Energy & Fuels 2000, 14, 533-538
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Two-Phase Flow Measurements and Combustion Tests of Burner with Continuously Variable Concentration of Coal Dust Minghou Xu,*,† Changdong Sheng, and Jianwei Yuan Huazhong University of Science and Technology, Wuhan 430074, P. R. China Received May 5, 1999
Problems commonly occur when the coal type and/or the load are changed in utility boilers. To solve these problems, a new pulverized coal burner, namely, a burner with continuously variable concentrations of coal dust by using a set of blades, is developed in this study. Gas-solid twophase flow downstream of the burner was measured by three-dimensional particle dynamics anemometer (3D PDA). The results show that the particle concentration biased degree of the burner can be continuously adjusted between 1 and 3 by changing the angle between the blades and the primary air flow, over a range of 0-20°. The turbulence characteristics in this range promote ignition and combustion of the pulverized coal. The results of combustion tests in a single burner furnace adopting the new burner verify that the temperature in both the near burner region and the furnace can be raised, which leads to better ignition and combustion characteristics of the pulverized coal than with an ordinary burner.
Introduction In recent year, the burning of low rank coals, with high efficiency but low pollution, has been investigated thoroughly in China. High concentration combustion of pulverized coal, which can meet the above requirements, has been realized as an advanced combustion technique and widely applied in power station boilers. Various means have been adopted to realize this kind of combustion.1-5 However, most of them have the disadvantage that the biased degree of the coal concentration in the primary air mixture is hard to adjust for burning coals of different ranks, properties, or boiler loads. Although the biased degree can be revised in some ways sometimes, the adjustment and operation are very complicated. To utilize the advantage of high concentration combustion and make the biased degree easily adjustable, a new pulverized coal concentrator is developed in this investigation. The burner uses a set of blades to make some of the pulverized coal deviate from their original direction and form a relatively high particle concentration downstream of the blades in the primary air. The motion of small particles carried along by a fluid flow has long been investigated theoretically and experimentally.6-8 In ref 9, an experimental study of the characteristics of the turbulent field upstream of a * Corresponding author. † Current address: Instituto Superior Te ´ cnico, Lisbon 1049-001, Portugal. (1) Cogoli, J. G., et al. Combust. Sci. Technol. 1977, 16, 165-171. (2) Wall, T. F., et al. Combust. Flame 1988, 72, 111-118. (3) Masayasu, S., et al. Technical Rev. 1986. (4) Xu, M.-H., Ph.D. Dissertation, Huazhong University of Science and Technology 1992. (5) Xu, M.-H.; Yuan, J.-W.; Han, C.-Y.; Zheng, C.-G. Fuel 1995, 74, 1913-1917. (6) Segre´, G.; Silberberg, A. J. Fluid Mech. 1962, 14, 115. (7) Proudman, I.; Pearson, J. R. A. J. Fluid Mech. 1957, 2, 237.
flat plate placed normal to the mean flow was performed when the integral scale of turbulence is small with respect to the characteristic dimension of the obstacle. Up to now, as far as the author knows, there has existed no experimental investigation on the combustion and flow behind a set of blades. The interactions between small particles and gasphase turbulence are extremely complex and, despite the importance of numerous industrial and natural particle-laden flows, many of the interactions remain poorly understood. The influence of particles on the gasphase turbulence is one problem area which bars successful prediction of complex particle-laden flows. Reasonably organizing the flow field of a burner is essential in stabilizing and strengthening the pulverized coal combustion, and it is also one of the criteria to evaluate the property of a burner. The gas-solid twophase flow characteristics at the burner outlet are very important, and somewhat difficult as mentioned above, for the design and operation of a biased firing burner. It is now well-known that moderate mass loading of fine particles can create very significant changes in the turbulence levels in the flow. In general, small particles can attenuate fluid turbulence, while large particles can augment it, as reviewed by Hetsroni10 and Gore and Crowe.11 The current study investigates the flow field and turbulence characteristics of the burner with continuously variable concentration of coal dust, both gasphase and solid-phase for particles, with a threedimensional particle dynamic anemometer. The results of combustion tests in a single burner furnace are also presented to show the overall properties of this burner. (8) Rubinow, S. I.; Keller, J. B. J. Fluid Mech. 1961, 11, 447. (9) Huot, J. P.; Rey, C.; Arbey, H. Phys. Fluids 1984, 27 (3). (10) Hetsroni, G. Int. J. Multiphase Flow 1989, 15, 735-746. (11) Gore, R. A.; Crowe, C. T. J. Fluids Eng. 1991, 113, 304-307.
10.1021/ef990082g CCC: $19.00 © 2000 American Chemical Society Published on Web 04/04/2000
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Figure 1. Mechanism of particle concentration of the burner.
Figure 2. Two installing modes of the concentrator: 1, approaching flow; 2, duct; 3, concentrator; 4, nozzle; 5, partition board.
Mechanism and Characteristics of the Burner As we know, when the fluid flows past a bluff-body, a pressure gradient will be generated downstream of the bluff-body to cause separated flows and flow recirculation. A similar result can be obtained when the fluid flows past a blade or a set of blades, as illustrated in Figure 1. If the fluid is a gas-solid mixture, more flow passing through the upper part will drag more particles in the downstream region. However, the gas can also flow through the gap between the blades while particles cannot easily do like the gas-phase due to their inertia and, therefore, create a fuel-rich region at the upper part and a fuel-lean one at the lower part, as shown in Figure 1b. In this case, a local higher particle concentration region will be produced. The structure of the system is so simple that the biased degree can be continuously adjusted by easily changing the angle (denoted as R in Figure 1) between the blade group and the approaching flow. There are two kinds of concentrator installation mode for this burner to realize biased firing. The blade group can be installed either near the burner outlet nozzle as in Figure 2a, or in the primary air duct as in Figure 2b. In the latter circumstance, a baffle is used to prevent the biased particle flow from re-mixing before it enters the furnace. It should be pointed out that the separated flows are quite sensitive to the upstream mean velocity profiles,12,13 and in this study, only results for the former mode are presented. If the burner is used in a tangentially fired system (as shown in Figure 3), a fuel concentrator of high biased degree should be adopted in the fuel transport line to separate horizontally the fuel/air mixture into two streams, namely, a fuel-rich and a fuel-lean stream. The two streams have a large fuel concentration difference at the same elevation, with an angle of 0-15° between their axes.14 The biased degree is so large that the fuel concentration in the fuel-rich stream is several times higher than that in the fuel-lean stream. On the side facing the high-temperature flame, the fuel-rich stream is injected to form the inner imaginary circle. Because (12) Eaton, J. K.; Johnston, J. P. Mechanical Engineering Department Report MD-39, Stanford University, Stanford, CA, 1980. (13) Simpson, R. L. Prog. Aerospace Sci. 1996, 32, 457-521. (14) Sun, S. Z.; Wu, S. H.; Yang, M. X.; Sun, E. Z.; Cheng, L. Z.; Wang, Z. J.; Sun, R.; Li, Z. Q.; Qin, Y. K. Proc. 3rd Int. Symp. Coal Combust. Sci. Technol., Beijing, China; 1995, 226-233.
Figure 3. Schematic diagram of the bias combustion system: 1, primary air stream; 2, concentrator; 3, fuel-rich stream; 4, fuel-lean stream; 5, inner imaginary circle; 6, outer imaginary circle; 7, water cooled wall; 8, oxidizing zone.
of the high fuel concentration, this stream acts as a powerful stabilizer and promotes the ignition, ensuring the stability of the overall flame and further combustion in the furnace. On the other side, the fuel-lean stream forms the outer imaginary circle. The fuel-lean nozzles are installed between the fuel-rich nozzles and the water cooling walls. Because of the existence of a relatively low fuel concentration, oxidizing zones will be obviously formed near the water cooling walls. Since the ash fusion temperature under such an oxidizing atmosphere is much higher than that under a reducing atmosphere, slagging and fouling resistance can be expected. Moreover, the corrosion of water cooling wall tubes can be reduced under the oxidizing atmosphere. Each of the two streams burns at chemical stoichiometry and the global chemical stoichiometry remains the same as that of a normal burner, ensuring higher combustion efficiency. The above characteristics make this new burner to be encouragingly utilized in the tangentially fired boiler system. Experimental Section The cold flow tests were conducted in a closed wind tunnel with a test section 1.5 m long. The properties of the burner structure are listed as following: W/B ) 0.24; L/B ) 0.65; and a/B ) 0.24. Where W, B, L, and a represent, respectively, the width of a single blade, the width of the primary stream channel, the streamwise distance, and the width between the midpoint of the two farmost blades, as shown in Figure 4. R and β represent, respectively, the angle between the blade and the approaching flow, and the angle between the linking-up line of the blade midpoint and the approaching flow. Particles are fed before the test section and the burner exit is right after the test section. In the wind tunnel, the free-stream velocity can be varied from 10 to 50 ms-1. The experimental setup is illustrated in Figure 5. The pilot-scale pulverized coal furnace for the combustion tests is shown in Figure 6. The single burner furnace is 4 m long and has a cross-section of 0.35 × 0.50 m2. Air preheaters
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Figure 4. Schematic diagram of the burner structure. Figure 7. Schematic diagram of PDA system: 1, argon optical laser; 2, injector; 3,4, optical distribution boxes; 5,6, filters; 7, signal processor; 8, computer; 9,10,11, photomultipliers; 12, receiving optical system; 13,15, optical probes; 14, experimental section. Table 1. Properties of the Particles
Figure 5. Schematic diagram of cold flow test apparatus: 1, particle feeder; 2, development section; 3, test section; 4, gasparticle separator; 5, exhaust.
Figure 6. Schematic diagram of combustion test apparatus: 1, blower; 2, air preheater; 3, air pipe; 4, secondary air distributor; 5, pulverized coal hopper; 6, coal feeder; 7, primary air valve; 8, electric motor; 9, secondary air valve; 10, thermocouple; 11, furnace; 12, sampling points; 13, cyclone separator. are employed to heat the primary air and secondary air. Temperatures along the center-line of the furnace are measured with thermocouples, from which the combustion characteristics of the burner could be analyzed. The fuel used in this study was Chinese Jiaozuo coal (VM 12.88 wt %, 23.54 MJ kg-1). To study the gas-solid two-phase flow characteristics of the burner, the particle dynamics anemometer (PDA) was used in this investigation. PDA is based on a phase Doppler anemometer, which is an extension of a laser Doppler anemometer (LDA). The system includes an argon ion laser, transmitter, fiber optics, receiving optics, signal processor, and computer system, as shown in Figure 7. PDA uses well-proven phase Doppler principle for simultaneous nonintrusive and real-time measurements of three velocity components and turbulence characteristics and makes use of a new method for the phase differences between Doppler signals received by three detectors located at different positions. The transmitting optics is based on 55X Modular LDA Optics. Several optical configurations with measuring distances from 50 to 600 mm are available. The system settings such as bandwidth and high voltage are controlled automatically. An analogue-digital converter allows the computer to read the anode current of the photomultipliers. It should be mentioned that the combination of photomultiplier and particle velocity correlation bias can contribute to uncertainty, but the error is likely to be small. The overall uncertainty in measured values of mean velocity and particle diameter are 1% and 4%,
particle
shape
density (kg m-3)
sphericity
size (µm)
0.69
0-200
0.86
0-150
plastic irregular ellipsoid 1200-1600 powder block pulverized ellipsoid 1200-1600 coal
respectively, and the range of measurable particle sizes can be 1 µm-10 mm. Successful use of PDA needs a reasonable choice of trace particles to model pulverized coal. Talc and alumina powder were rejected from this purpose while white polychloroethylene powder was selected because it has properties similar to those of pulverized coal and can keep the testing window relatively clean, which is essential to the PDA measurement.5,15 The properties of the trace particles and pulverized coal are listed in Table 1. Both titanium dioxide and the fume of mosquito-repellent incense were tested for gas flow tracers and finally the latter was selected. The diameter of the fume particles is less than 1 µm, which is suitable for tracing the gas phase in gas-solid mixture.16 To measure the velocity of the tracer particles in the presence of the large particles, a two-counter technique utilizing pedestal amplitude descrimination was employed.17
Results and Discussion The biased degree is defined as the ratio of pulverized coal concentration of the particle-dense side to that of the lean side. It is a key parameter of biased firing pulverized coal burner. The main purpose of the measurement in this paper is to search for the principle of the biased degree varying with the angle (R) between the blade and the approaching flow, and to determine an optimized angle to fit the necessity for the operation of the burner. The measurement of the biased degree involves in the gas phase as well as the particle phase in the flow. In addition, the turbulence of the gas phase is very important for heat and mass transfer in the pulverized coal combustion process. Hence the gasphase characteristics, especially the average velocities, turbulence, and their variations with R, are also among the main subjects of the measurement. (15) Sheng, C.-D. Ph.D. Dissertation, Huazhong University of Science and Technology, 1995. (16) Sheng, C.-D.; Gao, Q.; Cui, H.-P.; Xu, M.-H.; Yuan, J.-W.; Han, C.-Y. Proc. 2nd Int. Conf. Fluid Dynamic Measurement Applications, Beijing, China; 1994, 112-116. (17) Kulick, J. D.; Fessler, J. R.; Eaton, J. K. J. Fluid Mech. 1994, 277, 109-134.
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Figure 8. Effects of R on streamwise gas velocity distribution.
Cold Flow Tests. Gas Phase. Mean streamwise gas velocity can be obtained by taking the average velocities of gas tracers accepted in an elapsed time interval, which has a very simple form: N
Ui(x,y,z)]/N ∑ i)1
U(x,y,z) ) [
(1)
where x, y, and z are the coordinates of the measuring point, respectively; N is the number of the tracer samples; Ui(x, y, z) is the streamwise velocity of the ith gas tracer particle at the measuring point; U(x, y, z) is the mean streamwise velocity. It can be seen from the time-averaged Navier-Stokes equations for turbulent channel flow that the mean flow is essentially defined by three quantities, namely, the friction velocity, the kinematic viscosity, and the channel half-width.18 Well away from solid boundaries, the mean viscous stress is small compared to the Reynolds stress, and the flow here is practically independent of fluid viscosity. This property is usually termed Reynolds number similarity. Also, the streamwise turbulence intensity exhibits Reynolds number similarity.19-21 For the channel flow past a set of blades, the streamwise velocity and turbulence intensity has some differences compared with those of the common channel flow, which is presented below. The mean streamwise velocity profile (x coordinate direction) at the burner outlet is shown in Figure 8, where U0 ) 25.75 m/s is the centerline velocity of the approaching flow, B is the width of the burner. It can be seen from Figure 8 that U possesses a peak (U/U0 > 1) at each side and a valley (U/U0 < 1) downstream of the blade group. Moreover, the velocities increase at both sides and decrease downstream of the blade group with R increasing. Comparing velocities for the same R at both sides, we can find that U is only slightly higher at y/B > 0 than that at y/B < 0, but the larger the R, the larger the difference. These phenomena can be explained as follows. (18) Townsend, A. A. The structure of turbulent shear flow; Cambridge University Press: Cambridge, 1976. (19) Purtell, L. P.; Klebanoff, P. S.; Buckley, F. T. Phys. Fluids 1981, 24, 802. (20) Johansson, A. V.; Alfredsson, P. H. J. Fluid Mech. 1982, 122, 295. (21) Alfredsson, P. H.; Johansson, A. V. Phys. Fluids 1984, 27 (8), 1974-1980.
When the gas-solid two-phase flow reaches the blade group in the flow duct, most gas will bypass, only a minority will flow through the space between the blades. Thus the gas flux at both sides is higher than that of the approaching flow, leading to U/U0 > 1. However U/U0 < 1 appears downstream of the blade group. This is because the inclined blades have a leading action on the gas flow for such a special structure of the blade group. Most gas may flow through the side with y/B > 0, which results in the velocity at this side higher than that at another. But the space between the blades has the action of equilibrating the gas flow of both sides to lighten the above action to a slight degree. The space decreases with increasing R, although space effects may still exist, the blade group subtracts the gas flow area smaller and smaller. Finally, the velocity increases at both sides and decreases downstream of the blade group with increasing R. The biased firing burner demands particle concentration at one side being higher than that at another one. Higher gas flux or gas velocity is not in favor of the increment of particle concentration. Thus, it is expected that the existence of the blade group has less effects on gas flow in the two-phase flow, namely, the distribution of U to be as uniform as possible. It can be seen from Figure 8 that, the larger the angle R, the less uniform the distribution of U. When R > 25°, the mean streamwise velocity U is very low downstream of the blade group and very high at both sides. These results of gasphase measurement suggest that the angle R be less than 25° in order to satisfy the demand of the biased firing burner. Turbulent intensity is also important for a burner. Here the gas-phase turbulence kinetic energy (kg), defined as following, is presented.
1 kg ) (u2 + v2 + w2) 2
(2)
where u, v, and w are the fluid r.m.s fluctuating velocity components in x, y, and z directions, respectively. The distribution of the turbulence kinetic energy at the burner outlet and its relation with R are presented in Figure 9. For R < 20°, kg at the side of y/B > 0 is quite high, and the highest kg appears near R ) 10°. When the biased jet formed by this burner injects into the furnace of a boiler, such a high kg at this side will promote heat and mass transfer between the fuel and the hot flue gas, and strengthen the ignition and combustion process. When R > 20°, the kg distribution tends to be uniform, it may result from the increasing of gas velocity. Solid Phase. Particle concentration is an important property of the particle phase in two-phase flow and is expressed here as a relative volume concentration:
Cr )
πNd3p 6x(u2 + v2 + w2)∆t
(3)
where Cr is the relative volume concentration of particles, N is the number of particle samples, dp is the average diameter of particles accepted at the measuring point, ∆t is the elapsed time, and u, v, and w are the velocity components in x, y, and z directions, respectively.
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Figure 9. Effects of R on gas turbulence kinetic energy distribution.
Figure 10. Effects of R on particle concentration distribution.
The distribution of particle concentration at the burner outlet and its relation with R are presented in Figure 10, where Cr0 is the mean particle volume concentration in the approaching flow. It shows that the concentration distribution has a peak at the side of y/B > 0, while the concentration at another side is lower. Such a result made us calling the side of y/B > 0 as particle-dense side, another one as particle-lean side. The concentration downstream of the blade group is the lowest. It is obvious that the two-phase flow upstream of the blade group will attack the blades when R * 0. Because of their larger inertia, most particles will collide with the blades, and then change their moving direction to the dense side. Particles passing through the dense side include the particles both upstream of the blade group and the dense side, while particles passing through the lean side include only those upstream of the lean side. As revealed in Figure 8, the gas flux of both sides is required almost equal, thus the particle concentration of the dense side is higher than that of the lean one. On the other hand, only a few particles may traverse the blade group, although the gas flux downstream of the blade group is low, the particle concentration is still low there. When R increases, more particles will pass through the dense side to make the particle concentration increase, while the concentration downstream of the blade group decreases. These tendencies are clearly showed in Figure 10. As described previously, the particle concentration biased degree is defined as the ratio of the concentration at the dense side to that at the lean side. The biased degree for different R can be derived from Figure 10, as
Figure 11. Effects of R on particle turbulence kinetic energy distribution. Table 2. Biased Degrees for Different r angle R
10°
20°
25°
30°
35°
biased degree
1.8
3.1
2.5
2.8
2.1
listed in Table 2. It can be found that, with R increasing, the biased degree of this burner continuously increases from 1 to about 3, and then decreases when R > 20°. Therefore, the biased degree can be adjusted in the range 1-3 for this burner, and the reasonable adjusting range of R is 0 ∼ 20°. When R > 20°, a large biased degree can still be obtained, but the gas and particle velocity at both the dense and lean side will be too high and collision of the particles with the blades will be serious and unavoidable. Figure 11 shows the distribution of the turbulence kinetic energy of the particles, kp, (the same form as kg in eq 2, with u, v, and w as the particle r.m.s fluctuating
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mass transfer between particles at the dense side and the high-temperature flue gas very quickly. In addition, after the ignition and combustion of the fuel-rich stream at the dense side, some small particles which pass through the space between the blades will obtain their released heat from the ignited coal particles, then transfer heat to other particles at the fuel-lean side. The above process makes the overall primary air steam ignite easily, combustion stable, thus makes the new burner have better properties than the ordinary burner. Conclusions
Figure 12. Temperature distribution on the centerline of single burner furnace with the novel burner and an ordinary burner: 1, novel burner (C0 ) 0.80 kg-c/kg-a); 2, novel burner (C0 ) 0.55 kg-c/kg-a); 3, ordinary burner (C0 ) 0.80 kg-c/ kg-a).
velocity components in x, y, and z directions, respectively) at the burner outlet and its relation with R. It can be seen that kp is very high and varies with the changing of R. kp is large at the dense side and the downstream region for a small R. As R increases, the overall kp is increased and relatively high at both the dense and the lean side, which will promote the heating and ignition of the pulverized coal stream. When R > 25°, kp has a smooth distribution. Combustion Tests. Figure 12 shows the temperature distribution at the centerline of a pilot-scale single burner furnace for the burner with continuously variable concentration of coal dust and an ordinary burner. It can be seen that the temperature of the former is much higher than that of the latter. The high temperature near the burner outlet will especially promote the initial ignition of coal. The mechanism can be analyzed as follows. In the cold flow tests we have found that a dense fuel region exists at the outlet of the burner, this highly concentrated pulverized coal will be ignited easily, as described before. Moreover, the strong turbulence of both the gas and particle phase will make the heat and
From the studies of the gas-solid two-phase flow and combustion process of the burner with continuously variable concentration of coal dust, the following conclusions can be made. (1) The new pulverized coal burner can successfully realize a biased firing. The biased degree can be continuously adjusted within a special range by changing the angle R between the blade and the approaching flow. (2) The experimental results of two-phase flow by 3-D PDA reveal that reasonable range of R is 0-20°, which can cover all adjustable range of biased degree (for this burner, the range is 1-3). (3) The turbulence at the dense side is strong in the above angle range, which can promote the ignition and combustion process of pulverized coal. (4) The combustion tests show that the temperatures on the centerline of single burner furnace installed with the novel burner are higher than those installed with an ordinary burner, verifying better properties of the new burner. Acknowledgment. The authors acknowledge the financial support of the State Key Project of Fundamental Research on the Combustion of Coal and Oil of China, Grant 85-25-06-2-3, the State Key Project of Fundamental Research on Pollutant Control in Coal Combustion, Grant G1999022212, and the experimental work by Mr. Heping Cui and Ms. Qin Gao. EF990082G
