Energy Fuels 2010, 24, 1592–1602 Published on Web 02/18/2010
: DOI:10.1021/ef901243c
Experimental Investigations into Gas/Particle Flows in a Down-Fired Boiler: Influence of the Vent Air Ratio Feng Ren,† Zhengqi Li,*,† Zhichao Chen,† Zhenxing Xu,† and Guohua Yang‡ †
School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China, and ‡Marine College, Ningbo University, 818, Fenghua Road, Ningbo 315211, P.R. China Received October 29, 2009. Revised Manuscript Received February 1, 2010
To investigate the characteristics of air/particle flow in down-fired boilers, experiments were carried out using a particle dynamics anemometer (PDA) on a two-phase test facility of a small-scaled furnace for a down-fired pulverized-coal 300 MWe utility boiler. The experiments were carried out with four different vent air ratio settings. With these settings, the distributions of the mean velocity and the root-mean-square fluctuation velocity in the furnace were investigated. The results show that too large a vent air ratio will result in shortened fuel-rich flow penetration in the furnace. This results in an insufficient particle residence time in the furnace, which is disadvantageous for fuel ignition and causes burnout. Conversely, too low a vent air ratio leads to the possibility of slagging and high NOx emission. An optimized vent air ratio was chosen.
To achieve a full understanding of the combustion characteristics of the whole furnace, it was necessary to investigate the accurate aerodynamic field of the down-fired furnace. In practical situations, it is almost impossible to acquire process parameters such as velocity and turbulence intensity in a fullscale furnace. 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. studied the influence of different primary air momentum ratios on the whole furnace flow field.7 He et al. focused on the ratios among different tiers of secondary air.8 Xu et al. placed more emphasis on the influence of the tertiary air ratio.9 Li et al. studied the influence of secondary air damper opening.4 Up until now, for measurements of gas/solid two-phase flow characteristics in downfired furnaces, only Li et al. have performed experiments to study the influence of the secondary air angle.10 No reports have focused on the vent air ratio. A particle dynamics anemometer (PDA) is a useful tool for simultaneous measurements of the motion of liquid and solid particles in two-phase flows.11-14 Li et al. investigated the gas/ particle flows for radial bias combustion swirl burners using a two-dimensional PDA.15-17 Fan et al. measured particle
1. Introduction In China there is a large amount of anthracite. Down-fired combustion technology, one of the processes applied for burning hard-to-burn coals, is developing very quickly and has become a leading process for anthracite firing in the increasingly important central power station market. It enhances coal burnout rate by increasing particulate residence time in the furnace. The up-flowing hot gases provide heat to assist the ignition of the fuel and the stability of the flame (see Figure 1). However, practical down-fired boiler operations still suffer from the problem of unstable flame operation and high carbon content in the fly ash. To find the cause of these problems, Fan et al. performed research on the combustion characteristics of anthracite and obtained some useful conclusions.1 Fan et al. have performed numerical simulations to predict the combustion characteristics in a down-fired boiler.2,3 Li et al. performed experiments on a full-scale boiler.4-6 In Li’s investigations, it was found that the vent air valve is an important factor that affects the flame stability.2 The authors tried to explain this based on the limited measurements collected from the three monitoring ports in the full-scale furnace. However, these data were so limited in the locations and numbers of the measurement points that they were unlikely to reflect the operation characteristics of the whole furnace.
(7) Che, G.; Xu, T. M.; Xu, W. J.; Hui, S. E. J. Eng. Therm. Energy Power 2001, 91, 19-22. (In Chinese) (8) 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) (9) Xu, W. J.; Yan, X.; Sun, X. G.; Hui, S. E.; Xu, T. M. J. Xi’an Jiaotong University 2001, 1, 108-110. (In Chinese) (10) Li, Z. Q.; Ren, F.; Chen, Z. C.; Liu, G. K.; Xu, Z. X. Energy Fuels DOI: 10.1021/ef900679n. (11) Dai, G. Q.; Chen, W. M.; Li, J. M.; Chu, L. Y. Chem. Eng. J. 1999, 74, 211–216. (12) Fu, W. B.; Hou, L. Y.; Wang, L. P.; Ma, F. H. Fuel Process. Technol. 2003, 80, 9–21. (13) Bao, J.; Soo, S. L. Powder Technol. 1995, 85, 261–268. (14) You, C. F.; Zhou, Y. Energy Fuels 2006, 20, 1855–1861. (15) Li, Z. Q.; Chen, Z. C.; Sun, R.; Wu, S. H. J. Energy Inst. 2007, 80, 123–130.
*To whom correspondence should be addressed. Telephone: þ86 451 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) Fan, J. R.; Jin, J.; Liang, X. H.; Chen, L. H.; Cen, K. F. Chem. Eng. J. 1998, 71, 233–242. (3) Fan, J. R.; Liang, X. H.; Xu, Q. S.; Zhang, X. Y.; Cen, K. F. Energy 1997, 22, 847–857. (4) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Fuel 2007, 86, 2457–2462. (5) Ren, F.; Li, Z. Q.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (6) Ren, F.; Li, Z. Q.; Jing, J. P.; Zhang, X. H.; Chen, Z. C.; Zhang, J. W. Fuel Process. Technol. 2008, 89, 1297–1305. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 1592–1602
: DOI:10.1021/ef901243c
Ren et al.
two flows that are separately introduced into the two cyclones in the tangential direction. Because the flow of the fuel/air mixture is swirling in the cyclone, the pulverized coal in the primary air is centrifugally separated. Thus, there is a fuel-rich flow in the peripheral zone and a fuel-lean flow in the central zone. The fuel-rich flow is emitted into the furnace through the burner nozzle. The fuel-lean flow is emitted into the furnace from the vent air pipes. Thus the fuel-lean flow is also called the vent air and the vent air ratio is defined as the ratio of the vent air flux to the total primary air flux. A large vent air ratio leads to a fuel-rich flow with low velocity and a vent air with high velocity. A valve is located on the vent air pipe to control the vent air ratio. 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 secondary air to be fed into the furnace. More specific details on design and operation are described elsewhere.2,3 In theory, to lower NOx emission, the vent air valve should be opened as wide as possible to maximize the air passing through it into the furnace. However, in practical operation, opening the vent air valve wide usually leads to a series of problems such as flame instability.4 Thus, a proper vent air ratio should be selected. The cold flow experimental system is illustrated in Figure 2. It consists of an inducing fan, a powder feeder, a small-scale furnace, and a cyclone separator. When the induced-drawn fan is active, all air is drawn into the small-scale furnace through the primary and secondary air pipes. The airflow flux in these air pipes was measured using Venturi tube flowmeters. Measurement error for the airflow rate was less than 10%. Ordinarily, the diameters of the particles in the fuel rich flow should be larger than those in the fuel lean flow. However, the experiment of twophase flow characteristics in the cyclones shows that the portion of the particle in the vent pipe never exceeds 10% of the total even with the vent air valve is 100% open22 and the particle load of the fuel lean flow is only around 0.10 here. Thus, the influence of the particle in the vent air on the total aerodynamic field in the furnace is little. For the convenience of the experiment, we neglect the size difference between the particles in the two flows. Here, all the glass beads were fed via the powder feeder into the fuel-rich and vent ducts and then carried by the primary air flow into the small-scale furnace. The similarity criteria for particle laden flows are quite complex and difficult to satisfy. Usually we could only ignore some of the criteria of less importance and make approximate modeling. The main criteria are discussed as follow: (1) Geometric similarity: The ratio of the small scale apparatus to the full scale is 1:15. In the original furnace, each fuel rich and fuel lean nozzle has a small annular port around it. These ports are so small that they are very difficult to be modeled in the scale of 1:15. Thus, in the small scale furnace, the area of the annular ports has been added to the corresponding fuel rich or fuel lean nozzle. While calculating the momentum of the flows, the flow momentum of the annular air is also added to the fuel-rich and fuel lean flow. (2) Self-modeling flows: Table 1 lists the Reynolds number of the main flows in the furnace. Since the effects of the E secondary air flows on the aerodynamic field are very slight, the Reynolds numbers of this flow are ignored. It shows that the Reynolds numbers of the fuel-rich flow, vent air, and F-tier secondary air flow for both the small-scale and full-scale boiler are greater than 10 800 and the Reynolds number for the furnace is greater than 53 500. The limited Reynolds numbers for the flows through burner nozzles and the furnace are 10 000 and 30 000. We thus conclude that the flows are self-modeling. (3) Froude criterion: For most of the forced flows with smallsize particles, Froude number shows much less importance than other criterion such as Re. For the modeling of inertia equipment, with the diameters of the particles less than 200 μm, the influence
Figure 1. Schematic for furnace of the 300 MWe down-fired boiler.
interaction in the turbulent boundary layer for cross-flow over a tube.18 Chen et al. discussed the particle volume flux at various cross-sections within a centrally fuel-rich burner, a radial bias combustion burner, and volute burners.19,20 Fan et al. investigated the flow field in a tangential fired furnace using a PDA.21 In this research, a PDA system was used to investigate the influence of the vent air ratio on the gas/particle flow field in a small-scale furnace modeled on a 300 MW full-scale downfired boiler designed on the Foster-Wheeler technique. The results of these experiments will be of benefit in the future design and operation of similar boilers. 2. Experimental Facility Setup Figure 1 shows the schematic view of the 300 MW furnace. The arches divide the furnace into two parts: the lower furnace and the upper furnace. Cyclone burners are set on the arches to form the W-shaped flame. The primary air/fuel first flows downward to a certain point and then reverses up. Most of the up-flowing gases continue flowing up into the upper furnace, while the other gases flow back to the arch to assist in the ignition of the primary air/ fuel. The flow-back part of the gases is the so-called recirculating gas. The fundamental principle of this cyclone burner is fuel bias combustion, which can lower NOx emission compared with other common burners. The configuration of this type of combustion system can also be seen in Figure 1. Through the air/coal inlet pipe, the primary air and pulverized-coal mixture is divided into (16) Li, Z. Q.; Sun, R.; Chen, L. Z.; Wan, Z. X.; Wu, S. H.; Qin, Y. K. Fuel 2002, 81, 829–835. (17) Li, Z. Q.; Sun, R.; Wan, Z. X.; Sun, S.; Wu, S. H.; Qin, Y. K. Combust. Sci. Technol. 2003, 175, 1979–2014. (18) Fan, J. R.; Shi, J. M.; Zheng, Y. Q.; Cen, K. F. Chem. Eng. J. 1997, 66, 201–206. (19) Chen, Z. C.; Li, Z. Q.; Wang, F. Q.; Jing, J. P.; Chen, L. Z.; Wu, S. H. Fuel 2008, 87, 2102–2110. (20) Chen, Z. C.; Li, Z. Q.; Jing, J. P.; Wang, F. Q.; Chen, L. Z.; Wu, S. H. Fuel Process. Technol. 2008, 89, 958–965. (21) Lin, Z. C.; Fan, W. D.; Li, Y. Y.; Li, Y. H.; Zhang, M. C. Energy Fuels 2009, 23, 744–753.
(22) Zhang, J; Li, Z. Q.; Jing, J. P.; Ren, F. J. Eng. Therm. Energy Power 2007, 22, 65-68. (In Chinese.)
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Figure 2. Cold flow experimental system. Table 1. Reynolds Number for the Main Flows of the Furnacea vent air ratio (%)
furnace type
fuel-rich flow
fuel-lean flow
0
industrial scale lab scale industrial scale lab scale industrial scale lab scale industrial scale lab scale
268 773 42 689 224 768 37 824 198 552 30 405 150 802 22 135
70 336 10 857 108 158 16 108 157 900 24 324
15 30 50
a In all the cases, the Reynolds number of F-tier secondary air is 59 995 for the industrial scale and 24 479 for the laboratory scale. That of the furnace flow is 53 519 for the industrial scale and 133 769 for the laboratory scale.
of the Froude number can be ignored. Here for the furnace flow which also has certain amount of swirl, the influence of the Froude is limited and is ignored here. (4) Stokes criterion: Since the most particle Reynolds numbers Rep in the main flow zone of the full-scale furnace are between 2 and 5, the flows are in the transition region (also called Allen region, where 1 < Rep < 750), which is between the Stokes region and Newton region. Then the corresponding Stokes criterion is defined as St ¼
Fp ug 0:725 d 1:725 23:4μg 0:725 lFg 0:275
Figure 3. PDA setup.
Reynolds numbers of the glass beads with this diameter range from 20 to 40 in the main flow zone of the lab-scale furnace, leading the lab-scale flows in the same transition region as those in the full-scale ones. Ordinarily, the particle inertia force is not supposed to be ignored in the transition region, thus the choice of such large diameter in the experiment will make the experimental flows deviate from the original full-scale ones more or less. It could also be calculated that the relaxation times for the particles with diameters of 10 μm and that with diameter of 42 μm are 7.55 10-4s and 1.33 10-2s, respectively, both very small. This means the lab scale experimental results can still indicate the particle motion in the full-scale furnace, though not very accurately. (5) Momentum ratios between various air-flows in the smallscale furnace are the same as those of the full-scale version. Similarly, particle loads within air/particle flows of the smallscale furnace are the same as those for the full-scale version.
ð1Þ
where St is the Stokes number, Fp is the particle density, Fg is the gas density, ug is the gas velocity, d is the particle diameter, μg is the gas viscosity, and l is the characterized length. To ensure the lab-scale particle size meet the requirement of this criterion, the diameter to be chosen should be less than 10 μm. However, particles of this size are impractical in small-scale experiments because of high cost and nonrecoverability. Thus, when considering the feasibility of the experiment, glass beads with a mean diameter of 42 μm were chosen. The estimated particle 1594
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Table 2. Technique Data of PDA System in Operation parameter velocity range (m/s) diameter range (μm) probe volume angle of the receiving optics typical data rate max spherical deviation
Table 3. Parameters for All Air Inlets in the Four Setups
value
vent air ratio (%) 0 15 30 50 velocity (m/s) fuel-rich flow 35.10 31.10 25.00 18.20 fuel-lean flow 0.00 10.40 14.90 22.50 E tier secondary air 2.20 2.20 2.20 2.20 F tier secondary air 15.00 15.00 15.00 15.00 particle load fuel-rich flow 0.49 0.55 0.65 0.81 (particle/air, kg/kg) fuel-lean flow 0.10 0.11 0.11
-10 to 32 m/s 2-130 μm 8.08 10-2 mm3 15° 0.8 kHz 10%
smaller particles were lost due to the low efficiency of the cyclone separator. Thus, fine particles were frequently introduced. Each time after the adding of new particle, measurements were repeated at several old positions to make sure that the particle size distribution does not change much. Particles with diameters from 2 to 8 μm were used to trace the air flow, and particles with diameters from 10 to 100 μm were used to represent particle phase flow. The reason for choosing particles with diameters less than 8 μm representative of the gas phase is that the particle Reynolds number of these particles are less than 1, thus these particle flows are in the Stokes regime that where particles can trace the gas motions tightly. Particles with diameters between 2 and 100 μm were used for the analysis of the particle-volume flux. Altogether four situations were evaluated: the vent air ratio was set to (1) 0, (2) 15%, (3) 30%, and (4) 50%. The specific parameters are listed in Table 3.
A 2D PDA made by Dantec was used in this study. The instrument is composed of laser generator, light splitter, 2D transmitting probe, optical receiver, photoelectric multiplier tube (PMT), Doppler signal analyzer (DSA), and data process system (see Figure 3). One laser line is emitted from the laser generator to the light splitter which split the incoming ray into four green homogeneous light beams, two of which are then shifted to blue by the Bragg cell. Each pair of the colors is used to measure 1D parameter. They are all conveyed to the 2D transmitting probe and focused on the measure point. The light scattered from measurement point is received by two optical receivers. Doppler optical signal included in the scattered light is changed in the PMT and conveyed to the DSA, where it is processed. The principles of velocity and particle size measurement are as follow: 2.1. Velocity Measurement. When two coherent laser beams with the same frequency intersect, they will interfere in the volume of intersection. If the beams intersect in their waists, the interferance produces fringes, and the distance δf between them depend on the wavelength λ and the angle between the incident beams θ: λ ð2Þ δf ¼ 2 sinðθ=2Þ
3. Results and Discussion In the rest of the discussion below, Y denotes the depth along the downward-pointing vertical direction and X the width along the horizontal direction pointing to the right. The origin of the coordinates is set at the exit of the fuel-rich nozzle. Y0 is the height and X0 is the width of the downfurnace (see Figure 4). Within the furnace, the region extending between cross-sections for Y/Y0 = 0.153-0.194 is the airflow zone of the D-tier, while that extending over Y/Y0 = 0.194-0.249 is the airflow zone of the E-tier, and finally from 0.339 to 0.513 is the airflow zone of the F-tier. Gas/particle flow characteristics were measured at cross-sections of Y/Y0 = 0.08, 0.137, 0.193, 0.25, 0.306, 0.363, 0.419, 0.476, 0.533, and 0.588. At each cross-section, data were collected at several points along the X-direction. 3.1. Distribution of Gas/Particle Velocities. Figures 5 and 6 display vertical velocity distributions for both air and particles within the furnace. Vy denotes the vertical component of the velocity in a downward direction, Vx the horizontal component of the velocity toward the right, and Vy0 the velocity at the outlet of the nozzle in each case. They show that at all cross-sections in each of the four velocity settings, the 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 it is directed upward. Thus, a “W” shaped flow is formed in the small scale furnace. Around cross-sections Y/Y0 = 0.08 and Y/Y0 = 0.137, there are two peaks in the profiles. The higher peaked region is the fuel-rich flow zone; the lower region is the vent air flow zone. Below the cross-section Y/Y0 = 0.193, only the fuel-rich flow peak zone remains, indicating that the vent air flow decays rapidly even when the velocity of the fuel-lean flow is at the same level as that of the fuel-rich flow. The reason for this is that without the assistance of large-inertia-particles, the air in the fuel-lean flow interacts easily with the surrounding gases and thus diffuses very quickly, and in a short time has
When a particle is passing through the intersection area, the intensity of light scattered from the particle will vary with a frequency fD proportional to the a component of the particle velocity vi which is in the direction perpendicular to the bisector of the two incident laser beams: λfD vi ¼ fD δf ¼ ð3Þ 2 sinðθ=2Þ 2.2. Diameter Measurement. The difference in optical path length for the reflections from the two incident beams changes with the position of the optical receiver. This means that, when the particle passes through the measuring volume, both receivers receive a Doppler burst of the same frequency, but the phases of the two bursts vary with the angular position of the detectors the phase difference between the two Doppler bursts depends on the size of the particle. Then, the diameter of the particle can be calculated. The other parameters including the root-mean-square (rms) velocity, particle volume flux and particle number can be deduced from the velocity and particle size. More specific content could be obtained in other literatures.23,24 The combination of photomultiplier and particle velocity correlation bias can contribute to uncertainty, but the error is likely to be small. During the experiment, 500 samples were captured to calculate the mean and rms value of the parameters. This number 500 wass chosen from the compromise between measurement accuracy and time consumed. With the confidence of 95% and normal distribution, the statistic random error for the velocity is below (0.75 m/s, while that for particle size is (0.45 μm. Other technique data of PDA system are listed in Table 2 During the experiment, some of the smaller particles were lost due to the low efficiency of the cyclone separator. Some of the (23) Mathiesen, V.; Solberg, T.; Hjertager, B. H. Int. J Multiphase Flow 2000, 26, 387–419. (24) Dantec Dynamics A/S. BSA Low Software, Ver 4.10; Installation & User’s Guide; Denmark: 2006.
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Figure 4. Indication of the measurement area.
Figure 5. Vertical velocity distributions for air in the furnace.
Figure 6. Vertical velocity distributions for particles in the furnace.
the same low velocity as the air nearby. Thus this particle/air mixture flow has little influence on the whole flow field, whatever the vent air ratio. On the other hand, it can be seen
that in all cases, fuel-rich flow has an influence extending (at least to the cross-section 0.419; e.g., the middle of the F-tier airflow zone) into the lower depths of the furnace and surges 1596
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: DOI:10.1021/ef901243c
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Figure 7. Horizontal velocity distributions for air in the furnace.
Figure 8. Horizontal velocity distributions for particles in the furnace.
through the furnace center as it flows downward. However, the depths reached by the fuel-rich flow vary from each other. Because particles in the fuel-rich flow account for more than 90% of the total particle weight, flow and combustion characteristics in the down-fired boiler determine how the fuel-rich flow behaves. Figures 7 and 8 show horizontal velocity distributions for air and particles in the furnace. For these velocity settings, the figures show that in the region identified by the first two cross-sections from Y/Y0 = 0.08 to 0.137, there are fuel-rich and vent air zone peaks (except for the case with vent air ratio at 0), similar to the vertical velocity distributions. This is because at these two cross-sections all air movement in the horizontal direction is caused by the fuel-rich and vent air because no other high velocity airs are fed into the furnace. After this, only the fuel-rich zone remains. Another phenomenon displayed in the first two cross-sections is that in the zones between the fuel-rich zone and the furnace centers, most of the velocities are negative. This indicates that the air in these areas is flowing to the fuel-rich 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 up-flowing gas toward the primary air/fuel and ensure sufficient heat for the air/fuel flow for a timely ignition. In the cross-section Y/Y0 = 0.193, for the cases with the vent air ratio at 30% and 50%, all the horizontal velocities are above 0, which means that no recirculating gas exists to assist the ignition with the primary air in these two cases. In the cross-section Y/Y0= 0.25, recirculating gas exists only when the vent air ratio is 0. From Y/Y0 = 0.306 onward, the recirculating gas exists in none of the cases. These phenomena indicate that the range of the recirculating gases is restricted by the vent air ratio. A
large vent air ratio will more or less prevent the gas from flowing back to the primary air/fuel, disadvantaging the ignition of the fuel. Thus, it can be estimated in the cases with vent air ratios at 30 and 50% that the surrounding environment for fuel ignition is not as good as in the other two cases. In the work by Li et al. on a full-scale boiler,4 it was found that when the vent air valve is 100% open (vent air ratio at 33%), the flame becomes unstable. One reason for this could be the bad environment for ignition. The region with cross-sections from Y/Y0 = 0.363 to 0.419 is in the F-tier airflow zone. With the large quantity of horizontal secondary air, the horizontal velocities in these crosssections are much larger than in the previous cross-sections. Since the fuel-rich flow has much lower horizontal velocity than the F-secondary air, a velocity trough is formed in the fuel-rich zone. The cross-sections Y/Y0 = 0.533 and 0.588 are below the F-tier airflow zone, so the horizontal velocity is very low near the wall. As the secondary air spreads outside the position X/X0 = 0.2, velocities increase. In these two cross-sections, no valley or peak zones are formed in the fuelrich flow zone (around X/X0 = 0.1) indicating that the fuelrich flow cannot influence the area below the F-tier airflow zone. The most important factor affecting the aerodynamic field in a down-fired furnace is the depth that the fuel-rich flow reaches 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. Since in each cross-section the maximum vertical velocity (y-direction velocity component) appears in the fuelrich flow zone, we define Vymax as the y-direction velocity component for the fuel-rich flow in this cross-section. Then the decay curves for the dimensionless velocity Vymax/Vy01 in different cross-sections can be used for the description of the 1597
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Figure 9. Decay curves for the fuel-rich air/particle flow in the furnace.
depth that the fuel-rich flows reach into the furnace. Vy01 represents the y-direction velocity component at the outlet of the fuel-rich nozzle in the case with vent air ratio at 0. The distance between the point at which Vymax/Vy01 = 0.15 and the fuel-rich nozzle in the y-direction is defined as the depth that the fuel-rich flow reaches in the furnace. Figure 9 shows the vertical velocity decay curves of the fuel-rich and vent gas/particle flows for the three setups with vent air ratio. It can be observed that in the first three setups with vent air ratio at 0, 15%, and 30%, mixing flows decay very slowly at first. However, below the dimensionless depth Y/Y0 = 0.137, with the feeding of horizontal E-tier secondary air, the decay became much faster. Unlike the other three cases, with the vent air ratio at 50%, the fuel-rich flow seems to decay at a more constant speed. This is because this fuelrich flow with relatively lower velocity is much easier to diffuse before interacting with the secondary air. In all three setups, prior to the depth Y/Y0 = 0.53, all flow stops going down and reverses upward. This shows that the large quantity of F-tier secondary air can block fuel-rich flow totally from continuing downward. The fuel-rich flow cannot penetrate the F-tier airflow zone (Y/Y0 = 0.339-0.513). From the particle behavior, it shows that right after the fuel-rich flow has issued from the nozzle, particle velocities fall behind gas velocities. This is because, at this stage, particles are still at low velocities and are being accelerated by the primary air. As the fuel-rich stream continues flowing downward, particle velocities decay more slowly than for air because their inertia is much larger in all three cases. After a certain position, particle velocities begin to exceed air velocities and the velocity difference between the two increases as the fuel-rich flow continues downward. For the four setups, the velocity difference between air and particle is larger with the vent air ratio at 30% and 50% than with the ratio at 0 and 15%. This indicates that with the decrease of initial air/ particle velocity, air diffuses more quickly than the particles, due to its low density. This suggests that in the fuel-rich flow, air is more sensitive to the vent air ratio (e.g., the initial velocity at the outlet of the nozzle) than the particles. As the fuel-rich streams flow near the reversal point, the particle
Table 4. Dimensionless Depth for Air and Particle Penetration into the Furnace vent air ratio for fuel-rich flow air particle for fuel-lean flow air particle
0 0.51 0.53
15% 0.48 0.52 0.15 0.16
30% 0.39 0.41 0.17 0.17
50% 0.35 0.40 0.20 0.22
velocity also decays greatly and the slip velocity between air and particle become similar again. However, the particle velocity is still larger than air at this stage so that particles can penetrate deeper than air into the furnace. The specific depths that air and particles reach are listed in Table 4. From the table, an increase in vent air ratio decreases the depth for both air and particles, which implies that particle residence times will be shortened with the increase of the vent air ratio. A short residence time for particles in the down furnace of the full-scale boiler will result in lower temperature of the gases. Thus, the heat carried by the recirculating gas for the ignition of the fuel will be reduced. For the four cases, the dimensionless depths for the air and particles to reach in the cases with vent air ratio at 30 and 50% are much shorter than the other two. This is another reason why the flame is unstable with the vent air valve 100% open. Furthermore, with the vent air ratio at 30% and larger, the short particle residence time in the furnace can also lead to high levels of unburnt carbon in the fly ash. Table 4 also shows that in the case with vent air ratio at 0 and 15%, the depths for the particles are not much different from each other. What’s more, in the 15% case, the fuel-rich flow has larger particle load, which is advantageous for the primary combustion of fuel-rich flow, then it could be estimated that the coal burnout in the 15% at least will not be worse than the total closed case. In Li’s work, it also could be found the conclusion that with the opening of the vent air valve, the NOx could be lowered.4 Thus, comparing the two cases with vent air ratio at 0 and 15%, the latter is better. Li’s work also reveals that 100% opening of the vent air valve, for example, about 30% of the vent air ratio,22 led to intense fluctuation of the negative gas pressure in the furnace and unstable flame. From the discussion above, it can be deduced 1598
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Figure 11. Trajectory of the fuel-rich flow with different vent air ratio.
Figure 10. Vertical velocity distribution as the variation of particle diameter for the fuel-rich air/particle with vent air ratio at 15%.
temperature. Then the differences of the depths for the fuelrich flows among the four vent air valve setting should be sharper than the experimental results. Moreover, with burning of the pulverized coal in the fuel-rich flow, the particle diameter decreases, which makes the slip velocity between the two phase smaller. Thus, when extrapolating the results to the combustion environment, the specific data from the cold experiment should be corrected while the qualitative conclusions that fuel-rich flow cannot penetrate into the F-tier airflow zone, particles can reach deeper into the downfired furnace than air and an increase in the vent air ratio decreases the depth for both air and particles are still valid. Figure 10 shows the variation of the vertical velocity as the changing of particle diameter along the fuel-rich flow with vent air ratio at 15%. It could be found the vertical velocity varies with different particle size. In the first two crosssections, the vertical velocities are always higher with large diameters than those with smaller diameter. The reason for it is that in the exit zone of the fuel-rich flow, the small particles are easier to be accelerated by the high-velocity air. Large particles, due to the influence of inertia force, lag behind. After that, in the cross-sections Y/Y0 = 0.193 to 0.419, with the mixing of horizontal secondary air, the vertical velocities of small particles decay quickly and soon were exceeded by those of large particles. This is because, impacted by the secondary air, the small particles begin to flow in the horizontal direction and stop continuing downward while moving directions of large particles are not so easy to change as the small ones also due to the large inertia force. In a word, large particles respond more slowly to the exterior inspiration than small particles and have longer relaxation time. Another important factor in evaluating a flow field is whether it will cause slagging in the furnace. From experience with tangential-firing, 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,
that the reason for it may be the shallow depth that the fuel rich reaches in the furnace. Thus, the vent air ratio should be not more than 30%. Considering all the factors including coal burnout, NOx emission and flame stability, among the four cases, 15% should be the optimum air ratio. As for the vent air flow, it can be seen both in Figure 8 and Table 4 that although the depths for the air and particles rise with the increase of the vent air ratio, they do not show obvious differences. The vent airs with low particle loadings all diffuse very quickly even with the vent air ratio at 50%. This indicates that particles contribute most of the ability of penetration to the air/particle flow. Without the large-loading particles, vent air itself cannot penetrate very deep into the lower furnace, decaying very quickly and soon reversing upwards no matter how large the air velocity is. Therefore, the residence time for the particles carried out by the vent air in the furnace will be particularly short, which is disadvantageous to carbon burnout. From this point of view, the vent air ratio should not be set too large. It is quite certain that cold modeling with isothermal condition cannot accurately describe the complex physical and chemical progress of fuel combustion in the furnace. Thus, when predicting the flow field in the full-scale furnace by cold modeling, the influence of the temperature variation should also be taken into consideration. When the fuel-rich flow goes down in the lower furnace, its volume expands quickly as the continual increase of the temperature. Thus, the density of the fuel-rich flow also falls together with the flow momentum, which makes the fuel-rich flow easier to be blocked from continually downward by the relatively cold horizontal secondary air. Then it can be concluded that the actual depth for the fuel-rich flow to reach in the full-scale furnace is shallower than the lab-scale results. Also, with the opening of vent air valve, the particle load of the fuel-rich flow can be increased, leading to more rapid rise of the 1599
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: DOI:10.1021/ef901243c
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Figure 12. Vertical rms fluctuation velocity distributions for air in the furnace.
Figure 13. Vertical rms fluctuation velocity distributions for particles in the furnace.
causing slagging. Figure 11 graphs measurement points that have a maximum Vy value for particles in its cross-section. They indicate the trajectory of the fuel-rich flow in the downfired furnace. It shows that in all three setups, the fuel-rich streams decline toward the furnace center (positive X direction) and neither of them washes over the side wall. The significance is that the probability of particles reaching the wall and causing slagging is slight. The reason for this occurrence 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. With the increase of the vent air ratio, the fuel-rich flow is funneled into the furnace center further reducing the chance for slagging. This is because the momentum of the fuel-rich flow falls with the increase of the vent air ratio, leading to a decreasing rigidity of the fuel-rich flow. With low momentum, the fuel-rich flow trajectory will easily lean to the furnace center to be impacted by the horizontal secondary air, reducing conditions in which slag forms. Moreover, a higher momentum of the fuel rich flow (when the vent air ratio is lower) increases entrainment of the ambient air and induces a recirculation along the side wall, which favors deposition of hot particles and thus the formation of slag. From this point of view, the vent air valve should be open as wide as possible. Considering all the facts including the range of the flame stability, the unburnt carbon in the fly ash, the NOx emission, and the slag on the wall, 15% should be chosen as the optimized vent air ratio. 3.2. Distribution of Gas/Particle rms Fluctuation Velocities. Figures 12 and 13 show distributions of vertical rootmean-square (rms) fluctuation velocities for gas and particles, respectively, in the furnace. Vertical velocity distributions
Figure 14. Horizontal rms fluctuation velocity distributions for air in the furnace.
for both are very similar (see Figures 5 and 6). At crosssections Y/Y0 = 0.08 and 0.137, fuel-rich and fuel-lean peaks 1600
Energy Fuels 2010, 24, 1592–1602
: DOI:10.1021/ef901243c
Ren et al.
Figure 16. Particle size distributions in the furnace.
Figure 15. Horizontal rms fluctuation velocity distributions for particles in the furnace.
leads to a large fluctuation in this area. That is the source of large rms turbulence. The reason the fluctuation is larger in the zone X/X0 > 0.3 than in the zone X/X0 < 0.3 is that, in the latter zone, the horizontal secondary only shears the air nearby, but not collides directly as that occurs in the furnace center zone. It can be predicted that with large fluctuations at the furnace center, heat and mass transfers are strong and combustion is most intense in this area. In considering this situation, the vent air ratio should also not be too large so that the fuel-rich flow can face the furnace center directly favoring the absorption of heat from the high-temperature area. Another option is to exchange the current display for a new one with the fuel-rich flow close to the furnace center and the fuel-lean flow close to the wall. This work will be undertaken and reported in future studies. 3.3. Distribution of Particle Size, Particle Volume Flux, and Particle Number Concentration. Figure 16 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 with the vertical velocity distribution, the particle size distribution also has its fuelrich and fuel lean peak 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 flow, 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 flow due to larger inertia. The diffusion of small particles from air/particle flows to the surroundings raises the particle mean diameter
appear in the profiles. Below these cross-sections, only the fuel-rich peak remains. This is because the primary air/fuel flow contains particles with various diameters. The wide size range caused the particles bearing different velocities (shown in Figure 10). Thus, particles with different sizes will have a different dynamics in the gas flow, leading to wide velocity dispersion. As a consequence, large rms occurs for both large particles and small particles. Figures 14 and 15 show the distributions of horizontal gas and particle rms fluctuation velocities. Within the crosssection Y/Y0 = 0.08 to 0.193, there is an obvious peak in the profile caused by the fuel-rich flow. After the fuel-rich stream issues from the nozzle, the high velocity flow shears with the low velocity air nearby and diffuses into the surroundings. The shearing and spreading brings large horizontal fluctuations at the boundary between the fuel-rich flow and the surrounding air. In the following profiles, this peak is not so significant. The reason is that when the fuelrich stream flows downward, it diffuses quickly and rapidly attains the same velocity as the air nearby. The small velocity difference makes the shearing weak and the peaks almost disappear. Another phenomenon shown in the figure 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 other parts in all profiles while in the same zone, the velocity is low. The reason can be explained as follow. The horizontal secondary air jets from both side of the furnace with almost the same momentum flow to the furnace center in two completely opposite directions (as shown in the following figure). They collide with each other intensely in the furnace center in a way that most of the momentum was dissipated there. That is why the mean velocity is low in the furnace center. The great collision also 1601
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: DOI:10.1021/ef901243c
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than the originally fed from the powder feeder, 42 μm. As the fuel-rich flow advances into 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 fuel-rich zones and surroundings. Figure 17 shows the distribution of particle volume flux in the furnace. Similar with 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 5 and 6). Hence, the higher peak is caused by the particles in the fuel-rich flow and the lower is caused by the particles in the fuel-lean flow. Further down in the furnace only one peak has remained, which is caused by the fuel-rich flow. Figure 18 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 = 0-0.1) 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.
Figure 17. Particle volume flux distributions in the furnace.
4. Conclusion
Figure 18. Particle number concentration distributions in the furnace.
A PDA system was used to investigate the gas/particle flow characteristics of a small-scale furnace modeled from a 300 MW full-scale down-fired boiler. The influence of the vent air ratio on the aerodynamic field was also investigated. The results of these experiments will be of benefit to the design and operation of similar boilers. (1) In a down-fired furnace, a “W” shaped flow can be formed. Under the arches, there is a large recirculation zone on either side, which is favorable for the ignition of fuel. (2) With low momentum and particle load, the fuel-lean flow decays rapidly, whatever the vent air ratio. Fuel-rich flow cannot penetrate into the F-tier airflow zone, but particles can reach deeper into the down-fired furnace than air. (3) A large recirculation zone is formed on both sides of the center line of the furnace. The recirculating range of the gases is restricted by the vent air ratio. A large vent air ratio will more-or-less prevent the upflowing gas from flowing to the primary air/fuel, which is disadvantageous for the ignition of the fuel. (4) An increase in the vent air ratio decreases the depth for both air and particles, implying that particle residence times will be shortened with the increase of the vent air ratio, which is disadvantageous for flame stability and burnout of fuel. (5) The fuel-rich flow trajectory will keep leaning to the furnace center under the impact of the horizontal secondary air as the vent air ratio reduces, creating conditions in which slagging forms. (6) A setup with the vent air ratio at 15% is the best choice.
in the fuel-rich and fuel-lean zones and lowers it in other parts of the furnace. Thus, the measured particle mean diameter at the outlet of the fuel-rich nozzle is much larger
Acknowledgment. This work is sponsored by the Hi-Tech Research and Development Program of China (863 program) (Contract No. 2006AA05Z321). 1602
