Energy Fuels 2010, 24, 4893–4903 Published on Web 08/19/2010
: DOI:10.1021/ef1007209
Experimental Investigations on the Performance of a Coal Pipe Splitter for a 1000 MW Utility Boiler: Influence of the Vertical Pipe Length Hao Zhou,* Guiyuan Mo, Jiapei Zhao, Jianzhong Li, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received June 9, 2010. Revised Manuscript Received August 4, 2010
The objective of this study was to experimentally investigate the splitting characteristics of the coal pipe splitter using a fiberoptic measurement system. Four different vertical pipe lengths of 1D, 3D, 5D, and 18D were employed in the experiments to investigate the influences of the vertical pipe length on the splitting performance. The local solid concentration and particle size distributions at different cross-sections were obtained. Fiberoptic measurement systems showed their ability in the field measurements in this work. The measurements indicated that the length of the vertical pipe played an important role in the splitting performance. With the development of the gas-solid two-phase flow, the peak/lowest concentration ratio and the outer/inner mean concentration ratio decrease with an increase in the vertical pipe length. A split imbalance occurred when the vertical pipe length was 1D. The vertical pipe length of 5D resulted in a uniform performance of the particle concentration, and particle size distribution between the two legs was achieved; such a splitting characteristic is favorable for coal balancing and flame stability.
primary air burners are employed in the 1000 MW boiler, while 24 primary air burners are employed for the 600 MW boiler. Each pulverizer has four outlet coal pipes to convey the pulverized coal to the burners at the same elevation. As the boiler load changes, corresponding fuel elevations can be deployed with the deployment of the pulverizer. For the 600 MW boiler, there are six pulverizers to sever at six elevations, and every elevation has four corner-located burners. For the 1000 MW boiler, every elevation has eight burners, which results in a splitter that should be employed to split the coal-air stream in every pulverizer outlet coal pipe into two coal flows (as shown in Figure 1). Burner balancing is an important factor that influences the combustion process, which results in more efficient combustion, lower pollutant emissions, and fewer operational problems. Burner balancing includes equal distribution of both coal and air; the distribution of primary air can be controlled by the flow resistance of various coal pipes, such as orifices. The coal balancing is especially important for the 1000 MW tangentially fired furnace, because there are two fireballs in one furnace; the distribution of coal affects the heat release in the fireballs directly, which may result in the non-uniform distribution of the fluid temperature in the furnace waterwall and even the rupture of the waterwall tube. A significant pulverized coal flow non-uniformity is formed because of an upstream elbow as a result of the inertial effects.2 The particle rope carries most of the conveyed material in a small portion of the pipe cross-section close to the outer wall of the elbow, and the splitter installed close after an elbow cannot split the coal flow uniformly. A lot of studies on the roping phenomenon in lean-phase pneumatic conveying were conducted at Lehigh University.2-5
Introduction Coal retains its important role in power generation in the world. Most coal-fired power plants are now built in China and India, they are subjected to stringent pollution regulations and the demand of high efficiencies. For pulverized coal-fired boilers in China, a lot of supercritical plants have been built in the last several years. Large-capacity supercritical plants can achieve higher efficiencies, which results in lower fuel costs and lower generation power costs and reduces emissions per kilowatt of electricity produced. The efficiencies of coal-fired boilers based on the low heating value (LHV) expressed in terms of European parameters of approximately 52% can be achieved in power plants using advanced stream cycles with a stream temperature up to 760 °C.1 Several 1000 MW supercritical pulverized-coal fired boilers have been put into commercial operation since 2006, and dozens of 1000 MW boilers are in construction in China. In comparison to 600 MW or less capacity boilers, 1000 MW boilers have a much larger furnace, and the cross-section of the furnace becomes rectangle with a width/depth ratio of about 2, while the 600 MW or less capacity boilers have almost square cross-section furnaces. A stable fireball is formed in the center of the furnace of the 600 MW boiler, but there are two fireballs formed in the furnace of the 1000 MW boiler. These two fireballs have contrary rotation directions, which can decrease the flue gas temperature deviation at the outlet of the furnace. The number of burners of a 1000 MW boiler is twice as many as that of a 600 MW capacity boiler, but the number of the pulverizers is the same for both cases. For instance, the 1000 and 600 MW boilers all have six pulverizers, and 48 *To whom correspondence should be addressed. Telephone: þ86-571-87952598. Fax: þ86-571-87951616. E-mail: zhouhao@ cmee.zju.edu.cn. (1) Viswanathan, R.; Coleman, K.; Rao, U. Materials for ultrasupercritical coal-fired power plant boilers. Int. J. Pressure Vessels Piping 2006, 83, 778–783. r 2010 American Chemical Society
(2) Yilmaz, A.; Levy, E. K. Formation and dispersion of ropes in pneumatic conveying. Powder Technol. 2001, 114 (1-3), 168–185. (3) Yilmaz, A.; Levy, E. K. Roping phenomena in pulverized coal conveying lines. Powder Technol. 1998, 95, 43–48.
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Investigations on turbulent gas-solid two-phase flow measurements have been conducted by many researchers.9-11 On the basis of the principles of fluid mechanics and fluidization, optical techniques and tools for obtaining local information about particles have been developed. Laser doppler velocimetry (LDV), phase doppler anemometry (PDA), and fiberoptic sensors are the typical ones among them. The mean and fluctuating velocities of gas-solid two phases in coaxial jets were measured by Fan et al.,9 who employed LDV. The effect of solid particles on the flow characteristics was studied as well. Gas-solid two-phase flow in the pipe downstream of a fuel concentrator was measured using three-dimensional (3D) PDA. The characteristics of the concentrator with a continuously variable concentration of solid dust were investigated by this means.10 Li et al.11 studied the gas-particle flow characteristics of the centrally fuel-rich and enhanced ignition-dual-register burner using a 3D particle dynamics anemometer (PDA) system. Factors such as solids loading concentration and flow field geometry can affect the selection of the optical measurement method. For the gas-solid flow with a solid volume fraction greater than a few percent, the optical depth is limited to several particle diameters, which implies that measurements using LDV and PDA are impractical in relatively dense suspension. Woodhead12 reported that the upper limit of the particle volume fraction for light-phase pneumatic conveying of pulverized coal measurements with LDV was only 0.03%, because of the light attenuation by the particles. The typical volume fraction ratios (solid/air) range from 0.044 to 0.026% in industrial pulverized coal conveying pipes.8 Considering the local concentrating phenomenon near the outer wall of the elbow, the volume fraction ratios (solid/air) at many locations will exceed the value of 0.03%. Moreover, the PDA and LDV methods always have the limitation of a clean environment and the scale ratio of the model burner, which results in their inconvenience for measuring an industrial-scale burner jet in a dirty environment. For measurements in circulating fluidized beds (CFBs) and pneumatic conveyor systems, fiberoptic sensors offer an alternative approach, although it is an intrusive measurement technique. Fiberoptic probes have been used to measure the velocity and size of particle clusters, as well as the voidage in a CFB to identify various flow regimes.13 Morikawa et al.14 investigated particle velocity and concentration profiles in a fully accelerated horizontal pneumatic conveying line with a fiberoptic probe. A series of studies on the roping phenomenon in lean-phase pneumatic conveying using fiberoptic probes has been conducted at Lehigh University.2-5 In our previous investigations, the gas-solid jet downstream of the fuel/rich burner was studied by employing the fiberoptic measurement system.15-18
Figure 1. Schematic diagram of the burners in a 1000 MW tangentially fired boiler.
Laboratory experiments were performed to measure the characteristics of the particle rope in the pipe after a horizontal to vertical elbow,3 and the formation and dispersion of ropes in pneumatic conveying were experimentally and numerically investigated.4 Various types of flow mixtures, including nozzles, air jet injection, and swirl vanes, were employed to disperse the particle rope, and the rope dispersion characteristics of the mixing devices were studied.5 Chu and Yu performed numerical simulation of the gas-solid flow in pneumatic conveying elbows and analyzed the gas-solid, particle-particle, and particle-wall interaction forces to understand their role in governing the complicated flow.6 However, there are very few published data about the splitting characteristics of the splitter, especially the experimental work. Frank et al.7 used parallelized code MISTRAL/ PartFlow-3D to simulate pulverized coal particle flow in bifurcator-type flow splitters, and they focused their attention mainly on the numerical methods for complex problems. Giddings et al.8 made some investigations on the behavior of a scaled pneumatic rig representative of the typical bends and junctions used in a power station. They found that a number of bends prior to the junction print ensure an unbalanced distribution of power in the downstream pipes. The objective of the work described in this paper was to experimentally investigate the splitting characteristics of the splitter. Data about the local particle concentration and particle size distribution can be used to evaluate different design possibilities and to test theoretical modeling approaches. (4) Schallert, R.; Levy, E. K. Effect of a combination of two elbows on particle roping in pneumatic conveying. Powder Technol. 2000, 107 (3), 226–233. (5) Bilrgen, H.; Levy, E. K. Mixing and dispersion of particle ropes in lean phase pneumatic conveying. Powder Technol. 2001, 119 (2-3), 134– 152. (6) Chu, K. W.; Yu, A. B. Numerical simulation of the gas-solid flow in three-dimensional pneumatic conveying bends. Ind. Eng. Chem. Res. 2008, 47, 7058–7071. (7) Frank, T.; Schneider, H.; Bernert, K.; Pachlere1, K. Parallel numerical prediction of pulverized coal particle flow in bifurcator-type flow splitters. J. Pressure Vessel Technol. 2003, 125, l–6. (8) 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, 2195–2204. (9) Fan, J. R.; Zhao, H.; Jin, J. Two-phase velocity measurements in particle-laden coaxial jets. Chem. Eng. J. 1996, 63, 11–17. (10) Xu, M.; Sheng, C. D.; Yuan, J. Two-phase flow measurements and combustion tests of burner with continuously variable concentration of coal dust. Energy Fuels 2000, 14, 533–538. (11) Chen, Z. C.; Li, Z. Q.; Jing, J. P.; Chen, L. Z.; Wu, S. H.; Yao, Y. Gas/particle flow characteristics of two swirl burners. Energy Convers. Manage. 2009, 50, 1180–1191.
(12) Woodhead, S. R. The measurement of particle velocity and suspension density in pneumatic coal injection system. Ph.D. Thesis, University of Greenwich, London, U.K., 1992. (13) Nicolai, R.; Reh, L. Measurement of solids concentration, velocity and momentum distribution in a cold CFB unit. Proceedings of the 13th International Conference on Fluidized Bed Combustion; American Society of Mechanical Engineers (ASME): New York, 1995; Vol. 1. (14) Morikawa, Y.; Tsuji, Y.; Tanaka, T. Measurements of horizontal air-solid two-phase flow using a fiber optic probe. Bull. JSME 1986, 29, 802–809. (15) Zhou, H.; Cen, K. F.; Fan, J. Experimental investigation on flow structures and mixing mechanisms of a gas-solid burner jet. Fuel 2005, 84, 1622–1634.
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Figure 2. Fiberoptic measurement system.16 Figure 4. Sketch of the pilot-scale pneumatic laboratory facility.
Figure 3. Sketch of the fiberoptic probe.16
The present paper describes a laboratory investigation of the splitting characteristics of the splitter. A fiberoptic measurement system was employed to obtain the concentration distribution of solids across the cross-sections of the splitter. The local solids concentration and particle size distribution were obtained. Four different vertical pipe lengths of 1D, 3D, 5D, and 18D were employed in the experiments to investigate the influences of the vertical pipe length on the splitting performance.
Figure 5. Sketch of the test splitter and the measuring locations. Table 1. Operational Parameters of the Industrial and Laboratory Splitter
2. Fiberoptic Measurement System There are two types of fiberoptic sensors, i.e., intensity modulated and phase modulated, categorized according to their operational principle. Intensity-modulated fiberoptic sensors using the reflective or transmissive approach are usually used for measurements of gas-solid flows because of their accuracy, simplicity, and low cost. In the work presented here, a fiberoptic measurement system based on the transmissive, intensity-modulated concept was applied, as shown in Figure 2. This system consists of an optical probe, an integrated electronic circuit unit (with pre-amplification of the electrical signals), an analog-to-digital converter (ADC), and a personal computer with data collection and post-processing software. The optical probe configuration employed in the present study is demonstrated in Figure 3. Four optical glass fibers were used in this system, two of which were used to send light from the light-emitting diode (LED) into the gas-solid flow
gas velocity at the splitter inlet (m/s)
pipe diameter (mm)
air temperature (°C)
particle mean diameter (μm)
35 17
610 150
75 20
40 21.3
industrial laboratory
Table 2. Laboratory Experimental Cases vertical pipe length case (mm) 1 2 3 4
150 450 750 2700
relative vertical pipe length 1D 3D 5D 18D
measuring locations
gas velocity at splitter inlet (m/s)
particle mean diameter (μm)
A-E A-E A-E A-C
17 17 17 17
21.3 21.3 21.3 21.3
region. The two fibers were triangularly located in the laser emissive hole. On the opposite vertical face of the notch, two similar holes were opened to locate the other two fibers similarly. Fiber 3 transferred the transmitted light from fiber 1, and fiber 4 transferred the light from fiber 2, to the detector areas of two silicon photodiodes. There were two laser beams in the measuring zone; the first beam, emitted from fiber 1, was the upstream beam, and the second beam was the downstream beam.
(16) Zhou, H.; Cen, K. F. Experimental measurements of a gas-solid jet downstream of a fuel-rich/lean burner with a collision-block-type concentrator. Powder Technol. 2006, 170, 94–107. (17) Zhou, H.; Cen, K. F. Experimental investigations on performance of collision-block-type fuel-rich/lean burner: Influence of solid concentration. Energy Fuels 2007, 21, 718–727. (18) Zhou, H.; Cen, K. F.; Fan, J. R. Two-phase flow measurements of a gas-solid jet downstream of fuel rich/lean burner. Energy Fuels 2005, 19, 64–72.
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Table 3. Comparison of the Dimensionless Numbers for Industrial and Laboratory Scales
Reynolds Froude Stokes temperature (°C) gas velocity in the pipe (m/s) atmospheric pressure (bar) particle density (kg/m3) particle mean diameter (μm) internal pipe diameter (mm)
Re = FUD/μ Fr = U/(Dg)1/2 St = Fpdp2U/(18Dμ) T U P Fp dp D
industrial scale
laboratory scale
1.04 10 14.02 0.079 75 35 1 1200 40 610
1.76 105 14.31 0.066 20 17 1 2700 21.3 150
6
Figure 6. Particle concentration distributions measured in case 1.
where the cross-correlation function C(τ) becomes maximum. Then, local particle velocity can be readily computed from L ð2Þ Up ¼ τmax
The standard fast Fourier transform (FFT) algorithm with a cross-correlation technique was employed in measuring the local particle velocities. The flight time of particles from the upstream beam to the downstream beam was calculated as follows: Z 1 T u1 ðtÞu2 ðt þ τÞdt ð1Þ CðτÞ ¼ T 0
where L is the optical distance between the laser beams 1 and 2. Information on the local particle concentration is obtained from the mean values of the signals with and without particles present ð3Þ Ip ¼ Im - I0
where the terms u1(t) and u2(t) are particle flow signal waveforms. The flight time of particles (τmax) is the time lag (τ), 4896
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Figure 7. Particle concentration distributions measured in case 2.
where I, the transmissive light signal intensity, is computed from the analog signal u(t) for a time interval T. 1 I ¼ T
Z
T
uðtÞdt
measures at a point, it must be traversed over the pipe crosssection to obtain full information on the non-uniformities of the flow and the average particle concentration. Measurements with a fiberoptic probe have been compared to the measurements performed with an isokinetic sampling device, as reported in ref 20. This sampling device extracted a representative sample of particles to determine the mass flux; the suction velocity at the probe tip could be adjusted to be identical to the local gas stream velocity, so that the “isokinetic” condition could be achieved. The pressure fluctuation in the pipe occurred in the region of 5% static pressure. The detailed working principle of the isokinetic sampling probe can be found in ref 21. The measurements obtained by an isokinetic sampling device were used to compare to the fiberoptic probe measurements, as reported in our previous work.15 The comparisons showed that the local particle mass fluxes obtained using the two measurement techniques were in agreement within 10%. Two types of standard-size particles, 74 and 9 μm in diameter, were used for calibrating the particle size measurement.
ð4Þ
0
Earlier studies showed that the amount of light transmitted through a gas-solid flow was linearly dependent upon the volume concentration of particles. The particle size was obtained by analyzing the fluctuation characteristics of the optical signals. The readers can be referred to refs 19 and 20 for more description of the measuring principle. 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, both the particle concentration and particle size were measured, as reported in ref 3. A reasonably uniform particle distribution can be expected over the pipe cross-section, although a roping phenomena still exists. Because the probe (19) Cai, X. S.; Pan, Y. Z.; Ouyang, X.; Wu, W. L.; Yu, J.; Hu, J. The study of diagnosing the running condition of pulverized coal in pipe. Proc. Chin. Soc. Electr. Eng. 2001, 21 (7), 83–86 (in Chinese). (20) Cai, X. S.; Pan, Y. Z.; Wu, W. L.; Ouyang, X.; Su, M. X.; Yu, J. A study of on-line measurement technology for size, concentration and velocity of pulverized coal. Power Eng. 1999, 19 (6), 466–70 (in Chinese).
(21) 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.
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Figure 8. Particle concentration distributions measured in case 3.
For each particle size, six tests with different particle concentrations were performed. The results showed that the particle concentration and particle size measurements were nearly independent.
The investigated pneumatic conveying test facility consisted of an elbow, a vertical pipe, and a splitter, which then split two ways, as showed in Figure 5. There were two legs with the diameter of 115 mm downstream of the splitter. The coal pipe splitter and the elbow were in the same plane. The elbow radius was 175 mm, and the pipe diameter was 150 mm. Four different vertical pipe lengths of 1D, 3D, 5D, and 18D were employed in the experiments. The local solid concentration and particle size distributions were obtained at different test cross-sections A-E. The distance between the cross-sections A and B was 50 mm, while the distance was 85 mm between the cross-sections B and C. The measuring holes were illustrated in Figure 5. The cross-sections A-C were used to study the development of the gas-solid flow, and the cross-sections D and E were used to investigate the splitting characteristics. The instrument ports on the wall of the splitter allowed the fiberoptic probe to traverse the splitter cross-section in both the lateral and axial directions to measure the gas-solid flow in the splitter. For one cross-sectional measurement, more than 120 points were measured using the fiberoptic probe to obtain detailed information about the mixing characteristics of the splitter. As mentioned above, the use of fiberoptic sensors was an intrusive measurement technique. The percentage of the crosssectional area blocked by the probe in this work was ∼4%. At the
3. Experimental Facility The experiments described in this paper were performed in a 150 mm internal diameter pilot-scale pneumatic conveying carbon-steel pipes, as illustrated in Figure 4. 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 using a variable speed control. A cyclone was used to separate the solid particles from the air. The collected particles were 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 of operation. Sieve analysis showed no significant attrition for 100 h period covered. After 100 h run of the loop, the particles were renewed. 4898
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: DOI:10.1021/ef1007209
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is given as follows: Eu ¼
Δp FU
ð6Þ
where Δp is the pressure difference in the gas between the inlet and outlet of the flow. The Euler number represents the ratio of the pressure difference to the inertial force. The Froude number represents the ratio of inertial forces to gravitational forces on the airflow and is given by U Fr ¼ pffiffiffiffiffiffiffi ð7Þ Dg where g is the gravitational acceleration. The Stokes number is computed form St ¼ Fp dp 2 U=ð18DμÞ
where Fp is the particle density and dp is the particle diameter. When the flow enters the self-modeling flow region, the same flow phenomenon can be observed in both burners, even though the Reynolds number of the laboratory-scale burner is smaller than that of the industrial-scale burner. It has been reported that the flow enters the self-modeling flow region if the Reynolds number is greater than 104-105.22 The Reynolds number of the model burner in our work was equal to 1.04 106, 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. The Stokes number determined the density and mean size of the test particles in the laboratory test facility. A scaling ratio of 1:4.07 was chosen in this work to make the burner model fit the geometrical requirements. Talcum powder particles with RosinRammler size distribution (the mean diameter of 21.3 μm) and a true particle density of 2700 kg/m3 were used as the material conveyed in this work. The effects of the vertical pipe length on the concentrating performance of the coal pipe splitter were investigated, and four experimental cases were studied, as listed in Table 2. As reported by Wei et al.,23 in the primary air of a pulverized coal burner, the fuel/air mass ratio in the pneumatic pipe is normally ∼0.3-0.6 kg of coal/kg of air. Therefore, the average particle concentration in the primary air was kept at 0.32 kg/kg in the experimental test section in all cases. The velocities in the conveying pipe and at the splitter inlet were determined according to the scaling criteria. Table 3 shows the experimental parameters and a detailed comparison of the dimensionless numbers for the industrial and experimental scales.
Figure 9. Particle concentration distributions measured in case 4.
end of the chamber, the particles were separated from the air using a cyclone separator. 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. The full industrial-scale coal pipe splitter studied in this work was designed for a 1000 MWe capacity utility boiler. Table 1 shows the operational parameters of the industrial-scale burner. Owing to the geometrical restrictions of the test rig, the industrial burner was scaled so that it could be installed in the test facility. In this experiment, a scale ratio of 1:4.07 was employed to make a model coal pipe splitter. The aerodynamic scaling of the industrial burner should follow some similar criteria, as listed in (1) geometric similarity, (2) selfmodeling flows, (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. When the Reynolds number of the flow is greater than a certain value, the Euler number does not depend upon the Reynolds number; that is, the flow enters the “self-modeling flow” region. The Reynolds number represents the ratio of the inertial force to the viscous force and is defined as Re ¼
FUD μ
ð8Þ
4. Results and Discussion Figure 6 shows a contour plot of the particle concentration measured with the fiberoptic probe. Some contour labels have been shown in the figure. The magnitude of A-C represents the test cross-section in the splitter. These measurements, obtained when the vertical pipe length was 1D (case 1), showed that the concentration profile of the particle was uneven. The peak particle concentration was 0.55 kg/kg, and the rather concentrated rope was close to the outer wall. The lowest particle concentration was 0.1 kg/kg, occurring in the region of the inner side. A peak/lowest particle concentration ratio of 5.5 was achieved at the cross-section A. The difference of the particle concentration between the outer and (22) Cen, K. F. The Study Method and Measurement Techniques of Boiler Combustion Experiments; Water Conservancy and Electric Power Press: Beijing, China, 1987 (in Chinese). (23) 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.
ð5Þ
where F is the gas density, D is a representative length, U is the gas velocity, and μ is the viscosity of the gas. The Euler number 4899
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Figure 10. Particle size distributions measured in case 1.
inner side was due to the roping effect of an upstream elbow.3 At the location of cross-section B, the peak concentration reduced to 0.425 kg/kg, while the lowest particle concentration increased to 0.125 kg/kg. With the development of the gas-solid flow, the peak/lowest particle concentration ratio reduced to 3.4. The particle distribution measured at crosssection C was similar to cross-section B; the peak and the lowest particle concentration was 0.425 and 0.15 kg/kg, respectively. The area of the peak concentration decreased, and the peak/lowest concentration ratio was 2.8. The peak concentration occurred away from the outer wall but was still on the outer side. Panels d and e of Figure 6 show the contour plots of the particle concentration in the outer and inner leg of the splitter, respectively. The peak particle concentration at the outer leg was 0.5 kg/kg, while the peak concentration in the inner leg was 0.3 kg/kg, which resulted in an outer/inner mean concentration ratio of 1.6 at the outlet of the splitter. This will cause a split imbalance in the two legs downstream of the splitter with a vertical pipe length of 1D. Figure 7 presents the contour plots of the particle concentration measured at cross-sections A-E, with the vertical pipe length of 3D downstream of the elbow (case 2). The particle
concentrations were more uniform than those in case 1. The peak particle concentration was 0.5 kg/kg, and there rather concentrated zones were in the outer side. The lowest particle concentration was 0.15 kg/kg, occurring in the region of the inner side; a peak/lowest particle concentration ratio of 3.3 was achieved at the cross-section A. At the location of crosssection B, the peak concentration reduced to 0.475 kg/kg, while the lowest particle concentration increased to 0.175 kg/ kg. As for cross-section C, the area of rather concentrated zones became smaller and a more uniform concentration distribution was achieved. As illustrated in panels d and e of Figure 7, the peak particle concentrations at the outer and inner leg were 0.475 and 0.4 kg/kg, respectively, which resulted in an outer/inner mean concentration ratio of 1.2 at the outlet of the splitter. When the vertical pipe length was raised to 5D (case 3), the splitting performance was investigated and information of the particle distribution was obtained. Figure 8 shows the contour plots of the particle concentration measured at cross-sections A, B, and C with the fiberoptic probe. In the region of crosssections A, B, and C, the peak particle concentration was 0.55, 0.475, and 0.45 kg/kg, respectively, while the lowest particle concentration ranged from 0.15 to 0.2 kg/kg for all 4900
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Figure 11. Particle size distributions measured in case 2.
cross-sections. In comparison to case 1, the particle concentrations in case 3 were almost uniform and the peak concentrations decreased. The peak of the particle concentration in case 1 split into two peaks in case 3, and the peaks had shifted toward the center of the splitter. As shown in cross-section C, the peak concentration on the outer and inner side was 0.45 and 0.475 kg/kg, respectively. The peak value on the outer side was almost equal to that on the inner side. Because of the baffle plate at the bottom of the splitter, the peak concentrations of the outer and inner leg were both close to the outer wall; the zone of the peak particle concentration was still obvious at all of the cross-sections. As illustrated in panels d and e of Figure 8, the peak particle concentration in the outer leg was 0.45 kg/kg, while the peak concentration in the inner leg was also 0.45 kg/kg, which resulted in an outer/ inner mean concentration ratio of 1.08 at the outlet of the splitter. In comparison to cases 1 and 2, the peak concentration in the outer leg was equal to that in the inner leg in case 3; the two legs of the splitter could achieve a coal balance. With the vertical pipe length of 5D downstream of the elbow, a split balance was achieved; such a particle concentration of the primary air flow will lead to enhancement of the coal balancing, as well as air balancing, in each burner.
The straight vertical pipe with a length of 5D upstream of the splitter is favorable for burner balancing, which may result in the uniform distribution of the fluid temperature in the furnace waterwall and even the rupture of the tube. In case 4, lower peak particle concentrations can be obtained at various cross-sections of the coal pipe splitters. As illustrated in Figure 9, the peak concentrations measured at cross-sections of A, B, and C were 0.475, 0.45, and 0.425 kg/kg, respectively. The particle concentrations in case 4 were similar to those in case 3, and there was no difference in the particle concentration between the outer and inner sides of the splitter. As illustrated in Figure 9c, the rope was already dispersed and the concentration profile of the particle at both outer and inner sides is uniform. Figure 10-13 present the particle size distribution contours obtained at various cross-sections of the four cases. The particles on the outer side of the splitter always had larger mean diameters than those on the inner side in case 1. The peak particle size exists in the region adjacent to the outer wall of the splitter, and the values of the peak particle sizes were 22, 20, and 20 μm at the cross-sections of A, B, and C, respectively. The solid size and concentration distribution have the same tendency. The particles with a small diameter can 4901
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Figure 12. Particle size distributions measured in case 3.
respond quickly to fluid motion and are distributed uniformly in the flow field because of the smaller aerodynamic response time scale. The particles with bigger diameters showed a strong preferential concentration and congregate largely near the outer boundaries. In case 2, the particle size distributions were more uniform than in case 1. With the vertical pipe length of 5D in case 3, the mean size distribution was more uniform between the outer and inner sides than in case 1. The peak value of the peak particle size was 21 μm in both the outer side and inner leg. The measurements showed that the zone of the peak particle size was located at both the outer and inner sides; the particle size distributions became more homogeneous with the development of the gas-solid flow. Such a splitting characteristic was useful for coal balancing and flame stability. When the vertical pipe length was raised to 18D, particle size distributions were uniform in both sides. The splitting performance of the coal pipe splitter was affected by the length of the vertical pipe downstream of the elbow. As illustrated in Figure 14, the peak/lowest concentration ratios of cross-sections A, B, and C of the splitter decreased with the increase in the vertical pipe length, although the particle concentration contour profiles were similar in various cases. Figure 15 shows the outer/inner leg
mean concentration ratio of cross-sections D and E in three cases. The outer/inner leg mean concentration ratios were 1.6, 1.2, and 1.08 in the case of the vertical pipe length of 1D, 3D, and 5D, respectively. The outer/inner mean concentration ratio decreased with the increase in the vertical pipe length. When the vertical pipe length was raised to 5D, a good splitting performance was achieved. Other factors, such as gas velocity and solid loading ratio, may also influence the splitting characteristics, and our further efforts should be focused on investigating these factors. However, in the industrial pulverized coal conveying system of power plants, the vertical pipe length may be more variable and play a more important role in splitting performance and a splitting balance can be obtained with the vertical pipe length of 5D. 4. Conclusions A fiberoptic measurement system is an effective method to obtain the particle concentration and particle size distribution characteristics of the gas-solid two-phase flow. Because the fiberoptic measurement system can be used for high particle concentration flow measurements, it can directly be employed 4902
Energy Fuels 2010, 24, 4893–4903
: DOI:10.1021/ef1007209
Zhou et al.
Figure 14. Influence of the vertical pipe length (L/D) on the peak/lowest concentration ratio.
Figure 15. Influence of the vertical pipe length (L/D) on the outer/inner mean concentration ratio.
result in the uniform distribution of the fluid temperature in the furnace waterwall and even the rupture of the tube. The solid size and concentration distribution have the same tendency. The particles with big diameters show a strong preferential concentration and congregate largely near the outer boundaries. The particles with small diameter can respond quickly to fluid motion and are distributed uniformly in the flow field. With a vertical pipe length of 5D, a particle size distribution balance between the two legs is achieved; such a splitting characteristic is favorable for coal balancing and flame stability.
Figure 13. Particle size distributions measured in case 4.
for field measurements. Fiberoptic measurement systems show their ability in the field measurements in this paper. The splitting characteristics of the coal pipe splitter for the 1000 MW boiler and the effect of the length of the vertical pipe on splitting characteristics were investigated in this work. The peak/lowest concentration ratio decreases slightly with an increase in the vertical pipe length, which is caused by the development of the two-phase flow. The outer/inner mean concentration ratio between the two legs decreases with the increase in the vertical pipe length. The vertical pipe length of 5D results in a uniform performance of the particle concentration, which is useful for the burner balancing and may
Acknowledgment. This work was supported by the National Basic Research Program of China (2009CB219802), the Program for New Century Excellent Talents in University (NCET-070761), the Foundation for the Author of National Excellent Doctoral Dissertation of China (200747), the Zhejiang Provincial Natural Science Foundation of China (R107532), and the Zhejiang University K. P. Chao’s High Technology Development Foundation (2008RC001), and the Fundamental Research Funds for the Central Universities.
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