Numerical Simulation of a 600 MW Utility Boiler with Different

Aug 23, 2012 - characteristics of the corner tangentially fired furnace, slagging and high-temperature corrosion on the surfaces of water walls and...
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Numerical Simulation of a 600 MW Utility Boiler with Different Tangential Arrangements of Burners Linbo Yan, Boshu He,* Fang Yao, Rui Yang, Xiaohui Pei, Chaojun Wang, and Jingge Song School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, People’s Republic of China ABSTRACT: Corner tangentially fired boilers (CTFBs), in which the burners are tangentially arranged at the corners, have been widely used in the modern thermal power plants in China. However, because of the variation in coal and inherent characteristics of the corner tangentially fired furnace, slagging and high-temperature corrosion on the surfaces of water walls and large temperature deviation at the furnace outlet are often detected during operation. In comparison to CTFBs, wall tangentially fired boilers (WTFBs), including wall-center tangentially fired boilers (WCTFBs) and wall-off-center tangentially fired boilers (WOTFBs), in which the burners are tangentially arranged on the walls and pulverized coal burns stably, are believed to be able to solve the above-mentioned problems. The commercially available software package Fluent is used to simulate the performances of different tangential arrangements of burners, including CTFB, WCTFB, and WOTFB, in the flow and combustion characteristics of pulverized coal in a 600 MW utility boiler. The results show that, in contrast to the CTFB, the highspeed airflow erosion does not happen to the wall tangential burners. In the WCTFB, the high-temperature flame center is lowered, benefiting the burnout of coal particles, the decrease of the carbon content in fly ash, and the reduction of energy loss. In addition, combustion of coal in a local reducing atmosphere is avoided, which reduces the possibility of slagging and hightemperature corrosion. Because of the entrainment effects, uneven wall heating is eased. The flue gas temperature deviation and overheating at the furnace outlet are restrained, benefitted from the symmetric distributions of velocity and temperature. The WCTFB is concluded as the best tangential burner arrangement among the three arrangements studied in this work.

1. INTRODUCTION The utility boiler is one of the main pieces of equipment in the power plant. The corner tangentially fired boilers (CTFBs) with one burner set at each corner have been widely used in the modern power plants in China because of their good performances of flame stability and anti-slagging, low NOx emissions, and high operation efficiency.1,2 However, the local temperature near the middle of the water wall is much higher than that near the corner of the water wall because of the inherent design defects of these boilers, which may result in coking on the surface of the water walls. If the diameter of the actual tangential circle formed by the burner injection is too large or the deflection of burner injection is severe, slagging and high-temperature corrosion on the water wall may occur.3 Wall tangentially fired boilers (WTFBs) with one burner set on one side wall, however, do not suffer from these problems and have attracted increasing attentions. This arrangement of burners has not been widely used in the utility boilers; related studies are still in a preliminary stage. Some studies about the performances of WTFBs are with two burner sets on one side wall.4,5 WTFB has been found to have better performance according to the experiments and the preliminary simulation of a 1 MW wall tangentially fired furnace with horizontal bias burners.6,7 The distance between the burner and the center of the furnace of the WTFBs is much shorter than that of the CTFBs; therefore, the jet rigidity of the WTFBs is much higher than the latter, which can restrain the deflection of the jet flows and thereby restrain the slagging and high-temperature corrosion.8 In addition, because the burners of the WTFBs are arranged near the center of the water walls, the heat-absorbing action of the jet flows will lower the temperature near the center of the water walls. It is © 2012 American Chemical Society

reported that the temperature difference of the water wall in the WTFBs is 62 °C, while that in the CTFBs is 251 °C.9 Some models used for the numerical simulation of coal combustion were discussed in refs 10−12. Many efforts13−27 have been made on the characteristics of flow and heat transfer of pulverized coal combustion in furnaces. A 500 MW corner tangentially fired furnace was simulated, and the results were compared to the actual cold-flow field.13 Processes of flow, heat transfer, and combustion were simulated for a power plant furnace, and some useful results were obtained.14 The formation and production of NOx was simulated for a 300 MW utility boiler.15 The formation mechanism of carbon black in the furnace was studied by Fletcher.16 The combustion characteristics and NOx emissions were numerically analyzed for a 200 MW corner tangentially fired furnace.17,18 Cold experimental studies with regard to overlarge outlet steam temperature deviation were carried out for a 2008 ton/h furnace.19 A horizontally pulverized coal bias combustion process was simulated for a 200 MW tangentially fired furnace.20 Numerical simulations were implemented for a 600 MW tangentially fired pulverized coal furnace, investigating contrary modes of secondary air and analyzing concerns, such as superheater overheating, under temperature of reheat steam, and overlarge outlet steam temperature deviation with relevant solutions.21−25 In comparison to the well-performed corner tangential boilers, studies on the emerging wall tangential boiler are much scarce. The wall heat flux and tangential circle diameter Received: May 29, 2012 Revised: August 23, 2012 Published: August 23, 2012 5491

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To fully understand the flow and combustion characteristics of the WTFB, a CTFB shown in panels a−c of Figure 1 is selected as the benchmark and retrofitted into two different kinds of WTFBs, with the geometry parameters of the burner set shown in Figure 1b unchanged. The imaginary tangential circle diameters (ICDs) are also maintained the same during the retrofitting, as shown in panels c−e of Figure 1. A burner set contains several burners located in a vertical plane, with a single burner consisting of a primary fuel/air (referred to as primary air in Figure 1b) nozzle, sandwiched between secondary air nozzles above and below. There is significant separation between the nozzles. One of the retrofitting schemes is that the

were studied for a wall tangential boiler on a 1 MW experimental furnace.26,27 From the recent studies, it can be found that the WTFBs are more promising than the CTFBs for their unique arrangement of the burners. However, a detailed comparison of the characteristics among the CTFBs, wall-center tangentially fired boilers (WCTFBs), and wall-off-center tangentially fired boilers (WOTFBs) is rarely found in the literature. In this work, the commercially available software package Fluent28 is used to simulate the effects of different tangential arrangements of burners, including CTFB, WCTFB, and WOTFB, on the performance of a 600 MW utility boiler to find the best arrangement of the burners.

Figure 1. Schematic of the furnace and the arrangements of the burners: (a) right view of the furnace, (b) nozzle arrangement for a burner set, (c) CTFB, (d) WCTFB, and (e) WOTFB.

Figure 2. Meshing of the computational domain and burner regions: (a) meshing of the front elevation view of the furnace, (b) meshing of the central horizontal cross-section of burners for CTFB, (c) meshing of the central horizontal cross-section of burner injections for WCTFB, and (d) meshing of the central horizontal cross-section of burners for WOTFB. 5492

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Figure 3. Velocity distributions on the horizontal cross-sections at different levels of primary air nozzles: (a) CTFB, (b) WCTFB, and (c) WOTFB.

Figure 4. Contours of the velocity on the central vertical cross-sections for the furnaces at y = 0 m: (a) CTFB, (b) WCTFB, and (c) WOTFB.

Figure 5. Contours of the temperature on the central vertical cross-sections for the furnaces at y = 0 m: (a) CTFB, (b) WCTFB, and (c) WOTFB. 5493

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walls, as shown in Figure 1e. The characteristics of flow and combustion of pulverized coal in tangentially fired boilers, including CTFB, WCTFB, and WOTFB, are numerically presented in this paper. The distributions of wall heat flux distributions, the flue gas temperature contours, the velocity fields, and the species distributions, especially the combustible species concentrations at the furnace outlet of these tangentially fired boilers, are analyzed and compared to each other. The predicted results indicate that the WCTFB behaves the best among the tangentially fired boilers.

CTFB is retrofitted into WCTFB, in which burners are tangentially arranged at the centers of water-cooled walls, as shown in Figure 1d, and the other is WOTFB, in which burners are tangentially arranged at the off-centers of water-cooled Table 1. Mean and Maximum Temperatures on Central Vertical Cross-Sections at y = 0 m temperature (K) burner arrangement

mean

maximum

CTFB WCTFB WOTFB

1674 1613 1636

2148 2235 2119

2. SIMULATION OBJECT The geometric configuration of the tangentially fired boilers simulated in this work is shown in Figure 1a. The seat of a burner set mounted at one of the four corners or on one of the four walls of the furnaces is shown in Figure 1b between 16.637 and 30.306 m in height. The lowest nozzle is oil 1 located at 19.958 m high, which is for oil injection during the startup and for secondary air during normal operation of the furnace. The highest nozzle, located at 29.863 m, is over fire air 2 (OFA 2), which is for reverse secondary air during operation. There are 6 primary air nozzles, 12 secondary air nozzles, 3 oil nozzles, and 2 OFA nozzles in each corner or wall-mounted burner. The imaginary tangential circles of the CTFB, WCTFB, and WOTFB are shown in panels c, d, and e of Figure 1, respectively. The ICD formed by the jet flows of corners 1 and 3 of the CTFB is 1882 mm, while that of corners 2 and 4 is 1458 mm, as shown in Figure 1c. The ICDs formed by the jet flows from the front and rear walls (walls 1 and 3) of the WCTFB and WOTFB are 1882 mm, while those formed by the side walls (walls 2 and 4) of the WCTFB and WOTFB are 1458 mm, as shown in panels d and e of Figure 1. The injection angle of the primary and secondary air and oil at corners 1 and 3 is 39°, while that at corners 2 and 4 is 47° for the CTFB. The injection angle of the primary and secondary air and oil is 83° on wall 1 (front wall of the furnace) and wall 3 (rear wall of the furnace) and 85° on wall 2 (left wall) and wall 4 (right wall) for the WCTFB. The injection angle of the primary and secondary air and oil on all four walls is 90° for the WOTFB. All of the reversed tangential angles (RTAs) of the OFAs are deviated from the primary and

Figure 6. Change in the mean temperature on the horizontal crosssection along the furnace height.

Figure 7. Contours of the temperature on the horizontal cross-section of the first primary air of the burners: (a) CTFB, (b) WCTFB, and (c) WOTFB. 5494

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of burners, are arranged because of the complex arrangements of burner nozzles, as shown in panels c, d, and e of Figure 2 for boilers of CTFB, WCTFB, and WOTFB, respectively. The entire object is divided into several sections, which are meshed separately and coupled by interfaces. Grid dependence tests have been performed for the three mesh systems, and the current meshing is sufficient to provide gridindependent solutions. The flow jet angle between the velocity vector of the air from the burner nozzles and the Cartesian coordinate is about 45° in the simulation of the CTFB. To decrease the false diffusion during the numerical calculation, the grids in the burner region of the CTFB are generated with the pave method, as shown in Figure 2b. For the WCTFB and WOTFB, the direction of the air velocity is almost coincident with the Cartesian coordinate. Thus, grids in the burner regions of the two boilers are relatively regular, as shown in panels c and d of Figure 2. In addition, to further restrain the numerical diffusion, the governing equations are discretized by the QUICK difference scheme and solved with the SIMPLE algorithm. To make the simulations more time-efficient and easier to converge, a simulation strategy of the gasphase cold-flow simulation followed by the gas−solid-phase hot-flow simulation is used.

secondary air and oil by 10° for all of the three arrangements, as shown in panels c−e of Figure 1.

3. MATHEMATICAL MODELS AND COMPUTING PROCEDURE 3.1. Mathematical Models. Pulverized coal combustion in a furnace is a gas−solid two-phase flow with a variety of heat transfers and chemical reactions. Models describing physical and chemical processes in a furnace include those of gas−solid flow, gas−solid heat transfer, volatile pyrolysis, char combustion, etc. A non-premixed combustion model is adopted by comparison, with the realizable k−ε turbulence model to simulate the turbulent gas transport and discrete ordinates (DO) radiation model to calculate radiation heat transfer. The chemical reaction rate is determined by a simplified probability density function (PDF),29 which is commonly used in engineering.11,12,30−32 3.2. Meshing and Numerical Algorithms. The side view of a tangentially fired boiler is shown in Figure 1a. During the meshing of the boiler, some simplifications of the object are made because of its complex construction. A total of 6 front partition platens in the upper front of the furnace are simplified as 6 zero-thickness platens. A total of 24 rear platens near the outlet of furnace are simplified as 24 zerothickness platens. It is believed that this simplification has little effect on the real gas flow field in the boiler. The computational domain is from the furnace hopper to the inlet of the air preheater and is meshed, as shown in Figure 2a. The mesh systems of the CTFB, WCTFB, and WOTFB consist of 506 544, 748 904, and 756 394 cells, respectively. More grids in the burner region, especially at the outlets

4. RESULTS AND DISCUSSION 4.1. Velocity Field in the Furnaces. The gas flow field in a furnace has great impact on the combustion and burnout of the pulverized coal. The flow fields in the furnaces of the

Figure 8. Contours of the mass fractions of main species on the cross-sections of CTFB: (a) CO, (b) O2, and (c) CO2, with (1) the central vertical cross-section at y = 0 m and (2) the horizontal cross-section at z = 21.75 m. 5495

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the CTFB, but the impacts on the water-cooled walls by the jet flow are not detected. Figure 4 shows the velocity contours of gas flow on the vertical center sections of the three burner arrangement schemes. It can be seen that a maximum velocity up to 32 m/s can be reached on the center section for the CTFB; a larger highspeed region and a better symmetry of the velocity distribution can be achieved. The high-speed region is mainly in the burner region, with a maximum velocity of 45 m/s for the WCTFB. The velocity distribution is asymmetric in the furnace hopper zone for the WOTFB, but it is very symmetric above the burner region. In comparison to CTFB and WOTFB, a high gas flow velocity zone is found within the upper zone in WCTFB, as shown in Figure 4c. Unlike the CTFB in which high-speed regions are found near the walls, the WTFBs can effectively avoid the high-speed airflow erosions in the burner regions. 4.2. Temperature Field in the Furnaces. The temperature field is one of the key issues for the simulation of pulverized coal combustion in furnaces. The position of the boiler flame center is a key factor for the boiler operation because issues, such as local overheating, slagging, and high-temperature corrosion, may occur if it is too high or too low. Contours of the temperature distribution on the central vertical cross-sections in the furnaces are presented in Figure 5. The patterns of the temperature distribution in the three boilers are basically the same. The gas temperature in the hopper zone

CTFB, WCTFB, and WOTFB are calculated in this work, and the simulation results are shown in Figures 3 and 4. The horizontal cross-sections shown in Figure 3 include those at the heights of the first, third, and fifth primary air nozzles and OFA 2. It can be seen that a swirling gas flow field is quickly formed in the furnace under the effects of the impact extrusion of the gas flow, the furnace wall restriction, the centrifugal force, and the jet entrainment. Strong heat, mass, and momentum transfers take place. The tangential circle diameter of the gas flow increases with an increasing height. Near the bottom of the furnaces, the formation of the tangential circles is earlier in CTFB than in WOTFB and WCTFB. However, in the middle and upper zones, the tangential circle diameter in CTFB is larger than those in WOTFB and WCTFB, which leads to the flow jets inclined to impact the water-cooled wall. Although the tangential circle construction near the bottom of the WTFB is not apparent and the gas flow symmetry is worse than that of the CTFB, the tangential circle in the WTFB forms quickly when the gas flows upward, meaning that the turbulence intensity of the gas flow is strong in the WTFB and a symmetric gas flow field can be formed quickly. The shorter distance between burners and the center of the furnace in WTFB leads to larger jet rigidity, smaller tangential circle diameter in the upper furnace, and a diamond-shaped edge of the outer circle. The tangential circle diameters of the WTFBs have the same evolution trends as that of

Figure 9. Contours of the mass fractions of main species on the cross-sections of WCTFB: (a) CO, (b) O2, and (c) CO2, with (1) the central vertical cross-section at y = 0 m and (2) the horizontal cross-section at z = 21.75 m. 5496

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Figure 10. Contours of the mass fractions of main species on the cross-sections of WOTFB: (a) CO, (b) O2, and (c) CO2, with (1) the central vertical cross-section at y = 0 m and (2) the horizontal cross-section at z = 21.75 m.

is relatively low, and high-temperature regions are located in the burner region in the height range from 16.637 to 30.306 m. Then, the gas temperature decreases along the furnace height. It can also be seen from Figure 5 that the gas temperature in the burner zone of the CTFB is lower than those of the WTFBs, which can be attributed to insufficient combustion of coal particles in the burner zone in CTFB, as further discussed later. However, the gas temperature of the CTFB decreases more slowly than those of the WTFBs above the burner zone; the gas temperature of the CTFB is even higher in a small scope above the burner zone. Overall, the mean and maximum temperatures of the three boilers are about the same, as shown in Table 1. Figure 6 shows the change of the mean gas temperature on the horizontal cross-section along the furnace height. In the burner region, the average temperature of the WOTFB is slightly higher than that of the WCTFB; CTFB has the lowest average temperature. The mean gas temperature of the CTFB in a small region above the OFAs is higher than those of the WTFBs. The maximum temperature appears within the burner region for the WTFBs but above the burner region for the CTFB. It can be concluded from the above analysis that a stable high-temperature region can be formed in the burner region for all of the three arrangement schemes. In the lower burner region, the gas temperature of the WOTFB is significantly higher than those of the other two boilers and the peak gas temperature is almost at the middle of burner region, which is far away from

Figure 11. Variation of the mean mass fraction of CO on the horizontal cross-section along the height of the furnaces.

the furnace outlet. The peak temperature of the CTFB appears above the burner region, and the position of the high-temperature 5497

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flame center is higher than those of the WTFBs, which leads to a higher temperature of gas flow at the outlet of the furnace. The elevated position of the high-temperature flame center will also result in losses of unburned carbon, increased exhaust gas heat, and the risk of overheating superheaters. WTFBs provide the advantage of a lower position of the high-temperature flame center over CTFB. The gas temperature contours on the horizontal cross-section at the height of primary air 1 (Figure 1b) are shown in Figure 7.

It can be seen that a distinct tangential circle has been formed at the burner bottom for all of the three boilers. The maximum temperature in the first layer of the burner for the CTFB is 1900 K, which is lower than those of the WTFBs. In addition, although the jet flow adherence to the water-cooled walls is not detected, the tangential circle diameter of the CTFB is still very large, which is prone to high-temperature slagging and corrosion of water-cooled walls. For the WOTFB, the area of the high-temperature region is small and the gas flow field is

Figure 12. Contours of heat flux (W/m2) on the walls: (a) CTFB, (b) WCTFB, and (c) WOTFB. 5498

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flux distribution is not exactly the same on the four walls because of the interaction between air and coal particles. From the wall heat flux distribution contours of the three burner arrangements, it can be seen that the wall heat flux is lower in the hopper zone and reaches the peak value in the burner region. Wall heat flux gradually decreases along the furnace height above the burner. This corresponds to the highest gas temperature in the burner region. The water-cooled wall absorbs a large amount of heat. The water temperature in the water wall increases and is then kept constant in these boilers, while the gas temperature decreases with the furnace height increment; therefore, the heat flux through the water-cooled wall decreases with the furnace height. The heat loss of the hopper is small; therefore, the heat flux through the hopper wall is small. The steam temperature in the furnace top wall is relatively high, while the gas temperature near the top wall is relatively low; therefore, the heat flux through the top wall is relatively small (Table 2). It can be seen from Figure 12a that the corner tangential arrangement of the burners will cause the flame deflection and the local wall heat flux is very high on the flame inclination side. The maximum value can reach 5.93 × 105 W/m2. In the upper part of the furnace, the heat flux gradually becomes uniform along the furnace width. It can be seen from panels b and c of Figure 12 that the wall heat flux distributions are similar to that of the CTFB and higher in the burner regions. The maximum wall heat load is 7.07 × 105 and 6.69 × 105 W/m2 for the WCTFB and WOTFB, respectively. The peak heat flux appears near the wall centerline. The area with high heat fluxes decreases along the furnace width above the burners. This is due to the entrainment effect of the flow jets of the WTFBs that absorb heat and ease the uneven heating and the local overheating of the water-cooled wall. 4.5. Phenomena at the Furnace Outlets. For the tangentially fired furnaces, residual swirl velocity always exists at the furnace outlet, which causes velocity and temperature deviations here and, subsequently, causes high-temperature rupture of superheaters. The flue gas temperature and velocity distribution contours at the furnace outlets of the three boilers are shown in Figure 13. The velocity near the right side are higher than that near the left side, as shown in panel a2 of Figure 13, while the flue gas temperature has the opposite distribution, as shown in panel a1 of Figure 13. The temperature distribution is more symmetric, as shown in panels b1 and c1 of Figure 13, and there is no large temperature deviation. The velocity and temperature deviations at the furnace outlet of the CTFB are found more obvious than those of the WTFBs. The velocity (temperature) deviation factor, Ev (ET) defined by eq 1 and the velocity (temperature) uneven coefficient, Mv

asymmetric. On the basis of the above comparisons, WCTFB is more promising because of its more satisfactory tangential circle, larger high-temperature combustion region, and less risk of local high-temperature slagging and corrosion. 4.3. Component Concentration Field in the Furnaces. Contours of mass fractions of CO, O2, and CO2 in the center vertical cross-section of the CTFB, WCTFB, and WOTFB are shown in Figures 8, 9, and 10, respectively. In comparison to the distributions in the WTFBs, higher CO mass fractions in the burner region are found for the CTFB (Figure 8), which indicates that the combustion of pulverized coal in the burner region is not complete because of the lack of O2, especially in the center of the burner region. From Figures 9 and 10, the O2 concentration is high in the burner regions, which guarantees rapid ignition and combustion of the pulverized coal. Consequently, the CO concentration in the burner regions of the WTFBs is very low, and the coal combustion is relatively thorough. It can also be seen that O2 has almost been completely consumed at the furnace outlets of the three boilers. When the species distributions are compared on the horizontal cross-section at z = 21.755 m, which is the position of primary air 3 in three boilers, it can be found that coal burns more completely in the burner region of WCTFB than the other two boilers because the mass fractions of CO and O2 at this cross-section in WCTFB are the lowest, while the mass fraction of CO2 is the highest among the boilers. The variation in the mean CO mass fraction on the horizontal cross-section along the furnace height is presented in Figure 11. The CO concentration near the water-cooled wall is found significantly higher in the CTFB than that in the WTFBs. This indicates that the reductive atmosphere exists near the wall area where the flame is prone to adhere to the water-cooled wall, which makes the slagging and corrosion occur more easily. However, it is not the case in the WTFBs. Thus, the risk of slagging and corrosion can be avoided in WTFBs. The decreased CO mass fraction in WTFBs indicates that the carbon content in fly ash will be reduced and then more efficient coal combustion will be reached. The values of the char conversion ratio (CCR) of the three boilers are extracted to confirm this conclusion. The CCR values are 96.2, 99.3, and 98.95% for CTFB, WCTFB, and WOTFB, respectively. It can be found that the CCR value of WCTFB is the largest. This is because the high-temperature flame center is lowered in WCTFB, as mentioned before, which benefits the burnout of coal particles, the decrease of carbon content in fly ash, and the reduction of energy loss. 4.4. Heat Flux on Furnace Walls. Radiation heat transfer is absolutely dominant in the furnaces. The greater the wall heat flux, the more the heat exchanges. The wall heat flux distribution is shown in Figure 12, where the minus symbol means that heat flux is toward the walls. It is apparent that the wall heat flux distribution is uneven. Although the burner arrangement scheme is symmetrical, the wall heat Table 2. Mean and Maximum Values of Absolute Wall Heat Flux

absolute wall heat flux (W/m2) CTFB

WCTFB

WOTFB

walls

mean

maximum

mean

maximum

mean

maximum

front wall rear wall left wall right wall

386000 387000 362000 360000

520000 515000 636000 626000

398000 404000 393000 390000

676000 683000 707000 705000

430000 420000 394000 397000

669000 652000 595000 597000

5499

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Figure 13. Contours of the temperature and velocity at furnace outlets: (a) CTFB, (b) WCTFB, and (c) WOTFB, for the (1) temperature and (2) velocity.

(MT) defined by eq 2 are used to better explain the velocity and temperature deviations at the furnace outlet33 E = ϕR̅ /ϕL̅

where the subscript max denotes the maximum value. The Ev (ET) factors of the CTFB, WCTFB, and WOTFB are 1.260 (1.060), 1.083 (1.010), and 1.160 (1.045), respectively. The Mv (MT) coefficients of the CTFB, WCTFB, and WOTFB are 2.302 (1.193), 1.460 (1.176), and 1.521 (1.183), respectively. Through the comparisons of Ev (ET) factors and Mv (MT) coefficients, it can be seen that the velocity and temperature deviations at the furnace outlet of the WCTFB are the smallest among all of the three burner jet arrangements.

(1)

where ϕ̅ denotes the mean value of the velocity or temperature and the subscripts R and L denote the right and left sides of the furnace outlet, respectively M = ϕmax /ϕ ̅

(2) 5500

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Notes

The velocity near the right side is higher for the clockwise dominant flow in the furnace. The amount of gas increases along the height of the furnace with the coal burning, which causes the diameter of the tangential circle to increase along the furnace height. The swirl strength then becomes so strong that the reversed tangential air from the two OFAs (Figure 1b) is not able to counteract the swirling flow effectively. Thus, the residual swirl velocity can still exist at the furnace outlet, unless the OFAs are properly arranged.2,20 The same phenomena can be detected in WOTFB but not in WCTFB. The mean concentrations of CO and H2 at the furnace outlets for the three boilers are listed in Table 3. Mass fractions

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC, 51176009) for this work.

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Table 3. Mass Fractions of Combustible Gases at the Outlets of Furnaces mass fraction (%) burner arrangement

H2

CO

CTFB WCTFB WOTFB

0.033 0.002 0.004

0.152 0.116 0.249

of H2 and CO are 0.002 and 0.116%, respectively, for WCTFB, which are lower than those of the other two boilers. It is, therefore, favorable to the burnout of the combustible gases, which decreases the energy loss of the boiler.

5. CONCLUSION Tangentially fired furnaces with three different burner arrangements have be simulated and analyzed with regard to their gas flow, temperature profile, and heat transfer. The conclusions are as follows: (1) As far as the velocity field is concerned, the tangential circle is quickly formed in WTFB and its diameter increases gradually with the height of the furnace. The flow velocity distribution is more uniform in the upper part of the furnace, especially at the furnace outlet, which is beneficial to the reduction of flue gas velocity and temperature deviations. (2) CTFB has a higher position of the high-temperature flame center and a higher gas temperature at the furnace outlet, which increases the carbon content of the fly ash and the heat loss of the exhaust flue gas. However, the high-temperature flame center is located within the burner region in WCTFB, which avoids the above problems. (3) Combustion of coal in WTFBs is in a richer oxygen atmosphere, which reduces the possibility of slagging and high-temperature corrosion. (4) The peak heat flux appears near the centerline of the water-cooled wall in the WTFBs. The uneven heating on the water-cooled wall is eased because of the entrainment action of the jet flows. (5) The gas velocity and temperature distributions at the furnace outlet are more symmetric for the WCTFB. These distributions are beneficial to the burnout of the combustible gases and reduce the gas temperature deviation at the furnace outlet.

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REFERENCES

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