Design and Experimental Study of HTSG for Waste to Energy: Analysis of Pressure Difference

Article Design and Experimental Study of HTSG for Waste to Energy: Analysis of Pressure Difference

Jeachul Jang ID , Sunhee Oh, Chongpyo Cho and Seong-Ryong Park * ID Energy Efﬁciency and Materials Research Division, Korea Institute of Energy Research (KIER), Daejeon 34129, Korea; [email protected] (J.J.); [email protected] (S.O.); [email protected] (C.C.) * Correspondence: [email protected]; Tel.: +82-42-860-3224

Received: 18 June 2018; Accepted: 5 July 2018; Published: 11 July 2018

Abstract: The purpose of the present study is to analyze pressure difference changes inside a high-temperature steam generator (HTSG), which produces steam using the heat generated by waste incineration and decreases the pressure of the produced steam while increasing its temperature. The high-temperature, low-pressure steam produced by a HTSG is used for hydrogen production. Therefore, the steam temperature must be at least 700 ◦C, and the pressure must be lower than 300 kPa; hence, a device is needed to increase the steam temperature in the boiler and decrease the steam pressure. The physical behavior of the device was modeled and experimentally validated. The modeling and experimental results demonstrated good agreement when the steam was not preheated; however, an additional pressure drop required consideration of the opposite case.

Keywords: high-temperature steam generator (HTSG); superheater; waste to energy (WtE); sudden effect; pressure difference

1. Introduction Waste to energy (WtE) has the advantages of reducing waste and transforming the energy obtained during incineration to a new form of useful energy [1]. In addition, the waste heat that is dissipated during incineration can be used to generate steam in a waste heat boiler, which can then be used directly or supplied to a turbine for power generation [2,3]. In cases when steam is generated and used directly near demanding locations, the steam temperature typically ranges between 125 and 180 ◦C[4]. This temperature range is obtained without a superheater due to the requirements and economic feasibility. When a superheater is not installed, low-temperature corrosion needs to be considered because the water vapor in the air accelerates chemical changes [5,6]. WtE thermochemical technology typically includes incineration, thermal gasification, and pyrolysis methods. In the incineration method, waste is burned at temperatures above 1000 ◦C. However, the conventional temperature of the thermal gasification method is 750 ◦C, and the conventional temperature of the pyrolysis method ranges between 300 and 800 ◦C, which occurs at a higher pressure and in the absence of oxygen. However, the practical temperature and pressure at which the steam is produced in the waste heat boiler is approximately 400 ◦C and 40 bar, respectively, due to high-temperature corrosion problems [7,8]. Extensive research on materials has been recently conducted to overcome this challenge. Zielińskiet al. [9] introduced a material with an austenitic structure (HR6W, Sanicro 25) that has stability up to 750 ◦C. In particular, Sanicro 25 was proven to remain stable for more than 2000 h at 700 ◦C. Zeng et al. [10] investigated the tensile properties, microhardness, chemical composition distribution, and microstructure of the BTi-6431S titanium alloy, which can be used in applications up to 700 ◦C. Furthermore, Wright et al. [11] examined the properties of Alloy 800H and Alloy 617 for nuclear plants and asserted that their operating temperature range

Energies 2018, 11, 1815; doi:10.3390/en11071815 www.mdpi.com/journal/energies Energies 2018, 11, 1815 2 of 14 Energies 2018, 11, x FOR PEER REVIEW 2 of 14 temperaturewas 750–800 range◦C in thewas steam 750–800 generator. °C in the Dewa steam et generator. al. [12] also Dewa analyzed et al. the[12] fatigue also analyzed properties the offatigue Alloy properties617 and stated of Alloy that 617 its maximum and stated design that its temperature maximum design was 950 temperature◦C. was 950 °C. AmongAmong the the technologies technologies available available for for hydrogen hydrogen production, production, the the operating operating conditions conditions and and characteristicscharacteristics of of fuel fuel cells cells vary vary depending depending on on their their constituent constituent materials. materials. Some Some examples examples include include the the electrolyteelectrolyte-type‐type alkaline fuelfuel cell,cell, the the phosphoric phosphoric acid acid fuel fuel cell, cell, the the polymer polymer electrolyte electrolyte membrane membrane fuel fuelcell, cell, the the molten molten carbonate carbonate fuel fuel cell, cell, and and the the solid solid oxide oxide fuel fuel cell cell (SOFC). (SOFC). The The objective objective of of this this work work is isto to produce produce steam steam for for hydrogen hydrogen production production using using heat, heat, and and thus thus the the SOFC SOFC is considered,is considered, as as it hasit has an anoperating operating temperature temperature range range of of 600–1000 600–1000◦C[ °C13 [13].]. In In addition, addition, the the minimum minimum operating operating temperature temperature of ofthe the SOFC SOFC is is 700 700◦C, °C, and and the the efﬁciency efficiency increases increases along along with with the the heat heat source source temperature temperature [14 [14].]. TheThe use use of hydrogen andand powerpower generation generation technologies technologies that that exploit exploit the the steam steam obtained obtained from from the theheat heat generated generated during during waste waste incineration incineration is expected is expected to help to help reduce reduce the the effects effects of global of global warming warming and andenvironmental environmental pollution. pollution. Despite Despite their advantages,their advantages, hydrogen hydrogen production production technologies technologies that leverage that leveragewastes have wastes not have yet not been yet commercialized been commercialized owing owing to the to lack the lack of advanced of advanced technology technology required required to togenerate generate the the high-temperature high‐temperature steam steam (greater (greater than than 700 700◦C) °C) using using waste. waste. Therefore,Therefore, this this paper paper presents presents the the design design of of a a high high-temperature‐temperature steam steam generator generator (HTSG) (HTSG) having having aa structure structure that that can can be be inserted inserted into into an an incinerator incinerator to to produce produce high high-temperature‐temperature steam steam over over 700 700 °C.◦C. TheThe steam steam passing passing through through the the HTSG HTSG is is designed designed to to be be heated heated and and expanded expanded because because the the pressure pressure mustmust be be reduced reduced due due to to SOFC SOFC durability. durability. The The pressure pressure characteristics characteristics of of the the proposed proposed HTSG HTSG are are analyzedanalyzed herein. herein.

2.2. High High-Temperature‐Temperature Steam Steam Generator Generator (HTSG) (HTSG) Design WhenWhen the the steam steam generated generated using using waste waste incineration incineration heat heat is is used used directly, directly, the the steam steam temperature temperature rangesranges between between 125 125 and and 180 180 °C◦C[ [4].4]. Therefore, Therefore, the the design design temperature temperature considered considered herein herein is is 151.9 151.9 °C,◦C, whichwhich corresponds corresponds to to the the saturation saturation temperature temperature at at the the design design pressure pressure of of 500 kPa. The The processes processes illustratedillustrated inin Figure Figure1 are1 are required required to lower to lower the pressure the pressure below 300 below kPa while 300 kPa increasing while theincreasing temperature the temperatureabove 700 ◦C. above In Process 700 °C. A, In the Process steam is A, ﬁrst the heated steam atis thefirst proposed heated at inlet the conditions, proposed inlet and theconditions, pressure andis subsequently the pressure reduced.is subsequently Contrarily, reduced. Process Contrarily, B ﬁrst reduces Process the B first pressure reduces and the then pressure heats theand steam. then heatsProcess the A steam. generally Process exhibits A generally higher exergy exhibits than higher Process exergy B. However, than Process devices B. However, that expand devices the heated that expandsteam above the heated temperatures steam ofabove 700 ◦ Ctemperatures will face durability of 700 issues °C will and areface considerably durability issues more expensiveand are considerablythan those designed more expensive for relatively than those low-temperature designed for ranges.relatively As low a result,‐temperature the HTSG ranges. of this As a study result, is thedesigned HTSG consideringof this study the is designed aspects of considering both engineering the aspects and energy. of both engineering and energy.

FigureFigure 1. 1. TheThe processes processes A A and and B B on on the the T T-s‐s diagram. diagram.

In a typical closed‐loop system, the pressure and temperature rise simultaneously. Since the HTSG proposed herein heats the steam using the high waste incineration heat (1000 °C) [7], which is

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EnergiesIn 2018 a typical, 11, x FOR closed-loop PEER REVIEW system, the pressure and temperature rise simultaneously. Since3 of the 14 HTSG proposed herein heats the steam using the high waste incineration heat (1000 ◦C) [7], which is a a sufficient heat source, the heat transfer area can be sufficiently enlarged while forcing the pressure sufﬁcient heat source, the heat transfer area can be sufﬁciently enlarged while forcing the pressure to drop. to drop. Figure 2 depicts the proposed HTSG, wherein different variables are used to describe each Figure2 depicts the proposed HTSG, wherein different variables are used to describe each component for the purposes of modeling the pressure drops. The heat transfer area corresponds to component for the purposes of modeling the pressure drops. The heat transfer area corresponds to the the total height (), which is installed in the combustor for the experiment. The heat transfer area total height (H ), which is installed in the combustor for the experiment. The heat transfer area was was obtained fromtot Equation (1). Where, the heat loss is generally higher at high temperature ranges. obtained from Equation (1). Where, the heat loss is generally higher at high temperature ranges. In the In the combustor, the heat loss was assumed to be 30%, because the heat loss due to fouling is large combustor, the heat loss was assumed to be 30%, because the heat loss due to fouling is large [15,16]. [15,16]. . QHTSG=UtotAHTSG∆∆Tlm = ηm aveC,p,ave(To − Ti),, (1) (1)

Figure 2. The schematic diagram of the high-temperature steam generator (HTSG) geometry. Figure 2. The schematic diagram of the high‐temperature steam generator (HTSG) geometry.

Furthermore,Furthermore, the the overall overall heat heat transfer transfer coefficient coefficient was was computed computed using using Equation Equation (2). (2). 1 = , (2) Utot t , (2) 1+ + 1 hin kmt hex The Gnielinski correlation [17] was employed to compute the Nusselt number and obtain the internalThe and Gnielinski external correlation heat transfer [17] coefficients was employed (Equation to compute (3)). The the internal Nusselt and number external and fluids obtain were the internalassumed and to be external steam heatand air, transfer respectively. coefficients (Equation (3)). The internal and external fluids were assumed to be steam and air, respectively. , f (3) 8 .(Re− 1000 )Pr = Nu 1 , (3) f 2 2 Here, the friction factor, f, is given by Equation (4) [18].3 1 + 12.7 8 (Pr − 1) 0.79 ln 1.64, (4)

Here, the friction factor, f, is given by Equation (4) [18].

The ratio of the diameter () to the total height () of the HTSG was calculated using Equations −2 (1) and (4) and resulted in a ratio of f1:2.= (The0.79 design ln Re − lengths1.64) ,of the diameter and total height were (4) approximately 0.35 m and 0.7 m, respectively. In general, as the length of the total height increases, the streamline length increases; however, the outlet velocity decreases at the same time. Therefore, the diameter and the total height were selected such that the flow of steam was smooth. In this study, a laboratory scale experiment was conducted beforehand, and the HTSG material was SUS310. The thermal conductivity of SUS310 is similar to that of HR6W [9] at temperatures below 400 °C, as shown

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The ratio of the diameter (D) to the total height (Htot) of the HTSG was calculated using Equations (1) and (4) and resulted in a ratio of 1:2. The design lengths of the diameter and total height were approximately 0.35 m and 0.7 m, respectively. In general, as the length of the total height increases, the streamline length increases; however, the outlet velocity decreases at the same time. Therefore, the diameter and the total height were selected such that the ﬂow of steam was smooth. InEnergies this study, 2018, 11 a, laboratory x FOR PEER REVIEW scale experiment was conducted beforehand, and the HTSG material was4 of 14 SUS310. The thermal conductivity of SUS310 is similar to that of HR6W [9] at temperatures below 400in ◦FigureC, as shown 3, however, in Figure its3 conductivity, however, its differs conductivity from that differs of HR6W from that at temperatures of HR6W at temperatures above 400 °C. aboveFurther 400 analysis◦C. Further on this analysis discrepancy on this discrepancy would be necessary would be prior necessary to actual prior site to applications. actual site applications.

FigureFigure 3. 3.The The thermal thermal conductivities conductivities of of the the HR6W HR6W and and SUS310. SUS310.

3.3. Pressure Pressure Difference Difference TheThe size size of of each each HTSG HTSG component component was was determined determined to to generate generate a a reduced reduced pressure pressure while while using using the HTSG. The inner diameter (d ) at the inlet was approximately 0.008 m, the outlet inner diameter the HTSG. The inner diameter (i) at the inlet was approximately 0.008 m, the outlet inner diameter (do) was approximately 0.025 m, and ìt was designed to be longer than HHTSG. Moreover, the angle ( ) was approximately 0.025 m, and ℓ was designed to be longer than HHTSG. Moreover, the angle of the pipe from the entrance to the HTSG inlet was established as 5◦. This value was selected to of the pipe from the entrance to the HTSG inlet was established as 5°. This value was selected to minimize the heat loss and to avoid pressure stagnation inside the HTSG by immediately pushing the minimize the heat loss and to avoid pressure stagnation inside the HTSG by immediately pushing induced vortices toward the pipe outlet. If the inlet angle of the pipe is 0◦, the residence time of steam the induced vortices toward the pipe outlet. If the inlet angle of the pipe is 0°, the residence time of at the top of the HTSG increases. This decreases the velocity of steam at the outlet of the HTSG. steam at the top of the HTSG increases. This decreases the velocity of steam at the outlet of the HTSG. Figure4 features a schematic of the parts that affect the pressure variation inside the HTSG. In Part Figure 4 features a schematic of the parts that affect the pressure variation inside the HTSG. In A, the pressure drops sharply due to the sudden enlargement, and the steam flows into the HTSG Part A, the pressure drops sharply due to the sudden enlargement, and the steam flows into the body at the HTSG inlet in a tangential direction, which causes the swirling flow in Part B. The majority HTSG body at the HTSG inlet in a tangential direction, which causes the swirling flow in Part B. The of the heat transfer and pressure reduction occurs in Part B. Part C is designed to induce a vortex flow, majority of the heat transfer and pressure reduction occurs in Part B. Part C is designed to induce a which allows the fluid to exit directly towards the outlet; a slight pressure drop occurs due to friction. vortex flow, which allows the fluid to exit directly towards the outlet; a slight pressure drop occurs In Part D, the pressure is reduced by the sudden contraction. Subsequently, the pressure drop across due to friction. In Part D, the pressure is reduced by the sudden contraction. Subsequently, the each part can be computed as the total pressure drop, given by Equation (5). The additional pressure pressure drop across each part can be computed as the total pressure drop, given by Equation (5). drop (∆Pte) due to heating was obtained from the experiment. The additional pressure drop ∆ due to heating was obtained from the experiment. ∆P = ∆P + ∆P + ∆P + ∆P + (∆P ), (5) tot∆ ∆se ∆s f ∆v f ∆sc ∆te, (5)

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FigureFigure 4. The4.The The streamline streamline streamline of of the the HTSG and and typical typical pressure pressure profile. profile. proﬁle.

3.1. Sudden3.1. Sudden Enlargement Enlargement Part Part 3.1. Sudden Enlargement Part The Thepressure pressure decreases decreases abruptly abruptly when when thethe fluid exitsexits the the pipe pipe due due to tothe the sudden sudden enlargement enlargement The pressure decreases abruptly when the ﬂuid exits the pipe due to the sudden enlargement effecteffect [19,20]. [19,20]. The The HTSG HTSG proposed proposed in in this this study study features a a shape shape that that varies varies from from a narrow a narrow inlet inletarea area effect [19,20]. The HTSG proposed in this study features a shape that varies from a narrow inlet area to to a widerto a wider area, area, thus, thus, forcing forcing the the pressure pressure to to drop.drop. The Borda Borda‐Carnot‐Carnot equation equation was was used used to compute to compute a widerthe area,pressure thus, drop forcing (Equation the pressure (6)). to drop. The Borda-Carnot equation was used to compute the the pressure drop (Equation (6)). pressure drop (Equation (6)). ∆ 2, (6) ∆ γvi , (6) ∆Pse= ξ se , (6)

g where, is the coefficient of fluid resistance obtained2 from Karev’s experimental data [19]. where, is the coefficient of fluid resistance obtained from Karev’s experimental data [19]. where, ξse is the coefﬁcient of ﬂuid resistance obtained from Karev’s experimental data [19]. 3.2. Swirling Flow Part 3.2. Swirling Figure Flow 5 illustratesPart the steam in the HTSG that swirls downwards in the tangential direction. FigureThe length 55 illustratesillustrates of the streamline thethe steamsteam is obtained inin thethe HTSGbyHTSG the sum thatthat of swirlsswirls L, and downwardsdownwards uniform spacing inin thethe is assumed tangentialtangential between direction.direction. the lines. The pressure drop is then computed using the well‐known Darcy‐Weisbach equation The length of the streamline is obtained by the sum of L, and uniform spacing is assumed between (Equation (7)) [21]. the lines. The The pressure pressure drop drop is is then then computed using the well well-known‐known Darcy Darcy-Weisbach‐Weisbach equation (Equation (7)) [[21].21]. Δ ,, (7)

2 fHTSG vave ∆ΔPs f =ρave L,s f ,tot,, (7) (7) dave 2

Figure 5. The streamline of the swirling flow.

Figure 5. TheThe streamline streamline of the swirling flow. ﬂow.

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3.3. Vortex Flow Part After passing through Part B, the fluid forms a vortex flow and concentrates at the bottom center as it enters Part C. In this case, it was assumed that the streamline of Part C was extended from Part HHTSG B, and the streamline can be calculated using Equation (8) because it was assumed that n was constant. When the hemispherical bottom of the HTSG is differentiated infinitely, the length of each stream is linear, and the total streamline in the vortex flow region can be obtained by integration. HHTSG Here, n is assumed to be the same as that in the swirling flow. v u  2 u Z D u 2 u  2π x  Lv f ,tot = u1 + − r  dx, (8) 0  n 2  t D 2 2 − x where, n is the pitch number between the stream lines. The pressure drop was computed using the Darcy-Weisbach equation [21].

3.4. Sudden Contraction Part The sudden contraction as the fluid enters the pipe causes the pressure to drop, similar to the sudden enlargement pressure drop discussed previously [19,20]. The proposed HTSG is also designed to discharge the fluid that is concentrated at the center due to the vortex flow through the intermediate pipe. Additionally, the shape of the intermediate pipe varies from a wide to a narrower area. Subsequently, the pressure drop is obtained from Equation (9).

γv 2 ∆P = ξ o , (9) sc sc 2g where, ξsc was given by the sudden contraction data in Karev’s experimental data [19].

4. Experiments The HTSG experimental setup contained an external combustor to heat the HTSG, as depicted in Figure6. The steam boiler supplied steam to the HTSG, and a mass flowmeter (MF, vortex type flowmeter) was installed at the HTSG inlet. The temperature and pressure at the inlet and outlet of the HTSG, as well as those of the gas that served as a heat source, were measured. To minimize the heat loss, the combustor exterior and steam pipe were insulated with 150 mm of castable and 100 mm of fiberglass, respectively. The details of each sensor are presented in Table1. The experimental data were obtained by averaging the values for more than 30 min after the inlet/outlet temperatures, inlet/outlet pressures, and the flow rate of steam became constant. Additionally, the steam was supplied to the HTSG inlet at a constant pressure and flow rate using a pressure regulator.

Table 1. Speciﬁcation of the sensors.

Sensor Operating Range Uncertainty Temperature sensor −200–1250 ◦C ±0.02 ◦C (T, k-type thermocouple) Pressure transducer 0–1000 kPa ±4.71 kPa (P, diaphragm type) Mass ﬂowmeter 0.02–47.6 t/h ±0.35 kg/h (MF, vortex ﬂowmeter) Energies 2018, 11, x FOR PEER REVIEW 6 of 14

3.3. Vortex Flow Part After passing through Part B, the fluid forms a vortex flow and concentrates at the bottom center as it enters Part C. In this case, it was assumed that the streamline of Part C was extended from Part B, and the streamline can be calculated using Equation (8) because it was assumed that was constant. When the hemispherical bottom of the HTSG is differentiated infinitely, the length of each stream is linear, and the total streamline in the vortex flow region can be obtained by integration. Here, is assumed to be the same as that in the swirling flow.

, 1 , (8)

where, n is the pitch number between the stream lines. The pressure drop was computed using the Darcy‐Weisbach equation [21].

3.4. Sudden Contraction Part The sudden contraction as the fluid enters the pipe causes the pressure to drop, similar to the sudden enlargement pressure drop discussed previously [19,20]. The proposed HTSG is also designed to discharge the fluid that is concentrated at the center due to the vortex flow through the intermediate pipe. Additionally, the shape of the intermediate pipe varies from a wide to a narrower area. Subsequently, the pressure drop is obtained from Equation (9).

∆ , (9)

where, was given by the sudden contraction data in Karev’s experimental data [19].

4. Experiments The HTSG experimental setup contained an external combustor to heat the HTSG, as depicted in Figure 6. The steam boiler supplied steam to the HTSG, and a mass flowmeter (MF, vortex type flowmeter) was installed at the HTSG inlet. The temperature and pressure at the inlet and outlet of the HTSG, as well as those of the gas that served as a heat source, were measured. To minimize the heat loss, the combustor exterior and steam pipe were insulated with 150 mm of castable and 100 mm of fiberglass, respectively. The details of each sensor are presented in Table 1. The experimental data were obtained by averaging the values for more than 30 min after the inlet/outlet temperatures, inlet/outlet pressures, and the flow rate of steam became constant. EnergiesAdditionally,2018, 11, the 1815 steam was supplied to the HTSG inlet at a constant pressure and flow rate using7 of a 14 pressure regulator.

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Table 1. Specification of the sensors.

Sensor Operating Range Uncertainty Temperature sensor −200–1250 °C ±0.02 °C (T, k‐type thermocouple) Pressure transducer 0–1000 kPa ±4.71 kPa (P, diaphragm type) Mass flowmeter 0.02–47.6 t/h ±0.35 kg/h FigureFigure(MF, vortex6. 6. TheThe schematic schematicflowmeter) diagram diagram of of the the HTSG HTSG tester tester and and HTSG HTSG test test rig. rig.

5.5. Results Results TheThe HTSG HTSG pressure pressure drop drop model model was was used used to to obtain obtain the the pressure pressure drops drops as as functions functions of of different different inletinlet conditions. conditions. In In addition, addition, various various entrance entrance angles angles and and changes changes in in the the outlet outlet pipe pipe diameter diameter were were examinedexamined to to further further reduce reduce the the computed computed pressure drop.

5.1.5.1. Modeling Modeling Results Results FigureFigure 7 shows the the pressure pressure variation variation as as a function a function of the of theReynolds Reynolds number number at the at HTSG the HTSG inlet, whichinlet, which demonstrates demonstrates the effect the effectof the of sudden the sudden enlargement. enlargement. As the As Reynolds the Reynolds number number increases, increases, the higherthe higher fluid ﬂuid velocity velocity causes causes the magnitude the magnitude of pressure of pressure to drop. to drop.The magnitudes The magnitudes of the pressure of the pressure drops aredrops expected are expected to increase to increase further further if higher if higher flow rates ﬂow ratesthan those than those proposed proposed herein herein are implemented. are implemented.

Figure 7. The pressure difference according to the Reynolds number (sudden enlargement effect). Figure 7. The pressure difference according to the Reynolds number (sudden enlargement effect).

FigureFigure 8 displays the pressure variation inin the swirling flowflow region. The pressure drop decreases asas the ReynoldsReynolds number number at at the the inlet inlet increases increases due todue the to reduced the reduced friction friction factor with factor increased with increased Reynolds Reynoldsnumbers. numbers. Subsequently, Subsequently, the pressure the drop pressure will increase drop will further increase if the fluxfurther increases if the at flux higher increases inlet flow at higherrates. Figure inlet flow9 depicts rates. the Figure pressure 9 depicts variation the pressure in the swirling variation flow in the region swirling as a functionflow region of the as a inlet function pipe of the inlet pipe entrance angle. As the entrance angle decreases, the pressure drop increases. This is because the length of streamline increases as the angle becomes smaller. However, further reduction of the entrance angle will lead to fluid retention in the upper region of the HTSG, and the pressure will stagnate; hence, care must be taken in the design process.

Energies 2018, 11, 1815 8 of 14 entrance angle. As the entrance angle decreases, the pressure drop increases. This is because the length of streamline increases as the angle becomes smaller. However, further reduction of the entrance angle will lead to ﬂuid retention in the upper region of the HTSG, and the pressure will stagnate; hence, care EnergiesmustEnergies be 2018 2018 taken, ,11 11, ,x x inFOR FOR the PEER PEER design REVIEW REVIEW process. 88 of of 14 14

FigureFigure 8. 8. The The pressure pressure difference difference according according to to the the Reynolds Reynolds number number (swirling (swirling flow ﬂowflow effect). effect).effect).

FigureFigure 9. 9. The The pressure pressure difference difference according according to to the the entrance entrance angle angle at at the the swirling swirling flow ﬂowflow part. part.part.

FigureFigure 1010 depicts depictsdepicts the thethe pressure pressurepressure variation variationvariation in in thein thethe vortex vortexvortex ﬂow flowflow region. region.region. The pressureTheThe pressurepressure drop indropdrop this inin region thisthis regionisregion relatively isis relativelyrelatively smaller smaller thansmaller the thanthan other thethe parts otherother due partsparts to the duedue smaller toto thethe area, smallersmaller and the area,area, pressure andand thethe drop pressurepressure decreases dropdrop as decreasesthedecreases Reynolds asas thethe number ReynoldsReynolds at the numbernumber inlet increases atat thethe inlet dueinlet to increasesincreases the reason duedue discussed toto thethe reasonreason previously discusseddiscussed pertaining previouslypreviously to the pertainingswirlingpertaining ﬂow to to the the region. swirling swirling flow flow region. region.

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FigureFigure 10. 10. TheThe pressure pressure difference difference according according to to the the Reynolds Reynolds number number (vortex (vortex flow ﬂow effect). effect). Figure 10. The pressure difference according to the Reynolds number (vortex flow effect). Figure 11 shows the pressure variation in the sudden contraction region. This pressure drop is FigureFigure 11 11 shows shows the the pressure pressure variation variation in in the the sudden sudden contraction contraction region. region. This This pressure pressure drop drop is is also small due to the relatively large size of the discharging pipe. The pressure drop increases with alsoalso small small due due to to the the relatively relatively large large size size of of the the discharging discharging pipe. pipe. The The pressure pressure drop drop increases increases with with increasing Reynolds number, similar to the sudden enlargement region. Figure 12 depicts the increasingincreasing Reynolds number,number, similar similar to theto the sudden sudden enlargement enlargement region. region. Figure Figure 12 depicts 12 depicts the pressure the pressure variation in the sudden contraction region with respect to different outlet pipe diameters, pressurevariation variation in the sudden in the contraction sudden contraction region with region respect with to respect different to outletdifferent pipe outlet diameters, pipe diameters, where the where the pressure drop increases as the pipe diameter decreases. However, if the outlet pipe wherepressure the drop pressure increases drop as theincreases pipe diameter as the pipe decreases. diameter However, decreases. if the outletHowever, pipe if diameter the outlet is further pipe diameter is further reduced, the pressure inside the HTSG will stagnate; thus, careful consideration diameterreduced, is the further pressure reduced, inside the the pressure HTSG will inside stagnate; the HTSG thus, will careful stagnate; consideration thus, careful is required consideration in the is required in the design process. isdesign required process. in the design process.

Figure 11. The pressure difference according to the Reynolds number (sudden contraction effect). Figure 11. The pressure difference according to the Reynolds number (sudden contraction effect). Figure 11. The pressure difference according to the Reynolds number (sudden contraction effect).

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Figure 12. The pressure difference according to the outlet pipe diameter at the sudden contraction Figurepart. 12.12. TheThe pressure pressure difference difference according according to to the the outlet outlet pipe pipe diameter diameter at the at suddenthe sudden contraction contraction part. part. 5.2. Experimental Results 5.2. Experimental Results The experiment experiment conducted conducted herein herein considered considered various various temperatures temperatures at the at theHTSG HTSG inlet. inlet. The Thetemperature temperatureThe experiment was was controlled controlled conducted by byinstalling herein installing considered a heater a heater at at thevarious the HTSG HTSG temperatures inlet, inlet, and and the the at Reynolds Reynoldsthe HTSG number inlet. wasThe variedtemperaturevaried byby changingchanging was controlled thethe flowflow raterateby installing forfor a given a heater temperature. at the HTSG Figure inlet, 13 shows and athe graph Reynolds of the number temperature was variationvariedvariation by as changing a a function function the of flow of the the rate different different for a giventemperatures temperatures temperature. and and flow Figure flow rates. 13 rates. showsThe Thepressure a graph pressure drop of the drop increases temperature increases with withvariationthe Reynolds the Reynolds as a functionnumber number andof the HTSG and different HTSG inlet temperatures temperature inlet temperature dueand flowto due the rates. toincreased the The increased pressure fluid viscosity fluid drop viscosity increases at the HTSG at with the HTSGtheinlet. Reynolds inlet. number and HTSG inlet temperature due to the increased fluid viscosity at the HTSG inlet.

Figure 13. Comparisons of the pressure difference according to the inlet temperature. Figure 13. Comparisons of the pressure difference according to the inlet temperature. Figure 13. Comparisons of the pressure difference according to the inlet temperature. Figure 1414 compares compares the the two two cases, cases, one one where where the combustorthe combustor interior interior is heated is heated and the and other the where other it is not heated. When the HTSG surface is heated, the internal HTSG temperature increases, and an whereFigure it is not 14 heated.compares When the twothe HTSG cases, surfaceone where is heated, the combustor the internal interior HTSG is temperature heated and increases,the other whereand an it additional is not heated. pressure When drop the HTSGoccurs surface due to is the heated, high‐ temperaturethe internal HTSG steam temperatureresiding in theincreases, upper and an additional pressure drop occurs due to the high‐temperature steam residing in the upper

Energies 2018,, 11,, 1815x FOR PEER REVIEW 11 of 14 Energies 2018, 11, x FOR PEER REVIEW 11 of 14 region of the HTSG, thus, pushing out the steam that enters through the inlet. Such a phenomenon is regionadditionalherein oftermed the pressure HTSG, the thermo dropthus, occurs pushing‐extrusion due out to effect, the the steam high-temperature and thatFigure enters 15 shows through steam the residing the additional inlet. in Such the pressure upper a phenomenon region variations of the is hereinHTSG,due to termedthis thus, effect. pushing the thermo out the‐extrusion steam that effect, enters and through Figure the 15 inlet.shows Such the aadditional phenomenon pressure is herein variations termed duethe thermo-extrusionto this effect. effect, and Figure 15 shows the additional pressure variations due to this effect.

Figure 14. ComparisonsComparisons of the heating case and non non-heating‐heating case according to the Reynolds number. Figure 14. Comparisons of the heating case and non‐heating case according to the Reynolds number.

Figure 15. TheThe additional additional pressure difference by heating (thermo-extrusion(thermo‐extrusion effect). Figure 15. The additional pressure difference by heating (thermo‐extrusion effect). The thermo-extrusionthermo‐extrusion effect is caused by the changes in the density and viscosity as the ﬂuidfluid is The thermo‐extrusion effect is caused by the changes in the density and viscosity as the fluid is heated, as well as due to the fluid ﬂuid velocity change. Therefore, this effect can be described as a function heated, as well as due to the fluid velocity change. Therefore, this effect can be described as a function of the Reynolds number (Equation (10)). of the Reynolds number (Equation (10)). ∆ , (10) β ∆∆Pte =f(Re ) = αRe ,, (10) (10) where, α = 0.00001 and β = 1.3287 within the present experimental range. where,where,Figure αα = 0.00001 0.00001 16 shows and and βaβ =comparison= 1.3287 1.3287 within within of thethe present presentsimulation experimental experimental and experimental range. range. results. The dotted line representsFigure the16 linearizedshows a comparison experimental of results,the simulation and the solidand experimentalline denotes theresults. linearized The dotted simulation line represents the linearized experimental results, and the solid line denotes the linearized simulation

Energies 2018, 11, 1815 12 of 14

EnergiesFigure 2018, 1116, x showsFOR PEER a REVIEW comparison of the simulation and experimental results. The dotted12 lineof 14 represents the linearized experimental results, and the solid line denotes the linearized simulation results. The The discrepancy discrepancy increases increases with with increasing increasing Reynolds Reynolds numbers numbers due due to thermo to thermo-extrusion,‐extrusion, the theeffects effects of which of which must must be considered be considered in the in thedesign design process. process.

Figure 16. Comparisons of the calculation results and experimental test results (inlet temperature Figure 16. Comparisons of the calculation results and experimental test results (inlet temperature 151.9 ◦C approximately). 151.9 °C approximately). 6. Conclusions 6. Conclusions In this study, we examined the use of an HTSG to increase the steam temperature and reduce the In this study, we examined the use of an HTSG to increase the steam temperature and reduce pressure of the high-temperature steam that is obtained using waste incineration heat for hydrogen the pressure of the high‐temperature steam that is obtained using waste incineration heat for production. The structure of the HTSG was designed to facilitate maintenance and repair when hydrogen production. The structure of the HTSG was designed to facilitate maintenance and repair inserted into an actual waste incinerator, and the proposed design model was experimentally validated. when inserted into an actual waste incinerator, and the proposed design model was experimentally The structure of the proposed HTSG can be deduced from the pressure changes due to sudden validated. The structure of the proposed HTSG can be deduced from the pressure changes due to enlargement, swirling flow, vortex flow, and sudden contraction when the fluid temperature remains sudden enlargement, swirling flow, vortex flow, and sudden contraction when the fluid temperature constant. In the case when the HTSG is heated, however, the pressure variation as a function of remains constant. In the case when the HTSG is heated, however, the pressure variation as a function temperature must be considered. As a result, the inner diameters of the HTSG inlet and outlet and the of temperature must be considered. As a result, the inner diameters of the HTSG inlet and outlet and entrance angle must be reduced, and the streamline must be lengthened by extending the Htot of the the entrance angle must be reduced, and the streamline must be lengthened by extending the of HTSG body to increase the pressure drop. the HTSG body to increase the pressure drop. Author Contributions: J.J. conceived this paper and proposed the model; S.O. performed the experiments; C.C.Author advised Contributions: the proposed J.J. conceived paper; S.-R.P. this paper designed and proposed the experimental the model; device S.O. performed and contributed the experiments; to the entire C.C. manuscript.advised the proposed All authors paper; provided S.‐R.P. substantive designed the comments. experimental device and contributed to the entire manuscript. Funding:All authorsThis provided subject substantive is supported comments. by the “Technologies for Waste Recycling Policy” funded by the Korea Ministry ofFunding: Environment This subject (MOE) is as supported the “Public by Technology the “Technologies Program basedfor Waste on Environmental Recycling Policy” Policy” funded (2016000710007). by the Korea ConflictsMinistry ofof Interest:EnvironmentThe authors (MOE) declare as the no “Public conflict ofTechnology interest. Program based on Environmental Policy” (2016000710007). Nomenclature Conflicts of Interest: The authors declare no conflict of interest. A Area m2 CNomenclaturep Specfic heat at constant pressure kJ/kg·K D Diameter of HTSG body m A Area m2 d Diameter of pipe m Cp Specfic heat at constant pressure kJ/kg∙K f Friction factor D Diameter of HTSG body m g Gravitational acceleration m/s2 d Diameter of pipe m f Friction factor g Gravitational acceleration m/s2 H Height m

Energies 2018, 11, 1815 13 of 14

H Height m h Convection heat transfer coefficient kW/m2·K k Conduction heat transfer coefficient kW/m·K L Stream line length m ` Pipe length m m˙ Mass flow kg/s n Number of pitch EA Nu Nusselt number P Pressure kPa Pr Prandtl number Cpµ/k Q Heat capacity kW Re Reynolds number ρvd/µ T Absolute temperature K t Thichness m U Heat transfer coefficient kW/m2·K v Velocity m/s Greek Letters α Coefficient of thermo-extrusion β Exponent of thermo-extrusion ∆ Difference η Heat loss coefficient Θ Angle ◦ µ Viscosity kg/ms ξ Resistance coefficient ρ Density kg/m3 Subscriptions ave Average ex External HTSG High temperature steam generator i Inlet in Internal it Intermediate lm Log mean temperature difference mt Material o Outlet sc Sudden contraction se Sudden enlargement sf Swirling flow st Stream line te Thermo-extrusion tot Total vf Vortex flow

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