Experimental Investigations on Performance of Collision-Block-Type

718

Energy & Fuels 2007, 21, 718-727

Experimental Investigations on Performance of Collision-Block-Type Fuel-Rich/Lean Burner: Influence of Solid Concentration Hao Zhou* and Kefa Cen Zhejiang UniVersity, Institute for Thermal Power Engineering, State Key Laboratory of Clean Energy Utilization, Hangzhou, 310027, People’s Republic of China ReceiVed September 1, 2006. ReVised Manuscript ReceiVed December 1, 2006

The influences of solid concentration on the concentrating performance of a collision-block-type fuel-rich/ lean burner was experimentally investigated using a fiber-optic measurement system in a two-phase flow test facility of internal diameter 150 mm. The local solid concentration and particle size distributions were measured to investigate the separation performance of the fuel-rich/lean separator and the mixing performance of the parallel fuel-rich and fuel-lean streams. Three different solid concentrations of 0.31, 0.42, and 0.52 kg/kg were employed in the experiments to investigate the influences of solid concentration on the concentrating performance of the separator. The measurements indicated that the collision-block-type separator always gave good concentrating performance under various inflow solid concentrations. The fuel-rich and fuel-lean streams do not mix together soon after leaving the burner; this leads to better ignition and combustion characteristics of the pulverized coal as compared with the ordinary burner. The concentration ratio between the fuel-rich and fuel-lean streams decreases slightly with the increase in the solid concentration of the primary air, which was caused by inter-particle collisions in the fuel-rich/lean separator.

Introduction Coal remains the major source of energy resources in the world. Coal-fired boiler power stations are now subject to stringent pollution regulations and suffer penalties if the pollution emissions exceed the acceptable level. Furthermore, coal-fired power station efficiency affects the cost of power. Burner structures and parameters have significant effects on the combustion performance in the furnace. In recent years, a novel pulverized coal combustion technology called horizontal fuelrich/lean burner has been employed widely in power station boilers in China, with the benefits of burning low volatile coal more efficiently and decreasing the NOx emissions.1,2 In the conventional primary air of a pulverized coal burner, the mass ratio of the fuel/air mixture in the pneumatic pipe is normally about 0.3-0.6 kg (coal)/kg (air), but the optimum ratio for low volatile coals may be 0.6-1.4 kg (coal)/kg (air) as a result of the need to obtain a stable flame.3 When the horizontal fuel-rich/lean burner is used in a tangentially fired boiler, a fuelrich/lean separator needs to be employed in the fuel conveying line to divide the fuel/air mixture into two streams with various solid concentrations. Facing the high-temperature flame, the fuel-rich stream injects through the fuel-rich nozzle. This stream acts as a powerful stabilizer for its high fuel concentration. The fuel-lean stream injects through the fuel-lean nozzle locating between the fuel-rich nozzle and the furnace waterwall; oxidizing zones can be achieved near the waterwall, resulting in the resistance of the furnace against slagging and fouling by reason * Corresponding author. Tel: +86-571-87952598. Fax: +86-57187951616. E-mail: [email protected]. (1) Zhou, H.; Cen, K.; Fan, J. Experimental investigation on flow structures and mixing mechanisms of a gas-solid burner jet. Fuel 2005, 84, 1622-1634. (2) Zhou, H.; Cen, K.; Fan, J. Detached eddy simulation of particle dispersion in a gas-solid two-phase fuel rich/lean burner flow. Fuel 2005, 84, 723-731. (3) Wei, X.; Xu, T.; Hui, S. Burning low volatile fuel in tangentially fired furnaces with fuel rich/lean burners. Energy ConVers. Manage. 2004, 45, 725-735.

Figure 1. Sketch of collision block type fuel-rich/lean burner.

that oxidizing atmosphere results in a high ash fusion temperature. Seeing that the nonstoichiometry combustion occurs at the cross section of the furnace under fuel-rich/lean combustion conditions, the NOx emissions concentrations can also be reduced, as verified by field data.3,4 Various means have been adopted to separate the fuel/air mixture, such as louver-type,4 collision-block-type,2 and bend-type.3 In the work described here, the investigation focuses on a collision-block-type fuel-rich/lean burner. As shown in Figure 1, the collision-block diverts the primary air flow and makes the pulverized coal particles rebound, thus changing their directions. The coal particles are then carried by the flow to be concentrated in the fuel-rich side over the central partition plate. As a result, the fuel flow is divided into a fuel-rich flow in the upper zone beside the partition plate and a fuel-lean flow in the lower zone, as illustrated in Figure 1a. The fuel concentration ratio between the fuel-rich side and the fuel-lean side can be regulated continuously by adjusting the height of the collision block. The conventional pulverized coal conveying can be considered as dilute two-phase flows. As provided by Elghobashi,5 a two-phase system may be regarded as dilute for solid volume (4) Xu, M.; Sheng, C.; Yuan, J. Two-phase flow measurements and combustion tests of burner with continuously variable concentration of coal dust. Energy Fuels 2000, 14 (3), 533-538. (5) Elghobashi, S. On predicting particle-laden turbulent flows. In Proceedings of the 7th Workshop on Two-Phase Flow Predictions, 1994.

10.1021/ef0604442 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007

Collision-Block-Type Fuel-Rich/Lean Burner

fractions Rp up to 0.1%. In this regime, the influence of the inter-particle collisions can be neglected. In the dense regime (i.e., Rp > 0.1%), the inter-particle collisions become of importance. As mentioned above, the mass ratio of the fuel/air mixture in the pneumatic pipe is normally about 0.3-0.6 kg (coal)/kg (air), corresponding to the typical volume fraction ratios (solid to air) between 0.044 and 0.026%, which means that the particle-particle interactions in the pulverized coal pipe can be neglected. But with the applications of the fuel-rich/lean separator in the coal pipe, local concentrating occurs, especially downstream of the collision-block, the volume fraction ratios (solid to air) in those zones exceed the value corresponding to the dilute two-phase conveying. The gas-solid flow characteristics in the fuel-rich/lean burner and downstream of the burner exit are major criteria to evaluate the performance of the burner and the combusting system. Access to detailed information on the two-phase flow structure is helpful to understand the separation performance of the separator and the interactions between the fuel-rich and the fuellean streams downstream of the burner exit. The availability of local particle concentration and particle size distributions can be employed to evaluate the different design possibilities and to test the theoretical modeling approaches. Measurements of turbulent gas-solid two-phase flow have been conducted in several investigations. Optical techniques and tools for obtaining the measurements of local particle information were developed; These include Laser Doppler Velocimetry (LDV),6 Phase Doppler Anemometry (PDA),4 and fiber-optic sensors.7 The selection of the optical techniques and tools is affected by factors such as solids loading concentration and flow field geometry. In the gas-solid flow with a solid volume fraction greater than a few percent, the optical depth is limited to several particle diameters, which means that measurements using LDV and PDA are impractical in relatively dense suspension. As reported by Woodhead,8 the upper limit of particle volume fraction for light phase pneumatic conveying of pulverized coal measurements with LDV is only 0.03%, due to the light attenuation by the particles, while the typical volume fraction ratios (solid to air) are between 0.044 and 0.026% in the industrial pulverized coal conveying pipe.9 In view of the local concentrating phenomenon in the fuel-rich/lean combustion technology, the volume fraction ratios (solid to air) at many locations will exceed the value of 0.03%. In addition to this, the PDA and LDV methods always require the restriction of the scale ratio of the model burner and clean environments, which results in their inconvenience for measuring industrial scale burner jet in dirty environment. Fiber-optic sensors have offered an alternative measuring method in circulating fluidized beds (CFBs) and pneumatic conveying systems, although they are intrusive. Fiber-optic probes have been employed to measure the velocity and size of particle cluster and voidage in a CFB to identify the flow regimes. Cai et al. used a fiber-optical instrument to conduct an in-line measurement of the coal concentration in the (6) Fan, J.; Zhao, H.; Jin, J. Two-phase velocity measurements in particleladen coaxial jets. Chem. Eng. J. 1996, 63 (1), 11-17. (7) Schallert, R.; Levy, E. Effect of a combination of two elbows on particle roping in pneumatic conveying. Powder Technol. 2000, 107, 226233. (8) Woodhead, S. The measurement of particle velocity and suspension density in pneumatic coal injection system. University of Greenwich, 1992. (9) Giddings, D.; Aroussi, A.; Pickering, S. J.; Mozaffari, E. A 1/4 scale test facility for PF transport in power station pipelines. Fuel 2004, 83, 21952204.

Energy & Fuels, Vol. 21, No. 2, 2007 719

pulverized coal pipe using a light transmission fluctuation method.10 The characteristics of a particle rope in the coal conveying pipe following the elbow have been measured by fiber-optic probes.7,11-13 Laboratory experiments in these cases were performed with pulverized coal and air in a 0.154 m diameter pipe. The particle concentration was determined by the obscuring light over a small distance by comparison with calibration data. The measurements obtained showed the overall pattern of the “rope”sa concentrated stream of coal particles. The present work describes laboratory investigations of fuelrich/lean separation performance of the collision-block-type separator. A fiber-optic measurement system was employed to obtain the solid concentration distributions both in the separator and downstream of the burner exit. Local solid concentration and particle size distributions were obtained. Three different solid concentrations of 0.31, 0.42, and 0.52 kg/kg were employed in the experiments to investigate the influences of solid concentrations on the separation performance. The experimental results showed the mixing mechanism of two parallel gas-solid two-phase flows with large fuel concentration difference. The concentration ratios between the fuel-rich and fuellean streams decrease slightly with the increase in the inflow solid concentration of the primary air, which was caused by inter-particle collisions in the separator. Fiber-Optic Measurement System Measurements of local solids concentrations and mean particle sizes were obtained using a transmissive fiber-optic probe. The probe contained four glass fibers, which were placed in a 25 mm external diameter ceramic tube. Two of the fibers were used to transmit light into the region of particle gas flow (see Figure 2). They were parallel to the axis of the probe and located in holes on the vertical face of an opening notch 12 mm wide. The distance between the centers of fibers 1 and 2 was 7 mm. On the opposite face of the notch, two similar holes were opened to place the other two fibers. Fiber 3 transferred the transmitted light from fiber 1, and fiber 4 transferred the light from fiber 2 to the photodetectors. To avoid having the probe disturb the flow, the probe diameter must be sufficiently small. In the present work, the probe occupied ∼4% of the cross sectional area of the measuring section. Local particle velocities were measured with a crosscorrelation technique using a standard fast Fourier transform (FFT) algorithm. The flight time of particles from the upstream beam to the downstream beam was estimated as follows:

C(τ) )

1 T

∫0T u1(t)u2(t + τ) dt

(1)

where the terms u1(t) and u2(t) are particle flow signal waveforms obtained from the two fiber-optic beams 1 and 2. The flight time of particles (τmax) is the time lag (τ) where the cross-correlation function becomes maximum. Then, local particle velocity can be readily computed from

Up )

L τmax

(2)

(10) Cai, X.; Li, Junfeng; Ouyang, Xin; Zhao, Zhijun; Su, Mingxu. Inline measurement of pneumatically conveyed particles by a light transmission fluctuation method. Flow Meas. Instrum. 2005, 16, 315-320. (11) Bilrgen, H.; Levy, E. Mixing and dispersion of particle ropes in lean phase pneumatic conveying. Powder Technol. 2001, 119 (2-3), 134152. (12) Yilmaz, A.; Levy, E. Formation and dispersion of ropes in pneumatic conveying. Powder Technol. 2001, 114 (1-3), 168-185. (13) Yilmaz, A.; Levy, E. K. Roping phenomena in pulverized coal conveying lines. Powder Technol. 1998, 95, 43-48.

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

Zhou and Cen

Figure 2. Fiber-optic measurement system and detail of fiber-optic probe design.

where L is the optical distance between the laser beams 1 and 2. Information on local particle concentration is obtained from the mean values of the signals with and without particles present:

Ip ) I m - I 0

(3)

where I, the transmissive light signal intensity, is computed from the analog signal u(t) for a time interval T:

I)

1 T

∫0T u(t) dt

(4) Figure 3. Sketch of laboratory facility.

Earlier investigations with fiber-optic probes showed that the amount of light transmitted through a gas-solid flow was linearly dependent upon the volume concentration of particles.10 The fiber-optic probe was calibrated for particle concentration measurements conducted at an axial position z/D ) 18 downstream of the elbow exit in the vertical upflow region of a gas-solid two-phase flow loop. A reasonably uniform particle distribution can be expected over the pipe cross-section at this location.13 The probe was traversed over the pipe cross-section to obtain full information about flow non-uniformities and the average particle concentration. Measurements with the fiberoptic probe were compared with those obtained with an isokinetic sampling device. This sampling device extracted a representative sample of particles to determine the mass flux; the suction velocity at the probe tip could be adjusted so as to be identical to the local gas stream velocity, so that the “isokinetic” condition was achieved. The detailed working principle of the isokinetic sampling probe can be found in ref 14. The comparisons showed that the local particle mass fluxes obtained by the two measurement techniques were in agreement to within 10%. Information on particle size was obtained from analysis of the fluctuation characteristics of the optical signals. Assuming that the particle/beam size ratio is much smaller than one, and therefore that the “boundary effect” can be neglected, the particle size Dp can be estimated as follows:

Dp ≈

x

2Aext B ξh hI , Aext ) Ameas 2 , ξh ) -ln π I0 I φ(ξh)

(5)

0

where Ameas is the cross-section of the measured volume, Aext is the extinction cross-section of a single particle, and ξ is the turbidity, defined as the shadowed fraction of the cross-section (14) Nguyen, T.; Nguyen, A.; Nieh, S. An improved isokinetic sampling probe for measuring local gas velocity and particle mass flux of gas-solid suspension flows. Powder Technol. 1989, 59, 183-189.

NAext/Ameas. Here N is the total number of particles sensed. B is defined as follows:

B ) I02

Ameas φ(ξh) , N h ) ξh N h Aext

(6)

where φ(ξ) is a single-peaked function introduced by Shifrin.15 A more detailed description of the principle of measurement of the local particle size can be found in ref 10. Calibrations of the particle size measurement were made using two kinds of standard-size particles, one 74 µm in diameter and the other 9 µm in diameter. For each particle size, six tests with different particle concentrations were carried out. The results showed that the particle concentration and particle size measurements were nearly independent.16 Experimental Details The pneumatic conveying test facility consists of a 6-m-long horizontal pipe and a 3.5-m-long vertical pipe (see Figure 3). The pipes are made of carbon steel pipes (0.15 m i.d.). A forced-draft fan provided a maximum air velocity of 40 m/s in the pipes. Particles stored in the hopper were discharged into the inlet air stream by means of a screw feeder calibrated for the test solid materials. The metering of the solids flow rate was accomplished by using a variable speed control. A cyclone is used to separate the solid particles from the air. The collected particles are fed to the hopper by an airlock. The remaining fine particles were removed by a fabric filter and then fed back to the feeder hopper so that continuous operation and measurement were possible. Owing to the continuous circulation of particles in the loop, it is necessary to evaluate the attrition rate of the particles. In a series of experiments, the particles were extracted from the loop every 10 h (15) Shifrin, K. Physcial Optics of Ocean Water; AIP Translation Series; AIP: New York, 1988. (16) Cai, X.; Pan, Y.; Wu, W.; Ouyang, X.; Su, M.; Yu, J.; Hu, J. A study of on-line measurement technology for size, concentration and velocity of pulverized coal. Power Eng. (in Chinese) 1999, 19 (6), 466-470.



Table 1. Operational Parameters of the Industrial and Laboratory Burner

industrial laboratory

gas velocity at nozzle exit (m/s) 23 14

exit area (mm × mm)

air temp (°C)

atm pressure (bar)

particle mean diameter (µm)

433 × 325 160 × 120

268 20

0.848 1.0

40 14.3

of operation. Sieve analysis showed no significant attrition for 100 h period covered. After 100 h run of the loop, the particles were renewed. The instrument ports on the wall of the measuring chamber allowed the fiber-optic probe to traverse the chamber cross-section in both the lateral and the axial direction to measure the gas-solid jet flow downstream of the burner exit. For one cross-sectional measurement, more than 120 points were measured using the fiberoptic probe to obtain detailed information about the gas-solid structure of the jet. As mentioned above, the use of fiber-optic sensors is an intrusive measurement technique. The percentage of the cross-sectional area blocked by the probe in this work was ∼4% to avoid having the probe disturb the flow. The full industrial-scale fuel-rich/lean burner studied in the experiments was designed for a 300-MWe capacity utility boiler. It consisted of a collision-block-type concentrator and a coal nozzle in combination. The operational parameters of the industrial scale burner are illustrated in Table 1. Due to the geometrical restrictions of the test rig, the burner must be scaled so that it can be installed in the test facility. A scale ratio of 1/2.708 was employed to make a model burner with a nozzle width of 160 mm and a height of 120 mm. A 3-mm-thick partition plate was located at the center of the nozzle to separate the fuel-rich and fuel-lean streams. The aerodynamic scaling of the industrial burner should follow some similarity criteria, as listed in ref 17: (1) geometric similarity and (2) self-modeling flows. The Euler number Eu does not depend on the Reynolds number Re when Re of the flow is greater than a certain value; that is, the flow enters the “self-modeling flow” region. The Reynolds number is defined as Re )

FUD µ

(7)

where F is the gas density, D is a representative length, U is the gas velocity, and µ is the viscosity of the gas. Re represents the ratio of the inertial force to the viscous force. Eu represents the ratio of the pressure difference to the inertial force and is defined as Eu )

∆p FU

(8)

where ∆p is the pressure difference in the gas between the inlet and outlet of the flow. When the flow enters the self-modeling flow region, the same flow phenomenon can be observed in both burners even though Re of the laboratory scale burner flows are smaller than that of the industrial scale ones. It has been reported that the flow enters the self-modeling flow region if the Re is greater than 104-105.18 Re of the model burner flow in our work was equal to 1.518 × 105, which means that the flow pattern of the turbulent field in the modeling burner was similar to that in the flow field of an industrial burner with a similar geometrical configuration. Additional similarity criteria include (3) boundary condition similarity, (4) the condition that the Froude number remains constant with scale reduction, and (5) the condition that the Stokes number remains constant with scale reduction. The Froude number repre(17) Zhou, H.; Cen, K.; Fan, J. Two-phase flow measurements of a gassolid jet downstream of fuel rich/lean burner. Energy Fuels 2005, 19 (12), 64-72. (18) Cen, K. The Study Method and Measurement Techniques of Boiler Combustion Experiments; Water Conservancy and Electric Power Press: Beijing, 1987 (in Chinese).

Figure 4. Particle size distribution of the talcum powder. Table 2. Laboratory Experimental Cases

case

collision block height (mm)

inflow particle concn (kg/kg)

gas velocity at nozzle exit (m/s)

particle mean diameter (µm)

1 2 3

60 60 60

0.31 0.42 0.52

14 14 14

14.3 14.3 14.3

Table 3. Comparison of the Dimensionless Numbers for Industrial and Laboratory Scales industrial scale Reynolds

Re )

Froude

Fr )

FUD µ U

xDg Stokes St ) Fpdp2U/(18Dµ) temperature (°C) T gas velocity in pipe (m/s) U atm pressure (bar) P 3 particle density (kg/m ) Fp particle mean diameter (µm) dp internal pipe diameter (mm) D

laboratory scale

1.687 × 105 1.518 × 105 12.53

12.53

0.197 268 25 0.848 1200 40 406

0.172 20 15.2 1.0 2700 14.3 150

sents the ratio of inertial forces to gravitational forces on the airflow and is defined as U (9) xDg where g is the gravitational acceleration. The Stokes number St is defined as Fr )

St ) Fpdp2U/(18Dµ)

(10)

where Fp is the particle density and dp is the particle diameter. St determined the density and mean size of the test particles in the laboratory test facility. As a scaling ratio of 1/2.708 was chosen in this work, talcum powder particles with a mean diameter of 14.3 µm, and a true particle density of 2700 kg/m3 was used as the material conveyed to satisfy the similarity requirement of St. The particle size distribution of the talcum powder is shown in Figure 4. Three various solid concentrations in the primary air were employed in this work, as 0.31, 0.42, and 0.52 kg/kg, respectively. The velocities at nozzle exit and in the conveying pipe were determined according to the scaling criteria. The experimental parameters are listed in Tables 1 and 2, and the detailed comparison of the dimensionless numbers for the industrial and experimental scales are presented in Table 3. The collision block height was kept as 60 mm in all cases, which can lead to a good separation performance. The influence of collision block height on the separation performance can be found in refs 19 and 20.


Figure 5. Particle concentration distributions measured in case 1. (a) 50 mm; (b) 100 mm; (c) 200 mm; (d) 400 mm.


Zhou and Cen




Figure 8. Particle size distributions measured in case 1. (a) 50 mm; (b) 100 mm; (c) 200 mm; (d) 400 mm.


Zhou and Cen


Results and Discussion Figure 5a shows a contour plot of the particle concentration measured at 50 mm distance downstream of the nozzle exit with the fiber-optic probe. Some contour labels have been shown in the figures. These measurements, obtained when the particle concentration was 0.31 kg/kg (case 1), showed that a good fuelrich/lean separation was achieved. The peak particle concentration in the fuel-rich stream was 0.447 kg/kg, while the peak concentration in the fuel-lean stream was 0.26 kg/kg, which resulted in a fuel-rich/lean ratio of 1.722 at this cross-section. The fuel-rich/lean ratios were 1.785 and 1.798 at the crosssections 100 and 200 mm, respectively, downstream of the burner exit. With the scaling ratio of 1/2.708 used in the experiments, the location 100 mm downstream of the nozzle exit in the model represents a location 270.8 mm downstream of the exit in the industrial burner. In a real combustion furnace, the coal particles begin thermal pyrolysis and then ignite at this location; such a fuel concentration of the primary air flow will lead to enhancement of the ignition and further combustion of the coal particles as well as a decrease in the NOx emission. The fuel-rich and fuel-lean streams merged at the crosssection at 400 mm, but the location of the peak particle concentration deviated by ∼50 mm from the axis of the nozzle. Compared with the phenomenon of a centrally located particle concentration peak in an ordinary burner jet, such a deviation of the high-concentration zone is favorable for flame stability due to the high-concentration zone of the primary air is directly (19) Fan, J.; Xia, Z.; Zhang, X.; Cen, K. Numerical investigation on two-phase flow in rich/lean pulverized coal nozzles. Fuel 2000, 79 (14), 1853-1860. (20) Zhou, H.; Cen, K. Experimental measurements of a gas-solid jet downstream of a fuel-rich/lean burner with a collision-block-type concentrator. Powder Technol. 2006, 170 (2), 94-107.

faced the upstream hot gas of the chamber in a real combustion case and will result in strong heat transfer between the particles and the upstream hot gas in the chamber in a real combustion case, which can enhance the ignition performance of the particles and the burnout of the coal particles. Some retrofit work using the fuel-rich/lean combustion technique has verified its ability of flame stability and enhancement of burning velocity of coal particles.3 When the inlet particle concentration was raised to 0.42 kg/ kg (case 2), a similar fuel-rich/lean separation performance was obtained as compared with that of case 1. Figure 6 shows the contour plots of the particle concentration measured at 50, 100, 200, and 400 mm distance downstream of the nozzle exit with the fiber-optic probe. At the location of 50 mm downstream of the burner exit, the peak particle concentration in the fuel-rich stream was 0.607 kg/kg, while the peak concentration in the fuel-lean stream was 0.365 kg/kg, which resulted in a fuel-rich/ lean ratio of 1.66 at this cross-section. The fuel-rich/lean ratios were 1.765 and 1.7 at the cross-sections 100 and 200 mm, respectively. The fuel-rich and fuel-lean streams merged at the cross-section at 400 mm, but the location of the peak particle concentration deviated by ∼50 mm from the axis of the nozzle. As mentioned above, the mass ratio of the fuel/air mixture in the industrial pneumatic pipe is normally about 0.3-0.6 kg (coal)/kg (air), but the optimum ratio for low volatile coals may be 0.6-1.4 kg (coal)/kg (air) as a result of the need to obtain stable flame. In case 2, the fuel-rich/lean separator raised the peak solid concentration from 0.42 kg/kg to ∼0.6 kg/kg at the section of 100 mm downstream of the burner exit. Such a high coal particle concentration is favorable for the enhancement of the ignition of the low volatile coals. With an inflow solid concentration of 0.52 kg/kg (case 3), higher peak particle concentrations can be obtained at various




cross sections downstream of the burner exit. As illustrated in Figure 7, the fuel-rich/lean ratio was 1.512, 1.699, and 1.642 at the cross-sections of 50, 100, and 200 mm, respectively; the peak concentrations can be greater than 0.7 kg/kg at these sections. The fuel-rich and fuel-lean streams merged at the crosssection at 400 mm, as similar with those in cases 1 and 2. Contours of the particle size distribution for three cases were also obtained at various cross sections downstream of the burner exit. Figures 8-10 present the mean size distribution in the jet for various cases. The particles on the fuel-rich side always had larger mean diameters than those on the fuel-lean side. Take case 1, for example. The zone of peak particle size exists on the fuel-rich side of the jet flow, and the value of the peak particle size is 21.28 µm at the cross-section 50 mm downstream of the burner exit, which is 1.49 times the mean particle size. The zone with the highest particle size overlaps with the zone with the peak particle concentration, as illustrated in Figures 8a and 5a. At the cross-section 100 mm downstream of the burner exit, the zone of peak particle size is 20.65 µm, 1.44 times the mean size. The measured value at 200 mm is 19.43 µm, 1.36 times the mean particle size. It shows that the separator brought more quantity of the larger particles to the fuel-rich side than fuel-lean side; the larger particles can receive more intensive heating by the upstream flame as a result of facing the high-temperature flame, which can enhance the burnout of the larger coal particles. The measurements also showed that the peak particle size decreases and the particle size distributions become more homogeneous with the development of the burner jet. The two particle size peaks on the fuel-rich and fuel-lean sides merge into one peak at the cross-section 400 mm

downstream, which behaves in the same way as the particle concentration contour plots. Figure 11 shows plots of the particle concentration and mean size in the lateral central lines of the various cross sections. It illustrates that the concentration plots and mean-size plots are in good agreement. Furthermore, it can be found that the twin size peaks were less different than those in the concentration distribution graph. The separation performance of the collision-type-separator was affected by the inlet solid concentrations. As illustrated in Figure 12, the fuel-rich/lean concentration ratios downstream of the burner exit decreased slightly with the increase in the inlet particle concentrations, although the particle concentration contour profiles were similar in various cases. Such a phenomenon may be caused by the inter-particle collisions at the region downstream of the collision block. As shown in Figure 13, there is a high particle concentration ripple downstream of the block because the particles rebounded at the surface of the block. These numerical results, obtained by computational fluid dynamics (CFD) simulations, can be found in detail in ref 2. The particles that did not collide with the block surface have the opportunity to collide with the particles in the high particle concentration ripple and make them enter into the fuel-lean side of the separator. As pointed out by Sommerfeld,21 The interparticle collision probability depends mainly on the particle concentration, particle size, and the fluctuating motion of the particles. The higher the inlet solid concentration, the larger the probability of inter-particle collision. (21) Sommerfeld, M. Validation of a stochastic Lagrangian modelling approach for inter-particle collisions in homogeneous isotropic turbulence. Int. J. Multiphase Flow 2001, 27, 1829-1858.


Zhou and Cen

Figure 13. Numerical simulation results of the particle trajectories in and downstream of the separator.

Figure 11. (a) Local particle concentration and (b) particle mean size profiles in lateral central line.

Figure 12. Influence of inflow solid concentration on the fuel-rich/ lean concentration ratio.

Figure 14 illustrates the particle-particle collision schematic configuration downstream of the collision-block in the fuelrich/lean separator. Figure 14a shows the impact and rebound phenomenon of particles on wall of the collision-block. As pointed out by Jun and Tabakoff,22 the velocity of a rebounding particle is determined using the restitution coefficients, et and en. The rebound particle velocity components, V2t and V2n are then calculated as follows:

V2t ) etV1t

(11)

V2n ) enV1n

(12)

(22) Jun, Y.; Tabakoff, W. Numerical simulation of a dilute particulate flow (laminar) over tube banks. J. Fluids Eng. 1994, 116, 770-777.

Figure 14. Schematic particle-particle collision configuration in the separator. (a) Impact and rebound phenomena of particles on wall. (b) Particle-particle collision schematic configuration.

Owing to the energy loss during the impact process, the rebound velocity is always less than the impact velocity. Grant and Tabakoff obtained the experimental data on the restitution coefficients,23 which had the following expressions:

et ) en )

V2t ) 1.0 - 2.12β1 + 3.0775β12 - 1.1β13 V1t

(13)

V2n ) 1.0 - 0.4159β1 + 0.4994β12 - 0.292β13 V1n (14)

where β1 is an impact angle in radian as defined in Figure 14. As illustrated in Figure 14b, particle 1 is the representative of the particles that collide with the wall of the collision-block,


particle 2 is the representative of the particles that do not collide with the collision-block. If there is no collision between particles 1 and 2, particle 1 will rebound on the wall of the collisionblock and enter into the fuel-rich side. The solid concentration downstream of the collision-block rises when the inlet particle concentration rises; then the collision probability in this region becomes larger, which results in more collision between particle 1 and particle 2. If such a collision happens, particles 1 and 2 will both change their directions. As mentioned above, the velocity of particle 1 is always less than that of particle 2 due to the energy loss during the impact of particle 1 with the collision-block. As a result, particle 1 changes its trajectory and enters into the fuel-lean side if the particles 1 and 2 have a similar mass. Furthermore, particle 2 will still enter into the fuel-lean side because of the smaller moment of particle 1. The above description about the collision process is simple and schematic, which just provides a plain analysis of the experimental phenomenon that the separation performance of the fuel-rich/lean separator decreases with the increase in the inlet solid concentration. The most straightforward approach to account for the complicated inter-particle collisions is the direct simulation approach, which tracks all the particles in the flow field simultaneously. Thereby, the occurrence of collisions between any pair of particles can be judged by their relative motion and positions.24 Three-dimensional direct simulation of the large-scale two-phase jet flow is a subject of current investigations. (23) Grant, G.; Tabakoff, W. Erosion prediction in turbomachinery resulting from environmental solid particles. J. Aircraft 1975, 12 (5), AIAA Paper 74-16. (24) Laı´n, S.; Garcı´a, J. A. Study of four-way coupling on turbulent particle-laden jet flows. Chem. Eng. Sci. 2006, 61, 6775-6785.


Conclusions A fiber-optic measurement system is an effective method to obtain the particle concentration and particle size distribution characteristics of the gas-solid two-phase jet flow. Because the fiber-optic measurement system can be used for high particle concentration jet flow measurements, it can directly be employed for field measurements or the large-scale laboratory experiments, which model the practicable burner flow much better than the small-scale ones. The separation performance of the fuel-rich/lean separator and mixing characteristics of the gas-solid two-phase burner jet were obtained in this work. Local solid concentration and particle size distributions were obtained. The measurements indicated that the collision-block-type separator always gave good concentrating performance under various inflow solid concentrations. The fuel-rich stream and the fuel-lean stream can maintain their continuous structure for a long distance, ensuring that the benefits of the horizontal fuel-rich/lean combustion can be expected, which results in the flame stability and low NOx emissions. The solid concentration ratio between the fuel-rich and fuellean streams decreases slightly with the increase in the solid concentration of the primary air, which was caused by interparticle collisions in the separator. The solid size and concentration distribution have the same tendency. The particles with large diameter always exist in the zone where the particle concentration is high. Acknowledgment. Supported by National Natural Science Foundation of China (60534030, 50576081), program for Changjiang Scholars and Innovative Research Team in University. The authors would like to thank Zhanghua Zhou and Jianzhong Li for their assistance in the experiments. EF0604442

Experimental Investigations on Performance of Collision-Block-Type

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