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Effect of Bed Pressure Drop on Performance of a CFB Boiler Hairui Yang,† Hai Zhang,*,† Shi Yang,† Guangxi Yue,† Jun Su,‡ and Zhiping Fu§ Department of Thermal Engineering, Key Laboratory for Thermal Science and Power Engineering of Ministry Education, Tsinghua UniVersity, Beijing, 100084, China, Taiyuan Boiler Group Co. Ltd, Taiyuan, 030021, China, and Shanxi Lishi Datuhe Heat and Power Cogeneration, Lishi, 033000, China ReceiVed January 11, 2009. ReVised Manuscript ReceiVed April 4, 2009
The effect of bed pressure drop and bed inventory on the performances of a circulating fluidized bed (CFB) boiler was studied. By using the state specification design theory, the fluidization state of the gas-solids flow in the furnace of conventional CFB boilers was reconstructed to operate at a much lower bed pressure drop by reducing bed inventory and control bed quality. Through theoretical analysis, it was suggested that there would exist a theoretical optimal value of bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan energy consumption. The analysis was validated by field tests carried out in a 75 t/h CFB boiler. At full boiler load, when bed pressure drop was reduced from 7.3 to 3.2 kPa, the height of the dense zone in the lower furnace decreased, but the solid suspension density profile in the upper furnace and solid flow rate were barely influenced. Consequently, the average heat transfer coefficient in the furnace was kept nearly the same and the furnace temperature increment was less than 17 °C. It was also found that the carbon content in the fly ash decreased first with decreasing bed pressure drop and then increased with further increasing bed pressure drop. The turning point with minimal carbon content was referred to as the point with optimal bed pressure drop. For this boiler, at the optimum point the bed pressure was around 5.7 kPa with the overall excess air ratio of 1.06. When the boiler was operated around this optimal point, not only the combustion efficiency was improved, but also fan energy consumption and wear of heating surface were reduced.
1. Introduction During the past two decades, due to its merits in wide fuel flexibility, cost-effective emission control, and large range of load adjustment, CFB boiler technology has been developed rapidly. Nowadays, it becomes one of the main commercially applicable clean coal technologies around the world. In China, the total number of CFB boilers in operation is over 3000, playing an irreplaceable role in the market of small- and medium-scale industrial boilers. Moreover, in recent years, CFB boilers emerged in the market of large-scale boilers used in power generation. So far, a number of units of 300 MW electical class CFB boilers have been put in operation around the world and the 600 MW electrical supercritical CFB boiler, the largest unit in the world, is to be put into commissioning by the end of 2011 in China. However, the CFB boiler technology is still under development, facing a number of challenges. Among them, low combustion efficiency, high self-consumed service power, and severe heating surface erosion are three major ones. Compared with a conventional pulverized coal-fired boiler, due to the large amount of bed inventory M in the CFB furnace, the primary air fan and secondary air fan need higher pressure head, introducing 1-2% extra self-consumed service power. In addition, a CFB boiler usually has lower combustion efficiency mainly due to the lower combustion temperature and larger coal particle size. Application also showed that the wear of the * Correspondence author: E-mail:
[email protected]; phone: +86 010 62794129; fax: +86 010 62781743. † Tsinghua University. ‡ Taiyuan Boiler Group Co. Ltd. § Shanxi Lishi Datuhe Heat and Power Cogeneration.
heating surfaces, especially during low quality coal firing is much more severe, greatly reducing the boiler’s availability. How to resolve these problems, making the CFB technology more cost-effective and more reliable, is significant for its further development. To save the self-consumed power, it is straightforward to think about the method through reducing the pressure drop, namely, bed inventory M in the furnace. Since M is the integral of bed material along the furnace, before adjusting M, one should examine the flow characteristic in the furnace. In general, the flow regime inside CFB furnace is described as the superposition of a fast bed in the upper furnace and a bubbling bed or turbulent bed in the lower furnace.1,2 To maintain the fast bed regime in the upper furnace is important to the heat transfer. On the basis of previous studies,3-11 with (1) Yue, G.; Lu, J.; Zhang, H., et al. In Design Theory of Circulating Fluidized Bed Boilers. In 18th International Fluidized Bed Combustion Conference: Toronto, Canada, May 2005; Jia, L. Ed. (2) Bai, D.; Kato, K. J. Chem. Eng. Jpn. 1995, 28, 179–185. (3) Xu, G.; Gao, S. Powder Technol. 2003, 137, 63–76. (4) Rhodes, M. J.; Laussmann, P. Can. J. Chem. Eng. 1992, 70, 625– 630. (5) Chang, H.; Louge, M. Powder Technol. 1992, 70, 259–270. (6) Weinstein H.; Graff, R. A.; Meller, M.; et al. The Influence of the Imposed Pressure Drop across a Fast Fluidized Bed. In Fluidization; Kunii, D., Toei, R. Eds.; Engineering Foundation, New York, 1983; pp 299-306, Vol. IV. (7) Li, J.; Tung, Y.; Kwauk, M. Axial Voidage Profiles of Fast Fluidized Beds in Different Operating Regions. In Circulating Fluidized Bed Technology II; Large, J. F.; Basu, P. Eds.; Pergamon Press: Oxford, 1988; pp 193-203. (8) Mori, S.; Liu, D.; Kato, K. Powder Technol. 1992, 70, 223–227. (9) Li, Y. Chen, B.; Wang, F.; et al. Hydrodynamic Correlations for Fast Fluidization. Kwauk, M.; Kunni, D. Eds.; Science and Technology Press: Beijing, 1982.
10.1021/ef900025h CCC: $40.75 2009 American Chemical Society Published on Web 05/12/2009
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given geometric structure of each component in the circulating loop and bed material properties, the profiles axial voidage and bulk density in the fast fluidized bed is determined by fluidizing air velocity (Ug) and solid circulating rate (Gs), which can be adjusted by the bed inventory M. Fast fluidization regime at a certain Ug can be realized by keeping M more than a minimal critical values M*, at which Gs in the upper furnace reaches the saturated value G*s for pneumatically transport, while the bottom furnace becomes a dense bed.3,12 Once the upper riser is in the fast bed regime, the voidages in the dense zone and upper dilute zone are held stably, while the height of the dense zone ascends gradually and the voidage in the transition zone increases with the increasing M.3 In other words, it is only necessary to keep M e M* to maintain fast fluidization in the upper furnace; the further increase of M after M > M* in a CFB boiler mainly accumulates particles in the lower furnace covered with refractory. The increase of M is not beneficial to the heat transfer; instead, it introduces several disadvantages to boiler operation, such as severe wear on the membrane water-wall near the top of the tapered part and extra energy consumption of the primary air fan. Therefore, reducing M sounds like an effective way to reduce the service power consumption and wear severity. Since the feedstock in a CFB boiler normally has a relatively wide size distribution, spanning from 0 to 8.0 mm, the bed material also has a wide size distribution. Consequently, reducing M should take the size distribution in to account, namely coupling with bed quality control. With respect to different contribution to fluidization and heat transfer, bed material can be divided into two groups as effective material and ineffetive material. The effective bed material consists of the fine particles that can be entrained out the bottom turbulent bed and those particles in the fast bed in the upper furnace. The mass fraction of effective bed material is often defined as bed quality. The remaining particles, with relatively large size, are referred as the ineffetive material. Beacuse their terminal velocity is larger than Ug, these particles can not be entrained into the upper furnace, remaining in the turbulent bed in the lower furnace. Since the membrane water-wall in the lower furnace is covered with a refractory layer, the ineffective bed material has a minor effect on heat transfer. On the contrary, theses ineffective particles are in favor of membrane waterwall protection near the top of the tapered part. From the view of combustion efficiency, the ineffective particles possess enough residence time for burning out. However, these particles also increase the penetration resistance of the secondary air in the middle of the tapered part. Nevertheless, the carbon loss of ineffective particles only counts for a small portion in the total carbon loss, much less than the carbon content loss in the fly ash (LOI).1,2 It was found that the LOI in a CFB boiler is mainly affected by factors associated with the chemical reaction rate, including fuel reactivity, bed temperature,13 the residence time,14 and the gas mixing and diffusion resistance against oxygen to the surface of fuel. Besides, LOI is influenced by the bed pressure drop.15
Different from the effect on fan energy consumption and heating surface wear, the effect of bed pressure drop on the combustion efficiency is bifurcated.16 On one hand, the increase in bed pressure drop leads to larger average solid suspension density Fs in the furnace. As a result, more dispersed solid particles tend to agglomerate into clusters above the dense zone. The clusters would move downward if its determination velocity is higher than Ug and then be dispersed again with the interaction with surrounding solid-gas flow. Such phenomenon enhances local mixing intensity17,18 and prolongs the residence time of fine particles in the furnace.13,15 On the other hand, the high Fs in the furnace strongly weakens the gas-solid mixing. The momentum and rigidity of the upward gas-solid flow increase as Fs increases in the zone where the secondary air is injected and shortens its penetration depth.19 The oxygen in the center region can be very lean, close to zero, but near the wall the oxygen concentration may be high.1 This nonuniform oxygen distribution prevails at every cross section above the secondary air nozzles, inducing high LOI of fine carbon particles. The bifurcated effect of bed pressure drop on the combustion efficiency can be schematically summarized in Figure 1. Obviously, it is reasonable to expect that there is an optimal bed pressure drop. Around the optimal point not only the maximal combustion efficiency can be obtained, but also the fan energy consumption and the wear of the heating surface can be greatly reduced. The feasibility of the pressure drop adjustment in a CFB boiler is further supported by the state specification design theory.1 Based on the theory, the CFB boiler as an opening fluidization system with fast bed in the upper furnace can be operated at multiple states, and each state is “specified” by Ug and Gs. Moreover, a CFB boiler can operate at different states while keeping the upper furnace in the fast bed regime with a given Ug and dependent Gs by adjusting M and bed quality. When the designated state in a CFB boiler is changed from an old one to a new one, the flow characteristics in the bed are changed as well and the process is called state reconstruction. Obviously, the reconstruction should be validated by bed material balance based on the formation of ashes from the coal, the attrition properties of the inert materials, the cyclone efficiency,20 and the size of feed coal.21
(10) Kim, S.; Numkung, W.; Kim, S. Korean J. Eng. 1999, 16, 82–88. (11) Bai, D.; Jin, Y.; Yu, Z.; et al. Powder Technol. 1992, 71, 51–58. (12) Hu, N., Flow Regimes in the Ultra High CFB Riser; Bachelor Thesis, Tsinghua University: 2008. (13) Lu, J.; Jin, X.; Yang, H.; et al. J. Basic Sci. Eng. 2000, 8 (1), 97– 105. (14) Bursi, J. M. Lafanechere, L.; Jestin, L. Basic Design Studies for a 600 MWe CFB Boiler. In Proceedings of the 15th International Conference on Fluidized Bed Combustion: Savannah, GA, 1999; Reuther, R. B. Ed. (15) Yang, H.; Xiao, X.; Wang W.; et al. Powder Sys. Eng. (in Chinese) 2005, 21, 13-14.
(16) Xiao, X.; Yang, H.; Zhang, H.; et al. Energy Fuels 2005, 19, 1520– 1525. (17) Bai, D.; Jin, Y. J. Chem. Ind. Eng. (Chinese). 1991, 6, 697–703. (18) Fujima, Y.; Tagashira, K.; Takahashi, Y.; et al. Conceptual Study on Fast Fluidization Formation. In CFB Technology III; Basu, P.; Horio, M.; Hasatani, M. Eds.; Pergamon Press: 1990; pp 85-90. (19) Yang, J.; Yang, H.; Yue, G. J. Power Eng. 2008, 28, 509–513. (20) Yang, H.; Yue, G.; Xiao, X.; et al. Chem. Eng. Sci. 2005, 60, 5603– 5611. (21) Yang, H.; Wirsum, M.; Luh, J. Fuel Process. Technol. 2004, 85, 1403–1414.
Figure 1. Bifurcated effect of bed pressure drop on LOI.
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the primary and secondary air were maintained constant, and the fluidization velocity Ug was kept at 4.4-4.6 m/s. 3. Results and Discussion
Figure 2. Schematic of the 75 t/h CFB boiler with size and measure points; 1: distributor; 2: recycle point; 3: coal feed point; 4: secondary air inlet; 5: furnace roof; O: pressure point; +: temperature point.
On the basis of the above introduction and discussion, a novel CFB boiler combustion technology with optimal low bed pressure drop was proposed. It is expected that the new technology can resolve the three problems of low combustion efficiency, high self-consumed service power, and severe heating surface erosion in a conventional CFB boiler, as we mentioned earlier. 2. The Boiler Used for Field Test To validate the concept of reconstruction of the fluidization state, a series of field tests were carried out in several CFB boilers. In this paper, the results obtained from a 75 t/h CFB boiler are given and discussed. By utilizing the patented technology of Tsinghua University, a new type of 75 t/h CFB boiler with medium steam temperature and pressure was designed by Taiyuan Boiler Works, China. The main configuration of this CFB boiler was similar to that of other 75 t/h CFB boilers, as shown in Figure 2. However, some modifications in the arrangement of the heating surface were done based on recalculation of heat release distribution as a consequence of the reconstruction of the fluidization state. The boiler has being operated in Datuhe Power Station, Shanxi, China since 2006. The fuel was the middling coal from a local coal washery, and its properties are shown in Table 1. The particle size distribution of coal fed into the boiler was well controlled, and neither too coarse nor too fine particles were used. As shown in Figure 3, about 90% of the particles were finer than 4 mm in size, and about 60% of them were of 1-2 mm. The ash formation experiments showed that the percentage of ash particles in the size of 100 to 1000 µm was 60%, satisfying the requirement of the bed material balance.20 The field tests were done at full-load condition. The bed pressure drop in the furnace was adjusted at 3.2, 3.8, 5.6, and 7.3 kPa, respectively, by controlling the discharges of bottom ash and circulating ash. The circulating ash was discharged through a valve located at the loop seal. The boiler was stable at all these pressure drops. During the tests, the flow rates of
3.1. Axial Solids Suspension Density Profiles at Different Bed Pressure Drop. Neglecting the effects of acceleration of gravity and friction, the axial solids suspension density profile along the furnace can be calculated from the axial bed pressure drop profile. Figure 4 shows the axial profiles of solids concentration under different tested conditions. It can be seen that the profiles are influenced by the amount of M. In the cases with larger bed pressure drop, there were more solids in the middle furnace. Notably, the solids suspension density in the area near the top of the tapered part at 3.4 m from the bottom varied remarkably. The pressure drops of the loop seal in different cases are listed in Table 2. The flow rate of aeration air in the loop seal was nearly constant during the tests. Provided the resistance coefficient was constant and the particle flow rate Gs through the loop seal varied linearly with the imposed pressure, the Gs at different condition was estimated according the pressure drop of the loop seal.22 It can be seen that with the increment of the bed pressure drop, the height of dense zone increases, whereas Gs and upper Fs are nearly unchanged, shown in Figure 4 and Table 1. The results indicate that Gs ≈ G*, s and the flow of the fine solids is in fast fluidization regime.3,7 3.2. Temperature Profiles and Heat Transfer Coefficients in the Furnace. Although Fs in the upper dilute furnace, 16 m above the bottom, is nearly constant, the height of the dense bed zone in the lower furnace increases with the increment of the bed pressure drop, leading to an increase of the overall heat transfer coefficient between the gas-solid flow and the water membrane wall in the furnace. Consequently, the average bed temperature decreases. The overall heat transfer coefficient and the radiate heat transfer coefficient can be calculated from the heat absorped by the working medium and the bed temperature23 as shown in Table 3. The convective heat transfer coefficient increases slightly, proving that Fs in the upper furnace varies slightly. Figure 5 shows the temperature profiles along the furnace for different conditions. Consistent with Table 2, with an increase in the bed pressure drop, the average bed temperature falls from 911 to 895 °C. From the view of heat transfer, the fall in temperature is a consequence of the increase of suspension density. 3.3. Carbon Content in Fly Ash. Because the coarse particles in the feed coal were screened out, the size of the bottom ash was relatively fine. 90% of the particles in the bottom ash were finer than 2 mm. The fly ash was also finer: 50% of them was in the range of 30-50 µm, and more than 95% was finer than 100 µm. The results indicated that the cyclones had very high separation efficiency. Figure 6 shows the LOI, circulating ash and bottom ash at different bed pressure drops. The LOI decreases first with decreasing bed pressure drop and then reaches a minimum about 17% at a bed pressure drop of 5.7 kPa. The results validated the theoretical analyses. In other words, the 2-fold effect of bed pressure drop on the combustion efficiency mentioned above is the reason why the LOI has a minimal value. For this 75 t/h (22) Leung, L. S.; Chong, Y. O.; Lottes, J. Powder Technol. 1987, 49, 217–276. (23) Lu, J.; Zhang, J.; Yue, G. Heat Transfer s Asia Res. 2002, 31, 540–550.
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Table 1. Proximate and Ultimate Analyses of the Coal ultimate analyses (as received)
proximate analyses (as received)
C (%)
H (%)
O (%)
N (%)
S (%)
ash (%)
moisture (%)
VM (%)
LHV (kJ/kg)
50.37
2.94
5.92
0.67
0.74
40.03
1.09
14.29
19 039.9
Table 2. Resistance of Return Valve and Relative Circulating Rate case
1
2
3
4
bed pressure drop (Pa) resistance of return valve (Pa) relative solid flow rate
3220 9590 1
3830 10225 1.03
5680 10579 1.05
7330 10788 1.06
CFB boiler, 5.7 kPa is the optimal bed pressure drop for the given feedstock with the excess air ratio 1.06. 3.4. Power Consumptions of Fans. When the boiler operated with lower bed pressure drop, the resistance of the bed material became less and the pressure head of primary and secondry air fans became smaller. As a result, the power consumption of the fans decreased. The changes of the primary and secondry air fans, roots draft fan, and induced draft fan are listed in Table 4. It should be pointed out that the LOI during the tests was relatively high. The main reason was that the excess air ratio
Figure 3. Size distribution of coal and ash.
Figure 4. Suspension density profiles along the furnace.
Table 3. Heat Transfer Coefficients in the Furnace case
1
2
3
4
bed pressure drop, Pa furnace heat absorption, KJ/s average temperature in upper furnace, °C temperature at outlet of left cyclone, °C temperature in left loop seal, °C total heat transfer coefficient, W/m2.K radiation heat transfer coefficient, W/m2.K convective heat transfer coefficient, W/m2.K
3220 32 882 905
3830 33 483 911
5680 33 836 895
7330 33 902 894
932
939
917
915
919
924
907
905
146.8
152.7
153.8
154.6
98.6
101.6
101.1
101.6
48.2
51.1
52.7
53.0
was only 1.06 during the tests, too low for complete combustion. In the tests, the CO concentration at the furnace exit was up to 2000 ppm.
Figure 5. The temperature distribution in the furnace.
Figure 6. Carbon content in the ashes.
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Table 4. Currents of Fans at Full Load under Different Bed Pressure Drops wind chamber pressure (Pa)
primary air fan (A)
secondary air fan (A)
induced draft fan (A)
roots draft fan (A)
3220 3830 5680 7330
18.32 19.09 19.60 20.36
164.87 170.43 172.93 178.43
19.41 19.53 19.72 19.99
10.99 11.11 11.41 11.76
Based on the field test results, the boiler was adjusted to operate at an optimal bed pressure drop range of 5.0-5.8 kPa and an excess air ratio of 1.2. As a result, the LOI was reduced to 14%. Since the reconstruction of the fluidization state, the 75 t/h CFB boiler has being operated with a low optimal bed inventory. In 2006, the availability of the unit was as high as 95%, and the combustion efficiency and thermal efficiency of the boiler were remarkably improved. In addition, the service power of the unit was reduced by about 2.5%. About 10 000 t of middling coal and 1 GWh of electricity were saved every year per unit. Furthermore, the erosion of heating surfaces in the boiler and its auxiliaries was alleviated, and the cost of maintenance was reduced by as much as 10% per year. 4. Concluding Remarks Bed pressure drop, that is, bed inventory, is an important factor for flow dynamics in the furnace and performance of the CFB boiler. On the basis of the state specification design theory, the gas-solid flow in the furnace of a CFB boiler is regarded as the superposition of a turbulent bed formed by large particles in the lower furnace and a fast bed formed by fine particles in the upper furnace. The fluidization state of the gas-solids flow in the furnace can be reconstructed by adjusting the bed pressure drop. From the heat transfer point of view, the bed material in a CFB boiler can be classified into an effective material that circulates and an ineffective material that predominantly remains in the bottom. Given that the upper furnace remains in fast fluidization, reducing the bed pressure drop only slightly affects the heat transfer in the furnace.
However, the bed pressure drop remarkably affects the combustion efficiency of the coal particles, that is, carbon content in fly ash (LOI) in a CFB boiler. The effect is bifurcated. Most of the existing CFB boilers are operated with redundant bed material inventory. There is an optimal low value of bed pressure drop, for which, not only the maximal combustion efficiency is obtained, but also fan energy consumption and wear of the heating surface can be greatly reduced. The theoretical analysis of the effect of the bed pressure was validated by field tests carried out on a 75 t/h CFB boiler at full load. It was found that the boiler could be steadily operated with a bed pressure drop as low as 3.1 kPa by reconstructing the fluidization state of the solids-gas two-phase flow in the furnace. Within the optimal bed pressure drop of 5.0-5.8 kPa, the LOI was reduced from 23 to 17%, and the service power of the unit was reduced by 2.5%. At the same time, the erosion of the heating surfaces in the boiler and its auxiliaries was alleviated, and the availability of the unit reached 95%. For this boiler, the optimum point was with a bed pressure of 5.7 kPa and an excess air ratio of 1.06. The optimal bed pressure drop could be affected by the boiler geometry (mainly furnace height) and coal type. More researches such as the effect of the bed pressure drop on desulfurization, nitrous oxide formation, and more detailed flow dynamics, heat transfer, and combustion in the furnace are suggested. Acknowledgment. National Science Fund Committee (No. 50406002) provided financial support to this research.
Nomenclature Gs ) solid circulating rate, kg/s G*s ) Critical solid circulating rate for saturated entrainment, kg/s M ) bed inventory, kg/m2 M* ) Critical bed inventory to maintain fast fluidization condition, kg/m2 Ug ) fluidizing air velocity, m/s Fs ) solid suspension density, kg/m3 EF900025H
