based on Danish data from 2016, the specific particulate emissions of some heating modes are shown in Figure 1. that the emissions from solid fuels mainly depend on the operating equipment and the qualityTable of1. theUse fuelof fuel burned in inhabited in the dwellings equipment. in Hungary Based on (2011) Danish. data from 2016, the specific particulate emissionsNumber of some of Dwellings heating modes areProportion shown in Figureof Dwellings1. as a % of Total Fuel (Thousands) Inhabited Dwellings Table 1. Use of fuel in inhabited dwellings in Hungary (2011). Gas 2388 61.96 FuelCoal Number of Dwellings (Thousands)113 Proportion of Dwellings as a % of Total 2.93 Inhabited Dwellings GasElectricity 238876 61.96 1.97 CoalOil fuel 113 1 2.93 0.03 76 1.97 Oil fuelWood 11470 0.03 38.14 WoodSolar energy 1470 5 38.14 0.13 Solar energy 5 0.13 GeothermalGeothermal energy energy 3 3 0.08 0.08 PelletsPellets 22 0.05 0.05 OtherOther renewable renewable 3 3 0.08 0.08 Other fuel 4 0.10 All inhabitedOther dwellings fuel 3854 4 100.00 0.10 All inhabited dwellings 3854 100.00

Particalate Emissions (g/GJ) 930 900 740

413 270 155 206 29 1 1.3 5.5 0.05 1.3

FigureFigure 1.1.Particulate Particulate emissionsemissions ofof differentdifferent heating heating methods methods in in Denmark Denmark [ 1[1].].

BasedBased onon FigureFigure1 ,1, it it can can be be observed observed that that solid solid fuel fuel appliances, appliances, which which can can be be consid- consid- eredered obsolete,obsolete, havehave outstandingoutstanding emissionemission values.values. ByBy comparison,comparison, ananold old wood-burning wood-burning stovestove at at the the end end of of the the line line emits emits 715 715 times times more more pollutants pollutants than than the the PM2.5 PM2.5 dust dust emission emission of aof more a more than than ten-year-old ten-year-old truck; truck; however, howeve evenr, theeven environmentally the environmentally conscious conscious pellet boilerpellet givesboiler more gives than more 22 than times 22 the times value the [ 1value,5]. [1,5]. SeveralSeveral international studies studies have have been been conducted conducted on on the the combustion combustion of modern of modern pel- pelletslets or orwood wood chips chips for for boilers boilers of of household household size, size, or or with with a a nominal nominal power power of up to 50 kW. InIn thethe casecase ofof twotwo typestypes ofof pine-basedpine-based ,woodchips, it it was was shown shown that that increasing increasing the the excess excess airair factorfactor reducedreduced thethe emissionemission ofof pollutants,pollutants, butbut alsoalso reduced reduced the the maximum maximum extractable extractable performanceperformance [ 6[6].]. WhenWhen usingusing pelletpellet fuel,fuel, thethe recoverablerecoverable power power is is higher higher and and the the series series of of requirementsrequirements according according to to EN EN 14,785 14,785 can can be be met met [ 7[7].]. TakingTaking into into account account the the socio–economicsocio–economic andand infrastructuralinfrastructural developmentdevelopment ofof thethe Hun-Hun- gariangarian regions,regions, as as wellwell asas thethe reductionreduction inin thethe necessarynecessary environmentalenvironmental load, load, we weexamined examined aa conventional,conventional, manuallymanually fed,fed, household-sizedhousehold-sized solid-firedsolid-fired boilerboiler inin termsterms ofof extractable extractable outputoutput andand pollutantpollutant emissions.emissions.

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2. Operational Characteristics 2. OperationalEven with Characteristics conventional appliances, the amount of primary and secondary combustion air hasEven a significantwith conventional effect onappliances, the combustion the amount processes of primary in the and boiler secondary [8]. In combus- the case of open tionheating air has appliances a significant according effect on the to MSZcombustion EN 303-5, processes the requirements in the boiler [8]. according In the case to of EN 14,597 openmust heating be met: appliances according to MSZ EN 303-5, the requirements according to EN 14,597• Equipped must be met: with a temperature controller, • • EquippedEquipped with with a temperature safety temperature controller, limiter. • Equipped with safety temperature limiter. The safety temperature limiter may be omitted if the device cannot be switched off The safety temperature limiter may be omitted if the device cannot be switched off and the excess heat energy can be dissipated in the form of steam due to the connection to and the excess heat energy can be dissipated in the form of steam due to the connection the atmosphere. In most cases, manual dosing open heating appliances used in households to the atmosphere. In most cases, manual dosing open heating appliances used in house- holdsare not are connected not connected to a to heating a heating buffer buffer tank tank but but operate operate with with a a temperature temperature controlcontrol valve [9]. valveThe primary[9]. The primary purpose purpose of the of temperature the temperat controllerure controller is to is maximizeto maximize the the temperature tempera- of the tureheating of the medium heatingproduced medium produced by the boiler. by th Duringe boiler. operation, During operation, a valve withouta valve without auxiliary energy auxiliarycontrols energy the opening controls angle the opening of the draft angle control of the door,draft control depending door,on depending the power on thatthe changes powercontinuously that changes during continuously firing. Continuous during firing. intervention Continuous has intervention a significant has a effect significant on the quality effectof the on combustion the quality of process the combustion in the firebox, process and in the thus firebox, on the and emission thus on the of harmfulemission substances.of harmfulIn substances. the course of our laboratory measurements, we examined the operating characteris- ticsIn of the a solid course fuel of our boiler laboratory equipped measurem with aents, temperature we examined controller, the operating as well character- as the operating isticsparameters of a solid occurring fuel boiler duringequipped the with firing a temperature of different controller, fuel charges as well atas the specific operating draft control parametersdoor opening occurring angles. during the firing of different fuel charges at specific draft control door opening angles. 3. Measurement Procedure 3. Measurement Procedure Prior to the actual measurements, a load was burned in the boiler to eliminate the Prior to the actual measurements, a load was burned in the boiler to eliminate the errorserrors from from the the cold cold start, start, to toform form suitable suitable embers, embers, and to and warm to warm up our up system our system to operat- to operating ingtemperature temperature [10 [10].]. Our Our examined examined system system operatedoperated on the basis of of the the arrangement arrangement shown shownin Figure in Figure2. After 2. After preheating, preheating, 7.2 kg7.2 ofkg fuel of fuel was was uniformly uniformly loaded loaded through through the - firebox door boxshown door in shown the figure. in the figure. During During the tests, the tests, the totalthe total combustion combustion period period of of the the loadedloaded fuel was fuelmonitored was monitored in each in case. each Thecase. measured The measured parameters parameters are are shown shown in in Table Table2 .2.

Figure 2. Geometrical parameterization of draft control door (*: multiplication). Figure 2. Geometrical parameterization of draft control door. (*: multiplication) Table 2. Measured parameters. Table 2. Measured parameters. Sign of Measured Parameter UnitName of Measured Name of Measured Parameter Sign of Measured Parameter Unit O2 %Parameter Oxygen content of flue gas CO2 % Carbon dioxide content of flue gas O2 % Oxygen content of flue gas CO ppm Carbon monoxide content of flue gas CO2 NOx % ppm Carbon dioxide content Nitrogen of flue oxide gas content of flue gas CO SO2 ppm Carbonppm monoxide content Sulfur dioxideof flue gas content of flue gas ∆pchimney Pa Chimney draft NOx ppm ◦ Nitrogen oxide content of flue gas tfg C Combustion product temperature SO2 ppm Sulfur dioxide content of flue gas λ - Excess air factor qA % Combustion product loss mvíz L/min Heating medium mass flow ◦ tfw C Flow temperature ◦ tr C Returning medium temperature Energies 2021, 14, 2966 4 of 10

Different operations were performed for the cases without a working draft regulator (temperature controller) and without a draft regulator with different fixed draft door settings, and the effect of different fuel loads was also measured for fixed primary air supply cases. In the various measurement studies the cases according to Table3 were performed. In order to clearly define the opening of the draft control door of the device, a flow rate must be determined, which can be determined from the quotient of the free-flow cross section resulting from the opening of the door and the nominal free cross section, as shown in Figure2. Figure3 shows a schematic arrangement of the measuring station.

Table 3. Cases examined.

Fuel Mass Primary Air Control Door Operation Notation with draft controller 1st case

Wood 7.2 kg Cdraft = 0.093 Cdraft = 0.275 2nd case Cdraft = 0.440

Cdraft = 0.093 Energies 2021, 14, x FOR PEER REVIEW 7 kg Cdraft = 0.275 3rd case5 of 10

Cdraft = 0.440

Figure 3. Schematic arrangement of measuring station.

4. MeasurementGeneral geometric Results definition of free-flow cross section: Among the measured parameters according to Table 2, the development of carbon  1   1   1  monoxide emissions,Acs = L × which2 × His ×of sinkey importance∝ + H ×accordingsin ∝ to× theH MSZ× cos EN 303-5∝ standard, 2 2 2 was included among the primary pollutant components to be examined. In addition to the evolutionFrom the of quotient emissions, of the our free-flow important cross goal section was andto be the able nominal to extract cross the section, highest the possi- flow blerate energy for the yield draft from control the door device can while be determined: reducing emissions.

Acs 4.1. Evaluation of Case 1 Cdra f t = An In case 1, according to Table 3, dry logs with a moisture content not exceeding 15% where:were burned while the primary air door of the boiler was moved by an automatic draft control device. According to the aforementioned MSZ EN 303-5 standard, a maximum • Cdraft—flow number, permissible carbon monoxide emission of 5000 mg/m3, which means 4000 ppm in the case • Acs—the free-flow cross section, of CO, is allowed for solid fuel equipment not exceeding 50 kW and equipped with an • An—nominal flow cross section (An = H × L). automatic dosing system. The determined volume ratio (ppm) is converted to a mass flow In the case of the tested boiler: value (mg/m3). The following values apply as a conversion factor for conversion from ppm • H to mg/m=3: 14 fCO cm, = 1.25 [9]. Carbon monoxide emissions must be checked for the average value released• L = 12during cm. complete combustion. However, it is worth observing the evolution of CO released during the entire firing interval, as well as the recoverable power values shown in Figures 4 and 5.

25 0.25 Flow number Q

20 0.2

15 0.15

10Q (kW) 0.1 flow number([-)

5 0.05

0 0 0 20406080100 time (min)

Figure 4. Development of Q at different flow rates over the whole period.

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4. Measurement Results Among the measured parameters according to Table2, the development of carbon monoxide emissions, which is of key importance according to the MSZ EN 303-5 standard, was included among the primary pollutant components to be examined. In addition to the evolution of emissions, our important goal was to be able to extract the highest possible energy yield from the device while reducing emissions.

4.1. Evaluation of Case 1 In case 1, according to Table3, dry logs with a moisture content not exceeding 15% were burned while the primary air door of the boiler was moved by an automatic draft control device. According to the aforementioned MSZ EN 303-5 standard, a maximum

permissible carbon monoxide emission of 5000 mg/m3, which means 4000 ppm in the caseFigure of 3. CO, Schematic is allowed arrangement for solid of fuel measuring equipment station. not exceeding 50 kW and equipped with an automatic dosing system. The determined volume ratio (ppm) is converted to a mass flow4. Measurement value (mg/m Results3). The following values apply as a conversion factor for conversion 3 from ppmAmong to mg/mthe measured: fCO = 1.25parameters [9]. Carbon according monoxide to Table emissions 2, the must development be checked of forcarbon the averagemonoxide value emissions, released which during is completeof key importance combustion. according However, to the it MSZ is worth EN 303-5 observing standard, the evolutionwas included of CO among released the during primary the pollutant entire firing comp interval,onents as to well be asexamined. the recoverable In addition power to valuesthe evolution shown inof Figuresemissions,4 and our5. important goal was to be able to extract the highest possi- ble energyIn Figures yield4 and from5, it the can device be observed while thatreducing the automatic emissions. draft control door continuously reduces the flow rate in parallel with the increase in power (Q), and at the same time the CO4.1. emission Evaluation also of Case increases. 1 As it can be observed, under the construction firing stage, Q increases,In case but 1, CO according decreases. to AtTable this 3, interval, dry logs the with system a moisture approaches content the perfect not exceeding combustion 15% process,were burned but at while the same the time,primary it reaches air door the of set the maximum boiler was temperature, moved by an which automatic causes draft the draftcontrol regulator device. toAccording close. When to the the aforementioned load in the firebox MSZ enters EN 303-5 the decliningstandard, section,a maximum the controlpermissible device carbon begins monoxide to open emission the primary of 5000 air doormg/m to3, which maintain means the 4000 temperature ppm in the set case on theof CO, draft is regulator. allowed for The solid minimum fuel equipmen flow ratet ofnot almost exceeding 25 min 50 iskW due and to theequipped fact that, with due an toautomatic safe operation, dosing asystem. minimum The determined amount of combustionvolume ratio air (ppm) must is be converted provided to even a mass in flow the eventvalue of(mg/m a complete3). The following shutdown, values which apply means as a conv flowersion rate of factor 0.093 for in co thisnversion case. It from can alsoppm be observed that in the initial, developing phase of combustion, the instantaneous CO to mg/m3: fCO = 1.25 [9]. Carbon monoxide emissions must be checked for the average value emissionsreleased during increase complete sharply combustion. at the same timeHoweve as ther, it draft is worth control observing door is closed.the evolution For the of entireCO released firing time during interval, the entire the average firing interv CO emissional, as well was as 5973 the ppm,recoverable which power is more values than 1600 ppm higher than the limit allowed by the standard. shown in Figures 4 and 5.

25 0.25 Flow number Q

20 0.2

15 0.15

10Q (kW) 0.1 flow number([-)

5 0.05

0 0 0 20406080100 time (min)

FigureFigure 4. 4.Development Development of of Q Q at at different different flow flow rates rates over over the the whole whole period. period.

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FigureFigure 5. 5. DevelopmentDevelopment of of CO CO at at different different flow flow rates rates over over the the whole whole period period.. 4.2. Evaluation of Case 2 In Figures 4 and 5, it can be observed that the automatic draft control door continu- ouslyIt reduces can be the clearly flow seenrate in from parallel Figure with6 that the increase with a constant in power high-flow (Q), and at rate, the same the firing time theprocess CO emission takes place also inincreases. a short As time, it can and be the observed, developing under phase the construction is followed firing by a stage, rapid Qdeclining increases, phase. but CO In decreases. the case of At an this intermediate interval, the flow system rate, approaches the burn-out the time perfect increased com- bustionby almost process, one hour, but at and the the same developing time, it reaches phase was the characterizedset maximum bytemperature, a nearly constant which causespeak power the draft lasting regulator 10 min. to close. The When declining the phaseload in was the firebox prolonged enters in time.the declining With a sec- low tion,flow the rate, control the burn-out device begins time also to open lengthens, the primary but the air maximum door to maintain recoverable the power temperature is well setbelow on the the draft value regulator. of the previous The minimum setting parameter. flow rate of Compared almost 25 to min the is recoverable due to the fact power that, of Energies 2021, 14, x FOR PEER REVIEWFigure 4, the maximum recovered power was also higher. Figure7 shows the carbon7 of 10 due to safe operation, a minimum amount of combustion air must be provided even in monoxide emission values for the entire combustion stage at the flow rates described above. the event of a complete shutdown, which means a flow rate of 0.093 in this case. It can also be observed that in the initial, developing phase of combustion, the instantaneous CO emissions increase sharply at the same time as the draft control door is closed. For the 0.44 0.09 0.27 entire firing time interval, the average CO emission was 5973 ppm, which is more than 1600 ppm30 higher than the limit allowed by the standard. 25 4.2. Evaluation of Case 2 20 It can be clearly seen from Figure 6 that with a constant high-flow rate, the firing process15 takes place in a short time, and the developing phase is followed by a rapid de- Q (KW) clining 10phase. In the case of an intermediate flow rate, the burn-out time increased by almost one hour, and the developing phase was characterized by a nearly constant peak power lasting5 10 min. The declining phase was prolonged in time. With a low flow rate, the burn-out0 time also lengthens, but the maximum recoverable power is well below the value of the0 previous 102030405060708090 setting parameter. Compared to the recoverable power of Figure 4, the maximum recovered power was alsoTIME higher. (MIN) Figure 7 shows the carbon monoxide emission values for the entire combustion stage at the flow rates described above. FigureFigure 6. 6.Evolution Evolution of of the the yielded yielded power power at at different different constant constant flow flow rates. rates.

Figure 7. CO emission evolution for each flow rate.

The solid horizontal line indicates the permissible CO emission value according to the MSZ EN 303-5 standard. It can be observed that at the highest flow rate, the equipment operates above the permissible emission limit for almost the entire firing time. The air inlet resistance of the appliance is the lowest in this case, so the temperature of the flue gas, and at the same time, the draft in the chimney increase due to the rising temperature of the firebox. As a result of the combined effect of these phenomena, the amount of com- bustion air entering the firebox exceeds the amount required for ideal combustion, which results in poorer quality combustion and thus higher CO emissions. In the case of an in- termediate draft control door position, a monotonically increasing CO evolution is ob- served in the developing phase of the firebox; however, after the maximum output and ideal combustion at this preset, CO formation drops drastically and briefly exceeds the standard limit in the burnout phase. At the lowest flow rate, the CO emission takes on a similar character to the previous setting value, but higher carbon monoxide emission val- ues are typically observed over the time of total combustion. The average CO emission values obtained for each flow rate are given in Table 4.

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0.44 0.09 0.27

30 25 20 15 Q (KW) 10 5 0 0 102030405060708090 TIME (MIN) Energies 2021, 14, 2966 7 of 10 Figure 6. Evolution of the yielded power at different constant flow rates.

Figure 7. CO emission evolution for each flow flow rate.

The solid horizontalhorizontal lineline indicates indicates the the permissible permissible CO CO emission emission value value according according to the to theMSZ MSZ EN EN 303-5 303-5 standard. standard. It canIt can be be observed observed that that at at the the highest highest flow flowrate, rate, thethe equipmentequipment operates aboveabove thethe permissible permissible emission emission limit limit for for almost almost the the entire entire firing firing time. time. The airThe inlet air inletresistance resistance of the of appliance the appliance is the lowestis the lowest in this case,in this so case, the temperature so the temperature of the flue of gas,the flue and gas,at the and same at the time, same the time, draft the in draft the chimney in the ch increaseimney increase due to thedue risingto the temperaturerising temperature of the offirebox. the firebox. As a result As a result of the combinedof the combined effect ofeffect these of phenomena,these phenomena, the amount the amount of combustion of com- bustionair entering air entering the firebox the exceeds firebox theexceeds amount the requiredamount required for ideal combustion,for ideal combustion, which results which in resultspoorer in quality poorer combustion quality combustion and thus higherand thus CO higher emissions. CO emissions. In the case In of the an case intermediate of an in- termediatedraft control draft door control position, door a monotonicallyposition, a monotonically increasing CO increasing evolution CO is evolution observed inis theob- developing phase of the firebox; however, after the maximum output and ideal combustion served in the developing phase of the firebox; however, after the maximum output and at this preset, CO formation drops drastically and briefly exceeds the standard limit in the ideal combustion at this preset, CO formation drops drastically and briefly exceeds the burnout phase. At the lowest flow rate, the CO emission takes on a similar character to the standard limit in the burnout phase. At the lowest flow rate, the CO emission takes on a previous setting value, but higher carbon monoxide emission values are typically observed similar character to the previous setting value, but higher carbon monoxide emission val- over the time of total combustion. ues are typically observed over the time of total combustion. The average CO emission values obtained for each flow rate are given in Table4. The average CO emission values obtained for each flow rate are given in Table 4. Table 4. Average CO emission.

CO Average Difference CO Operation avg max (ppm) (ppm) Draft crtl. 5973.03 1606.96 Cdraft = 0.09 4017.14 −348.93 Cdraft = 0.27 3368.54 −997.53 Cdraft = 0.44 4879.00 512.93

Thus, it can be stated that the automatic draft control is the most unfavorable in terms of carbon monoxide production, while the draft control door with a constant value of 0.27 flow rate is the most favorable. On average, a reduction in CO emissions of more than 2600 ppm can be achieved, which is almost half that of the permissible average CO emission limit. In the case of Figure8, the excess air factor can be observed under different drafts, and in the case of the draft regulator door. At 0.27 flow rates, it is experienced for the longest time, a nearly constant value, for which the control also reflects other parameters of firing. Energies 2021, 14, x FOR PEER REVIEW 8 of 10

Table 4. Average CO emission.

COavg Average Difference COmax Operation (ppm) (ppm) Draft crtl. 5973.03 1606.96 Cdraft = 0.09 4017.14 −348.93 Cdraft = 0.27 3368.54 −997.53 Cdraft = 0.44 4879.00 512.93

Thus, it can be stated that the automatic draft control is the most unfavorable in terms of carbon monoxide production, while the draft control door with a constant value of 0.27 flow rate is the most favorable. On average, a reduction in CO emissions of more than 2600 ppm can be achieved, which is almost half that of the permissible average CO emis- Energies 2021, 14, 2966 sion limit. 8 of 10 In the case of Figure 8, the excess air factor can be observed under different drafts, and in the case of the draft regulator door. At 0.27 flow rates, it is experienced for the longest time, a nearly constant value, for which the control also reflects other parameters Atof 0.09firing. and At 0.44, 0.09 theand excess 0.44, the air factorexcess valuesair factor rise values steeply, rise reflecting steeply, rapid reflecting burnout rapid and burnout a 21% increaseand a 21% in oxygenincrease levels. in oxygen levels.

Excess air factor under different drafts

14 12 10 8 0.44 0.09 6 0.27 4 Excess air factor (-) air factor Excess draft regulator 2 0 0 20406080 Time (min)

Figure 8. Excess air factor under different drafts. Figure 8. Excess air factor under different drafts.

4.3.4.3. EvaluationEvaluation ofof CaseCase 33 InIn casecase 3,3, thethe procedureprocedure waswas thethe samesame asas before.before. ForFor thethe threethree flowflow rates, rates, the the carbon carbon monoxidemonoxide emissionemission andand energyenergy yieldyield valuesvalues shownshownin in Figures Figures9 9and and 10 10 were were obtained. obtained. It can be observed that when burning with briquette fuel, the CO emissions can meet the maximum permissible average carbon monoxide emission limit value indicated by the dashed line at any preset value. In the case of briquettes, we obtained the lowest emission value with a flow rate of 0.27, which is almost half of the value compared to log firing. However, in the case of wood burning, the average energy yield is 17.1 kWh, compared to 14.5 kWh obtained for briquettes. However, in the case of briquettes in the operating state belonging to the maximum opening, a higher energy yield of 16.1 kWh was obtained, with a minimum increase in carbon monoxide emissions. An outstanding difference compared Energies 2021, 14, x FOR PEER REVIEWto log burning was that in the case of the CO emission limit value that is met even at9 of the 10

lowest flow rate, we achieved almost twice the energy yield in the case of briquettes.

5000.00

4000.00

3000.00

2000.00 Wood CO (ppm) Briquette 1000.00

0.00 Cdraft=0.09 Cdraft=0.27 Cdraft=0.44 Flow number

FigureFigure 9. 9.Average Average CO CO emissions emissions for for different different fuels. fuels.

35.00

30.00

25.00

20.00

15.00 Wood E (kWh) E 10.00 Briquette

5.00

0.00 Cdraft=0.09 Cdraft=0.27 Cdraft=0.44 Flow number

Figure 10. Average energy yields for different fuels.

It can be observed that when burning with briquette fuel, the CO emissions can meet the maximum permissible average carbon monoxide emission limit value indicated by the dashed line at any preset value. In the case of briquettes, we obtained the lowest emission value with a flow rate of 0.27, which is almost half of the value compared to log firing. However, in the case of wood burning, the average energy yield is 17.1 kWh, compared to 14.5 kWh obtained for briquettes. However, in the case of briquettes in the operating state belonging to the maximum opening, a higher energy yield of 16.1 kWh was obtained, with a minimum increase in carbon monoxide emissions. An outstanding difference com- pared to log burning was that in the case of the CO emission limit value that is met even at the lowest flow rate, we achieved almost twice the energy yield in the case of briquettes.

5. Summary In the course of our investigations, we performed the operational tests of a mixed- fired boiler for use in detached houses. During the tests, the flow rate characteristic of the draft control door was determined, with the help of which the operating parameters oc- curring during the operation of the device were measured at different presetting values. Seven separate cases were examined with two fuels. In the first case, the effect of a draft control door continuously controlled by the temperature limiter was analyzed in the case of log firing.

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5000.00

4000.00

3000.00

2000.00 Wood CO (ppm) Briquette 1000.00

0.00 Cdraft=0.09 Cdraft=0.27 Cdraft=0.44 Flow number Energies 2021, 14, 2966 9 of 10

Figure 9. Average CO emissions for different fuels.

35.00

30.00

25.00

20.00

15.00 Wood E (kWh) E 10.00 Briquette

5.00

0.00 Cdraft=0.09 Cdraft=0.27 Cdraft=0.44 Flow number

FigureFigure 10.10. AverageAverage energyenergy yieldsyields for for different different fuels. fuels.

5. SummaryIt can be observed that when burning with briquette fuel, the CO emissions can meet the maximumIn the course permissible of our investigations, average carbon we monoxi performedde emission the operational limit value tests indicated of a mixed- by the fireddashed boiler line forat any use preset in detached value. houses.In the case During of briquettes, the tests, we the obtained flow rate the characteristic lowest emission of thevalue draft with control a flow door rate was of 0.27, determined, which is with almost the half help of of the which value the compared operating to parameters log firing. occurringHowever, during in the thecase operation of wood ofburning, the device the wereaverage measured energy atyield different is 17.1 presetting kWh, compared values. Sevento 14.5 separate kWh obtained cases were for briquettes. examined withHowever, two fuels. in the In case the firstof briquettes case, the in effect the ofoperating a draft controlstate belonging door continuously to the maximum controlled opening, by the a hi temperaturegher energy limiter yield of was 16.1 analyzed kWh was in obtained, the case ofwith log a firing. minimum increase in carbon monoxide emissions. An outstanding difference com- paredIt to can log be burning stated fromwas that the in measurement the case of the results CO emission that this limit type va oflue regulation that is met has even an unfavorableat the lowest effectflow rate, on thewe achieved carbon monoxide almost twice emission the energy values yield of in the the device case of and briquettes. on the recoverable energy yield, and therefore it cannot be considered as an optimal solution from the5. Summary point of view of environmental protection and energy consumption. Subsequently,In the course of in our the investigations, case of logs and we briquettes, performed the the recoverable operational energy tests of yield a mixed- and thefired carbon boiler monoxide for use in emissiondetached werehouses. examined During the at three tests, different the flow constantrate characteristic flow rates. of We the founddraft control that, with door the was exception determined, of one with case, the help CO emission of which limits the operating specified inparameters the relevant oc- standardcurring during for a permanent the operation draft of control the device door canwere be measured met at a higher at different energy presetting yield than values. in the caseSeven of separate continuous cases draft were control. examined with two fuels. In the first case, the effect of a draft controlIn the door case continuously of log burning, controlled higher by CO the emissions temperature were limiter achieved was with analyzed all tested in the presets case thanof log in firing. the case of briquette burning. When burning briquettes, we obtain the highest energy yield with a low flow rate and carbon monoxide emissions within the limit value. The effect of the draft regulator on dust forms a further part of our study, which is one of the main pollutants in solid fuel equipment. It is more technically complicated due to the difficult implementation of isokinetic sampling.

Author Contributions: All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by NRDI Fund (TKP2020 IES, Grant No. BME-IE-MISC) based on the charter of bolster issued by the NRDI Office under the auspices of the Ministry for Innovation and Technology. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available on request ([email protected]). Conflicts of Interest: The authors declare no conflict of interest. Energies 2021, 14, 2966 10 of 10

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