外文翻譯--循環(huán)流化床鍋爐中飛灰含量的研究（英文）

1、Research on Carbon Content in Fly Ash from Circulating Fluidized Bed BoilersXianbin Xiao,* Hairui Yang, Hai Zhang, Junfu Lu, and Guangxi YueDepartment of Thermal Engineering, Tsinghua University, Beijing 100084, ChinaRec

2、eived December 12, 2004. Revised Manuscript Received March 9, 2005The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, which directly influences the co

3、mbustion efficiency. In the present paper, carbon burnout was investigated in both field tests and laboratory experiments. The effect of coal property, operation condition, gas-solid mixing, char deactivation, residence

4、time, and cyclone performance are analyzed seriatim based on a large amount of experimental results. A coal index is proposed to describe the coal rank, having a strong effect on the char burnout. Bad gas-solid mixing in

5、 the furnace is another important reason of the higher carbon content in the fly ash. Some chars in the fly ash are deactivated during combustion of large coal particles and have very low carbon reactivity. Several sugge

6、stions are made about design, operation, and modification to reduce the carbon content in the fly ash.IntroductionWith the advantages of fuel flexibility and low pol- lutant emission, circulating fluidized bed (CFB) com-

7、 bustion technology has been developed rapidly in power generation. The capacity of CFB power plants has been growing steadily ever since the commercialization of the technology in the late 1970s. Currently, the maximum

8、capacity of a single CFB utility boiler is on the order of 300 MWe and more large capacity units of 600 and 800 MWe supercritical pressure CFB boilers are developing. The development of CFB boilers in China started in th

9、e 1980s, and the maximum capacity for a single unit has been increasing year after year, as shown in Figure 1. There are over 1000 CFB boilers in operation up to 2004, including over 20 units above 135 MWe. In addition,

10、80 units of 135 MWe and 10 units of 300 MWe CFB boilers are in construction or on order.However, it is a fact that the combustion efficiency of CFB boilers is lower than that of pulverized coal fired (PC) boilers, though

11、 high combustion efficiency was reported in the CFB market abroad.1 Brown coals, with high activity, are commonly burned abroad, which is the main reason for low carbon content in the fly ash. In China, on the contrary,

12、most CFB boilers burn hard coals such as anthracite, bituminous, and coal wastes; the carbon content in the fly ash is much higher than expected,2 especially for the large-capacity CFB boilers manufactured with imported

13、technology. At the sametime, the high carbon content in the fly ash limits the potential utilization as cement materials.3 It has become the bottleneck reducing the competitive power of the CFB boiler. Coal combustion pr

14、ocesses in CFB boiler are very complex, undergoing the following interrelated se- quences:4 heating and drying, devolatilization and volatile combustion, swelling and fragmentation, and burning of char. The unburned carb

15、on content in the fly ash is believed to be the final results for character- izing the combustion efficiency. To optimize the process * To whom correspondence should be addressed. E-mail: xiaoxianbin00@mails.tsinghua.edu

16、.cn. (1) Daladimos, G.; Hirschfelder, H. Long Term Experience with Commercially Operating Large Steam CFB Generators. In Proceedings of 4th International Conference on Circulating Fluidized Beds, Pitts- burgh, PA, 1993;

17、Avidan, A. A., Ed.; 1993; pp 174-182. (2) Li, Y.; Yue, G.; Lu, J.; et al. An Investigation of Carbon Loss of Boilers Burning Hard Coals. In Proceedings of the 16th International Conference on Fluidized Bed Combustion, Re

18、no, NV; Geiling, D. W., Ed.; American Society for Mechanical Engineers: Fairfield, NJ, 2001; Paper FBC01-0064.(3) Tyson, S. S.; Blackstock, T. H. Overview of Coal Ash Use in Construction and Related Application. Fuel Ene

19、rgy Abstr. 1995, 36 (5), 337. (4) Basu, P. Combustion of Coal in Circulating Fluidized-Bed Boilers: A Review. Chem. Eng. Sci. 1999, 54, 5547-5557. (5) Zheng, Q.; Liu X.; Jin Y. Study of Combustion Characteristics of Char

20、 Particles in Circulating Fluidized Bed Combustor. J. Eng. Thermophys. 1995, 16 (1), 106-110 (in Chinese).Figure 1. Maximum capacity for a single unit.1520 Energy & Fuels 2005, 19, 1520-152510.1021/ef049678g CCC: $30

21、.25 © 2005 American Chemical Society Published on Web 05/03/2005layer resistance, surface temperature, residence time, and burnout time are different, resulting in the non- homogeneous distribution. Bed Temperature.

22、 Generally speaking, the increase of the bed temperature can promote the chemical reaction rate and then increase the combustion ef- ficiency. Experiments were carried out for six kinds of coal in three 75 t/h CFB boiler

23、s, which are manufac- tured by the same boiler manufacture and have nearly the same structure. It can be seen in Figure 5 that, for all kinds of coals, the carbon content in the fly ash has a strong relationship with the

24、 bed temperature where the bed inventory, the excess air ratio, and the air supply are kept as constant as possible.6 For the six different coals, the increase of bed temperature will cause the decrease of the carbon con

25、tent. It clearly proves the promotion of the bed temperature can reduce the carbon content in the fly ash and increase the combustion efficiency. However, other disadvantages, such as lower desulfurization efficiency and

26、 more NOx emission, would limit the increasing temperature.7,8 Thus, it is not an advisable way for performance optimization of CFB boilers. Coal Rank. A CFB boiler can be designed to burn almost any kind of solid fuels,

27、 but, as noted above, thepractical experience of CFB boiler in China proves that the carbon content in the fly ash is not as low as expec- ted. Figure 5 also shows that the coal property will in- fluence the carbon conte

28、nt in the fly ash. It is believed that the key factor affecting the combustion efficiency is the coal characteristics, including volatile content, heat value, char reactivity, and char structure, etc. The high volatile c

29、oals, such as brown coal and bituminous coal, usually have higher reactivity and are easy to burn out, while the low volatile and high ash coals, such as an- thracite and lean coal, are usually on the contrary. Oper- ati

30、ng conditions, carbon content in the fly ashes, and coal properties from 17-unit 220 t/h CFB boilers are shown in Table 1. It is clear that the carbon content in the fly ash depends on the coal type strongly. A coal inde

31、x I was defined as the volatile content, Vdaf (dry ash-free basis, magnitude on 1-basis) divided by the lower heating value, Qar,net,p (MJ/g):And, the relationship between the carbon content in the fly ash and the coal i

32、ndex can be easily seen in Figure 6. Although the furnace temperature in boiler P, burning anthracite, is higher than other boilers, the carbon content in the fly ash is still the highest. Actually, it is also the genera

33、l experience in Chinese CFB boilers burning anthracite that the carbon content in the fly ash is always excessively high. Even for some kinds of bituminous, the carbon content in the fly ash is still relatively high. On

34、the contrary, the CFB boilers burn- ing brown coal, which has high coal index, normally have low carbon content. The coal index, presented here, is suggested a useful parameter to represent the coal reactivity and to ana

35、lyze the char burnout. Gas-Solid Mixing and Air Supply. A large amount of solid particles are elutriated from the dense bed during the operation of CFB boilers, and the high solid loading in the gas has a strong impact o

36、n the gas-solid mixing. The oxygen concentration and solid suspensionFigure 5. Effect of bed temperature on carbon content in fly ash.Table 1. Operating Conditions and Carbon Content in Fly Ash from CFB Boilersboiler fur

37、nace temp (°C) excess air ratio carbon content in fly ash (%) Vdaf Qar,net,p (MJ/kg)A 880-905 1.21 17.17 21.49 22.76 B 880-905 1.23 13.74 20.55 20.52 C 860-890 1.23 8.91 22.51 19.83 D 880-910 1.22 7.05 28.88 20.80 E

38、 892-915 1.21 8.72 30.52 19.87 F 900-915 1.26 22.20 11.25 21.00 G 885-900 1.31 8.38 27.79 20.10 H 900-910 1.24 6.79 35.54 18.00 I 890-900 1.25 6.91 46.65 17.10 J 890-920 1.28 8.72 30.52 18.84 K 875-885 1.32 6.30 29.93 18

39、.82 L 880-900 1.26 5.61 40.58 18.00 M 890-915 1.26 5.30 45.58 13.13 N 880-900 1.26 18.01 13.00 21.56 O 875-895 1.26 5.01 49.59 15.90 P 900-930 1.25 27.12 5.98 21.39 Q 860-890 1.26 16.3 19.28 22.70Figure 6. Relationship b