Energy & Fuels 2009, 23, 2565–2569
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Bed Inventory Overturn in a Circulating Fluid Bed Riser with Pant-leg Structure Jinjing Li, Wei Wang, Hairui Yang,* Junfu Lv, and Guangxi Yue Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua UniVersity, Beijing, 100084, China ReceiVed October 6, 2008. ReVised Manuscript ReceiVed January 14, 2009
The special phenomenon, nominated as bed inventory overturn, in circulating fluid bed (CFB) riser with pant-leg structure was studied with model calculation and experimental work. A compounded pressure drop mathematic model was developed and validated with the experimental data in a cold experimental test rig. The model calculation results agree well with the measured data. In addition, the intensity of bed inventory overturn is directly proportional to the fluidizing velocity and is inversely proportional to the branch point height. The results in the present study provide significant information for the design and operation of a CFB boiler with pant-leg structure.
Introduction Pant-leg bottom furnace and fluidized bed heat exchanger (FBHE) are the patent features of large-scale circulating fluid bed (CFB) boilers in Alstom technology,1 which was imported by the top three boiler manufacture companies in China. So far, about 8 units of 300 MWe CFB boilers have been in operation. Though the operation of 300 MWe CFB boilers proves that the pant-leg structure can improve the air and solid mixing in the furnace and reduce the carbon content in fly ash greatly, the two independent distributors at the bottom of the pant leg will always cause bed inventory imbalance between the two legs. During the operation, one special phenomenon, nominated as bed inventory overturn, usually occurs. When bed inventory overturn occurs, the pressure drop in one leg decreases while the air flow rate increases, until the bed materials in this leg is blown out and transferred into the other leg. The pressure drop in the other leg increases until the bed inventory is too vast to be fluidized by the primary air. Once bed inventory occurs, the boiler operator has to adjust the air valve to improve the air flow rate in the leg with defluidization but turn down the flow rate in the leg with little bed inventory. If the capacity of the air fan is large enough, this operation works to blow the bed material back the original. Due to hysteresis of control and adjustment, continuous adjustments are needed to keep the bed inventory balance between two legs, and such adjustments usually cause the shuttle of bed inventory between the two legs. So far, more and more attention is paid to the bed inventory overturn,2,3 because of its considerable negative influence on * To whom correspondence should be addressed. E-mail: yhr@ mail.tsinghua.edu.cn. Phone: +86-10-62773384. Fax: +86-10-62781743. (1) Marchetti, M. M.; Czarnecki, T. S.; Semedard, J. C.; Devroe, S.; Lemasle, J.-M. ALSTOM’s Large CFBs and Results. Proceedings of the 17th International Conference on Fluidized Bed Combustion; 2003, 673– 683. (2) Le Guevel, T.; Thomas, P. Fuel Flexibility and Petroleum Coke Combustion at Provence 250 MW CFB. Proceedings of the 17th International Conference on Fluidized Bed Combustion; 2003, 643–649. (3) Shevtchenko, V. A.; Franke, W.; Gummel, P.; Kotrus, M.; Von Wedel, G. Ukraine’s first large-scale CFB 200 MWE anthracite fired Starobeshevo Power Plant. Proceedings of the 18th International Conference on Fluidized Bed Combustion; 2005, 65–74.
Figure 1. Schemes of the material balance in the main loop.
CFB boiler operation security. Li et al.4 believed that the bed inventory overturn is caused by the mass balance between combustor and standing pipe. Another viewpoint5 proposed that the main reason of bed inventory overturn is the lateral solid motion in the upper region in the furnace where the two legs join together. According to Fick’s Law (eq 7), the lateral particle flow rate may be influenced by the branch point (Figure 1) height and the fluidizing velocity, which are the important factors in CFB boiler design. In order to study effects of these two factors, a compounded pressure drop mathematic model was developed and a cold experimental study was conducted to verify the model. Theoretical Model A mathematical model of compounded pressure drop in a CFB boiler was proposed by Li et al.6 The furnace was (4) Li, Q.; Zhao, K.; Mi, Z. Research on Overturn Beds of CFB Boiler with Double Feet Structure. N. China Electr. Power 2007, 7, 43–51. (5) Li, J.; Li, Y.; Liu, S.; Yue, G.; Li, Z. Numerical simulation of the bed material unbalance between the breeches-legs in a circulating fluidized bed boiler. J. Power Eng. 2008, 28, 28–32.
10.1021/ef800835z CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
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∆pf_1 + ∆pMs_1 ) ∆pf_2 + ∆pMs_2 ) p0-p-
(3)
where ∆pMs is the pressure drop caused by the bed inventory in the furnace. The pressure drop across the valve, air manifold, and air distributor, ∆pf, is assumed to be a function of the superficial fluidizing velocity in the furnace: ∆pf ) Cf u20
Figure 2. Scheme of the air-gas system.
considered to contribute to the pressure drop in the air-gas system, because of its considerable bed inventory of solid particles. The pressure drop across the furnace is proportional to the total bed inventory of solid particles. The increasing of bed inventory could cause larger resistance to air flow through the furnace, consequently reducing the flow rate of primary air. In this paper, a mathematical model describing the gas solid flow in a CFB boiler with pant-leg furnace was proposed based on the previous model. The model consists of several sub models to describe the process of particle transfer in CFB furnace. The entire furnace is divided symmetrically into two half-parts, each has one leg. Either part is divided into a number of cells as shown in Figure 1. The characteristics of gas and solid in each cell are assumed to be uniform. There is lateral mass transfer between the two parts at a rate, Winter, through the interface. For inventory balance, one can have dMs_1 ) Wash_1 - Wslag_1 - Wfl_1 - Winter dt dMs_2 ) Wash_2 - Wslag_2 - Wfl_2 + Winter dt
(1)
where u0 is the superficial fluidizing velocity in the furnace, and Cf is the resistance constant. So far, the difference of pressure drops in the cyclones is neglected in the present model due to the relative smaller value compared to that in the furnace.7 The pressure drop in the furnace, ∆pMs, is mainly dependent on the bed inventory in the furnace, neglecting the contribution of friction and acceleration. The pressure drop in the furnace, ∆pMs, can be calculated by the axial voidage profile along the furnace. Yue et al.8 found that the gas solid flow regime in the furnace of a CFB boiler is the superposition of bubbling bed flow of coarse particles and the fast fluidized bed flow of the fine particles that circulate through the loop. At the stable state, these coarse solids can not be entrained over the branch point, and the amounts of coarse solids in each legs should be nearly equal, so during the overturn process, the coarse solids in each legs will not change greatly. It is reasonable to assume the coarse solids have nearly no contribution to the horizontal solids flow in the upper furnace. In the present model, bed inventory is assumed to be composed of only fine solids. For the voidage profile along the furnace, several approaches were proposed, such as the Li-Kwauk9 model, the Smolders-Baeyens model,10 and the Horio model.11 In this model, the equation of Li and Kwauk was applied to each part furnace:
(
ln
where Winter is the total lateral flow rate of particles. When the particles transferred from the left leg to the right one, Winter is in a positive value, and vice versa. The subscripts _1 and _2 refer to the left side and the right side of the furnace, respectively. In the CFB boiler with pant leg structure, the primary air, provided by one main pipe, splits into two separate air manifolds after heated in the air preheater. So, at the outlet of the air preheater, the two air streams have the same pressure head, p0. Similarly, the two streams rejoin together at the outlets of the cyclones, so the pressure head, p-, at the cyclone outlets is also the same. Upon the assumption that pressure heads at the outlets of air preheater and furnace, p0 and p- are unchanged in the boiler operation, bed inventory and air flow rate in one part of the furnace will vary inversely. As shown in Figure 2, the pressure balance between the two air (flue) streams is:
)
z - zi ε - εa )ε′-ε Z0
(5)
where is the voidage at the furnace height, z; a and ′ are the asymptotic value of downside voidage and upside voidage, respectively; zi is the height of voidage profile inflection between the dense zone and dilute zone; and Z0 stands for the length of the transition zone. All four of these parameterssa, ′, zi, and Z0sare functions of boiler operation conditions. In general, the pressure drop in furnace (∆pMs) can be expressed as ∆pMs )
(2)
(4)
∑ (1 - ε )F g∆z j
s
j
(6)
j
where j is the voidage in cell j; Fs is the particle density, in kg/m3; g is the acceleration of gravity, 9.806 m/s2; and ∆zj is the distance in height of cell j. With the four parameters optimized from the experimental data in a 300 MWe CFB boiler in China, the axial voidage profiles in the boiler furnace were calculated and are shown in Figure 3. It is obvious that when the fluidizing velocity increases, more particles are entrained to the upper region in the furnace. (6) Li, Z.; Ni, W.; Yue, G.; Sun, X.; Zhang, W. The compounded pressure drop mathematical model of circulating fluidized bed boilers. Power Eng. 1997, 17, 13–16. (7) Li, S.; Yang, S.; Yang, H.; Zhang, H.; Liu, Q.; Lv, J.; Yue, G. Particle holdup and average residence time in the cyclone of a circulating fluidized bed boiler. Chem. Eng. Technol. 2008, 31, 224–230. (8) Yue, G.; Lu, J.; Zhang, H.; Yang, H.; Zhang, J.; Liu, Q.; Li, Z. Design Theory of Circulating Fluidized Bed Boilers. 18th International Conference on Fluidized Bed Combustion; 2005, 1–12. (9) Li, Y.; Kwauk, M. The Dynamics of Fast Fluidization; Plenum Press: New York, 1976; pp 537-544.
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Figure 3. Axial voidage profile in a CFB furnace.
When the gas solid flow in the two half-furnaces are different, the lateral solid motion occurs, which causes the bed inventory imbalance between both legs. Borrowing the view of fundamental mass diffusion, the lateral solid motion is impelled by the discrepancy of suspension solid concentration between two half-furnaces.12 According to Fick’s law, the lateral solid flow rate, Winter, is in the expression of Winter ) Kd∆FA
Figure 4. The cold experimental system.
(7)
where Kd is the coefficient of lateral solid flow rate, in m/s; A is the area of interface, in m2; and ∆F is the solid concentration difference between the two half-furnaces: ∆F ) F_1 - F_2
(8)
where F_1 and F_2 are the average suspension concentrations in upper furnace of left and right sides above the branch point, respectively. The previous study13 to decide the magnitude of the lateral solid flow coefficient, Kd, indicated that Kd is in the range of 1.0∼2.0 m/s. So, the value of Kd was assumed as 1.0 m/s in this numerical simulation.
Figure 5. Pressure drop of the air distributors.
Experimental Setup and Methodology As shown in Figure 4, the experimental test rig consists of the air supply system, the cold CFB loop, and the measurement system. The air is supplied by a Roots blower and then split into two parallel pipelines, which includes control valve, air manifold, and air distributor. The CFB cold model with pant leg structure consists of a riser with a rectangle section (above the branch point) of 0.08 m2 and the height of 2.5 m, two cyclone and two loop seals. Two distributors with bell jar nozzles were used to provide uniform air flow in both sides of the CFB riser. The resistance properties of the two distributors are shown in Figure 5. The pressure was measured with the water manometers, and the fluidizing velocity in each half-furnace was measured by the pitot tube. Since the process of bed inventory overturn is quite fast (on the order of 101∼102 s), the snapshot technology was employed to record the variation of pressure head in the water manometers with a sampling frequency of once per 5-7 s. (10) Smolders, K.; Beayens, J. Hydrodynamic modeling of circulating fluidized beds. AdV. Powder Technol. 1998, 9, 17–38. (11) Horio, M.; Morishita, K.; Tachibana, O.; Murata, N.; Solid Distribution and MoVement in Circulating Fluidized Beds. Basu, P., Large, J. F. Eds.; Circulating Fluidized Bed Technology II; Pergamon Press: Oxford, 1988; pp 147-154. (12) Davidson, J. F. Circulating fluidized bed hydrodynamics. Powder Technol. 2000, 113, 249–260. (13) Li, J.; Zhang, H.; Yang, H.; Lv, J.; Yue, G. Modeling of asymmetric flow dynamics in the furnace of a large scale CFB boiler with pant-legs structure. 9th International conference on CFB; 2008, 147–152.
To study the effect of branch point height on bed inventory overturn, a partition wall was set as a means for varying height of the branch point. It separates the CFB riser into two parts symmetrically but incompletely. There was a gap of 0.1 m height at the top of the CFB riser, which conjunct two parts together. Considering the scale law of fluidized beds,14 fine iron powder with an average diameter of 50 µm (terminal velocity ut ) 0.43 m/s, transition velocity into fast fluidization utr ) 1.5 m/s, calculated with the equations reviewed by Van de Velden et al.15) was used as solid materials. There are two reasons for this selection. First, a large density of bed material can provide an appreciable pressure drop in the CFB riser. Second, even in a low gas velocity, fine particles can form a CFB flow regime. Figure 6 shows the mass fraction distribution of iron powder used in this study. The impacts of fluidizing velocity and branch point height on the bed inventory overturn were studied under four operation cases, listed in Table 1. In each case, the valves were adjusted to provide equal gas velocity in each half-riser without solid material. Then, the same weight (6 kg) of iron powder was put into either leg of the CFB riser. Once the Roots blower started up, the variation of (14) Glicksman, L. R. Scaling Relationships for Fluidized beds. Chem. Eng. Sci. 1984, 39, 1373–1379. (15) Van de Velden, M.; Baeyens, J.; Seville, J. P. K., et al. The solids flow in the riser of a circulating fluidized bed (CFB) viewed by positron emission particle tracking (PEPT). Powder Technol. 2008, 183, 290–296.
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Figure 8. Difference of pressure drop between both sides of the CFB riser (case 2). Figure 6. Mass fraction distribution of iron powder.
Figure 9. Impact of branch point height on the characteristic time. Figure 7. Pressure drop in both sides of the CFB riser (case 2). Table 1. Experimental Cases fluidization velocity s-1
0.54 m 0.45 m s-1
with partition wall
without partition wall
case 1 case 3
case 2 case 4
pressure was recorded continuously with a sampling time of 5∼7 s until the process of bed inventory overturn completed.
Results and Discussion Because the bed material particles are so fine and the fluidizing velocity is larger than the terminal velocity, most solid particles can be entrained out the riser and then returned back through loop seals. The pressure drop in the riser, which is directly proportional to the amount of bed inventory, is regarded as the indication of bed inventory overturn. Figure 7 shows the variations of the pressure drop in both half-sides of the CFB riser in case 2. The trends of pressure drop in the other cases are similar to that shown in Figure 7. The tendency of pressure drop in each half-side furnace side is monotonic while reverse to each other. When the pressure drops in both side are same, it is the ideal state called the balance state in CFB boiler operation. Unfortunately, such balance state is also an unstable state, because any tiny disturbance in operation parameters can cause the bed inventory unbalance until overturn occurs. In this study, the bed materials in both sides of the riser can be fluidized even after the bed inventory overturn. We can regard the state after bed inventory overturn as a stable but unbalance state. A characteristic time (Tc), the time that pressure drops in both half-furnaces change from the balance state to the stable state at the largest slope of their curve, was employed to describe
the intensity of later mass transfer from one leg to the other. The shorter the characteristic time is, the faster the bed material moves laterally. As shown in Figure 8, the characteristic time (Tc) can be determined from the curve of time and the pressure drop difference between both sides. For example, the Tc of bed inventory overturn in case 2 is about 33 s. Figure 9 shows the impact of branch point height on the intensity of bed inventory overturn. When the branch point height is increased by inserting partition wall in case 1, Tc also becomes longer than that in case 2. The interface area (A) becomes smaller with branch point height increases. In addition, because the solid concentration decreases with the riser height, the average solid concentrations (F_1 and F_2) above the branch point height decrease when the branch point height increases. As a result, the difference of average solid concentration (∆F) becomes smaller. The above two factors decrease the lateral solid flow rate, Winter, which is reversely proportional to the characteristic time. Paying attention to the difference of pressure drop between both sides of the CFB riser after bed inventory overturn (Figure 9), a larger pressure drop difference can be found in the case with larger characteristic time. This is due to the limited branch point height in this cold experimental test rig. More solid particles accumulate in one leg, when the branch point becomes higher. Therefore, there is a larger difference of pressure drops after bed inventory overturn when a partition wall is set in the CFB riser. The effect of fluidizing velocity on characteristic time of bed inventory overturn is shown in Figure 10. At lower fluidizing velocity, the solid circulating rate is smaller than that at higher fluidizing velocity. The average solid concentration in the upper region of the riser decreases as well as the difference of pressure
Bed InVentory OVerturn in a CFB Riser
Figure 10. Impact of fluidizing velocity on the characteristic time.
drop between both sides of the CFB riser. Consequently, Tc increases when the fluidizing velocity decreases. At lower fluidizing velocity, most solid particles stay at the bottom of the riser, and only a limited amount of solid can be entrained above the branch point height. So the amount of solids transfer from one side to other side is smaller than that in case with higher fluidizing velocity. So the difference of pressure drops between two sides after bed inventory turnover is lower than that in the case with higher fluidizing velocity. In addition, the comparisons of the model calculation and experimental data under different operation cases prove that the model developed in this paper can describe the process of bed inventory overturn well. As bed inventory overturn is the inevitable result of hydrodynamic unbalance in CFB furnace with pant-leg structure, a real-time adjustment is needed to prevent the hydrodynamics in both sides of the furnace far away from the balance state. This requires the characteristic time of bed inventory overturn to be as long as possible. The experimental data show that the
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characteristic time of bed inventory overturn is directly proportional to the height of branch point and inversely proportional to the fluidizing velocity. In other words, increasing the branch point height and decreasing the fluidizing are two means reducing the intensity of the bed inventory overturn. Based on the discussion above, an automatic control system should pay attention to the following parameters: (I) pressure drops at both sides of the upper funace. A greater difference between these two pressure drops may cause a larger horizontal solid flow rate. (II) Air flow rates in both legs of the furnace. Air flow rate is an important parameter to maintain the pressure drop balance between both sides of the furnace. Further researches are required to determine the automatic control strategy in operations. Conclusion The process of bed inventory overturn in CFB riser with pant leg structure was studied with model calculation and experimental work. A compounded pressure drop mathematic model was developed and validated with the experimental data in a cold experimental test rig. The model calculation results agree well with the measured data. The characteristic time of bed inventory overturn is directly proportional to the height of branch point and inversely proportional to the fluidizing velocity. In other words, increasing the branch point height and decreasing the fluidizing will reduce the intensity of the bed inventory overturn. Acknowledgment. Financial supports of this work by Key Project of the National eleventh-Five Year Research Program of China (2006BAA03B02) and National Science Fund Committee (50406002) are gratefully acknowledged. EF800835Z
