Energy Fuels 2010, 24, 3884–3889 Published on Web 06/04/2010
: DOI:10.1021/ef100293f
Influence of the Outer Secondary Air Vane Angle on the Flow Field of a Down-Fired Pulverized-Coal 300 MWe Utility Boiler with Swirl Burners Zhengqi Li,* Subo Fan, Wei Su, Zhichao Chen, and Yukun Qin School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China Received March 12, 2010. Revised Manuscript Received May 14, 2010
Using an IFA300 constant temperature anemometer system, cold air experiments on a scale model of a down-fired pulverized-coal 300 MWe utility boiler with swirl burners were performed to investigate the influence of outer secondary air vane angles on the flow characteristics in the furnace. For a vane angle of 20°, the reach of the downward airflow is deep and the mixing with vent air and staged air is good, thereby favoring coal combustion but at the expense of airflow washing out at the entrance of the upper furnace. For vane angles of 25° and 30°, the reach of the downward airflow is short, mixing with the staged air weakens, and thus, the recirculation zone below the arches is small, although washing out at the entrance ceases. For a 35° vane angle, the reach of the airflow further shortens and the mixing of the airflow and vent air is weaker. At even larger vane angles of 40° and 50°, the recirculation zone appears in the burner nozzle region and the airflow is directed upward near the burner nozzle region, creating a short flame circulation. Considering all of the above, conditions with a vane angle setting in the range of 25° and 30° are considered as optimal in the operation of the boiler.
on flow characteristics, combustion, and NOx formation of burners.9,10 With this arrangement, the swirl burner aerodynamic characteristics are key factors determining the aerodynamic field in the furnace. Their structural and operational parameters have an influence on the aerodynamic field. Li et al. have completed experimental studies on these characteristics by varying the outer secondary vane angles.11 The burner model was set in an open space. However, no measurements of aerodynamic characteristics in the furnace of a down-fired boiler with swirl burners have yet been reported. Using an IFA300 constant temperature anemometer system, we studied the influence of the outer secondary air vane angle on the aerodynamic characteristics in a small-scale furnace of a Babcock and Wilcox (B&W) down-fired pulverized-coal 300 MWe utility boiler.
1. Introduction Down-fired combustion technology is one of several techniques capable of burning anthracite and lean coal. At present, there are two types of down-fired pulverized-coal utility boilers: one with tangentially fired burners installed and the other with swirl burners on the furnace arch. However, both suffer from problems of high carbon content in the fly ash and high NOx emissions. The study of pulverized-coal combustion and aerodynamic characteristics are of great importance. Performing experiments with small-scale models of the furnace is a widely popular means to obtain approximate aerodynamic characteristics.1-5 With a down-fired furnace and tangentially fired burners, Li et al. have studied aerodynamic characteristics in a small-scale model furnace of a Foster Wheeler (FW) boiler having direct flow pulverized-coal burners enhanced by circularly arranged primary air nozzles and a Mitsui Babcock Energy Limited (MBEL) boiler enhanced by cyclones.6-8 With a down-fired furnace with swirl burners, Fan et al. have performed numerical research
2. Experimental Section Figure 1 shows a schematic diagram of the furnace structure of the boiler and the combustion system arrangement. The subcritical pressure boiler is controlled by natural circulation and has a rated load of 300 MW. Q The boiler consists of a single furnace with double arches in a -type arrangement with 16 enhanced ignition axial control low-NOx (EI-XCL) swirl burners symmetrically fixed on the two arches (Figure 2). The coal powder system is of middle storage type with a hot air feeding powder system. Vent air is piped in from the front and rear walls, at an angle of 28° to the horizontal. Under the vent-air pipes, there are a group of pipes directed at an angle of 30° connected with the secondary air box. So-called staged air is fed by these pipes into the furnace from the front and rear walls. The air volume of the staged air can be adjusted by damper openings. The number of vent-air and stagedair pipes matches the number of burners on the arches, with each pipe aligned vertically below a burner.
*To whom correspondence should be addressed. Telephone: þ86451-86418854. Fax: þ86-451-86412528. E-mail:
[email protected]. (1) Zhou, Y. G.; Xu, T. M.; Hui, S. E.; Zhang, M. C. Appl. Therm. Eng. 2009, 29, 732–739. (2) Li, Y. Q.; Zhou, H. C. Flow Meas. Instrum. 2006, 17, 113–122. (3) He, B. S.; Chen, M. Q.; Liu, S. M.; Fan, L. J.; Xu, J. Y.; Pan, W. P. Exp. Therm. Fluid Sci. 2005, 29, 537–554. (4) Zhou, Y. G.; Zhang, M. C.; Xu, T. M.; Hui, S. E. Energy Fuels 2009, 23, 5375–5382. (5) Lin, Z. C.; Fan, W. D.; Li, Y. Y.; Li, Y. H.; Zhang, M. C. Energy Fuels 2009, 23, 744–753. (6) Ren, F.; Li, Z. Q.; Chen, Z. C.; Wang, J. J.; Chen, Z. Energy Fuels 2009, 23, 2437–2443. (7) Ren, F.; Li, Z. Q.; Zhang, Y. B.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (8) Kuang, M.; Li, Z. Q.; Han, Y. F.; Yang, L. J.; Zhu, Q. Y.; Zhang, J.; Shen, S. P. Energy Fuels 2010, 24, 1603–1610. (9) Fan, J. R.; Jin, J.; Liang, X. H.; Chen, L. H.; Cen, K. F. Chem. Eng. J. 1998, 71, 233–242. r 2010 American Chemical Society
(10) Fan, J. R.; Zha, X. D.; Cen, K. F. Energy Fuels 2001, 15, 776–782. (11) Fan, S. B.; Li, Z. Q.; Yang, X. H.; Liu, G. K.; Chen, Z. C. Fuel 2010, 89, 1525–1533.
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Energy Fuels 2010, 24, 3884–3889
: DOI:10.1021/ef100293f
Li et al.
in operation, air is drawn from every duct. All airflow rates were measured in the furnace by Venturi tube flowmeters, with errors of less than 10%. Measurement errors associated with the IFA300 constant temperature anemometer system used were less than 2%. Normally, three-dimensional anemometer systems can only be used in velocity measurements in an unconfined space. Because the experimental system is an enclosed space, a two-dimensional anemometer system is used to measure air velocity. In the burner nozzle, measured air velocities are taken within two different sections of the burner to eliminate errors in tangential velocities arising in the calculations. In other regions, air velocities of weaking jets were measured directly with the twodimensional anemometer system. The flows in the model furnace are scaled appropriately, and the momentum ratios between airflows within the small-scale furnace are the same as those at the full-scale furnace. The influence of the outer secondary air vane angle on airflow fields reported here was investigated under conditions of constant momentum ratios between airflows at room temperature (i.e., 20 °C). Airflow velocity parameters are listed in Table 1. Given these fixed velocity settings, the outer secondary air vane angle was adjusted in turn to 20°, 25°, 30°, 35°, 40°, and 50°. These angles are the included angle between the outer secondary air vanes and the axes of the burner. For simplicity, we shall refer to the outer secondary air vane angle as the vane angle and is the only adjustible angle in the experiment.
3. Results and Discussion Cold aerodynamic field flow rates were measured along longitudinal cross-sections intersecting the vertical centerline of one of the primary air nozzles. Airflow velocity fields for the various vane angles are shown in Figure 4. With increasing vane angle, swirl flow increases at the burner nozzle. Airflow axial velocities decrease and the penetration of the flow weakens; therefore, the main airflow is directed upward further up the burner. However, when the staged-air nozzle declination angle is 30°, staged air can be deflected into the middle hopper region and no stagnant areas occur. Varying the vane angle has little effect on the flow field. For a vane angle of 20°, the airflow from the burner can mix directly with the vent airflow. Although the effect that vent airflow has is small, the recirculation zone is large enough to encroach upon the central area of the furnace and even sweep clean the throat of the furnace. Upon increasing the vane angle to 25°, airflow from the burner circulates in the same area but has less of a perturbing effect on the upward flow. The width of the recirculation zone is obviously smaller, significantly diminishing the sweeping effect at the furnace throat. At a vane angle of 30°, this zone continues to reduce in width. The main airflow is diverted upward within the upper area of the vent nozzle. For the larger vane angles of 35° and 40°, the downward airflow obviously becomes deflected near the vent-air nozzle, showing that the axial velocity of the airflow from the burner has completely decayed. With the declination angle of the vent-air nozzle at 28°, vent air continues going downward along with a portion of the airflow, creating a small recirculation zone within this area. At a 50° vane angle, there is clearly a recirculation center near the burner nozzle area. Airflow above the nozzle is redirected upward. With the appearance of the recirculation center, vent airflow is redirected, creating a small recirculation zone near the nozzle area. In comparison to the vent airflow, staged air remains steady. Figure 5 shows the longitudinal velocity component profiles for the various vane angle settings at two cross-sections. Here, Vy is the velocity component along the y direction; V0 signifies the outlet velocity at the primary air nozzle along the y direction; X0 is the horizontal distance between the front and
Figure 1. Diagram of the furnace and combustion system of the down-fired boiler (dimensions in millimeters).
Figure 2. Schematic of the EI-XCL burner and position of the monitoring pipe (dimensions in millimeters): (1) primary air duct, (2) secondary air valve, (3) inner secondary air duct, (4) inner secondary air duct vanes, (5) adjustable outer secondary air vanes, (6) fixed outer secondary air vanes, (7) outer secondary air duct, and (8) monitoring pipe.
Figure 2 shows a schematic diagram of the EI-XCL burner. Each burner has 14 radial vanes in the inner secondary air duct as well as in the outer secondary air duct. The inner and outer secondary airflows have the same swirling direction. 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 primary air tube wall, thereby resulting in a coal-rich flow in the peripheral zone of primary air and a coallean flow in the central zone. The small-scale model furnace is a 1:15 scaled version of the original. The experimental system is depicted in Figure 3. It consists of an induced draft fan, a small-scale furnace model, and an IFA300 constant-temperature anemometer. With the fan 3885
Energy Fuels 2010, 24, 3884–3889
: DOI:10.1021/ef100293f
Li et al.
Figure 3. Schematic of the small-scale cold experiment system and layout of the measurement points.
the flow in this region is primarily that of staged air and the velocity component remains essentially the same. Figure 6 shows the variation with the vane angle in the turbulence intensity distribution at the two chosen depths, i.e., H/H0 = 0.245 and 0.413. The turbulence intensity T is defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V 0x2 þ V 0y2 T ¼ ð1Þ =Vm 2
Table 1. Fixed Velocity Parameters for All Outer Secondary Air Vane Angle Settings air velocity (m/s) primary air inner secondary air outer secondary air vent air staged air 27
18
37
26
23
rear walls in the lower furnace; and X is the distance from the furnace center to the measurement points along the x direction. H0 is the vertical distance between the outlet of the primary air nozzle and the upper edge of the dry bottom hopper, while H is the vertical distance from the measurement point to the outlet of the primary air nozzle. The cross-section at H/H0 = 0.245 lies in the exit zone of the vent-air ports, while the cross-section at H/H0 = 0.413 lies in the exit zone of the staged-air ports (see Figure 3). Figure 5a corresponds to the cross-section H/H0 = 0.245; the longitudinal velocity component nearer the wall decreases with an increasing vane angle. Increasing the vane angle causes the swirl strength of the airflow to increase, and thus, the axial velocity of the burner jet decays rapidly. For the smaller vane angles of 20°, 25°, and 30°, the arch airflow can penetrate down into this area and mix with the fuel-lean flow. Conversly, for the larger vane angles of 35°, 40°, and 50°, this region is essentially composed of fuel-lean flow because the arch airflow has already been diverted this way; therefore, the longitudinal velocity component is smaller. However, the longitudinal velocity component of the upward airflow in the furnace center remains the same. Figure 5b corresponds to the crosssection H/H0 = 0.413; the longitudinal velocity component of the airflow near the wall is higher for vane angle of 20°, while near the center of the furnace, the velocity component declines rapidly as the main airflow is directed upward into this region. With the vane angle set at 25°, the main airflow is already diverted away and, thus, the trend in the radial velocity component gently declines. For the larger vane angle settings,
where V0 x and V0 y are the respective velocity fluctuations in the x and y directions and Vm is the mean velocity at a point in the velocity field. As shown in Figure 6a, at the cross-section H/H0 = 0.245, the turbulence intensity decreases with an increasing vane angle. For a vane angle of 20°, the arch airflow can penetrate downward to this depth and mix with the fuel-lean flow. Thus, turbulence intensities of the flow are higher near the wall, with peaks forming at radii X/X0 = (0.3846, and decline toward the furnace center, with minimums at X/X0 = 0.05. At a vane angle of 25°, maxima occur at X/X0 = 0.3846 and -0.4135 and intensities strictly decay rapidly because of the downward flowing airflow. At this depth, vent air mixing is weaker; therefore, turbulence intensities decline slightly compared to the smaller 20° vane angle. At a 30° vane angle, the downward airflow has already been diverted and only a small amount of arch airflow flows into the area. Turbulence intensities decrease, and peaks move slightly toward the rear wall. At the larger vane angles of 35°, 40°, and 50°, the main airflow falls short of reaching the region and, thus, the turbulence intensity distribution remains uniform. Figure 6b shows the variation of the turbulence intensity distribution with the vane angle at cross-section H/H0 = 0.413, which is below the staged-air ports. Only when the vane angle is 20° does the downward airflow reach this region, and turbulence intensities can only attain their largest values at this angle, forming peaks at X/X0 = (0.4135. At a vane angle of 25°, turbulence intensities 3886
Energy Fuels 2010, 24, 3884–3889
: DOI:10.1021/ef100293f
Li et al.
Figure 4. Continued
decrease in this area and peak values move toward the furnace center. With larger vane angles, turbulence intensities change only slightly. Figure 7 shows the decay curve for the longitudinal velocity component of the downward airflow. Here, (Vy)max is the largest longitudinal velocity component of the primary airflow at a fixed depth below the front and rear arches. The distance between the point at which (Vy)max/V0 = 0.4 and the outlet of the primary air nozzle in the y direction is defined as the airflow penetration depth where downward airflow reaches into the lower furnace. The dimensionless airflow depth is the ratio of the airflow penetration depth to H0. Table 2 lists the airflow dimensionless depths for the various vane angles. As seen in Figure 7, the largest longitudinal velocity component of the airflow decreases as the vane angle increases. As vane angles vary, (Vy)max/V0 decreases at first with the downward flowing airflow, then increases, and peaks at the cross-section H/H0 = 0.245, after which it decreases again. This behavior is due to staged-air mixing with vent air. At a
vane angle of 20°, the velocity decreases continuously, while (Vy)max/V0 is less than 0.4 below the depth of H/H0 = 0.413, indicating that the airflow is directed upward in this area. At the larger vane angles of 30°, 35°, 40°, and 50°, the redirection occurs at H/H0 = 0.3, 0.192, 0.13, and 0.11, respectively. When the vane angle decreases, the recirculation zone below the arches reduces in size and perturbation of the upward airflow at the center of the furnace becomes weak. At the vane angle of 20°, the downward airflow can reach the staged-air port region; therefore, mixing between vent air and staged air is high, favoring the combustion of the coal powder. However, because the recirculation zone below the arch is so large, the airflow is able to wash out the entrance of the upper furnace. At intermediate vane angles of 25°, 30°, and 35°, velocities of the downward airflow decay rapidly, penetration depths decrease, and the airflow can fall short of reaching the staged-air injection area; therefore, mixing is weaker. For larger vane angles of 40° and 50°, the recirculation zone appears near the outlet of the burner, allowing the coal 3887
Energy Fuels 2010, 24, 3884–3889
: DOI:10.1021/ef100293f
Li et al.
Figure 4. Aerodynamic fields associated with different outer secondary air vane angles.
powder and the hot smoke gas to mix earlier and be ready for firing. However, because airflow velocities decay too quickly, the downward airflow is redirected before it reaches the ventair nozzle and the stroke of the coal powder become shorter. This is counter to the mixing of the main airflow with the vent air and staged air and burning out of the coal powder.
the turbulence intensities in these two cross-sections are at their highest, indicating that the main airflow mixes intensely between vent air and staged air, thereby further enhancing the combustion of coal powder. When the vane angles are increased to 25° and 30°, the degree of mixing between the main airflow and vent air decreased slightly and also mixed poorly with staged air. At 35°, the main airflow again mixed poorly with the vent air, resulting in downward airflow falling short of reaching the staged-air area. For the larger vane angles of 40° and 50°, the turbulence intensity near the vent-air nozzle is very weak, indicating poor mixing of the main airflow with vent air and, in consequence, inducing no further combustion of coal powder. (3) With an increasing vane angle, the longitudinal velocity component of the airflow from the burner decayed rapidly, while the depth of the downward airflow decreased. For a vane angle of 20°, the airflow can reach a depth of H/H0 = 0.5 and the downward airflow diverted upward in the area under the staged-air nozzle. With vane
4. Conclusion (1) With the vane angle increasing from 20° to 50°, the rigidity of downward airflow slackened and the recirculation zone below the arch decreased. For the smaller vane angle of 20°, washing out of the airflow at the entrance of the upper furnace appeared. At the larger vane angles of 40° and 50°, the recirculation zone appeared near the outlet of the burner. The influence of the vane angle setting on the flow field is very weak. (2) When the outer secondary vane angle is increased, turbulence intensities of the airflow near the vent-air and staged-air nozzles weaken. For the smallest vane angle of 20°, 3888
Energy Fuels 2010, 24, 3884–3889
: DOI:10.1021/ef100293f
Li et al.
Figure 5. Distribution of the longitudinal velocity component at two cross-sections for various outer secondary air vane angles.
Figure 6. Turbulence intensity distribution along different cross-sections for the cases with different outer secondary air vane angles. Table 2. Airflow Dimensionless Depths Attained at Different Outer Secondary Air Vane Angles outer secondary air vane angles (deg)
20
airflow dimensionless depth
0.5 0.354 0.3 0.192 0.13 0.11
25
30
35
40
50
airflow was redirected upward at an intermediate area between vent-air and staged-air nozzles. At a vane angle of 35°, the depth was H/H0 = 0.192 and airflow reverted upward on the upside of the vent-air nozzle. For vane angles of 40° and 50°, the depths are H/H0 = 0.13 and 0.11 and the airflow was redirected near the outlet of the burners. (4) At larger outer secondary vane angle settings, the stroke of the coal powder shortened, while mixing of the main flow with vent air and staged air worsened. However, when the vane angle is too small, the recirculation zone below the arch became too large, resulting in the airflow washing out at the entrance of the upper furnace. Consideration of all of the above factors suggests that the outer secondary vane angle setting should be between 25° and 30° during boiler operations.
Figure 7. Decay curve for the longitudinal velocity component of the down airflow with different outer secondary air vane angles.
Acknowledgment. This work is sponsored by the Hi-Tech Research and Development Program of China (863 Program, Contract 2006AA05Z321).
angles of 25°and 30°, the depth of the downward airflow decreased to H/H0 = 0.354 and 0.3, respectively, and the 3889
