Energy Fuels 2009, 23, 5846–5854 Published on Web 09/15/2009
: DOI:10.1021/ef900679n
Experimental Investigations into Gas/Particle Flows in a Down-Fired Boiler: Influence of Down-Draft Secondary Air Zhengqi Li,* Feng Ren, Zhichao Chen, Guangkui Liu, and Zhenxing Xu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China Received July 2, 2009. Revised Manuscript Received August 28, 2009
Down-fired boiler operations suffer problems of high carbon content in the fly ash. This is because horizontally fed secondary air restrains the fuel-rich flow from penetrating into the lower levels of the furnace. Experiments were conducted on a small-scaled furnace for a down-fired pulverized-coal 300 MWe utility boiler on a gas/solid two-phase test facility. Modifications to the furnace permitted secondary air to be directed downward giving five different F-tier secondary air angle settings. Investigations at various settings were performed to obtain distributions of the mean velocity, root-mean-square (rms) fluctuation velocity, particle size, particle volume flux, and particle number concentration within the furnace. Results show that, with increasing angle settings, greater depths were attained for both air and particles within the furnace and slip velocities also increased between air and particles. At a 25° angle setting, the particle residence time in the furnace can be increased, while slagging within the furnace hopper can be avoided.
criteria, cold-flow fields in small-scale furnaces can more or less describe aerodynamic characteristics in full-scale versions. Thus, pulverized coal combustion dynamics can be predicted to a certain extent by small-scale cold-flow modeling. On this basis, optimized parameters can be obtained with the assistance of furnace combustion numerical simulations. Therefore, to analyze the influence of aerodynamic behavior on pulverized coal combustion, researchers have studied the aerodynamic behavior of full-scale furnaces by performing single flow experiments in small-scale models. For down-fired furnaces, Che et al. have studied the influence of different primary air momentum ratios on the whole furnace flow field.5 He et al. have focused on corresponding ratios between different tiers of secondary air.6 Xu et al. placed more emphasis on the influence of the tertiary air ratio.7 In these studies, a few improvements have been sought to increase the penetration depth of fuel-rich air flow into the furnace. Li et al. have proposed a down-draft of F-tier secondary air and have confirmed its usefulness in a small-scale single-phase test facility.8 Results show that the fuel-rich air flow then penetrates deeper into the down furnace (as far as the hopper zone) by applying this adjustment. However, particle behavior still remains unknown in this type of flow field. To unravel this behavior, velocity measurements are taken with a particle dynamic anemometer (PDA), an instrument incorporating phase Doppler anemometry that arose out of laser Doppler anemometry (LDA). Velocities are measured from frequencies of Doppler bursts as for LDA.9 Using PDA,
1. Introduction Anthracite deposits represent a sizable fraction of the total coal reserves in the world. Researchers everywhere have conducted many experiments studying its properties. These show that this type of coal is usually of low-volatile content, high fuel ratio, and small access pore volume. Thus, anthracite generally has the lowest combustion reactivity compared to other types of coals, and difficulties develop in burnout. Aside from reactivity, coal burnout also depends upon char particle size, partial pressure of reactants, furnace temperature, and particle residence time in the furnace. Down-fired combustion technology is one of the techniques applied in enhancing anthracite burnout rates by increasing residence times in the furnace.1-3 However, in practice, down-fired boiler operations still develop problems from high carbon content in the fly ash. This is because the fuel-bearing air flow does not penetrate sufficiently into the lower half of the furnace.4 Thus, residence times for coal particles within the furnace are still insufficient. Aerodynamic characteristics are of great significance in pulverized coal combustion. Different aerodynamic fields result in wide variations in the coal combustion process. In practice though, process parameters, such as velocity and turbulence intensity, in a full-scale furnace are almost impossible to obtain. Cold-flow fields are commonly believed to be unable to model accurately enough actual hot flows because of differing heat expansions within the flows in the full-scale furnace. However, following certain similarities in modeling
(5) Che, G.; Xu, T. M.; Xu, W. J.; Hui, S. E. J. Eng. Therm. Energy Power 2001, 91, 19–22 (in Chinese). (6) 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). (7) 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). (8) Ren, F.; Li, Z. Q.; Chen, Z. C.; Wang, J. J.; Chen, Z. Energy Fuels 2009, 23, 2437–2443. (9) Pickett, L. M.; Jackson, R. E.; Tree, D. R. Combust. Sci. Technol. 1999, 143, 79–106.
*To whom correspondence should be addressed. Telephone: þ86451-86418854. Fax: þ86-451-86412528. E-mail:
[email protected]. (1) Fan, W. D.; Lin, Z. C.; Li, Y. Y.; Kuang, J. G.; Zhang, M. C. Energy Fuels 2009, 23, 111–120. (2) Zhang, L. X.; Huang, J. J.; Fang, Y. T.; Wang, Y. Energy Fuels 2006, 20, 1201–1210. (3) Pappano, P. J.; Schobert, H. H. Energy Fuels 2009, 23, 422–428. (4) Ren, F.; Li, Z. Q.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 5846–5854
: DOI:10.1021/ef900679n
Li et al.
Figure 1. Schematic for the furnace of the 300 MWe down-fired boiler.
not only velocities but also particle sizes and particle number concentrations of two-phase air flows can be measured.10-13 Here, a PDA system was used to investigate the influence of down-draft secondary air on the gas/solid flow field in a smallscale furnace modeled on a 300 MW full-scale down-fired boiler designed on the Foster-Wheeler technique. The results of these experiments will be beneficial for future design and operation of similar boilers.
Figure 2. Combustion system for the down-fired boiler (dimensions in millimeters). Table 1. Reynolds Number for the Main Flows of the Furnace flow industrial scale lab scale
fuel-rich flow F-tier secondary air flow furnace flow 239172 37824
59995 24497
53519 133769
tier F are 25% closed. In the paper, we only investigate the effect of opening the E-tier damper and finding its optimal setting. Consequently, only the secondary air from the F tier can largely influence the flow field in the furnace. More specific details on design and operation are described elsewhere.4,14 Originally, the direction of the F-tier secondary air was horizontal. It is very likely that this setup prevents fuel-rich flow penetrating deep into the furnace and therefore results in brief particle residence times in this part of the furnace. Li et al. have proposed an adaptation involving down-draft F-tier secondary air.8 Here, angle optimization of the F-tier secondary air is investigated by studying gas and particle behavior in the furnace. The cold-flow experimental system is illustrated in Figure 2. It consists of an induced fan, a powder feeder, a small-scale furnace, and a cyclone separator. When the fan is active, all air is drawn into the small-scale furnace through the primary and secondary air pipes. Air-flow flux in these air pipes was measured using Venturi tube flowmeters. Measurement errors for air-flow rates were less than 10%. Glass beads were fed via the powder feeder into the fuel-rich and fuel-lean ducts and then carried by the primary air flow into the small-scale furnace. The small-scale furnace used in our cold-flow experimental studies is modeled after the equipment described above. The similarity criteria for scaling are as follows. (1) Geometric similarity: the ratio of the small scale apparatus to the full scale is 1:15. (2) Self-modeling flows: Table 1 listed the Reynolds numbers of the main flows in the furnace. Because the effects of the fuel-lean flows and E secondary air flows on the aerodynamic field are very slight, the Reynolds numbers of these flows are neglected. It shows that the Reynolds numbers of the fuel-rich flow and F-tier secondary air flow for both the small- and fullscale boiler are greater than 24 000 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. We thus conclude that the flows are self-modeling. (3) Stokes criterion: the Stokes number of fuel-rich
2. Experimental Facility Setup The 1025 tph boiler used with the 300 MWe unit was made by Dongfang Boiler Group Ltd. and employs technology from FW Ltd. This involves a Π-type layout, double arches, and a single furnace. Figure 1 shows a schematic view of the 300 MW furnace. The arches divide the furnace into two: below the arches is the fuelburning zone, and above the arches is the fuel-burnout zone. Cyclone burners are arranged on the arches specifically to form the W-shaped flame. 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 the cyclone of the burner in a tangential direction. Because the fuel/air mixture is a swirling flow in the cyclone, pulverized coal in the primary air is centrifugally separated. Thus, fuel-lean flow concentrates in the central zone, while fuel-rich flow converges in the peripheral zone. The fuel-lean flow is then fed into the furnace through vent air pipes, while the fuel-rich flow is fed into the furnace through the burner nozzle. The secondary air box is divided into six parts: A, B, C, D, E, and F. The damper at the inlet of each box can be varied to change the secondary air-flow rate. Secondary air from the A boxes is fed into the furnace through annular ports around the fuel-lean nozzles, while air from the B boxes is supplied 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. The secondary air from the D, E, and F boxes is fed into the furnace through these tier ports. Secondary air from the A and B boxes cools the fuel-rich and fuel-lean nozzles but has little influence on coal combustion in the furnace because of its low flux. The dampers of the C boxes are totally closed when oil guns are out of service. In the current operations, the dampers for both D and E tier secondary-air boxes are totally closed, while those for (10) Moon, S.; Bae, C.; Choi, J.; Abo-Serie, E. Fuel 2007, 86, 400–409. (11) Hubner, A. W.; Tummers, M. J.; Hanjalic, K.; vander Meer, Th. H. Therm. Fluid Sci. 2003, 27, 481–489. (12) Aı´ sa, L.; Garcia, J. A.; Cerecedo, L. M.; Garcı´ a Palacı´ n, I.; Calvo, E. Int. J. Multiphase Flow 2002, 28, 301–324. (13) Sommerfeld, M.; Qiu, H. H. Int. J. Multiphase Flow 1993, 19, 1093–1127.
(14) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Fuel 2007, 86, 2457–2462.
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Table 2. Prefixed Parameters for All Air Inlets velocity (m/s)
particle load (particle/air, kg/kg)
fuel-rich flow fuel-lean flow E-tier secondary air F-tier secondary air fuel-rich flow fuel-lean flow
31.1 10.4 2.2 15.0 0.55 0.10
flows for the industrial scale is 0.76. To ensure that the lab-scale particle size meets the requirement of this criterion, a diameter of 5 μm should be chosen. However, particles of this size are impractical in small-scale experiments because of the high cost and non-recoverability. Many papers have shown that particles of different diameters show similar velocity and turbulence distribution characteristics in small-scale furnace flow field measurements when their diameters are below 100 μm. Slip velocities between different sized particles are small.1,16,17 These characteristics can also be found in the following measurement of this paper. Thus, in considering the feasibility of the experiment and according to previous experience, glass beads with a mean diameter of 42 μm were chosen. (4) Momentum ratios between various air flows in the small-scale furnace are the same as those of the full-scale version. Similarly, particle loads within air/ particle flows of the small-scale furnace are the same as those for the full-scale version. A three-dimensional PDA made by Dantec was used in this study. The instrument includes an argon ion laser, a transmitter, fiber optics, receiver optics, signal processors, a traversing system, a computer system, and a three-dimensional auto-coordinated rack. The PDA uses proven phase Doppler principles for simultaneous non-intrusive and real-time measurements of each velocity component and turbulence characteristics and uses new triangulation methods using phase differences between Doppler signals received by three detectors located in different positions. The instrument uses 60 fiber flow optics and 57 10 PDA receiver optics. Several optical configurations up to 500 mm are available. All instrument settings, such as bandwidth and voltage, are computer-controlled. An analog-digital converter allows the computer to read the anode current of the photomultipliers. The combination of the photomultiplier and particle velocity correlation bias can contribute to measurement uncertainty, but the error is likely to be small. Overall uncertainties for measured values of the mean velocity, particle diameter, and particle volume flux are 1, 4, and 30%, respectively, and the measurable ranges for size and velocity are 0.5-1000 μm and from -500 to 500 m/s, respectively. Particles with diameters up to 8 μm were used to trace the air flow, and particles with diameters from 10-100 μm were used to represent particle phase flow. Particles with diameters below 100 μm were used for an analysis of the particle volume flux, particle size, and particle number concentration. Altogether five settings were evaluated: the angle of F-tier secondary air flow below the horizontal was adjustable to 0°, 15°, 25°, 35°, and 45°. From here on, the unqualified “angle setting” will only be in reference to the F-tier secondary air. Other fixed parameters for both primary and secondary air inlets are listed in Table 2.
Figure 3. Cold-flow experimental system.
Figure 4. Vertical velocity distributions for air in the furnace.
3. Results and Discussion In the rest of the discussion below, the origin of the coordination is set at the same height as the exit of the fuelrich nozzle, Y denotes depth below the nozzle, and X denotes horizontal displacements with positive values to the right. Y0 and X0 correspond to maximum depth and width,
respectively, of the down-furnace (see Figure 3). Within the furnace, the region extending between cross-sections Y/Y0 = 0.153 and 0.194 is the air-flow zone of the D tier, while that extending from Y/Y0 = 0.194 to 0.249 is the air-flow zone of the E tier, from 0.339 to 0.513 is the air-flow zone of the F tier, and finally, from 0.602 to 1.0 is the furnace hopper zone. Gas/particle flow characteristics were measured at cross-sections Y/Y0 = 0.08, 0.137, 0.193, 0.25, 0.306, 0.363, 0.419, 0.476, 0.533, 0.588, 0.677, and 0.792. At each crosssection, data were collected at several points along the X direction.
(15) Chen, Z. C.; Li, Z. Q.; Wang, F. Q.; Jing, J. P.; Chen, L. Z.; Wu, S. H. Fuel 2008, 87, 2102–2110. (16) Wang, G. Z.; Wu, S. H.; Chen, L. Z.; Qiu, P. H. Chin. J. Sci. Instrum. 2005, 26, 199–202 (in Chinese). (17) Wang, G. Z.; Wu, S. H.; Qiu, P. H.; Chen, L. Z.; Qin, Y. K. Proc. Chin. Soc. Electr. Eng. 2006, 26, 26–30 (in Chinese).
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measurement point. The figures show that, at all crosssections for each of the five angle settings, velocities near the furnace center zone (zones around X/X0 = 0.5) are negative and those near the fuel-rich flow zone (zones around X/X0 = 0.1) are positive. This means that, near the fuel-rich zone, the air/particle flow is downward while near the furnace center, the flow is directed upward. Thus, a “W”shaped flow pattern is formed in the small-scale furnace. At cross-sections extending from Y/Y0 = 0.08 to 0.25, the vertical velocity distributions are nearly the same in all five angle settings, indicating that downward-directed F-tier secondary air barely affects air-flow patterns above the F-tier zone. At cross-sections Y/Y0 = 0.08 and 0.137, there are two peaks in the profiles. The higher peaked region corresponds to the fuel-rich flow zone, and that of the lower falls within the fuel-lean flow zone. Below cross-section Y/Y0 = 0.193, only the fuel-rich flow peak zone remains, indicating that the fuel-lean flow decays rapidly. Alternatively, it can be seen that the fuel-rich flow has an influence extending deep into the furnace and the fuel-rich flow zone keeps leaning toward the furnace center as the flow moves downward. Because particles in the fuel-rich flow account for 96% of the total particle weight, flow and combustion characteristics in the down-fired boiler determines how the fuel-rich flow behaves. After cross-section Y/Y0 = 0.363, because of the different impact effects from the F-tier secondary air at differing angles, the vertical velocity curves begin to separate, especially in the zones between the fuelrich flow (around X/X0 = 0.15) and the wall (X/X0 = 0), where the F-tier secondary air has most influence. In these zones, the decay of the vertical velocity slows down as the angle increases. The most important factor affecting the aerodynamic field in a down-fired furnace is the depth that the fuel-rich flow extends into 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. Because in each cross-section, the maximum vertical velocity (y-direction velocity component) appears in the
3.1. Distribution of Gas/Particle Velocities. Figures 4 and 5 display vertical velocity distributions for both air and particles within the furnace. We define Vy,0 as the y-direction velocity component for the fuel-rich flow at the outlet of the nozzle and Vy as the y-direction velocity component at each
Figure 5. Vertical velocity distributions for particles in the furnace.
Figure 6. Decay curves for the fuel-rich air/particle flow in the furnace.
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Figure 8. Horizontal velocity distributions for the particle in the furnace.
Figure 7. Horizontal velocity distributions for air in the furnace. Table 3. Dimensionless Penetration Depth for Air and Particles within the Furnace setup
0°
15°
25°
35°
45°
dimensionless depth for air dimensionless depth for particles
0.50 0.52
0.55 0.58
0.57 0.64
0.59 0.67
0.59 0.70
fuel-rich flow zone, we define Vy,max as the y-direction velocity component for the fuel-rich flow at this crosssection. Decay curves for the dimensionless velocity Vy,max/ Vy,0 at different cross-sections can then be used in quantifying the depth that the fuel-rich flow extends into the furnace. The vertical distance between the point at which Vy,max/Vy,0 = 0.15 and the fuel-rich nozzle is defined as the penetration depth of the fuel-rich flow within the furnace. Figure 6 displays the dependence upon angle setting of the vertical velocity decay curves of the fuel-rich gas/particle flows. It can be observed that mixing flows with both the air and particles decay very slowly at first for all angle settings. However, below depth Y/Y0 = 0.193, the fuel-rich flow decays much faster when supplied with E- and F-tier secondary air and finally stops extending downward. This shows that E- and F-tier secondary air takes up nearly all of the fuel-rich flow momentum. This momentum transfer shows that, immediately after the fuel-rich flow has been issued from the nozzle, particle velocities lag behind gas velocities. That is because coal particles have greater inertia despite being accelerated by the primary air. As the fuel-rich flow continues flowing downward, particle velocities decay more slowly than for air, again because their inertia is much
Figure 9. Trajectory of the fuel-rich flow.
greater. Consequently, as seen in all angle settings, particle velocities first approach and then exceed air velocities; at this point, slip velocities between the two become even higher. Upon reaching the F-tier air-flow zone, slip velocities diminish because both air and particles are buffeted by mixing secondary air and, hence, velocities decay rapidly. Besides, 5850
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Figure 10. Particle velocity angle along the fuel-rich flow.
Figure 12. Vertical rms fluctuation velocity distributions for particles in the furnace.
increase, its impact on particle flow weakens and particle velocity decay slows. Relatively speaking, the decay in the vertical component of air velocities slows down not as obviously as for particles. Thus, slip velocities rise with increasing angle. Table 3 also shows that, with greater angles, penetration depths change much less for air than for particles, which also supports the conclusion that particle dynamics are more sensitive to changes in secondary-air angle than air-flow dynamics. Figure 7 and 8 show horizontal velocity distributions for air and particles in the furnace. For all angle settings, the figures show that, in the region identified by cross-sections Y/Y0 = 0.08-0.193, no obvious differences between the horizontal velocity distributions are evident. In addition, fuel-rich and fuel-lean zone peaks appear similar to the vertical velocity distributions. This is because, in this region, all air movement in the horizontal direction is caused by the fuel-rich and fuel-lean flow because no vertical secondary air is fed into the furnace. Another phenomenon displayed in this region is that, in zones between the fuel-rich zone and the furnace center, velocities on the whole are negative. This indicates that the air in these areas is flowing toward the fuelrich flow from the furnace center. Thus, a large recirculation zone is formed on both sides of the center line of the furnace, as shown in Figure 1. From the viewpoint of combustion, the existence of this recirculation can direct hot upward flowing gas toward the primary air/fuel and ensure sufficient heat for the air/fuel flow for timely ignition. Below cross-section
Figure 11. Vertical rms fluctuation velocity distributions for air in the furnace.
particle velocities are always higher than air velocities in the F-tier air-flow zone and below. The inference is that particles can penetrate deeper than air within the furnace. Table 3 lists the specific depths reached by air and particles. From the table, an increase in angle setting increases the depth for both air and particles, which implies that particle residence times can be prolonged with down-draft F-tier secondary air. Figure 6 also shows that increasing the angle raises slip velocities between the air and particle in the F-tier air-flow zone and below. Moreover, particle dynamics are very sensitive to changes in secondary air angles. When angles 5851
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Figure 13. Horizontal rms fluctuation velocity distributions for air in the furnace.
Figure 14. Horizontal rms fluctuation velocity distributions for particles in the furnace.
Y/Y0 = 0.25, with the supply of E- and F-tier secondary air, the horizontal velocity distribution curves begin to separate from one another. At cross-section Y/Y0 = 0.306, the horizontal velocities for angle 0° are higher than the other angle settings. Between cross-sections Y/Y0 = 0.363 and 0.419, the horizontal velocities for angle 15° rise to the same level as those for 0°. At cross-section Y/Y0 = 0.476, the highest horizontal velocities are for angle 15°; those at 25° and 35° begin to exceed those for 0°. At cross-section Y/Y0 = 0.533, the highest horizontal velocities occur for angles 25° and 35°, while at Y/Y0 = 0.588, the horizontal velocities for angles 25° and 35° are the highest. In the final two crosssections, horizontal velocities for angle 45° rise and attain their highest values. The above phenomenon reveals overall that, as the angle setting increases, the zone lowers under the influence of the larger volume of secondary air. Larger horizontal velocities more easily result in the reversal of fuel-rich flow. Thus, it can also be concluded that, when angles increase, the position where this flow reversal begins drops and particle penetration depth within the furnace increases. With regard to coal burnout, this would be advantageous in the full-scale furnace. Another important factor in evaluating a flow field is whether field conditions will cause slagging in the furnace. Experience with tangential-firing shows that the zone between the primary air/coal flow and wall is often at low pressure, which allows primary air/fuel to flow easily and wash over the side wall, causing slagging. Figure 9 graphs
positions of maximum Vy value for particles with respect to cross-section. These positions indicate the trajectory of the fuel-rich flow in the down-fired furnace. Above cross-section Y/Y0 = 0.306, all five trajectories coincide but begin to separate below this level. With an increasing angle, the trajectory slants slightly toward the wall. However, on the whole, both fuel-rich flows slant toward the furnace center (positive X direction) and neither washes over the side wall. The significance is that the probability is slight that particles reach the side wall and cause slagging. The reason for this is that a certain amount of secondary air is fed into the furnace from the wall under the arch and allows the zone between the wall and fuel-rich flow to maintain a relatively larger pressure than the furnace center area. When the fuel-rich flow advances into the furnace hopper zone, angles between the flow and hopper wall are now significantly different because the hopper is bevelled. Also from Figure 9, when the angle setting is 35° and 45°, the flow trajectory is directed toward the hopper, which may lead to a large probability that particles will reach the hopper wall and promote slagging. While the angle is at 25° or less, the trajectory is directed away from the furnace hopper and reduces conditions in which slagging forms. Figure 10 shows the angular dependence of particle velocities along the trajectories of the fuel-rich flow. The velocity angle is defined as the angle between the velocity direction and the horizontal. This figure indicates that, above crosssection Y/Y0 = 0.306, all velocity angles are around 80°, with 5852
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Figure 15. Particle size distributions in the furnace. Figure 16. Particle volume flux distributions in the furnace.
the reason being that the burner nozzle is oriented 10° from the vertical. Below this cross-section, the particle velocity angles begin to fall from the impact of secondary air. With increasing angle setting, the impact of secondary air on the fuel-rich flow becomes weaker and, as a result, the fall in the particle velocity angle slows. The figure also shows that, with angle settings of 35° and 45°, the velocity angle of particles in the fuel-rich flow within the furnace hopper zone is larger than 50°. Because the angle between the hopper and the horizontal is 55°, the probability that particles wash over the hopper wall is large. However, when angle settings are less than 25°, this velocity angle is less than 41° in the furnace hopper zone and the possibility that slagging occurs in the hopper is reduced. In view of both penetration depth of the fuel-rich flow within the furnace and slagging within the furnace hopper, the optimal angle setting is 25° for F-tier secondary air. 3.2. Distribution of Gas/Particle Root-Mean-Square (rms) Fluctuation Velocities. Figures 11 and 12 show distributions of vertical rms fluctuation velocities for gas and particles, respectively, in the furnace. Vertical velocity distributions for both are very similar (see Figures 4 and 5). At crosssections Y/Y0 = 0.08 and 0.137, fuel-rich and fuel-lean peaks appear in the profiles. Below these cross-sections, only the fuel-rich peak remains. This is because, in the down-fired boiler, velocity fluctuations in the vertical direction for the main part are caused by high-velocity vertical air flows. No other factors can affect the vertical fluctuation. Thus, where vertical velocities are large, vertical fluctuations are also large.
Figures 13 and 14 show the distributions of horizontal gas and particle rms fluctuation velocities, respectively. Between cross-sections Y/Y0 = 0.08 and 0.193, there is an obvious peak in the profile caused by the fuel-rich flow. After the high-velocity flow is issued from the nozzle, it shears with the low-velocity air nearby and diffuses into the surroundings. The shearing and diffracting brings large horizontal fluctuations over the boundary between the fuel-rich flow and the surrounding air. In the following profiles, this peak is not so significant because, as fuel-rich flow advances downward, it rapidly diffuses and quickly attains the same velocity as the air nearby. The small velocity difference weakens shearing, and the peaks almost disappear. Another phenomenon shown in the figures is that, apart from the peak zone caused by the fuel-rich flow, the horizontal rms fluctuation velocities at the furnace center are always higher than along either side of the center for all profiles. The reason is air and particles from both sides of the furnace colliding with each other at the furnace center. Thus, in this area, air and particle velocities fluctuate greatly in the horizontal direction. The figures also show that, in the furnace hopper zone (between cross-sections Y/Y0 = 0.677 and 0.792), the horizontal rms velocity rises with the increasing angle setting. Thus, downward-directed F-tier secondary air can intensify flow fluctuations in the furnace hopper zone. With regard to combustion, the expectation is that, with large fluctuation in the furnace hopper zone, heat and mass transfers will be strong and combustion will intensify. Such effects would be advantageous if the furnace volume was fully used. However, 5853
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the furnace, the peak becomes less pronounced. This is because the large particles also spread into the surroundings, which lowers the particle size differences between the fuelrich zones and surroundings. Figure 16 shows the distribution of particle volume flux in the furnace. Similar to the velocity distribution, in the first few cross-sections, two peak zones also appear in the profiles. It can be observed that these two peaks are precisely at the same positions as the vertical velocity peaks (see Figures 4 and 5). Hence, the higher peak is caused by the particles in the fuel-rich flow, and the lower peak is caused by the particles in the fuel-lean flow. Further down the furnace, only one peak has remained, which was caused by the fuel-rich flow. Figure 17 depicts particle number concentration distributions in the furnace. At cross-section Y/Y0 = 0.08, the values of the particle number concentration in the fuel-rich zone (around X/X0 = 0.1) and the fuel-lean zone (around X/X0 = 0.15) are at a minimum. This phenomenon indicates that, although the fuel-rich and fuel lean flows have high particle volume fluxes, they can be attributed to large particles. These particles occupy large volumes but are a small fraction of the total particle number. A great number of small particles diffuse quickly to the surroundings. As a consequence, the particle number concentrations in these two zones are low. Because the area between the fuel-rich flow and the wall (X/X0 = 0 to 0.1) is small but contains a relatively large number of small particles spreading from the fuel-rich flow, the particle number concentration in this region is especially high. 4. Conclusion Figure 17. Particle number concentration distributions in the furnace.
A PDA system was used to investigate the gas/particle flow characteristics of a small-scale furnace that had been modeled on a 300 MW full-scale down-fired boiler. The influence of the F-tier secondary air angle on the aerodynamic field was also investigated. The results of these experiments will be beneficial to the design and operation of similar boilers. Specific conclusions are as follows: (1) In a down-fired furnace, a “W”shaped flow can be formed. Under the arches, a large ring recirculation zone forms, providing favorable conditions for fuel ignition. (2) With low momentum and particle load, the fuel-lean flow decays rapidly for all angle settings investigated. Fuel-rich flow can penetrate deep within the furnace, but particles can penetrate to a greater depth than air. Downwarddirected F-tier secondary air barely influences the area above the air-flow zone of the F tier. (3) With an increasing F-tier secondary air angle, penetration depths for both primary air and particles within the furnace are increased. Residence times for particles in the furnace can be prolonged, which is advantageous for coal burnout. (4) With an increasing F-tier secondary air angle, flow fluctuations in the furnace hopper zone intensify and the probability for particles to wash over the hopper wall increases, promoting slagging in the hopper. (5) Particles with small diameters spread more rapidly from the fuel-rich and fuel-lean flows than the larger sized particles. (6) A setup with a F-tier secondary air angle at 25° offers the optimal choice to reduce slagging.
if the combustion is too strong, slagging may occur within the furnace hopper. At cross-section Y/Y0 = 0.792, fluctuations with angle settings of 35° and 45° are much higher than for other angle settings, suggesting that with all likelihood slagging would result. With these considerations, an angle setting of 25° appears optimal for F-tier secondary air. 3.3. Distribution of Particle Size, Particle Volume Flux, and Particle Number Concentration. Figure 15 shows particle mean diameter distributions within the furnace. This particle mean diameter is the arithmetic mean diameter for particles ranging from 0 to 100 μm. Similar to the vertical velocity distribution, the particle size distribution also has its fuelrich and fuel-lean peaks between cross-sections Y/Y0 = 0.08 and 0.137. In the region from Y/Y0 = 0.137 to 0.476, fuellean flow has already stopped moving down and reverses upward; thus, only one peak remains. The position of the particle size peak is almost the same as that of the vertical velocity peak. This indicates that, in the fuel-rich and fuellean flows, the particle mean diameter is higher than that in other parts of the furnace. This occurs because, after the particles are fed into the furnace, smaller particles are dispersed quickly into the surroundings, while larger particles still remain in the fuel-rich and fuel-lean flows because of a larger inertia. The diffusion of small particles from air/ particle flows to the surroundings raises the particle mean diameter in the fuel-rich and fuel-lean zones and lowers it in other parts of the furnace. As the fuel-rich flow advances into
Acknowledgment. This work is sponsored by the Hi-Tech Research and Development Program of China (863 program) (Contract 2006AA05Z321).
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