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h CFB boiler with water-cooled square cyclones
Fuel Processing Technology 88 (2007) 129 – 135 www.elsevier.com/locate/fuproc
Performance evaluation of a 220t/h CFB boiler with water-cooled square cyclones Lu Junfu ⁎, Zhang Jiansheng, Zhang Hai, Liu Qing, Yue Guangxi Thermal Engineering Department, Tsinghua University, Beijing 100084, China Received 9 September 2004; received in revised form 20 December 2004; accepted 22 December 2004
Abstract The first Chinese 220 t/h CFB boiler was successfully demonstrated. The boiler was compactly designed with Tsinghua-patented, water-cooled square cyclones with curved inlet based on previous experience of similar CFB boilers of smaller capacities. The demonstration showed that the boiler possesses excellent performance in start-up, fuel flexibility, flexibility to frequent turn-down ratio variation, facility availability and reliability. The performance of the water-cooled square cyclone was compared with that of other cyclones through fly ash analysis. The results showed that the overall performance of the square cyclone in such capacity is compatible to that of the round cyclone, meeting the requirements for material balance and efficient combustion in CFB boilers. The demonstration was a milestone for CFB boiler scaling-up in China. © 2006 Elsevier B.V. All rights reserved. Keywords: Water-cooled square cyclone; CFB boiler; Performance; Scaling-up; 220 t/h
1. Introduction Circulating fluidized bed (CFB) has been considered as one of the prevailing clean coal combustion technologies in China since the mid 1980s. Cyclone, the gas–solid separator in a CFB boiler, is a key factor for capacity scaling-up, which is one of the major challenges in commercialization. In the early 1990s, Ahlstrom Corporation, Finland introduced the first square cyclone, named as Pyroflow COMPACT to CFB boilers [1]. Compared with the traditional round cyclone with the same capacity, the square cyclone was of smaller size, shorter start-up time, simpler and less-costly construction and engineering, though it scarified some degree of separation efficiency. Later, supported by the State Economy and Trade Committee of China (SETCC), the Tsinghua University in Beijing, China developed and patented an advanced water-cooled square cyclone featuring with a curved inlet to accelerate particles to increase the separation efficiency (shown in Fig. 1) [2] and successfully applied it in boilers of capacities of 75 t/h and 150 t/h [3–6]. The application showed that the Tsinghua-patented cyclone gener-
⁎ Corresponding author. Tel./fax: +86 10 62781743. E-mail address: [email protected] (L. Junfu). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2004.12.008
Fig. 1. Schematic of an advance square cyclone with a curved inlet patented by Tsinghua University.
130
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135 Table 1 Approximate analysis of the design coal Car
Har
Oar
Nar
Sar
Aar
Mad
Vdaf
Qar,net,p
53.85
2.64
3.86
0.75
1.18
30.92
6.80
30.50
20.58
Note: composition in wt.%, and Qar,net,p in MJ/kg.
To demonstrate the performance and the scalability of the advance water-cooled square cyclone, also supported by SETCC, a CFB boiler of 220 t/h steam capacity was established in Weihai Heat and Power Cogeneration Plant, Shandong, China, and successfully put into commercial operation in the end of 2001. Since then, a series of industrial experiments in the studies of such as start-up performance, fuel flexibility, load performance, and size and carbon content distributions of fly ash and bottom ash, have been conducted. The results are important to assess the performance of the patented cyclone and the entire boiler. 2. Design of the 220 t/h CFB boiler with advance watercooled square cyclones As shown in Fig. 2, the demonstrated boiler is of 43.6 m in height, 21.4 m in width and 20.7 m in depth. It is of natural circulation type with single drum, which is mounted at the elevation of 39.6 m. The furnace, made by membranes of 0.06 m × 0.08 m pitch, is of net height about 30 m and its cross section about 55 m2 at freeboard. The distributor plate and the windbox are also made by water wall membrane, acting as an extension of the front wall of the furnace. The distributor is about 25 m2 in area. Two square cyclones made of planar membranes are located between the furnace and the second pass. The front wall of the square cyclone functions as the rear wall of the furnace, and the rear wall of the square cyclone functions as the front wall of the second pass. The expansion joint, which might become a weakest part of a CFB boiler, is not used for connection. Instead, the entrance of the Table 2 Design parameters of the 200 t/h CFB boiler
Fig. 2. Layout of a 220 t/h circulating fluidized bed boiler with water-cooled square cyclone.
ated much less pressure drop, while remaining compatible high efficiency, availability and performance compared with the traditional round hot cyclone [7–9]. Moreover, the patented cyclone is expected to be superior for scale-up because of its full water membrane structure from furnace to the second pass and the separators [10].
Item
Value
Main steam flow rate, t/h Main steam pressure, MPa Main steam temperature, °C Feed water temperature, °C Exhaust gas temperature, °C Ambient temperature, °C Primary air temperature, °C Secondary air temperature, °C Spraying water flow, t/h Ratio of fly ash to bottom ash Bed temperature, °C Carbon content in fly ash, % Carbon content in bottom ash, % Boiler heat efficiency, % CO emission, ppm NOX emission, ppm SO2 emission, ppm⁎
220 5.29 485 150 134 20 134 220 3.0 7:3 894 b10 b2 ∼ 90 b250 b200 b250
Note: ⁎Ca/S molar ratio is designed as 2.33.
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
131
Table 4 Operation parameters of the 220 t/h CFB boiler
Fig. 3. Spatial variation of bed temperature during start-up process.
cyclone is connected directly with the furnace, and the exit of the cyclone is connected directly with the second pass. The equivalent diameter of the square cyclone is chosen as about 5.40 m. Since water-cooling is used, the studs which are connected with the cyclone membranes are of rather low temperature. Thus they can be made of relatively low grade carbon steel. The compacted and water-membrane enclosed design of the cyclone provides a big advantage in boiler manufacture, engineering and operation. The design fuel is the domestic lean coal. Its approximate analysis is shown in Table 1. Some design parameters of the boiler are shown in Table 2, including not only the boiler thermal efficiency, but also the limits of carbon content in fly ash (LOI) and bottom ash. The carbon content in fly ash, measured from many industrial CFB boilers in China, had been found to be much higher than expectation especially when low quality coals were fired. To decrease the LOI, relatively high bed temperature is adopted. In the demonstration boiler, the bed temperature is designed as 894 °C. 3. Results and discussion 3.1. Start-up performance Reliable and quick start-up is important for CFB operation. Based on the design and operational experiences from previous 75 t/h and 130 t/h boilers, we concluded that it is thermal stress but erosion that is mainly attributed to the refractory break. Therefore, the thickness of refractory layer can be optimized. For the lower part of furnace, the thickness of refractory layer is designed as 0.10 m, which is thinner than the thickness used in some other designs. The thin refractory of the furnace and cyclone provides an advantage of faster heat up rate than that for boilers using hot cyclone where thick refractory is used.
Item
Case I
Case II
Case III
Case IV
Main steam flow rate, t/h Main steam pressure, MPa Main steam temperature, °C Feed water temperature, °C Exhaust gas temperature, °C Ambient temperature, °C Primary air temperature, °C Secondary air temperature, °C Spraying water flow, t/h Ratio of fly ash to bottom ash Bed temperature, °C Carbon content in fly ash, % Carbon content in bottom ash, % Boiler heat efficiency, %⁎ CO emission, ppm NOX emission, ppm SO2 emission, ppm⁎⁎
218.5
219.9
240.3
222.6
5.19
5.16
5.07
5.15
475
473
473
475
145.4
138.5
136.5
139.4
131
129
138
130
16
14
15
17
130
133
136
123
223
224
228
233
1.1
0.5
2.0
2.8
7:3
6:4
7:3
6:4
898 9.08
886 9.70
902 7.44
891 6.44
1.27
0.47
0.97
0.74
90.22
90.28
90.59
91.12
123 68 628
76 121 664
38 55 582
88 69 475
Note: ⁎based on the low heat caloric value; ⁎⁎no limestone was injected into the combustor for desulphurization.
An oil-fired duct burner was used for the under-bed ignition during the boiler start-up, as the practice used in many CFB boilers in China. The start-up process is shown in Fig. 3. First, in about 1.5 h, the furnace was gradually heated up by hot flue gas exiting from the oil-fired duct burner and the consumption rate of the assist oil gradually increased with bed temperature. Then coal feeding began when the furnace temperature reached about 500 °C, and its amount was controlled according to the variations of bed temperature and oxygen concentration in flue gas while oil consumption remained at the maximum rate of
Table 3 Approximate analyses of different coals used in operation Case
Car
Har
Oar
Nar
Sar
Aar
Mad
Vdaf
Qar,net,p
I II III IV
52.91 50.92 52.14 42.20
3.33 3.26 3.36 3.00
7.20 7.07 7.48 8.31
0.81 0.79 0.81 0.64
0.84 0.90 0.82 0.75
25.91 27.26 25.19 29.50
9.00 9.80 10.2 15.6
34.58 35.46 35.97 32.44
19.83 19.17 19.62 16.17
Note: Composition in wt.%, and Qar,net,p in MJ/kg.
Fig. 4. Load variation of the boiler in 4000 h.
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L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
Fig. 5. Variation of bed temperature with boiler load.
about 1000 kg/s. Finally, once bed temperature reached or became close to the design value, and coal feeding was set at normal design rate, the assist oil could be gradually reduced or even completely shut off. The start-up was reliable and the time from a cold state to full load was only about 3–4 h. 3.2. Fuel flexibility Four kinds of coals, whose approximate analysis are listed in Table 3, were tested during commissioning and commercial operation. Table 4 gives the corresponding operation parameters when MCR was near 100%. The results showed that the major performance of the cyclone and the boiler matched the design requirements even though the coal characteristics varied in certain range. The carbon content in fly ash, CO and NOX emission was lower than guarantee values. The fuel flexibility becomes more and more in demand since the coal supply in China has been uncertain. Like most other CFB boilers operating in China, however, no limestone was added for desulphurization.
Fig. 7. Temperature comparison between the inlet and outlet of the separator.
Moreover, the boiler was not only capable to operate steadily under wide range of MCR, but also capable to vary its load frequently according to the demand of power and steam. Fig. 4 shows the 5-month load record of the boiler. The maximum load even occasionally reached as high as 260 t/h, about 15% more than the rated output. The average bed temperature, obtained from eight thermalcouples embedded in dense bed, was found to be a function of the load as shown in Fig. 5. A few data were clearly not consistent with the majority at the same load. The authors believe this might be caused by the variation of coal properties and the operation style for different operators. Since automatic load control system for this boiler was not equipped and no limestone was added, the bed temperature was not tightly controlled. The linear variation of circulating ash temperature with the boiler load is shown in Fig. 6. The circulating ash temperature also can be used as a reference of furnace temperature at a certain load. 3.4. Temperature distribution in boiler
3.3. Load performance The achievement of rated output had been a pain for many early CFB clients. For the demonstration boiler in the Weihai Power Plant, this criterion was satisfied in the commissioning.
The circulating ash temperature in the loop seal was lower than that at the cyclone inlet, averagely about 40 °C, shown in Fig. 7. No slagging or agglomeration was found in separator and loop seal. This is a big advantage for the water-cooled cyclone
Fig. 6. Variation of circulating ash temperature with boiler load.
Fig. 8. Typical temperature distribution in main circulating loop.
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
133
Fig. 9. Weight losses of the bottom ash and fly ash from different fields of ESP for Case IV.
over the insolated hot cyclone, where agglomeration in loop seal is a “headache” problem at a high bed temperature. The axial temperature distribution inside the furnace was relatively uniform when the air ratio was properly kept. Fig. 8 shows the axial temperature distribution in furnace at load of 232 t/h while air was split as: under bed primary air 45%, upper primary air 10%, secondary air 44% and loop seal fluidizing air 1%. The temperature distribution in main loop was reasonable, indicating sound solid loading in the CFB boiler [10–12]. The peak temperature appeared at 3–4 m above the distributor, where most volatile heat was released. Temperature difference was also found in the dense bed, higher at upper level, which is typical for high volatile coal combustion. 3.5. Fly ash analysis Fig. 9 depicts the carbon content in bottom ash and fly ashes for Case IV from three different fields of the electrostatic precipitator (ESP). The weight loss represents the carbon content in ash. Assuming that the separation efficiency is 80% for each field of the ESP, then the amount of fly ash in the first, second and third field is respectively 80%, 16% and 3.2% of the total amount of fly ash. Thus, the average unburned carbon content in fly ash can be estimated as: 0.8C1 + 0.16C2 + 0.032C3,
Fig. 11. Comparison of fly ash size distributions of three 220 t/h CFB boilers with different separators.
where C1, C2 and C3 are respectively the unburned carbon content in the first, second and third field of the ESP. As a result, the unburned carbon content was typically less than 1.5% in bottom ash, and less than 10% in fly ash. The ash with the highest unburned carbon content was from the third field of ESP, agreeing well with the other observations. The size distributions and unburned carbon content of the fly ashes from present square cyclone CFB are compared with those from CFB boilers with insolated hot cyclones and steamcooled round cyclones, and the results are respectively shown in Figs. 10 and 11. In Fig. 10, a parameter called frequency percent Pi is used to describe ash size distribution and is defined as Pi = xi / (di+1 − di) where di and di+1 are the diameters of ash particles of i and i + 1 groups respectively, and xi is mass percentage in a certain size interval [di, di+1] with respect to the mass of total ash. Correspondingly, the average diameter in [di, di+1] is defined as square root of the product of di and di+1. The frequency percent is more convenient to describe the size distribution when different size intervals are used in sampling and easier to determine the peak values of the mass fraction. Table 5 and Fig. 12 respectively present the approximate analyses and size distributions of the coals used in the three boilers, and Table 6 shows some operational parameters of the three boilers. Some slight difference existed among the approximate analysis, size distribution and bed temperature, but in general the coals' properties and operational parameters of the three boilers were similar. Though bed temperature plays an important role in coal particle burnout, it mainly affects the cyclone efficiency through the variation of fluid viscosity. Table 5 Approximate analyses of the coals respectively used in three 220 t/h CFB boilers with different kinds of cyclones Cyclone type
Fig. 10. Comparison of unburned carbon content distributions in fly ashes of three 220 t/h CFB boilers with different separators.
Car
Har
Oar
Nar
Sar
Aar
Mad
Vdaf
Qar,net,p
Square water- 52.91 3.33 7.20 0.81 0.84 25.91 9.00 34.58 19.83 cooled Round hot 53.10 2.42 2.65 0.81 2.79 29.4 8.84 29.44 20.99 Round steam- 53.98 3.20 8.62 0.68 0.70 19.25 13.57 33.47 20.24 cooled Note: composition in wt.%, and Qar,net,p in MJ/kg.
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L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
Fig. 13. Particle size distributions of fly ash and circulating ash from a 220 t/ h boiler. Fig. 12. Particle size distributions of coals respectively used in three 220 t/h CFB boilers with different kinds of cyclones.
Therefore, the marginal bed temperature difference among these boilers only played a minor effect on cyclone efficiency. Given that ash formation is dependent on the coal properties, such as coal type, the initial size distribution, fragmental and attrition characteristics, and the operational conditions of CFB boiler [13] and the coal properties and operational conditions of three boiler were similar, the size distributions of the fly ash represent, at least in certain degree, the collection characteristics of individual cyclone. Shown in Fig. 10, for the square cyclone and the steam-cooled round cyclone, size distributions of the fly ashes are nearly identical, while for the round hot cyclone, the particle diameter corresponding to peak mass fraction shifted from of 40 μm and 20 μm. In general, the fly ash size distributions shown in Fig. 10 are similar and thus we can conclude that the three kinds of cyclones possess similar separation performance. Of course, more detailed experimental and experimental studies are of interests to fully compare the performance among various kinds of cyclones. It is noticed that, as shown in Fig. 11, the peak values of carbon content are dramatically higher for the insolated hot cyclone CFB boiler than those of the other two. The result implies that in addition to the collection efficiency of cyclone and coal character there are some other factors dominating the burnout of carbon in CFB boiler. Though more investigation is needed, poor mixing and oxygen dispersion are highly suspected [4]. The comparison also indicates that both fly ash size distribution and carbon burnout for CFB boilers with square cyclones are compatible to those for boilers with round cyclones.
3.6. Collection efficiency of square cyclone The collection efficiency of square cyclone has being debated for many years, especially for CFB scaling up. The practice for 220 t/h CFB boiler in Weimar Heat and Power Plant is encouraging. Though it was not directly measured, the collection efficiency can be indirectly estimated from the size distributions of the fly ash and the circulating ash. As shown in Fig. 13, the estimated cut size (d50) is 30 μm and the estimated critical size (d99) is 130 μm. Meanwhile, both pressure drops of the square cyclone and furnace were measured as less than 800 Pa, approximated to those from CFB boilers of similar capacities with round cyclones [9,14]. Fig. 12 compares the particle size distributions of fly ash and circulating ash from present boiler with those of a 75 t/h CFB boiler with square cyclone (the equivalent diameter was 3.0 m) [5]. Here the parameter frequent percent is used again as we did in Fig. 10. The comparison shows that the size distributions of fly ashes from two boilers were almost identical, implying that the cut sizes were nearly the same. The peak value of circulating ash size for 75 t/h CFB boiler was 100 μm, but 120 μm for present 220 t/h CFB boiler. This was reasonable, because even for the round cyclone, if the equivalent diameter increased, e.g., from 3.0 m to 5.0 m, the critical size (d99) was expected to increase (Fig. 14).
Table 6 Operation parameters of three 220 t/h CFB boilers with different kinds of cyclones Item
Square watercooled
Round hot
Round steamcooled
Bed temperature, °C Capacity, t/h Primary air temperature, °C Secondary air temperature, °C
888 220 130
934 220 185
875 220 190
225
230
220
Fig. 14. Particle size distributions of fly ash and circulating ash from a 75 t/ h boiler.
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
4. Conclusions A 220 t/h CFB boiler with advanced water-cooled square cyclones with curved inlet was successfully demonstrated in Shandong, China, with a short commissioning time of 12 days. The boiler is of compact structure fully enclosed by water membrane. The performance of the boiler was assessed with a series of experiments. Both the cyclone and the boiler showed excellent behaviors in start-up, fuel flexibility, load performance, temperature distribution and carbon burnout. Compared with CFB with round cyclones with respect to the fly ash and the carbon burnout, the present CFB boiler with square cyclones was of compatible performance. Inspections after seven months of commercial operation, continuously operated over 5000 h, found no abrasion on heat surfaces or damage of the refractory in furnace and in square cyclones. The only remarkable problem of the boiler was that the main steam temperature was slightly lower than the design value when the boiler load was lower than 170 t/h. The problem was resolved by the adjustment of heat surfaces in the updated design. Overall, the success of this demonstration was a milestone for the CFB boilers scaling up in China. Acknowledgments The project is financially supported by the National Key Basic Research Special Fund (No.2000026309). References [1] R. Gamble, T. Hyppanen, T. Kauranen, Pyroflow Compact a second generation CFB boiler by Ahlstrom Pyropower, Proceeding of the 12th International Conference on Fluidized Bed Combustion, 1993, pp. 751–760.
135
[2] G.X. Yue, X.Y. Zhang, Y. Li, et al., The water cooled square separator with an acceleration inlet, Chinese Patent, No. 93235842.X. [3] J.F. Lu, G.X. Yue, Q. Liu, et al., The design and operation of 75 t/h circulating fluidized bed boiler with water cooled separator, Chinese Electrical Power 32 (4) (1999) 61 (in Chinese). [4] J.F. Lu, G.X. Yu, Q. Liu, et al., The progress of the water-cooled separator CFB boiler in China, Proceeding of the 15th International Conference on Fluidized Bed Combustion, 1999, CD Rom. [5] G.X. Yue, Y. Li, X.X. Zhao, et al., The first pilot compact CFB boiler with water cooled separator in China, Proceeding of the 14th International Conference on Fluidized Bed Combustion, 1997, pp. 497–506. [6] J.F. Li, G.X. Yue, Q. Liu, et al., The operation experience of a 130 t/h circulating fluidized bed boiler with water cooled square cyclone, Chinese Electrical Power 32 (4) (2001) 19 (in Chinese). [7] J.S. Zhang, J.F. Lu, G.X. Yue, et al., Performance of large scale CFB boilers in China, Proceeding of the 4th International Symposium on Coal Combustion, 1999, pp. 493–498. [8] J.S. Zhang, J.F. Lu, G.X. Yue, et al., Solid suspension density distribution in the furnace of 75 t/h circulating fluidized bed boiler with water-cooled square separator, Proceeding of the 4th International Symposium of Multiphase Flow and Heat Transfer, 1999, pp. 276–284. [9] J.F. Lu, L. Yu, J. Zhang, et al., Comparison of collection efficiency for various types separator of a circulating fluidized bed boiler, Boiler Manufacture 2 (2000) 7 (in Chinese). [10] J.F. Lu, Y.G. Bai, Q. Liu, et al., The performance prediction of the scaling up square cyclone with particle accelerating inlet, Journal of Basic Science and Engineering 8 (2) (2000) 207 (in Chinese). [11] X.Z. Jin, J.F. Lu, Q. Liu, Comprehensive mathematical model for coal combustion in the circulating fluidized bed combustor, Tsinghua Science and Technology 6 (4) (2001) 319 (in Chinese). [12] X.Z. Jin, J.F. Lu, J.S. Zhang, et al., Experimental investigation on heat transfer in industrial-scale circulating fluidized bed boilers, Proceeding of the 6th International Conference on Circulating Fluidized Bed Technology, 1999, pp. 356–361. [13] H.R. Yang, M. Wirsum, J.F. Lu, X.B. Xiao, G.X. Yue, Semi-empirical technique for predicting ash size distribution in CFB boilers, Fuel Processing Technology 85 (12) (2004) 1403. [14] X.D. Lin, H.W. Cheng, L. Yu, Long, et al., Operation of 220 t/h CFB boilers, Power System Engineering 16 (4) (2000) 217 (in Chinese).
Performance evaluation of a 220t/h CFB boiler with water-cooled square cyclones Lu Junfu ⁎, Zhang Jiansheng, Zhang Hai, Liu Qing, Yue Guangxi Thermal Engineering Department, Tsinghua University, Beijing 100084, China Received 9 September 2004; received in revised form 20 December 2004; accepted 22 December 2004
Abstract The first Chinese 220 t/h CFB boiler was successfully demonstrated. The boiler was compactly designed with Tsinghua-patented, water-cooled square cyclones with curved inlet based on previous experience of similar CFB boilers of smaller capacities. The demonstration showed that the boiler possesses excellent performance in start-up, fuel flexibility, flexibility to frequent turn-down ratio variation, facility availability and reliability. The performance of the water-cooled square cyclone was compared with that of other cyclones through fly ash analysis. The results showed that the overall performance of the square cyclone in such capacity is compatible to that of the round cyclone, meeting the requirements for material balance and efficient combustion in CFB boilers. The demonstration was a milestone for CFB boiler scaling-up in China. © 2006 Elsevier B.V. All rights reserved. Keywords: Water-cooled square cyclone; CFB boiler; Performance; Scaling-up; 220 t/h
1. Introduction Circulating fluidized bed (CFB) has been considered as one of the prevailing clean coal combustion technologies in China since the mid 1980s. Cyclone, the gas–solid separator in a CFB boiler, is a key factor for capacity scaling-up, which is one of the major challenges in commercialization. In the early 1990s, Ahlstrom Corporation, Finland introduced the first square cyclone, named as Pyroflow COMPACT to CFB boilers [1]. Compared with the traditional round cyclone with the same capacity, the square cyclone was of smaller size, shorter start-up time, simpler and less-costly construction and engineering, though it scarified some degree of separation efficiency. Later, supported by the State Economy and Trade Committee of China (SETCC), the Tsinghua University in Beijing, China developed and patented an advanced water-cooled square cyclone featuring with a curved inlet to accelerate particles to increase the separation efficiency (shown in Fig. 1) [2] and successfully applied it in boilers of capacities of 75 t/h and 150 t/h [3–6]. The application showed that the Tsinghua-patented cyclone gener-
⁎ Corresponding author. Tel./fax: +86 10 62781743. E-mail address: [email protected] (L. Junfu). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2004.12.008
Fig. 1. Schematic of an advance square cyclone with a curved inlet patented by Tsinghua University.
130
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135 Table 1 Approximate analysis of the design coal Car
Har
Oar
Nar
Sar
Aar
Mad
Vdaf
Qar,net,p
53.85
2.64
3.86
0.75
1.18
30.92
6.80
30.50
20.58
Note: composition in wt.%, and Qar,net,p in MJ/kg.
To demonstrate the performance and the scalability of the advance water-cooled square cyclone, also supported by SETCC, a CFB boiler of 220 t/h steam capacity was established in Weihai Heat and Power Cogeneration Plant, Shandong, China, and successfully put into commercial operation in the end of 2001. Since then, a series of industrial experiments in the studies of such as start-up performance, fuel flexibility, load performance, and size and carbon content distributions of fly ash and bottom ash, have been conducted. The results are important to assess the performance of the patented cyclone and the entire boiler. 2. Design of the 220 t/h CFB boiler with advance watercooled square cyclones As shown in Fig. 2, the demonstrated boiler is of 43.6 m in height, 21.4 m in width and 20.7 m in depth. It is of natural circulation type with single drum, which is mounted at the elevation of 39.6 m. The furnace, made by membranes of 0.06 m × 0.08 m pitch, is of net height about 30 m and its cross section about 55 m2 at freeboard. The distributor plate and the windbox are also made by water wall membrane, acting as an extension of the front wall of the furnace. The distributor is about 25 m2 in area. Two square cyclones made of planar membranes are located between the furnace and the second pass. The front wall of the square cyclone functions as the rear wall of the furnace, and the rear wall of the square cyclone functions as the front wall of the second pass. The expansion joint, which might become a weakest part of a CFB boiler, is not used for connection. Instead, the entrance of the Table 2 Design parameters of the 200 t/h CFB boiler
Fig. 2. Layout of a 220 t/h circulating fluidized bed boiler with water-cooled square cyclone.
ated much less pressure drop, while remaining compatible high efficiency, availability and performance compared with the traditional round hot cyclone [7–9]. Moreover, the patented cyclone is expected to be superior for scale-up because of its full water membrane structure from furnace to the second pass and the separators [10].
Item
Value
Main steam flow rate, t/h Main steam pressure, MPa Main steam temperature, °C Feed water temperature, °C Exhaust gas temperature, °C Ambient temperature, °C Primary air temperature, °C Secondary air temperature, °C Spraying water flow, t/h Ratio of fly ash to bottom ash Bed temperature, °C Carbon content in fly ash, % Carbon content in bottom ash, % Boiler heat efficiency, % CO emission, ppm NOX emission, ppm SO2 emission, ppm⁎
220 5.29 485 150 134 20 134 220 3.0 7:3 894 b10 b2 ∼ 90 b250 b200 b250
Note: ⁎Ca/S molar ratio is designed as 2.33.
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
131
Table 4 Operation parameters of the 220 t/h CFB boiler
Fig. 3. Spatial variation of bed temperature during start-up process.
cyclone is connected directly with the furnace, and the exit of the cyclone is connected directly with the second pass. The equivalent diameter of the square cyclone is chosen as about 5.40 m. Since water-cooling is used, the studs which are connected with the cyclone membranes are of rather low temperature. Thus they can be made of relatively low grade carbon steel. The compacted and water-membrane enclosed design of the cyclone provides a big advantage in boiler manufacture, engineering and operation. The design fuel is the domestic lean coal. Its approximate analysis is shown in Table 1. Some design parameters of the boiler are shown in Table 2, including not only the boiler thermal efficiency, but also the limits of carbon content in fly ash (LOI) and bottom ash. The carbon content in fly ash, measured from many industrial CFB boilers in China, had been found to be much higher than expectation especially when low quality coals were fired. To decrease the LOI, relatively high bed temperature is adopted. In the demonstration boiler, the bed temperature is designed as 894 °C. 3. Results and discussion 3.1. Start-up performance Reliable and quick start-up is important for CFB operation. Based on the design and operational experiences from previous 75 t/h and 130 t/h boilers, we concluded that it is thermal stress but erosion that is mainly attributed to the refractory break. Therefore, the thickness of refractory layer can be optimized. For the lower part of furnace, the thickness of refractory layer is designed as 0.10 m, which is thinner than the thickness used in some other designs. The thin refractory of the furnace and cyclone provides an advantage of faster heat up rate than that for boilers using hot cyclone where thick refractory is used.
Item
Case I
Case II
Case III
Case IV
Main steam flow rate, t/h Main steam pressure, MPa Main steam temperature, °C Feed water temperature, °C Exhaust gas temperature, °C Ambient temperature, °C Primary air temperature, °C Secondary air temperature, °C Spraying water flow, t/h Ratio of fly ash to bottom ash Bed temperature, °C Carbon content in fly ash, % Carbon content in bottom ash, % Boiler heat efficiency, %⁎ CO emission, ppm NOX emission, ppm SO2 emission, ppm⁎⁎
218.5
219.9
240.3
222.6
5.19
5.16
5.07
5.15
475
473
473
475
145.4
138.5
136.5
139.4
131
129
138
130
16
14
15
17
130
133
136
123
223
224
228
233
1.1
0.5
2.0
2.8
7:3
6:4
7:3
6:4
898 9.08
886 9.70
902 7.44
891 6.44
1.27
0.47
0.97
0.74
90.22
90.28
90.59
91.12
123 68 628
76 121 664
38 55 582
88 69 475
Note: ⁎based on the low heat caloric value; ⁎⁎no limestone was injected into the combustor for desulphurization.
An oil-fired duct burner was used for the under-bed ignition during the boiler start-up, as the practice used in many CFB boilers in China. The start-up process is shown in Fig. 3. First, in about 1.5 h, the furnace was gradually heated up by hot flue gas exiting from the oil-fired duct burner and the consumption rate of the assist oil gradually increased with bed temperature. Then coal feeding began when the furnace temperature reached about 500 °C, and its amount was controlled according to the variations of bed temperature and oxygen concentration in flue gas while oil consumption remained at the maximum rate of
Table 3 Approximate analyses of different coals used in operation Case
Car
Har
Oar
Nar
Sar
Aar
Mad
Vdaf
Qar,net,p
I II III IV
52.91 50.92 52.14 42.20
3.33 3.26 3.36 3.00
7.20 7.07 7.48 8.31
0.81 0.79 0.81 0.64
0.84 0.90 0.82 0.75
25.91 27.26 25.19 29.50
9.00 9.80 10.2 15.6
34.58 35.46 35.97 32.44
19.83 19.17 19.62 16.17
Note: Composition in wt.%, and Qar,net,p in MJ/kg.
Fig. 4. Load variation of the boiler in 4000 h.
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Fig. 5. Variation of bed temperature with boiler load.
about 1000 kg/s. Finally, once bed temperature reached or became close to the design value, and coal feeding was set at normal design rate, the assist oil could be gradually reduced or even completely shut off. The start-up was reliable and the time from a cold state to full load was only about 3–4 h. 3.2. Fuel flexibility Four kinds of coals, whose approximate analysis are listed in Table 3, were tested during commissioning and commercial operation. Table 4 gives the corresponding operation parameters when MCR was near 100%. The results showed that the major performance of the cyclone and the boiler matched the design requirements even though the coal characteristics varied in certain range. The carbon content in fly ash, CO and NOX emission was lower than guarantee values. The fuel flexibility becomes more and more in demand since the coal supply in China has been uncertain. Like most other CFB boilers operating in China, however, no limestone was added for desulphurization.
Fig. 7. Temperature comparison between the inlet and outlet of the separator.
Moreover, the boiler was not only capable to operate steadily under wide range of MCR, but also capable to vary its load frequently according to the demand of power and steam. Fig. 4 shows the 5-month load record of the boiler. The maximum load even occasionally reached as high as 260 t/h, about 15% more than the rated output. The average bed temperature, obtained from eight thermalcouples embedded in dense bed, was found to be a function of the load as shown in Fig. 5. A few data were clearly not consistent with the majority at the same load. The authors believe this might be caused by the variation of coal properties and the operation style for different operators. Since automatic load control system for this boiler was not equipped and no limestone was added, the bed temperature was not tightly controlled. The linear variation of circulating ash temperature with the boiler load is shown in Fig. 6. The circulating ash temperature also can be used as a reference of furnace temperature at a certain load. 3.4. Temperature distribution in boiler
3.3. Load performance The achievement of rated output had been a pain for many early CFB clients. For the demonstration boiler in the Weihai Power Plant, this criterion was satisfied in the commissioning.
The circulating ash temperature in the loop seal was lower than that at the cyclone inlet, averagely about 40 °C, shown in Fig. 7. No slagging or agglomeration was found in separator and loop seal. This is a big advantage for the water-cooled cyclone
Fig. 6. Variation of circulating ash temperature with boiler load.
Fig. 8. Typical temperature distribution in main circulating loop.
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
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Fig. 9. Weight losses of the bottom ash and fly ash from different fields of ESP for Case IV.
over the insolated hot cyclone, where agglomeration in loop seal is a “headache” problem at a high bed temperature. The axial temperature distribution inside the furnace was relatively uniform when the air ratio was properly kept. Fig. 8 shows the axial temperature distribution in furnace at load of 232 t/h while air was split as: under bed primary air 45%, upper primary air 10%, secondary air 44% and loop seal fluidizing air 1%. The temperature distribution in main loop was reasonable, indicating sound solid loading in the CFB boiler [10–12]. The peak temperature appeared at 3–4 m above the distributor, where most volatile heat was released. Temperature difference was also found in the dense bed, higher at upper level, which is typical for high volatile coal combustion. 3.5. Fly ash analysis Fig. 9 depicts the carbon content in bottom ash and fly ashes for Case IV from three different fields of the electrostatic precipitator (ESP). The weight loss represents the carbon content in ash. Assuming that the separation efficiency is 80% for each field of the ESP, then the amount of fly ash in the first, second and third field is respectively 80%, 16% and 3.2% of the total amount of fly ash. Thus, the average unburned carbon content in fly ash can be estimated as: 0.8C1 + 0.16C2 + 0.032C3,
Fig. 11. Comparison of fly ash size distributions of three 220 t/h CFB boilers with different separators.
where C1, C2 and C3 are respectively the unburned carbon content in the first, second and third field of the ESP. As a result, the unburned carbon content was typically less than 1.5% in bottom ash, and less than 10% in fly ash. The ash with the highest unburned carbon content was from the third field of ESP, agreeing well with the other observations. The size distributions and unburned carbon content of the fly ashes from present square cyclone CFB are compared with those from CFB boilers with insolated hot cyclones and steamcooled round cyclones, and the results are respectively shown in Figs. 10 and 11. In Fig. 10, a parameter called frequency percent Pi is used to describe ash size distribution and is defined as Pi = xi / (di+1 − di) where di and di+1 are the diameters of ash particles of i and i + 1 groups respectively, and xi is mass percentage in a certain size interval [di, di+1] with respect to the mass of total ash. Correspondingly, the average diameter in [di, di+1] is defined as square root of the product of di and di+1. The frequency percent is more convenient to describe the size distribution when different size intervals are used in sampling and easier to determine the peak values of the mass fraction. Table 5 and Fig. 12 respectively present the approximate analyses and size distributions of the coals used in the three boilers, and Table 6 shows some operational parameters of the three boilers. Some slight difference existed among the approximate analysis, size distribution and bed temperature, but in general the coals' properties and operational parameters of the three boilers were similar. Though bed temperature plays an important role in coal particle burnout, it mainly affects the cyclone efficiency through the variation of fluid viscosity. Table 5 Approximate analyses of the coals respectively used in three 220 t/h CFB boilers with different kinds of cyclones Cyclone type
Fig. 10. Comparison of unburned carbon content distributions in fly ashes of three 220 t/h CFB boilers with different separators.
Car
Har
Oar
Nar
Sar
Aar
Mad
Vdaf
Qar,net,p
Square water- 52.91 3.33 7.20 0.81 0.84 25.91 9.00 34.58 19.83 cooled Round hot 53.10 2.42 2.65 0.81 2.79 29.4 8.84 29.44 20.99 Round steam- 53.98 3.20 8.62 0.68 0.70 19.25 13.57 33.47 20.24 cooled Note: composition in wt.%, and Qar,net,p in MJ/kg.
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Fig. 13. Particle size distributions of fly ash and circulating ash from a 220 t/ h boiler. Fig. 12. Particle size distributions of coals respectively used in three 220 t/h CFB boilers with different kinds of cyclones.
Therefore, the marginal bed temperature difference among these boilers only played a minor effect on cyclone efficiency. Given that ash formation is dependent on the coal properties, such as coal type, the initial size distribution, fragmental and attrition characteristics, and the operational conditions of CFB boiler [13] and the coal properties and operational conditions of three boiler were similar, the size distributions of the fly ash represent, at least in certain degree, the collection characteristics of individual cyclone. Shown in Fig. 10, for the square cyclone and the steam-cooled round cyclone, size distributions of the fly ashes are nearly identical, while for the round hot cyclone, the particle diameter corresponding to peak mass fraction shifted from of 40 μm and 20 μm. In general, the fly ash size distributions shown in Fig. 10 are similar and thus we can conclude that the three kinds of cyclones possess similar separation performance. Of course, more detailed experimental and experimental studies are of interests to fully compare the performance among various kinds of cyclones. It is noticed that, as shown in Fig. 11, the peak values of carbon content are dramatically higher for the insolated hot cyclone CFB boiler than those of the other two. The result implies that in addition to the collection efficiency of cyclone and coal character there are some other factors dominating the burnout of carbon in CFB boiler. Though more investigation is needed, poor mixing and oxygen dispersion are highly suspected [4]. The comparison also indicates that both fly ash size distribution and carbon burnout for CFB boilers with square cyclones are compatible to those for boilers with round cyclones.
3.6. Collection efficiency of square cyclone The collection efficiency of square cyclone has being debated for many years, especially for CFB scaling up. The practice for 220 t/h CFB boiler in Weimar Heat and Power Plant is encouraging. Though it was not directly measured, the collection efficiency can be indirectly estimated from the size distributions of the fly ash and the circulating ash. As shown in Fig. 13, the estimated cut size (d50) is 30 μm and the estimated critical size (d99) is 130 μm. Meanwhile, both pressure drops of the square cyclone and furnace were measured as less than 800 Pa, approximated to those from CFB boilers of similar capacities with round cyclones [9,14]. Fig. 12 compares the particle size distributions of fly ash and circulating ash from present boiler with those of a 75 t/h CFB boiler with square cyclone (the equivalent diameter was 3.0 m) [5]. Here the parameter frequent percent is used again as we did in Fig. 10. The comparison shows that the size distributions of fly ashes from two boilers were almost identical, implying that the cut sizes were nearly the same. The peak value of circulating ash size for 75 t/h CFB boiler was 100 μm, but 120 μm for present 220 t/h CFB boiler. This was reasonable, because even for the round cyclone, if the equivalent diameter increased, e.g., from 3.0 m to 5.0 m, the critical size (d99) was expected to increase (Fig. 14).
Table 6 Operation parameters of three 220 t/h CFB boilers with different kinds of cyclones Item
Square watercooled
Round hot
Round steamcooled
Bed temperature, °C Capacity, t/h Primary air temperature, °C Secondary air temperature, °C
888 220 130
934 220 185
875 220 190
225
230
220
Fig. 14. Particle size distributions of fly ash and circulating ash from a 75 t/ h boiler.
L. Junfu et al. / Fuel Processing Technology 88 (2007) 129–135
4. Conclusions A 220 t/h CFB boiler with advanced water-cooled square cyclones with curved inlet was successfully demonstrated in Shandong, China, with a short commissioning time of 12 days. The boiler is of compact structure fully enclosed by water membrane. The performance of the boiler was assessed with a series of experiments. Both the cyclone and the boiler showed excellent behaviors in start-up, fuel flexibility, load performance, temperature distribution and carbon burnout. Compared with CFB with round cyclones with respect to the fly ash and the carbon burnout, the present CFB boiler with square cyclones was of compatible performance. Inspections after seven months of commercial operation, continuously operated over 5000 h, found no abrasion on heat surfaces or damage of the refractory in furnace and in square cyclones. The only remarkable problem of the boiler was that the main steam temperature was slightly lower than the design value when the boiler load was lower than 170 t/h. The problem was resolved by the adjustment of heat surfaces in the updated design. Overall, the success of this demonstration was a milestone for the CFB boilers scaling up in China. Acknowledgments The project is financially supported by the National Key Basic Research Special Fund (No.2000026309). References [1] R. Gamble, T. Hyppanen, T. Kauranen, Pyroflow Compact a second generation CFB boiler by Ahlstrom Pyropower, Proceeding of the 12th International Conference on Fluidized Bed Combustion, 1993, pp. 751–760.
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