Influence of the Secondary Air-Box Damper Opening on Airflow and

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Energy & Fuels 2007, 21, 668-676

Influence of the Secondary Air-Box Damper Opening on Airflow and Combustion Characteristics of a Down-Fired 300-MWe Utility Boiler Feng Ren, Zhengqi Li,* Yubin Zhang, Shaozeng Sun, Xiaohui Zhang, and Zhichao Chen School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, P.R. China ReceiVed April 27, 2006. ReVised Manuscript ReceiVed September 10, 2006

Experiments with a small-scale furnace for a down-fired pulverized-coal 300-MWe utility boiler were carried out on a single-phase test facility to investigate the influence of different secondary air distributions on the aerodynamic field in the furnace. When the secondary air flux of tier E increased within a suitable range, it did not reverse the fuel-rich flow or shorten the residence time of coal particles in the furnace. Industrial experiments were also performed on a full-scale boiler. The gas temperature distribution along the primary air and coal mixture flow and in the furnace, and gas components such as O2, CO, CO2, and NOx in the near-wall region, were measured with damper openings of the E-tier secondary air box at 0% and 30%. At 0%, ignition of the primary air and pulverized coal mixture was delayed and the gas temperature peak was above the burner arch, with high NOx emission. Increasing the damper opening to 30% provided the oxygen necessary for the initial combustion. This was advantageous for stable combustion and also lowered NOx emissions and carbon content in the fly ash.

1. Introduction Reserves of anthracite are abundant and are distributed globally. With its low volatile content, anthracite shows difficulties in ignition and burnout. At present, down-fired combustion technology is one of the techniques applied for burning this fuel. However, practical down-fired boiler operation suffers from problems of high carbon content in the fly ash and high NOx emissions. It is difficult to maintain a stable flame, especially when the load is low.1 Up to the 1960s, down-fired boilers were used to burn low-grade coal in Spain.2 Burdett carried out industrial tests to investigate the effects of air staging on NOx emissions from a 500-MWe down-fired boiler unit.3 Fan et al. and Liang et al. carried out numerical research into NOx formation in the furnaces of down-fired boilers.4,5 Accurate knowledge of the behavior of char particles in pulverized-coal combustion systems is critical to understanding the fundamental process that occurs during heterogeneous combustion. Combustion data for full-scale equipment can give the characteristics of real combustors, in particular the turbulent flow of industrial coal flames. As a result, studies in full-scale equipment are required. In the field of swirl and tangential combustion, measurements were made of the mean concentrations of local gas species (O2, CO, CO2, and NOx), gas temperature, and char burnout at several ports in utility boilers.6-17 However, in down-fired boilers, no measurements of gas temperature and gas species concentrations at several ports of the furnace have been reported. * Corresponding author: Tel.: +86-451-86413231, ext 806. Fax: +86451-86412528. E-mail address: [email protected]. (1) Garcia-Mallol, J. A.; Steitz, T.; Chu, C. Y.; Jiang P. Z. Ultra-low NOx advanced FW arch firing: Central power station applications. In Proceedings of the 2nd U.S. China NOx and SO2 control workshop, Dalian, China, Aug 25-28, 2005. (2) Blas, J. G. Combustion 1970, 42, 6-13. (3) Burdett, N. A. J. Inst. Energy 1987, LX, 103-107. (4) Fan, J. R.; Zha, X. D.; Cen, K. F. Energy Fuels 2001, 15, 776-782. (5) Liang, X. H.; Fan, W. C.; Fan, J. R.; Cen, K. F. Int. J. Energy Res. 1999, 23, 707-717.

Different aerodynamic fields lead to variations in the coal combustion process. To analyze the influence of aerodynamic behavior on pulverized-coal combustion, researchers have studied the aerodynamic behavior of full-scale furnaces by performing experiments with small-scale models.18-21 Cold air-flow experiments in a small-scale furnace model were carried out. In situ experiments were carried out on a 1025t/h (tph) down-fired pulverized-coal boiler. Measurements of the distribution of the furnace gas temperature, ignition of the primary air/fuel mixture flow, and the gas components in the furnace were first made for a full-scale boiler. The influence of the damper opening of the E-tier air box on combustion in the (6) Costa, M.; Azevedo, J. L. T; Carvalho, M. G. Combust. Sci. Technol. 1997, 129, 277-293. (7) Costa, M.; Silva, P.; Azevedo, J. L. T. Combust. Sci. Technol. 2003, 175, 271-289. (8) Vikhansky, A.; Bar-Ziv, E.; Chudnovsky, B.; Talanker, A.; Eddings, E.; Sarofim, A. Int. J. Energy Res. 2004, 28, 391-401. (9) Butler, B. W.; Webb, B. M. Fuel 1991, 70, 1457-1464. (10) Bonin, M. P.; Queiroz, M. Combust. Flame 1991, 85, 121-133. (11) Bonin, M. P.; Queiroz, M. Fuel 1996, 75, 195-206. (12) Queiroz, M.; Bonin, M. P.; Shirolkar, J. S.; Dawson, R. W. Energy Fuels 1993, 7, 842-851. (13) Tree, D. R.; Webb, B. W. Fuel 1997, 76, 1057-1066. (14) Black, D. L.; McQuay, M. Q. Combust. Sci. Technol. 1998, 132, 37-74. (15) Butler, B. W.; Wilson, T.; Webb, B. M. Proc. Combust. Inst. 1999, 24, 1333-1339. (16) Fan, J. R.; Sun, P.; Zheng, Y. Q.; Ma, Y. L.; Cen, K. F. Fuel 1999, 78, 1387-1394. (17) Li, Z. Q.; Yang, L. B.; Qiu, P. H.; Sun, R.; Chen, L. Z.; Sun, S. Z. Int. J. Energy Res. 2004, 28, 511-520. (18) Beltagui, S. A.; MacCallum, N. R. L. J. Inst. Fuel 1976, 49, 183193. (19) Tucker, A. C. N. J. Inst. Fuel 1969, 42, 118-121. (20) Beltagui, S. A.; MacCallum, N. R. I. Combustion aerodynamics of a gas field furnace with peripheral fuel injection. In Proceedings of the 3rd World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Honolulu, HI, Oct 3-Nov 5, 1993; p 1051. (21) Beltagui, S. A.; MacCallum, N. R. L. J. Inst. Fuel 1976, 49, 193200.

10.1021/ef060181b CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007

Airflow and Combustion in a Utility Boiler

Energy & Fuels, Vol. 21, No. 2, 2007 669

Figure 1. Furnace of the 1025-tph down-fired boiler (dimensions in milimeters).

furnace was determined. The results of these experiments will be of benefit for the design and operation of similar boilers, and the data have value because of the support they can lend to theoretical and numerical calculations. 2. Utility Boiler The 1025-tph boiler with a 300-MWe unit used was made by Dongfang Boiler Group Ltd., using the technology of FW Ltd. This involves a Π-type layout, double arches, and a single furnace. Double-cyclone pulverized-coal burners are set on the arches to organize the W-shaped flame. A double-entry double-exit ball mill and a positive-pressure direct-fired system are used to supply pulverized coal. Figure 1 shows the furnace. The zone below the arches is the fuel-burning zone, and that above the arches is the fuel-burnout zone. The cross section of the fuel-burnout zone is rectangular, and that of the fuel-burning zone is octagonal with wing walls. A membrane water-cooled wall, with a tube diameter of 76 mm and pitch of 95.5 mm, was used. Figure 1 also shows the index, ports, and nozzles of the burners. A total of 24 doublecyclone burners are arranged on the arches, each of which has two fuel-rich and two fuel-lean nozzles. There is also a secondary air port to provide air for oil ignition on the arch corresponding to each double-cyclone burner. Four double-entrance double-exit ball mills supply pulverized coal to the burners, so that there is one mill for every six burners. Mill A supplies pulverized coal to burners

A1-A6, mill B, to burners B1-B6, mill C, to burners C1-C6, and mill D, to burners D1-D6. An FW-type double-cyclone pulverized-coal burner is used. The fundamental principle of this burner is fuel bias combustion, which shows lower NOx emissions compared with common burners. The configuration of this type of combustion system is shown in Figure 2. The primary air and pulverized coal mixture is fed through the air/coal inlet pipe into a riffle distributor. The flow is then divided into two flows, which separately enter the two cyclones of a burner in the tangential direction. Because the fuel/air mixture is a swirling flow in the cyclone, the pulverized coal in the primary air is centrifugally separated. Thus, the fuel-lean flow is in the central zone and the fuel-rich flow is in the peripheral zone. The fuel-lean flow is emitted into the furnace from the vent air pipes. The fuelrich flow is emitted into the furnace through the burner nozzle. This method solves the problem of ignition and combustion for hard-to-burn fuels. The secondary air box is divided into six parts: A, B, C, D, E, and F. Every part has 24 small boxes, each corresponding to a double-cyclone burner on the arch. The damper at the inlet of each box can be varied to change the secondary airflow rate. Secondary air from the A boxes is sent to the furnace through annular ports around the fuel-lean nozzles, while that from B boxes is sent through annular ports around the fuel-rich nozzles. Oil secondary air from the C boxes is sent to the furnace through the oil secondary-air port (Figure 2). The front and rear walls below the arches contain three tiers of secondary air ports, as shown in

670 Energy & Fuels, Vol. 21, No. 2, 2007

Ren et al.

Figure 2. Combustion system for the down-fired boiler (dimensions in milimeters).

Figure 3. Small-scale cold experiment system and layout of the measurement points on the side wall.

Figure 2; the width of the wide slot is 114.5 mm and that of the narrow slot is 38.3 mm. The secondary air from the D, E, and F boxes is sent to the furnace through these tier ports. The secondary air from the A and B boxes is to cool the fuelrich and the fuel-lean nozzles. It has little influence on the coal combustion in the furnace. The dampers of the C boxes are totally

closed when the oil guns are out of service. So, only the secondary air from D, E, and F boxes largely influences the coal combustion in the furnace. In current operation, the dampers for both the tier D and E secondary air boxes are totally closed and that for tier F is closed 25%. In this paper, we only investigate the effect of the damper opening for tier E and discover the optimum setting for it.



Figure 4. Decay curve for the longitudinal velocity component of the fuel-rich flow: (×) case 1, secondary air velocity of tier E is 2.14 m/s; (0) case 2, secondary air velocity of tier E is half that of tier F; (2) case 3, secondary air velocity of tier E is the same as that of tier F. Figure 6. Aerodynamic fields for the three cases: (a) case 1, velocity of the E-tier secondary air is 2.14 m/s; (b) case 2, secondary air velocity of the E tier is half that of the F tier; (c) case 3, secondary air velocity of the E tier is the same as that of the F tier.

Figure 5. Changes in the transverse velocity component of the fuelrich flow: (×) case 1, secondary air velocity of tier E is 2.14 m/s; (0) case 2, secondary air velocity of tier E is half that of tier F; (2) case 3, secondary air velocity of tier E is the same as that of tier F. Table 1. Parameters for the Small-Scale Cold Aerodynamic Experimentsa parameter air temperature of the flow (°C) air velocity (m/s) fuel-rich flow fuel-lean flow D-tier secondary air E-tier secondary air F-tier secondary air MD:ME:MF momentum flow rate ratio

case 1

case 2

case 3

28.97 12.35 0 2.14 20 0:1:260

20 31.91 12.57 0 9.62 20 0:1:12.9

32.16 11.6 0 17.32 18 0:1:3.1

Figure 7. Sketch map of the direction of the mixed jet.

a Note: M , M , and M denote the exit momentum flow rate for tiers D E F D, E, and F, respectively.

3. Small-Scale Cold Aerodynamic Experiment In the utility boiler, combustions in the longitudinal sections are independent of one another. Thus, the small-scale furnace model only represents the central part of the full-scale furnace, with 8 burners on the arches instead of 24. The experimental system is shown in Figure 3. The airflow flux in the primary and secondary air pipes was measured using Venturi tube flowmeters. The error for measurement of the airflow rate was less than 10%. An IFA300 constant-temperature anemometer system was used to measure the air velocity at the measurement points. A 1240-type probe with two hot-film sensors was used in the experiments. The error for velocity measurements was less than 2%. Three cases were evaluated: the secondary air velocity of tier E was set to (1) 2.14 m/s, (2) half that of tier F, and (3) the same value as for tier F, respectively. The specific parameters are listed in Table 1. The most important factor for the aerodynamic field in a down-fired furnace is the depth that the fuel-rich flow reaches

Figure 8. Turbulence intensity distribution in the airflow zone of tier E: (×) case 1, secondary air velocity of tier E is 2.14 m/s; (0) case 2, secondary air velocity of tier E is half that of tier F; (2) case 3, secondary air velocity of tier E is the same as that of tier F.

in the furnace. This determines the residence time for coal particles in the furnace and whether the goal of increasing the burnout rate in the W-shaped fire will be achieved. Figure 4 shows a decay curve for the downward dimensionless velocity component Vz/Vmax of the fuel-rich flow along the dimensionless coordinates H/H0 in the z direction. Along the y-coordinate, Vz stands for the z-direction velocity component at the measurement points


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Table 2. Angles between the Fuel-Rich Flow and the Vertical Direction airflow zone tier D tier E tier F

calculated result experimental result calculated result experimental result calculated result experimental result

case 1

case 2

case 3

10° 10.4° 10.82° 22.5° 75.75° 70.35°

10° 12.99° 22.97° 38.26° 72.19° 75.96°

10° 34.38° 46.02° 70.02° 70.82° 82.4°

Table 3. Proximate and Ultimate Analysis of the Experimental Coal Proximate Analysis (Air-Dried weight percent) volatile matter

ash

moisture

fixed carbon

net heating value (kJ/kg)

8.17

29.4

0.7

61.73

23390

Ultimate Analysis (Air-Dried weight percent) carbon 63.65

hydrogen

sulfur

nitrogen

oxygen

2.36 1.45 0.86 1.58 pulverized-coal fineness: R90 ) 5%, R74 ) 7%

along the fuel-rich flow. Vmax stands for the maximum Vz along the z-direction. Along the x-coordinate, H is the distance between the fuel-rich nozzle and the velocity measurement pointing along the z-direction. H0 is the distance between the fuel-rich nozzle and the upper edge of the furnace hopper (see Figure 3). The distance between the point at which Vz/Vmax ) 0.15, and the fuel-rich nozzle in the z-direction is defined as the depth that the fuel-rich flow reaches in the furnace. It can be observed in Figure 4 that, in cases 1 and 2, the dimensionless depth that the fuel-rich flow reaches is 0.46 and 0.44, respectively. The difference between the two cases is only slight. When tier E secondary air has the same velocity as that for tier F, the dimensionless depth is 0.32, which is clearly less than for the other two cases. Thus, increasing the damper opening of the E-tier secondary air box and the velocity of tier E secondary air within a suitable range does not shorten the depth that the fuel-rich flow reaches in the furnace. In all three cases, the dimensionless depth reached by the fuel-rich flow is below 0.5. The fuel-rich flow cannot continue any farther down into the airflow zone of tier F, so nearly half of the furnace cannot be used for coal combustion. Thus, the gas temperature in the fuel-burning zone is low. This is disadvantageous for the ignition of the fuel-rich flow. Figure 5 shows changes in the transverse dimensionless velocity component Vx/Vmax of the fuel-rich flow along the dimensionless coordinates H/H0. Vx stands for the x-direction velocity component at the measurement points along the fuelrich flow. It is evident that as the fuel-rich flow descends into the furnace, the transverse velocity increases. This illustrates that while flowing downward, the fuel-rich flow is deflected. The transverse velocity increases more rapidly for case 2 compared to case 1. In other words, deflection of the fuel-rich flow to the furnace center is greater in the former case than in the latter. However, the maximum transverse velocity of the fuel-rich flow for both cases is at H/H0 ) 0.6, i.e., in both cases, the fuel-rich flow can reach the airflow zone of tier F. This also illustrates that increasing the damper opening of the tier E air box to a proper extent does not affect the depth that the fuel-rich flow reaches in the furnace. When the secondary air of tier E has the same velocity as that of tier F, the transverse velocity of the fuel-rich flow reaches a maximum at H/H0 ) 0.32. This indicates that, for case 3, the fuel-rich flow reverses and rises much earlier than for the other two cases, which may lead to an unstable flame in the full-scale furnace. Figure 6 shows the flow field for the three cases. It is evident that as the secondary air velocity of tier E increases, the fuel-

rich flow shows significantly greater deflection. For case 2, the deflection is slight and the depth that the fuel-rich flow reaches in the furnace is only slightly different from case 1. The fuelrich flow can only reach the airflow zone of tier F. When tiers E and F have the same secondary air velocity, the depth that the fuel-rich flow reaches is shorter. It is also evident from this figure that because of secondary air flowing into the furnace from the front and rear walls, the fuel-rich flow is not deflected to the walls. Thus, it can be predicted that slagging and hightemperature corrosion will not appear on the front and rear walls in the full-scale furnace. Although gas recirculation may exist below the arches, the gas flow does not erode the arches. Thus, slagging will not appear on the arches either. For two intersecting airflows, the direction of the mixed airflow can be calculated by the parallelogram law using the Euler method. Suppose β is the angle between the main and the mixed airflows (shown in Figure 7); then, the following relation can be obtained:

β ) arctan

sin R M1 + cos R M2

where R is the angle between the main and the impact airflows and M1 and M2 are the momentum of the main and impact airflows, respectively. The angle between the fuel-rich flow mixed with several tiers of secondary air and the vertical direction can be calculated according to the above expression. The calculated and experimental results are listed in Table 2. The calculated results coincide with the experimental results in cases 1 and 2. The error is relatively larger in case 3. This is because the processes of attenuation, collision, diffusion, convection transport, and penetration are not considered by the parallelogram law. However, both the calculated and experimental results show that, as the momentum of the tier E secondary air increases, the deflection angle of the fuel-rich flow in the airflow zone of tier E increases. In particular, for case 3, the deflection angle calculated for the fuel-rich flow is greater than 45°. The transverse momentum of the fuel-rich flow is greater than the downward momentum, which reverses the fuelrich flow. Figure 8 shows changes in the turbulence intensity of the tier E airflow zone along the x-direction for the three cases. The turbulence intensity T is defined as

T)

x

V′x2 + V′z2 /Vmax 2

where V′x and V′z are velocity fluctuations in the x- and z-directions, respectively. Figure 8 shows that the turbulence intensity is lower for case 1 compared to the other two cases. This is because when the airflow zone is supplied with a certain amount of secondary air through the tier E slots, the fuel-rich flow and the secondary air interact with each other and are well mixed. Intense fluctuations in this zone can enhance heat and mass transfer in the combustion process, which is advantageous for ignition of the primary air/coal mixture and for strong combustion. The results of the small-scale cold aerodynamic experiment show that if the secondary air flow in tier E is suitably controlled, the fuel-rich flow will not reverse and rise back out of the furnace. Increasing the secondary air flux can also strengthen turbulence fluctuations in the tier E airflow zone, which is advantageous for ignition and strong combustion.



Figure 9. Measurement points in the furnace (dimensions in milimeters). (a) Monitoring ports on the side wall. (b) Monitoring ports on the front wall. (c) Monitoring ports on the rear wall. (d) Angle at which the sight pipe is inserted into the furnace. Note: Level 0 mm is located at the bottom of the slag pool.

4. In situ Industrial Experiments In situ experiments were carried out using a 1025-tph downfired pulverized-coal boiler to investigate the combustion process and NOx formation in the furnace. At a load of 300 MWe, the furnace excess O2 remained stable. Soot blowing and sewer bleeding were avoided during the experiments. The coal used in the experiments was Yangquan anthracite. The analysis shown in Table 3 confirms that the coal is anthracite with a high heating value and low volatile content. The following parameters were measured. (1) The gas temperature of the furnace was measured with a leucoscope through monitoring ports in the front, rear, and side walls. The layout of the monitoring ports is shown in Figure 9a-c. The measurement error is 50 °C. (2) The gas temperature of the furnace was measured with a nickel chromium-nickel silicon thermocouple through monitoring ports 1 and 2 shown in Figure 9a. Port 1 is in the airflow zone of the tier D and E slots, and port 2 is in the airflow zone of the tier F slots (Figure 2). (3)

As shown in Figure 9a, a thermocouple was inserted along the line parallel to the axis of the cyclone to measure the primary airflow/temperature distribution. The specific position at which the thermocouple was inserted is shown in Figure 2. In addition, along the sight pipe, the thermocouple was inserted into the furnace to measure the gas temperature of the fuel-burning zone. Figure 9d shows the angle at which the sight pipe was inserted into the furnace. (4) A water-cooled gun was inserted into the furnace through monitoring port 2 shown in Figure 9a, and gas drawn out using the gun was analyzed online using a Testo 350M instrument. The flue gas was also analyzed online. The measurement error for O2 and CO2 was 1%, while the error for CO and NOx was 1 ppm. Two settings of the damper opening for tier E air boxes were evaluated: 0% and 30%. The main operating parameters are listed in Table 4. In addition, according to operating experience, when the damper opening for tier E is 100%, negative gas pressure in the furnace fluctuates significantly, which leads to


Ren et al.

Figure 10. Gas temperature distribution in burner D1 region with different damper openings of the E-tier air box: (×) 0%; (]) 30%. (a) End of burner D1. (b) End of sight pipe D1.

Figure 11. Gas temperature distribution in the zone near the watercooled wall with different damper openings of the E-tier air box: (×) 0%; (]) 30%. (a) Through monitoring port 1. (b) Through monitoring port 2.

Table 4. Main Operating Parameters of the Boiler damper opening of the E-tier secondary air box parameter mill A primary air mill B flux at the mill mill C 3 exit (N m /h) mill D mill A mass flow flux mill B of pulverized coal mill C at the mill exit (kg/h) mill D temperature of the primary air (°C) total flux of the secondary air (N m3/h) temperature of the secondary air (°C) damper opening for tier D air boxes damper opening for tier F air boxes

0%

30%

37270 40022 42022 42022 29353 27767 29353 27767 105 574630 320 0 25

40429 37330 35443 41768 28134 28986 29839 27281 105 580996 320 0 25

unstable coal combustion and, thus, boiler operation. For this reason, 100% opening was not investigated. This agrees with the results of the small-scale cold aerodynamic experiments. When the damper opening for tier E was too large, the flow of secondary air stops the fuel-rich flow from flowing down into the furnace and reverses it, so that the residence time of coal particles in the furnace is shortened. Figure 10 shows the gas temperature distribution in the fuelburning zone along the cyclone axes of the burners and the sight pipes, where the zero points are set to the ends of the fuel-rich nozzle and the sight pipe in the furnace. Figure 11 shows the furnace gas temperature measured using a thermocouple inserted through monitoring ports 1 and 2 (Figure 9). Figure 12 shows the furnace temperatures measured using a leucoscope. Some

temperature results are missing because the monitoring ports were covered with charred coal during measurement. Figure 13 shows gas components in the zone near monitoring ports 1, 2, and 3 in the furnace. Figure 10a shows that for damper opening of 0%, at the measurement point 1000 mm away from the ends of the fuelrich nozzle along the cyclone axes, the temperature reached only 300 °C. The ignition position of the primary air and pulverized coal mixture was far away from the fuel-rich nozzle. However, the ignition distance for a typical boiler while burning anthracite is 300-500 mm. This shows that this type of burner layout is disadvantageous for ignition of the primary air and pulverized coal mixture. Figure 10 also shows that for a damper opening of 30%, at measurement points less than 1000 mm away from the ends of the fuel-rich nozzle along the cyclone axis, the gas temperature is very close to that for 0% opening. Along the axis of the sight pipe, at measurement points less than 1500 mm away from the end of the sight pipe, the same results were obtained. This indicates that, for a damper opening of 30%, the ignition position of the primary air and coal mixture is also very far from the fuel-rich nozzle. From the aerodynamic fields of the small-scale model experiments, it is evident that regardless of the damper opening, the fuel-rich flow cannot descend into the airflow zone of tier F. Thus, nearly half of the furnace cannot be used for coal combustion. The gas temperature in the fuel-burning zone is low. Thus, the ignition position of the fuel-rich flow is far away from the fuel-rich flow nozzles, and the mixture flow of pulverized coal and primary air cannot be ignited in time. Figure



Figure 12. Various cross-section gas temperature distributions in the furnace with different damper openings of the E-tier air box (values in °C) at levels of (a) 18.3, (b) 21, (c) 27, (d) 30, (e) 37.5, and (f) 43.4 m.

11 shows that, for a damper opening of 30%, the gas temperature in the zone near port 1 is higher than that for a damper opening of 0%. In the zone near port 2, the gas temperature is also higher for a damper opening of 30% compared to that of 0%. The combustion is intense in this zone. This indicates that for a damper opening in tier E of 30%, the fuel-rich flow can reach the airflow zone of tier F and does not reverse ahead of time. This also validates the results of the small-scale cold aerodynamic experiments. When the damper opening of the tier E air box is controlled within a suitable range, the fuel-rich flow does not reverse and rise back out of the furnace ahead of time. A gradual supply of air is more advantageous for the coal combustion process. Thus, at a damper opening of 30%, with a certain amount of secondary air fed into the furnace from the secondary air slots of tier E, there is sufficient oxygen to maintain combustion of the pulverized coal after ignition. When reaching the airflow zone of tier F, there is also sufficient oxygen for coal combustion. Thus, the gas temperature in the zones near ports 1 and 2 is higher than for the case of a damper opening of 0%. In general, the gas temperature peak is in the fuel-burning zone for a boiler in service when the distance from the position of the gas temperature peak to the exit of the furnace is relatively large. Coal combustion time in the higher temperature zone is greater, which is advantageous for fuel burnout. If the gas

Figure 13. Gas components in the zone near monitoring port 2 with different damper openings of the E-tier air box (×) 0%; (]) 30%.

temperature peak is in the fuel-burnout zone, the distance from the position of the gas temperature peak to the exit of the furnace is relatively shorter. The coal combustion time in the higher temperature zone is shorter, which is disadvantageous for fuel burnout. Thus, the gas temperature peak should be in the fuelburning zone. Figure 12 shows that, for damper openings of both 0% and 30%, the position of the temperature peak in the furnace is at the 27-m cross section, which is in the fuel-burnout zone, lapsing from the original design. This is because the coal/ air ignition position is far away from the end of the fuel-rich nozzle, which means that it is harder for coal to burn out in the furnace. Figure 13 shows the distribution of O2, CO, CO2, and NOx contents in the zone near monitoring port 2, i.e., the airflow zone of tier F. With a damper opening for tier E slots of 0%, the O2 content is about 10% and the CO2 content is about 12%. This indicates that combustion is intense in this zone, and the fuel-rich flow can reach the airflow of tier F. The CO content


Ren et al.

Table 5. Gas Components and Carbon Content in the Fly Ash damper opening of the E-tier air box

O2 content at the furnace exit (%)

O2 content in the flue gas (%)

CO content in the flue gas (ppm)

NOx content in the flue gas (mg/m3) (at 6% O2 dry)

carbon content in the fly ash (%)

0% 30%

2.82 3.07

4.17 6.49

13.75 157.33

2101.0 1725.9

7.84 6.54

is at a low level (

Influence of the Secondary Air-Box Damper Opening on Airflow and

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