Oxy-fuel Combustion in a 600 MW Gaseous Fuel Tangentially Fired

Sep 25, 2017 - The boiler consists of a furnace and a return tube bank. The furnace and return tube bank are classified according to the heat-transfer...
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Oxy-fuel Combustion in a 600 MW Gaseous Fuel Tangentially Fired Boiler R. Ben-Mansour, M. A. Habib,* N. A. A. Qasem, and A. Abuelyamen Mechanical Engineering Department, KFUPM, Dhahran 31261, Saudi Arabia ABSTRACT: Tangentially fired furnaces are vortex-combustion burners preferably used for oxy-fuel combustion in order to minimize the CO2 emission and control the boiler temperature. The objective of the present study is to investigate numerically the combustion and emission characteristics of oxy-fuel combustion of turbulent reacting flows in a three-dimensional model furnace of a 600 MW tangentially fired boiler. Numerical calculations of the flow field and thermal fields as well as the species concentrations for the oxy-fuel combustion in the furnace of a gaseous fuel tangentially fired boiler were conducted. Three cases of oxy-fuel combustion were investigated. These were 73%, 79%, and 65% of volumetric inlet CO2 while the remaining inlet volume was O2. The results of these three oxy-fuel cases are compared to the air-fired combustion. In the present model, the radiation heat transfer was solved by the weighted sum gray gases approach, whereas the chemical reaction model was facilitated by a simple lumped reaction model. The results show that the oxy-fuel case of 79% CO2 can preferably minimize the combustion temperature for highly sustainable materials. However, the 65% CO2 oxy-fuel case generates high temperature values for improving the heat characteristics. The 73% CO2 oxy-fuel and the air-fired cases are fairly close to each other in thermal behavior.

1. INTRODUCTION Climate change is one of the greatest environmental concerns of the 21st century. About 80% of current energy demand is satisfied by fossil fuels. Burning fossil fuels generates CO2, which is considered to be the largest contributor among the greenhouse gases. CO2 emissions in large power plants will be the biggest source of CO2 in the few coming decades.1 Carbon capture is an effective solution to continue using fossil fuels while minimizing the CO2 emissions into the atmosphere and thereby diluting the climate change. Several approaches have focused on CO2 capture in the utility industry including precombustion and postcombustion capture and oxy-fuel combustion. Oxy-fuel combustion can be used for existing or new power plants. In oxy-fuel combustion, the fuels are burnt in a nitrogen-free environment. The flue gases in the products of the combustion are composed mainly of carbon dioxide and water vapor. The simple condensation of H2O enables to separate it from CO2 simply and inexpensively. The carbon dioxide can, then, be captured easily.2 Tangentially fired furnaces are those units that normally have a square cross section in which the fuel and oxidizer are tangentially supplied from the corners of the furnace. The flames in these furnaces are directed to create a vortex that moves upward. The vortex motion that results from the pressure gradient in these furnaces may, with proper design, eliminate or reduce the slagging and erosion in the walls of the furnace as a result of impingement and local overheating. Because of the well-distributed temperature field in these furnaces, the NOx formation becomes low. All of these facts about tangentially fired furnaces and tangentially fired boilers give the advantages to successfully use them for steam generation. The limits of ignition depend on the vortex size and the swirl angle.3 In tangentially fired boilers, the tangential velocities increase with increasing radii as a result of the vortex motion of gases. As a result, a pressure difference is generated between the periphery and the vortex center. This pressure © XXXX American Chemical Society

gradient is significantly required for the stabilization of the flame inside the furnace. The well-mixed gases and heat distribution inside the furnace lead to uniform heat flux to the furnace walls. In an early work,4 it was recommended that the swirl angle from the diagonals of the furnace should be 4−6° for square furnaces. The study indicated that occupation of the combustor by the flame is improved by increasing the number of the burner levels. Robinson5 provided prediction and empirical modeling of tangentially fired boilers and showed vortex formation in the core of the furnace and recirculation zones of recirculation close to the bottom and side walls. Chen et al.6 modeled two operating furnaces to investigate incomplete fuel burnout. The results of the burnout predictions studied for seven operating conditions agreed very well with the experimental data. Experimental and numerical investigations of the reduction of NOx emissions from the combustion of a tangentially fired boiler under different operating conditions were conducted.7 In their work, a simplified NOx formation mechanism model was developed, and their results compared well with the experimental data. Some researchers have numerically investigated the oxy-fuel combustions in order to propose new designs of oxy-fuel burners, investigating the recycling of CO2 on temperature control, or determining a suitable radiation model to accurately estimate the heat transfer inside the boilers.8−23 A comparison between the performance of a boiler utilizing oxy-fuel combustion and another boiler operated on air−fuel combustion to produce a net power output of 452 MW was studied by Shah and Christie.24 The reduction in the efficiency was reported as 20%, and the reduction of emitted CO2 was 90%. Buhre et al.19 reviewed previous work on the oxy-fuel Received: August 17, 2017 Revised: September 23, 2017 Published: September 25, 2017 A

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Figure 1. Construction of the water-tube boiler: (a) longitudinal section of the boiler, (b) cross section of a furnace model, and (c) burner arrangement.

combustion of pulverized coal. The authors identified the issues of heat transfer, gaseous emissions, and flame stability of the combustion. CO2 and H2O have higher emissivity which produces more radiation. However, CO2 and H2O have high heat capacities compared with that of N2. As a result, the heat transfer increased in the convection heat-transfer sections because of this increase of the thermal heat. Kakaras et al.25 considered flue gas recirculation to moderate the furnace temperature. They studied the influence of oxy-fuel combustion

on the heat exchange surfaces design and indicated higher radiative heat transfer because of the high concentrations of carbon dioxide and water vapor in the flue gas. Wall et al.23 found that, in oxy-fuel combustion, oxygen in the near burner region can increase flame stability and is an advantage in highvelocity burners. Ditaranto and Hals26 investigated the thermoacoustic instabilities in the oxy-fuel combustor with recycling CO2 and showed that increasing CO2 amounts allows more potential for “zoning” the flame. It was deduced that the flame B

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tube. In choosing the values in Table 1, the actual boiler typical fuel flow rate was considered. Based on the ratios of O2, N2, and CO2 in column 1, the values of other columns were calculated accordingly.

properties in CO2/O2 system are required in order to predict the flame structure. Other researchers27,28 have recorded the characteristics of the flame stability and pollution formation. Based on the literature search presented in this section, it is clear that the combustion characteristics of the oxy-fuel combustion of gaseous fuels in tangentially fired furnaces are not well-understood and need further investigation. The present work is aimed at investigating numerically the problem of oxy-fuel combustion in the furnace of a tangentially fired boiler. The proposed work is aimed also at a through understanding of the stability and safety of the ignition of the air−fuel mixture and subsequent reliable operation. The variables affecting flow, thermal, and concentration fields are the air-to-fuel ratio and the input flow rates. This problem is very essential for the power production sectors because of its relation to steam generation in large boiler furnaces used in thermal power plants. The proposed work is aimed also at obtaining a better understanding of oxy-fuel combustion in these furnaces. In the proposed work, numerical calculation of the flow and heat fields as well as the species concentrations for the oxy-fuel combustion in the furnace of a tangentially firedboiler are provided. The investigation covers a range of different parameters including the flow rates and carbon dioxide recirculation percent. The details of the flow, heat, and combustion fields are obtained from the solution of mass, momentum, heat, species, turbulence, combustion, and radiation equations.

3. MATHEMATICAL FORMULATION 3.1. Flow, Energy, and Species Governing Equations. The conservation of mass, momentum, energy, and species transport can generally be estimated by the following equation:29,30 ∂ ∂ ⎡⎢ ∂Φ ⎤⎥ (ρ ̅ Uj̅ Φ + ρ ̅ ujϕ) = Γϕ + ρ ̅ SΦ ∂xj ∂xj ⎣⎢ ∂xj ⎦⎥

where ρ̅ is the fluid density, uj the velocity of a gas component, Φ the general dependent variable, ΓΦ a coefficient of the diffusion, and SΦ the source term. Φ can change eq 1 to be a mass conservation equation when Φ = 1, a momentum conservation equation when Φ is a component of velocity, an energy equation when Φ is a stagnation enthalpy, or a transport equation when Φ is a scalar variable. The turbulent flow model has been represented by the k−ε model.26 The relationship between Reynolds stresses, turbulent scalar fluxes, mean velocities, and scalar variable are formulated as shown by eqs 2 and 3.31 ⎛ ∂U̅ ∂Uj̅ ⎞ ⎟⎟ − 2 ρkδij −ρuiuj = μt ⎜⎜ i + ∂xi ⎠ 3 ⎝ ∂xj

2. BOILER DESCRIPTION

−ρuiϕ = ΓΦ

A water tube boiler with four levels of burners, each level with eight burners, is considered in this study. The boiler consists of a furnace and a return tube bank. The furnace and return tube bank are classified according to the heat-transfer mechanism as radiation and convection sections, respectively. The cross section area of the combustion chamber is about 26 × 10.4 (m2) and the height is 29.68 m. A detailed schematic diagram of the boiler is shown in Figure 1. The 32 burners were distributed in 4 levels and located at elevation of 4.3, 5.515, 6.731, and 9.161 m above the bottom of the furnace vertical wall surfaces. For conforming central vortexes and so increasing the flow turbulences, Figure 1b, the burners take place at the furnace corners each with a swirl angle of θ as shown in Figure 1c. The boiler is really used for the electricity production. The fuel (natural gas, CH4), oxygen, and nitrogen or carbon dioxide mass flow rates for both secondary and binary burners are clearly shown in Table 1. The gases inlet conditions have been assigned at 101.3 kPa and 400 K. Different percentiles of CO2 are used instead of N2 (Table 1) to address an optimal condition of suppressing the exacerbated temperature, while sufficiently providing a required heat to water

(3)

μt = cμρfμ K 2/ε

(4)

where cμ, fμ, and σΦ are constants. Therefore, the turbulent viscosity is calculated from the solution of k and ε equations (eqs 5 and 6). For obtaining more accurate results for the vortex flow, the renormalized group (RNG) turbulence model32 was exploited. 3.2. Turbulence Model Equations. The turbulence model equations for k and ε are given as follows:29,30 ∂ ∂ ⎛ μeff ∂k ⎞ (ρUk ⎜ ⎟ + Gk − ρε j̅ ) = ∂xj ∂xj ⎝ σk ∂xi ⎠

(5)

∂ ∂ ⎛ μeff ∂ε ⎞ ε ε2 (ρUj̅ ε) = ⎜ ⎟ + C1Gk − C2*ρ k k ∂xj ∂xi ⎝ σε ∂xi ⎠

(6)

fuel (kg/s)

O2 (kg/s)

N2 or CO2 (kg/s)

primary secondary

14.29

61.85 3.92

203.62 12.91

265.47 16.83

primary secondary

14.29

61.85 3.92

318.89 20.21

380.74 24.13

Gk = −ρuiuj

primary secondary

14.29

61.85 3.92

229.27 14.53

291.12 18.45

G2* = C2 + C3

primary secondary

14.29

61.85 3.92

157.46 9.98

219.31 13.9

case Air (21% O2, 79% N2 by volume) 21% O2, 79% CO2 by volume 27% O2, 73% CO2 by volume 35% O2, 65% CO2 by volume

∂Φ ∂xj

(2)

where μt denotes the turbulent viscosity and the diffusion coefficient, ΓΦ, equals μt/σΦ. The value of the turbulent viscosity is calculated by eq 4.

Table 1. Burner Gases (Fuel + O2 + CO2/N2) Amounts for Stoichiometric Oxygen-to-Fuel Ratio 1.15 mixture (O2 + N2/CO2) (kg/s)

(1)

where Gk is the generation of turbulent kinetic energy caused by the mean velocity gradients and σk and σε are the effective Prandtl numbers for k and ε, respectively. Gk and C2* are evaluated by eqs 7 and 8,31 respectively. ∂Uj̅ ∂xi

(7) (8)

where C1 and C2 are constants with values C1 = 1.42 and C2 = 1.42, and C3 is a function of the term k/ε. The wall functions scheme plays a role in establishing the link between the flow C

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The net source of chemical species i due to reaction is computed as the sum of the Arrhenius reaction sources over the two reactions that the species participate in

variables at the near-wall cells and the corresponding quantities at the wall. More details of the wall functions are provided in refs 33 and 34. 3.3. Species Transport Equations. The conservation equation of the local mass species, ml, can be expressed as follows: ∂ ∂ (ρUm J + Rl i̅ l ) = − ∂xi ∂xi l , i

2

R i = M w, i ∑ R̂ i , r r=1

where Mw,i is the molecular weight of species i and R̂ i , r is the Arrhenius molar rate of creation/destruction of species i in reaction r. The forward rate constant is computed using the Arrhenius expression:

(9)

where Rl is the creation or depletion mass rate caused by chemical reaction of the species l; Jl,i is the diffusion flux of species l and can be evaluated by ⎛ μ ⎞ ∂m Jl , i = −⎜ρDl , m + t ⎟ l Sct ⎠ ∂xi ⎝

⎛ −E ⎞ kc , i = A r T βr exp⎜ r ⎟ ⎝ RT ⎠

(10)

(11)

where I is the total intensity of the radiation which relies on the vector of the position (r) and the length of the path (s), κ the coefficient of the radiation absorption, and σs the coefficient of the scatted radiation. The Planck law is used to evaluate the mean absorption coefficient as

specified value of K /U̅ 2 equal to 0.1 and a length scale (L) equal to the characteristic length of the inlet pipe/annulus. All velocity components values are zero at the boundary walls according to the no-slip conditions. The turbulence model equation is used to determine the kinetic energy of turbulence and its dissipation rate. Saturated water temperature is assumed for the water tube walls. The production of k and ε are supposed to be equal in the wall-adjacent control volume. Because the ε equation is not estimated at the wall-adjacent cells, the k at the node near the wall is taken to be proportional to the square of the wall shear stress. The dissipation rate (ε) at the node near the wall is considered proportional to k1.5.31 3.7. Solution Procedure. An iterative procedure38 is used to solve the set of all differential equations along with the boundary condition. The calculation procedures are reported in detail in previous works.38−40 Computational fluid dynamics (CFD) packages of Fluent were used to carry out the calculations.

∞

κP(T , P) =

∫0 κλ(λ , T , P) ebλ(λ , T )dλ σT 4

(12)

where ebλ is the spectral emissive power of blackbody and σ is the Stefan−Boltzmann constant. The availability of gases such as CO2 and H2O inside the combustor influences the calculation of the radiation heat transfer because of the contribution of these gases in adsorbing some radiation heat. The most commonly used model to consider the effect of gray gas on the radiation heat transfer is the weighted sum gray gases (WSGG) model.36 This model has been exploited in the present modeling. The radiation and convection heat transfer are considered in the boiler including furnace, superheaters (radiative, platen, and convective sections), reheaters, and economizer. On the other side, the participating percentage of the heat transfer may vary from section to another relying on their location regarding the flame propagation and exhaust gases. 3.5. Reaction Kinetics Model. The chemical reactions have been carried out using a two-step chemical reaction; they can be expressed as 2CH4 + 3O2 ⇒ 2CO + 4H 2O

(13)

2CO + O2 ⇒ 2CO2

(14)

(16)

where Ar is pre-exponential factor (consistent units); βr is temperature exponent (dimensionless); Er is activation energy for the reaction, J Kg mol−1; R is universal gas constant (8313), J kmol−1 K−1.The value of Ar was 2.119 × 1011 and 1 × 105 for the first and second steps, respectively. Er was 2.027 × 108 and 1 × 108 for the first and second steps, respectively. The values of βr were zero for both steps. The two-step equation model presents a compromise between the simple one-step model and other detailed reaction mechanisms with respect to lengthy calculations and accuracy. 3.6. Boundary Conditions. The furnace consists of two symmetry parts as shown in Figure 1c, so that the domain of the simulation is sufficient for one part. Each part consists of 16 burners divided into 4 levels. Uniform velocity distribution is considered at fuel and oxidizer inlets with velocity directions in the same directions of the burner nozzle axis. Kinetic and dissipation energy values are assigned through a

where Dl,m is the diffusion coefficient for species l in the mixture; Sct is the turbulent Schmidt number, and μt/ρDt is equal to 0.7. In this work, we used the EDC that relies on turbulence chemistry interaction35 to provide the reaction rate of combustion products. 3.4. Radiation Heat-Transfer Modeling. The radiative transfer equation (RTE) equation (eq 11) is utilized to precisely predict the temperature distribution in the furnace involving the radiation energy that is absorbed, emitted, and scattered by the participating medium. d I (r , s ) = κIb − (κ + σs)I(r , s) ds

(15)

4. RESULTS AND DISCUSSION 4.1. Mesh Independence. The furnace domain was meshed using structure quadrilateral elements as shown in Figure 2. Grid independency was carried out for the air-fired combustion case. Figure 3 shows the temperature values at the center of burners along the z-axis for four different numbers of cells (0.45 × 106, 1.23 × 106, 1.67 × 106, and 2.32 × 106 cells). It is clear that all selected grids have shown close temperature values, indicating that the smallest number of cells (0.45 × 106) was enough to present an accurate simulation.

The reaction model is described in detail by ANSYS Fluent.37 D

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• Primary stream: air flow rate is 37 m3/h at swirl angle of 45° and inlet temperature of 300 K. • Secondary stream: air flow rate is 45 m3/h, at swirl angle about 15° and inlet temperature of 300 K. Oxy-fuel: • Primary stream: 27% vol O2 and 73% vol CO2 mixture. Flow rate of mixture is 29 m3/h at swirl angle 45° and inlet temperature of 300 K. • Secondary stream: 27% vol O2 and 73% vol CO2 mixture. Flow rate of mixture is 48 m3/h at swirl angle 15° and inlet temperature of 300 K. Figure 5 shows the temperature profiles for radial axis at 0.553 m from the burner for air-fired combustion and at 0.384

Figure 2. Mesh distribution of studied furnace.

Figure 3. Mesh independence as a comparison of temperature at the middle of burners along the z-axis for 0.45 × 106, 1.32 × 106, 1.67 × 106, and 2.32 × 106 cells.

4.2. Validation. Validation of the present modeling was carried out through a comparison of the present calculations with the experimental results.41 The combustion chamber is shown in Figure 4. The center jet is used to introduce the fuel

Figure 5. Temperature profile comparison between the experimental work41 and the present simulation for the (a) air-fired and (b) oxy-fuel (27% O2 and 73% CO2) combustion.

m from the burner for oxy-fuel (73% CO2) for both the experimental work41 and the present simulation. The results of the present work appear to be in fair agreement with those of the experimental work within the experimental uncertainty41 of ±70 °C. 4.3. General Features of the Flow Field. In order to investigate the effect of the CO2 circulation on the flow and heat characteristics of the oxy-fuel combustion process, the main results of the CO2 circulation ratios (79% CO2, 73% CO2, and 65% CO2) have been compared to those of the air-fired combustion. The main results involve temperature, velocity, and CO2 and CH4 friction contours as well as turbulence viscosity, and total and radiation surface heat flux profiles. Figure 6 represents temperature contours for the four cases at the vertical plane (x−z plane) and y = 6.5 m from the origin, Figure 1 (having four burners for each level). Generally, the highest temperature values have appeared close to the burners. However, these temperature values slightly decrease at the

Figure 4. Oxy-fuel combustion unit.41

while the oxidizer is two jets (primary and secondary streams) surrounding the center jet. The flow swirl angle and the airfired and oxy-fuel operating conditions are as follows: • Fuel is propane: inlet mass flow rate is 0.001726 kg/s at 300 K. Air-fired: E

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Figure 6. Temperature contours at z−x plane, y = 6.5 m.

Figure 7. Temperature contours at level 1 (x−y plane), z = 4.3 m from the bottom wall.

Figure 8. Temperature contours at level 4 (x−y plane), z = 9.161 m from bottom wall.

middle of the furnace. The temperature values also drop gradually as the combustion gases pass through superheater, reheater, and economizer. It is clear from Figure 6 that replacing N2 with CO2 (79%) has significantly reduced the temperature values. In the air-fired combustion process, the combustion gases enter the water tubes zone with an average temperature of 1500 K, while in the oxy-fuel case (21% O2, 79% CO2), the temperature value drops to about 1350 K. The lower values of temperature associated with the 79% CO2 case refers to the fact that the CO2 has high specific thermal capacity values in comparison to those of nitrogen. Although the carbon dioxide gas absorbs more incident radiation energy especially in the hottest zone (close to the burners), the temperature has preferably shown quite low values due to the large variations in the specific thermal capacity.

In addition, the comparison of the three oxy-fuel combustion cases (79% CO2, 73% CO2, and 65% CO2) shows that the temperature values increase more with decreasing CO2 percentages. This increasing in temperature values is mainly due to the increase of O2 percentages in the mixture. Therefore, a high concentration of CO2 such as 79% is supremely important to limit the increase of the temperature values inside the furnace for a sustainable operation without damaging the construction materials of the boiler. For more perspective, the temperature contours at the horizontal plane level 1 is exhibited in Figure 7. Because of the swirl angles of the burners (as shown earlier in Figure 1c), the flame tends toward the furnace walls resulting in low isothermal circles in the center of the furnace. The minimum recorded temperature values for oxy-fuel gases are 1347, 1435, and 1558 F

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Figure 9. Velocity magnitude (m/s) at z−x plane, y = 6.5 m.

Figure 10. Velocity magnitude at level 4 (x−y plane), z = 9.161 m from bottom wall.

Figure 11. Velocity vectors at z−x plane, y = 6.5 m.

burner level 4 are higher than those of level 1. For instance, the temperature values at the center of level 4 reach 1347, 1558, and 1663 K for CO2 recirculation percentages of 79%, 73%, and 65%, respectively. Figures 9 and 10 present the velocity magnitudes in the vertical plane (z−x plane, y = 6.5 m from the origin, Figure 1) and the horizontal plane crossing level 4, respectively. In addition, the corresponding velocity vectors are also shown in Figures 11 and 12. In this study, the mass flow rates of fuel and O2 are kept the same for all the cases, as shown in Table 1.

K for carbon dioxide recirculation of 79%, 73%, and 65%, respectively, while the combustion gases near the furnace walls in the burning direction have high temperature values. Again, the higher the temperature values, the lower the concentration of CO2. As the combustion gases move up, their temperature values increase because of the contribution of buoyancy. Figure 8 shows temperature contours at level 4 (z = 9.161 m from the origin, Figure 1) where the upper burners are located. The minimum temperatures recorded in the furnace center of G

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Figure 12. Velocity vectors at level 4 (x−y plane), z = 9.161 m from bottom wall.

Figure 13. CH4 fraction at z−x plane, y = 6.5 m.

Figure 14. CH4 fraction at level 4 (x−y plane), z = 9.161 m from bottom wall.

velocity less than 5 m/s while in the other cases, they leave the economizer at velocity less than 10 m/s. Moreover, oxy-fuel combustion influences the burning rate of the methane gas. Figures 13 and 14 present the fuel mass fraction (CH4) in the vertical plane (z−x plane, y = 6.5 m) and horizontal plane at level 1, respectively. For all cases, the fraction of CH4 starts to vanish gradually far from the burners while the highest concentration of the fuel is near the burners. As the recirculating carbon dioxide increases, high concentration of the fuel starts to appear close to the burners as shown in the horizontal plane (Figure 13). Additionally, replacing N2 with CO2 results in a high level of CO2 inside the furnace as indicated in Figure 15. The fraction of the produced CO2

Thus, in order to have a different percentage for the recirculating carbon dioxide, its mass flow rate would be changing. Consequently, the mixture velocity will change. Replacing N2 with CO2 of the same molar fraction results in almost the same velocity magnitudes in the reaction zones, whereas the air-fired case has higher values in the central zones of the furnace (Figure 9) due to the high values of temperature. As the molar fraction of the carbon dioxide reduces from 79% to 65%, the velocity magnitudes also reduce specially in the superheater, reheater, and economizer zones because of the reduction of the temperature values. For instance, in the case of the smallest percentage of recirculating carbon dioxide (35% O2, 65% CO2), the combustion gases leave the furnace with a H

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Figure 15. CO2 fraction at level 4 (x−y plane), z = 9.161 m from bottom wall.

Figure 16. Turbulent viscosity profile at a line (y = 6.625 m, z = 9.161 m) through the x-direction.

changes proportionally with the recirculating percentage of CO2 gas. For the 79% CO2 case, a fraction of 0.84−0.9 of CO2 dominates across the combustion zone whereas the low CO2 fraction appears close to the burners. As the recirculating CO2 percentage reduces to 73%, the CO2 fraction values decrease to between 0.81 and 0.88. The higher values are distributed close to the burners. Generally in turbulent flow, the diffusion mechanism is performed by molecular and eddies motion. Thus, the turbulent viscosity term has been added to the molecular viscosity in the momentum and energy equations. Accordingly, the velocity and heat transfer would be affected by the turbulent viscosity term. Figure 16 shows the turbulent viscosity for both the air−fuel and the oxy-fuel cases across a line through level 4 (y = 6.625 m and z = 9.161 m) in the x-direction. It is clear from the figure that the turbulent viscosity is directly proportional to the percentage of recirculated CO2. Therefore, the highest mixing rate of gases occurs for the CO2 79% case. This clarifies the reason for the noticeably low concentrations of CH4 near the burners where there are large amounts of the carbon dioxide (Figure 13). Because turbulent viscosity affects heat transfer, it helps to demonstrate the low temperature level in oxy-fuel combustion as CO2 percentage increases (Figure 8). In a comparison between oxy-fuel 79% CO2 and 65% CO2, the first case has a higher mixing rate along with large CO2 mass flow rate (Table 1); thus, it minimizes the exaggerated increase of the temperature values. Figure 17 represents the total heat transfer along with the percentage of radiation heat transfer in different zones across the furnace. In the combustion zone, it is clear that the radiation heat transfer is the dominant mechanism of all the

Figure 17. Heat transfer in different zones: (a) total heat transfer and (b) percentage of radiation heat transfer.

I

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Figure 18. Heat transfer flux across a line (x = 5.57 m, y = 0 m) through the z-direction.

Figure 19. Heat-transfer flux at a line (y = 6.625 m, z = 28.68 m) across the x-direction: (a) total heat flux and (b) radiative heat flux.

heat transfer. Around 71−93% of the total heat transfer in this region occurs by radiation. Replacing N2 with CO2 in the base case with the same volume percentage results in reducing the percentage of the radiation heat transfer from 92.2% to 84.4%. Nonetheless, this slight drop in the radiation percentage produces a significant reduction in the total heat transfer (105 MW) in the combustion region. Moreover, the increase of the

percentage of O2 (by reduced recirculated CO2 percentage) leads to an increase in the fraction of the radiation heat transfer from 84.4% to 90.1% and then decreases to 70.9% for 79% CO2, 73% CO2, and 65% CO2 oxy-fuel combustions, respectively. However, the total heat transfer in the combustion zone grows sharply from 120 to 171 MW and up to 310 MW for 79% CO2, 73% CO2, and 65% CO2 oxy-fuel combustions, J

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percentage of the radiative heat transfer in the place of x = 7.5− 10.5 m as shown in Figure 19b. Additionally, the total heat transfer increases sharply at the entrance of the third part of the superheater (Figure 19a) because of the domination of the convective heat transfer as explained above. Then, it decreases gradually until the gases leave the superheater and enter the reheater zone where the number of plates is much less than that in the superheater zone. Hence, the heat-transfer coefficient decreases in the reheater zone because of the relatively large distances between the plates. Consequently, the decrease of the convective heat transfer causes a reduction in the total heat flux as shown in Figure 19a. This also explains the sudden jump in the radiative heat-transfer percentage in Figure 19b (x = 10.5 m), where the convective heat transfer drops considerably.

respectively. The gradually increases in the heat transfer is mainly due to the increase of the temperature values in the combustion zone as a result of reducing the recirculating CO2 percentage, which works as a coolant. Furthermore, the high percentage of the radiation heat transfer in the combustion region can be explained as being due to the high temperature values of the products, besides the absence of any obstacles which prevent viewing faces to each other. In the superheater zone, the radiation heat transfer is only 20.7−30.5% of the total heat transfer. This happens because the incident radiation is blocked by the plates and tubes in the upper heater region. However, these plates increase the heat-transfer coefficient and also offer large surface area for the convection heat transfer; therefore, the total amount of heat transfer in the superheater zone becomes large (∼240 MW). Reheater and economizer zones have a relatively small amount of the total heat transfer compared to the other zones. This heat is mainly dominated by the convective heat transfer as shown in Figure 17 b. Figure 18 shows the heat flux across a vertical line in the front wall of the furnace (x = 5.57 m, y = 0 m, through the zdirection). It can be noticed that the middle area (3.5−10 m elevation) of combustion zone has the highest total heat flux in all cases. That is mainly due to the high temperature values of the burning. Thus, the region near combusted gases would have high heat flux value which reduces as the gases leave the combustion region. In addition, for the combustion zone (0− 20.68 m elevation), the heat flux profiles are semisymmetric about the peak values. The symmetry behavior comes mainly from the dominant radiation heat transfer mode in this region. However, the small deviation of symmetry profiles in the upper z-direction happens because of convective heat transfer. As the product gases move up toward the superheater, they exchange more heat to the side walls of the upper direction than to the lower direction walls because the flow is moving up. Accordingly, more heat flux would occur in the upper direction than in lower direction. Therefore, the total heat flux reduces gradually from peak values (in the middle) in the upper direction and then drops sharply toward the bottom wall of the furnace. In addition, it can be noticed from the figure (Figure 18) that the oxy-fuel of 65% CO2 records the highest heat flux profile, whereas the 79% CO2 case records the lowest heat flux profile. The basic case (air−fuel) shows a heat flux profile similar to that of the 65% CO2 oxy-fuel. In order to characterize the performance of this tangentially fired boiler furnace, the heat-transfer flux and the percentage of radiative heat transfer are observed across a line (y = 6.625 m, z = 28.68 m) at the top of the furnace through the x-direction as shown in Figure 19. This line passes over the superheater zone (which consist of three sections) and upper part of the reheater. The first two sections of the superheater are open directly to the combustion zone, so their upper surfaces are exposed to a high intensity of the radiation heat compared to the other sections (third part of the superheater and the reheater). Hence, the radiation in the first and second sections of the superheater (x = 0−7.5 m) recorded a high percentage of radiative heat-transfer flux (70−86%) as indicated in Figure 19b. However, the total heat flux is small (50−75 kW/m2) compared with the peak value near the burners (450 kW/m2 for oxy-fuel of 65% CO2, Figure 18). The third section of the superheater contains a large number of sheet plates. In view of that, convective heat transfer improves significantly because of the small distances between the plates. Hence, the convective heat transfer dominates in this region which explains the small

5. CONCLUSION The heat transfer and oxy-fuel combustion characteristics of a tangentially fired boiler are investigated in this study. The impact of the CO2 circulation on the heat and flow characteristics of the oxy-fuel combustion was explored by investigating three cases of oxy-fuel combustion to compare with air−fuel combustion. The investigated O2/CO2 ratios were 21%/79%, 27%/73%, and 35%/65% per volume. The results show that the temperature values are lowest in the case of 79% CO2 oxy-fuel and greatest in the 65% CO2 oxy-fuel case in comparison to those of the air−fuel combustion case. However, the 73% CO2 oxy-fuel case shows temperature and heat levels close to those of the air-fired combustion. High values of temperature in the 65% CO2 oxy-fuel case can help in increasing the efficiency of the burner by increasing the heat characteristics. However, this is applicable for the highly sustainable materials of which the combustors are made. Otherwise, the oxy-fuel case (79% CO2) is preferable to suppress the excessive temperature. The results show high rates of combustion in the 65% CO2 oxy-fuel case as compared to other combustion cases because of the low amount of recirculated CO2 (65%) with increasing molar fraction of O2 (35%). It is also shown that the heat flux is much higher in the combustion zone for the case of 65% CO2 oxy-fuel, and that is also higher in the reheater and the economizer for the 79% CO2 oxy-fuel case; in the superheated zone, all combustion cases have almost the same heat flux amounts. The contribution of the radiation heat transfer is dominant in the combustion zone and slows sharply in the superheaters, reheater, and economizer zones, sequentially.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

M. A. Habib: 0000-0003-3459-1462 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support received from King Fahd University of Petroleum and Minerals (KFUPM) and SABIC for funding this work through Project No. SB141005.

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REFERENCES

(1) World Energy Council, 2004. http://www.worldenergy.org. (2) Wall, T. F. Proc. Combust. Inst. 2007, 31 (1), 31−47.

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DOI: 10.1021/acs.energyfuels.7b02411 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.7b02411 Energy Fuels XXXX, XXX, XXX−XXX