Particle Flows in a Down-Fired

Purchase temporary access to this content. ACS Members purchase additional access options · Ask your library to provide you and your colleagues site-w...
0 downloads 0 Views 2MB Size
Download PDF
Energy Fuels 2010, 24, 3498–3509 Published on Web 06/02/2010

: DOI:10.1021/ef100247v

Experimental Investigations into Gas/Particle Flows in a Down-Fired Boiler: Influence of Secondary Air Momentum Feng Ren,† Zhengqi Li,*,† Zhichao Chen,† Qunyi Zhu,† and Guohua Yang‡ † School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China, and ‡Marine College, Ningbo University, 818 Fenghua Road, Ningbo 315211, People’s Republic of China

Received March 4, 2010. Revised Manuscript Received May 6, 2010

To investigate characteristics of air/particle flow in down-fired boilers, experiments were carried out with a small-scaled furnace for a down-fired pulverized-coal 300-MWe utility boiler on a two-phase test facility using a phase Doppler anemometer. The experiments are carried out with three different E-tier secondary air velocity settings. With these settings, the distributions of the mean velocity, root-mean-square fluctuation velocity, and particle volume flux in the furnace are investigated. The results show that fuelrich flow cannot penetrate the F-tier airflow zone, thus making the particle residence time in the furnace relatively short. When the E-tier secondary air velocity was set at a proper value of half of the F-tier velocity, the depth for the particles in the down-fired furnace will not be shortened, while slagging on the side wall can be avoided. Under these conditions, particle concentrations in the fuel-rich flow will undergo little influence.

For down-fired furnaces, Che et al. have 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 Ren et al. studied the influence of the secondary air damper opening.3 Until now, few measurements of the gas/solid two-phase flow characteristics in down-fired furnaces have been reported. A phase Doppler anemometer (PDA) is a useful tool for simultaneous measurements of the motion of liquid and solid particles in two-phase flows.10-13 Li et al. investigated the gas/particle flows for radial bias combustion swirl burners using a three-dimensional PDA.14-16 Fan et al. measured particle interaction in the turbulent boundary layer for crossflow over a tube.17 Chen et al. discussed the particle volume flux at various cross sections within a central fuel-rich burner, a radial bias combustion burner, and volute burners.18,19 Lin et al.

1. Introduction In China, there is a large amount of anthracite and lean coal, which provide more than 40% of the generated electricity. Down-fired combustion technology, being one of the processes applied for burning hard-to-burn coal, is developing very quickly in China. It enhances the coal burnout rate by increasing the particulate residence time in the furnace. However, practical down-fired boiler operations still suffer from the problem of high carbon content in the fly ash. To find the cause of this problem, Fan et al. performed research on the combustion characteristics of anthracite and obtained some useful conclusions.1 Li et al.2 and Ren et al.3,4 have made some efforts to lower the carbon content in the fly ash by performing experiments on a full-scale boiler. Fan et al. have performed some numerical simulations to predict the combustion characteristics in a down-fired boiler.5,6 However, to achieve a full understanding of why unburnt carbon in the fly ash is high, it is also necessary to investigate the aerodynamic field of the down-fired furnace. In practical situations, process parameters such as the velocity and turbulence intensity in a full-scale furnace are almost impossible to obtain. Therefore, to analyze the influence of the 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.

(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. Chinese 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 Univ. 2001, 1, 108-110 (in Chinese). (10) Eskin, D. Chem. Eng. Sci. 2005, 60, 655–663. (11) Marko, L.; Pasi, M.; Ville, A.; Juhani, A. Chem. Eng. Sci. 2007, 62, 721–740. (12) Morud, K. E.; Hjertager, B. H. Chem. Eng. Sci. 1996, 61, 233– 249. (13) Su, Y. X. Chem. Eng. Sci. 2006, 61, 1505–1514. (14) Li, Z. Q.; Sun, R.; Chen, L. Z.; Wan, Z. X.; Wu, S. H.; Qin, Y. K. Fuel 2002, 81, 829–835. (15) Li, Z. Q.; Sun, R.; Wan, Z. X.; Sun, S.; Wu, S. H.; Qin, Y. K. Combust. Sci. Technol. 2003, 175, 1979–2014. (16) Li, Z. Q.; Chen, Z. C.; Sun, R.; Wu, S. H. New low NOx, low grade coal fired swirl stabilized technology. J. Energy Inst. 2007, 80, 123–130. (17) Fan, J. R.; Shi, J. M.; Zheng, Y. Q.; Cen, K. F. Chem. Eng. J. 1997, 66, 201–206. (18) Chen, Z. C.; Li, Z. Q.; Wang, F. Q.; Jing, J. P.; Chen, L. Z.; Wu, S. H. Fuel 2008, 87, 2102–2110. (19) 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.

*To whom correspondence should be addressed. Tel.: þ86 451 8641 8854. Fax: þ86 451 8641 2528. 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) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Fuel 2007, 86, 2457–2462. (3) Ren, F.; Li, Z. Q.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (4) Ren, F.; Li, Z. Q.; Jing, J. P.; Zhang, X. H.; Chen, Z. C.; Zhang, J. W. Fuel Process. Technol. 2008, 89, 1297–1305. (5) Fan, J. R.; Liang, X. H.; Xu, Q. S.; Zhang, X. Y.; Cen, K. F. Energy 1997, 22, 847–857. (6) Fan, J. R.; Jin, J.; Liang, X. H.; Chen, L. H.; Cen, K. F. Chem. Eng. J. 1998, 71, 233–242. r 2010 American Chemical Society

3498

pubs.acs.org/EF

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 2. Cold-flow experimental system.

pulverized coal to meet the requirements of a PDA measurement.19 Stokes and Froude criterions are also taken into consideration, though they are hard to satisfy. Momentum ratios and particle loads within the air/particle flows of the small-scale furnace are the same as those of a full-scale one. The specific details are described elsewhere.21 A two-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 autocoordinated rack. A PDA is an instrument based on phase Doppler anemometry, which is an extension of laser Doppler anemometry. The velocity is measured from the frequency of the Doppler burst as for laser Doppler anemometry.22 Using a PDA, the velocity, size, and concentration of a two-phase flow can be measured.23-26 The principle and accuracy of the measurements have been described in detail elsewhere.21 During the experiment, some of the smaller particles were lost because of the low efficiency of the cyclone separator. The particle material had to be frequently renewed to maintain the same particle size distribution as closely as possible. Particles with diameters from 0 to 8 μm were used to trace the airflow, and particles with diameters from 10 to 100 μm were used to represent particle phase flow. Particles with diameters between 0 and 100 μm were used for analysis of the particle volume flux. Altogether, three situations were evaluated: the secondary air velocity of tier E was set to (1) 2.14 m/s, (2) half that of tier F, and (3) the same value as tier F. The specific parameters are listed in Table 1.

Figure 1. Schematic for the furnace of the 300MWe down-fired boiler.

investigated the flow field in a tangential fired furnace using a PDA.20 Here, a PDA system was used to investigate the gas/particle flow characteristics of a small-scale furnace modeled on a 300MW full-scale down-fired boiler designed by the FosterWheeler technique. Because the secondary air momentum is one of the most important factors that affect the aerodynamic field in the down-fired furnace, the influence of the E-tier secondary air velocity is also investigated. 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 300MW furnace. The arches divide the furnace into two parts. The part below the arches is the fuel-burning zone, and that above is the fuel-burnout zone. Cyclone burners are set on the arches to organize the W-shaped flame. By the centrifugal effect, the cyclone burner separates the primary air/coal stream into two flows: the fuel-rich flow and the fuel-lean flow. 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 the design and operation are described elsewhere.2,3 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 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. The measurement error for the airflow rate was 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 airflow into the small-scale furnace. This small-scale furnace, which was used in cold-flow experimental work, is modeled on the equipment described above at a ratio of 1:15. In the cyclone burner of the full-scale furnace, there is always a special device to eliminate the swirl in the fuel-rich flow, leading the fuel-rich flow direct. Thus, to simplify the experimental method, the original cyclone burner were replaced by fuel-rich and fuel-lean pipes. All streams (both primary and secondary air) flowing into the furnace are self-modeling. The glass beads with a mean diameter of 42 μm are used instead of

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 coordination is set at the same level as the exit of the fuel-rich nozzle. Y0 is the height and X0 is the width of the down-fired furnace (see Figure 2). Within the furnace, the region extending between cross sections, Y/Y0 = 0.153 and 0.194, (21) Ren, F.; Li, Z. Q.; Chen, Z. C.; Xu, Z. X.; Yang, G. H. Energy Fuels 2010, DOI:10.1021/ef901243c, in press. (22) Pickett, L. M.; Jackson, R. E.; Tree, D. R. Combust. Sci. Technol. 1999, 143, 79–106. (23) Aı´ sa, L.; Garcia, J. A.; Cerecedo, L. M.; Garcı´ a Palacı´ n, I.; Calvo, E. Int. J. Multiphase Flow 2002, 28, 301–324. (24) H€ ubner, A. W.; Tummers, M. J.; Hanjalic, K.; vander Meer, Th. H. Therm. Fluid Sci. 2003, 27, 481–489. (25) Moon, S.; Bae, C.; Choi, J.; Abo-Serie, E. Fuel 2007, 86, 400–409. (26) Sommerfeld, M.; Qiu, H. H. Int. J. Multiphase Flow 1993, 19, 1093–1127.

(20) Lin, Z. C.; Fan, W. D.; Li, Y. Y.; Li, Y. H.; Zhang, M. C. Energy Fuels 2009, 23, 744–753.

3499

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Table 1. Parameters for All Air Inlets in the Three Setups velocity (m/s)

particle load (particle/air, kg/kg)

setup

setup 1: Ev = 2.2 m/s

setup 2: Ev = 0.5Fv

setup 3: Ev = Fv

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

31.4 10.02 6.9 13.7 0.55 0.10

31.2 9.3 11.9 11.9 0.56 0.10

Figure 3. Vertical velocity distributions for air in the furnace.

is the airflow zone of the D tier, while that extending from Y/Y0 = 0.194 to 0.249 is the airflow zone of the E tier and finally that from 0.339 to 0.513 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 3 and 4 display vertical velocity distributions for both air and particles within the furnace. They show that, at all cross sections in each of the three 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. At cross sections Y/Y0 = 0.08 and 0.137, there are two peaks in the profiles. The higher peaked region is the fuel-rich flow zone; the lower one is the fuel-lean flow zone. Below the cross section Y/Y0 = 0.193, only the fuel-rich flow peak zone remains, indicating that the fuellean flow decays rapidly. The reason for this is that the low

momentum and particle load makes the fuel-lean flow diffuse very quickly, and in a short time, it has the same low velocity as the air nearby. Thus, this particle/air mixture flow has little influence on the whole flow field. Alternatively, it can be seen that the fuel-rich flow has an influence very deep into the furnace and surges through the furnace center as it flows downward. Because particles in the fuelrich flow account for 96% of the total particle weight, flow and combustion characteristics in the down-fired boiler determine how the fuel-rich flow behaves. Figures 5 and 6 show horizontal velocity distributions for air and particles in the furnace. For the three velocity settings, the figures show that, in the region identified by the first three cross sections from Y/Y0 = 0.08 to 0.193, there are also fuel-rich and fuel-lean zone peaks, similar to the vertical velocity distributions. This is because, at these three cross sections, 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 these three cross sections is that, in zones between the fuel-rich zone and the furnace centers, most of the velocities are negative. This indicates that the air in these 3500

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 4. Vertical velocity distributions for particles in the furnace.

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 upflowing gas toward the primary air/fuel and ensure sufficient heat for the air/fuel flow for a timely ignition. Below cross section Y/Y0 = 0.25, with the supply of E-tier secondary air, the horizontal velocity near the wall (X/X0 = 0) rises quickly and exceeds that in the fuel-rich and fuel-lean zones for the second and third situations where the E-tier secondary air velocity is half of the F tier and the same as the F tier, respectively. Thus, no obvious peak can be found. The cross section Y/Y0 = 0.306 is between the E- and F-tier airflow zones, and no secondary air is fed into this area. Also at this cross section, the fuel-lean flow has already stopped moving down air; the average values of the horizontal velocities in these three cross sections reverse direction upward. Thus, only the fuel-rich peak remains. 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 cross sections are much larger than those in the previous cross sections. Because the fuel-rich flow has a much lower horizontal velocity than the F-tier 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 secondary air spreads, outside the position X/X0 = 0.2, velocities rise. 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 fuel-rich 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. Because in each cross section the maximum vertical velocity (y-direction velocity component) appears in the fuel-rich flow zone, we define Vymax as the y-direction velocity component for the fuel-rich flow in this cross section. Then decay curves for the dimensionless velocity Vymax/Vy0 in different cross sections can be used for the description of the depth that the fuel-rich flows reach into the furnace. Vy0 represents the y-direction velocity component at the outlet of the fuel-rich nozzle. The distance between the point at which Vymax/Vy0 = 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 7 shows the vertical velocity decay curves of the fuel-rich gas/particle flows for the three setups with different E-tier secondary air velocities. It can be observed that in all three setups mixing flows decay very slowly at first. However, below the dimensionless depth Y/Y0 = 0.193, with the feeding of E- and F-tier secondary air, the decay became much faster. Prior to depth Y/Y0 = 0.505, all flow stopped going down and reversed upward. This shows that the E-tier secondary air more or less consumes the momentum of the fuel-rich flow, while the large quantity of F-tier secondary air totally blocks the fuel-rich flow from continuing downward. The fuel-rich flow cannot penetrate the F-tier airflow zone (Y/Y0 = 0.339 to 0.513), leading to a relatively shallow depth into the furnace. This conclusion agrees with that obtained in 3501

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 5. Horizontal velocity distributions for air in the furnace.

Figure 6. Horizontal velocity distributions for particles in the furnace.

3502

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 7. Decay curves for the fuel-rich air/particle flow in the furnace.

Figure 8. Trajectory of the fuel-rich flow.

Table 2. Dimensionless Depth for Air and Particle Penetration into the Furnace setup dimensionless depth for air dimensionless depth for particles

time in the furnace. Only when the amount of E-tier secondary air fed is too large does the residence time for the particle in the furnace shorten, which is disadvantageous for coal burnout. Considering the situation in a full-scale furnace, feeding of a certain amount of secondary air into the furnace from E-tier secondary air ports can achieve the goal of a gradual supply of oxygen, which is beneficial in the combustion of pulverized coal. Thus, the recommendation is that the velocity of the E-tier secondary air should be set to half of that of the F tier so that both the coal residence time and the graduation combustion time can be ensured. It is quite certain that cold modeling with isothermal conditions cannot accurately describe the complex physical and chemical progress of fuel combustion in the furnace. Thus, for prediction of 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 with 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 block from a continual downward flow 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 laboratory-scale 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 phases smaller. Thus, during extrapolation of the results to the combustion environment, the specific data from the cold-flow 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 down-fired furnace than air, and a certain amount of E-tier secondary air will not influence much the particle residence time in the furnace are still valid. 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, causing slagging. Figure 8 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 down-fired furnace.

Ev = 2.2 m/s Ev = 0.5Fv Ev = Fv 0.38 0.39

0.35 0.38

0.29 0.32

the single-phase flow experiment on this kind of down-fired boiler performed by Li et al.3 From some numerical simulations on the flow field in full-scale down-fired boilers, it also shows that the air/fuel flow stops flowing downward before the coordination Y/Y0 = 0.5 and cannot reach a deeper position.5 The short depth for the fuel-rich flow to reach in the furnace will result in the short residence time for the particle in the furnace and the high carbon content in the fly ash. From this particle behavior, it shows that right after the fuel-rich flow has issued from the nozzle, particle velocities fall behind gas velocities. That is because at this stage particles are still at low velocity and are being accelerated by the primary air. As the fuel-rich stream continues flowing downward, particle velocities decay more slowly than those for air because their inertia is much larger in all three cases. After reaching the F-tier airflow zone, particle velocities begin to exceed air velocities, so that particles can penetrate deeper than air in the furnace. The specific depths that air and particles reach are listed in Table 2. Figure 7 also shows that, with an increase of the E-tier secondary air, velocity decays became faster for both air and particles. When velocities at the E-tier reach half of that at the F tier, i.e., the second setup, the air velocity of the fuel-rich flow decays much more quickly than that in the first setup when the E-tier secondary air is 2.2 m/s. Meanwhile, the particle velocity decay curves of these two setups show much less difference. Only when the E-tier secondary air velocity reaches the same value as that for the F tier does the particle velocity decay much more rapidly and show a great difference compared to the other two setups. This suggests that, for the different inertia components in the fuel-rich flow, particles are less sensitive to the E-tier secondary air velocity than air is. Table 2 shows a similar situation. In the first two cases, the depths for the particle to reach in the down-fired furnaces are almost the same. Only in the third setup does the depth for the particles become shallower and the residence time shorten. All of the above indicates that a certain amount of E-tier secondary air will not influence much the particle residence 3503

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 9. Vertical rms fluctuation velocity distributions for air in the 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 wall and fuel-rich flow to maintain a relatively larger pressure than the furnace center area. With an increase of the E-tier secondary air, the fuel-rich flow is funneled into the furnace center, further reducing the chance for slagging. A certain amount of E-tier secondary air is advantageous in reducing the slagging on the furnace wall. 3.2. Distribution of Gas/Particle Root-Mean-Square (rms) Fluctuation Velocities. Figures 9 and 10 show distributions of vertical rms fluctuation velocities for gas and particles in the furnace. Vertical velocity distributions for both are very similar (see Figures 3 and 4). At cross sections 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 most of the velocity fluctuations in the vertical direction is caused by the high-velocity vertical airflows. No other factors can affect the vertical fluctuation. Thus, where vertical velocities are large, vertical fluctuations are also large. Figures 11 and 12 show the distributions of horizontal gas and particle rms fluctuation velocities. Within cross sections Y/Y0 = 0.08-0.193, 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 lowvelocity air nearby and diffuses into the surroundings. The shearing and spreading bring large horizontal fluctuations

on 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 fuel-rich 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 the other parts in all profiles. The reason is that air and particles from both sides of the furnace impact each other at the furnace center. Thus, in this area, air and particles fluctuate greatly in the horizontal direction. With regard to combustion, 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. Considering this situation, the current display with the fuel-rich flow close to the wall and the fuel-lean flow close to the furnace center (see Figures 1 and 2) is not reasonable. The fuel-rich flow is separated by the fuel-lean flow from the high-temperature furnace center, conditions which do not favor ignition. Thus, retrofitting of the combustion system is needed. This work will be performed in a future study. 3.3. Distribution of the Particle Size. Figure 13 shows particle-mean-diameter distributions in 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 fuel-rich and fuel-lean peaks within cross sections Y/Y0 = 0.08 and 0.137. In the region from Y/Y0 = 0.137 to 0.476, the fuel-lean flow has already stopped going down and reverses upward, thus only one peak remains. The position of the 3504

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 10. Vertical rms fluctuation velocity distributions for particles in the furnace.

Figure 11. Horizontal rms fluctuation velocity distributions for air in the furnace.

3505

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 12. Horizontal rms fluctuation velocity distributions for particles in the furnace.

Figure 13. Particle size distributions in the furnace.

3506

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 14. Particle volume flux distributions in the furnace.

This is because the large particles also spread into the surroundings, which lowers the particle size differences between the fuel-rich zones and surroundings. 3.4. Distribution of the Particle Volume Flux. Figure 14 shows the distribution of the 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 3 and 4). Hence, the higher peak is caused by the particles in the fuelrich flow and the lower peak caused by the particles in the fuel-lean flow. However, this latter peak lasts much longer than the velocity peak. At cross section Y/Y0 = 0.363, the two-peak distribution still persists. This stems from the fact that, although the air/particle velocities soon decay to the same level as the air nearby, particles do not spread to the surroundings that quickly and still concentrate in a relatively small area, maintaining its downward flow until reaching the middle of the E-tier airflow zone. After that, only one peak remained, which was caused by the fuel-rich flow. Figure 15 shows the decay curves of the maximum particle volume fluxes at different cross sections. Because in each cross section the particle volume flux peak appears just at the same positions as the vertical velocity peak, the maximum particle volume flux lies in the fuel-rich flow zone. Thus, these curves indicate the particle volume flux decay along the fuel-rich flow. It shows that in the first stage, without the mixing of secondary air, the particles do not spread much into the surroundings and its volume flux decays slowly. However, after the mixing of secondary air, particles diffuse rapidly into the surrounding environment and its volume flux decays more quickly. When the E-tier secondary air is

Figure 15. Particle volume flux distributions along the fuel-rich flow.

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 those that are small are dispersed quickly into the surroundings while the larger ones still remain in the fuel-rich and fuel-lean flow because of its large 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 moves forward into the furnace, the peak becomes less pronounced. 3507

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

Figure 16. Particle number concentration distributions in the furnace.

increased, the diffusion of particles becomes faster. When the velocity of the E-tier secondary air increases from 2 m/s to half of the F-tier one, the particle diffusion changes slightly. This indicates that supplying a certain amount of secondary air into the furnace from the E-tier secondary air ports has little effect on the concentration of the particles in the fuelrich flow and can ensure timely ignition of pulverized coal in a full-scale furnace. However, when the air velocity increases from a value half that for the F tier to the same value, particle diffusion differs greatly, occurring much more quickly than that in the other two situations considered. This may result in the delay of coal ignition. From the above analysis on the particle penetration into the furnace, slagging on the wall, and particle diffusion, this second setup is the best and preferred choice. 3.5. Distribution of the Particle Number Concentration. Figure 16 depicts particle number concentration distributions in the furnace. At cross section Y/Y0 = 0.08, it shows that 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 large particle volume fluxes, most of them are 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. The area between the fuel-rich and fuel-lean flows presents the same characteristics. A great

number of small particles from the nearby air/fuel flows are concentrated in this small region. 4. Conclusion A PDA system was used to investigate the gas/particle flow characteristics of a small-scale furnace modeled from a 300MW full-scale down-fired boiler. The influence of the E-tier secondary air ratio on the aerodynamic field is also investigated. The results of these experiments will be of benefit for 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 ignition of fuel. 2 With low momentum and particle load, the fuel-lean flow decays rapidly in all situations investigated. Fuelrich flow cannot penetrate into the F-tier airflow zone, but particles can reach deeper into the down-fired furnace than air. 3 When the E-tier secondary air velocity was set at either 2 m/s or half of the F-tier air velocity, particle penetration depths into the furnace were almost the same. Only when velocities were at the same value as the F-tier velocity was the particle depth shallower. A certain amount of E-tier secondary air is advantageous for lessening slagging on the furnace wall. 4 Apart from the fuel-rich flow zone, the horizontal rms fluctuation velocities in the furnace center were always greater than those in other parts. 5 Particles with small diameters spread more rapidly from the fuel-rich and fuel-lean flows than the larger-sized particles. 3508

Energy Fuels 2010, 24, 3498–3509

: DOI:10.1021/ef100247v

Ren et al.

6 Mixing of secondary air made particles in the fuel-rich flow diffuse into the surrounding environment more quickly and its volume flux decay more rapidly. Supplying a certain amount of secondary air into the furnace from the E-tier secondary air ports will have little effect on the concentration of particles in fuel-rich flow; too larger quantities of E-tier secondary air will only

induce a more rapid diffusion of particles in the fuel-rich flow. 7 A setup with the E-tier secondary air velocity at half that of the F tier is the best choice. Acknowledgment. This work is sponsored by the Hi-Tech Research and Development Program of China (863 program; Contract 2006AA05Z321).

3509