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Effects of riser height and total solids inventory on the gas–solids in an ultra-tall CFB riser
Powder Technology 196 (2009) 8–13
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Effects of riser height and total solids inventory on the gas–solids in an ultra-tall CFB riser Nan Hu, Hai Zhang, Hairui Yang ⁎, Shi Yang, Guangxi Yue, Junfu Lu, Qing Liu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China
a r t i c l e
i n f o
Article history: Received 24 November 2008 Received in revised form 19 June 2009 Accepted 19 June 2009 Available online 27 June 2009 Keywords: CFB Riser height Solid inventory Voidage profile Fast fluidization
a b s t r a c t More and more CFB boilers with large capacity and ultra-tall furnaces are used for power generation. Understanding the fluid dynamics in the ultra-tall furnace is important. However, existing studies on fluid dynamics in the CFB furnace are limited to the risers with rather short height. An experimental study was conducted with a cold CFB test rig of 240 mm in I.D. and 38 m and 54 m in height respectively. The influences of total solid inventory Iv, and fluidizing gas velocity Ug on the axial voidage profile along the riser and solid circulation rate Gs were investigated. Experimental results showed that when Ug exceeded the transport velocity, an S-shaped voidage profile characterized by fast fluidization was established in the riser. In such circumstance, the voidage at top dilute section kept constant and Gs reached saturation carrying capacity (Gs = Gs⁎) and inappreciably change with riser height and Iv. Moreover, Gs⁎ increased from 40 kg to 50 kg when the riser height increased from 38 m to 54 m. The results indicated that even for the 600 MWe supercritical CFB boiler with a 54 m tall furnace, only a modest increase of Iv and power of forced draft fans is needed to obtain high enough Gs to meet the requirements of heating surfaces arrangement in furnace and the circulation loop. The necessary conditions to form the S-shaped profile of voidage in the riser were also discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Featured with excellent fuel flexibility and cost-effective emission control, circulating fluidized bed (CFB) boiler combustion technology is regarded as a clean coal technology, under rapid development around the world, especially in China. Up to now, nearly 3000 CFB boilers are in operation in China. With the development of the technology, CFB boilers are not only limited to the steam or combined heat and power generation, but also used in power generation, competing with the traditional pulverized coal fired boilers. Consequently, the unit capacity of the CFB boiler keeps increasing. So far, tens of subcritical 300 MWe CFB boilers have been put into operation in USA and China [1]. A 460 MWe supercritical CFB boiler made by Foster Wheeler Corporation, USA will be in commissioning in later 2009 at Lagisza Power Plant, Poland. A 600 MWe supercritical CFB boiler has been designed and is to put into commissioning in 2012 in Sichuan, China [2]. With the increase of unit capacity, the geometry of the CFB furnace enlarges, and the furnace height, Hr normally increases. Table 1 gives the typical Hrs of the CFB boilers with different unit capacity [1]. It can be seen that for CFB boilers used for power generation, the Hrs are of
⁎ Corresponding author. Tel.: +86 10 62773384; fax: +86 10 62788523. E-mail address: [email protected] (H. Yang). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.06.011
large values, much greater than that of the CFB risers used in laboratory experiments. In general, the furnace with a height over 30 m is called an ultra-tall furnace. It is curious, from a scientific view, to know how much difference in fluid mechanics exists as the riser height increases to such a large value. Moreover, from an engineering view, to properly design and operate the large capacity CFB boilers, it is of first priority to understand the gas–solid two-phase flow characteristics in the ultra-tall furnace. In a CFB boiler, the feedstock and thereby the bed material in the furnace normally has a relatively wide size distribution. Consequently, with Ug about 4–5 m/s at full boiler load, it is expected that a fast fluidized bed formed by fine particles can be formed in the upper furnace, as the superposition of a dense turbulent bed formed by large particles at the bottom [3]. Forming a fast bed in the upper furnace is important to obtain the desired axial profile of the solid suspension density, ρb or voidage εb along the height. The axial ρb profile is a key factor for heat transfer, and thereby the arrangement of heating surfaces. Meanwhile, it affects the penetration depth and diffusion rate of the secondary air, as well as the residence time, thereby the Table 1 Typical furnace height for CFB boilers with different unit capacity. Unit capacity (MW) Furnace height (m)
135 35–38
200 38–41
300 35–38
460 48
600 54–58
N. Hu et al. / Powder Technology 196 (2009) 8–13
entrance is very weakly restricted and when the riser is sufficiently high [26]. In a CFB boiler, Iv is normally a controllable variable. However, so far, the impact of Iv on gas–solid flow characteristics in the riser is still in controversy [12,16–25]. Table 2 summarizes the experimental studies and conditions on the Iv effect found in the literature. Based on experimental results obtained in a cold CFB rig with a riser of 10 m in height and 90 mm in diameter, Li et al. [12] concluded that at given Ug and Gs, the height of the bottom dense bed increases with increasing Iv but the εb in top dilute section is constant. To realize fast fluidization regime at a certain Ug, Iv should be maintained above a critical values I⁎v, such that Gs in the upper riser reaches the saturated value Gs⁎ for pneumatic transport, and a dense zone appears in the bottom of the riser [17–21]. On the contrary, some researchers argued that εb distribution in the riser does not change with Iv, and instead it is only a function of Ug and Gs [22–26,29]. It is suspected that the controversy of the Iv effect on the flow characteristics was at least partially attributed to the height of experimental rig, the restriction of the entrance and the structure of standpipe [9,28]. This controversy will be further discussed in Section 3.4 with the data retrieved from published literatures and present study. Based on the above introduction, the existing studies on the axial ρb or εb profile in the CFB risers are mostly conducted with the rather lower height, i.e., Hr b 20 m. From the view of both academic research and engineering application, it is interesting and important to validate if the experimental data and conclusions obtained in the conventional risers can be extended to the ultra-tall furnace in the large capacity CFB boiler. Consequently, in this study, a cold CFB test rig with a riser of 240 mm in I.D. and 38 m and 54 m in height respectively was built. The influence of operational conditions, such as Hr, Iv and Ug, on the axial εb profile along the riser was assessed.
Nomenclature Gs G⁎s Hr Iv I⁎v Ug εb ρb
9
solid circulation rate, kg/m2 s saturation carrying Gs, kg/m2 s height of riser, m solid inventory, kg saturation carrying Iv, kg fluidizing gas velocity, m/s voidage, (–) solid suspension density, kg/m3
combustion efficiency [4]. In addition, the ρb profile is directly coupled with the solid inventory Iv and thus the pressure drop of the furnace and the selection of the primary air fan. The key parameters affecting the formation of fast fluidization include the minimum transport velocity, Utr [5–10] and saturated solid circulation rate, Gs⁎ [11–15]. Utr refers to a critical gas velocity, at which a transition occurs from a turbulent fluidized bed towards the fast fluidization regime. It can be determined experimentally [5,6] by the methods such as the empty time (blow-off) one developed by Perales et al. [6]. Gs⁎ is a critical value for pneumatically transport. When Ug N Utr and Gs N Gs⁎, a dense zone appears in the bottom of the furnace [11,12]. Some studies found that G⁎s is the maximal Gs at choking velocity when no solid accumulation at the riser bottom occurs, independent of furnace height [13,14]. Bai and Kato [15], however, found that G⁎s is smaller than the choking value. The discrepancy might be caused by the different definition of G⁎s and the relative lower riser height used in the experiments. The voidage distribution in riser of a fast fluidized bed is influenced by not only the operational parameters such as Ug, Gs, solid particle properties and Iv, but also the geometric structure of the components in the circulating loop (e.g., the inlet and exit structure and bed diameter) [16–37]. Li et al. [12] found that the εb profile typically has an S-shape, i.e. dense phase at bottom, dilute phase at the top section and a point of inflection dividing the two sections. This discovery was proved by several researchers [16–21]. However, a simple exponential shaped distribution in εb rather than the S-shaped one was observed in some other studies [e.g., 22–25]. Comparing the experimental systems with each other, one can find that the restrictions of the riser geometry, i.e. structure of inlet and outlet are different. Furthermore, the risers with exponential voidage distribution are shorter than those with S-shaped voidage distribution. Previous studies found that the restriction could greatly influence the shape of the axial εb distribution [26,27], and the S-shaped distribution only appears when the riser
2. Experiments The experimental CFB test rig was installed next to a 300 MWe CFB boiler located in Baima Power Plant, Sichuan, China, and the schematic is shown in Fig. 1. The main body consisted of a riser, a cyclone, a return leg and a loop seal. The riser had inner diameter of 240 mm. The cyclone was installed close to the riser roof, with its inlet at the height of 38 m and 54 m above the distributor respectively. The return leg was of 75 mm I.D., and was made of transparent plexiglass for observation. A butterfly valve and three-way valve were installed along the return leg. The fluidizing air was induced from the secondary air fan of the boiler, with pressure head of 40 kPa. The air distributor was composed of an orifice plate and stainless sieve. The flow rate of the fluidizing air was controlled by an adjustable valve and online measured by a vortex flow meter. Solid particles were carried by the fluidizing air upward in
Table 2 Experimental conditions used in studying the effect of solid inventory. Investigator
Year
Particles
ρp
dp
Riser
kg/m3
μm
I.D/mm
H/m
External loop
m/s
kg/(ms)
49 54 55 54 166 74.9 109 57 38 70
152 90 50 90 97 152 – 100
8.5 10 4.97 10 3 5.5 – 5.5
Slow bed, butterfly valve V-valve U-valve V-valve Slow bed, butterfly valve Reservoir, L-vale Butterfly valve Screw feeder
2.9–3.4 1.5–2.6 1.5–4.0 2.1 2.45–4.48 2.2–4.0 – 1.32–3.56
71–140 14–193 10–170 24.1–96.3 6–65 4.9–50.6 – 5.3–79.4
6.1 7.62 38/54
Gate valve
4.0–8.0
18–425
210
76.2 102 240
Loop seal
3.9
2.6–11.4
Weinstein et al. [16] Li J. et al. [12] Mori et al. [17] Xu et al. [19]
1983 1988 1992 2003
Rhodes et al. [22] Chang et al. [23] Hirama et al. [24]
1992 1992 1992
HFZ-20 FCC FCC FCC Silica sand FRF5 Uncoated FCC
Issangya et al. [25]
1997
FCC
1450 930 729 930 2220 2456 2530 930 750 1600
In this work
2008
Silica sand
2630
Ug
Gs
10
N. Hu et al. / Powder Technology 196 (2009) 8–13
Fig. 3. Time required for all solid inventory to leave the riser at different fluidizing gas velocity.
Fig. 1. Schematic diagram of experimental apparatus.
the riser and then into the cyclone. After separation, nearly all of them were collected and returned to the riser through the return leg and the loop seal. All the experiments were carried out at ambient temperature and pressure. Solid material used was quartz sand with the diameter ranged from 150 μm to 250 μm. It has the real density of 2630 kg/m3 and bulk density of 1550 kg/m3. The axial εb profile was derived by pressure drops measured at different pressure taps located along the riser height. The lowest pressure tap P0 was installed right below the air distributor and the next one P1 was installed 5 cm above the distributor. The rest pressure taps were installed with 2 m interval along the riser. At the same time, the voidage (or solid concentration) at 27 m height (from the air distributor) was measured by a fiber optic probe.
Two methods were used to measure the solid circulation rate, Gs. One was based on the time accounting for the returning particles to reach a certain height in the return leg after a sudden close of the butterfly valve. The other method, i.e. the online weighting method, is illustrated in Fig. 2. When the system was in a steady operation for at least 5 min, the three-way valve on the return leg was suddenly switched from the normal position to the pipe connected with a measuring tank. The weight of the tank was measured by a weighting sensor and recorded by a computer. Against the transient method developed by Bai et al. [30], during the weighting process, Ivs in the riser kept decreasing, and the variation could be observed from the change of pressure drops. This method is similar to the transient one developed by Arena et al. [18] and Monazam et al. [31]. To ensure the gas–solid flow in the riser was in fast fluidization, Ug was kept above the transport velocity, Utr [7]. Based on the empty time (blow-off) method, on the curve of blow-off time verse Ug, as shown in Fig. 3, the bending point corresponds to Utr [6]. Fig. 3 shows Utr measured in 38 m and 54 m riser respectively when Iv = 40 kg. It can be seen that for the 54 m riser, at the same Ug, the blowoff time is about 20% longer when the riser is in the pneumatic transport regime, and 30%–40% longer in the fast bed regime. However, the difference of Utr is much less sensitive to the riser height. It is about 3.5 m/s for both risers. Due to the different characteristics of solid particles and riser structures, the predicted values calculated by different correlations are greatly different, as shown in Table 3. Consequently, experimental determination is regarded as the most reliable method to obtain Utr. In the present study, Utr was chosen as 3.9 m/s. 3. Results and discussion 3.1. The influence of Iv and Hr on the axial voidage profiles Fig. 4a and b shows the axial εb profiles at different solid inventories, Ivs, in the 38 m riser and 54 m riser respectively. εb at the bottom dense bed and transition zone dramatically decreases when Iv increases. On the contrary, εb in the top zone obviously decreases when Iv is small and then becomes nearly constant when Iv is greater than a certain value. When Hr = 38 m, the transient happens as Iv N 40 kg, and when Hr = 54 m, the transient happens as Iv N 50 kg. Table 3 The calculated values of Utr with correlations in literature.
Fig. 2. Schematic diagram of online weighing system.
Authors Utr, m/s
Matsen [8] 7.0
Li Youchu [9] 5.3
Adanez [5] 2.87
Bai [10] 2.24
This paper 3.5
N. Hu et al. / Powder Technology 196 (2009) 8–13
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Table 4 G*s, kg/m2 s, calculated by different experimental correlations. Bai [32] 99.9
Fig. 4. Variations of axial voidage profile with solid inventory and riser height.
Though the data are relatively sparse due to the arrangement of pressure measurement points, they are sufficient to validate the similarity of the εb profiles between the 54 m riser and 38 m riser. The only difference exists for the critical Iv corresponding to the invariant εb in the upper riser. For a taller riser, the critical Iv is larger. 3.2. Influence of Iv and Hr on Gs Fig. 5 shows the variations of axial pressure drop, solid fraction measured at 27 m high and Gs with Iv respectively. Based on the results measured by fiber optic probe at height 27 m (shown in Fig. 5b), it can be seen that for both risers, the solid fraction linearly increases with Iv, and then becomes nearly constant as Iv exceeds a critical value, I⁎v. Combined with the results shown in Fig. 4, it can be concluded that the εb above 25 m inappreciably changes with riser height.
Monazam [33] 1.51
Bi [34] 20.5
Xu [35] 10.7
Namkung [36] 60.6
This paper 9–10
At a given Ug, the variations of Gs with Iv in the risers with different height are plotted in Fig. 5c. Clearly, each variation curve has two stages. In the first stage, Gs increases rapidly with Iv, and in the second stage, Gs is nearly constant as Iv N I⁎v. Gs cross the two stages is named as the saturated G⁎s. In the second stage, further increase of Iv mainly accumulates particles in the bottom riser. Moreover, from Fig. 5b and c, it can be seen that with different riser height, G⁎s keeps nearly the same, about 9–10 kg/m2 s, and so does the corresponding voidage. However, I⁎v is larger for the taller riser. When Hr = 38 m, I⁎v= 40 kg and when Hr = 54 m, I⁎v= 50 kg. The total pressure drops of the riser are 7.5 kPa and 9 kPa, respectively, as shown in Fig. 5a. The above results are consistent with those found by Li et al. [12] and Xu et al. [13]. Though the classical S-shape εb profile for a fast fluidization bed was not found in present study, the phenomena of constant εb in top dilute section and invariant Gs with Iv prove that the gas–solid flow reaches the saturated carrying capacity. Table 4 lists the predicted values by different correlations. Because the riser used in Bai et al. [32] and Namkung et al. [36] are relatively short, the corresponding values are much higher than average. The present experimental data are consistent with the correlation used by Xu et al. [35], which was proposed with the consideration of the influences of various solid characteristic and riser size. The results have significant meaning to the design and operation of the 600 MWe supercritical CFB boiler. They indicate that choosing the bed pressure drop of 300 MWe CFB boiler (about 10 kPa) can obtain the same εb and Gs. The power consumption of the force draft fans is expected to increase within a small margin.
3.3. Discussion on the results of online weighting Shown in Fig. 4, the εb profiles are in exponential, instead of S-shaped. Since gravity acceleration effect and the wall friction are normally strong in the bottom part of the CFB riser, the solid fraction measured pressure drop method could be overestimated [26]. This overestimation could be corrected with the online weighting method introduced in the experimental section. Fig. 6 shows the temporal variations of pressures or pressure drops at different parts during the online weighing experiment in the 54 m
Fig. 5. Variations of pressure drop across the riser, solid fraction at 27 m high, and Gs with solid inventory.
Fig. 6. Temporal variations of pressures or pressure drops at different parts.
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riser when Ug = 3.9 m/s and Iv = 90 kg. With time elapsed, the gas– solid flow in the system experienced four successive periods: Stage I. Steady operation. In this stage, the baffle of the valve was held on normal position, Position b, as shown in Fig. 2. The CFB system was in stable and the pressures at different locations were also stable. Stage II. Transition period. When the baffle was switched to Position A, the pressure balance in the system was broken abruptly and then reached a new balance. Stage III. Constant Gs period. The weight of solids in the tank increased constantly during a certain time, about 80 s. This indicates that Gs was constant in this period. In addition, the pressure drop across the cyclone was nearly constant, also indicating the constant Gs when Ug was held constant [37]. At the same time, the pressure drops from 50 m to 52 m, from 44 m to 54 m constantly maintain at about 0.2 kPa and 1.0 kPa respectively. These results showed that the local εb above 44 m height was nearly same and inappreciably changed as Iv decreased. It can also be seen that the pressure drop from 4 m to 8 m is greater than that from 50 m to 52 m, and does not change with the decreasing of Iv for a period about 60 s. This means that the height of bottom dense bed was greater than 8 m at the beginning of online weighting. With decreasing of Iv in the system, the height of the bottom dense bed decreased below the 8 m level, and the pressure drop from 4 m to 8 m decreased gradually. Stage IV. Blowing off period. As Iv decreases, Gs b G⁎s and the gas– solid flow regime changed into dilute-phase pneumatic transport. The pressure drop across the riser decreased gradually until the riser was particle free. In the same CFB riser and at the same Ug, when Iv = 40 kg, the stage with stable Gs was not observed during the online weighting experiments. From the beginning of the online weighting, the pressure or pressure drop in the riser continuously decreased with decreasing of Iv. The results of online weighting were consistent with that shown in Fig. 5. When Ug = 3.9 m/s, the measured I⁎v= 50 kg in the 54 m riser. When Iv = 90 kg, S-shaped εb profiles appeared in the riser for the certain period during the online weighting, The εb in the top dilute section and bottom dense section were constants, and the height of dense bed decreased with decreasing of Iv. When Iv b I⁎v, the εb in the riser and Gs dropped gradually with the decreasing of Iv in the riser. 3.4. Discussion By analyzing the structure of the experimental rigs and operational conditions in the published literatures, it is believed that the controversy in the Iv effect on gas–solid flow characteristics in the riser lies in the structure difference in the external loop, especially the restriction of return valve [9]. When the return valve is weakly restricted, i.e., when the flow resistance of the valve is insensitive to Gs, the height of dense bed in a S-shaped εb profile is affected significantly by Iv while the voidage in the bottom dense section and in the upper dilute section remain nearly constant [16–21]. On the contrary, if return valve is strongly restricted, at given Ug and Gs, the increment of Iv accumulates in the standpipe compensates the resistance of the return valve. Consequently, the variation of Iv plays minimal effect on the axial voidage profile [22,23,29]. When a solid hopper or a bubbling bed with large dimension is arranged in the external loop, the effect of Iv is suppressed [22]. The loop seal used in present study had low flow resistance (about 1.0 kPa), and only in the case of Iv b 15 kg could it be used to control the solid flow rate by aeration. When the pressure drop in the riser or in the return leg was far greater than the flow resistance of the valve, the valve lost the controllability. Consequently, same effect of Iv on the εb profile in the riser was found as in the literatures [16–19]. On the other hand, for a short riser, just like the riser of 3.2 m height used by Yang et al. [28], the gas–solid flow in the upper dilute
region could not reach the saturated carrying capacity at certain Ug because the freeboard length was shorter than TDH [15]. No matter what kind of control valve is used, the S-shaped profile cannot be formed, unless Ug is decreased to reach the saturated carrying capacity. For a tall riser that can keep the freeboard length larger than TDH at certain Ug, when a return valve of weak constraint is equipped, S-shaped εb profile is formed and the variation of Iv would have little influence on εb on top dilute region or Gs. Moreover, with the increasing of the riser height, the flow resistance of return valve becomes relatively smaller compared with the pressure drop in the riser, and the constraint becomes weaker. Li et al. [9] pointed out that the higher is the riser, the higher formation tendency the S-shaped profile will be, which was then simulated by Wang et al. [21] with multi-scale EMMS approach. In CFB boilers, a loop seal with low gas solid flow resistance is commonly used. The furnace height is normally higher than 20 m. When Ug = 4–5m/s and Iv b I⁎v, fast fluidization regime can be realized, i.e., gas–solid flow in the upper dilute furnace reaches the saturation state [19]. 4. Conclusions To properly design and operate the large capacity CFB boilers used in power generation, it is of significance to conduct experimental study on the fluid dynamics in the riser with ultra-high heights (N30 m). In this study, a cold CFB test rig with a riser 38 m and 54 m in height respectively was built to investigate the influence of operational conditions, including Iv and Ug, on the axial voidage profile along the riser and Gs in these risers. Experimental results showed that when Ug is greater than the transport velocity and Iv is above a critical value, the S-shaped voidage profile, as a feature of fast fluidization appears in the riser. The voidage in top dilute section and Gs does not change with Hr nor Iv. The critical Iv for saturated carrying capacity increases from 40 kg to 50 kg when Hr increases from 38 m to 54 m. The pressure drops in the riser are about 7.5 kPa and 9.0 kPa, respectively. For the 600 MWe supercritical CFB boiler with 54 m tall furnace, only a modest increment in Iv and power of forced draft fans is needed to obtain high enough Gs to meet the requirements of heating surfaces arrangement in furnace and the circulation loop. The distribution of gas–solid concentration in the riser is affected not only by Ug and Gs, but also by Iv. When the riser is tall enough and the return valve is weakly restricted, the tendency to form fast fluidization with S-shaped voidage profile is high in the riser. Acknowledgements Financial supports of this work by the National Science Fund Committee (No.50406002) and High Technology R&D (863) (2009AA05Z302) are gratefully acknowledged. References [1] G.X. Yue, Development and future of the circulating fluidized bed combustion technology, in: G.X. Yue (Ed.), 1st Fluidization Combustion and Technology in China, 2006, pp. 1–30. [2] Y.X. Wu, J.F. Lu, J.S. Zhang, et al., Conceptual design of an 800 MW supercritical pressure circulating fluidized bed, in: K.F. Cen (Ed.), Circulating Fluidized Bed Technology VIII, International Academic Publisher, Hangzhou, 2005, pp. 529–544. [3] G.X. Yue, J.F. Lu, H. Zhang, et al., Design theory of circulating fluidized bed boilers,18th International Fluidized Bed Combustion Conference, Toronto Canada, May 18–21 2005. [4] X.B. Xiao, H.R. Yang, H. Zhang, et al., Research on carbon content in fly ash from circulating fluidized bed boilers, Energy & Fuels 19 (2005) 1520–1525. [5] J. Adanez, L.F. de Diego, P. Gayan, Transport velocities of coal and sand particles, Powder Technology 77 (1993) 61–68. [6] J.F. Perales, T. Coll, M.F. Liop, et al., On the transition from bubbling to fast fluidization regimes, in: P. Basu, M. Horio, M. Hasatami (Eds.), Circulating Fluidized Bed Technology III, Pergamon, Oxford, 1991, pp. 73–78.
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Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Effects of riser height and total solids inventory on the gas–solids in an ultra-tall CFB riser Nan Hu, Hai Zhang, Hairui Yang ⁎, Shi Yang, Guangxi Yue, Junfu Lu, Qing Liu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China
a r t i c l e
i n f o
Article history: Received 24 November 2008 Received in revised form 19 June 2009 Accepted 19 June 2009 Available online 27 June 2009 Keywords: CFB Riser height Solid inventory Voidage profile Fast fluidization
a b s t r a c t More and more CFB boilers with large capacity and ultra-tall furnaces are used for power generation. Understanding the fluid dynamics in the ultra-tall furnace is important. However, existing studies on fluid dynamics in the CFB furnace are limited to the risers with rather short height. An experimental study was conducted with a cold CFB test rig of 240 mm in I.D. and 38 m and 54 m in height respectively. The influences of total solid inventory Iv, and fluidizing gas velocity Ug on the axial voidage profile along the riser and solid circulation rate Gs were investigated. Experimental results showed that when Ug exceeded the transport velocity, an S-shaped voidage profile characterized by fast fluidization was established in the riser. In such circumstance, the voidage at top dilute section kept constant and Gs reached saturation carrying capacity (Gs = Gs⁎) and inappreciably change with riser height and Iv. Moreover, Gs⁎ increased from 40 kg to 50 kg when the riser height increased from 38 m to 54 m. The results indicated that even for the 600 MWe supercritical CFB boiler with a 54 m tall furnace, only a modest increase of Iv and power of forced draft fans is needed to obtain high enough Gs to meet the requirements of heating surfaces arrangement in furnace and the circulation loop. The necessary conditions to form the S-shaped profile of voidage in the riser were also discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Featured with excellent fuel flexibility and cost-effective emission control, circulating fluidized bed (CFB) boiler combustion technology is regarded as a clean coal technology, under rapid development around the world, especially in China. Up to now, nearly 3000 CFB boilers are in operation in China. With the development of the technology, CFB boilers are not only limited to the steam or combined heat and power generation, but also used in power generation, competing with the traditional pulverized coal fired boilers. Consequently, the unit capacity of the CFB boiler keeps increasing. So far, tens of subcritical 300 MWe CFB boilers have been put into operation in USA and China [1]. A 460 MWe supercritical CFB boiler made by Foster Wheeler Corporation, USA will be in commissioning in later 2009 at Lagisza Power Plant, Poland. A 600 MWe supercritical CFB boiler has been designed and is to put into commissioning in 2012 in Sichuan, China [2]. With the increase of unit capacity, the geometry of the CFB furnace enlarges, and the furnace height, Hr normally increases. Table 1 gives the typical Hrs of the CFB boilers with different unit capacity [1]. It can be seen that for CFB boilers used for power generation, the Hrs are of
⁎ Corresponding author. Tel.: +86 10 62773384; fax: +86 10 62788523. E-mail address: [email protected] (H. Yang). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.06.011
large values, much greater than that of the CFB risers used in laboratory experiments. In general, the furnace with a height over 30 m is called an ultra-tall furnace. It is curious, from a scientific view, to know how much difference in fluid mechanics exists as the riser height increases to such a large value. Moreover, from an engineering view, to properly design and operate the large capacity CFB boilers, it is of first priority to understand the gas–solid two-phase flow characteristics in the ultra-tall furnace. In a CFB boiler, the feedstock and thereby the bed material in the furnace normally has a relatively wide size distribution. Consequently, with Ug about 4–5 m/s at full boiler load, it is expected that a fast fluidized bed formed by fine particles can be formed in the upper furnace, as the superposition of a dense turbulent bed formed by large particles at the bottom [3]. Forming a fast bed in the upper furnace is important to obtain the desired axial profile of the solid suspension density, ρb or voidage εb along the height. The axial ρb profile is a key factor for heat transfer, and thereby the arrangement of heating surfaces. Meanwhile, it affects the penetration depth and diffusion rate of the secondary air, as well as the residence time, thereby the Table 1 Typical furnace height for CFB boilers with different unit capacity. Unit capacity (MW) Furnace height (m)
135 35–38
200 38–41
300 35–38
460 48
600 54–58
N. Hu et al. / Powder Technology 196 (2009) 8–13
entrance is very weakly restricted and when the riser is sufficiently high [26]. In a CFB boiler, Iv is normally a controllable variable. However, so far, the impact of Iv on gas–solid flow characteristics in the riser is still in controversy [12,16–25]. Table 2 summarizes the experimental studies and conditions on the Iv effect found in the literature. Based on experimental results obtained in a cold CFB rig with a riser of 10 m in height and 90 mm in diameter, Li et al. [12] concluded that at given Ug and Gs, the height of the bottom dense bed increases with increasing Iv but the εb in top dilute section is constant. To realize fast fluidization regime at a certain Ug, Iv should be maintained above a critical values I⁎v, such that Gs in the upper riser reaches the saturated value Gs⁎ for pneumatic transport, and a dense zone appears in the bottom of the riser [17–21]. On the contrary, some researchers argued that εb distribution in the riser does not change with Iv, and instead it is only a function of Ug and Gs [22–26,29]. It is suspected that the controversy of the Iv effect on the flow characteristics was at least partially attributed to the height of experimental rig, the restriction of the entrance and the structure of standpipe [9,28]. This controversy will be further discussed in Section 3.4 with the data retrieved from published literatures and present study. Based on the above introduction, the existing studies on the axial ρb or εb profile in the CFB risers are mostly conducted with the rather lower height, i.e., Hr b 20 m. From the view of both academic research and engineering application, it is interesting and important to validate if the experimental data and conclusions obtained in the conventional risers can be extended to the ultra-tall furnace in the large capacity CFB boiler. Consequently, in this study, a cold CFB test rig with a riser of 240 mm in I.D. and 38 m and 54 m in height respectively was built. The influence of operational conditions, such as Hr, Iv and Ug, on the axial εb profile along the riser was assessed.
Nomenclature Gs G⁎s Hr Iv I⁎v Ug εb ρb
9
solid circulation rate, kg/m2 s saturation carrying Gs, kg/m2 s height of riser, m solid inventory, kg saturation carrying Iv, kg fluidizing gas velocity, m/s voidage, (–) solid suspension density, kg/m3
combustion efficiency [4]. In addition, the ρb profile is directly coupled with the solid inventory Iv and thus the pressure drop of the furnace and the selection of the primary air fan. The key parameters affecting the formation of fast fluidization include the minimum transport velocity, Utr [5–10] and saturated solid circulation rate, Gs⁎ [11–15]. Utr refers to a critical gas velocity, at which a transition occurs from a turbulent fluidized bed towards the fast fluidization regime. It can be determined experimentally [5,6] by the methods such as the empty time (blow-off) one developed by Perales et al. [6]. Gs⁎ is a critical value for pneumatically transport. When Ug N Utr and Gs N Gs⁎, a dense zone appears in the bottom of the furnace [11,12]. Some studies found that G⁎s is the maximal Gs at choking velocity when no solid accumulation at the riser bottom occurs, independent of furnace height [13,14]. Bai and Kato [15], however, found that G⁎s is smaller than the choking value. The discrepancy might be caused by the different definition of G⁎s and the relative lower riser height used in the experiments. The voidage distribution in riser of a fast fluidized bed is influenced by not only the operational parameters such as Ug, Gs, solid particle properties and Iv, but also the geometric structure of the components in the circulating loop (e.g., the inlet and exit structure and bed diameter) [16–37]. Li et al. [12] found that the εb profile typically has an S-shape, i.e. dense phase at bottom, dilute phase at the top section and a point of inflection dividing the two sections. This discovery was proved by several researchers [16–21]. However, a simple exponential shaped distribution in εb rather than the S-shaped one was observed in some other studies [e.g., 22–25]. Comparing the experimental systems with each other, one can find that the restrictions of the riser geometry, i.e. structure of inlet and outlet are different. Furthermore, the risers with exponential voidage distribution are shorter than those with S-shaped voidage distribution. Previous studies found that the restriction could greatly influence the shape of the axial εb distribution [26,27], and the S-shaped distribution only appears when the riser
2. Experiments The experimental CFB test rig was installed next to a 300 MWe CFB boiler located in Baima Power Plant, Sichuan, China, and the schematic is shown in Fig. 1. The main body consisted of a riser, a cyclone, a return leg and a loop seal. The riser had inner diameter of 240 mm. The cyclone was installed close to the riser roof, with its inlet at the height of 38 m and 54 m above the distributor respectively. The return leg was of 75 mm I.D., and was made of transparent plexiglass for observation. A butterfly valve and three-way valve were installed along the return leg. The fluidizing air was induced from the secondary air fan of the boiler, with pressure head of 40 kPa. The air distributor was composed of an orifice plate and stainless sieve. The flow rate of the fluidizing air was controlled by an adjustable valve and online measured by a vortex flow meter. Solid particles were carried by the fluidizing air upward in
Table 2 Experimental conditions used in studying the effect of solid inventory. Investigator
Year
Particles
ρp
dp
Riser
kg/m3
μm
I.D/mm
H/m
External loop
m/s
kg/(ms)
49 54 55 54 166 74.9 109 57 38 70
152 90 50 90 97 152 – 100
8.5 10 4.97 10 3 5.5 – 5.5
Slow bed, butterfly valve V-valve U-valve V-valve Slow bed, butterfly valve Reservoir, L-vale Butterfly valve Screw feeder
2.9–3.4 1.5–2.6 1.5–4.0 2.1 2.45–4.48 2.2–4.0 – 1.32–3.56
71–140 14–193 10–170 24.1–96.3 6–65 4.9–50.6 – 5.3–79.4
6.1 7.62 38/54
Gate valve
4.0–8.0
18–425
210
76.2 102 240
Loop seal
3.9
2.6–11.4
Weinstein et al. [16] Li J. et al. [12] Mori et al. [17] Xu et al. [19]
1983 1988 1992 2003
Rhodes et al. [22] Chang et al. [23] Hirama et al. [24]
1992 1992 1992
HFZ-20 FCC FCC FCC Silica sand FRF5 Uncoated FCC
Issangya et al. [25]
1997
FCC
1450 930 729 930 2220 2456 2530 930 750 1600
In this work
2008
Silica sand
2630
Ug
Gs
10
N. Hu et al. / Powder Technology 196 (2009) 8–13
Fig. 3. Time required for all solid inventory to leave the riser at different fluidizing gas velocity.
Fig. 1. Schematic diagram of experimental apparatus.
the riser and then into the cyclone. After separation, nearly all of them were collected and returned to the riser through the return leg and the loop seal. All the experiments were carried out at ambient temperature and pressure. Solid material used was quartz sand with the diameter ranged from 150 μm to 250 μm. It has the real density of 2630 kg/m3 and bulk density of 1550 kg/m3. The axial εb profile was derived by pressure drops measured at different pressure taps located along the riser height. The lowest pressure tap P0 was installed right below the air distributor and the next one P1 was installed 5 cm above the distributor. The rest pressure taps were installed with 2 m interval along the riser. At the same time, the voidage (or solid concentration) at 27 m height (from the air distributor) was measured by a fiber optic probe.
Two methods were used to measure the solid circulation rate, Gs. One was based on the time accounting for the returning particles to reach a certain height in the return leg after a sudden close of the butterfly valve. The other method, i.e. the online weighting method, is illustrated in Fig. 2. When the system was in a steady operation for at least 5 min, the three-way valve on the return leg was suddenly switched from the normal position to the pipe connected with a measuring tank. The weight of the tank was measured by a weighting sensor and recorded by a computer. Against the transient method developed by Bai et al. [30], during the weighting process, Ivs in the riser kept decreasing, and the variation could be observed from the change of pressure drops. This method is similar to the transient one developed by Arena et al. [18] and Monazam et al. [31]. To ensure the gas–solid flow in the riser was in fast fluidization, Ug was kept above the transport velocity, Utr [7]. Based on the empty time (blow-off) method, on the curve of blow-off time verse Ug, as shown in Fig. 3, the bending point corresponds to Utr [6]. Fig. 3 shows Utr measured in 38 m and 54 m riser respectively when Iv = 40 kg. It can be seen that for the 54 m riser, at the same Ug, the blowoff time is about 20% longer when the riser is in the pneumatic transport regime, and 30%–40% longer in the fast bed regime. However, the difference of Utr is much less sensitive to the riser height. It is about 3.5 m/s for both risers. Due to the different characteristics of solid particles and riser structures, the predicted values calculated by different correlations are greatly different, as shown in Table 3. Consequently, experimental determination is regarded as the most reliable method to obtain Utr. In the present study, Utr was chosen as 3.9 m/s. 3. Results and discussion 3.1. The influence of Iv and Hr on the axial voidage profiles Fig. 4a and b shows the axial εb profiles at different solid inventories, Ivs, in the 38 m riser and 54 m riser respectively. εb at the bottom dense bed and transition zone dramatically decreases when Iv increases. On the contrary, εb in the top zone obviously decreases when Iv is small and then becomes nearly constant when Iv is greater than a certain value. When Hr = 38 m, the transient happens as Iv N 40 kg, and when Hr = 54 m, the transient happens as Iv N 50 kg. Table 3 The calculated values of Utr with correlations in literature.
Fig. 2. Schematic diagram of online weighing system.
Authors Utr, m/s
Matsen [8] 7.0
Li Youchu [9] 5.3
Adanez [5] 2.87
Bai [10] 2.24
This paper 3.5
N. Hu et al. / Powder Technology 196 (2009) 8–13
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Table 4 G*s, kg/m2 s, calculated by different experimental correlations. Bai [32] 99.9
Fig. 4. Variations of axial voidage profile with solid inventory and riser height.
Though the data are relatively sparse due to the arrangement of pressure measurement points, they are sufficient to validate the similarity of the εb profiles between the 54 m riser and 38 m riser. The only difference exists for the critical Iv corresponding to the invariant εb in the upper riser. For a taller riser, the critical Iv is larger. 3.2. Influence of Iv and Hr on Gs Fig. 5 shows the variations of axial pressure drop, solid fraction measured at 27 m high and Gs with Iv respectively. Based on the results measured by fiber optic probe at height 27 m (shown in Fig. 5b), it can be seen that for both risers, the solid fraction linearly increases with Iv, and then becomes nearly constant as Iv exceeds a critical value, I⁎v. Combined with the results shown in Fig. 4, it can be concluded that the εb above 25 m inappreciably changes with riser height.
Monazam [33] 1.51
Bi [34] 20.5
Xu [35] 10.7
Namkung [36] 60.6
This paper 9–10
At a given Ug, the variations of Gs with Iv in the risers with different height are plotted in Fig. 5c. Clearly, each variation curve has two stages. In the first stage, Gs increases rapidly with Iv, and in the second stage, Gs is nearly constant as Iv N I⁎v. Gs cross the two stages is named as the saturated G⁎s. In the second stage, further increase of Iv mainly accumulates particles in the bottom riser. Moreover, from Fig. 5b and c, it can be seen that with different riser height, G⁎s keeps nearly the same, about 9–10 kg/m2 s, and so does the corresponding voidage. However, I⁎v is larger for the taller riser. When Hr = 38 m, I⁎v= 40 kg and when Hr = 54 m, I⁎v= 50 kg. The total pressure drops of the riser are 7.5 kPa and 9 kPa, respectively, as shown in Fig. 5a. The above results are consistent with those found by Li et al. [12] and Xu et al. [13]. Though the classical S-shape εb profile for a fast fluidization bed was not found in present study, the phenomena of constant εb in top dilute section and invariant Gs with Iv prove that the gas–solid flow reaches the saturated carrying capacity. Table 4 lists the predicted values by different correlations. Because the riser used in Bai et al. [32] and Namkung et al. [36] are relatively short, the corresponding values are much higher than average. The present experimental data are consistent with the correlation used by Xu et al. [35], which was proposed with the consideration of the influences of various solid characteristic and riser size. The results have significant meaning to the design and operation of the 600 MWe supercritical CFB boiler. They indicate that choosing the bed pressure drop of 300 MWe CFB boiler (about 10 kPa) can obtain the same εb and Gs. The power consumption of the force draft fans is expected to increase within a small margin.
3.3. Discussion on the results of online weighting Shown in Fig. 4, the εb profiles are in exponential, instead of S-shaped. Since gravity acceleration effect and the wall friction are normally strong in the bottom part of the CFB riser, the solid fraction measured pressure drop method could be overestimated [26]. This overestimation could be corrected with the online weighting method introduced in the experimental section. Fig. 6 shows the temporal variations of pressures or pressure drops at different parts during the online weighing experiment in the 54 m
Fig. 5. Variations of pressure drop across the riser, solid fraction at 27 m high, and Gs with solid inventory.
Fig. 6. Temporal variations of pressures or pressure drops at different parts.
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N. Hu et al. / Powder Technology 196 (2009) 8–13
riser when Ug = 3.9 m/s and Iv = 90 kg. With time elapsed, the gas– solid flow in the system experienced four successive periods: Stage I. Steady operation. In this stage, the baffle of the valve was held on normal position, Position b, as shown in Fig. 2. The CFB system was in stable and the pressures at different locations were also stable. Stage II. Transition period. When the baffle was switched to Position A, the pressure balance in the system was broken abruptly and then reached a new balance. Stage III. Constant Gs period. The weight of solids in the tank increased constantly during a certain time, about 80 s. This indicates that Gs was constant in this period. In addition, the pressure drop across the cyclone was nearly constant, also indicating the constant Gs when Ug was held constant [37]. At the same time, the pressure drops from 50 m to 52 m, from 44 m to 54 m constantly maintain at about 0.2 kPa and 1.0 kPa respectively. These results showed that the local εb above 44 m height was nearly same and inappreciably changed as Iv decreased. It can also be seen that the pressure drop from 4 m to 8 m is greater than that from 50 m to 52 m, and does not change with the decreasing of Iv for a period about 60 s. This means that the height of bottom dense bed was greater than 8 m at the beginning of online weighting. With decreasing of Iv in the system, the height of the bottom dense bed decreased below the 8 m level, and the pressure drop from 4 m to 8 m decreased gradually. Stage IV. Blowing off period. As Iv decreases, Gs b G⁎s and the gas– solid flow regime changed into dilute-phase pneumatic transport. The pressure drop across the riser decreased gradually until the riser was particle free. In the same CFB riser and at the same Ug, when Iv = 40 kg, the stage with stable Gs was not observed during the online weighting experiments. From the beginning of the online weighting, the pressure or pressure drop in the riser continuously decreased with decreasing of Iv. The results of online weighting were consistent with that shown in Fig. 5. When Ug = 3.9 m/s, the measured I⁎v= 50 kg in the 54 m riser. When Iv = 90 kg, S-shaped εb profiles appeared in the riser for the certain period during the online weighting, The εb in the top dilute section and bottom dense section were constants, and the height of dense bed decreased with decreasing of Iv. When Iv b I⁎v, the εb in the riser and Gs dropped gradually with the decreasing of Iv in the riser. 3.4. Discussion By analyzing the structure of the experimental rigs and operational conditions in the published literatures, it is believed that the controversy in the Iv effect on gas–solid flow characteristics in the riser lies in the structure difference in the external loop, especially the restriction of return valve [9]. When the return valve is weakly restricted, i.e., when the flow resistance of the valve is insensitive to Gs, the height of dense bed in a S-shaped εb profile is affected significantly by Iv while the voidage in the bottom dense section and in the upper dilute section remain nearly constant [16–21]. On the contrary, if return valve is strongly restricted, at given Ug and Gs, the increment of Iv accumulates in the standpipe compensates the resistance of the return valve. Consequently, the variation of Iv plays minimal effect on the axial voidage profile [22,23,29]. When a solid hopper or a bubbling bed with large dimension is arranged in the external loop, the effect of Iv is suppressed [22]. The loop seal used in present study had low flow resistance (about 1.0 kPa), and only in the case of Iv b 15 kg could it be used to control the solid flow rate by aeration. When the pressure drop in the riser or in the return leg was far greater than the flow resistance of the valve, the valve lost the controllability. Consequently, same effect of Iv on the εb profile in the riser was found as in the literatures [16–19]. On the other hand, for a short riser, just like the riser of 3.2 m height used by Yang et al. [28], the gas–solid flow in the upper dilute
region could not reach the saturated carrying capacity at certain Ug because the freeboard length was shorter than TDH [15]. No matter what kind of control valve is used, the S-shaped profile cannot be formed, unless Ug is decreased to reach the saturated carrying capacity. For a tall riser that can keep the freeboard length larger than TDH at certain Ug, when a return valve of weak constraint is equipped, S-shaped εb profile is formed and the variation of Iv would have little influence on εb on top dilute region or Gs. Moreover, with the increasing of the riser height, the flow resistance of return valve becomes relatively smaller compared with the pressure drop in the riser, and the constraint becomes weaker. Li et al. [9] pointed out that the higher is the riser, the higher formation tendency the S-shaped profile will be, which was then simulated by Wang et al. [21] with multi-scale EMMS approach. In CFB boilers, a loop seal with low gas solid flow resistance is commonly used. The furnace height is normally higher than 20 m. When Ug = 4–5m/s and Iv b I⁎v, fast fluidization regime can be realized, i.e., gas–solid flow in the upper dilute furnace reaches the saturation state [19]. 4. Conclusions To properly design and operate the large capacity CFB boilers used in power generation, it is of significance to conduct experimental study on the fluid dynamics in the riser with ultra-high heights (N30 m). In this study, a cold CFB test rig with a riser 38 m and 54 m in height respectively was built to investigate the influence of operational conditions, including Iv and Ug, on the axial voidage profile along the riser and Gs in these risers. Experimental results showed that when Ug is greater than the transport velocity and Iv is above a critical value, the S-shaped voidage profile, as a feature of fast fluidization appears in the riser. The voidage in top dilute section and Gs does not change with Hr nor Iv. The critical Iv for saturated carrying capacity increases from 40 kg to 50 kg when Hr increases from 38 m to 54 m. The pressure drops in the riser are about 7.5 kPa and 9.0 kPa, respectively. For the 600 MWe supercritical CFB boiler with 54 m tall furnace, only a modest increment in Iv and power of forced draft fans is needed to obtain high enough Gs to meet the requirements of heating surfaces arrangement in furnace and the circulation loop. The distribution of gas–solid concentration in the riser is affected not only by Ug and Gs, but also by Iv. When the riser is tall enough and the return valve is weakly restricted, the tendency to form fast fluidization with S-shaped voidage profile is high in the riser. Acknowledgements Financial supports of this work by the National Science Fund Committee (No.50406002) and High Technology R&D (863) (2009AA05Z302) are gratefully acknowledged. References [1] G.X. Yue, Development and future of the circulating fluidized bed combustion technology, in: G.X. Yue (Ed.), 1st Fluidization Combustion and Technology in China, 2006, pp. 1–30. [2] Y.X. Wu, J.F. Lu, J.S. Zhang, et al., Conceptual design of an 800 MW supercritical pressure circulating fluidized bed, in: K.F. Cen (Ed.), Circulating Fluidized Bed Technology VIII, International Academic Publisher, Hangzhou, 2005, pp. 529–544. [3] G.X. Yue, J.F. Lu, H. Zhang, et al., Design theory of circulating fluidized bed boilers,18th International Fluidized Bed Combustion Conference, Toronto Canada, May 18–21 2005. [4] X.B. Xiao, H.R. Yang, H. Zhang, et al., Research on carbon content in fly ash from circulating fluidized bed boilers, Energy & Fuels 19 (2005) 1520–1525. [5] J. Adanez, L.F. de Diego, P. Gayan, Transport velocities of coal and sand particles, Powder Technology 77 (1993) 61–68. [6] J.F. Perales, T. Coll, M.F. Liop, et al., On the transition from bubbling to fast fluidization regimes, in: P. Basu, M. Horio, M. Hasatami (Eds.), Circulating Fluidized Bed Technology III, Pergamon, Oxford, 1991, pp. 73–78.
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