Energy Fuels 2010, 24, 5349–5358 Published on Web 09/09/2010
: DOI:10.1021/ef100682s
Numerical Simulation of Combustion Characteristics and NOx Emissions in a 300 MWe Utility Boiler with Different Outer Secondary-Air Vane Angles Lingyan Zeng, Zhengqi Li,* Guangbo Zhao, Shanping Shen, and Fucheng Zhang School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China Received June 2, 2010. Revised Manuscript Received August 19, 2010
We experimentally and numerically studied the pulverized-coal combustion process and NOx emissions for a 300 MWe wall-fired boiler burning bituminous coal. Measurements of the polar components of the gas velocity made with a particle-dynamics anemometer and calculated values are found to be in good agreement. Good agreement was also found for the CO2 concentration, NOx concentration, and combustible material content in the fly ash between simulation and industrial experiments performed on full-scale boilers. For enhanced ignition-dual register (EI-DR) burners, pulverized coal is spread widely from the burner. Some pulverized-coal particles are thrown to the furnace hopper, and much pulverized coal is swept to the furnace hopper at the bottom of the boiler. As the outer secondary-air vane angle increases, the temperature of the water-cooled tube wall region near the burners and furnace hopper increases. Changes in the NOx concentration at the furnace outlet are not simply linear with changes in the outer secondary-air vane angle. The NOx concentration at the furnace outlet was lowest (420.2 ppm at 6% O2) for an outer secondary-air vane angle of 35° and highest (468.2 ppm at 6% O2) for an outer secondaryair vane angle of 30°. This represents an increase of 11.4%.
results.4 CFD modeling has been applied to simulate a number of large-scale combustion facilities.5-8 The outer secondary-air vane angles of the burner greatly affect the capacity of flow rotation and further influence the formation of the central recirculation zone and its size, which affect the process of pulverized-coal ignition and combustion and the quantity of NOx generated in the combustion process.9 In this paper, we simulate the coal combustion and NOx emission characteristics of a 300 MWe wall-fired boiler installed with EI-DR burners with different outer secondary-air vane angles. When concentrations of O2, CO, and NOx, gas temperatures in the furnace, temperatures of the furnace outlet, and combustible material content in the fly ash were compared, we can carry out a comparative analysis of the burner arrangements with different outer secondaryair vane angles under actual operation and provide a theoretical basis for the operation of utility boilers in real situations.
1. Introduction NOx emissions contribute to acid rain formation and the production of photochemical smog. The primary emissions of coal-fired power plants into the air are the main sources of NOx and generally must be controlled to meet governmental standards. The best approach is to abate NOx formation at its source, the burner. Low-NOx pulverized-coal burner technologies are an efficient solution. One such technology, the enhanced ignition-dual register (EI-DR) burner, has been widely applied,1 e.g., to utility boilers in China. Computational fluid dynamics (CFD) models are powerful tools for studying and characterizing complex processes that take place in the boiler. CFD provides much precise numerical data on velocity, temperature, and concentration fields, irradiation profiles, the heat-transfer distribution, and pollutant formation. Sarlej et al.2 demonstrated the application of CFD to experimental burner design. Taglia et al.3 presented and discussed optimization studies based on computer simulations of a 450 kWe combustion chamber. Habib et al. numerically investigated the influence of combustion parameters on NOx production in an industrial boiler. The CFD package Fluent was used to carry out calculations in that study. Their CFD model predictions were in good agreement with experimental
2. Utility Boiler A B&W B-1025/16.8-M boiler with a 300 MWe unit was made by Babcock and Wilcox Beijing Co., Ltd. The opposite wall-fired, pulverized-coal boiler with a dry-ash furnace is equipped with 20 EI-DR burners. There are 12 EI-DR burners arranged in three rows on the front wall of the furnace. The other eight burners are arranged in two rows on the rear wall, opposing the top and bottom rows on the front wall. Five medium-speed mills and a positivepressure direct-fired system are used to supply pulverized coal to the burners. At full load, the medium-speed mills and 16 EI-DR burners are used. Figure 1 is a schematic diagram of the EI-DR burner, showing eight radial vanes in the inner secondary-air duct and 12 tangential
*To whom correspondence should be addressed. Telephone: þ86451-86418854. Fax: þ86-451-86412528. E-mail:
[email protected]. (1) Larue, A. D.; Cioffi, P. L. Mod. Power Syst. 1988, 8, 42–47. (2) Sarlej, M.; Petr, P.; Hajek, J.; Stehlik, P. Appl. Therm. Eng. 2007, 27 (16), 2727–2731. (3) Taglia, C. D.; Gass, J.; Dreher, H. Prog. Comput. Fluid Dyn. 2001, 1, 117–130. (4) Habib, M. A.; Elshafei, M.; Dajani, M. Comput. Fluids 2008, 37 (1), 12–23. (5) Saario, A.; Oksanen, A. Energy Fuels 2008, 22, 297–305. (6) Yin, C.; Rosendahl, L.; Kær, S.; Clausen, S.; Hvid, S. L.; Hille, T. Energy Fuels 2008, 22, 1380–1390. r 2010 American Chemical Society
(7) Diez, L. I.; Cortes, C.; Pallares, J. Fuel 2008, 87 (7), 1259–1269. (8) Miltner, M.; Makaruk, A.; Harasek, M.; Friedl, A. Clean Technol. Environ. Policy 2008, 10 (2), 165–174. (9) Jing, J. P.; Li, Z. Q.; Liu, G. K.; Chen, Z. C.; Ren, F. Energy Fuels 2010, 24 (1), 346–354.
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3.1. Mathematical Models. Commercial CFD codes, such as Fluent, have proven to be powerful and effective tools in looking at control operations and analyzing processes that occur within coal-fired utility boilers.10-12 The commercially available Fluent 6.3.26 software can be used to calculate, in particular, flow fields of boilers employing a range of widely used numerical models. In this work, gas turbulence was specifically taken into account by the so-called realizable k-ε model.13 In comparison to the standard model, the realizable k-ε model provides new dissipation equations and realizable eddy-viscosity equations but still contains the k-ε model, which describes adequately the isotropic nature
of turbulence. The Lagrangian stochastic tracking model was applied to analyze the gas/particle flow field,14 while calculations of gas/particle two-phase coupling employed the particle-source-in-cell method.15 Radiation was described using the P-1 model,16 and devolatilization was modeled with the two-competing-rate Kobayashi model.17 The combustion of volatiles was modeled by employing a probability density function theory,17 and char combustion was modeled by employing a diffusion/kinetics model.18 The formation of NOx includes thermal NOx and fuel NOx but hardly any prompt NOx. Here, only the production of NO was taken into account because NOx emitted into the atmosphere from combusting fuels consists mostly of NO, with there being much lower concentrations of NO2 and N2O. The concentration of thermal NOx was calculated using the extended Zeldovich mechanism (specifically, N2 þ O f NO þ N, N þ O2 f NO þ O, and N þ OH f NO þ H). The fuel NOx concentration was calculated using De Soete’s model.19 The formation of prompt NOx was neglected in calculations. Table 1 presents the detailed mathematical model for numerical simulation and corresponding model parameters. 3.2. Verification Simulations. 3.2.1. Verification of Cold Gas/Particle Flow Simulations. To verify the validity and feasibility of grid divisions, the calculation model and methods, and the correctness of boundary conditions, simulation and experimental data were compared. The full-scale industrialsized burner was a burner for a 1025 ton/h lean-coal-fired boiler. The scale of the model burner was 1:7. In a cold air/particle twophase experimental test facility, a three-dimensional particle dynamics anemometer was used to measure gas/particle flows in the region near the model EI-DR burner.20 A three-dimensional computer model based on the burner model was configured for use in the cold gas/particle flow numerical calculation. The geometric model for simulations was set up to closely mimic the experimental rig. The realizable k-ε model was adopted to simulate the gas turbulent flow. The Lagrangian stochastic tracking model was applied to analyze the gas/particle flow field, while calculations of gas/particle two-phase coupling employed the particle-source-in-cell method. Figure 2 shows the simulation results and data acquired from experiments, where x and d represent the distance between measuring points and the burner water-cooled wall and the diameter of the outermost cone within the burner, with d = 173 mm, respectively. Figure 2 shows that the axial, radial, and tangential gas velocities obtained in the cold experiment and simulation are in good agreement. The simulations reproduced the distribution of the gas velocity, supporting the conclusion that the calculation model can simulate the swirling gas/particle flows of the burner. 3.2.2. Verification of Reacting Flow Simulations. We studied the average gas temperatures along the furnace height in the case of an outer secondary-air vane angle of 35° for full-scale furnaces in both in situ experiments and simulation.
(10) Dong, C. Q.; Yang, Y. P.; Yang, R.; Zhang, J. J. Appl. Energy 2010, 87, 2834–2838. (11) Vuthaluru, H. B.; Vuthaluru, R. Appl. Energy 2010, 87, 1418– 1426. (12) Zeng, L. Y.; Li, Z. Q.; Cui, H.; Zhang, F. C.; Chen, Z. C.; Zhao, G. B. Energy Fuels 2009, 23 (10), 4893–4899. (13) Shih, T. H.; Liou, W. W.; Shabbir, A. Comput. Fluids 1995, 24 (3), 227–238. (14) Gosman, A. D.; Loannides, E. Aspects of computer simulation of liquid-fuelled combustors. Proceedings of the American Institute of Aeronautics and Astronautics (AIAA) 19th Aerospace Science Meeting; St. Louis, MO, 1981; pp 81-323.
(15) Crowe, C. T.; Sharma, M. P.; Stock, D. E. J. Fluids Eng. 1977, 99 (2), 325–332. (16) Cheng, P. AIAA J. 1964, 2, 1662–1664. (17) Smoot, L. D.; Smith, P. J. Pulverized Coal Combustion and Gasification; Plenum Press: New York, 1985. (18) Zhou, L. X. Theory and Numerical Modeling of Turbulent GasParticle Flows and Combustion; CRC Press, Inc.: Boca Raton, FL, 1993. (19) De Soete, G. G. Proceedings of the 15th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1975; pp 991-1195. (20) Chen, Z. C.; Li, Z. Q.; Jing, J. P.; Chen, L. Z.; Wu, S. H.; Yao, Y. Energy Convers. Manage. 2009, 50, 1180–1191.
Figure 1. EI-DR burner and the position of the monitoring pipe (dimensions in meters): (1) primary-air duct, (2) inner secondary-air duct, (3) outer secondary-air duct, (4) water-cooled wall, (5) tangential vanes, (6) radial vanes, (7) monitoring pipe, and (8) conical diffuser.
vanes in the outer secondary-air duct. The swirling directions of the inner and outer secondary-air flows are identical. Under the influence of the particle deflector and conical diffuser, pulverized coal carried by primary air diffuses radially and gathers in the region close to the wall of the primary-air tube, which results in coal-rich flow in the peripheral zone of primary air and coal-lean flow in the central zone. When primary air streams through the burner nozzle and into the furnace, the fuel-rich flow in the peripheral zone of primary air premixes with secondary air, which shifts most of the pulverized coal into low-temperature secondary air. The flow that has lost coal mass in the peripheral zone of primary air and the fuel-lean flow in the central zone both enter the central recirculation zone. Thus, there is a low concentration of pulverized coal in the high-temperature central recirculation zone.1
3. Numerical Simulation Method
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Table 1. Mathematical Model and Model Parameters for Numerical Simulation
Figure 3 compares the experiment and calculation results. The figure of gas temperatures shows that the experiment and calculation results differ, with the calculated values being higher than the experiment values, but the trends of the temperature are the same. Therefore, the simulation method can evaluate pulverized-coal combustion in the furnace. 3.3. Computational Domain Specifications and Calculated Parameters. The simulated boiler is a 300 MWe wall-fired boiler with a “Π”-type furnace arrangement. Owing to the mirror symmetry plane along its width, the furnace can be divided into two equal parts, with the burners of the two parts rotated in opposite directions. Therefore, only half of the furnace is needed for the simulation domain, and the
boundary type of the mirror symmetry plane is specified as “symmetry”. According to experience garnered from previous simulations,21 a simplification of the domain can reduce the number of cells and shorten computing time but has little effect on the actual distributions of the main variables, such as temperature and concentration. The simulated domain of the furnace is shown in Figure 4. This numerical domain extended from the bottom of the cold ash hopper to the furnace outlet. The dimensions of the reacting flow region were 15 780 mm in length, 46 800 mm in height, and 6500 mm in width, which covered half of the furnace. The coordinate axes and origin are also displayed in Figure 4. A three-dimensional mesh with 1 080 486 cells was allocated over the computational domain. A grid independence test was performed by conducting several simulation tests for different mesh sizes, to study the flow and temperature
(21) Huang, L. K.; Li, Z. Q.; Sun, R.; Zhou, J. Fuel Process. Technol. 2006, 87, 363–371.
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Figure 2. Comparison of calculated gas velocities with measurement data. Table 2. Design Parameters of Swirl Burners in the Utility Boiler inner secondary air
outer secondary air
quantity
primary air
exit area (m2) temperature (°C) mass flow rate (kg s-1) inner secondary-air vane angles (deg) outer secondary-air vane angles (deg) swirl number
0.1979 75 5.80 60
0.5648 353 3.59
30
35
40
45
0.407
0.364
0.329
0.312
0.6677 353 8.37
22
Chigier and Beer, the degree of swirl in swirling flow is usually characterized by the swirl number Gθ ð16Þ S0 ¼ RGx Figure 3. Comparison of average gas temperatures calculated along the furnace height with measurement data.
where Gθ is the axial flux of the tangential momentum, Gx is the axial flux of the axial momentum, and R is the outer radius of the annulus. The parameters Gθ and Gx can be expressed as22,23 Z R Gθ ¼ ðWrÞFU2πrdr ð17Þ
profile. A refined grid was constructed in regions of maximum interest, where a sudden change in fluid flow is expected. For example, close to the burner exit, the shape of this part is cuboid, and thus, the mesh in this area is also cuboid. The finest grid with the greatest density of cells was constructed near the burner region. The burner was assigned a fine mesh according to its structural shape. Hexahedral and wedge-shaped meshes were employed for the blast pipe and flaring of the burner, and a tetrahedral mesh was employed for the region of the conical diffuser in the primary air duct. A rough grid was constructed in regions of less interest, where a sudden change in the fluid flow is not expected. Hexahedral and wedge-shaped meshes were employed for the bottom of the cold ash hopper, the furnace arch, and the furnace outlet, which are irregular in shape. The operational parameters are listed in Table 2, while the coal proximate and ultimate analyses are presented in Table 3. The experimental coal was bitumite. The average particle diameter of the experimental coal was 50 μm according to the test result. The swirl number is an important parameter characterizing combustion in a furnace. In this paper, we changed the swirl number of the burner by changing the outer secondaryair vane angle (see Table 3). As was originally proposed by
0
Z
R
Gx ¼ 0
Z 2πrFU 2 dr þ
R
2πrpdr
ð18Þ
0
where U, W, and p are the axial and tangential components of the velocity and static pressure, respectively. It is difficult to directly estimate the pressure integral term because static pressure is strongly dependent upon the geometry of the swirl. According to Beer and Chigier,24 Martin,25 and Weber and Dugue,26 the axial momentum flux, Gx, can be wellapproximated by eliminating the pressure term in eq 18. Thus, (22) Chigier, N. A.; Beer, J. M. J. Basic Eng. 1964, 788–796. (23) Sheen, H. J.; Chen, W. J.; Jeng, S. Y.; Huang, T. L. Exp. Therm. Fluid Sci. 1996, 12, 444–451. (24) Bedr, J. M.; Chigier, N. A. Combustion Aerodynamics; Wiley, Inc.: New York, 1972. (25) Martin, C. A. American Society of Mechanical Engineers (ASME) Paper 87-GT-139; ASME: New York, 1987. (26) Weber, R.; Dugue, J. Prog. Energy Combust. Sci. 1992, 18, 349–367.
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Figure 4. Schematic diagrams of the structure of half of the furnace (dimensions in meters). Table 3. Burned Coal Characteristics proximate analysis (as received, wt %)
fixed carbon
ash
moisture
volatiles
net heating value (kJ kg-1)
40.82
27.13
11.8
33.15
17790
ultimate analysis (as received, wt %)
carbon
hydrogen
sulfur
nitrogen
oxygen
48.05
2.51
1.23
0.54
8.74
burners with an outer secondary-air vane angle of 35°. The figure shows that pulverized coal is affected by the conical diffuser in the primary air duct of a burner and is seen to disperse around the burner after streaming into the furnace. The region near the burner axis does not become a fuel-rich zone. Some particles are carried by the strongly rotating secondary air and rejected to the water-cooled wall, readily leading to a higher temperature and slagging in the watercooled wall region. A large number of pulverized-coal particles wash over the hopper area, resulting in slagging in the hopper. Figure 6 shows the results of gas-temperature field simulations for a cross-section through the burner center at a height of 9.77 m. It is seen that, at different outer secondary-air vane angles, the distribution regularities of the temperature field near the burner are similar. Each burner organizes combustion independently. The gas temperature increases rapidly within a short distance of the burner nozzle. This is because bituminous coal ignites quickly in the presence of hightemperature gas, and with the addition of secondary air and the burnout of fuel, the gas temperature then begins to decline after a period of sustained temperature. In the outlet areas of the burners, the temperatures are high and the gradients change greatly. This is mainly due to pulverized coal spreading to the region close to the wall of the primary air duct after the primary air/coal mixture of the burners has passed through the conical diffuser in the primary air tube. The pulverized coal spreads to the surrounding wall of the burner after the pulverized coal is injected into the furnace and then quickly burns in the presence of high-temperature flue gas from the recirculation zone. When cases with different outer secondary-air vane angles are compared, the temperature at the burner exit gradually decreases from
Figure 5. Calculated particle trajectory for four burners with outer secondary-air vane angles of 35°.
the modified swirl number S defined in eq 16 is generally expressed as RR FUWr2 dr Gθ ¼ 0R R ð19Þ S0 ¼ RR 2πR 0 FU 2 rdr R 0 FU 2 rdr 4. Results and Discussion 4.1. Analysis of Combustion Inside the Furnace. Figure 5 shows the results of particle trajectory simulation for four 5353
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Figure 6. Calculated temperature fields over a cross-section through the burner center at a height of 9.77 m (dimensions in kelvin).
1000 to 600 K with an increase in the outer secondary-air vane angle. This is because, with an increasing outer secondaryair vane angle, the swirl number decreases gradually, the entrainment capacity of the air flow decreases, the size of the central recirculation region decreases, the amount of entrainment hot gas decreases, and the ignition is postponed. A temperature at the burner exit of 600 K adversely affects the ignition and combustion stability; the temperature near the water-cooled wall gradually decreases from 1600 to 1000 K with an increase in the outer secondary-air vane angle. This is because, with an increasing vane angle, the swirl capability of the outer secondary air decreases, the quantity of pulverized coal carried by the outer secondary air decreases, the quantity of particles rejected to the water-cooled wall decreases accordingly, and the temperature in the water-cooled wall region decreases, which reduces the probability of slagging on the water-cooled wall. Figure 7 shows the results of O2 concentration (%) simulations for a cross-section through the burner center at a height of 9.77 m. At the burner exit, with the ignition and combustion of coal, huge amounts of O2 are consumed and the O2 concentration decreases gradually. The O2 concentration at the burner exit decreases slowly with an increase qin the outer secondary-air vane angle, and there is a more obvious gradient change. This is because the ignition is postponed with an increasing outer secondary-air vane angle. The O2 concentration distribution is consistent with the temperature distribution in Figure 6. Figure 8 shows results of CO concentration (%) simulations for a cross-section through the burner center at a height of 9.77 m. When pulverized coal is injected into the furnace through the burner, it diffuses in the surroundings. Some of the pulverized coal combusts in the oxygen-deficient recirculation region and produces more CO, while the remainder combusts in the oxygen-rich secondary-air region, and char is easily oxidized to CO2. Figure 8 shows that the CO
concentration is symmetrical around the burner. The CO concentration is highest around the burner exit mainly because the recirculation zone, which extends the residence time of pulverized coal and has less oxygen, oxidizes a large amount of char to CO. With the injection of secondary air, CO is oxidized to CO2, and the CO concentration decreases rapidly. Comparing simulations of the CO concentration with different outer secondary-air vane angles, we find that the exit CO concentration increases around the burner with an increase in the outer secondary-air vane angle. This is because, with an increase in the outer secondary-air vane angle, the swirl number decreases, the air rotation capability decreases, the quantity of pulverized coal carried into the secondary-air zone decreases, and thus, more fuel combusts in the oxygen-deficient recirculation region. Therefore, more CO is generated around the burner, and the CO concentration increases. Figure 9 shows the simulation results for the NOx concentration (%) at a cross-section through the burner center at a height of 9.77 m. As seen in the figure, the NOx concentration is consistent for the four vane angles. With volatile devolatilization, the NOx concentration increases until char combustion and reaches a peak. The NOx concentration at the burner center is lower than that near the water-cooled wall. One reason for this is that pulverized coal is affected by the conical diffuser in the primary-air pipe and diffuses to the surroundings, resulting in a low pulverized-coal concentration, high CO concentration, and strongly reducing atmosphere,27 which inhibits NOx formation. A second reason is that there is much pulverized coal around the burner that promotes fuel NOx formation, and at the same time, because of pulverized-coal combustion, the temperature around the burner is high, in some areas more than 1800 K, which (27) van der Lans, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23, 349–377.
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Figure 7. Calculated O2 concentrations (%) over a cross-section through the burner center at a height of 9.77 m.
Figure 8. Calculated CO concentrations (%) over a cross-section through the burner center at a height of 9.77 m.
promotes the formation of thermal NOx. At the burner exit and with an outer secondary-air vane angle of 30°, the NOx concentration is 600 ppm, which is significantly higher than the concentration of 400 ppm for the other three vane angles. This is because, with an outer secondary-air vane angle of 30°, with a large recirculation region, the primary air and second air strongly premix, pulverized coal ignites early, the temperature is high, and more NOx is generated. With an outer secondary-air vane angle of 45°, a large portion of pulverized coal that was not burnt in the early stage begins to
ignite, resulting in the NOx concentration in the region far from the burner nozzle to increase rapidly, and the NOx concentration at a later stage is higher than for the other three vane angles. Among the four vane angles, the NOx concentration in the cross-section at a height of 9.77 m is lowest for an outer secondary-air vane angle of 35°. Figure 10 shows distributions of the average gas temperatures and concentrations along the furnace height. The trend of the flue gas temperature in the simulation is the same in the four vane angles. With an increasing furnace height, the 5355
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Figure 9. Calculated NOx concentration (ppm) over a cross-section through the burner center at a height of 9.77 m.
Figure 10. Distributions of the average gas temperatures and concentrations along the furnace height for the four vane angles.
temperature of the flue gas first increases and then decreases and the temperature peaks at 1750 K between the heights of Y = 15 and 20 m. At the height of the outlet of the burner, the temperature of the flue gas decreases obviously. This is because the mixture of cold air and pulverized coal pours into the burner and the temperature of the flue gas gradually increases with the burnout of the pulverized coal. At a furnace height between Y = 0 and 9 m, with the pulverized
coal igniting and combusting, the temperature of the flue gas gradually increases as the height increases and the flue gas temperature decreases as the outer secondary-air vane angle increases. This is mainly because most of the pulverized coal streams from the EI-DR burners at Y = 9.77 m, then moves to the hopper (see Figure 5), and burns there, which increases the temperature of this region rapidly. Moreover, as the outer secondary-air vane angle increases, the rotation ability 5356
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Table 4. Experimental Data and Simulation Results of the Gas Temperature and Concentrations at the Furnace Outlet outer secondary-air vane angles (deg) 30 35 40 45
simulation experiment simulation simulation simulation
flue gas temperature (K)
O2 concentration (%)
CO concentration (ppm)
CO2 concentration (%)
NOx concentration (ppm at 6% O2)
combustible material content in the fly ash (%)
1140
4.71 6.4 4.82 4.87 4.92
24.13 6.83 26.12 43.25 51.37
13.08 12.84 12.99 12.81 12.73
468.2 411.5 420.2 441.6 445.3
6.31 6.54 6.39 6.52 6.57
1155 1162 1173
flue gas analyzer at the furnace outlet, and the results show that the O2 concentration is 6.4%, the CO concentration is 6.83 ppm, the CO2 concentration is 12.84%, and the NOx concentration is 411.5 ppm, at 6% O2. For an outer secondary-air vane angle of 35°, the numerical simulation shows that the combustible material content in the fly ash at the furnace outlet is 6.39%, while the combustible material content in the fly ash at the furnace outlet is 6.54% in the experiment. The difference between experimental and simulation results is slight. This validates the application of the model in evaluating pulverized-coal combustion in this furnace. Table 4 shows that the gas temperature at the furnace outlet increases as the outer secondary-air vane angle increases. In addition, the O2 concentration increases; the CO2 concentration decreases; and the combustible material content in the fly ash gradually increases. As the outer secondary-air vane angle increases, the swirl number decreases and the recirculation region becomes smaller. The pulverized-coal burnout rate in the burner zone also decreases. In addition, more combustible material is burnt in the upper furnace, resulting in a higher temperature and O2 concentration at the furnace outlet. The simulation results show that changes in the NOx concentration at the furnace outlet with changes in the outer secondary-air vane angle are not simply linear. The NOx concentration at the furnace outlet was lowest (420.2 ppm at 6% O2) for an outer secondary-air vane angle of 35° and highest (468.2 ppm at 6% O2) for an outer secondary-air vane angle of 30°. This represents an increase of 11.4%.
of the flow weakens. At the same time, there are fewer pulverized-coal particles carried by secondary air and fewer pulverized-coal particles thrown to the furnace hopper, and the temperature of the flue gas decreases. At a height of Y > 20 m, the gas temperature decreases gradually with the burnout of coal. Figure 10 presents distributions of the O2 concentration. It is seen that the variations are similar for the four vane angles. At the height of the burner inlet, much air streams into the burner and increases the O2 concentration. The pulverized coal ignites and combusts near the three rows of burners (between Y = 9.77 and 17.12 m), consuming a lot of oxygen, and at the same time, a large quantity of O2 is injected into the furnace through the secondary-air pipe, resulting in a large change in the O2 concentration. Between Y = 0 and 9.77 m, the O2 concentration increases gradually as the outer secondary-air vane angle increases. This is because there are fewer pulverized coal particles in the furnace hopper and less oxygen is consumed by pulverized coal as the outer secondaryair vane angle increases. Between Y = 17.12 and 38 m, no air is injected into the furnace, and hence, the O2 concentration decreases gradually with further burning of pulverized coal but later becomes gradually stable. Figure 10 shows distributions of the CO concentration. The variation is similar for the four vane angles. With increasing furnace height, the CO concentration initially rises and then falls. In front of the three rows of burners (between Y = 9.77 and 17.12 m), pulverized coal ignites and combusts, releasing a great amount of CO. Thus, the CO concentration rises, and furthermore, the smaller the outer secondary-air vane angle, the more CO released. For Y > 17.2 m, CO is continuously oxidized to CO2 with the burning out of pulverized coal, and thus, the CO concentration gradually decreases. Figure 10 shows distributions of the calculated NOx concentration. The variation is similar for the four vane angles. There is a higher NOx concentration near the three rows of burners (between Y = 9.77 and 17.12 m). Large amounts of high-temperature pulverized coal ignite and burn out in this region, and much fuel NOx and thermal NOx are generated. For Y > 17.2 m, the NOx concentration decreases as the pulverized coal burns out. For an outer secondary-air vane angle of 30°, the NOx generation is greatest and the NOx concentration is highest. The NOx concentration is lowest for an outer secondary-air vane angle of 35°. 4.2. Distribution Characteristics of the Gas Temperature and Concentrations at the Furnace Outlet. Table 4 presents experimental and simulation data of the gas temperature at the furnace outlet, distributions of O2, CO, CO2, and NOx concentrations, and combustible material content in the fly ash. For an outer secondary-air vane angle of 35°, the numerical simulation shows that the O2 concentration at the furnace outlet is 4.82%, the CO concentration is 26.12 ppm, the CO2 concentration is 12.99%, and the NOx concentration is 420.2 ppm, at 6% O2. The concentrations of gas species were measured with a
5. Conclusions From numerical simulations of the pulverized-coal combustion process and NOx emissions for a 300 MWe wall-fired boiler, the following conclusions are drawn: (1) Measurements of the polar components of gas velocities made with a particle-dynamics anemometer and calculated values are in good agreement. In addition, the simulation results of the CO2 concentration, NOx concentration, and combustible material content in the fly ash are in good agreement with the results of industrial experiments performed for full-scale boilers. In general, the concordance shows that the numerical model describes the pulverized-coal combustion process and NOx emissions reasonably well. (2) For EI-DR burners, pulverized coal is spread widely from the burner. Some of the pulverized coal is carried by strongly rotating secondary air and deposited on the water-cooled wall, and the region of the water-cooled tube wall of the burner attains high temperatures. Much pulverized coal is swept into the furnace hopper at the bottom of the boiler. The smaller the outer secondaryair vane angle, the higher the temperature of the water-cooled tube wall region; the temperature of the water-cooled tube wall region is about 1600 K for an outer secondary-air vane angle of 30°. (3) As the outer secondary-air vane angle increases, the temperature of the furnace outlet, the O2 5357
Energy Fuels 2010, 24, 5349–5358
: DOI:10.1021/ef100682s
Zeng et al.
concentration, and the carbon content in the fly ash all increase gradually. (4) Changes in the NOx concentration at the furnace outlet are not simply linear with changes in the outer secondary-air vane angle. The NOx concentration at the furnace outlet was lowest (420.2 ppm at 6% O2) for an outer secondary-air vane angle of 35° and highest (468.2 ppm at 6% O2) for an outer secondary-air vane angle of 30°. This represents an increase of 11.4%. (5) The suggested outer secondary-air vane angle for boiler operation is 35°.
T = temperature (K) t = time (s) Cμ, C1, and C2 = empirically determined constants σk and σε = turbulent Prandtl and Schmidt numbers m = mass (kg) g = gas p = particle Fx = additional acceleration (force/unit particle mass) term gx = component of the acceleration due to gravity in the x direction P0 = total pressure (Pa) PO2 = partial pressure of oxygen (Pa) K = reaction rate SP = release rate of the volatiles mN = nitrogen mass fraction in the coal (kg) M = molecular weight (kg/kmol) X = mole fraction (mol/mol) AE = external surface area of char (m2/kg) b = radius of the CO flame sheet (m) S0 = swirl number Gθ = axial flux of the tangential momentum Gx = axial flux of the axial momentum U = axial component of the velocity W = tangential component of the velocity
Acknowledgment. This work was supported by the Hi-Tech Research and Development Program of China (Contract 2007AA05Z301), Heilongjiang Province, via 2005 Key Projects (Contract GC05A314).
Nomenclature Sφ = gas source term for variable φ Sp,m = particle source term for variable φ Γφ = generalized effective transport coefficient u, v, and w = instantaneous axial, radial, and tangential velocities (m/s) Gk = generation rate of gas turbulent kinetic energy (kg m-1 s-3) F = density (kg/m3) μ = viscosity
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