Energy & Fuels 2006, 20, 1855-1861
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Effect of Operation Parameters on the Slagging near Swirl Coal Burner Throat Changfu You* and Yong Zhou Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua UniVersity, Beijing, 100084, China ReceiVed January 9, 2006. ReVised Manuscript ReceiVed June 10, 2006
Fluid flow, heat transfer, coal combustion, and slagging processes had been numerically simulated near a swirl burner throat. The effect of the ratio distribution of each burner air, their swirling numbers, and the coal character on the slagging process had been analyzed. The computation results indicate that the maximal stickingparticle numbers occur at the uppermost waterwall, while the sticking-particle number at neither waterwall near the swirl burner outlet is very small. The swirling number has a significant effect on the number of the sticking particle. The sticking-particle number increases rapidly with the increment of the outer secondary air and the primary air-swirling numbers, respectively, because it can strengthen the flow entrainment ability to carry more particles to the waterwall. The inner secondary air has a complicated influence on the slagging process. When the inner secondary air-swirling number is about middle intensive degree (about 0.9), the stickingparticle number reaches maximum. If the inner secondary air-swirling number continues increasing, then the coal particles will combust completely and reduce the particle concentration, thus decrease the sticking-particle number. The ratio of each air has a slight influence on the sticking-particle number relative to the swirling number. The coal particles with small mean diameter combust completely, which can reduce the stickingparticle number.
1. Introduction Coal is globally used as the energy resource of the thermal power plants, mostly as pulverized coal. As for the pulverized coal fired boilers, emissions and ash troubles are a main concern in designing and operation. Slagging is one of the ash troubles sometimes found on the furnace wall, typically near the burner throat. In the case that the slagging becomes large, it may plug the throat partially or the bottom of the furnace may suffer damage hit by fallen slagging. Swirl burner has been broadly used for many coal combustion processes with its low pollutant emission and good combustion stability. However, in some practical applications, the slagging problem near the burner throat cannot be absolutely eliminated, which is a potential risk for the boiler operation. It is wellknown that the slagging formation is strongly related to ash properties (ash-fusion temperature, etc.), coal particle size, the flow pattern near the burner, thermal conditions, etc., and to control these conditions will be necessary to reduce slagging on the burner throat. Recently, considerable advances have been made in developing models to predict ash deposition behavior.1-4 Wang1 had conducted the modeling of ash deposition in large-scale coal * To whom correspondence should be addressed. Telephone: +86-1062781740. E-mail:
[email protected]. (1) Wang, H. F. Modeling of Ash Formation and Deposition in PC Fired Utility Boilers. Ph.D. Thesis, Brigham Young University, Provo, UT, 1998. (2) Fan, J. R.; Zha, X. D.; Sun, P. Simulation of ash deposit in a pulverized coal-fired boiler. Fuel 2001, 80, 645-654. (3) Lee, F. C. C.; Lockwood, F. C. Modeling ash deposition in pulverized coal-fired applications. Prog. Energy Combust. Sci. 1999, 25, 117-132. (4) Costen, P. G.; Lockwood, F. C.; Siddique, M. M. Mathematical modeling of ash depostion in pulverized fuel-fired combustors. In Proceedings of the Combustion Institute; Combustion Institute: Pittsburgh, PA, 2000; pp 2243-2250.
Figure 1. Schematic diagram of the burner model.
combustion facilities. Detailed analyses of coal ash deposits were performed with the use of scanning electron microscopy (SEM), X-ray, and image analysis to characterize local deposit properties as a function of the position. Fan et al.2 developed a model to simulate deposition growth under slagging conditions and simulated the coal combustion and slagging process in the whole furnace. The predictions indicate that the numerical model can be used to optimize the design and operation of pulverized coalfired boilers. In regard to the slagging process nearby the throat of the swirl burner, it should be affected by the operation parameters of the burner, such as the swirling number. The present research is focused on this topic. The simplified structure scheme of the general swirl burner is displayed in Figure 1. The pulverized coal carried by air passes through the primary air tube into the furnace. Both primary and secondary airs are all rotating flows. The primary air rotates when it passes through the volute drew
10.1021/ef0600107 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/11/2006
1856 Energy & Fuels, Vol. 20, No. 5, 2006
You and Zhou accuracy of the turbulence model used in the research, the experiment had been conducted. The PDPA technique made by DANTEC Corporation was used to measure the velocity near the burner throat. The fog generator machine used in theatrics had been used to make flow tracers whose mean diameter was about 7 µm. The burner model and measurement points are shown in Figure 2, where the measurements had been made on the lines. In the measurements, as the flow is swirling, a strong centrifugal force resulted in many fog particles sticking on the window, which affected the laser penetration and then affect the measurement accuracy. According to the experience of the earlier PDPA applications, the measurement error was estimated to be about 5%.
Figure 2. Measurement layout for PDPA.
in the left part of the schematic Figure 1 before entering the furnace. The secondary air rotates when it passes through the tangential vanes. There are two vanes installed in the air vent pipe to produce the inner and outer secondary airs, which were shown in the right side in Figure 1 expressed with X. The tertiary air is nonswirl-flow and passes through the central tube into the furnace. The slagging process on the waterwall near the swirl burner outlet had been numerically simulated. To validate the numerical results, phase Doppler particle analyzer (PDPA) had been used to measure the flow field near the burner outlet. The effect of the ratio distribution of each burner air, their swirling numbers, and coal character on the slagging process had been studied. 2. Experimental Measurement and Computational Validation The flow near the burner throat is strong swirling flow, which is a very anisotropic turbulence flow.5 The current turbulence models should be validated before their applications. To test the
The same computational domain as the actual experimental rig had been numerically simulated. The inlet boundary conditions had been obtained with measurement of IFA300-4 hot-wire machine made by TSI Corporation. A realizable κ- turbulence model is the best model to simulate the strong swirling flow because it improves the turbulence viscosity coefficient µT and combines the angle deformation rate, i.e., rotation motion, into the equation.6 Mean velocities between experimental data and computation results were compared at each measuring point along the radial direction at each plane displayed in Figure 2 to validate the computation model as precisely as possible. Figures 3 and 4 respectively show the comparison of the axial and tangential velocities at each plane between the experimental data and computation results in the condition of 30% opening of the outer secondary air vane. In the figures, the horizontal ordinate represents the radial direction symbolized by R/D, where R is the actual radial size and D is the diameter of the burner outlet (D ) 0.16 m). The vertical ordinate represents either the axial velocity or the tangential velocity. Generally, the agreement of global tendency of both axial and tangential velocity change for this case is good, especially near the outlet of the swirl burner model. Using the simulation method can get believable result.
Figure 3. Comparison of axial velocities between experimental data and computational results in the condition of 30% opening of outer secondary air vane.
Figure 4. Comparison of tangential velocities between experimental data and computational results in the condition of 30% opening of outer secondary air vane.
Operation Parameters on the Slagging
Energy & Fuels, Vol. 20, No. 5, 2006 1857 Table 1. Proximate and Element Analysis of Coal as Input
Figure 5. Computational domain.
3. Computation of the Flow Field near the Burner Throat 3.1. Determination of the Computational Domain. Because the boundary condition of the computational domain has a great effect on the flow field, the boundary condition should be determined as close as possible to the actual physics condition. In the present study, the flow pattern near the burner throat had been focused. The assumption had been applied that the flow and temperature fields far away from the burner have no effect on the slagging on the waterwall near the burner. On the basis of this assumption, the computational domain had been selected as displayed in Figure 5, in which the axis represents the axis of the swirl burner and the symmetry represents the confluence field of the two burners arranged on the two opposite walls. The inlets of the computational domain were from the outlets of the burner. The calculations were carried out in an axisymmetric 2D domain with a swirling velocity component. The slope represents the flow direction after confluence of two flows, and it is similar to the actual physics phenomenon.
Vdaf (%)
Cdaf (%)
Hdaf (%)
Odaf (%)
Ndaf (%)
Sdaf (%)
Qnet.daf (MJ/kg)
30.4
82.633
4.716
10.598
1.383
0.669
31.856
The effect of three sloping degrees (s ) 1, 1.5, and 2 m in Figure 5, corresponding to cases numbers 1, 2, and 3) on the flow field near the burner outlet had been tested. Figure 6 gives the comparison of velocities between three structures, while Ω (swirling number defined in the literature7) is 1.7. For the real furnace, the radius of the burner outlet, rb, is 2 m and R is the actual radial size in Figure 6. The swirling numbers of the primary air, the inner secondary air, and the outer secondary air are at the same level. In these figures, all of the results of these three computational domains show excellent agreement. In the current research, the number 2 computational domain had been used to conduct the following computations. 3.2. General Analysis of the Combustion Character. The PDF combustion model had been used to simulate the volatile combustion, where Bate PDF integration was used to obtain more accurate results than the Delta PDF approach. The equilibrium system consisted of 13 chemical species: C, C (solid char), CH4, CO, CO2, H, H2, H2O, N, N2, O, O2, and OH. The effects of distribution of each air, their swirling numbers, and coal characters on the combustion process were studied. The input data of the coal for the combustion model is listed in Table 1, where the data of the proximate and element analysis are given. The moisture, ash, and fixed carbon are 6.62, 16, and 46.98%, respectively. A kinetics/diffusion-limited surface reaction model had been used in the calculations to simulate char combustion. This model assumes that the surface reaction rate is controlled by the chemical reaction rate and diffusion rate.8,9 A model developed by the literature10 had been used to simulate the radiation heat exchange between the gas and particle phases. Figure 7 shows the furnace temperature field, carbon and oxygen concentration field, and other parameter distribution of
Figure 6. Comparison of velocities between three structures when Ω is 1.7.
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You and Zhou
Figure 7. Global combustion character.
one case near the swirl burner outlet. The inner and outer secondary air-swirling number is 0.5; the ratio of the outer secondary air is 65%; the ratio of the primary air is 10%; and the ratio of the tertiary air is 3%. Figure 7a gives axial velocity vector distribution, and it can be seen that, nearby the axis, the axial velocity is negative and can reflow high-temperature flue gas for stable combustion. In Figure 7b, it can be seen that a high local temperature area appears in the joint of the secondary air outlet and primary air outlet, and then this area broadens along the radial direction. In this area, the temperature is high up to 1600 °C. In Figure 7c, there is a high carbon concentration area that appears in the same location with a high temperature area. In Figure 7d, the oxygen has been consumed very rapidly in the same domain. On the basis of the above results, a conclusion can be obtained that there are three “high” areas (high temperature and high carbon and high oxygen concentrations) near the joint of the secondary air outlet and primary air outlet and it reflects coal combust intensively in this area. In parts c and d of Figure 7, there are also a high oxygen concentration and low carbon density near the waterwall. In comparison with the oxygenlean situation (reducing case), the oxygen-rich phenomenon (oxidation case) increases the melting point of ash and reduces (5) Li, Z. Q.; Sun, R.; Chen, L. Z.; Wan, Z. X.; Wu, S. H.; Qin, Y. K. Effect of primary air flow types on particle distributions in the near swirl burner region. Fuel 2002, 81, 829-835. (6) You, C. F.; Qi, H. Y.; Xu, X. C.; Baudoin, B. Numerical simulation of flow field in tangentially fired boiler using different turbulence models and discretization schemes. Power Eng. (in Chinese) 2001, 21, 1128-1131. (7) Rong, L. N.; Yuan, Z. F.; Liu, Z. M. Principle of Electric Power Boiler, in Chinese; China Electric Power Press: Beijing, China, 1997; p 141. (8) Baum, M. M.; Street, P. J. Predicting the combustion behavior of coal particles. Combust. Sci. Technol. 1971, 3, 231-243. (9) Field, M. A. Rate of combustion of size-graded fraction of char from a low rank coal between 1200K-2000K. Combust. Flame 1969, 13, 237252. (10) Siegel, R.; Howell, J. R. Thermal Radiation Heat Transfer; Hemisphere Publishing Co.: Washington, D.C., 1992.
Figure 8. Effect of inner secondary air-swirling number on the slagging.
the particle number sticking to the waterwall.11 It indicates that the burner operated in this condition can decrease the slagging problem to some extent. When the flow progresses along the axis, the outer secondary air rapidly supplies into the main flow to strengthen the later combustion. 4. Simulation and Discussion of the Slagging on the Waterwall 4.1. Slagging Model. Several slagging models had been developed recently to predict the slagging and fouling to systems firing pulverized coal.12 On the basis of the application review (11) Cen, K. F.; Fan, J. R.; Chi, Z. H. PreVention Theory and Calculation of Ash Accumulation, Slagging, Erosion and Corrosion of Boiler and Heat Exchanger, in Chinese; Science Press: Beijing, China, 1994. (12) Wang, H. F.; Harb, J. N. Modeling of ash deposition in large-scale combustion facilities burning pulverized coal. Prog. Energy Combust. Sci. 1997, 23, 267-282.
Operation Parameters on the Slagging
Energy & Fuels, Vol. 20, No. 5, 2006 1859
Figure 9. Distribution of sticking-particle number along the waterwall.
of the slagging models, a conclusion can be shown that there are many factors influencing the slagging process, for example, particle or waterwall surface temperature, particle incident angle and velocity, and particle component. The particle viscosity is considered primarily, and it is the most important factor. Currently, most research used critical viscosity to study the effect of the particle viscosity on the slagging progress that can be denoted as the sticking-particle number. If the particle viscosity is smaller than the critical viscosity, it can be regarded that this particle can completely stick on the waterwall and define the sticking probability as 1. If the particle viscosity is larger than critical viscosity, the sticking probability is the rate of the critical and particle viscosities. A detailed formula is followed as eq 1.
pi(Tps) )
µref µ > µref µ
(1)
pi(Tps) ) 1 µ e µref Here, pi is the sticking probability of the particle group having the average viscosity µ marked as i. Tps is the temperature of the particle group i. The outstanding advantage of this model is simple and fit to the engineering application. All current studies used this model to simulate the slagging progress to get the sticking-particle number.
Generally, the critical viscosity is regarded as the constant during the slagging progress. The value is commonly selected as 104 Pa s. In the current research, a temperature subarea method13 had been used to compute the particle viscosity. The formula of the high- and low-temperature viscosities are different, and then the maximum viscosity of these two viscosities is selected. This model is precise enough if the particle critical viscosity in the interval is 104-109 Pa s, and it is just in the range of the current selection viscosity; therefore, this model can supply the creditable result and has a wide application. On the basis of the computed temperature field, a stochastic trajectory model had been used to compute the particle trajectory and count the sticking-particle number. The total particle trajectory number was 1 000 000. 4.2. Effect of the Inner Secondary Air-Swirling Number. All of the work conditions for the slagging simulations were the same as the combustion cases. Figure 8 displays the effect of the inner secondary air-swirling number on the slagging. It can be seen that the sticking-particle number first increases with the increment of the inner secondary air-swirling number. Then, (13) Senior, C. L.; Srinivasachar, S. Viscosity of ash particles in combustion systems for prediction of particle sticking. Energy Fuels 1995, 9, 277-283.
1860 Energy & Fuels, Vol. 20, No. 5, 2006
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Figure 10. Effect of outer secondary air ratio on the slagging. Figure 13. Effect of primary air-swirling number on the slagging.
Figure 11. Effect of outer secondary air-swirling number on the slagging.
Figure 14. Effect of tertiary air ratio on the slagging.
Figure 15. Effect of coal mean diameter on the slagging. Figure 12. Effect of primary air ratio on the slagging.
the number decreases to a certain level with the increment of the inner secondary air-swirling number. When the inner secondary number is 0.9, there is a maximum value. Figure 9 shows the distribution of the sticking-particle number along the waterwall. For these cases, the maximal stickingparticle numbers occur at the uppermost waterwall and, generally, the sticking-particle numbers at the upper waterwall are much larger than those near the burner outlet. Generally, the ratio of the sticking-particle number out of the total particles is very small, and the maximum value is 0.3% of the total particles. It can be regarded that the inner secondary air has little effect on the particle sticking. Because the axial velocity near the
waterwall is very small but the tangential velocity is large, it is easy to reflow the particle to the main flow and prevent particles moving from the main flow to the vicinity of the waterwall. 4.3. Effect of the Outer Secondary Air Ratio. Figure 10 displays the effect of the outer secondary air ratio on the slagging. It can be seen that the sticking-particle number first increases with the increment of the ratio. Then, the number decreases to a certain level. When the ratio is 70%, there is a maximum value of the sticking-particle number. In all computations, the swirling number of the outer secondary air was kept constant; therefore, the tangential velocity was increased with the increment of the outer secondary air axial velocity to maintain the swirling number. A large tangential velocity results
Operation Parameters on the Slagging
Energy & Fuels, Vol. 20, No. 5, 2006 1861
Figure 16. Comparison of the number of sticking particles between some parameters.
into the flow moving toward the waterwall direction, and it makes more particles stick to the waterwall. As the outer secondary air ratio continues to increase, the rigidity of the air is the dominant force of the flow movement, and it carries more particles to the latter furnace. This phenomenon reduces the chance of the particle impaction to the wall. Generally, the ratio of the sticking-particle number out of the total particle is very small, and the maximum value is 0.8% of the total number. 4.4. Effect of the Outer Secondary Air-Swirling Number. Figure 11 gives the effect of the outer secondary air swirling on the slagging number. The sticking number gradually increases with the increment of the outer secondary air-swirling number. The reason is that a large tangential velocity results in the flow moving toward the waterwall direction, and it makes more particles stick to the wall. 4.5. Effect of the Primary Air Ratio. Figure 12 gives the effect of the primary air ratio on the slagging. The sticking number slowly increases with the increment of the primary air ratio. The sticking-particle numbers in all cases are at the same level. When the primary air ratio accounts of 20% of all flow amounts, the rate of the sticking number is about 0.2%. 4.6. Effect of the Primary Air-Swirling Number. Figure 13 gives the effect of the primary air-swirling number on the slagging. The sticking number quickly increases with the increment of the primary air-swirling number. When the primary air-swirling number is 1.7, the sticking-particle number is several times larger than that of the small swirling numbers. The reason is that a large primary air-swirling number produces a large centrifugal force, which will lead most coal particles moving toward the outside of the primary air, forming dense coal flow, and the rotating secondary air carries more coal particles to the waterwall. This phenomenon causes more sticking-particle numbers. However, the ratio of the sticking-particle number out of the total particle is still very small, and the maximum rate is 0.7% of the total particles. 4.7. Effect of the Tertiary Air Ratio. Figure 14 gives the effect of the tertiary air ratio on the slagging. The stickingparticle number first decreases with the increment of the tertiary air ratio. Then, the number maintains at a certain level and then increases again with the increment of the tertiary air ratio. Generally, the sticking-particle numbers are almost at the same level in all cases. It can be regarded that the tertiary air has little effect on the sticking-particle number. 4.8. Effect of the Coal Mean Diameter. Distribution of the coal diameter satisfies the Rosin-Rammler formula. Figure 15
gives the effect of the coal mean diameter on the slagging. The sticking number increases with the increment of the coal mean diameter. For the particle larger than 75 µm in diameter, the increment degree of the sticking-particle number is varies gently. 4.9. Comparison of the Number of Sticking Particles between Some Parameters. When all of the results are summerized, Figure 16 gives the comparison of the number of sticking particles between some parameters. It can be seen that the swirling number of the outer secondary air has a significant effect on the number of particles sticking on the waterwall because it can strengthen the flow entrainment ability to carry more particles to the wall. In the real case, to reduce the slagging, more attention should be paid to the outer secondary air. 5. Conclusion On the basis of the numerical results of the coal combustion for the swirl burner, a sticking model had been used to compute the number of particles sticking to the waterwall. The slagging process on the waterwall near the swirl burner outlet had been numerically simulated. The effect of the ratio distribution of each burner air, their swirling numbers, and the coal character on the slagging process had been studied. The computation results indicate that the maximal stickingparticle numbers occur at the uppermost waterwall, while the sticking-particle number at neither waterwall near the swirl burner outlet is relatively small. The swirling number has a significant effect on the number of the sticking particle. The sticking-particle number increases rapidly with the increment of the outer secondary air and the primary air-swirling numbers, respectively, because it can strengthen the flow entrainment ability to carry more particles to the waterwall. The inner secondary air has a complicated influence on the slagging process. When the inner secondary air-swirling number is at about middle intensive degree (about 0.9), the sticking-particle number reaches a maximum. If the inner secondary air-swirling number continues to increase, the coal particles will combust completely and reduce the particle concentration, thus decrease the sticking-particle number. The ratio of each air has a slight influence on the sticking-particle number relative to the swirling number. The coal particles with a small mean diameter combust completely, which can reduce the sticking-particle number. EF0600107
