Energy & Fuels 2009, 23, 2437–2443
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Influence of the Down-Draft Secondary Air on the Furnace Aerodynamic Characteristics of a Down-Fired Boiler Feng Ren, Zhengqi Li,* Zhichao Chen, Jingjie Wang, and Zhao Chen School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China ReceiVed NoVember 24, 2008. ReVised Manuscript ReceiVed February 6, 2009
The operation of down-fired boilers can suffer from problems of high carbon content in the fly ash. This is because horizontally fed secondary air keeps the fuel-rich flow from going deep down into the lower furnace and the recirculation zones in the furnace hopper area are too large. To improve the burnout of coal in downfired boilers, a retrofit modification was devised and validated. The modification lowered the angle of flow of the secondary air to a down-draft. Experiments were carried out on a single-phase test facility to investigate the influence of down-draft secondary air on the aerodynamic field in the furnace. The depth reached by the fuel-rich flow in the down-furnace, the volume of dead recirculation zone, the angle of the mixed air in the airflow zone of secondary air, and the turbulence intensity in certain cross sections were investigated. The results show when the flow of secondary air was lowered to an optimized angle, the primary air can reach a deeper position in the lower furnace without washing the furnace hopper, and consequently the dead recirculation zone shrinks. The influence of the secondary air ratio distribution on the flow field was also investigated.
1. Introduction Down-fired combustion technology is one of the techniques applied for burning hard-to-burn coal. It enhances coal burnout rate by increasing residence time in the furnace. However, practical down-fired boiler operation still suffers from problems of high carbon content in the fly ash. This is because the distance for the fuel-rich flow to go down into the lower furnace is too short.1 Aerodynamic characteristics are of great significance in pulverized coal combustion. Different aerodynamic fields lead to variation in the coal combustion process. However, it is very difficult to collect aerodynamic parameters such as velocities and turbulence intensities in full-scale furnaces. Therefore, to analyze the influence of aerodynamic behavior on pulverized coal combustion, most experiments are performed in small-scale models. The results of small-scale cold experiments show certain differences from those performed on full-scale hot furnaces. However, using certain modeling similarity criteria, cold flow fields in a small-scale model furnace can describe the aerodynamic characteristics in the full-scale one. Thus, the pulverized coal combustion can be analyzed by small-scale cold modeling. On the basis of the cold modeling, certain optimized parameters can be obtained with the assistance of numerical simulation of furnace combustion. For the down-fired furnaces, Che et al. studied the influence of different primary air momentum ratios on the whole furnace flow field.2 He et al. focus on the ratios among different tiers of secondary air.3 Xu et al. emphasize * To whom correspondence should be addressed. Tel.: +86 451 86418854. Fax: +86 451 86412528. E-mail:
[email protected]. (1) Ren, F.; Li, Z. Q.; Zhang, Y. B.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (2) Che, G.; Xu, T. M.; Xu, W. J.; Hui, S. E. J. Eng. Therm. Energy Power 2001, 91, 19–22 (in Chinese). (3) He, L. M.; Zhang, J. B.; Li, X. Y.; Che, G.; Xu, T. M.; Xu, W. J.; Hui, S. E. Chin. J. Appl. Mech. 2002, 1, 18–22 (in Chinese).
the influence of the tertiary air ratio.4 Zhou et al. study the effect of swirl number of the primary air/coal mixture.5 However, changing these operating parameters does not effectively increase the flow distance for the fuel-rich flow in a down-fired furnace. In this paper, a retrofit modification to lower the angle of flow of the secondary air into a down-draft furnace was devised and validated. Cold air-flow experiments were conducted in a small-scale furnace modeled from a 300MW full-scale down-fired boiler. The influence of the F-tier secondary air angle on the flow field in a down-fired furnace was determined. The influence of the E-tier secondary air flux on the aerodynamic field with downdraft secondary air was also investigated. The results of these experiments will be of benefit for the design and operation of similar boilers. 2. Experimental Facility Setup Figure 1 shows the schematic view of the 300MW furnace. The arches divide the furnace into two parts. The part below the arches is the fuel-burning zone, and that above the arches is the fuel-burnout zone. Cyclone burners are set on the arches to organize a W-shaped flame. The cyclone burner centrifugally separates the primary air/coal stream into two flows: the fuel-rich and the fuellean. Under the arch there are three tiers of slots (D, E, and F) between the vertical waterwall tubes in the front and rear walls for the secondary air to be fed into the furnace (see Figure 2). More specific detail is described elsewhere.1,6 Since there are no extra air ports to feed air into the furnace hopper region, two large dead recirculation zones form. Very little pulverized coal can reach these two dead recirculation zones, leading to an under utilization of the furnace volume. (4) Xu, W. J.; Yan, X.; Sun, X. G.; Hui, S. E.; Xu, T. M. Acad. J. Xi’an Jiaotong UniV. 2001, 1, 108–110 (in Chinese). (5) Zhou, Z. J.; Zhu, Z. L.; Zhao, X.; Yao, Q.; Cao, X. Y.; Cen, K. F. Power Eng. 1999, 3, 33–37 (in Chinese). (6) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Fuel 2007, 86, 2457–2462.
10.1021/ef8010146 CCC: $40.75 2009 American Chemical Society Published on Web 03/16/2009
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Figure 2. Secondary air slots.
Figure 1. Furnace of the 1025-tph down-fired boiler.
The cold flow experimental system is illustrated in Figure 3. It consists of an induced draft fan, a small-scale furnace model, and an IFA300 constant temperature anemometer. The small-scale furnace in the cold flow experimental system is modeled from the equipment described above. The similarity criteria for the scaling are as follows: (1) The first is geometric similarity. The ratio of the small scale apparatus to the full scale is 1:15. (2) The second is self-modeling flows. The Reynolds numbers of the primary and secondary airflow for both the small-scale and full-scale boiler are greater than 45 000, and the Reynolds number for the furnace is greater than 76 000. The limited Reynolds numbers for the flows through burner nozzles and the furnace are 10 000 and 30 000, respectively. So we conclude that the flows are self-modeling. (3) The momentum ratios among the airflows of the small-scale furnace are the same as that of the full-scale. An IFA300 constant-temperature anemometer system produced by TSI was used to measure air velocity. The constant-temperature anemometer comprises a bridge and amplifier circuit that controls a thin film sensor at constant temperature. As the fluid flow passes over the heated sensor, the amplifier senses the bridge off-balance and adjusts the voltage to the top of the bridge, keeping the bridge in balance. The voltage on top of the bridge can therefore be related to flow velocity. The bridge voltage is sensitive to temperature as well as velocity, and so the built-in thermocouple circuit is attached to a thermocouple to measure fluid temperature. This temperature reading is then used to correct the results, minimizing the effect of temperature. Here, a 1240-type probe with two hot-film sensors was used giving a velocity measurement error of less than 2%. When the induction fan was started, all the air was drawn into the pilot-scale furnace through the primary and secondary air pipes. The airflow flux in the primary and secondary air pipes was measured using Venturi tube flowmeter. The error for measurement of the airflow rate by this method was less than 10%.
3. Results and Discussion (1) Influence of the F-Tier Secondary Air Angle. In this part, the influence of the F-tier secondary air angle on the flow field is investigated. The velocities of the primary and secondary air are listed in Table 1. At these velocities, the angle of the secondary air was adjusted to 0°, 15°, 25°, 35°, and 45°. The most important factor affecting the aerodynamic field in a down-fired furnace is the depth that the fuel-rich flow reaches in the furnace. This determines the residence time for coal particles in the furnace and hence whether the goal of increasing the burnout rate in the W-shaped fire will be achieved. However, if the fuel-rich flow goes too deep into the furnace, it will wash the furnace hopper and cause slagging. Here decay curves for the downward dimensionless velocity component Vz/Vmax of the
fuel-rich flow along the dimensionless coordinates H/H0 in the z direction are used to estimate the depth reached by the fuelrich flows. Along the y-coordinate, Vz stands for the z-direction velocity component at the measurement points along the fuelrich flow. Vmax stands for the Vz at the outlet of the fuel-rich nozzle along the z-direction. Along the x-coordinate, H is the distance between the fuel-rich nozzle and the velocity measurement point 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. If the dimensionless depth reached by the fuel-rich flow is less than 0.5, the fuel-rich flow will not wash the furnace hopper. Figure 4 shows the decay curves of the fuel-rich flows for five cases with different F-tier secondary air angles. It can be observed that when the angle of the secondary air is set at 0°, the dimensionless depth that the fuel-rich flow reaches is only 0.36snot very far from the nozzle tip. This is because the large amount of F-tier secondary air impacts the fuel-rich flow in the x-direction and prevents it from continuing down. The fuelrich flow then reverses and goes up ahead of time. The unburned carbon content is likely to be high in this situation, because the residence time for reaction in the furnace is too short. When the angle of the F-tier secondary air declines to 15° and 25°, the depths increase to 0.42 and 0.48, respectively. This means that the time for the fuel-rich flow in the lower furnace becomes longer, lowering the carbon content of the fly ash. Between the two, with the angle set at 25°, the fuel-rich flow can go to a deeper position into the furnace. When the F-tier secondary air angle turned to 35°, the dimensionless depth increases to 0.55. With the angle at 45°, the depth is greater than 0.6. In these two cases, the fuel-rich flow will wash the furnace hopper and cause slagging. Therefore, 25° is the optimized angle for the F-tier secondary air. 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 5); then the following relationship can be obtained: β ) arctan
sin R M1 + cos R M2
(1)
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. The angle between the mixed air (fuel-rich flow mixed with several tiers of secondary air) and the horizontal direction can be calculated according to the above expression. The calculated and experimental results are shown in Figure 6. It can be seen that the calculated angle of the mixed flow (mixture of the fuel-rich flow and F-tier secondary air) near the F-tier
Down-Draft Secondary Air Influence on Down-Fired Boiler
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Figure 3. Small-scale cold experiment system and layout of the measurement points on the side wall. Table 1. Parameters for the Cases with Different F-Tier Secondary Air Angles primary air velocity (m/s)
secondary air velocity (m/s)
fuel-rich flow
fuel-lean flow
D
E
F
28.97
12.35
0
2.14
20
secondary air ports shows a linearized increase as the declined angle of the F-tier secondary air at its inlet rises from 0° to 45°. The change of experimental declined angle also increases linearly but shows an obvious transition at the point where the F-tier secondary air angle is 25°. In the first range as the F-tier secondary air angle rises from 0° to 25°, the angle of the mixed air increases more rapidly than the calculated one. This is because no air inlets are configured in the region below the airflow zone of F-tier, leading to low air pressure in this part of the furnace. Thus the mixed air tends to decline downward more than predicted. In the second range, when the angle of the F-tier secondary air rises from 25° to 45°, the mixed air angle increases very slowly. This is because in this range, the declined angle of the mixed air becomes very near or larger than 55° which is the angle between the furnace hopper wall and the horizontal direction. In this case the furnace hopper prevents the mixed
Figure 5. Sketch map of the direction of the mixed jet.
Figure 6. Declined angles of the mixed air near the F-tier secondary slots with the increase of F-tier secondary air angle.
Figure 4. Decay curve for the longitudinal velocity component of the fuel-rich flow with different F-tier secondary air angles.
air from declining at the original rate. It can also be deduced that when the F-tier secondary air is larger than 25°, the mixed air washes the furnace hopper which may cause slagging. Thus 25° is the optimized angle for the F-tier secondary air.
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Figure 7. Aerodynamic fields for the cases with different secondary air declining angles (the thick dashed lines are the boundaries between the main flow zone and the dead recirculation zone). Table 2. Volume Ratio of the Recirculation Zone to the Whole Lower Furnace with Different F-Tier Secondary Air Angles declined angle of F tier secondary air volume ratio
0° 0.57
15° 0.44
25° 0.35
35° 0.29
45° 0.26
Figure 7 shows the flow field for all five cases. It can be seen more clearly that when the F-tier secondary air is not flowing downward, the fuel-rich flow stops at the upper part of the F-tier secondary air. The two large dead recirculation zones in the furnace hopper region show a large waste of the furnace space for coal combustion. The ratio of the dead recirculation zone to the whole down-fired furnace is listed in Table 2. With the F-tier secondary air not flowing downward, the dead recirculation zone occupied 57% volume of the whole downfurnace. As the declining angle increases, the fuel-rich flow can penetrate the airflow zone of F-tier and the two recirculation zones also become smaller. As the angle increases to 25°, the volume ratios of the recirculation zones are 35%. The utilization of the furnace space for coal combustion is raised. And as the angle is set at 35° and 45°, from the flow field it can be found that though the recirculation zones in the furnace hopper zone
Figure 8. Turbulence intensity distribution in the cross sections H/H0 ) 0.35 for the cases with different secondary air declined angles.
continue shrinking, the fuel-rich flows are very likely to wash the furnace hopper. From this aspect, 25° is also the optimized angle. Figure 8 shows the turbulence intensity distribution in the cross section where H/H0 ) 0.35 along the X/X0 direction for these five cases. Here, X0 is the distance from the front wall to the rear wall in the down-furnace and X is the distance from the measurement point to the front wall. This cross section is in the middle of airflow zone of F tier. The turbulence intensity T is defined as T)
/
V'x2 + V'z2 Vmax 2
(2)
where Vx′ and Vz′ are velocity fluctuations in the x- and z-directions, respectively. From Figure 8, it can be observed that as the F-tier secondary air is not flowing downward and only declined at 15°, the turbulence intensities are at the same level. When the angle increases to 25°, the turbulence intensity also rises. The reason for it is that in the former two cases, with the small declined angles, most of the fuel-rich flow momentum is consumed in the upper half of the airflow zone of the F tier. Thus, the impact of the F-tier secondary air to the fuel-rich flow is not so intense in the measured cross section, leading to relatively weak turbulence intensity. As the angle increases to 25°, more momentum remained when the fuel-rich flow passes through the measured cross section H/H0 ) 0.35. Thus, the turbulence intensity is larger in this case. 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. So compared with the case where the F-tier secondary air angle is set at 0° and 15°, the 25° setting gives optimal combustion conditions. When the angle increases to 35°, for the same reason mentioned previously, the turbulence intensities are larger than with the 25° setting. It can be predicted that combustion is also enhanced in this case. However, as previously mentioned, the fuel-rich flow will wash the furnace hopper. Then, although the
Down-Draft Secondary Air Influence on Down-Fired Boiler Table 3. Parameters for the Cases with Different E-Tier Secondary Air Ratios air velocity fuel-rich flow (m/s) fuel-lean flow (m/s) D-tier secondary air (m/s) E-tier secondary air (m/s) F-tier secondary air (m/s) a
Energy & Fuels, Vol. 23, 2009 2441 Table 4. Declined Angles of the Mixed Air near the F-Tier Secondary Slots in the Cases with Different E-Tier Secondary Air Ratios
0.029a
0.063a
0.118a
0.165a
0.207a
parameter
0.029a
0.063a
0.118a
0.165a
0.207a
28.97 12.35 0 2.14 20
29.7 12.3 0 4.76 19.8
31.91 12.57 0 9.62 20
32.12 12.1 0 13.7 19
32.16 11.6 0 17.32 18
calculated angle experimental angle
35.76° 51.22°
36.33° 53.15°
36.75° 49.81°
35.83° 49.52°
33.79° 52.13°
a
E-tier secondary air ratios.
E-tier secondary air ratios.
Figure 10. Distribution of transverse air velocity component in the region clinging to the front wall.
Figure 9. Decay curve for the longitudinal velocity component of the fuel-rich flow with different E-tier secondary air ratios.
combustion is enhanced, slagging becomes even more serious. With the angle at 45°, turbulence intensity is lower than that when the angle is set to 35°. This is because with the F-tier secondary air angle at 45°, the angle between the fuel-rich flow and the F-tier secondary is very small and the impact between the two streams becomes weak; thus, the turbulence intensity falls. However, it is still much higher than that with the angle at 0°, 15°, and 25°, so the slagging is not much reduced. From this aspect, 25° is also the optimized angle. (2) Influence of the E-Tier Secondary Air Flow Ratio. In practical boiler operation, the damper of the E-tier secondary air is often set at a certain opening to organize a better aerodynamic field for efficient coal combustion and low NOx emission. In this part, with the down-draft angle of the F-tier secondary air set at 25°, five cases were performed. In these cases, the E-tier secondary air ratio was set to five different values: 0.029, 0.063, 0.118, 0.165, and 0.207. The direction of E-tier secondary air was kept horizontal. The E-tier secondary air ratio is the ratio between the mass flow rate of the E-tier secondary air and that of the total air fed into the furnace. The specific parameters for all the ports are listed in Table 3. Ordinarily, the primary air ratio in the full-scale boiler operation is maintained at a fixed value, so the cold-flow experimental primary air ratio is also fixed here. Another fixed value is the F-tier secondary air velocity. Thus as the increase of the E-tier secondary air ratio, the total secondary air flux rises. To maintain the primary air ratio, the primary air velocity also rises (see Table 3). Figure 9 shows the decay curves of the fuel-rich flows for these five cases. It can be observed that as the increase of E-tier secondary air flow ratio, the time for the fuel-rich flow to decay is brought forward. This is because the air from the E-tier secondary slots impacts the fuel-rich flow. More and more momentum was consumed in this region as the E-tier air was increased. Li et al. have carried out a similar experiment with the F-tier secondary air not flowing down.1 They concluded that
whatever the flow rate of the E-tier secondary air is, the fuelrich flow cannot penetrate the airflow zone of F-tier. However, in contrast when the F-tier secondary air declined by 25°, the fuel-rich flow can penetrate the air-flow zone of the F-tier. The depths of penetration into the lower furnace in these cases range from 0.49 to 0.55. This means that the decline of the F-tier secondary air can increase the depth that the fuel-rich flow reaches when the secondary air flow rate of E-tier is not more than 0.207. This is because after penetrating the airflow zone of E-tier, no matter how much the rest of the momentum remains, the fuel-rich flow can be brought down to a much deeper place by the declined F-tier secondary air but not blocked by it. So by feeding certain amount of air into the E-tier airflow zone, the fuel-rich flow will not go up too soon. Also, in theory a certain amount of the secondary air fed into the furnace will achieve the goal of stage combustion and gradual supply of oxygen, which is good for NOx reduction and stable coal combustion. Table 4 lists the angle of mixed air in the airflow zone of F-tier. It can be seen that the experimental angles are larger than the calculated ones, the reason for which has been mentioned above. It can also be observed that with the increase of E-tier secondary air flow ratio, both the calculated and experimental angle varies in a small range. From this aspect, it can also be deduced that the depths for the mixed air to reach in the lower furnace are almost the same in these five cases. Figure 10 shows the distribution of the transverse velocity component Vwx in the region clinging to the front wall along the dimensionless coordinates H/H0. When Vwx < 0, the air washes the wall, which may cause slagging on the front and rear walls. When Vwx > 0, the air flows away from the wall, which is the ideal situation. From this figure, it can be observed that in the two cases with the E-tier secondary air ratio at 0.029 and 0.063, most of the Vx in the near-wall region is
