- Email: [email protected]
particle flow characteristics of a centrally fuel rich swirl coal combustion burner
Available online at www.sciencedirect.com
Fuel 87 (2008) 2102–2110 www.fuelfirst.com
Gas/particle flow characteristics of a centrally fuel rich swirl coal combustion burner Zhichao Chen *, Zhengqi Li, Fuqiang Wang, Jianping Jing, Lizhe Chen, Shaohua Wu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China Received 27 April 2007; received in revised form 3 October 2007; accepted 5 October 2007 Available online 29 October 2007
Abstract A three-component particle-dynamics anemometer is used to measure the characteristics of two-phase gas/particle flows in the nearburner region for a centrally fuel rich swirl coal combustion burner using a gas/particle two-phase test facility. Velocities, mean particle diameters and particle-volume flux profiles were obtained. The primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, the mean radial velocities and tangential velocities are low and a zone of high particle-volume flux and large particle size is formed. The influence of gas/particle flow characteristics on combustion is analyzed. 2007 Elsevier Ltd. All rights reserved. Keywords: Burner; Gas/particle flow; Coal combustion; Three-dimensional PDA
1. Introduction Chinese coals are mostly of low grade with low calorific value. They either have small amounts of volatile matter or high moisture and/or ash content. Generally the flame from these coals is not stable. The ashes also have low ash fusion points and thus there is a tendency to slag in the furnace. The power industry requires coal combustion techniques that show flame stability, no slagging propensity and high combustion efficiency that also meet pollution control standards. The quality of coal provided to power plants often fluctuates and is usually of low grade. Kurose et al. investigated the influence of coal type on combustion characteristic and NOx formation of burner [1–4]. It is very difficult to maintain a stable flame with this type of coal, especially when the load is low. This also lowers the combustion efficiency. When burning low-grade coal, low-NOx burners designed to burn high-grade coal often *
Corresponding author. Tel.: +86 451 86418854; fax: +86 451 86412528. E-mail address: [email protected] (Z. Chen). 0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.10.005
cannot meet power industry requirements such as flame stability and no slagging propensity [5–7]. To solve these problems, Li and co-workers proposed a new burner, the centrally fuel rich (CFR) swirl coal combustion burner (Fig. 1a) based on the radial bias combustion (RBC) burner [8–10] and enhanced ignition-dual register burner [11] in 2003. Both the inner and outer secondary air is swirling. The CFR burner has no central pipe compared to the RBC burner. The primary air–coal mixture duct is in the center of the burner and the primary air is non-swirling. Cone separators are installed in the primary air–coal mixture duct to concentrate the pulverized coal into the central zone of the burner. A particle-dynamics anemometer (PDA) is useful for simultaneous measurement of the motion of liquid and solid particles in two-phase flow [12–14]. Zhou et al. investigated the flow structures and mixing mechanisms of a two-phase gas–solid jet for a burner in a utility boiler [15]. Black and McQuay collected a detailed set of particle data in a co-axial jet and a swirling flow to assess the influence of particle shape on the particle dynamics using a PDA system [16].
Z. Chen et al. / Fuel 87 (2008) 2102–2110
a
outer secondary air inner secondary air cone separators
primary air axial vanes tangential vanes fuel-lean primary air fuel-rich primary air and glass beads
φ 176
b
inner secondary air outer secondary air Fig. 1. CFR burner and CFR burner model (dimensions in mm).
A three-dimensional PDA system was used to study the gas/particle flow characteristics of the CFR burner. It is virtually impossible to replicate all the physical and chemical processes of a full-sized industrial burner in a scaled down model used in research, but it would be too expensive to conduct experiments in a full-sized burner. However, results from a great number of different small scale coldflow tests compared to those of full-size burner tests show that reliable predictions can be made from the scaled down model tests [17–19]. For example, Pickett et al. [19] found that the velocity profiles for reacting flow showed similar trends and patterns to those observed in cold-flow experiments.
2103
between Doppler signals received by three detectors located in different positions. The instrument uses 60· fiber flow optics and 57 · 10 PDA receiver optics. Several optical configurations from 0 to 500 mm are radially available. All instrument settings, such as bandwidth and voltage, are computer controlled. An analog-digital converter allows the computer to read the anode current of the photomultipliers. The combination of photomultiplier and particle velocity correlation bias can contribute to uncertainty, but the error is likely to be small. Overall uncertainties for measured values of the mean velocity, particle diameter and particle-volume flux are 1%, 4% and 30%, respectively, and the measurable ranges for size and velocity are 0.5–1000 lm and 500 to 500 m/s, respectively. The test facility is illustrated in Fig. 2. It consists of a suction device, a feeder, a burner model, a test chamber and a cyclone separator. An air flow is induced from the wind box to the burner by the suction device. Glass beads are fed via the feeder into the fuel-rich primary air duct. The degree of sphericity of glass beads is larger than 95%. The density of the glass beads used was 2500 kg/m3. The particle size distribution obtained by PDA is shown in Fig. 3. The mean diameter was 42 lm. Since coal particles cannot meet the steradian and reflectance characteristics required for PDA particle measurements, glass beads, which do meet these requirements, were used instead. The full industrial-scale CFR burner studied in the experiments was designed for a 1025 t/h coal-fired boiler. For installation in the test facility, the burner had to be scaled. A scale ratio of 1:7 was employed for the model burner. No enricher was mounted in the CFR burner model (see Fig. 1b) and glass beads were fed only into the fuel-rich duct. This simulates the extreme case in which particles in the primary air are all concentrated into the central zone of the burner. The primary air velocity was 10.0 m/s, the inner secondary air velocity was 10.58 m/s, and the outer secondary air velocity was 16.07 m/s. The fuel-rich primary air particle mass concentration, which is
6
2. Experimental
7
A three-dimensional PDA made by Dantec was used in this study. The instrument includes an argon ion laser, a transmitter, fiber optics, receiver optics, signal processors, a traversing system, a computer system and a three-dimensional auto-coordinated rack. The PDA is an instrument based on phase Doppler anemometry, which is an extension of laser Doppler anemometry (LDA) [20]. The velocity is measured from the frequency of the Doppler burst as for LDA [19]. Using the PDA, the velocity, size and concentration of two-phase flow can be measured [21–24]. The PDA uses the proven phase Doppler principle for simultaneous non-intrusive and real-time measurements of three velocity components and turbulence characteristics, and makes use of new methods for phase differences
11
8 9 10
r x
5 4 3 2 1
12 13
(1) wind box, (2) valve, (3) electronicscale, (4) feeder, (5) particle reservoir, (6) burner model, (7) test chamber, (8) platform, (9) equalizing hole, (10) bracket, (11) cyclone separator, (12) brief flapper, (13) sucker. Fig. 2. Schematic drawing of the test facility.
2104
Z. Chen et al. / Fuel 87 (2008) 2102–2110
balance. Therefore, the total particle mass flow rate at the inlet was obtained by integrating the mass flux profile. The global mass flow rate was obtained by weighing particles collected during a certain time period. In addition, a correction factor was applied to the mass flux measurements for all other cross-sections [8–10,24].
6
particle number %
5 4 3
3. Results and discussion
2
Gas/particle flow characteristics were measured for cross-sections of x/d = 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, and 2.5, where x is the distance to the exit of the burner along the jet flow direction (Fig. 1b), and d is the outer diameter of the outer secondary air duct, which is B176 mm.
1
0
30
60
90
120
150
Dμm
3.1. Distributions of gas/particle velocities
Fig. 3. Particle size distribution.
defined as the ratio of particle mass flow rate to primary air mass flow rate, was 0.20 kg (fuel)/kg (air), and that of the fuel-lean concentration was 0 kg (fuel)/kg (air). During the experiment some of the smaller particles were lost due to the low efficiency of the cyclone separator. The particle material had to be frequently renewed to maintain the same particle size distribution as closely as possible. The ratio of the test chamber diameter to the nozzle diameter of the secondary air outflow was 4.91. Ratios greater than 3 are considered to be low-confinement flows [25]. Particles with diameters from 0 to 8 lm were used to trace the air flow, and particles with diameters from 10–100 lm were used to represent the particle phase flow [8–10]. Particles with diameters between 0 and 100 lm were used for analysis of the particle-volume flux. The main factors that determine an accurate concentration value are the setting parameters of PDA system, the properties of particles and particle concentration. In the experiment, the degree of sphericity of glass beads employed as seeds is larger than 95%, which is advantageous in obtaining an accurate concentration. The fuel-rich primary air particle mass concentration was 0.20 kg (fuel)/ kg, which corresponds to dispersed two-phase flows. Experiment indicates that the PDA technique is suitable for spherical particle concentration and local flux measurements in dispersed two-phase flows [23]. In the same crosssection, the setting parameters of the PDA system are the same. Therefore, the difference in the error in the same cross-section is small. Thus, the measured values for mass flux can indicate a relative value. For example, from Fig. 6a in the cross-section x/d = 0.1, the measured particle-volume flux in the radius from 0 to 20 mm is much larger than that in the radius from 20 to 350 mm. Glass beads were fed only into the fuel-rich primary air duct (see Fig. 1b). The measured particle-volume flux distribution shows the real distribution. Owing to uncertainties in the determination of the cross-section of the control volume, the measured mass flux was corrected using global mass
Fig. 4 shows profiles of the gas/particle mean axial velocities for the CFR burner. In four cross-sections from x/d = 0.5 to 1.5, in a radius range from 0 to 20 mm, there is a slip velocity between the gas and particle velocities, and the particle axial velocity is greater than the air axial velocity. This means that the gas velocity lags behind the particle velocity. The measured gas/particle slip velocity is big near the centerline. The behavior of particles with different diameters is strongly governed by inertial effects in this complex flow. The larger particles have the highest velocities due to their larger inertia. The particles maintain a higher velocity than the air flow in the core region of the test section and within the central recirculation zone [24,26]. The measured gas/ particle slip velocity is quite small far from the centerline. It is caused by the fact that particles from completely different directions cross a certain location. This means that both particles issuing straight from the inlet, which still have positive velocities, and recirculating particles from the central recirculation zone and the recirculation zone near the wall, which comes from further downstream, are sampled at these locations. It is a consequence of the so-called ‘‘history effect’’ [24,26]. From the burner jet to the x/d = 0.7 cross-section there are two peaks in the profiles of the mean axial velocities for the gas and particles: the peak near the center is the primary flow zone for the gas/particle mixture and the peak near the wall is the secondary air flow zone. The peak near the center is always greater than that near the wall. With diffusion of the primary gas/particle mixture into the secondary air and diffusion of the secondary air towards the wall, both peaks gradually decrease and the peak near the wall moves towards the wall. For the x/d = 1.0 cross-section, the peak near the wall diminishes, which indicates that the primary and secondary air mix comparatively quickly. Fig. 4 shows profiles of the gas/particle mean radial velocities for the CFR burner. From the burner jet to the x/d = 0.7 section, the profiles show two peaks: the peak near the burner center is the primary gas/particle mixture flow zone, and the peak near the wall is the secondary air flow zone. The peak near the wall is always greater than
Z. Chen et al. / Fuel 87 (2008) 2102–2110
2105
Fig. 4. Profiles of the mean velocity for the gas (–) and particles (s).
the peak near the burner center. With diffusion of the primary gas/particle mixture into the secondary air and diffusion of the secondary air towards the wall, the two peaks move towards the wall. With jet development, the profiles of the gas/particle mean radial velocities become flat. Near the center of the test chamber from the burner jet to the x/ d = 0.7 cross-section, the mean radial velocities are negative. This means that the primary gas/particle mixture flows towards the test chamber center, which enhances the particle-volume flux near the center.
Fig. 4 shows profiles of the gas/particle mean tangential velocities for the CFR burner. Because the primary air is non-swirling, the mean tangential velocities are relatively small in the x/d = 0.1 cross-section (r 6 50 mm). In the x/ d = 0.3 cross-section, the distribution of the mean tangential velocities is a Rankine-type vortex, which is a combination of a solid-body rotational core and a free vortex. Downstream from the x/d = 0.5 cross-section, the peak mean tangential velocities move towards the center of the test chamber, which indicates that the gas/particle mixture
2106
Z. Chen et al. / Fuel 87 (2008) 2102–2110
near the central region begins to swirl, driven by secondary air. With jet development, profiles of the gas/particle mean tangential velocities become flat. Fig. 5 shows profiles of the gas/particle RMS axial velocities for the CFR burner. In cross-sections of x/ d = 0.1–1.5, the profiles show two peaks. In regions inside and outside the peak zone near the test chamber center, RMS axial velocities are comparatively high. The region inside the peak zone is the mixture zone between the primary gas/particle mixture and the reverse flow. The region
outside the peak zone is the mixture zone between the primary gas/particle mixture and the secondary air. The peak near the wall is the secondary air diffusion zone and the RMS axial velocities are comparatively high. The two peaks indicate that there is relatively high axial turbulent diffusion in these regions. With jet development, the two peaks for RMS axial velocities gradually decrease, the peak near the wall diffuses towards the wall and the profiles become flat. The peak near the test chamber center is always greater than the peak near the wall.
Fig. 5. Profiles of RMS velocity for the gas (–) and particles (s).
Z. Chen et al. / Fuel 87 (2008) 2102–2110
Fig. 5 shows profiles of the gas/particle RMS radial velocities for the CFR burner. In the cross-sections from x/d = 0.1 to 0.5, there is a peak for the RMS radial velocities, which indicates that there is comparatively high radial turbulent diffusion in this region. With jet development, the secondary air diffuses towards the wall, the peak gradually decreases and profiles of the gas/particle RMS radial velocities become flat. Fig. 5 shows profiles of the gas/particle RMS tangential velocities for the CFR burner. Gas/particle RMS tangential velocities are high, particular in the secondary zone in the x/d = 0.1 cross-section, where there are two peaks for the RMS tangential velocity in the inner and outer secondary air zones. This indicates that there is large turbulent diffusion ability in these two zones. In the inner secondary air zone of the x/d = 0.3 cross-section, with diffusion of the primary gas/particle mixture into the secondary air, the peak RMS tangential velocity near the center diminishes. With diffusion of the secondary air towards the wall, the peaks gradually decrease and move towards the wall. 3.2. Particle-volume flux Fig. 6 shows the profiles of particle-volume flux in the range from 0 to 100 lm in different cross-sections of CFR, RBC and volute burners. The RBC and volute burners have a central air duct. The primary air of the RBC burner is non-swirling. The secondary air of the RBC burner is divided into the inner swirl secondary air and outer non-swirl secondary air. In the volute burner, the primary air is swirled by the volute and the secondary air by the axial vanes. All three burners have back flows near their walls. In the four cross-sections from x/d = 0.1 to x/d = 0.7, the particle-volume flux shows two peaks and two troughs for the CFR burner. With movement of the secondary air towards the wall, the two peaks gradually decrease and the peak near the wall moves towards the wall. The peak zone near the center of the test chamber is the primary gas/particle mixture flow zone and that near the wall is the secondary air flow zone. The peak near the center is much greater than the peak near the wall, and the absolute value of the trough near the center is also greater than that of the trough near the wall. Resulting from the burner structure and particle inertia (Fig. 1), particles in the fuel-rich primary air duct are ejected directly into the central zone and form the peak for the particle-volume flux near the jet axis. With jet development, in the cross-sections for x/d > 0.5, the particle-volume flux near the jet axis gradually decreases. In the x/d = 1.0 crosssection, the peak near the wall diminishes and the two troughs combine into one. In the x/d = 0.7 cross-section (168 < r < 218 mm) the particle-volume flux is positive in the peak zone near the wall, while in the x/d = 1.0 crosssection in the radius range 32–278 mm, the particle-volume flux is entirely back flow. This means that particles
2107
in the peak zone near the wall partially penetrate the central recirculation zone. As particles have comparatively high tangential and radial velocities at the center of test chamber, with jet development in cross-sections from x/d = 1.0 to x/d = 2.5 in the radius range 0– 248 mm, the particle-volume flux is negative; therefore, there is a peak for particle-volume flux near the wall. For the RBC swirl burner, in radially measured crosssection fields of x/d = 0.1–1.02, the particle-volume flux profiles show two peak zones due to the burner structure and particle inertia. One peak zone is near the burner center and the other is near the edge of the central recirculation zone; with stream development, the peak values are always greater. For the volute burner, the radial stirring motion provides strong transport ability. Most particles are distributed in the wall zone, which is far from the center of the burner. In the burner central zone, the particle-volume flux is very small [5]. Compared with the RBC and volute burners, for the same cross-section, the peak particle-volume flux for the CFR burner is much closer to the center of the test chamber. In cross-sections from x/d = 0.3 to x/d = 2.5, some particle-volume fluxes for the CFR burner are negative in the radial direction, which indicates that these particles partially penetrate the central recirculation zone and are then deflected radially and flow back. This circuitous particle path prolongs the particle residence time inside the central recirculation zone. The second peak for the CFR burner in the x/d = 0.7 cross-section is close to zero and in cross-sections from x/d = 1.0 to x/d = 2.5 in the radius range 0–248 mm the particle-volume flux is negative. This means that particles in the second peak zone have flowed into the central recirculation zone. With stream development, the peak particle-volume flux in the secondary air zone of the RBC burner is always greater. This indicates that particles in the second peak zone do not flow into the central recirculation zone or all the particles completely penetrate the central recirculation zone and the residence time is shorter than for the CFR burner. 3.3. Particle diameter Fig. 7 shows mean diameter profiles for particle size in the range 0–100 lm. The mean particle diameter (d10) is the arithmetic mean diameter. In four cross-sections from x/d = 0.1 to x/d = 0.7, the particle diameter near the center of the test chamber is greater than those in other positions. This is because particles are ejected from the center of the fuel-rich primary air duct, and the larger a particle is; the greater its inertia is. Small particles can easily be transferred to the central recirculation zone by air. Particles near the center have small tangential velocities and axial velocities. With jet development, the positions of larger particles show little change. With further jet development, the particle diameter distribution gradually approaches a uniform value.
2108
Z. Chen et al. / Fuel 87 (2008) 2102–2110
Fig. 6. Particle-volume flux profiles for the (a) CFR, (b) RBC and (c) volute burners.
3.4. Influence of gas/particle flow characteristics on combustion The particle residence time in the burner’s central recirculation zone affects particle burnout in the burner region. For the CFR burner, the primary gas/particle mixture partially penetrates the central recirculation zone and is then deflected radially. The residence time of particles in the reducing atmosphere is prolonged. Experiments with a single annular-orifice burner and a single central-orifice burner in a large-scale laboratory combustor [27,28]
indicated that burnout quality in the burner region is primarily influenced by the particle residence time in the inner recirculation zone. Some experiments showed that increasing the fuel concentration within a defined range can increase the flame velocity [29,30]. Pulverized coal is concentrated in the central zone of the burner and thus a zone of high temperature and high fuel concentration is formed. With increasing fuel concentration, the degree of blackness of the fuel stream increases. When radiation heat, which is absorbed from the high-temperature flame of the furnace, then increases,
Z. Chen et al. / Fuel 87 (2008) 2102–2110
2109
Fig. 7. Profiles of the mean particle diameter.
the gas temperature increases. In the central region of the CFR burner, there is a large particle-volume flux, high gas temperature, and a zone with large RMS axial velocities, which exhibits intense heat convection with the central recirculation zone. This is advantageous for coal heating, firing and flame stability. The particle-volume flux and particle diameter are large at the center of the test chamber, and large particles are mainly resident in the burner’s central recirculation zone, where the temperature is high. Thus, the CFR burner is advantageous for burnout and producing a stable flame. The particle-volume flux is large in the CFR central recirculation zone. The primary gas/particle mixture partially penetrates the central recirculation zone and is then deflected radially. The residence time of pulverized coal in the reducing atmosphere is prolonged. The central recirculation zone consists of hot burned gases with a low amount of O2. Devolatilization occurs in the zone and hydrocarbons compete with nitrogen for the available substoichiometric O2. In this reducing environment, NO formation is low and most of the reactive nitrogen is converted to N2. This inhibits the formation of fuel–NOx. Staged mixing of the secondary air decreases the temperature in the near-burner region, which reduces the amount of thermal NO simultaneously with fuel nitrogen-derived NO [31]. Particles are ejected from the center of the CFR burner and have small tangential and radial velocities, with major particles gathering in the burner central zone. This is beneficial for the formation of an oxidizing atmosphere near the water-cooled wall, which increases the ash fusion point and for resisting slagging and high-temperature corrosion.
4. Conclusions In the CFR burner, particles partially penetrate the central recirculation zone and are then deflected radially. Profiles of the gas/particle mean radial velocities show two peak zones. In the x/d = 0.3 cross-section, the distribution
of the mean tangential velocities is a Rankine-type vortex configuration. Profiles of the particle-volume flux show two peaks; with diffusion of the secondary air towards the wall, the two peaks gradually decrease and the peak near the wall moves towards the wall. Four cross-sections from x/d = 0.1 to x/ d = 0.7 show two peak zones and two trough zones. There is a large particle-volume flux at the center of the test chamber. Most of the larger particles are resident at the center and the residence time is prolonged. The gas/particle flow characteristics of the CFR burner are advantageous for restraining NOx formation and resisting slagging and high-temperature corrosion. Acknowledgements This work was supported by Hi-Tech Research and Development Program of China (Contract No. 2007AA05Z01), the Ministry of Education of China via the 2004 New Century Excellent Talents in University (Contract No. NECT-04-0328), Heilongjiang Province via 2005 Key Projects (Contract No. GC05A314), Key Project of the National Eleventh-Five Year Research Program of China (Contract No. 2006BAA01B01) and the National Basic Research Program of China (Contract No. 2006CB200303). References [1] Kurose R, Ikeda M, Makino H. Combustion characteristics of high ash coal in a pulverized coal combustion. Fuel 2001;80:1447–55. [2] Kurose R, Tsuji H, Makino H. Effects of moisture in coal on pulverized coal combustion characteristics. Fuel 2001;80:1457–65. [3] Kurose R, Makino H, Suzuki A. Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner. Fuel 2004;83:1777–85. [4] Kurose R, Ikeda M, Makino H, Kimoto M, Miyazaki T. Pulverized coal combustion characteristics of high-fuel-ratio coals. Fuel 2004;83:693–703. [5] Rees DP, Douglas Smoot L, Hedman PO. Nitrogen oxide formation inside a laboratory pulverized coal combustor. In: 18th international
2110
[6]
[7]
[8]
[9]
[10]
[11] [12] [13]
[14] [15]
[16]
[17]
[18]
Z. Chen et al. / Fuel 87 (2008) 2102–2110
symposium on combustion. The Combustion Institute; 1981. p. 1305– 11. Maier H, Spliethoff H, Kicherer A, Fingerle A, Hein KRG. Effect of coal blending and particle size on NOx emission and burnout. Fuel 1994;73:1447–52. Asay BW, Douglas Smoot L, Hedamn PO. Effect of coal moisture on burnout and nitrogen oxide formation. Combust Sci Technol 1983;35:15–31. Li ZQ, Sun R, Chen LZ, Wan ZX, Wu SH, Qin YK. Effect of primary air flow types in particle distributions in the near swirl burner region. Fuel 2002;81:829–35. Li ZQ, Wan ZX, Sun R, Sun SH, Chen LZ, Wu SH, et al. Influence of division cone angles between the fuel-rich and the fuel-lean ducts on gas-particle flow and combustion near swirl burners. Energy 2002;27:1119–30. Li ZQ, Sun R, Wan ZX, Sun S, Wu SH, Qin YK. Gas-particle flow and combustion in the near-burner zone of the swirl-stabilized pulverized coal burner. Combust Sci Technol 2003;175:1979–2014. Larue AD, Cioffi PL. Low NOx burner development in the USA. Mod Power Syst 1988;8:42–7. Dai GQ, Chen WM, Li JM, Chu LY. Experimental study of solid– liquid two-phase flow in a hydrocyclone. Chem Eng J 1999;74:211–6. Gillandt I, Fritsching U, Bauckhage K. Measurement of phase interaction in dispersed gas/particle two-phase flow. Multiphase Flow 2001;27:1313–32. You CF, Zhou Y. Effect of operation parameters on the slagging near swirl coal burner throat. Energy Fuel 2006;20:1855–61. Zhou H, Cen KF, Fan JR. Experimental investigation on flow structures and mixing mechanisms of a gas–solid burner jet. Fuel 2005;84:1622–34. Black DL, McQuay MQ. Laser-based particle measurements of spherical and nonspherical particles. Int J Multiphase Flow 2001;27: 1333–62. Weber R. Scaling characteristics of aerodynamics, heat transfer, and pollutant emissions in industrial flames. In: Proceedings of the 26th international symposium on combustion, vol. 2; 1996. p. 3343–54. Shin D, Park S, Jeon B, Yu T, Hwang J. Effect of swirling flow by normal injection of secondary air on the gas residence time and
[19]
[20] [21] [22]
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30] [31]
mixing characteristics in a lab-scale cold model combustor. J Mech Sci Technol 2006;20:2310–7. Pickett LM, Jackson RE, Tree DR. LDA measurements in a pulverized coal flame at three swirl ratios. Combust Sci Technol 1999;143:79–106. Zemlianskii VM. A new phase method of measuring particle size with laser Doppler system. J Aerosol Sci 1996;27(Suppl 1):325–6. Moon S, Bae C, Choi J, Abo-Serie E. The influence of airflow on fuel spray characteristics from a slit injector. Fuel 2007;86:400–9. Hu¨bner AW, Tummers MJ, Hanjalic´ K, vander Meer ThH. Experiments on a rotating-pipe swirl burner. Therm Fluid Sci 2003;27: 481–9. Aı´sa L, Garcia JA, Cerecedo LM, Garcı´a Palacı´n I, Calvo E. Particle concentration and local mass flux measurements in two-phase flows with PDA. Application to a study on dispersion of spherical particles in a turbulent air jet. Int J Multiphase Flow 2002;28:301–24. Sommerfeld M, Qiu HH. Characterization of particle laden, confined swirling flows by phase-Doppler anemometry and numerical calculation. Int J Multiphase Flow 1993;19:1093–127. Weber R, Visser BM, Bousan F. Assessment of turbulence modeling for engineering prediction of swirling vortices in the near burner zone. Int J Heat Fluid Flow 1990;11:225–35. Sommerfeld M, Qiu HH. Detail measurements in a swirling particulate two-phase flow by a phase-Doppler anemometer. Int J Heat Fluid Flow 1991;12(1):20–8. Godoy S, Hassan MA, Ismail MA, Lockword FC. Measurements of char burnout in a large scale laboratory combustor. Combust Sci Technol 1991;80:137–50. Abbas T, Costa M, Costen P, Gody S, Lockwood FC. NOx formation and reduction mechanisms in pulverized coal flames. Fuel 1994;73:1423–36. Godoy S, Hirji K, Lockwood FC. Stability limits of pulverized coal firing. J Inst Energy 1986;59:38–44. Horton MD, Goodson FP, Smoot LD. Characteristics of flat, laminar coal-dust flames. Combust Flame 1977;28:187–95. Song YH, Pohl JH, Bee´r JM, Sarofim AF. Nitric oxide formation during pulverized coal combustion. Combust Sci Technol 1982;28: 31–9.
Fuel 87 (2008) 2102–2110 www.fuelfirst.com
Gas/particle flow characteristics of a centrally fuel rich swirl coal combustion burner Zhichao Chen *, Zhengqi Li, Fuqiang Wang, Jianping Jing, Lizhe Chen, Shaohua Wu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China Received 27 April 2007; received in revised form 3 October 2007; accepted 5 October 2007 Available online 29 October 2007
Abstract A three-component particle-dynamics anemometer is used to measure the characteristics of two-phase gas/particle flows in the nearburner region for a centrally fuel rich swirl coal combustion burner using a gas/particle two-phase test facility. Velocities, mean particle diameters and particle-volume flux profiles were obtained. The primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, the mean radial velocities and tangential velocities are low and a zone of high particle-volume flux and large particle size is formed. The influence of gas/particle flow characteristics on combustion is analyzed. 2007 Elsevier Ltd. All rights reserved. Keywords: Burner; Gas/particle flow; Coal combustion; Three-dimensional PDA
1. Introduction Chinese coals are mostly of low grade with low calorific value. They either have small amounts of volatile matter or high moisture and/or ash content. Generally the flame from these coals is not stable. The ashes also have low ash fusion points and thus there is a tendency to slag in the furnace. The power industry requires coal combustion techniques that show flame stability, no slagging propensity and high combustion efficiency that also meet pollution control standards. The quality of coal provided to power plants often fluctuates and is usually of low grade. Kurose et al. investigated the influence of coal type on combustion characteristic and NOx formation of burner [1–4]. It is very difficult to maintain a stable flame with this type of coal, especially when the load is low. This also lowers the combustion efficiency. When burning low-grade coal, low-NOx burners designed to burn high-grade coal often *
Corresponding author. Tel.: +86 451 86418854; fax: +86 451 86412528. E-mail address: [email protected] (Z. Chen). 0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.10.005
cannot meet power industry requirements such as flame stability and no slagging propensity [5–7]. To solve these problems, Li and co-workers proposed a new burner, the centrally fuel rich (CFR) swirl coal combustion burner (Fig. 1a) based on the radial bias combustion (RBC) burner [8–10] and enhanced ignition-dual register burner [11] in 2003. Both the inner and outer secondary air is swirling. The CFR burner has no central pipe compared to the RBC burner. The primary air–coal mixture duct is in the center of the burner and the primary air is non-swirling. Cone separators are installed in the primary air–coal mixture duct to concentrate the pulverized coal into the central zone of the burner. A particle-dynamics anemometer (PDA) is useful for simultaneous measurement of the motion of liquid and solid particles in two-phase flow [12–14]. Zhou et al. investigated the flow structures and mixing mechanisms of a two-phase gas–solid jet for a burner in a utility boiler [15]. Black and McQuay collected a detailed set of particle data in a co-axial jet and a swirling flow to assess the influence of particle shape on the particle dynamics using a PDA system [16].
Z. Chen et al. / Fuel 87 (2008) 2102–2110
a
outer secondary air inner secondary air cone separators
primary air axial vanes tangential vanes fuel-lean primary air fuel-rich primary air and glass beads
φ 176
b
inner secondary air outer secondary air Fig. 1. CFR burner and CFR burner model (dimensions in mm).
A three-dimensional PDA system was used to study the gas/particle flow characteristics of the CFR burner. It is virtually impossible to replicate all the physical and chemical processes of a full-sized industrial burner in a scaled down model used in research, but it would be too expensive to conduct experiments in a full-sized burner. However, results from a great number of different small scale coldflow tests compared to those of full-size burner tests show that reliable predictions can be made from the scaled down model tests [17–19]. For example, Pickett et al. [19] found that the velocity profiles for reacting flow showed similar trends and patterns to those observed in cold-flow experiments.
2103
between Doppler signals received by three detectors located in different positions. The instrument uses 60· fiber flow optics and 57 · 10 PDA receiver optics. Several optical configurations from 0 to 500 mm are radially available. All instrument settings, such as bandwidth and voltage, are computer controlled. An analog-digital converter allows the computer to read the anode current of the photomultipliers. The combination of photomultiplier and particle velocity correlation bias can contribute to uncertainty, but the error is likely to be small. Overall uncertainties for measured values of the mean velocity, particle diameter and particle-volume flux are 1%, 4% and 30%, respectively, and the measurable ranges for size and velocity are 0.5–1000 lm and 500 to 500 m/s, respectively. The test facility is illustrated in Fig. 2. It consists of a suction device, a feeder, a burner model, a test chamber and a cyclone separator. An air flow is induced from the wind box to the burner by the suction device. Glass beads are fed via the feeder into the fuel-rich primary air duct. The degree of sphericity of glass beads is larger than 95%. The density of the glass beads used was 2500 kg/m3. The particle size distribution obtained by PDA is shown in Fig. 3. The mean diameter was 42 lm. Since coal particles cannot meet the steradian and reflectance characteristics required for PDA particle measurements, glass beads, which do meet these requirements, were used instead. The full industrial-scale CFR burner studied in the experiments was designed for a 1025 t/h coal-fired boiler. For installation in the test facility, the burner had to be scaled. A scale ratio of 1:7 was employed for the model burner. No enricher was mounted in the CFR burner model (see Fig. 1b) and glass beads were fed only into the fuel-rich duct. This simulates the extreme case in which particles in the primary air are all concentrated into the central zone of the burner. The primary air velocity was 10.0 m/s, the inner secondary air velocity was 10.58 m/s, and the outer secondary air velocity was 16.07 m/s. The fuel-rich primary air particle mass concentration, which is
6
2. Experimental
7
A three-dimensional PDA made by Dantec was used in this study. The instrument includes an argon ion laser, a transmitter, fiber optics, receiver optics, signal processors, a traversing system, a computer system and a three-dimensional auto-coordinated rack. The PDA is an instrument based on phase Doppler anemometry, which is an extension of laser Doppler anemometry (LDA) [20]. The velocity is measured from the frequency of the Doppler burst as for LDA [19]. Using the PDA, the velocity, size and concentration of two-phase flow can be measured [21–24]. The PDA uses the proven phase Doppler principle for simultaneous non-intrusive and real-time measurements of three velocity components and turbulence characteristics, and makes use of new methods for phase differences
11
8 9 10
r x
5 4 3 2 1
12 13
(1) wind box, (2) valve, (3) electronicscale, (4) feeder, (5) particle reservoir, (6) burner model, (7) test chamber, (8) platform, (9) equalizing hole, (10) bracket, (11) cyclone separator, (12) brief flapper, (13) sucker. Fig. 2. Schematic drawing of the test facility.
2104
Z. Chen et al. / Fuel 87 (2008) 2102–2110
balance. Therefore, the total particle mass flow rate at the inlet was obtained by integrating the mass flux profile. The global mass flow rate was obtained by weighing particles collected during a certain time period. In addition, a correction factor was applied to the mass flux measurements for all other cross-sections [8–10,24].
6
particle number %
5 4 3
3. Results and discussion
2
Gas/particle flow characteristics were measured for cross-sections of x/d = 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, and 2.5, where x is the distance to the exit of the burner along the jet flow direction (Fig. 1b), and d is the outer diameter of the outer secondary air duct, which is B176 mm.
1
0
30
60
90
120
150
Dμm
3.1. Distributions of gas/particle velocities
Fig. 3. Particle size distribution.
defined as the ratio of particle mass flow rate to primary air mass flow rate, was 0.20 kg (fuel)/kg (air), and that of the fuel-lean concentration was 0 kg (fuel)/kg (air). During the experiment some of the smaller particles were lost due to the low efficiency of the cyclone separator. The particle material had to be frequently renewed to maintain the same particle size distribution as closely as possible. The ratio of the test chamber diameter to the nozzle diameter of the secondary air outflow was 4.91. Ratios greater than 3 are considered to be low-confinement flows [25]. Particles with diameters from 0 to 8 lm were used to trace the air flow, and particles with diameters from 10–100 lm were used to represent the particle phase flow [8–10]. Particles with diameters between 0 and 100 lm were used for analysis of the particle-volume flux. The main factors that determine an accurate concentration value are the setting parameters of PDA system, the properties of particles and particle concentration. In the experiment, the degree of sphericity of glass beads employed as seeds is larger than 95%, which is advantageous in obtaining an accurate concentration. The fuel-rich primary air particle mass concentration was 0.20 kg (fuel)/ kg, which corresponds to dispersed two-phase flows. Experiment indicates that the PDA technique is suitable for spherical particle concentration and local flux measurements in dispersed two-phase flows [23]. In the same crosssection, the setting parameters of the PDA system are the same. Therefore, the difference in the error in the same cross-section is small. Thus, the measured values for mass flux can indicate a relative value. For example, from Fig. 6a in the cross-section x/d = 0.1, the measured particle-volume flux in the radius from 0 to 20 mm is much larger than that in the radius from 20 to 350 mm. Glass beads were fed only into the fuel-rich primary air duct (see Fig. 1b). The measured particle-volume flux distribution shows the real distribution. Owing to uncertainties in the determination of the cross-section of the control volume, the measured mass flux was corrected using global mass
Fig. 4 shows profiles of the gas/particle mean axial velocities for the CFR burner. In four cross-sections from x/d = 0.5 to 1.5, in a radius range from 0 to 20 mm, there is a slip velocity between the gas and particle velocities, and the particle axial velocity is greater than the air axial velocity. This means that the gas velocity lags behind the particle velocity. The measured gas/particle slip velocity is big near the centerline. The behavior of particles with different diameters is strongly governed by inertial effects in this complex flow. The larger particles have the highest velocities due to their larger inertia. The particles maintain a higher velocity than the air flow in the core region of the test section and within the central recirculation zone [24,26]. The measured gas/ particle slip velocity is quite small far from the centerline. It is caused by the fact that particles from completely different directions cross a certain location. This means that both particles issuing straight from the inlet, which still have positive velocities, and recirculating particles from the central recirculation zone and the recirculation zone near the wall, which comes from further downstream, are sampled at these locations. It is a consequence of the so-called ‘‘history effect’’ [24,26]. From the burner jet to the x/d = 0.7 cross-section there are two peaks in the profiles of the mean axial velocities for the gas and particles: the peak near the center is the primary flow zone for the gas/particle mixture and the peak near the wall is the secondary air flow zone. The peak near the center is always greater than that near the wall. With diffusion of the primary gas/particle mixture into the secondary air and diffusion of the secondary air towards the wall, both peaks gradually decrease and the peak near the wall moves towards the wall. For the x/d = 1.0 cross-section, the peak near the wall diminishes, which indicates that the primary and secondary air mix comparatively quickly. Fig. 4 shows profiles of the gas/particle mean radial velocities for the CFR burner. From the burner jet to the x/d = 0.7 section, the profiles show two peaks: the peak near the burner center is the primary gas/particle mixture flow zone, and the peak near the wall is the secondary air flow zone. The peak near the wall is always greater than
Z. Chen et al. / Fuel 87 (2008) 2102–2110
2105
Fig. 4. Profiles of the mean velocity for the gas (–) and particles (s).
the peak near the burner center. With diffusion of the primary gas/particle mixture into the secondary air and diffusion of the secondary air towards the wall, the two peaks move towards the wall. With jet development, the profiles of the gas/particle mean radial velocities become flat. Near the center of the test chamber from the burner jet to the x/ d = 0.7 cross-section, the mean radial velocities are negative. This means that the primary gas/particle mixture flows towards the test chamber center, which enhances the particle-volume flux near the center.
Fig. 4 shows profiles of the gas/particle mean tangential velocities for the CFR burner. Because the primary air is non-swirling, the mean tangential velocities are relatively small in the x/d = 0.1 cross-section (r 6 50 mm). In the x/ d = 0.3 cross-section, the distribution of the mean tangential velocities is a Rankine-type vortex, which is a combination of a solid-body rotational core and a free vortex. Downstream from the x/d = 0.5 cross-section, the peak mean tangential velocities move towards the center of the test chamber, which indicates that the gas/particle mixture
2106
Z. Chen et al. / Fuel 87 (2008) 2102–2110
near the central region begins to swirl, driven by secondary air. With jet development, profiles of the gas/particle mean tangential velocities become flat. Fig. 5 shows profiles of the gas/particle RMS axial velocities for the CFR burner. In cross-sections of x/ d = 0.1–1.5, the profiles show two peaks. In regions inside and outside the peak zone near the test chamber center, RMS axial velocities are comparatively high. The region inside the peak zone is the mixture zone between the primary gas/particle mixture and the reverse flow. The region
outside the peak zone is the mixture zone between the primary gas/particle mixture and the secondary air. The peak near the wall is the secondary air diffusion zone and the RMS axial velocities are comparatively high. The two peaks indicate that there is relatively high axial turbulent diffusion in these regions. With jet development, the two peaks for RMS axial velocities gradually decrease, the peak near the wall diffuses towards the wall and the profiles become flat. The peak near the test chamber center is always greater than the peak near the wall.
Fig. 5. Profiles of RMS velocity for the gas (–) and particles (s).
Z. Chen et al. / Fuel 87 (2008) 2102–2110
Fig. 5 shows profiles of the gas/particle RMS radial velocities for the CFR burner. In the cross-sections from x/d = 0.1 to 0.5, there is a peak for the RMS radial velocities, which indicates that there is comparatively high radial turbulent diffusion in this region. With jet development, the secondary air diffuses towards the wall, the peak gradually decreases and profiles of the gas/particle RMS radial velocities become flat. Fig. 5 shows profiles of the gas/particle RMS tangential velocities for the CFR burner. Gas/particle RMS tangential velocities are high, particular in the secondary zone in the x/d = 0.1 cross-section, where there are two peaks for the RMS tangential velocity in the inner and outer secondary air zones. This indicates that there is large turbulent diffusion ability in these two zones. In the inner secondary air zone of the x/d = 0.3 cross-section, with diffusion of the primary gas/particle mixture into the secondary air, the peak RMS tangential velocity near the center diminishes. With diffusion of the secondary air towards the wall, the peaks gradually decrease and move towards the wall. 3.2. Particle-volume flux Fig. 6 shows the profiles of particle-volume flux in the range from 0 to 100 lm in different cross-sections of CFR, RBC and volute burners. The RBC and volute burners have a central air duct. The primary air of the RBC burner is non-swirling. The secondary air of the RBC burner is divided into the inner swirl secondary air and outer non-swirl secondary air. In the volute burner, the primary air is swirled by the volute and the secondary air by the axial vanes. All three burners have back flows near their walls. In the four cross-sections from x/d = 0.1 to x/d = 0.7, the particle-volume flux shows two peaks and two troughs for the CFR burner. With movement of the secondary air towards the wall, the two peaks gradually decrease and the peak near the wall moves towards the wall. The peak zone near the center of the test chamber is the primary gas/particle mixture flow zone and that near the wall is the secondary air flow zone. The peak near the center is much greater than the peak near the wall, and the absolute value of the trough near the center is also greater than that of the trough near the wall. Resulting from the burner structure and particle inertia (Fig. 1), particles in the fuel-rich primary air duct are ejected directly into the central zone and form the peak for the particle-volume flux near the jet axis. With jet development, in the cross-sections for x/d > 0.5, the particle-volume flux near the jet axis gradually decreases. In the x/d = 1.0 crosssection, the peak near the wall diminishes and the two troughs combine into one. In the x/d = 0.7 cross-section (168 < r < 218 mm) the particle-volume flux is positive in the peak zone near the wall, while in the x/d = 1.0 crosssection in the radius range 32–278 mm, the particle-volume flux is entirely back flow. This means that particles
2107
in the peak zone near the wall partially penetrate the central recirculation zone. As particles have comparatively high tangential and radial velocities at the center of test chamber, with jet development in cross-sections from x/d = 1.0 to x/d = 2.5 in the radius range 0– 248 mm, the particle-volume flux is negative; therefore, there is a peak for particle-volume flux near the wall. For the RBC swirl burner, in radially measured crosssection fields of x/d = 0.1–1.02, the particle-volume flux profiles show two peak zones due to the burner structure and particle inertia. One peak zone is near the burner center and the other is near the edge of the central recirculation zone; with stream development, the peak values are always greater. For the volute burner, the radial stirring motion provides strong transport ability. Most particles are distributed in the wall zone, which is far from the center of the burner. In the burner central zone, the particle-volume flux is very small [5]. Compared with the RBC and volute burners, for the same cross-section, the peak particle-volume flux for the CFR burner is much closer to the center of the test chamber. In cross-sections from x/d = 0.3 to x/d = 2.5, some particle-volume fluxes for the CFR burner are negative in the radial direction, which indicates that these particles partially penetrate the central recirculation zone and are then deflected radially and flow back. This circuitous particle path prolongs the particle residence time inside the central recirculation zone. The second peak for the CFR burner in the x/d = 0.7 cross-section is close to zero and in cross-sections from x/d = 1.0 to x/d = 2.5 in the radius range 0–248 mm the particle-volume flux is negative. This means that particles in the second peak zone have flowed into the central recirculation zone. With stream development, the peak particle-volume flux in the secondary air zone of the RBC burner is always greater. This indicates that particles in the second peak zone do not flow into the central recirculation zone or all the particles completely penetrate the central recirculation zone and the residence time is shorter than for the CFR burner. 3.3. Particle diameter Fig. 7 shows mean diameter profiles for particle size in the range 0–100 lm. The mean particle diameter (d10) is the arithmetic mean diameter. In four cross-sections from x/d = 0.1 to x/d = 0.7, the particle diameter near the center of the test chamber is greater than those in other positions. This is because particles are ejected from the center of the fuel-rich primary air duct, and the larger a particle is; the greater its inertia is. Small particles can easily be transferred to the central recirculation zone by air. Particles near the center have small tangential velocities and axial velocities. With jet development, the positions of larger particles show little change. With further jet development, the particle diameter distribution gradually approaches a uniform value.
2108
Z. Chen et al. / Fuel 87 (2008) 2102–2110
Fig. 6. Particle-volume flux profiles for the (a) CFR, (b) RBC and (c) volute burners.
3.4. Influence of gas/particle flow characteristics on combustion The particle residence time in the burner’s central recirculation zone affects particle burnout in the burner region. For the CFR burner, the primary gas/particle mixture partially penetrates the central recirculation zone and is then deflected radially. The residence time of particles in the reducing atmosphere is prolonged. Experiments with a single annular-orifice burner and a single central-orifice burner in a large-scale laboratory combustor [27,28]
indicated that burnout quality in the burner region is primarily influenced by the particle residence time in the inner recirculation zone. Some experiments showed that increasing the fuel concentration within a defined range can increase the flame velocity [29,30]. Pulverized coal is concentrated in the central zone of the burner and thus a zone of high temperature and high fuel concentration is formed. With increasing fuel concentration, the degree of blackness of the fuel stream increases. When radiation heat, which is absorbed from the high-temperature flame of the furnace, then increases,
Z. Chen et al. / Fuel 87 (2008) 2102–2110
2109
Fig. 7. Profiles of the mean particle diameter.
the gas temperature increases. In the central region of the CFR burner, there is a large particle-volume flux, high gas temperature, and a zone with large RMS axial velocities, which exhibits intense heat convection with the central recirculation zone. This is advantageous for coal heating, firing and flame stability. The particle-volume flux and particle diameter are large at the center of the test chamber, and large particles are mainly resident in the burner’s central recirculation zone, where the temperature is high. Thus, the CFR burner is advantageous for burnout and producing a stable flame. The particle-volume flux is large in the CFR central recirculation zone. The primary gas/particle mixture partially penetrates the central recirculation zone and is then deflected radially. The residence time of pulverized coal in the reducing atmosphere is prolonged. The central recirculation zone consists of hot burned gases with a low amount of O2. Devolatilization occurs in the zone and hydrocarbons compete with nitrogen for the available substoichiometric O2. In this reducing environment, NO formation is low and most of the reactive nitrogen is converted to N2. This inhibits the formation of fuel–NOx. Staged mixing of the secondary air decreases the temperature in the near-burner region, which reduces the amount of thermal NO simultaneously with fuel nitrogen-derived NO [31]. Particles are ejected from the center of the CFR burner and have small tangential and radial velocities, with major particles gathering in the burner central zone. This is beneficial for the formation of an oxidizing atmosphere near the water-cooled wall, which increases the ash fusion point and for resisting slagging and high-temperature corrosion.
4. Conclusions In the CFR burner, particles partially penetrate the central recirculation zone and are then deflected radially. Profiles of the gas/particle mean radial velocities show two peak zones. In the x/d = 0.3 cross-section, the distribution
of the mean tangential velocities is a Rankine-type vortex configuration. Profiles of the particle-volume flux show two peaks; with diffusion of the secondary air towards the wall, the two peaks gradually decrease and the peak near the wall moves towards the wall. Four cross-sections from x/d = 0.1 to x/ d = 0.7 show two peak zones and two trough zones. There is a large particle-volume flux at the center of the test chamber. Most of the larger particles are resident at the center and the residence time is prolonged. The gas/particle flow characteristics of the CFR burner are advantageous for restraining NOx formation and resisting slagging and high-temperature corrosion. Acknowledgements This work was supported by Hi-Tech Research and Development Program of China (Contract No. 2007AA05Z01), the Ministry of Education of China via the 2004 New Century Excellent Talents in University (Contract No. NECT-04-0328), Heilongjiang Province via 2005 Key Projects (Contract No. GC05A314), Key Project of the National Eleventh-Five Year Research Program of China (Contract No. 2006BAA01B01) and the National Basic Research Program of China (Contract No. 2006CB200303). References [1] Kurose R, Ikeda M, Makino H. Combustion characteristics of high ash coal in a pulverized coal combustion. Fuel 2001;80:1447–55. [2] Kurose R, Tsuji H, Makino H. Effects of moisture in coal on pulverized coal combustion characteristics. Fuel 2001;80:1457–65. [3] Kurose R, Makino H, Suzuki A. Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner. Fuel 2004;83:1777–85. [4] Kurose R, Ikeda M, Makino H, Kimoto M, Miyazaki T. Pulverized coal combustion characteristics of high-fuel-ratio coals. Fuel 2004;83:693–703. [5] Rees DP, Douglas Smoot L, Hedman PO. Nitrogen oxide formation inside a laboratory pulverized coal combustor. In: 18th international
2110
[6]
[7]
[8]
[9]
[10]
[11] [12] [13]
[14] [15]
[16]
[17]
[18]
Z. Chen et al. / Fuel 87 (2008) 2102–2110
symposium on combustion. The Combustion Institute; 1981. p. 1305– 11. Maier H, Spliethoff H, Kicherer A, Fingerle A, Hein KRG. Effect of coal blending and particle size on NOx emission and burnout. Fuel 1994;73:1447–52. Asay BW, Douglas Smoot L, Hedamn PO. Effect of coal moisture on burnout and nitrogen oxide formation. Combust Sci Technol 1983;35:15–31. Li ZQ, Sun R, Chen LZ, Wan ZX, Wu SH, Qin YK. Effect of primary air flow types in particle distributions in the near swirl burner region. Fuel 2002;81:829–35. Li ZQ, Wan ZX, Sun R, Sun SH, Chen LZ, Wu SH, et al. Influence of division cone angles between the fuel-rich and the fuel-lean ducts on gas-particle flow and combustion near swirl burners. Energy 2002;27:1119–30. Li ZQ, Sun R, Wan ZX, Sun S, Wu SH, Qin YK. Gas-particle flow and combustion in the near-burner zone of the swirl-stabilized pulverized coal burner. Combust Sci Technol 2003;175:1979–2014. Larue AD, Cioffi PL. Low NOx burner development in the USA. Mod Power Syst 1988;8:42–7. Dai GQ, Chen WM, Li JM, Chu LY. Experimental study of solid– liquid two-phase flow in a hydrocyclone. Chem Eng J 1999;74:211–6. Gillandt I, Fritsching U, Bauckhage K. Measurement of phase interaction in dispersed gas/particle two-phase flow. Multiphase Flow 2001;27:1313–32. You CF, Zhou Y. Effect of operation parameters on the slagging near swirl coal burner throat. Energy Fuel 2006;20:1855–61. Zhou H, Cen KF, Fan JR. Experimental investigation on flow structures and mixing mechanisms of a gas–solid burner jet. Fuel 2005;84:1622–34. Black DL, McQuay MQ. Laser-based particle measurements of spherical and nonspherical particles. Int J Multiphase Flow 2001;27: 1333–62. Weber R. Scaling characteristics of aerodynamics, heat transfer, and pollutant emissions in industrial flames. In: Proceedings of the 26th international symposium on combustion, vol. 2; 1996. p. 3343–54. Shin D, Park S, Jeon B, Yu T, Hwang J. Effect of swirling flow by normal injection of secondary air on the gas residence time and
[19]
[20] [21] [22]
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30] [31]
mixing characteristics in a lab-scale cold model combustor. J Mech Sci Technol 2006;20:2310–7. Pickett LM, Jackson RE, Tree DR. LDA measurements in a pulverized coal flame at three swirl ratios. Combust Sci Technol 1999;143:79–106. Zemlianskii VM. A new phase method of measuring particle size with laser Doppler system. J Aerosol Sci 1996;27(Suppl 1):325–6. Moon S, Bae C, Choi J, Abo-Serie E. The influence of airflow on fuel spray characteristics from a slit injector. Fuel 2007;86:400–9. Hu¨bner AW, Tummers MJ, Hanjalic´ K, vander Meer ThH. Experiments on a rotating-pipe swirl burner. Therm Fluid Sci 2003;27: 481–9. Aı´sa L, Garcia JA, Cerecedo LM, Garcı´a Palacı´n I, Calvo E. Particle concentration and local mass flux measurements in two-phase flows with PDA. Application to a study on dispersion of spherical particles in a turbulent air jet. Int J Multiphase Flow 2002;28:301–24. Sommerfeld M, Qiu HH. Characterization of particle laden, confined swirling flows by phase-Doppler anemometry and numerical calculation. Int J Multiphase Flow 1993;19:1093–127. Weber R, Visser BM, Bousan F. Assessment of turbulence modeling for engineering prediction of swirling vortices in the near burner zone. Int J Heat Fluid Flow 1990;11:225–35. Sommerfeld M, Qiu HH. Detail measurements in a swirling particulate two-phase flow by a phase-Doppler anemometer. Int J Heat Fluid Flow 1991;12(1):20–8. Godoy S, Hassan MA, Ismail MA, Lockword FC. Measurements of char burnout in a large scale laboratory combustor. Combust Sci Technol 1991;80:137–50. Abbas T, Costa M, Costen P, Gody S, Lockwood FC. NOx formation and reduction mechanisms in pulverized coal flames. Fuel 1994;73:1423–36. Godoy S, Hirji K, Lockwood FC. Stability limits of pulverized coal firing. J Inst Energy 1986;59:38–44. Horton MD, Goodson FP, Smoot LD. Characteristics of flat, laminar coal-dust flames. Combust Flame 1977;28:187–95. Song YH, Pohl JH, Bee´r JM, Sarofim AF. Nitric oxide formation during pulverized coal combustion. Combust Sci Technol 1982;28: 31–9.