Efficiency analysis of air-fuel and oxy-fuel combustion in a reheating furnace

International Journal of Heat and Mass Transfer 121 (2018) 1364–1370

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Efficiency analysis of air-fuel and oxy-fuel combustion in a reheating furnace Sang Heon Han a,⇑, Yeon Seung Lee b,⇑, J.R. Cho b, Kyun Ho Lee c a

NEXTfoam Co., 32, Digital-ro 9 gil, Geumcheon-gu, Seoul 08512, Republic of Korea Department of Naval Architecture and Ocean Engineering, Hongik University, 2639, Sejong-ro, Jochiwon-eup, Sejong 30016, Republic of Korea c Department of Aerospace Engineering Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 27 August 2017 Received in revised form 20 December 2017 Accepted 22 December 2017

Keywords: Reheating furnace Oxy-fuel combustion Efficiency Radiative slab heating

a b s t r a c t This study numerically verified the enhanced efficiency of a steel reheating furnace when applying oxyfuel combustion instead of air-fuel combustion. Only radiation heat transfer was considered to analyze the periodically transient slab heating for an axial-fired furnace. The radiation field was computed without flow field calculation by dividing the entire furnace into ten subzones of which the temperatures were calculated by taking the overall heat balance for all the subzones. A total of five cases, 2 for airfuels and 3 for oxy-fuels, was analyzed to compare the slab heating behavior between air-fuel and oxy-fuel combustion. The modified 5-gas WSGGM was used for oxy-fuel combustion cases to fulfill the characteristics of CO2 and H2O enriched medium, while ordinary 4-gas WSGGM was used for air-fuel combustion cases. From the efficiency analysis for the total of five cases, it was predicted that oxy-fuel combustion gave an approximately 50% enhancement in efficiency compared to air-fuel combustion. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction In an integrated steel mill, 80% of annual CO2 emissions are produced by the melting of ore while the remainder is mostly produced in the heating slabs or billets in reheating furnaces. Because the steel mill industry is the third largest CO2 emissions sector in South Korea [1], there is a compelling case to be made for steel mill companies to take necessary actions to reduce CO2 emissions in the near future. As is typically found in other thermal plants, the use of renewable energy sources can appear to offer a suitable choice for CO2 reduction in reheating furnaces. However, large integrated steel mills produce an abundant amount of byproduct gases, which are sufficient to run their own energy facilities. Such steel mills do not need a renewable energy source, and it is, therefore, desirable to develop an alternative approach. Reduction of CO2 emissions from reheating furnaces can be achieved by enhancing the thermal efficiency. This can be achieved by a reduction of heat loss through the furnace walls, heat recovery of exhaust gases, complete combustion, a redesign of the geometric configuration of reheating furnaces, and oxy-fuel combustion. However, the first four methods are well developed. The heat loss ⇑ Corresponding authors at: NEXTfoam CO., 32, Digital-ro 9 gil, Geumcheon-gu, Seoul 08512, Republic of Korea. E-mail addresses: [email protected] (S.H. Han), [email protected] (Y.S. Lee). https://doi.org/10.1016/j.ijheatmasstransfer.2017.12.110 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved.

through furnace walls can be managed to <3% of total heat input [2,3]. A recuperator is used to elevate the combustion air temperature using the heat from exhaust gases. Recuperators and burners are currently well developed and show only marginal potential for upgrade. Highly efficient burners can attain almost complete combustion. Because more than 90% of heat transfer to slabs or billets occurs by radiation in reheating furnaces [4,5], heating efficiency can be enhanced by oxy-fuel combustion [6]. The oxy-fuel combustion is free from the burden of heating up nitrogen and can give higher flame temperatures compared to ordinary air-fuel combustion [7]. In addition, oxy-fuel combustion produces a more radiatively active medium by removing a non-participating component, nitrogen. It has been reported that oxy-fuel combustion (OFC) is competitive to air-fuel combustion (AFC) including compensation for the cost of oxygen generation. However, the reports issued have been mostly from commercial stakeholders, such as Linde Gas and Praxair. Including not only the cost saved in reheating the furnace but also the cost of oxygen generation, it is likely that OFC is less effective, or only slightly more effective, if any, than AFC. However, OFC may play a key role in reheating furnaces in the near future, as a part of three CCS (CO2 capture and storage) processes [8,9]: pre-combustion, OFC, and post-combustion. Precombustion is not worth consideration in a reheating furnace. When comparing the cost between OFC and post-combustion, it is clear that OFC has a disadvantage at the stage before CO2

S.H. Han et al. / International Journal of Heat and Mass Transfer 121 (2018) 1364–1370

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Nomenclature ak be;k;j f h I _ m qR Q !

r s T

!

k-th absorption band, m1 polynomial coefficient for the WSGGM model mass flow rate factor specific enthalpy, J=kg radiation intensity, W=ðm2  srÞ mass flow rate per unit depth, kg=s=m radiative heat flux, W=m2 rate of heat transfer or heat generation per unit depth, W=m position vector, m unit direction vector, m temperature, K

separation and an advantage at the CO2 separation stage. If the cost of oxygen generation is fully compensated (or nearly compensated) by the enhanced efficiency as in reheating furnaces, OFC is absolutely superior to post-combustion. Kanniche et al. [8] suggested that each CCS process has its own optimum application part and OFC should be used together with a pulverized coal boiler (PC); many OFC studies are focused on PC [10–12]. Buhre et al. reviewed various issues for OFC: heat transfer, environmental issues, gaseous emissions, ash-related issues, ignition and flame stability. Wall et al. reported the current international status of the technology, contributions of current demonstrations, and a roadmap for commercial deployment. Xiong et al. carried out a detailed thermo-economic cost analysis for a 600 MW pulverized-coal-fired power plant operating under an OFC environment. In 1990, OFC was first introduced into a steel-reheating furnace at Timken by Linde gas. Advanced technologies including the socalled flameless combustion, direct-flame impingement, and reheating furnaces operating under OFC environment were adopted later [13–17]. Murakami et al. [18] carried out OFC in a small scale furnace with coke oven gas (COG). Ebeling et al. [19] presented technical and economic considerations for using OFC in steel-reheating furnaces and provided general guidelines as to when OFC is an appropriate alternative. Praxair has investigated the application of flameless oxy-fuel combustion as applied to reheating furnaces [20]. Oliveira et al. [21] performed a cost analysis for variations of preheating temperature in an OFC reheating furnace. This study has compared the thermal efficiency between AFC and OFC in an axial-fired reheating furnace. To analyze the efficiency, a numerical approach was adopted to estimate the radiation heat transfer into the slab generated by the combustion gas and to predict the periodically transient behavior of slabs. Based on the author’s previous work [3], only radiation was considered for the slab heating, and radiation was accessed by splitting the

Z ij

subzone ij

Greek symbols xe;k emissivity weighting factor for the k-th gray gas X solid angle Subscripts b boundary or black body i, j, ij indices k index for gray gas w wall

furnace into several subzones without flow-field calculation. Because the standard 4-gas WSGGM (weighted sum of gray gas model) [22] is known to give an error in the case of an oxygenenriched medium [23], a modified 5-gas WSGGM, as suggested by Johansson et al. [23], was used in the calculation for OFC. 2. Mathematical formulation 2.1. Overall heat balance equations Fig. 1 shows the schematic frontal view of the furnace of this study. It is 14.1 m deep and equipped with 51 axial burners: 24 in the lower part and 27 in the upper part. It contains 31 slabs and a new slab is inserted from the left side wall. Each slab has the width of 1.16 m and thickness of 0.23 m. The furnace is divided into three zones: preheating, heating, and soaking zones. The entire furnace is divided into 10 subzones to specify the temperature distribution of the gas medium and furnace wall. The axial burners are installed in the fuel feed subzones - Z11, Z12, Z13, Z21, Z22, and Z23. The following assumptions are introduced to analyzed the heating characteristics of the reheating furnace. I. Fuel is fully combusted inside each feeding subzone. I. The combustion gas flow of the lower part does not enter the upper part until it reaches Z15. II. Wall and medium temperature of each subzone remain constant. III. All the slabs are same in material, shape, and insertion interval. IV. It takes no time for slabs to move to next position. The mass and energy balance of an arbitrary subzone, Zij, satisfy the following conservation equations.

_ ij;in þ m _ ij;fuel _ ij;out ¼ m m

Fig. 1. Configuration of the furnace.

ð1Þ

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Table 1 Composition of mixtures (%). Species

H2

CH4

C2H6

CO

CO2

O2

H2O

N2

COG Flue gas (Air-fuel) Flue gas (Oxy-fuel)

56.4 0 0

26.6 0 0

2.9 0 0

8.4 0 0

3.1 0.08 0.26

0.3 0.01 0.03

0.0 0.21 0.70

2.3 0.70 0.01

Table 2 Fuel feed rate per unit depth (m2/h).

Air-fuel I Air-fuel II Oxy-fuel I Oxy-fuel II Oxy-fuel III

Z11

Z13

Z15

Z21

Z23

Z25

Total (f)

155.4 207.2 129.5 142.5 155.4

416.8 555.7 347.3 382.0 416.8

478.6 638.1 398.8 438.7 478.6

138.6 184.8 115.5 127.1 138.6

372.6 496.8 310.5 341.6 372.6

439.7 586.2 366.4 403.0 439.7

2001.6 2668.8 1667.9 1834.8 2001.6

Table 3 Properties of slabs. Temperature [K]

Conductivity [W/(mK)]

Specific heat [J/(kgK)]

Emissivity

T < 473 473 < T < 673 673 < T < 873 873 < T < 1073 1073 < T < 1273 1273 < T

60.57 51.17 41.74 34.04 28.08 29.81

504.0 577.9 712.3 892.1 730.8 672.0

0.5 0.5 0.5 0.5 0.6 0.6

the k-th gray gas emissivity with absorption coefficient, ak , and partial pressure-path length product, PS. The weighting factor, xe;k , can be expressed as a temperature dependent polynomial of order J  1:

xe;k ¼

J X be;k;j T j1

ð6Þ

j¼1

mij;out hij;out ¼ Q ij;comb þ mij;in hij;in þ mij;fuel hij;fuel  Q ij;skid  Q ij;boundary

where be;k;j is referred to as the emissivity gas temperature polynomial coefficient. Refer to Refs. [22,23] for ak and be;k;j of WSGGM models used in this study; 4-gas ordinary WSGGM for AFC and 5gas modified WSGGM for OFC. The total intensity can be expressed as the sum of all the gray gas intensities, Ik .

ð2Þ Q ij;skid ¼ cij;skid ðQ ij;comb þ mij;in hij;in þ mij;fuel hij;fuel Þ

ð3Þ

_ ij;in , and m _ ij;fuel are the rates of mass outflow, mass inflow, _ ij;out , m m and fuel feed of the ij subzone, respectively. hij;out , hij;in , hij;fuel , Q ij;comb , and Q ij;boundary represent the specific enthalpy of outflow, specific enthalpy of inflow, specific enthalpy of fuel and oxidizer, combustion heat, and radiative heat loss though the boundary of the ij subzone, respectively. A constant coefficient, cij;skid , is introduced to evaluate the skid loss. The skid loss is determined by the product of the skid loss coefficient and energy added to the subzone. The skid loss coefficient is 0.12 in this study. The medium temperature of each subzone is obtained by balancing Eqs. (1)–(3). Wall temperature of each subzone is determined by directly applying wall boundary conditions.

qRw ¼ kw ðT w  T w;outside Þ=L

ð4Þ

kw ; T w;outside , and L represent thermal conductivity of furnace wall, ambient temperature outside the furnace, thickness of the furnace wall, which are 1.06 W=ðm  KÞ, 343 K, and 0.3 m, respectively. 2.2. Radiation heat transfer In WSGGM, radiation intensity is grouped based on absorption !

band. The RTE for a gray band intensity, Ik , at any position, r , along !

a path, s , through an absorbing, emitting and non-scattering medium is given by [22,24]: ! !

! ! ! dIk ð r ; s Þ ¼ ak Ik ð r ; s Þ þ ak xk Ib ð r Þ ds

(1.2) (1.54) (1.0) (1.1) (1.2)

ð5Þ

Here, ak is the absorption coefficient of the medium, Ib is the black body intensity, xe;k denotes the emissivity weighting factor for the k-th gray gas based on gas temperature. The bracketed quantity is

I¼

K X Ik

ð7Þ

k¼0

In this study, Ik is solved with FVM radiation solving method for gray medium [25–27] in which the total solid angle (= 4p steradians) is split into finite directions. 3. Results and discussion COG is used for the fuel of the reheating furnace. Table 1 shows the typical compositions for COG, which has a combustion heat of 18.75 MJ/N m3. The fuel and oxidizer are fed into the furnace at the temperature of 300 K and 693 K, respectively. The oxidizer is fed into each fuel inlet port in 6% excess. The compositions of flue gas for the two different combustion modes are also specified in Table 1. A total of 5 cases are analyzed in this study and their fuelfeeding conditions are specified in Table 2. The fuel-feeding proportion of each fuel-feed subzone is the same for all five cases. The mass flow rate factor, f, is introduced to specify the fuel-feed rate for each case using Oxy-fuel I as the reference. Air-fuel I and Oxy-fuel III are set to have the same fuel-feed rate of f = 1.2 for an intuitive comparison between the two different combustion modes. The grid size for the entire domain is 350  30 and all slabs have the same grid size of 13  6. The solid angle is divided into 4  12 sections (azimuthal and polar directions) for radiation solving. Slabs move every 256 s and reside in the furnace for 8448 s. The time step for each calculation is 16 s. The conductivity and specific heat of the slab have piecewise linear profiles as listed in Table 3. Fig. 2(a) shows the medium temperature of subzones for Airfuel I and Oxy-fuel III. Except for the 5th subzones, Oxy-fuel III has higher medium temperatures than Air-fuel I because OFC has

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2200

2200 Air-fuel I Oxy-fuel III

1800

1600

1400

1200

1000

800

Air-fuel I Oxy-fuel III

2000

Wall Temperature (K)

Medium Temperature (K)

2000

1800

1600

1400

1200

1000

Z 11 Z 12 Z 13 Z 14 Z 15

800

Z 21 Z 22 Z 23 Z 24 Z 25

Z 11 Z 12 Z 13 Z 14 Z 15

Subzone

Z 21 Z 22 Z 23 Z 24 Z 25

Subzone

(a) Medium

(b) Wall

Fig. 2. Medium and wall temperature of subzones for Air-fuel I and Oxy-fuel III.

: 200 kw/m2

(a) Air-fuel I

(b) Oxy-fuel III Fig. 3. Radiative heat flux vector for Air-fuel I and Oxy-fuel III.

much higher adiabatic flame temperature than AFC. The temperature decreases drastically in no fuel-feed subzones. Oxy-fuel III experiences steeper temperature decrease than Air-fuel I because it has 0.267 times less flue gas flow rate than Air-fuel I. Medium temperatures of Oxy-fuel III become lower than those of Air-fuel I in the 5th subzones. The medium temperature difference between the two cases is high upstream and continues to decrease as the flow moves downstream. The largest temperature difference between the two cases is 200 K at the Z21. At the 4th subzones, the medium temperatures become almost identical to each other. Finally, the medium temperatures are reversed at the 5th subzones just before the flue gas exits the furnace. The Z25 medium temperature of Oxy-fuel III is 76.8 K lower than that of Air-fuel I. Wall-temperature profiles show similar behavior to that found for the medium-temperature profiles; Oxy-fuel III has higher wall temperature than Air-fuel I except in the 5th subzones, the difference between wall temperatures for the two cases continues to decrease as flow moves downstream, the wall temperature profiles

between the two cases is reversed at the 5th subzones. In view of local maximum, wall-temperature profiles are a little bit different from medium-temperature profiles. All the medium-temperature profiles have their local maxima at the 3rd subzones; however, the wall-temperature profiles do not show such a consistent behavior as the medium-temperature profiles in view of local maxima. Fig. 3 shows radiative flux vector plots for Air-fuel I and Oxyfuel III. The figure shows the clear difference in magnitude of radiative flux vector between Air-fuel I and Oxy-fuel III. In addition to having a higher adiabatic flame temperature, OFC has a more radiatively active medium compared to AFC. Approximately 96% of the flue gas is involved in radiation in OFC, while only 29% of the flue gas is involved for AFC. Therefore, except for the 5th subzone, the radiative flux vector strength of Oxy-fuel III is much larger than that of Air-fuel I. The largest flux magnitudes, as found in Z23, are 99.0 kW/m2 and 160.4 kW/m2 for Air-fuel I and Oxy-fuel III, respectively. Fig. 4 shows the radiative heat transfer rate to each slab. Although OFC cases have larger heat transfer rate than AFC cases

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240

9.0 Air-fuel Air-fuel

210

Air-fuel

8.5

Oxy-fuel

Oxy-fuel

Oxy-fuel

180

Radiative Heat Transfer (Mw/m)

Radiative Heat Transfer (kw/m)

Oxy-fuel

150

120

90

60

30

0

8.0

7.5

7.0

6.5

6.0

5.5

5

10

15

20

25

5.0

30

1

1.1

1.2

Slab

in the heating zone, a lower heat transfer rate is found in the early stage of heating. This is because OFC cases have lower medium temperatures than AFC cases in the 5th subzones, as shown in Fig. 2(a). Therefore, reversions between any two profiles of different combustion modes are necessary, and occur downstream of the 3rd subzones; the first reversion occurs between Oxy-fuel III and Air-fuel I at the 5th slab and the last reversion occurs between Air-fuel II and Oxy-fuel I at the 11th slab. This type of reversion does not occur between the profiles of the same combustion mode. The maximum radiation heat transfer rate occurs between the 13th and 15th slabs in the 3rd subzones. This is because the medium temperatures are highest at the 3rd subzones, as shown in Fig. 5(a), and slab temperatures in the 3rd subzones are absolutely lower than those of upstream subzones, which means the temperature difference between medium and slabs is highest there. Although Air-fuel I and Oxy-fuel III have the same fuel-feeding rate, they show a large difference in the maximum heat transfer rate. The maximum heat transfer rate of Air-fuel I and Oxy-fuel III are 92.6 kW/m and 149.3 kW/m, respectively. Even Air-fuel II has lower maximum heat transfer rate than Oxy-fuel I. Fig. 5 shows total heat transfer rate to all slabs. All the Oxy-fuel cases have greater total heat transfer rate than any Air-fuel case. Comparing two cases of identical fuel-feed rate, Oxy-fuel III has a 1.35 times larger total heat transfer rate than Air-fuel I, which results in the former to have a much higher slab emission temperature than the latter, as shown in Fig. 8. Air-fuel II has a 1.54 times larger fuel-feed rate than Oxy-fuel I, but has 0.2% less total heat transfer than Oxy-fuel I. Consequently, Air-fuel II and Oxy-fuel I give almost identical slab emission temperature, as shown in Fig. 8. These two comparisons clearly show the superiority of OFC in heating slabs. Fig. 6 shows the profiles of the mean slab temperature. In the early stage of heating, AFC cases have higher mean slab temperature than any other three OFC cases due to the characteristics of radiative heat transfer rate, as discussed in Fig. 4. However, OFC cases have higher slab emission temperature than AFC cases. As in the radiation heat transfer rate profiles, reversion occurs

1.4

1.5

1.6

f Fig. 5. Total radiative heat transfer rate into all the slabs inside the furnace.

2100 Air-fuel Air-fuel Oxy-fuel

1800

Oxy-fuel Oxy-fuel

1500

Temperature (K)

Fig. 4. Radiative heat transfer rate to the slabs inside the furnace.

1.3

1200

900

600

300

5

10

15

20

25

30

Slab Fig. 6. Mean slab temperature for the slabs inside the furnace.

between any two cases of different combustion mode. The first reversion occurs between Oxy-fuel III and Air-fuel I at the 6th slab. The last reversion occurs between Oxy-fuel I and Air-fuel II at the 24th slab. Regarding the slab emission temperature, the higher the total radiation heat transfer rate the higher the slab emission temperature. Fig. 7 shows the profile of the temperature difference, which is just the difference between the maximum and minimum temperatures inside a slab. The profiles are found to be very similar to the

S.H. Han et al. / International Journal of Heat and Mass Transfer 121 (2018) 1364–1370

600 Air-fuel Air-fuel Oxy-fuel

500

Oxy-fuel Oxy-fuel

Temperature (K)

400

300

200

100

0

5

10

15

20

25

30

Slab Fig. 7. Temperature difference for the slabs inside the furnace.

profiles of radiative heat transfer rate to the slab; the AFC cases have higher temperature difference than OFC cases in the early stage of heating, OFC cases have larger profile maxima than AFC cases, and reversion occurs between any two profiles of OFC and AFC cases. The profile maxima are between 250 K and 360 K and occur between the 14th and 15th slabs. Although the profiles show a wide distribution in the heating zone, they all fall into a relatively narrow range just before the slabs are emitted. This shows that if the fuel-feed proportions among burners are all the same, the slab temperature uniformity is not very dependent on the fuel-feed rate or combustion mode. In the strict sense, OFC cases are slightly more favorable than AFC cases in view of temperature uniformity.

2000

100 Temperature (Air-fuel) Temperature (Oxy-fuel) Efficiency (Air-fuel) Efficiency (Oxy-fuel)

80 1600

60

1400

1200 40

1000

0.8

1

1.2

1.4

1.6

1.8

f Fig. 8. Slab emission temperature and heating efficiency.

Efficiency (%)

Emission Temperature (K)

1800

1369

Fig. 8(a) shows the slab emission temperature for all the cases. In the figure, the dashed lines are the lower and upper limit of the slab emission temperature requirement, 1373 K and 1573 K. All the oxy-fuel combustion cases satisfy the slab emission temperature requirement. In the case of air-fuel combustion, Air-fuel II satisfies the slab emission temperature requirement, whereas Air-fuel I fails to satisfy the requirement. The slab emission temperature of Oxy-fuel I is slightly above the lower limit, while Oxy-fuel III has a value near the upper limit. In the case of f = 1.2, OFC can heat slabs up to the upper requirement temperature, whereas AFC fails to elevate the slab temperature even up to the lower limit. OFC gives a 357 K higher slab emission temperature than AFC for the same fuel-feed rate of f = 1.2. The efficiency is defined as the percentage of the net heat transferred to slabs divided by the total heat input summed over the enthalpy of fuel, enthalpy of oxidizer, and combustion heat. Fig. 8(b) shows the superiority of OFC in efficiency. The efficiency of OFC cases is between 71% and 74%, while AFC cases have efficiencies between 43% and 47%. Oxy-fuel I and Air-fuel II have a performance of 71% and 47.2%, respectively. Considering that Air-fuel II and Oxy-fuel I give almost the same slab emission temperature, it can be said that Oxy-fuel I is 54% more fuel efficient than Air-fuel II. The slab emission temperature and efficiency have opposite trends. The larger the fuel-feed rate, the lower the efficiency. A low fuel-feed rate means a lower heat content of the medium, which results in a lower medium temperature of Z25. This implies that the medium experiences the largest temperature decrease from the adiabatic flame temperature in a lower fuel-feed rate. It is evident that the lower the temperature of the flue gases at the stack, the higher the efficiency. As shown in Fig. 7, the slab temperature difference is not much different between the cases just before emission, and it is recommended to use a low fuel-feed rate as long as the slab emission temperature requirement is satisfied.

4. Conclusion The efficiency enhanced by applying oxy-fuel combustion to a reheating furnace was predicted quantitatively using a numerical approach. A total of five cases, listed in Table 2, were analyzed to compare the thermal efficiency between air-fuel and oxy-fuel combustion. The resulting conclusions are as follows: When the two identical fuel-feed rate cases for Oxy-fuel III and Air-fuel I are compared, oxy-fuel combustion was found to have a higher medium temperature than air-fuel combustion (except in the 5th subzones) because it has a higher adiabatic flame temperature. The medium temperature is reversed between the two cases in the 5th subzones because oxy-fuel combustion provides more heat transfer to slabs upstream of the 5th subzone whereas it has a relatively low total mass flow rate. Oxy-fuel III shows far larger radiative heat transfer to slabs than Air-fuel I except in the early stage of heating. It is because oxy-fuel combustion has higher medium temperature upstream of the 5th subzone and a more radiative active medium than air-fuel combustion. Oxy-fuel III has a 1.5 times larger total heat transfer to all the slabs than Air-fuel I. Because of the large difference in the total heat transfer rate, Oxy-fuel III gives a 120 K larger slab emission temperature than Air-fuel I. Air-fuel combustion does not have an identical slab emission temperature to oxy-fuel combustion until it feeds 1.54 times more fuel than oxy-fuel combustion. Oxy-fuel cases have an efficiency range between 71.1% and 74.2%, while the two air-fuel cases have efficiencies between 43.0% and 47.1%. Based on the computational result that Oxy-fuel I and Air-fuel II give almost the same slab emission temperature, Oxy-fuel I is 54% more fuel efficient than Air-fuel II.

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