Influence of Air Infiltration on Combustion Process Changes in A

processes

Article Inﬂuence of Air Inﬁltration on Combustion Process Changes in a Rotary Tilting Furnace

Róbert Dzur ˇnák 1,* , Augustin Varga 1, Gustáv Jablonský 1 , Miroslav Variny 2 ,Réne Atyaﬁ 1, Ladislav Lukáˇc 1 , Marcel Pástor 1 and Ján Kizek 3

1 Department of Thermal Technology and Gas Industry, Institute of Metallurgy, Faculty of Materials, Metallurgy and Recycling, Technical University of Kosice, Letna 9, 042 00 Kosice, Slovakia; [email protected] (A.V.); [email protected] (G.J.); rene.atyaﬁ@tuke.sk (R.A.); [email protected] (L.L.); [email protected] (M.P.) 2 Department of Chemical and Biochemical Engineering, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinskeho 9, 812 37 Bratislava, Slovakia; [email protected] 3 Department of Process Technique, Faculty of Manufacturing Technologies of the TU of Kosice with a Seat in Presov, Technical University of Kosice, Sturova 31, 080 01 Presov, Slovakia; [email protected] * Correspondence: [email protected]

Received: 24 September 2020; Accepted: 13 October 2020; Published: 15 October 2020

Abstract: Air infiltration into the combustion chambers of industrial furnaces is an unwanted phenomenon causing loss of thermal efficiency, fuel consumption increase, and the subsequent increase in operating costs. In this study, a novel design for a rotary tilting furnace door with improved construction features is proposed and tested experimentally in a laboratory-scale furnace, aimed at air infiltration rate reduction by decreasing the gap width between the static furnace door and the rotating body. Temperatures in the combustion chamber and oxygen content in the dry flue gas were measured to document changes in the combustion process with the varying gap width. Volumetric flow values of infiltrating air calculated based on measured data agree well with results of numerical simulations performed in ANSYS and with the reference calculation procedure used in relevant literature. An achievable air infiltration reduction of up to 50% translates into fuel savings of around 1.79 to 12% of total natural gas consumption of the laboratory-scale furnace. The average natural gas consumption increase of around 1.6% due to air infiltration into industrial-scale furnaces can thus likewise be halved, representing fuel savings of almost 0.3 m3 per ton of charge.

Keywords: air inﬁltration; rotary furnace; combustion; energy savings; thermal eﬃciency

1. Introduction Reduction of the carbon footprint of industry is a primary objective of the European Union [1,2]. Energy efficiency, one of the pillars of the EU Energy Union strategy, has therefore been proposed as a solution, namely as a highly effective pathway to improve the economic competitiveness and sustainability of the European economy. Energy efficiency can help reduce the reliance on imports of fossil fuels, thereby bolstering energy security in the short- as well as long-term in a cost-effective way [3,4]. Currently, the metallurgical industry is one of the largest emitters of pollutants [5–8]. One of the possibilities of reducing the release of emissions is to increase the efficiency of combustion units, which leads to a decrease in fuel consumption [9]. Oxy-combustion technology or combustion at elevated oxygen concentrations in the oxidizing agent has been proposed as a novel and promising means to achieve this objective [10]. This type of combustion is referred to as oxygen-enhanced combustion (OEC) and has many benefits including increased processing rates [11], higher heat transfer efficiency [12,13], improved flame characteristics [14], reduced production of emissions [15], reduced

Processes 2020, 8, 1292; doi:10.3390/pr8101292 www.mdpi.com/journal/processes Processes 2020, 8, 1292 2 of 16 equipment cost, and last but not least, improved product quality. It also represents an interesting route for decarbonization via carbon capture and storage [16,17]. OEC technology is most often applied in rotary tilting furnaces used in the processing of nonmetallic materials [18,19]. However, examples of how the iron and steelmaking industry can benefit from this technology as well have been recently summed up in [17]. Individual OEC technology applications in this industrial sector, related either to the proposed continuous scrap-melting rotary furnace, potentially replacing the electric arc furnace, or flameless oxy-fuel burners, are analyzed in detail in [20–22]. The rotary tilting furnace charge consists of a mixture of nonferrous wastes and dross, mostly based on aluminum and alumina [23]. The dross comes from primary aluminum production using bauxite. The processing of aluminum wastes using OEC technology has been published in more detail in studies [24,25]. Rotary tilting furnaces are characterized by having both the burner and the flue gas duct located in the door of the furnace, which remains static as the furnace body rotates [26]. To remove the flue gas, an exhaust fan is needed, which creates negative pressure in the combustion chamber. This causes the ambient air to infiltrate into the chamber, which significantly affects the combustion and heating of the charge in rotary tilting furnaces [27]. When OEC technology is used, the amount of penetrating air (79% vol. N2) increases the nitrogen concentration in the combustion chamber, thereby removing heat from the process and decreasing the combustion temperatures reached in the furnace [28,29]. Excessive air infiltration can, therefore, significantly lessen the advantages of using OEC technology and decrease the thermal efficiency of these furnaces. On the other hand, oxygen content of the infiltrating air causes an increase in the oxygen concentration in the combustion chamber. Therefore, the charge material (aluminum) tends to react with the flue gas components [30]. The main reason for this is the high affinity of aluminum for oxygen. The surface of solid and liquid aluminum is always covered with an oxidation layer, which serves as protection against further oxidation. The chemical reaction in which aluminum reacts with oxygen to form alumina (Al2O3) is a self-propagating reaction [31]. Its initiation can be prevented by decreasing the oxygen content in the flue gas, which means regulating combustion with a low excess of the oxidizing agent and reducing air penetration into the furnace. Prevention of air infiltration into the furnace in the furnace design stage is difficult due to the rotary movement of the kiln putting strain on the static door and leaving it with an improperly sealed opening [32]. A gap is gradually created through which air increasingly penetrates into the combustion chamber. The amount of air getting into the combustion chamber is influenced by the pressure conditions inside the chamber and by the width of the gap through which the air infiltrates. The amount of air penetrating into the combustion chamber can be estimated using the Bernoulli equation. In their studies, Varga [33] and Baukal [34] describe the method of calculating the amount of air getting into the combustion chamber and the associated energy required to heat the penetrating air up to the temperature of flue gas. The physical measurement of the amount of infiltrating air and the definition of its impact on the combustion process under real conditions are difficult. Analyzing the amount of infiltrating air is possible only on the basis of flue gas composition and/or combustion control, by means of which the analysis of dry flue gas (O2, CO2, and CO) can determine the amount of excess combustion air. At present, there is very little literature available that deals with the impact of air infiltration on heat transfer and charge heating in rotary tilting furnaces. The above-mentioned consequences of air penetration result in changes in the temperature field distribution inside the combustion chamber. Consequently, the charge heating process is significantly prolonged and its thermal efficiency is reduced. Attention should therefore be paid to this issue when evaluating the overall effect of air infiltration into furnace chambers. A solution to this issue lies in the modification of the furnace door design, which can reduce air infiltration through better sealing of the door opening, thus decreasing the gap through which air infiltrates into the furnace. In this way, a contribution can be made to achieving greater efficiency of oxygen use in furnaces, leading to an increased technological process efficiency and Processes 2020, 8, x FOR PEER REVIEW 3 of 17

Processes 2020, 8, 1292 3 of 16 cal process efficiency and reduced greenhouse-gas emissions due to fuel savings. The authors of this article designed and created an experimental laboratory-scale model of a rotary tilting furnace reducedwith a novel greenhouse-gas design of emissionsthe furnace due door to fuel to investigate savings. The the authors impact of of this air article infiltration designed on andthe combus- created antion experimental process and laboratory-scale the resulting fuel model consumption. of a rotary tiltingOxygen-enhanced furnace with combustion a novel design was of applied the furnace with dooroxygen to investigateconcentrations the impactranging of from air infiltration21 to 35% vo onl. The the combustioncomputational process fluid anddynamics the resulting (CFD) model fuel consumption.of the experimental Oxygen-enhanced device was combustiondesigned and was constructed applied with for oxygenthe better concentrations understanding ranging of the from impact 21 toof 35% air vol.infiltration The computational on the combustion fluid dynamics process. (CFD) The obtained model of theresults experimental confirm the device ability was of designed the new anddoor constructed design to forreduce the better air penetration understanding by up of to the 50% impact and of to air reduce infiltration the related on the fuel combustion consumption process. by The1.79% obtained to 12%, results clearly confirm demonstrating the ability its of superior the new doorperformance design to compared reduce air to penetration the traditional by up door to 50%design. and to reduce the related fuel consumption by 1.79% to 12%, clearly demonstrating its superior performanceThe rest compared of the paper to the is traditionalorganized dooras follows: design. First, the modified furnace door design is intro- ducedThe and rest its of thefeatures paper are is organizedexplained, as along follows: with First, a description the modified of furnacethe laboratory-scale door design is furnace introduced used andin the its featuresexperiments are explained,and the instrumentation along with a description employed.of The the method laboratory-scale for calculating furnace the used air infiltra- in the experimentstion rate is presented, and the instrumentation followed by the employed. results ob Thetained method and the for calculatingrelated discussion. the air infiltration The CFD simula- rate is presented,tion and its followed results are by thepresented results obtainedin the next and part, the corroborating related discussion. the experimental The CFD simulation findings. andConclu- its resultssions sum are presented up the key in findings. the next part, corroborating the experimental findings. Conclusions sum up the key findings. 2. Experimental Device 2. Experimental Device 2.1. Modification of the Door 2.1. Modification of the Door The proposed furnace door modification aimed to reduce the gap between the furnace door andThe the proposedfurnace body furnace itself, door through modification which air aimed penetr to reduceates into the the gap furnace. between The the existence furnace doorof the and gap theresults furnace from body the itself,industrial through tilting which furnace air penetrates design, because into the the furnace. furnace The door existence remains of thestatic gap while results the fromfurnace the industrialbody rotates tilting (see furnace Figure 1). design, The size because of th thee gap furnace area through door remains which static air is while sucked the inside furnace the bodyfurnace rotates can (seebe defined Figure 1as). Thethe area size of the gapcylinder area mantle. through The which novel air door is sucked design inside is based the furnace on elon- cangating be defined the perimeter as the areasurround of the of cylinder the door, mantle. which The resu novellts in door the partial design covering is based onof the elongating rotating thefur- perimeternace body. surround This, in of turn, the door,leads whichto reduction results in thethe partialgap area, covering as it results of the rotatingfrom Equations furnace body.(1)–(3), This, and inin turn, the leadsend, reduces to reduction air infiltration, in the gap area, as schematicall as it results fromy shown Equations in Figure (1)–(3), 2. andAdditional in the end, air reducesinfiltration air infiltration,reduction asresults schematically both from shown the increased in Figure 2relative. Additional roughness air infiltration of the opposing reduction walls results and both from from the thelonger increased path the relative penetrating roughness air must of the flow opposing through walls to andget inside from thethe longer furnace. path The the key penetrating feature of airthe mustproposed flow throughnovel door to get design inside is the that furnace. the elongated The key part feature of the of thedoor proposed surround novel is parallel door design and close is that to thethe elongated wall of the part rotating of the furnace door surround body. is parallel and close to the wall of the rotating furnace body.

FigureFigure 1. 1.( a(a)) Traditional Traditional design design of of door, door, ( b(b)) proposed proposed design design of of door. door.

A schematic visualization of the gap width estimation is provided in Figure 2. The width of the gap between the door and the furnace body can be calculated using Equation (1). If the distance

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between the furnace door and body is reduced, the gap width can be calculated using Equations (1)–(3). For a better understanding, please see the auxiliary scheme provided in Figure 2. It follows from Equations (1) and (2) that the gap width reduction depends on the sine of angle α. In the ideal case, with no contact between the furnace door and the furnace body itself, the theoretical angle value is 0°. In this case, there is no gap and, consequently, zero air infiltration. While this situation is unreal, it still holds that as the angle α approaches zero, so does the gap width as well. The modified door design in the experiments yielded an angle α equal to 24.2°. Such experimental alignment produced a gap width of 4.1 mm, with the distance between the furnace door and the front of the furnace itself remaining at 10 mm. Please refer to Figure 1 as well. Figure 3 shows the modified furnace door design for better visualization.

𝑥 =𝑠𝑖𝑛𝛼×ℎ (m) (1)

where 𝑥—width of gap through which air infiltrates into furnace (m); ℎ—distance between extended door surround and rotary kiln body (m); 𝛼—angle between sloping body of rotary kiln and axis of furnace (°).

ℎ =ℎ −∆ℎ (m) (2)

Processes 2020, 8, 1292 4 of 16 𝑥 =𝑥 × (m) (3)

Figure 2. Visualization of gap width estimation. Figure 2. Visualization of gap width estimation. A schematic visualization of the gap width estimation is provided in Figure2. The width of the gap between the door and the furnace body can be calculated using Equation (1). If the distance between the furnace door and body is reduced, the gap width can be calculated using Equations (1)–(3). For a better understanding, please see the auxiliary scheme provided in Figure2. It follows from Equations (1) and (2) that the gap width reduction depends on the sine of angle α. In the ideal case, with no contact between the furnace door and the furnace body itself, the theoretical angle value is 0◦. In this case, there is no gap and, consequently, zero air infiltration. While this situation is unreal, it still holds that as the angle α approaches zero, so does the gap width as well. The modified door design in the experiments yielded an angle α equal to 24.2◦. Such experimental alignment produced a gap width of 4.1 mm, with the distance between the furnace door and the front of the furnace itself remaining at 10 mm. Please refer to Figure1 as well. Figure3 shows the modified furnace door design for better visualization. x = sinα h (m) (1) 0 × 1 where x0—width of gap through which air infiltrates into furnace (m); h1—distance between extended door surround and rotary kiln body (m); α—angle between sloping body of rotary kiln and axis of furnace (◦). h = h ∆h (m) (2) 2 1 − h2 x1 = x0 (m) (3) × h1 Figure4 provides the values of the gap area through which air penetrates into the furnace with the traditional and with the new door design as a function of the furnace door distance from the furnace itself. As it can be seen from this figure, a reduction of around 50% in the gap area was achieved with the new door design. Theoretical calculations using the Bernoulli equation yielded the expected 50% reduction in air infiltration. The gap area in industrial-size rotary tilting furnaces is several times larger [13,35] compared with that in the laboratory furnace model, and the proposed door design modification can lead to a significant reduction in air penetration, improvement in combustion conditions, and subsequently increased thermal efficiency of the furnace. Processes 2020, 8, x FOR PEER REVIEW 5 of 17

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Figure 3. Door of the experimental furnace model.

Figure 4 provides the values of the gap area through which air penetrates into the furnace with the traditional and with the new door design as a function of the furnace door distance from the furnace itself. As it can be seen from this figure, a reduction of around 50% in the gap area was achieved with the new door design. Theoretical calculations using the Bernoulli equation yielded the expected 50% reduction in air infiltration. The gap area in industrial-size rotary tilting furnaces is several times larger [13,35] compared with that in the laboratory furnace model, and the proposed door design modification can lead to a significant reduction in air penetration, improvement Figure 3. Door of the experimental furnace model. in combustion conditions, andFigure subsequently 3. Door of the increased experimental thermal furnace efficiency model. of the furnace.

Figure 4 provides the values of the gap area through which air penetrates into the furnace with the traditional and with the new door design as a function of the furnace door distance from the furnace itself. As it can be seen from this figure, a reduction of around 50% in the gap area was achieved with the new door design. Theoretical calculations using the Bernoulli equation yielded the expected 50% reduction in air infiltration. The gap area in industrial-size rotary tilting furnaces is several times larger [13,35] compared with that in the laboratory furnace model, and the proposed door design modification can lead to a significant reduction in air penetration, improvement in combustion conditions, and subsequently increased thermal efficiency of the furnace.

Figure 4. GapGap area area dependence on gap width in the tr traditionaladitional and the proposed new door design.

2.2. Experimental Model Experiments werewere carriedcarried outout in in a a tilting tilting rotary rotary furnace furnace (Figure (Figure5) with5) with an insidean inside diameter diameter of 305 of mm305 andmm lengthand length of 607 mm.of 607 The mm. angle The of inclinationangle of inclination of the experimental of the experimental model during model the measurement during the wasmeasurement 7%. In total, was 16 thermocouples7%. In total, 16 of typethermocouples K (NICr-NiAl) of PTTK-TKb-60-2-SPtype K (NICr-NiAl) (Meratex PTTK-TKb-60-2-SP s.r.o., Košice, Slovakia)(Meratex s.r.o., were locatedKošice, inSlovakia) the furnace. were Thelocated measurement in the furnace. uncertainty The measurement stated by the uncertainty manufacturer stated is atby thethe levelmanufacturer of +/ 2.5 is◦C at at the the level temperature of +/−2.5 of°C 1200 at the◦C. temperature The objective of 1200 of the °C. thermocouples The objective wasof the to − continuouslythermocouples monitor was to thecontinuously changes in monitor the temperature the changes field in in the the temperature combustion field chamber, in the lining,combustion stack, andchamber, charge.Figure lining, 4. The Gap stack, experimental area dependence and charge. model on The gap was experimentalwidth set-up in the statically traditional model due was and to theset-up the proposed placement statically new of doordue thermocouples todesign. the place- aroundment of thethermocouples circumference around of the casing,the circumference and it did not of rotatethe casing, during and the it measurements. did not rotate The during flue gasthe 2.2.composition Experimental was Model analyzed using a TESTO-350XL flue gas analyzer (K-Test, s.r.o., Košice, Slovakia).

The positionExperiments of the were ﬂue gascarried analyzer out in and a tilting distribution rotary offurnace the thermocouples (Figure 5) with are an shown inside in diameter Figure5. of 305 mm and length of 607 mm. The angle of inclination of the experimental model during the measurement was 7%. In total, 16 thermocouples of type K (NICr-NiAl) PTTK-TKb-60-2-SP (Meratex s.r.o., Košice, Slovakia) were located in the furnace. The measurement uncertainty stated by the manufacturer is at the level of +/−2.5 °C at the temperature of 1200 °C. The objective of the thermocouples was to continuously monitor the changes in the temperature field in the combustion chamber, lining, stack, and charge. The experimental model was set-up statically due to the placement of thermocouples around the circumference of the casing, and it did not rotate during the

Processes 2020, 8, x FOR PEER REVIEW 6 of 17

measurements. The flue gas composition was analyzed using a TESTO-350XL flue gas analyzer (K- Test, s.r.o., Košice, Slovakia). The position of the flue gas analyzer and distribution of the thermocouples are shown in Figure 5. Flow rates of all gaseous media were measured using Bronkhorst-designed MASS-VIEW series thermal mass flow meters/regulators. The devices operate on the principle of direct thermal mass flow measurement. The models utilized during these experiments were MV-308, MV-106, and MV- 306 (AREKO s.r.o., Bratislava, Slovakia). Technical specifications of these models are listed in Tables 1 and 2. Other relevant parameters related to the furnace operation were recorded by the furnace control system.

Table 1. Technical specification of Bronkhorst flowmeters MASS-VIEW MV-106, MV-306, and MV- 308 [36]. FS = full scale; RD = reading.

MV-106, MV-306 MV-308 ±2% RD for >50% of max. capacity Accuracy ±(2% RD + 0.5% FS) on lower flows Repeatability <0.2% FS typical <0.6% FS typical Operating pressure 0–10 bar Operating temperature 0–50 °C Response time 2 s

Table 2. Flow rate specification of Bronkhorst flowmeters MASS-VIEW MV-106, MV-306, and MV- 308 [36].

Model Air, O2 (L/min) CH4 (L/min) range 1 2–200 1–100 MV-106, range 2 1–100 5–50 MV-306 range 3 0.5–50 0.2–20 range 4 0.4–20 0.2–10 range 1 5–500 2.5–250 range 2 2–200 1.25–125 MV-308 range 3 1–100 0.625–62.5 Processes 2020, 8, 1292 range 4 1–50 0.5–25 6 of 16

Figure 5. Experimental model of tilting rotary kiln. Figure 5. Experimental model of tilting rotary kiln. Flow rates of all gaseous media were measured using Bronkhorst-designed MASS-VIEW series Effects of 21% to 35% oxygen concentration in the oxidizing agent on heat transfer in the fur- thermal mass flow meters/regulators. The devices operate on the principle of direct thermal mass nace, flue gas temperature, and emissions produced were studied. At the same time, external effects flow measurement. The models utilized during these experiments were MV-308, MV-106, and MV-306 such as air infiltration into the furnace were monitored. Table 3 shows the gas and combustion air (AREKO s.r.o., Bratislava, Slovakia). Technical specifications of these models are listed in Tables1 and 2. Other relevant parameters related to the furnace operation were recorded by the furnace control system.

Table 1. Technical speciﬁcation of Bronkhorst ﬂowmeters MASS-VIEW MV-106, MV-306, and MV-308 [36]. FS = full scale; RD = reading.

MV-106, MV-306 MV-308 2% RD for >50% of max. capacity Accuracy ± (2% RD + 0.5% FS) on lower ﬂows ± Repeatability <0.2% FS typical <0.6% FS typical Operating pressure 0–10 bar Operating temperature 0–50 ◦C Response time 2 s

Table 2. Flow rate speciﬁcation of Bronkhorst ﬂowmeters MASS-VIEW MV-106, MV-306, and MV-308 [36].

Effects of 21% to 35% oxygen concentration in the oxidizing agent on heat transfer in the furnace, flue gas temperature, and emissions produced were studied. At the same time, external effects such as air infiltration into the furnace were monitored. Table3 shows the gas and combustion air and oxygen flow rates required to achieve the desired oxygen concentration in the oxidizing agent. Oxygen was added to the air input upstream of the burner using a diffuser to produce its uniform distribution. The addition of oxygen to the air stream has been described by Baukal and is intended for lower levels of Processes 2020, 8, 1292 7 of 16 oxygen enrichment of combustion air [6]. Natural gas was provided from the distribution network. The composition of this natural gas is presented in Table4.

Table 3. Volumetric ﬂows of inlet media depending on oxygen concentration in the oxidizing agent.

Concentration of Oxygen (%) 21 22.5 25 27.5 30 32.5 35 Air Flow (m3 h 1) 14.07 12.88 11.22 9.86 8.72 7.77 6.94 · − Oxygen Flow (m3 h 1) 0 0.25 0.6 0.88 1.12 1.32 1.49 · − Fuel Consumption (m3 h 1) 1.3 1.3 1.3 1.3 1.3 1.3 1.3 · − Air Excess coeﬃcient (-) 1.1 1.1 1.1 1.1 1.1 1.1 1.1

Table 4. Natural gas composition in % vol. (adapted from [37]).

CH4 C2H6 C3H8 C4H10 C5H12 C6H14 CO2 N2 %%%%%%%% 95.171 2.7131 0.8729 0.2772 0.0486 0.0207 0.2266 0.6697

2.3. Calculation of Air Infiltration The volumetric flow of air penetrating into the furnace can be calculated based on the material balance of the combustion process. The Testo 350XL flue gas analyzer provides concentrations of flue gas constituents (mainly oxygen) in the produced dry flue gas, which enables air excess coefficient calculation. Equations (4) and (5) present the calculation of the air excess coefficient and the volumetric flow of air infiltration. VFG,D,min,yo O m = 1 + 2 2 ( ) (4) L , × Y O − min,yo2 O2 − 2 3 1 V = m.L B V m h− (5) AI min × − 1 × × where m is the air excess coefficient (-); VFG,D,min,yo2 is the theoretical volume of flue gas (dry) if m = 1 (m3 m 3); L is the theoretical volume of the oxidizing agent if m = 1 (m3 m 3); O is the · − min,yo2 · − 2 oxygen content in dry flue gases determined by means of flue gas analysis (% vol.); YO2 is the required oxygen concentration in oxygen-enriched combustion air for experimental measurement (%); VAI is the volumetric flow of air infiltration (m3 h 1); B is the consumption of fuel (m3 h 1); V is the volumetric · − · − 1 flow of oxidizing agent determined on the basis of Table3 (m 3 h 1). · − The pressure difference between the ambient air and the inside of the furnace is related to the volumetric flow of air infiltration by Equation (6) if the frictional pressure losses are neglected. Their effect transposes into the measured oxygen content in the flue gases and, following that, into the calculated air infiltration. 2 VAL/3600 S ρAL ∆p = x × (Pa) (6) 2 2 where ∆p is the pressure difference (Pa); Sx is the gap area (m ); ρAL is the density of ambient air (kg m 3). · − 3. Results and Discussion Figure6 shows the measured oxygen concentration values (% vol.) in dry flue gas at various adjusted gap widths. Its linear increase with the increasing gap width can be observed, while the use of OEC had no visible effect on it. Based on Equations (4)–(6), it can be stated that underpressure in the furnace does not depend on the gap width; thus, the linear oxygen concentration increase results from the increasing gap area (see Figure4). The oxygen concentration increase in the combustion air due to OEC technology use reduces the pressure difference between the furnace interior and the ambient air and reduces the air infiltration accordingly. The optimal oxygen concentration range in the dry flue gas with varying oxygen content in the enriched air (21 to 35% vol.) at the optimum air excess coefficient of Processes 2020, 8, x FOR PEER REVIEW 8 of 17 combustion air due to OEC technology use reduces the pressure difference between the furnace interior and the ambient air and reduces the air infiltration accordingly. The optimal oxygen concentration range in the dry flue gas with varying oxygen content in the enriched air (21 to 35% vol.) Processes 2020, 8, x FOR PEER REVIEW 8 of 17 Processesat the optimum2020, 8, 1292 air excess coefficient of m = 1.1 is depicted in Figure 6 (orange and red lines). 8Oxy- of 16 combustiongen concentration air due values to OEC above tec hnologythe red line use indicate reduces air the infiltration pressure differenceinto the furnace. between The the difference furnace interiorbetween and the thered ambientline and airthe andmeasured reduces oxygen the air concen infiltrationtration accordingly. value at zero The gap optimal width (dooroxygen touch- con- m = 1.1 is depicted in Figure6 (orange and red lines). Oxygen concentration values above the red line centrationing the furnace range body) in the results dry flue from gas the with imperfect varying equipment oxygen content geometry, in the worn-down enriched air refractory (21 to 35% lining, vol.) indicate air infiltration into the furnace. The difference between the red line and the measured oxygen ator thepossibly optimum from air air excessinfiltration coeffici throughent of manother, = 1.1 is undetected, depicted in gap. Figu re 6 (orange and red lines). Oxy- concentration value at zero gap width (door touching the furnace body) results from the imperfect gen concentrationOn the basis ofvalues flue gasabove analysis, the red the line values indicate of air air infiltration infiltration volumetric into the furnace. flow were The calculateddifference equipment geometry, worn-down refractory lining, or possibly from air infiltration through another, betweendepending the on red the line gap and width the measuredand oxygen oxygen concentrat conceniontration in the value enriched at zero air gap(21 towidth 35% (doorvol.), touch-stand- undetected, gap. ingardized the furnace to fuel consumptionbody) results offrom B = the1 (m imperfect3·h−1). The equipment obtained trends geometry, are shown worn-down in Figure refractory 7. lining, or possibly from air infiltration through another, undetected, gap. On the basis of flue gas analysis, the values of air infiltration volumetric flow were calculated depending on the gap width and oxygen concentration in the enriched air (21 to 35% vol.), standardized to fuel consumption of B = 1 (m3·h−1). The obtained trends are shown in Figure 7.

Figure 6. Oxygen concentration in dry ﬂueflue gas in % vol.vol. measured by ﬂueflue gas analyzer at varying gap width values. Orange Orange and and red red lines indicate th thee range of values corresponding to an optimal combustion air excess of m = 1.1 at 21 to 35% oxygen content in combustion air.

On the basis of flue gas analysis, the values of air infiltration volumetric flow were calculated Figure 6. Oxygen concentration in dry flue gas in % vol. measured by flue gas analyzer at varying dependinggap width on the values. gap widthOrange and and oxygen red lines concentration indicate the inrange the enrichedof values aircorresponding (21 to 35% vol.), to an standardized optimal to fuel consumption of B = 1 (m3 h 1). The obtained trends are shown in Figure7. combustion air excess of m = 1.1· at− 21 to 35% oxygen content in combustion air.

Figure 7. Dependence of calculated specific air infiltration volumetric flow (m3 per m−3 of combusted natural gas) on gap width.

Results depicted in Figures 6 and 7 demonstrate the possible situation occurring in real furnaces, where the expected gap width between the furnace door and the furnace3 body−3 is much larger Figure 7. Dependence of calculated specific air infiltration volumetric flow (m3 per m 3 of combusted than that (10 mm)Dependence in the laboratory of calculated furnace specific em air infiltrationployed during volumetric aluminum flow (m meltingper m −experiments.of combusted natural gas) on gap width.

Results depicted inin FiguresFigures6 6 and and7 demonstrate7 demonstrate the th possiblee possible situation situation occurring occurring in in real real furnaces, furnac- wherees, where the expectedthe expected gap widthgap width between between the furnace the furn doorace and door the and furnace the furnace body is muchbody largeris much than larger that (10than mm) that in (10 the mm) laboratory in the laboratory furnace employed furnace em duringployed aluminum during aluminum melting experiments. melting experiments.

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Air infiltration increases combustion air excess and leads to increased thermal losses in flue gases Air infiltration increases combustion air excess and leads to increased thermal losses in flue exiting the furnace. At the same time, it leads to a decrease in the overall oxygen concentration in the gases exiting the furnace. At the same time, it leads to a decrease in the overall oxygen concentra- oxidizing agent (sum of combustion air and air infiltration). tion in the oxidizing agent (sum of combustion air and air infiltration). Figure8 shows computational results of how the oxygen concentration in the oxidizing agent Figure 8 shows computational results of how the oxygen concentration in the oxidizing agent changes as air infiltrates into the combustion chamber. The optimum oxygen concentration in the changes as air infiltrates into the combustion chamber. The optimum oxygen concentration in the oxidizing agent for melting aluminum in a rotary tilting furnace was found to be 35% based on oxidizing agent for melting aluminum in a rotary tilting furnace was found to be 35% based on experimental studies [19,24]. However, in the case of the estimated calculated air infiltration of up to experimental studies [19,24]. However, in the case of the estimated calculated air infiltration of up 4.5 m3 h–1 (see Figure7), this concentration value can be reduced by up to 14%. to 4.5 m· 3·h–1 (see Figure 7), this concentration value can be reduced by up to 14%.

Figure 8. Dependence of oxygen concentrationconcentration andand combustion combustion air air excess excess on on the the amount amount of of inﬁltrating infiltrat- ingair (mathematicalair (mathematical modeling). modeling).

The effects effects of the change in oxygen concentrationconcentration in the oxidizing agent and that in excess combustion air air can can be transposed to the change in combustion temperatures through the statics of combustion. Figure 99 illustrates illustrates the the impact impact of of air air infiltration infiltration on on natural natural gas gas combustion combustion temperature. temperature.Theoretical Theoretical temperature, temperature, including including the dissociation the dissociation of CO2 ofand CO H22 Oand components, H2O components, is shown is byshown the blue by theline. blue It is line. evident It is that evident the theoretical that the theoretical temperature temperature decreases bydecreases more than by 500more◦C than as the 500 combustion °C as the combustionconditions change conditions due to change air infiltration. due to air Actual infiltration. temperature Actual in temperature the furnace was in the calculated furnace by was applying calcu- latedthe pyrometric by applying effi ciencythe pyrometric factor of theefficien experimentalcy factor modelof the (ηexperimentalpyr = 0.45) determined model (ηpyr by = experimental 0.45) deter- minedmeasurements. by experimental Similarly, measurements the actual temperature. Similarly, inthe the actual furnace temperature decreases in by the more furnace than decreases 200 ◦C with by morethe increasing than 200 air°C leakage.with the The increasing charge materialair leakage. is thus The heated charge by material cooler flue is thus gases, heated which by increases cooler flue the gases,processing which time increases of the charge, the processing reduces the time furnace of the productivity, charge, reduces and increases the furnace the fuelproductivity, consumption. and increases the fuel consumption.

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Figure 9. Dependence of temperature change with air infiltration.

Figure 10 provides measured flue gas temperature data from an experimental run during which the gap between the furnace door and body was decreased. Thermocouples T1 to T4 were placed in axial positions in the furnace; thus, they had the quickest response to changes in combustion conditions. Measured flue gas temperatures of around 700 °C increased to around 900 °C after the reduction in air infiltration intensity, which is in good agreement with the calculated temperatures shown in Figure 9. Axial distances of individual thermocouples from the burner are listed in Table 5.

Table 5. Axial distances of individual thermocouples from the burner.

Thermocouple T1 T2 T3 T4 Figure 9. Dependence of temperature change with air infiltration. DistanceFigure (m) 9. Dependence of temperature0.05 change with0.2 air inﬁltration.0.35 0.5

Figure 10 provides measured flueflue gas temperature data from an experimentalexperimental run during which The effect of decreasing air infiltration intensity during an experimental run can be further the gapgap betweenbetween thethe furnacefurnace doordoor andand bodybody waswas decreased.decreased. Thermocouples T1 T1 to T4 were placed in monitored by thermocouple T1 showing a decrease in flue gas temperature. The explanation is giv- axial positionspositions inin thethe furnace; furnace; thus, thus, they they had had the the quickest quickest response response to changesto changes in combustion in combustion conditions. condi- en above (p. 2): Penetrating air can be considered as secondary air, which accelerates creation of the Measuredtions. Measured flue gas flue temperatures gas temperatures of around of around 700 C increased700 °C increased to around to 900aroundC after 900 the°C after reduction the reduc- in air combustible mixture by enhanced mixing of the◦ fuel with the oxidizing ◦agent. The flame front dis- infiltrationtion in air infiltration intensity, which intensity, is in which good agreementis in good withagreement the calculated with the temperaturescalculated temperatures shown in Figure shown9. tance from the burner is reduced as a result. Decreasing air infiltration intensity slows down the Axialin Figure distances 9. Axial of distances individual of thermocouplesindividual thermocouples from the burner from the are burner listed in are Table listed5. in Table 5. creation of the combustible mixture and the flame front moves further away from the burner. Table 5. Axial distances of individual thermocouples from the burner.

Thermocouple T1 T2 T3 T4 Distance (m) 0.05 0.2 0.35 0.5

The effect of decreasing air infiltration intensity during an experimental run can be further monitored by thermocouple T1 showing a decrease in flue gas temperature. The explanation is given above (p. 2): Penetrating air can be considered as secondary air, which accelerates creation of the combustible mixture by enhanced mixing of the fuel with the oxidizing agent. The flame front distance from the burner is reduced as a result. Decreasing air infiltration intensity slows down the creation of the combustible mixture and the flame front moves further away from the burner.

Figure 10. ChangesChanges in in flue ﬂue gas gas temperatures temperatures in in combustion combustion chamber chamber as asa result a result of decreasing of decreasing air airin- inﬁltrationfiltration intensity intensity (experimental (experimental measurements). measurements).

Table 5. Axial distances of individual thermocouples from the burner.

Thermocouple T1 T2 T3 T4 Distance (m) 0.05 0.2 0.35 0.5

The effect of decreasing air infiltration intensity during an experimental run can be further monitored by thermocouple T1 showing a decrease in flue gas temperature. The explanation is given above (p. 2): Penetrating air can be considered as secondary air, which accelerates creation of the Figure 10. Changes in flue gas temperatures in combustion chamber as a result of decreasing air infiltration intensity (experimental measurements).

Processes 2020, 8, 1292 11 of 16 combustible mixture by enhanced mixing of the fuel with the oxidizing agent. The flame front distance from the burner is reduced as a result. Decreasing air infiltration intensity slows down the creation of the combustible mixture and the flame front moves further away from the burner. Measurement results in Figure7 show that the operation of rotary tilting furnaces is inevitably coupled with significant air infiltration into the combustion chamber. The proposed new design of the furnace door leads to an around 50% decrease in the volumetric flow of air penetrating into the furnace, which represents a substantial change in the furnace operation. The resulting fuel saving rate can be partly calculated using Equations (7) and (8). Additional fuel savings, resulting from the shorter charge melting time, must be estimated experimentally case by case.

. 1 Q = L Cp ∆T(kJ day− ) (7) AI AI × AI × ·

QAI 3 1 x = (m day− ) (8) QnNG × · . 3 1 where LAI is the flow rate of air infiltration per day (m day− ); cpAI is the specific heat capacity of 3 1 · infiltrating air (kJ m− K− ); ∆T is the difference in temperature (K); QnNG is the calorific value of 3 3 1 natural gas (kJ m− ); x is the natural gas equivalent (m day− ). Data provided in Figure7 (at oxygen concentration of 35%) at the ∆T value of 650 K, estimated by means of experimental measurements, permit the deduction that 0.6 to 3.68 m3/day of natural gas can be saved by the proposed novel furnace door design. This represents around 1.79 to 12% of the total daily natural gas consumption of the laboratory furnace. In the case of industrial-sized rotary tilting furnaces, the gap area is approximately 2.5 to 6 times larger compared to the laboratory-scale furnace used in this study [38] depending on the furnace throughput. The average natural gas consumption in this type of furnace is 35 m3 per ton of charge. Applying the results obtained from the laboratory-scale furnace model, the air infiltration rate into a typical industrial furnace ranges between 17 and 120 m3 h 1. The · · − calculated air infiltration rate into a real furnace [6] was 23 m3/h, which led to an additional natural gas consumption of 10 m3 day 1. This represents approximately 1.6% of the average natural gas · − consumption in that furnace, or around 0.56 m3 of natural gas per ton of charge. Halving this amount by means of the proposed novel furnace door design produces a saving of almost 0.3 m3 per ton of charge on average. In the case of furnaces using pure oxygen for natural gas combustion, the oxygen concentration in the combustible mixture is reduced to around 80% of its original value due to air infiltration. The theoretical combustion temperature, taking CO2 and H2O dissociation into account, then drops by around 150 ◦C.

4. Mathematical Modeling The effects of air infiltration into a furnace can be illustrated using mathematical modeling with CFD simulation. Nieckele et al. [38] described the numerical modeling of an industrial aluminum melting furnace. Zhou et al. [39] performed the modeling of aluminum scrap processing in a rotary furnace. Khoei et al. [40] modeled the operation of a rotary furnace in aluminum recycling processes. Rimar et al. [41] developed a mathematical model of a heating furnace implemented with volumetric fuel combustion. The mathematical model of the furnace shown in Figures3 and5 was developed in ANSYS. Boundary conditions were defined based on the heat balance result and in accordance with Table3. Gas and oxidizing agent were defined in terms of the mass flow rate. The boundary condition of the chimney was specified as an opening with pressure condition “ 5 Pa,” which was − defined as negative pressure created by the exhaust fan. The furnace border (surround) was defined by the boundary condition “Opening,” where the specified pressure was equal to the atmospheric one, and the air composition was 21% of O2 and 79% of N2. The mesh of the mathematical model was created from tetrahedral, hexahedral, and prism cells. Meshing in the vicinity of the furnace walls was supported by the inflation function with the inflation option “Smooth transition” and a maximum of Processes 2020, 8, 1292 12 of 16

ﬁve layers, and growth rate of 1.2. Figure 11 provides a visualization of the mesh of the mathematical model. The total number of cells was 2,086,018. In order to simplify the mathematical model, the ProcessesfurnaceProcesses 2020 was2020, 8,, modeled8x, FORx FOR PEER PEER in REVIEW theREVIEW empty state, i.e., with no charge inside. 1212 of of 17 17

FigureFigureFigure 11. 11. 11. MeshMesh Mesh of ofof the thethe mathematical mathematical mathematical model. model. model.

ResultsResultsResults of of thethe the mathematicalmathematical mathematical modeling modeling modeling presented presented presented in Figurein in Figure Figure 12 confirm12 12 confirm confirm that whenthat that when meltingwhen melting melting aluminum alu- alu- minuminminum the physicalin in the the physical physical model, model, air model, infiltrates air air infiltrates infiltrates into the into furnaceinto the the furnace combustionfurnace combustion combustion chamber. chamber. chamber. Analysis Analysis Analysis of the resultsof of the the resultsshowsresults shows that shows the that actualthat the the measurementactual actual measurement measurement of emissions of of emissi emissi withonsons the with probewith the the in probe theprobe stack in in the ofthe stack the stack physical of of the the physical model physical is modelalsomodel influenced is is also also influenced influenced by air infiltration. by by air air infiltration. infiltration. There is There a There layer is ofis a a unburnedlayer layer of of unburned unburned oxygen closeoxygen oxygen to theclose close chimney to to the the chim- walls,chim- neywhichney walls, walls, is diluted which which with is is diluted diluted flue gases with with created flue flue gases duringgases crea crea naturaltedted during during gas combustion. natural natural gas gasThus, co combustion.mbustion. the optimum Thus, Thus, location the the opti- opti- of mumthemum flue location location gas analysis of of the the probeflue flue gas gas is alonganalysis analysis the probe stackprobe axisis is along along near the the the stack combustion stack axis axis near near chamber, the the combustion combustion i.e., exactly chamber, chamber, where it i.e.,wasi.e., exactly placedexactly where during where it our itwas was experiments. placed placed during during our our experiments. experiments.

Figure 12. Model results: Distribution of oxygen mass fraction contours. FigureFigure 12. 12. Model Model results: results: Distribution Distribution of of oxygen oxygen mass mass fraction fraction contours. contours. Due to the small pressure difference between the ambient air and the furnace interior, the pressure DueDue to to the the small small pressure pressure difference difference between between th the eambient ambient air air and and the the furnace furnace interior, interior, the the pres- pres- contours range had to be rescaled to 5 Pa. Figure 13 clearly shows the overpressure in the burner, suresure contours contours range range had had to to be be rescaled rescaled± to to ±5 ±5 Pa Pa. Figure. Figure 13 13 clearly clearly shows shows the the overpressure overpressure in in the the which quickly diminishes due to the flue gas outflow into the stack. Areas with lowest pressure are burner,burner, which which quickly quickly diminishes diminishes due due to to the the flue flue gas gas outflow outflow into into the the stack. stack. Areas Areas with with lowest lowest pressurepressure are are located located close close to to the the burner burner and and further further toward toward the the stack stack as as a aresult result of of the the flue flue gas gas fan fan operationoperation drawing drawing off off produced produced flue flue gas. gas.

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located close to the burner and further toward the stack as a result of the ﬂue gas fan operation drawing

ProcessesoProcessesﬀ produced 2020 2020, 8,, 8x ﬂue, xFOR FOR gas. PEER PEER REVIEW REVIEW 1313 of of 17 17

FigureFigureFigure 13. 13. 13. ModelModel Model results: results: results: Pressure PressurePressure contours contourscontours distribution. distribution. distribution.

From the results of the mathematical model, the extent to which air infiltration affects the natural FromFrom the the results results of of the the mathematical mathematical model, model, the the extent extent to to which which air air infiltration infiltration affects affects the the natu- natu- gas combustion process can also be determined. Figure 14 shows the distribution of infiltrating air in ralral gas gas combustion combustion process process can can also also be be determined. determined. Figure Figure 14 14 shows shows the the distribution distribution of of infiltrating infiltrating the combustion chamber by means of streamlines. The exhaust fan creates negative pressure in the airair in in the the combustion combustion chamber chamber by by means means of of streamlines. streamlines. The The exhaust exhaust fan fan creates creates negative negative pressure pressure chimney, and therefore, a large portion of the penetrating air is drawn directly into the flue gas, creating inin the the chimney, chimney, and and therefore, therefore, a alarge large portion portion of of th the epenetrating penetrating air air is is drawn drawn directly directly into into the the flue flue an air layer on the chimney walls. In proximity of the burner, the penetrating air is propelled into the gas,gas, creating creating an an air air layer layer on on the the chimney chimney walls. walls. In In proximity proximity of of the the burner, burner, the the penetrating penetrating air air is is combustion chamber by the flue gas flow. Thus, an excess of oxygen is generated in the combustion propelledpropelled into into the the combustion combustion chamber chamber by by the the flue flue gas gas flow. flow. Thus, Thus, an an excess excess of of oxygen oxygen is is generated generated chamber, creating an oxidizing atmosphere above the charge. inin the the combustion combustion chamber, chamber, creating creating an an oxidizing oxidizing atmosphere atmosphere above above the the charge. charge.

FigureFigureFigure 14. 14. 14. ModelModel Model results: results:results: Infiltrating InﬁltratingInfiltrating air airair flow ﬂowflow contours contourscontours in inin the thethe combustion combustioncombustion chamber. chamber.chamber.

DataDataData presented presented inin in FigureFigure Figure 15 15 15validate validate validate the the the model model model and and and calculation calculation calculation results results results of air of of inﬁltrationair air infiltration infiltration volumetric volu- volu- metricﬂowmetric through flow flow through through their comparison their their comparison comparison with values with with obtainedvalues values obtained usingobtained the using methodusing the the proposedmethod method proposed byproposed Baukal by [by35 Baukal ].Baukal Both [35].modeling[35]. Both Both modeling approachesmodeling approaches approaches yielded similaryielded yielded results similar similar to results results those to fromto those those the from from calculation the the calculation calculation based onbased based experimental on on exper- exper- imentalmeasurements.imental measurements. measurements. Thus, the Thus, Thus, method the the usedmethod method for mathematicalused used for for mathematical mathematical model creation model model in creation ANSYScreation in canin ANSYS ANSYS be considered can can be be consideredasconsidered correct, as and as correct, correct, the results and and the obtainedthe results results are obtained obtained relevant ar are and erelevant relevant can be and and used can can for be be fuel used used consumption for for fuel fuel consumption consumption reduction reductionestimationreduction estimation resultingestimation fromresulting resulting theproposed from from the the proposed furnace proposed door furnace furnace design door door change. design design change. change.

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Figure 15. ComparisonComparison of ofair air infiltration infiltration volumetric volumetric flow: flow: Calculated Calculated using using experimental experimental data data(or- (orange);ange); obtained obtained from from modeling modeling in ANSYS in ANSYS (yellow) (yellow);; and and calculated calculated using using the theapproach approach proposed proposed by byBaukal Baukal [35] [35 (green).] (green).

5. Conclusions The authors have proposed and experimentally tested an improved design for a rotary tiltingtilting furnace door.door. ItsIts abilityability toto reducereduce airair infiltrationinfiltration intointo thethe furnace furnace was was confirmed confirmed both both experimentally experimental- andly and by numericalby numerical simulations simulations in ANSYS. in ANSYS. Temperatures Temperatures in the in combustion the combustion chamber chamber and oxygen and contentoxygen incontent dry flue in dry gas flue were gas measured were measured in order in to order document to document the changes the changes in the in combustion the combustion process process with varyingwith varying gap width gap aroundwidth around the furnace the door.furnace The door. calculated The calculated air infiltration air volumetricinfiltration flowvolumetric values based flow onvalues the measuredbased on the data measured agree well data with agree the resultswell with of numericalthe resultssimulations of numerical performed simulations in ANSYS,performed as wellin ANSYS, as with theas well reference as with calculation the reference procedure calculation used in relevant procedure literature. used Thein relevant achieved literature. air infiltration The reductionachieved air of upinfiltration to 50% translates reduction into of up fuel to savings 50% translates amounting into to fuel around savings 1.8 toamounting 12% of the to total around natural 1.8 gasto 12% consumption of the total in thenatural laboratory-scale gas consumption furnace. in Applying the laboratory-scale the results obtained furnace. here Applying to industrial-scale the results furnacesobtained permitshere to usindustrial-scale to state that typical furnaces air permits infiltration us to rates state vary that in typical the range air frominfiltration 17 to 120rates m 3varyday in1 · − andthe range cause from a fuel 17 consumption to 120 m3·day increase−1 and ofcause 1.6% aon fuel average, consumption which representsincrease of around 1.6% on 0.56 average, m3 of natural which gasrepresents per ton around of charged 0.56 material.m3 of natural Based gas on per these tonexperimental of charged material. results, Based it can beon concludedthese experimental that this valueresults, can it can be halved be concluded by means that of this the value novel can furnace be halved door design. by means of the novel furnace door design. Based onon thethe resultsresults of of both both experimental experimental measurement measurement and and mathematical mathematical modeling modeling of theof the rotary ro- tiltingtary tilting furnace furnace device, device, it can it can be concluded be concluded that that the the infiltration infiltration of airof air into into the the combustion combustion chamber cham- causesber causes serious serious impact impact on the on combustion the combustion process process and reduces and reduces the energy the eenergyfficiency efficiency of rotary of furnaces. rotary Fromfurnaces. the environmentalFrom the environmental point of view, point the of main view, disadvantage the main disadvantage is the associated is the increase associated in greenhouseincrease in gasgreenhouse emissions gas into emissions the atmosphere into the and atmosphere the increase and in the fuel increase consumption. in fuel The consumption. results obtained The results reflect theobtained trends reflect in EU the energy trends policy in EU aimed energy strictly policy at industrial aimed strictly carbon at footprint industrial reduction, carbon footprint and research reduc- in thistion, area and is,research therefore, in this very area useful is, ther andefore, highly very topical. useful and highly topical.

Author Contributions:Contributions: Conceptualization,Conceptualization, A.V.A.V. and and J.K.; J.K.; methodology, methodology, R.D. R.D. and and J.K.; J.K.; validation, validation, M.P. M.P. and L.L.;and investigation,L.L.; investigation, R.A. andR.A. R.D.; and resources,R.D.; resources, G.J.; data G.J.; curation, data curation, L.L. and L.L. J.K.; and writing—original J.K.; writing—original draft preparation, draft prepara- R.D., M.V.tion, andR.D., G.J.; M.V. writing—review and G.J.; writing—review and editing, and M.V., editing, M.P. and M.V., R.A.; M.P. visualization, and R.A.; visualization, R.D. and M.V.; R.D. funding and M.V.; acquisition, fund- A.V.ing acquisition, and G.J. All A.V. authors and haveG.J. All read au andthors agreed have read to the and published agreed to version the published of the manuscript. version of the manuscript. Funding: This research was funded by the VEGA 1/0691/18 project and by the Slovak Research and Development Funding: This research was funded by the VEGA 1/0691/18 project and by the Slovak Research and Develop- Agency under contract no. APVV-15-0148 and the obtained results are part of the solution of these grant projects. ment Agency under contract no. APVV-15-0148 and the obtained results are part of the solution of these grant Conﬂictsprojects. of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript, nor in the decision to publishConflicts the of results. Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript, nor in the decision to publish the results.

References

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