ARTICLE pubs.acs.org/IECR
Commercial Operation Test and Performance Analysis of a 200 MWe Super-High-Pressure Circulating Fluidized Bed Boiler Y. K. Sun,† Q. G. Lu,† S. L. Bao,† Y. J. Na,† Z. Y. Gao,‡ J. S. Gu,‡ G. Z. Shao,‡ Y. G. Shen,‡ and H. G. Wang*,† † ‡
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190 Shanghai Boiler Works Limited Company, ShangHai, China, 20025 ABSTRACT: A 200 MWe circulating fluidized bed (CFB) boiler with three cyclone separators was built in Inner Mongolia in China, and tests were carried out with the coal combustion process. The objective of this research is to investigate the effect of primary (secondary) air flow rate on the combustion process and the effect of multicyclone separator arrangement on solids circulation characteristics. Temperature and pressure drops at different positions in the boiler chamber and cyclone separator diplegs were measured. On the basis of the measured temperature and pressure, heat flux and solids void fraction distribution in the chamber are derived and analyzed. The profiles of temperature, pressure, heat flux, and solids void fraction along the height of the CFB boiler chamber and in the dipleg of cyclone separators at different operation conditions are given and discussed. The secondary air penetration depth in the dense area was tested at different operation conditions, and the effect of secondary air on the combustion process has been investigated. The preliminary test results indicate that the solid distribution in the three parallel cyclone separators is nonuniform and future research is necessary to optimize the arrangement of multicyclone separators for large scale CFB boilers. Key issues on superhigh pressure CFB boiler design, scaling up of CFB boilers, and further work on computational fluid dynamics (CFD) simulation for hydrodynamic behavior in the CFB boiler and cyclone separators are given at the end of this paper.
1. INTRODUCTION AND BACKGROUND Reducing air emissions and concern about air emissions and the effect on global warming is one of the key factors for development and implementation of new advanced energy production solutions today. One state-of-the-art solution is circulating fluidized bed combustion technology combined with a high efficiency gas-solids separation circulation.1-3 Development work is under way to offer CFB technology up to 600-800 MWe capacities with ultrasuper high steam parameters.1,4-7 Simultaneously, the technology to provide capability for air and oxy-combustion flexible operation to capture carbon is being developed. Four 200 MWe superhigh pressure circulating fluidized bed (CFB) boilers have been built in Shenhua Yili Power Plant in Inner Mongolia in China, and performance tests have been carried out with the combustion process.8 To meet the demand for high operation parameters and large thermal capacity, high efficiency gas-solid separation is a key to achieving high combustion efficiency, reducing limestone consumption, and high sulfur capture efficiency.1,9 As is wellknown, the cyclone efficiency decreases when the cyclone size is increased, due to a reduction in centrifugal force.10 With scalingup of the CFB boiler, the size of the cyclone is increased accordingly up to a point where the size negatively impacts gas velocities and solids circulation. To cope with this problem more smaller cyclones are placed when increasing boiler size.3,11 However, there is nonuniform solids distribution among each cyclone.8,11,12 The maximum difference of solids concentration can be reached to 17% between different cyclone separators.8 To overcome the nonuniform distribution, a great deal of research has been reported, and some methods have been patented.3,11,13 ALSTOM13,14 provided a type of multicyclone separators system which can achieve uniform solids distribution in different cyclone r 2011 American Chemical Society
separators based on the test in a cold CFB rig with the scale of 0.45 m2 in square cross-sectional area with a height of 10.5 m. Foster-Wheeler put additional membrane tube wall in the top of the boiler chamber to provide uniform solids distribution among separators.13 Tsinghua University and Dongfang Boiler (Group) Co. Ltd. designed a type of nonuniform separators arrangement, and a cold flow test and CFD simulation have been carried out.13 A cold CFB test rig with symmetrical arrangements of six cyclone separators in two sides was built in Beijing by the Institute of Engineering Thermophysics, Chinese Academy of Sciences. Experiments as well as CFD simulation has been carried out to investigate the above problems.8 Four commercial CFB boilers with 200 MWe capacities have been set up in Shenhua Yili Power Plant in Inner Mongolia in China, and commercial operation has been started. In the cold CFB test rig in Beijing, there are three parallel arranged cyclone separators at the top of the boilers. The objective of this research is to investigate the combustion performance of the 200 MWe superhigh pressure CFB boilers with commercial operation. The test includes the effect of the boiler load on the temperature profile, different flow rates and loads on the thermal flux density, and pressure drop and solids void fraction along the boiler chamber height. The paper includes four parts, namely, background, test conditions, results analysis, and conclusions and future works. The ultimate objectives of the above test are to find the thermal characteristics and fluid hydrodynamics behavior for large scale CFB boilers with multicyclone separators and provide valuable Received: June 12, 2010 Accepted: January 20, 2011 Revised: December 28, 2010 Published: February 15, 2011 3517
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Figure 1. Schematic diagram of a 200 MWe CFB boiler (1 primary air, 2 secondary air, 3 coal feeding points, 4 boiler chamber, and 5 cyclone separator).
Table 1. Dimensions of CFB Boiler and Cyclone Separator parameter CFB boiler
Cyclone separator
Table 2. Fuel Composition
units (m)
composition
units
design value
coal sample 1
coal sample 2
width
7.2
Car
%
42.82
39.48
34.38
depth height
22.7 38.2
Har
%
2.81
2.54
2.33
3.0
Oar Nar
% %
11.17 0.40
8.99 0.49
8.78 0.45
inlet port width
3.2
St.ar
%
0.37
0.38
0.42
inlet port height
5.3
Aar
%
21.30
28.62
35.64
diameter of cylindrical part of cyclone
7.2
Mt
%
21.12
19.50
18.00
cylindrical part length
9.0
Vdaf
%
41.18
40.36
conical part diameter in the bottom
1.36
Qnet.ar
kJ/kg
vortex tube diameter
conical part length
15650
14170
41.83 12350
14.2
information and foundation theory for scaling-up of supercritical pressure CFB boilers, for example, the design and commercialization of 600 MWe and 800 MWe supercritical pressure CFB boilers.
Table 3. Limestone Characteristics composition
unit
value
LOI
%
42.04
CaCO3
%
91.60
2. CFB FACILITIES AND MEASUREMENT SETUP
SO3
%
0.07
CaO
%
51.34
2.1. CFB Test Rig and Operation Conditions. The schematic diagram of a 200 MWe CFB boiler is shown in Figure 1a. The boiler chamber is 38.2 m tall, and in the lower part, up to 9 m is converted with refractory and air distributor plates. The boiler chamber has a cross-sectional area of 7.8 m 22.7 m in the middle part. The main dimensions of the CFB boiler and cyclone separators are listed in Table 1. The walls consist of a membrane of tubes. The fuels are fed by six ports in the front wall and limestone by pneumatic pipes. There are three parallel arranged cyclone separators in one side at the top of the boiler chamber as shown in Figure 1b. The primary air is fed through the windbox below the air distributor, and it is within the range of 2.0 105 m3/h to 3.2 105 m3/h. The secondary air is fed through the
MgO
%
2.15
Fe2O3
%
0.64
SiO2
%
1.87
Al2O3 K2O
% %
0.11 0.24
upmost port in the conical part of the chamber. The fuels were low grade lignite, and the compositions are summarized in Table 2. Limestone was used to in-suit remove SO2 during combustion, and the characteristics are listed in Table 3. The median particle size (D50) of fuel and limestone is 1.5 mm and 0.18 mm, respectively. The main design parameters for the 3518
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200 MWe CFB boilers are as follows. The design evaporating water rate is 690 t/h, the steam temperature is 540 °C, the steam pressure is 13.73 MPa, and the inlet water temperature is 244 °C. 2.2. Measurement Parameters. In this research, two basic parameters, namely, temperature and pressure, are measured. The other two parameters, heat flux and solids void fraction, are derived from the measured temperature and pressure separately and defined as follows. 2πðt2 - ts Þ ð1Þ q¼ lnðr2 =r1 Þ 1 þ μ s1 λ r1 R where λ is the thermal conductivity of membrane tube, R is the convective coefficient inside the tube, ts is the saturated steam temperature inside the tube, t2 is the outside wall temperature of the membrane tube, μ is the thermal coefficient, and it is a function of Bi number and dimensions of the tube, that is, the inner and outer diameters of the tube, and tube length. It can be found in the look-up table. s1 is the gap between two tubes. The Bi number is defined as follows.15 Rl ð2Þ Bi ¼ λ
Figure 2. Primary and secondary air flow rate change with boiler loads.
In practice, the convective heat transfer coefficient R inside the tube is much higher than that outside the tube, and the temperature of the tube is nearly closed to the steam temperature. With the above assumption, eq 1 can be simplified into 2πλðt2 - ts Þ q¼ ð3Þ μs1 lnðr2 =r1 Þ With the assumption that the inside temperature is the steam temperature in the condition of saturation, we can get the heat flux by the outside wall temperature. The local temperature is measured by PT thermocouple probes welded to the membrane tube wall. The probes are located in seven levels in the boiler. There are a total of 135 probes used in this test, and all the signals are connected to PC, with the the corresponding data automatically saved to the PC. The solids void fraction in the boiler chamber can be derived from the measured pressure and defined as follows.15 ð1 - εÞ ¼ Δp=ðFp gΔlÞ ð4Þ
Figure 3. Fluidisation air flow rate for cyclone diplegs at changing boiler load.
where, hε is the average voidage in the cross-sectional area and Δl is the distance between two pressure sensor points.
3. TEST RESULTS 3.1. Primary and Secondary Air Change with Load. Figure 2 shows the change of primary and secondary air flow rates with the load of the CFB boiler. With the increase in both primary and secondary air flow rates, boiler load increases. The total flow rate of secondary air is always higher than that of the primary air during the whole process. The fluctuation of primary air flow is nearly kept constant when the boiler load reaches to steady operation condition. However, the secondary air flow has slight fluctuation, and the change of coal type may be the reason for such fluctuation. Figure 3 demonstrates the changes of total fluidization air flow rate with boiler load in the windbox below the cyclone separator dipleg. It decreases with the increase in boiler load and meets the design criteria. The maximum fluctuation of fluidization air flow rate is less than 400 m3/h, and it tends
Figure 4. Temperature curves with process time in the boiler chamber.
to keep constant when the boiler load reaches to the design point with the load of 200 MWe. 3.2. Temperature Profile in the Boiler Chamber. Figure 4 shows the average temperature in the bottom and middle levels of the boiler chamber when the load is in steady operation condition. Both of them keep nearly constant with time, and the temperature in the bottom is about 40 °C higher than that in the middle level of the chamber. Figure 5 displays temperature curves in three diplegs of the cyclone separator. The temperature on the right side of dipleg is higher than that on the left side with low boiler load. With high boiler load, the tendency is inverse, which 3519
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Figure 5. Temperatures change with boiler loads in the cyclone separators. Figure 7. Pressure drop in the windbox of cyclone separator diplegs.
Figure 6. Temperatures change with boiler loads on the back wall of the boiler chamber.
means the temperature on the left side is higher than that on the right side with 4-5 °C difference. However, the above phenomenon is not always kept the same and will change in some tests. To reveal the statistical characteristics of temperature distribution in the dipleg of the cyclone separator with boiler load, more measurement data should be provided. Figure 6 presents the temperature changes in the rear wall of the boiler chamber and below the return ports of the cyclone separator. The five measurement points are depicted in Figure 1b. The temperatures at the right and left points are lower than that at the other three points at 30 °C. Point 2 at the right side is 10 °C higher than that at point 2 in the left side of the boiler chamber. 3.3. Pressure Drop in the Dipleg of the Cyclone Separator. Figure 7 shows the pressure change in the windbox of the cyclone separator dipleg. From this figure, it can be seen that the pressure increases with the increase in boiler load and keeps constant when the boiler loads reach a steady value. There are differences among the three windboxes. The right one is 1 kPa higher than the left one and 3 kPa higher than the middle one. The pressure measurement results indicate that the ash recycling rate in the right one is the maximum and the middle is the smallest one. The ash recycling rate is nonuniform distribution among the three cyclone separators. To achieve high separation efficiency, optimum arrangement of multicyclone separators is necessary to design and commercialize super- or ultra-high pressure CFB boilers. 3.4. Profiles of Pressure and Solids Void Fraction in the Boiler Chamber. Figure 8 shows the pressure profiles in the boiler chamber for different boiler loads. Figure 9 provides the
Figure 8. Pressure profile in the chamber of CFB boiler.
Figure 9. Solids void fraction profile in the CFB boiler.
solids void fraction profiles calculated from the pressure profile shown in Figure 8 by eq 4. From the pressure profiles, it can be seen that the pressure reduces along the height of boiler exponential. At rated load conditions, the pressure change has 3520
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Figure 10. Heat flux in the 200 MWe CFB boiler.
Table 4. Test conditions case
time
valve in front wall ports
valve in rear wall ports
1
8:30-11:00
100% open
100% open
2
11:00-12:00
100% open
60% open
3 4
12:00-13:00 12:00-13:00
75% open 50% open
100% open 100% open
a big gradient below the level of 25 m in the boiler and the corresponding solids void fraction is higher in this region. Above that region, the change of pressure is smooth and the solids void fraction is only in the range of 1-2 kg/m3. A similar phenomenon also appears for the profiles of pressure and solids void fraction at other load conditions. At specific CFB boiler chamber and load conditions, the fuel ash concentration is closely related to the solids void fraction distribution and the above test results can be used for CFB boiler design with the same fuel characteristics. 3.5. Heat Flux Profile. The test for heat flux in the boiler is operated in the conditions of boiler loads 53 MWe, 100 MWe, 150 MWe, and 200 MWe, respectively. For each condition, the test runs for two hours and all the measured data are automatically recorded and saved to the PC by an IMP data acquisition system. The outside wall temperature of the membrane tube is the time averaged value in the steady operation condition period, and the inside tube temperature is the saturated pressure temperature. To get more accurate results, the heat flux calculated by eq 2 will be recalculated considering the effect of the tube fins as well as the whole thermal balance of the boiler. The heat flux along the height of the boiler is shown in Figure 10. In the dense area, the heat flux is low and only increases 20% when the load increases from 27% to 100%. The maximum heat flux is in the middle area above the refractory layer, and it reaches to 130 kW/m2 at 100% load. The heat flux in the top area of the boiler
Figure 11. Temperature signals in different test conditions: (a) temperature in the inner wall of the boiler and (b) temperature in the inlet port of cyclone separators.
chamber reduces gradually along the height of the boiler. Compared with pulverized coal (PC) combustion the CFB boiler has smoother heat flux distribution along the height.3 3.6. Secondary Air Penetration Depth Test. The test conditions are divided into four periods, and each period is summarized in Table 4. The total flow rate for the secondary air with 100% valve open condition is 268 103 m3/h. For each case, the CFB boiler continuously runs for one hour and starts to record data after 15 min when each case switches on. Two samples are taken for each case from the filter bag connected with the outlet ports of the cyclone separators. Figure 11a shows the temperature change in the dense solid area, and Figure 11b presents the temperature in the outlet port of the cyclone separator. It can be seen that the secondary air flow has little effect on the temperature profile in the dense area and it does not affect the combustion process in this area at all. The temperature at the outlet port of cyclone separators nearly stays the same for the different test cases given in Figure 11b. Table 5 provides the residual carbon content in fly ash analysis results for different test conditions, and there is no obvious change for the fly ash. The samples were 3521
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Table 5. Residual Carbon Content Analysis case 1 (reference case)
case 2
case 3
case 4
0.40
0.38
0.67
0.76
ash content (%)
extracted by a filter bag from a small gas-solid stream extracted from the outlet ports of cyclone separators.
4. CONCLUSIONS AND DISCUSSION Four 200 MWe circulating fluidized bed (CFB) boilers with three cyclone separators were built in Inner Mongolia in China, and tests were carried out with the coal combustion process. Key process parameters have been successfully measured, and the main conclusions are summarized as follows. 1. The measurement results indicate that the solids concentration in three cyclone separators is nonuniform distribution. The solids flow rate in the right side is higher than that in the other two cyclones, and the left one has the smallest flow rate. Further work should be addressed to optimize the design for the multicyclone separators arrangements. To achieve the optimum objective, CFD simulation and solid concentration measurement can provide more detailed information for the above purpose and solve real engineering problems. 2. For lignite coal, the secondary air flow has little effect on the temperature profile in the dense area at the bottom of the CFB boilers. However, future tests should be conducted for different types of coal. In addition, the secondary air flow penetration depth should be investigated for different types of coal and different sizes of CFB boiler chambers. 3. With the increase in both primary and secondary air flows, boiler load increases. The pressure also increases with the increase in boiler loads and keeps constant when the boiler load reaches a steady value. In the dense area, the heat flux is low and only increases 20% when the boiler load increases from 27% to 100%. The maximum heat flux is in the middle area above the refractory layer, and it reaches 130 kW/m2 at 100% load. 4. The heat flux calculated from measured temperature has smoother distribution along the height of the CFB boiler than that of PC boiler. The maximum heat flux is in the middle area above the refractory layer. The heat flux in the top area of the boiler reduces gradually along the height of boiler. As a result of large scale of dimension and complex gas-solids flow dynamic behavior, the design of the supercritical pressure fluidized bed is more difficult compared with the small scale fluidized bed. To meet the demand for high efficient performance and green-house gas emission reduction, future work is still needed to research the fluid behavior in the CFB system, for example, the flow in the windbox system and cyclone separators. Computational fluid dynamic (CFD) simulation is a useful tool in the above research for scaling-up the supercritical pressure CFB boiler, and this remains as further work.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 0086-10-82543140. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors would like to thank the Ministry of Science and Technology of the People’s Republic of China (the 11th Fiveyear Plan for R&D Infrastructure and Facility Development Program) and the National Natural Foundation of China (No. 61072001) for financially supporting this research. Shanghai Boiler Works Limited Company and Shenhua Yili Power Plant Ltd. are thanked for their collaboration and for providing the 200 MWe CFB commercial test. ’ NOMENCLATURE A cross-sectional area of bed (m2) Bi Biot number (-) g gravity (m/s2) h height (m) r tube diameter (m) T temperature (°C) P pressure (kPa) q heat flux (kw/m2) Greek Symbols
r λ μ ε
convective heat transfer coefficient (W/(m2 3 K)) thermal conductive coefficient (W/(m 3 K)) viscosity coefficient (kg/s2) volume fraction (%)
’ REFERENCES (1) Reh, L. Development potentials and research needs in circulating fluidised bed combustion. China Particuol. 2003, 1, 185–200. (2) Koornneef, J.; Junginger, M.; Faaij, A. Development of fluidised bed combustion— An overview of trends, performance and cost. Prog. Energy Combust. Sci. 2007, 33, 19–55. (3) Hack, H.; Hotta, A.; Kettunen, A.; Eriksson, T. Ultra-superhigh CFB technology to meet the challenge of climate change; Foster Wheeler Energia Oy: Finland, 2008. (4) Luo, Z. Y.; Cen, K. F. Research and development on circulating fluidised combustion technology in China. Proceedings of the 8th International Conference on Circulating Fluidised Beds; World Publishing Corporation: Beijing, China, 2005; pp 56-67. (5) Lv, J. F.; Yu, L.; Zhang, Y. J.; Yue, G. X.; Li, Z. Y.; Wu, Y. X. A 600 MW superhigh circulating fluidised bed boiler. J. Power Eng. 2007, 27, 497–501. (6) Fan, Z.; Goidich, S.; Robertson, A. 800 MWe circulating fluidised bed boiler with 1300°F superhigh steam. Presented at the 33rd International Technical Conference on Coal Utilization & Fuel Systems, June 1-5, 2008, Clearwater, FL, USA. (7) Chen, Y. M.; Zhou, X. L.; Zheng, C. H.; Wang, L. H.; Fang, M. X.; Shi, Z. L.; Luo, Z. Y.; Cen, K. F. Development of large-scale circulating fluidised bed boiler. J. Power Eng. 2008, 28, 817–826. (8) Liu, Z. C.; Sun, Y. K.; Na, Y. J.; Bao, S. L.; Shao, J. Z.; Guo, J. S.; Xiao, F.; Wang, D. F. Numerical simulation of cyclones arrangements of 600 MW superhigh CFB boiler. J. Eng. Thermophys. 2009, 30, 1949–1952. (9) Morin, J. X. Recent Alstom power large CFB and scale up aspects including steps to superhigh, Presented at the 47th International Energy Agency Workshop on Large Scale CFB, Zlotnicki, Poland, 2003. (10) Armistead, T. F.; Schwieger, R.; Reina, P. Boiler technology pursues goals of efficiency and lower emissions-circulating fluidised-bed equipment pushes size limits. Power & industrial—Powerplants; McGraw Hill Construction, ENR Magazine: New York, 2002. (11) Yue, G. X.; Yang, H. R.; Nie, L.; Wang, Y.; Zhang, H. Hydrodynamics of 300 MWe and 600 MWe cfb boilers with asymmetric cyclone layout. Presented at Circulating Fluidised Bed Technology IX, May 13-16, 2008, Germany. (12) Li, Z. G.; Liu, Z. C.; He, J.; Na, Y. J.; Lu, Q. G. Influence of Inlet Duct Layouts on Cyclone Performance. Proc. CSEE 2009, 29, 1–7. 3522
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(13) Morin, J. X. Recent Alstom power large CFB and scale up aspects including steps to supercritical, 47th International Energy Agency Workshop on Large Scales CFB, Zlotnicki, Poland, October 13, 2003. (14) Marchetti, M. M.; Czarnecki, T. S.; Semedard, J. C.; Devroe, S.; Kemaske, J. M. Alstom’s large CFBs and Results. Presented at FBC 2003, 17th International Fluidised Bed Combustion Conference, Jacksonville, FL, USA, May 18-21, 2003. (15) Kunii, D.; Levenspiel, O. Fluidization Engineering; ButterworthHeinemann: Boston, 1991; p 69.
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