Energy Fuels 2009, 23, 5375–5382 Published on Web 08/26/2009
: DOI:10.1021/ef900558e
Effect of Opposing Tangential Primary Air Jets on the Flue Gas Velocity Deviation for Large-Scale Tangentially Fired Boilers Yuegui Zhou,*,† Mingchuan Zhang,† Tongmo Xu,‡ and Shien Hui‡ † Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Minhang District, Shanghai 200240, China, and ‡Department of Thermal Energy Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Received May 31, 2009. Revised Manuscript Received August 9, 2009
An experimental and numerical study on the effect of opposing tangential primary air jets with different biased angles on aerodynamic fields in the furnace and the flue gas velocity deviation in the horizontal flue gas pass was carried out in a tangentially fired boiler model of a 600 MW unit. The results show that the flow fields in the left and right sides of the upper furnace are obviously different and the gas velocity deviation in the horizontal flue gas pass is large for the original tangentially fired system. However, the actual tangential circle diameter in the furnace, the residual swirl intensity at the furnace exit, and the flue gas velocity deviation in the horizontal flue gas pass will be reduced when the appropriate opposing tangential angle of primary air jets is adopted in a new tangential firing system. The appropriate opposing tangential angle of primary air jets is 10-15° relative to the original primary air jets, and the ratio of opposing tangential momentum flux moment to tangential momentum flux moment XJ should be controlled at the low limit of 0.92 to decrease the flue gas velocity deviation in the horizontal flue gas pass.
600 MW assessment utility boilers in the Pingwei power plant, China.2 Moreover, some effective measures were put forward to weaken residual swirl at the furnace exit by opposing some tangential air flows.2-4 Yuan et al.,5 Xu et al.,6 and Xu et al.7 investigated the effects of opposing tangential secondary air jets on the flue gas velocity and temperature non-uniformity numerically and concluded that residual air flow swirl and platen superheaters were two important reasons for the flue gas temperature deviation. Zhou et al.8,9 investigated the flue gas velocity deviation in a large-scale tangentially fired boiler furnace with experimental and numerical methods and found that the flow fields were obviously different in the lateral directions of the upper furnace. Yin et al.10,11 investigated the flue gas temperature deviation in large-scale tangentially fired boilers and also pointed out that residual air flow swirl at the furnace exit was believed to be one important factor for the
1. Introduction Tangentially coal-fired boilers are widely used in modern power plants because of good flame stability in the furnace, good adaptability for a wide variety of coal ranks and loads, and low NOx emission. However, their shortcomings include fouling and slagging in the burner zone and upper furnace, high-temperature corrosion at the water cool furnace wall, and heat imbalance and gas temperature deviation in the horizontal flue gas pass. Especially the flue gas temperature deviation in the horizontal flue gas pass significantly increases with the capacities of tangentially fired boilers, which is typically 100-150 K for 200 MW boilers, 150-200 K for 300 MW boilers, and 200-250 K for 600 MW boilers.1 This serious flue gas temperature deviation may cause the local overheating and pipe failure of superheaters and reheaters for large-scale tangentially fired boilers. It is well-known that the flue gas temperature deviation in the horizontal flue gas pass is inherent for corner tangentially fired boilers because the air jets from the burner nozzles form intensive vortex motion in the furnace and residual swirl is still powerful at the entry of the platen zones in the upper furnace. Thus, residual swirl in the upper furnace may directly affect the flow fields in the upper furnace and cause the flue gas velocity and temperature non-uniformity in the horizontal flue gas pass. The similarity modeling tests showed that there were fairly intensive residual swirl at the furnace exit and large flue gas velocity deviations in the horizontal flue gas pass for
(3) Zhang, X.; Xu, T. M.; Hui, S. E. J. Eng. Therm. Energy Power 1996, 11, 262–266 (in Chinese). (4) Dou, W. Y.; Zhou, Y. G.; Zhou, Q. L.; Hui, S. E.; Xu, T. M. J. Xi’an Jiaotong Univ. 1999, 33, 36–39 (in Chinese). (5) Yuan, J. W.; Xu, M. H.; Han, C. Y. Numerical study on gas temperature deviation in a tangentially coal fired boiler of 600 MW unit. In Proceeding of 3rd International Symposium on Coal Combustion, Beijing, China, 1995; p 241. (6) Xu, M. H.; Yuan, J. W.; Ding, S. F.; Cao, H. D. Comput. Methods Appl. Mech. Eng. 1998, 155, 369–380. (7) Xu, T. M.; Hui, S. E.; Zhou, Q. L.; Dou, W. Y.; Zhou, Y. G. The effect of opposing tangential circle of air flow on flow characteristics in large boiler furnace. In Proceeding of 4th International Symposium on Coal Combustion, Beijing, China, 1999. (8) Zhou, Y. G. Experimental and numerical investigation on flue gas deviation for large scale corner tangentially fired boilers. Ph.D. Dissertation, Xi’an Jiaotong University, Xi’an, China, 1999. (9) Zhou, Y. G.; Zhang, M. C.; Xu, T. M.; Hui, S. E. Proc. CSEE 2001, 21, 68–72 (in Chinese). (10) Yin, C. G.; Cailat, S.; Harion, J. L.; Baudoin, B.; Perez, E. Fuel 2002, 81, 997–1006. (11) Yin, C. G.; Rosendahl, L.; Thoma, J. C. Fuel 2003, 82, 1127– 1137.
*To whom correspondence should be addressed. Telephone: þ86-2134206769. Fax: þ86-21-34206115. E-mail:
[email protected]. (1) Xu, T. M.; Hui, S. E.; Guo, H. S.; Che, D. E. The developing tendency and countermeasure of the combustion in furnace of large pulverized-coal boilers. In Proceeding of International Conference on Power Engineering (ICOPE), Shanghai, China, 1995; p 305. (2) Guo, H. S.; Xu, T. M.; Hui, S. E. Power Eng. 1996, 16, 9–13 (in Chinese). r 2009 American Chemical Society
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flue gas temperature deviation. He et al. investigated the optimum counter-flow mode of air jets in a large utility furnace and showed that the separated counter-flow mode can efficiently prevent flow attachment to the furnace walls, reduce the angular moment flow rate during flow attachment, and control heat imbalance in the horizontal flue gas pass. Zhou et al.13 carried out an experimental and numerical study on the flow fields in the upper furnace for large-scale tangentially fired boilers and showed different flow characteristics in the lateral sides of the upper furnace and large flue gas velocity deviations in the horizontal flue gas pass. Additionally, some new furnace structure arrangements were proposed to decrease the flue gas velocity deviation for large-scale tangentially fired boilers. On the other hand, the opposing tangential primary air technology has been used to retrofit the existing large-scale tangentially coal-fired boilers to greatly improve flame stability for low volatile coal, reduce flue gas heat imbalance at the furnace exit, and alleviate furnace slagging potential as well as high-temperature corrosion.14-16 Chi et al.17 investigated the effect of opposing tangential primary air on stabilizing coal combustion, decreasing the flue gas temperature deviation in the horizontal flue gas pass, and preventing slagging in the furnace for the concentric firing system using the theory of turbulent multiphase flow. However, there are obviously differences in the angles and layer numbers of opposing tangential primary air jets for different coal-fired power stations. Therefore, it is necessary to investigate the effect of opposing tangential primary air jets on flow patterns in the furnace and the flue gas velocity deviation in the horizontal flue gas pass. It is also expected to obtain the criterion number to control the flue gas temperature deviation in large-scale tangentially fired boilers through similarity modeling and numerical simulation.8 The objective of the present paper is to investigate the effect of opposing tangential primary air jets on aerodynamic fields in the furnace and the flue gas velocity deviation in the horizontal flue gas pass with experimental and numerical methods and to elucidate the relationship between the residual swirl intensity at the furnace exit and the flue gas velocity deviation coefficient in the horizontal flue gas pass for large-scale tangentially fired boilers.
Figure 1. Diagrammatic sketch of a 600 MW tangentially fired boiler model: (a) tangentially fired boiler furnace and (b) arrangement of superheaters and reheaters in the upper furnace.
The primary and secondary air jets are injected into the furnace from the burner nozzles located at four corners at set angles of 45° and 36°, respectively, to form a counter-clockwise tangential fired system, as shown in Figure 2. In each corner, there are 15 burner nozzles, including 6 primary air nozzles, 6 secondary air nozzles, 2 top secondary air nozzles, and 1 over fired air nozzle. The lowest primary air nozzle and secondary air nozzle are closed in practical operation. The velocity of primary air jets is 14.1 m/s, and the ratio of the dynamics head of primary air to that of secondary air is 1.09. A TSI 1050A hot wire anemometer was used to measure gas velocities in the boiler furnace and in the horizontal flue gas pass. It was calibrated in a standard wind tunnel before the measurements with the relative error of (1.5%. The solid lines of the primary air and secondary air jets in Figure 2 indicate the counter-clockwise imaginary tangential circles in the furnace for the original tangentially fired system. In the new tangentially fired system, opposing tangential primary air jets are injected into the furnace from the burner at set biased angles to form clockwise imaginary tangential circles. The dash lines in Figure 2 show the opposing tangential primary air jets with a certain biased degree of R relative to the original primary air jets. The main operation parameters of different opposing tangential primary air jets at different cases are listed in Table 1. The parameter XJ is defined as the ratio of opposing tangential momentum flux moment to the tangential one later.
2. Experimental Section 2.1. Experimental Setup. The cold model of a 600 MW tangentially coal-fired boiler is shown in Figure 1 based on geometric similarity modeling of 1:25. Six pieces of division platen superheaters, rear platen superheaters, and rear reheaters are located in the upper furnace. The measuring points A1-A5 and B1-B5 are located at different cross-sections of the burner zone and upper furnace, respectively. The measuring points P1-P3 are located at different heights of the middle channel of the platen zone, and C1-C3 are located at different heights of the horizontal flue gas pass at the exit of rear reheaters. (12) He, B. S.; Chen, M. Q.; Yu, Q. M.; Liu, S. M.; Fan, L. J.; Sun, S. G.; Xu, J. Y.; Pan, W. P. Comput. Fluids 2004, 33, 1201–1223. (13) Zhou, Y. G.; Xu, T. M.; Hui, S. E.; Zhang, M. C. Appl. Therm. Eng. 2009, 29, 732–739. (14) Zhang, M. C.; Lie, J. P.; Ou, J. T.; Duan, B. W. China Electr. 1995, 2, 38–40, 73 (in Chinese). (15) Zhang, Z. R.; Zhang, M. C. Proc. CSEE 1997, 17, 54–57 (in Chinese). (16) Zhao, Z. R.; Zhang, M. C. J. Combust. Sci. Technol. 1997, 3, 88– 96 (in Chinese). (17) Chi, Z. H.; Zhou, H.; Xia, J. J.; Jiang, X.; Li, F. R.; Cen, K. F. Proc. CSEE 1998, 18, 135–139 (in Chinese).
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Figure 2. Schematic diagram of two concentric firing systems: 1, primary air jets; 1O, opposing tangential primary air jets; 2, secondary air jets; 3, main stream rotation direction; 4, opposing tangential primary air jet trajectory; R, biased degree of opposing tangential primary air jets relative to the original primary air jets. Table 1. Parameters of Different Opposing Tangential Primary Air Jets case
R (deg)
XJ
case 1 case 2 case 3 case 4 case 5 case 6
0 5 10 15 20 25
0 0.04 0.47 0.92 1.35 1.77
2.2. Main Parameters of Tangentially Fired Boilers. The following parameters are used to describe the gas flow characteristics in the furnace and in the horizontal flue gas pass for tangentially fired boilers in this paper. 2.2.1. Actual Tangential Circle Diameter. The actual tangential circle diameter in the furnace is one of the most important parameters to describe the aerodynamic fields in tangentially fired boilers. The dimensionless tangential circle diameter is defined by eq 1 d ¼
d1 A
þ dB2 2
Figure 3. Schematic calculation of the swirl intensity in the furnace.
ð1Þ
2.2.3. Ratio of Opposing Tangential Momentum Flux Moment to the Tangential One XJ. A criterion number XJ is defined as the ratio of the momentum flux moment of opposing tangential air jets to that of tangential air jets, expressed as2 P ðFQWRÞOT XJ ¼ P ð3Þ ðFQWRÞT
where A and B are the width and length of the furnace crosssection, respectively. d1 and d2 are defined as the intervals between maximum tangential velocities in the width and length direction of the furnace cross-section, respectively. 2.2.2. Swirl Intensity in the Furnace. The swirl intensity in the furnace is characterized by swirl momentum flux moment P,7,13 shown in Figure 3 n1 n2 X X P ¼ FQuR ¼ ½ ðFui 2 ΔxΔHLi Þþ ðFvj 2 ΔyΔHLj Þ=2 ð2Þ i ¼1
where F is the gas density, Q is the flue gas flow rate, W is the velocity of primary air or secondary air from the burner, and R is the imaginary tangential radium of different air jets in the furnace. 2.2.4. Flue Gas Velocity Deviation Coefficients. The gas velocity deviation coefficients E and M are used to evaluate the non-uniformity of the velocity field at the entry of the horizontal flue gas pass for tangentially fired boilers. The value E indicates the difference between gas mean velocities in the lateral sides of the horizontal flue gas pass, defined as the
j ¼1
where F is the gas density, ui and vj are gas tangential velocities in the x and y direction, respectively, Δx and Δy are measuring point or grid spacings, Li and Lj are the distances between measuring points or grid points and the furnace center, n1 and n2 are the numbers of measuring points or grids in the width and length directions, and ΔH is the unit of the furnace height. 5377
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Table 2. Eulerian Conversation Equations and Identification of Terms in eq 6 φ
conservation equation
Γ
continuity
1
0
x-direction momentum
u
μeff
y-direction momentum
v
μeff
Sφ
z-direction momentum
w
μeff
0 Dp D Du Dv þ DyD μeff Dx þ DzD μeff Dw - Dx þ Dx μeff Dx Dx D Du D Dv D Dw - Dp Dy þ Dx μeff Dy þ Dy μeff Dy þ Dz μeff Dy D D D Du Dv Dw - Dp Dz þ Dx μeff Dz þ Dy μeff Dz þ Dz μeff Dz
turbulent kinetic energy
k
μeff =σk
G - Fε
μeff =σk
ε k ðc1 G -c2 FεÞ
dissipation rate of turbulent kinetic energy ε 2 2 2 2 2 2 Du Dv Dv Dw Du Dv Dw G ¼ μeff f2 Dx þ Dy þ Dw g þ Du Dz Dy þ Dx þ Dx þ Dz þ Dz þ Dy μeff = μ þ μt, μt = cμFk2/ε, cμ = 0.09, c1 = 1.44, c2 = 1.92, σk = 1.0, σε = 1.3
bottom, and top. The power-law difference scheme was used to discretize the convective terms. The source term was negatively linearized in the discretization. Equation 7 is solved by the semi-implicit method for pressure linked equations (SIMPLE) algorithm with an appropriate under-relation technique and TDMA line-by-line iterations. A three-dimensional numerical program was developed to simulate the isothermal flow in a 600 MW tangentially fired boiler model. The boiler furnace geometry was illustrated in Figure 1, and the burners and the air jets arrangements were denoted in Figure 2. The main operation parameters of opposing tangential primary air jets at different cases were also listed in Table 1. The computations were carried out on a 26 26 68 non-uniform structured and staggered grid system. Grid-dependent tests were conducted, and the specified grid was fine enough to give grid-independent solutions. The grid spacings were finer at the burner zone, furnace arch zone, and the platen zones and changed smoothly to minimize the deterioration of the formal accuracy of the finite-volume scheme because of variable grid spacings. The sloping sides of the ash hopper and furnace arch were approximately dealt with as step grids. Six pieces of division platen superheaters, rear platen superheaters, and rear reheaters in the upper furnace shown in Figure 1b were treated as solid domains by the domain extension method with a large coefficient method19 in the computational domain. The boundary conditions for the tangentially fired boiler model were treated as follows: (1) Inlet boundary: the gas velocities at the inlet of burner nozzles were fixed, and k and ε were calculated as
ratio of gas mean velocity in the right side vm,R to that in the left side vm,L vm, R E ¼ ð4Þ vm, L Maximum velocity deviation coefficient M indicates the maximum local velocity non-uniformity in the whole crosssection of the horizontal flue gas pass, expressed as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n P 1 vm þ3 n -1 ðvi -vm Þ2 M ¼
i ¼1
vm
ð5Þ
where vi is v velocity at each measuring or grid point, n is the number of the measuring or grid points, and vm is the arithmetic mean velocity in each cross-section of the horizontal flue gas pass.
3. Mathematical Model and Numerical Method The gas phase flow in the tangentially fired boiler furnace is described by elliptic, partial differential conservation equations for a Newtonian fluid in Cartesian coordinates. The gas phase conservation equations are time-averaged and solved using a finite-volume method for Navier-Stokes equations. The value of the eddy viscosity and subsequent closure of the equations can be achieved using the standard two-equation k-ε model.18 Each of these equations has the same general form and can be cast into the standard form divðFvBφÞ -divðΓgrad φÞ ¼ Sφ
ð6Þ
where φ is the particular dependent variable, vB is the velocity vector, F is the fluid density, Γ is the effective diffusivity, and Sφ is the generalized source term for φ, which depends upon the geometry, transport coefficients, and other dependent variables. Table 2 identifies the gas-phase variables, together with the corresponding expressions for the effective diffusivity Γ and the source term Sφ used in eq 6. Equation 6 can be discretized using the finite-volume method to obtain the following algebraic equations:19,20 X ap φp ¼ anb φnb þSc ð7Þ
kin ¼ 0:005Vin 2 3=2 εin ¼ c3=4 =0:07L μ k
where L is the hydraulic diameter of the burner nozzle. Dw (2) Outlet boundary: Du Dy joutlet ¼ 0, Dy joutlet ¼ 0, and v was determined by the total mass conservation and exit velocity lifting method, and k and ε were treated using the local coordinates unilateralization.19 (3) Solid wall: impenetrable and no velocity slip. The wall function method was used to account for the near-wall effects in the flow field for the high Reynolds number k-ε twoequation model.19 The convergence criteria demands a maximum relative residual for each variable to less than 10-5.
where ap and anb are the coefficients of the discretized equations for φ, subscript p stands for the main grid (node) point, and subscript nb indicates six neighboring points surrounding the p node, including east, west, north, south, (18) Launder, B. E.; Spalding, D. B. Mathematical Models of Turbulence; Academic Press: New York, 1972. (19) Tao, W. Q. Numerical Heat Transfer, 2nd ed.; Xi'an Jiaotong University Press: Xi'an, China, 2001; pp 194-251, 332-362. (20) Patankar, S. V. Numerical Heat Transfer and Fluid Flow; Hemisphere Publishing Corporation: Washington, D.C., 1980.
4. Results and Discussion 4.1. Aerodynamic Fields in the Furnace and in the Horizontal Flue Gas Pass at Original Conditions. The tangential 5378
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Figure 4. Predictions and measurements of tangential velocity distributions in different cross-sections of the boiler furnace.
Figure 6. Predicted and experimental results of the horizontal velocity distributions at the entry of the flue gas pass: (a) measuring point C1 at the lower part, h1 = 0.20; (b) measuring point C2 at the middle part, h2 = 0.57; and (c) measuring point C3 at the upper part, h3 = 0.80. (b) Experimental data and (;) model predictions.
left side of the upper furnace reversed toward the front wall, and then most of the air flow entered the horizontal flue gas pass from the upper part of the platen zone. There was a low velocity recirculating zone over the furnace arch in the left side. However, the air flow in the right side of the platen zone directly flowed toward the rear wall and entered the horizontal flue gas pass. This was because there was fairly intensive residual swirl at the entry of the platen zone, which caused different flow fields in the lateral sides of the upper furnace and resulted in the flue gas velocity deviation in the width and height directions of the horizontal flue gas pass.13 Figure 6 shows the predicted and experimental results of the horizontal velocity distribution at different heights of the horizontal flue gas pass at case 1. The dimensionless height hi h¼ hhi indicates the ratio of the height of each measuring point hi to the height of the entrance cross-section h in the horizontal flue gas pass. The results showed that the gas velocity in the right side of the horizontal flue gas pass was obviously larger than that in the left side. The gas velocity deviation coefficients E and M were used to evaluate the nonuniformity of the velocity fields at the entry of the horizontal
Figure 5. Predicted velocity fields in the left and right vertical planes along the furnace width direction: (a) the left side and (b) the right side.
velocity distributions in representative cross-sections A2, A3, and B1 of the boiler furnace at the original design conditions (case 1) were shown in Figure 4. Numerical results agreed well with the experimental ones. It can be seen that the diameters of the actual tangential circles in the furnace are fairly large and the gas flow is apt to attach the furnace wall at the burner zone. The air jets from the burner deflected toward the wall at the actions of upstream and the whole swirling air flow, and the diameters of the actual tangential circles became large because of the large ratio of the height to the width of the burner nozzles and great difference of supplying air flows in both sides of air jets for large-scale tangentially fired boilers. Figure 5 shows the predicted aerodynamic fields in the left and right vertical planes along the furnace width direction at case 1. It was found that the flow fields in the lateral sides of the upper furnace are obviously different. The gas flow in the 5379
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Figure 7. Effect of the opposing tangential angle of primary air jets on tangential circle diameters in the furnace.
Figure 8. Effect of the opposing tangential angle of primary air jets on swirl intensity in the furnace.
flue gas pass. The values E and M were 1.125 and 1.803 at original design conditions, respectively, which showed large gas velocity deviations in the horizontal flue gas pass for the large-scale tangentially fired boiler at the original design. Therefore, some effective measures of opposing tangential primary air jets were taken to decrease the flue gas velocity deviation in the horizontal flue gas pass for the large-scale tangentially fired boiler in this paper. 4.2. Effect of the Opposing Tangential Angle of Primary Air Jets on Tangential Circle Diameters in the Furnace. Figure 7 shows the variations of relative tangential circle diameters d with the furnace height for different opposing tangential angles of primary air jets. The relative tangential circle diameters at the burner zone decreased with the increase of the opposing tangential angle of primary air jets from 0° to 15° because opposing tangential primary air jets weakened the whole air stream swirl intensity in the furnace. It was observed during the experimental process that the whole air stream in the furnace was unstable when the opposing tangential angle of primary air jets increased to some extent and the rotation direction of the whole air stream in the furnace was dependent upon the start-up subsequence of the primary air and secondary air. However, the rotation direction of the whole air stream in the furnace changed from the original counter-clockwise direction to the clockwise direction when the opposing tangential angle of primary air jets increased to 20° and 25°. Thus, the relative tangential diameters in the furnace inversely increased with a further increase of the opposing tangential angle of primary air jets. In addition, the numerical tangential circle diameters in the furnace agreed well with the experimental ones. 4.3. Effect of the Opposing Tangential Angle of Primary Air Jets on Swirl Intensity in the Furnace. Figure 8 shows the effect of the opposing tangential angle of primary air jets on swirl intensity in the furnace. The total swirl intensity in the furnace was significantly weakened when the opposing tangential angle of primary air jets increased from 0° to 15°. It was because the inverse momentum flux moment increased with the increase of the opposing tangential angle of primary air jets, which weakened the total swirl intensity in the furnace. However, the inverse momentum flux moment of primary air jets was dominant to change the rotation direction of the whole air stream in the furnace from the counter-clockwise direction to the clockwise direction, and then the total swirl intensity in the furnace inversely increased when the opposing tangential angle of primary air jets increased to 20° and 25°. The residual swirl intensity at the furnace exit was also significantly weakened with the
Figure 9. Effect of the residual swirl intensity on the gas mean velocity deviation coefficient E in the horizontal flue gas pass.
increase of the opposing tangential angle of primary air jets. It can be clearly seen that the residual swirl intensity at the furnace exit is minimum when the opposing tangential angle of primary air jets ranged from 10° to 15°. 4.4. Effect of the Residual Swirl Intensity on the Flue Gas Velocity Deviation in the Horizontal Flue Gas Pass. Figure 9 shows the effect of the residual swirl intensity on the flue gas mean velocity deviation coefficient E in the horizontal flue gas pass. The residual swirl intensity at the furnace exit was considered negative when the rotation direction of the whole air stream in the furnace changed from the counter-clockwise direction to the clockwise direction. The flue gas mean velocity deviation coefficient E was less than 1.0 when the whole air stream in the furnace was clockwise, which meant that the flue gas mean velocity in the left side of the horizontal flue gas pass was larger than that in the right side. It can be seen that the flue gas mean velocity in the right side of the horizontal pass was larger than that in the left side and the flue gas mean velocity deviation coefficient E decreased with the decrease of the residual swirl intensity at the furnace exit. However, the flue gas mean velocity deviation coefficient E was less than 1.0 when the residual swirl intensity at the furnace exit inversely increased, and a less value of E indicated that the flue gas velocity deviation in the lateral sides of the horizontal pass was inversely larger. Figure 10 shows the effect of the residual swirl intensity on the flue gas maximum velocity deviation coefficient M in the horizontal flue gas pass. It can be seen that the flue gas maximum velocity deviation coefficient M decreased with the decreasing residual swirl intensity at the furnace exit when the opposing tangential angle of primary air jets increased from 0° to 15°. On the other hand, the flue gas maximum velocity deviation coefficient M increased when the residual swirl intensity at the furnace exit inversely 5380
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lean coal, and anthracite. On the basis of the Results and Discussion in this paper, the appropriate opposing tangential angle of primary air jets was 10-15° relative to the original primary air jets and the ratio of opposing tangential momentum flux moment to tangential momentum flux moment XJ should be controlled at the low limit of 0.92. This work will be applied and validated in actual large-scale tangentially fired boilers. 5. Conclusion The effect of different opposing tangential angles of primary air jets on aerodynamic fields in the furnace and on the flow gas velocity deviation in the horizontal flue gas pass was carried out by experimental and numerical methods for the tangentially fired boiler model of a 600 MW unit. The agreements between the calculated and measured velocity distributions are satisfactory. The results show that the flow patterns in the left and right sides of the upper furnace are obviously different and the gas velocity deviation in the horizontal flue gas pass is large for the original design of the large-scale tangentially fired boiler. The average velocity deviation coefficient is 1.125, and the maximum velocity deviation coefficient is 1.803. Moreover, the diameter of the relative tangential circles in the furnace and the residual swirl intensity at the furnace exit were reduced when the appropriate opposing tangential angle of primary air jets was adopted in large-scale tangentially fired boilers. As a result, the flue gas velocity deviation in the horizontal flue gas pass was also alleviated. The appropriate opposing tangential angle of primary air jets is 10-15° relative to the original primary air jets, and the ratio of opposing tangential momentum flux moment to tangential momentum flux moment XJ should be controlled at the low limit of 0.92. Therefore, it is expected to decrease the flue gas velocity and temperature deviation in the horizontal flue gas pass, to prevent furnace slagging, and to improve pulverized coal ignition flame stability when the optimum opposing tangential angle of primary air jets is adopted in actual large-scale tangentially fired boilers.
Figure 10. Effect of the residual swirl intensity on maximum velocity deviation coefficient M in the horizontal flue gas pass.
decreased. The results showed the correlation between the residual swirl intensity at the furnace exit and the flue gas velocity deviation coefficients in the horizontal flue gas pass for large-scale tangentially fired boilers. Therefore, it can be concluded that the residual swirl intensity at the furnace exit and the flue gas velocity deviation in the horizontal flue gas pass are minimum when the opposing tangential angle of primary air jets is 15° relative to the original primary air jets. At this condition, the ratio of opposing tangential momentum flux moment to tangential momentum flux moment XJ was 0.92. 4.5. Discussion. As mentioned above, the opposing tangential angles of primary air jets had an important influence on the flow patterns in the furnace and the flue gas velocity deviation in the horizontal flue gas pass. When the suitable opposing tangential angle of primary air jet was adopted, the relative tangential circle diameter at the burner zone, residual swirl intensity at the furnace exit, and the flue gas velocity deviation in the horizontal flue gas pass decreased. It was beneficial to decrease the flue gas temperature imbalance in the horizontal flue gas pass and to prevent fouling and slagging at the burner zone. However, the flow streams in the furnace may either be unstable or change the rotation direction, and the flue gas velocity deviation in the horizontal flue gas pass inversely increased when the opposing tangential angle of primary air jets was too large. According to the close chain loop theory of pulverized coal mixture ignition in the furnace for corner tangentially fired boilers,21 it is not enough to only increase flue gas recirculation flux to enhance the ignition of the pulverized coal mixture and to improve flame stability. It is necessary to intensify the combustion of the pulverized coal mixture at the early stage to form an effective high-temperature ignition atmosphere. The primary air mixture initially flows in the direction of jet trajectory when it is injected into the furnace opposite the whole rotation ball, and then it is retarded by the main swirling gas stream in the furnace, as shown in Figure 2. Thus, the residence time of the pulverized coal mixture at the initial ignition stage is elongated to enhance heat release of the pulverized coal combustion in the ignition zone and to improve pulverized coal ignition stability. Subsequently, the rotation direction of opposing tangential primary air jets is the same as main flue gas stream at the action of the whole swirling gas stream in the furnace. Therefore, opposing tangential primary air jets can also improve the ignition stability of the pulverized coal mixture especially for low-volatile coal, such as inferior bituminite,
Acknowledgment. The authors are grateful for support provided by the Machinery Industry Technique Development Foundation of the People’s Republic of China (95JB1101) and Chenxing Young Scholarship of Shanghai Jiao Tong University.
Nomenclature A = width of the furnace cross-section (m) a = coefficient of the discretized equations for φ B = length of the furnace cross-section (m) d = dimensionless tangential circle diameter d1 = actual tangential circle diameter in the width direction of the furnace (m) d2 = actual tangential circle diameter in the length direction of the furnace (m) E = mean velocity deviation coefficient H = furnace height (m) k = turbulent kinetic energy (m2 s-2) L = distance between the grid point and furnace center or hydraulic diameter of burner nozzles (m) M = maximum velocity deviation coefficient n = number of measuring point or grid p = pressure (Pa) P = momentum flux moment (N m)
(21) Zhang, M. C. Pulverized coal ignition and flame stability analysis for tangential fired utility boilers. Ph.D. Dissertation, Tsinghua University, Beijing, China, 1990.
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F = gas density (kg m ) μ = viscosity (kg m-1 s-1) μt = turbulent viscosity (kg m-1 s-1) μeff = effective viscosity (kg m-1 s-1) ν = kinematics viscosity (m2 s-1) σk,ε = turbulent Prandtl number Γ = diffusion coefficient (kg m-1 s-1) ε = dissipation rate of the turbulent kinetic energy (m2 s-3) ΔH = unit of furnace height (m) Δx = grid spacings in the x direction (m) Δy = grid spacings in the y direction (m)
Q = flue gas flow rate in each burner nozzle (m s ) R = imaginary tangential radium (m) Sφ = gas source term for variable φ u = gas velocity in the x direction (m s-1) vB = gas velocity vector (m s-1) v = gas velocity in the y direction (m s-1) W = gas velocity at the exit of the burner nozzle (m s-1) w = gas velocity in the z direction (m s-1) x = horizontal coordinate XJ = ratio of opposing tangential momentum flux moment to the tangential one XL = furnace width (m) y = horizontal coordinate YL = furnace length (m) z = vertical coordinate
Subscript i = number of measuring point or grid in x direction j = number of measuring point or grid in y direction nb = neighboring points surrounding the p node OT = opposing tangential air jets p = main grid (node) point T = tangential air jets
Greek Letters R = biased degree of opposing tangential primary air jets relative to original primary air jets (deg) φ = general variable
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