Research on Carbon Content in Fly Ash from Circulating Fluidized

May 3, 2005 - In China, on the contrary, most CFB boilers burn hard coals such as anthracite, bituminous, and coal wastes; the carbon content in the f...
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Research on Carbon Content in Fly Ash from Circulating Fluidized Bed Boilers Xianbin Xiao,* Hairui Yang, Hai Zhang, Junfu Lu, and Guangxi Yue Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Received December 12, 2004. Revised Manuscript Received March 9, 2005

The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, which directly influences the combustion 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 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 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 suggestions are made about design, operation, and modification to reduce the carbon content in the fly ash.

Introduction With the advantages of fuel flexibility and low pollutant emission, circulating fluidized bed (CFB) combustion 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 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 the 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, 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 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, 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 technology. At the same * To whom correspondence should be addressed. E-mail: [email protected]. (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, Pittsburgh, PA, 1993; 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, Reno, NV; Geiling, D. W., Ed.; American Society for Mechanical Engineers: Fairfield, NJ, 2001; Paper FBC01-0064.

Figure 1. Maximum capacity for a single unit.

time, 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 processes in CFB boiler are very complex, undergoing the following interrelated sequences:4 heating and drying, devolatilization and volatile combustion, swelling and fragmentation, and burning of char. The unburned carbon content in the fly ash is believed to be the final results for characterizing the combustion efficiency. To optimize the process (3) Tyson, S. S.; Blackstock, T. H. Overview of Coal Ash Use in Construction and Related Application. Fuel Energy 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 Particles in Circulating Fluidized Bed Combustor. J. Eng. Thermophys. 1995, 16 (1), 106-110 (in Chinese).

10.1021/ef049678g CCC: $30.25 © 2005 American Chemical Society Published on Web 05/03/2005

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Figure 2. Gas sampling probe. reactivity measurement was performed by thermal gravimetric analysis (TGA). More details about the experimental methods, instruments, and processes are described in refs 6, 9, 10, and 12.

Figure 3. Solid bulk density probe.

Results and Discussion Carbon Content in the Fly Ash from CFB Boilers. The distribution of carbon content in the fly ash is nonhomogeneous, and the higher carbon content is concentrated within the size range of 30-50 µm. A typical example from one boiler is shown in Figure 4.

and make predictions of the combustor performance in a broad range of operating conditions, it is necessary to understand the combustion efficiency, especially the unburned carbon in the fly ash. The carbon burnout in Chinese CFB boilers was investigated in both field tests and laboratory experiments to identify the major factors affecting the carbon content in the fly ash. Experimental Section The experimental investigations include a great deal of field tests and laboratory experiments, such as sampling of coal and fly ash, control and measurements of the operation parameters, measurements of oxygen concentration and solid suspension density, size distribution measurement of fly ash, measurement of carbon content in the fly ash, coal analysis, and char reactivity measurement, etc. The field tests have been carried out in more than 20 units of commercial CFB boilers since several years ago. Coal and fly ash were sampled respectively from 17-unit 220 t/h boilers and then analyzed in the laboratory, to compare the effect of coal rank. The carbon content in the fly ash was measured by the loss of ignition (LOI) method. On bed temperature, bed pressure, and excess air ratio, experiments were carried out for six kinds of coal in three 75 t/h boilers in detail, which are manufactured by the same boiler manufacturer and have nearly the same structure. On gas-solid mixing, the oxygen concentration and solid suspension density in the furnace were investigated in one 75 t/h boiler. The cross-section of the furnace is 3 m (depth) and 6 m (width). Four measuring points were located at the centerline of the side wall along the height. The gas sampling probe and solid bulk density probe were inserted through the side wall, which are illustrated in Figure 2 and Figure 3. On char deactivation, char samples were prepared in a tube furnace at various temperatures for various durations, and the (6) Lu, J.; Jin, X.; Yang, H.; et al. Coal Combustion Efficiency in Circulating Fluidized Bed. J. Basic Sci. Eng. 2000, 8 (1), 97-105. (7) Lyngfelt, A.; Leckner, B. Sulphur Capture in Circulating Fluidized-Bed Boilers: Can the efficiency Be Predicted? Chem. Eng. Sci. 1999, 54, 5573-5584

Figure 4. Distribution of carbon content in fly ash.

The formation of carbon content in the fly ash relates the mass transfer, heat transfer, and the formation of ash layer during combustion.5 For different size, the ash (8) Åmand, L. E.; Leckner, B. Emissions of Nitrogen Oxide from a Circulating Fluidized BoilersThe Influence of Design Parameters. In Proceedings of the 2nd International Conference on Circulating Fluidized Beds, Compiegne, France, 14-18 March 1988; Basu, P., Large, J. F., Eds.; Pergamon Press: New York, 1988; pp 457. (9) Lu, J.; Yue, G.; Liu, Q.; et al. The Progress of the Water Cooled Separator CFB Boiler in China. In Proceedings of 15th International Conference of Fluidized Bed Combustion, Savannah, GA; Reuther R. B., Ed.; American Society for Mechanical Engineers: Fairfield, NJ, 1999; Paper FBC99-0038. (10) Jin, X.; Lu, J.; et al. Gas Concentration Profiles in the Furnace of Large CFB Boilers with Water-Cooled Square Separator. In Proceeding of 1st International Conference of Engineering Thermophysics; Cai, R., Xu, J., Eds.; Academy Press: Beijing, 1999; pp 286-275. (11) Yang, H.; Wirsum, M.; Lu, J.; et al. Semiempirical Technique for Predicting Ash Size Distribution in CFB Boilers. Fuel Process Technol. 2004, 12, 1403-1414. (12) Li, Y.; Zhang, J.; Liu, Q.; et al. A Study of the Reactivity and Formation of the Unburnt Carbon in CFB Fly Ashes. Dev. Chem. Eng. Miner. Process. 2001, 9, 3-4.

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Figure 5. Effect of bed temperature on carbon content in fly ash. Table 1. Operating Conditions and Carbon Content in Fly Ash from CFB Boilers boiler

furnace temp (°C)

excess air ratio

carbon content in fly ash (%)

Vdaf

Qar,net,p (MJ/kg)

A B C D E F G H I J K L M N O P Q

880-905 880-905 860-890 880-910 892-915 900-915 885-900 900-910 890-900 890-920 875-885 880-900 890-915 880-900 875-895 900-930 860-890

1.21 1.23 1.23 1.22 1.21 1.26 1.31 1.24 1.25 1.28 1.32 1.26 1.26 1.26 1.26 1.25 1.26

17.17 13.74 8.91 7.05 8.72 22.20 8.38 6.79 6.91 8.72 6.30 5.61 5.30 18.01 5.01 27.12 16.3

21.49 20.55 22.51 28.88 30.52 11.25 27.79 35.54 46.65 30.52 29.93 40.58 45.58 13.00 49.59 5.98 19.28

22.76 20.52 19.83 20.80 19.87 21.00 20.10 18.00 17.10 18.84 18.82 18.00 13.13 21.56 15.90 21.39 22.70

layer resistance, surface temperature, residence time, and burnout time are different, resulting in the nonhomogeneous distribution. Bed Temperature. Generally speaking, the increase of the bed temperature can promote the chemical reaction rate and then increase the combustion efficiency. Experiments were carried out for six kinds of coal in three 75 t/h CFB boilers, which are manufactured 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 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 content. 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 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, but, as noted above, the

Figure 6. Relationship between carbon content in fly ash and coal index.

practical experience of CFB boiler in China proves that the carbon content in the fly ash is not as low as expected. Figure 5 also shows that the coal property will influence the carbon content 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 coals, 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 anthracite and lean coal, are usually on the contrary. Operating 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 index 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):

I)

Vdaf Qar,net,p

(1)

And, the relationship between the carbon content in the fly ash and the coal index 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 general 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 the contrary, the CFB boilers burning 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 analyze 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 on the gas-solid mixing. The oxygen concentration and solid suspension

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Figure 9. Schematic distribution of oxygen zones.

Figure 7. Oxygen concentration at various heights above the distributor.

Figure 10. Carbon content in the fly ash by improvement of secondary air nozzles.

Figure 8. Solid suspension density at 4.5 m above the distributor.

density above the secondary air in the furnace were investigated in one 75 t/h boiler.9 Measurement results are shown in Figures 7 and 8. In the center of the furnace, the solid suspension density is relatively low, while in the wall region, it is higher because of the solid down flow near the wall. The oxygen concentration in the center region is almost close to zero with the oxygen-rich zone near the wall. This oxygen distribution occurs at almost all heights above the secondary air nozzles. The distribution of the lean and rich oxygen zones could be shown schematically in Figure 9. The lean oxygen core was observed above the secondary air, because the secondary air could not penetrate sufficiently deeper in the furnace. This would evidently influence the carbon burnout of fine particles in the core region. To improve the mixing of the secondary air, the design of the nozzles was modified in the same boiler. A dramatic decrease of the carbon content in the fly ash occurred while the velocity of the secondary air was increased, shown in Figure 10.10 Increasing the velocity and rigidity of the secondary air could extend the penetration depth and induce more

oxygen into the furnace center. Better gas-solid mixing will weaken the lean oxygen core area and increase char combustion efficiency. The excess air ratio also influences the carbon content in the fly ash. The higher the excess air ratio is, the lower the carbon content in the fly ash is, shown in Figure 11, where other operation parameters are kept constant.6 Operation experiences suggest that at least 20% excess air ratio is required for higher combustion efficiency. When the coal is difficult to burn out, 25% excess air ratio is suggested. When the fluidizing air velocity in furnace is constant, the change of bed inventory will influence the solid density along furnace as well as the gas-solid mixing. The bed inventory should have influence on the carbon content in the fly ash. But the influence tendencies were found to be ambivalent in different experiments. Figure 12 shows the experiments carried out in three 75 t/h CFB boilers,6 the carbon content in the fly ash decreases with bed inventory increasing, while in one 465 t/h CFB boiler, the influence is on the contrary, shown in Figure 13. The bed inventory is corresponding with the bed pressure drop, and other operating conditions were kept as constant as possible. Such ambivalent phenomena may be explained in two aspects. With the bed pressure drop increasing, the solid concentration in the furnace increases and more clusters

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Figure 11. Effect of excess air ratio on carbon content in fly ash.

Figure 12. Effect of bed inventory on carbon content in the fly ash (for three 75 t/h units).

Figure 13. Effect of bed inventory on carbon content in fly ash (for one 465 t/h unit).

form. The formation and descent of more clusters would increase the internal circulation and extend the residence time of char particles, which increases char combustion efficiency. On the other hand, denser solid

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Figure 14. Char reactivity with residence time.

distribution would restrict the penetration of the secondary air and not be beneficial to gas-solid mixing, as described above. Because of the complicated behavior, the experiment restriction on site, it could not be understood clearly. More reliable experiments are needed to understand the effect of the bed inventory on carbon content in the fly ash. Another important phenomenon is noticed from Figures 5, 11, and 12. For each coal, the carbon content in fly ash varies with bed temperature, bed pressure, and excess air ratio, respectively. But the relative relations of magnitude for the six coals have almost kept the same tendency, corresponding to the coal rank. For example, the carbon content for coal C1 is always the least under each condition. That is another evidence to confirm the important effect of coal property. Residence Time and Char Deactivation. The analysis of the carbon in the fly ash particles shows that the fine char particles could be divided into two groups according to their reactivity.2 One group is “fresh” char particles with high reactivity and a certain amount of volatile content. On the contrary, the other group of char particles has experienced sufficient combustion time both in the furnace and in the cyclone, with nearly no volatility. The char particles with the same size in the fly ash may have different “age” and different formation sources. The “fresh” char particles may come from the fine coal particles, from char primary and secondary fragmentation, respectively. These char particles have relatively short residence time in the furnace. The “old” particles probably come from the large coal particles by the attrition process11 and will experience longer residence time in the furnace before the large particles reduce into size so small to be elutriated away from cyclone. The char deactivation during heat treatment has been investigated in previous research.12 The char reactivity is a function of residence time and pyrolysis temperature. It decreases rapidly at the first stage and then changes more slowly, approaching an asymptotic value, which relates to the heating temperature and coal type. As shown in Figure 14, a lower constant reactivity value was obtained under higher pyrolysis temperature. Under the CFB combustion condition (around 1173 K), the pronounced deactivation of the char occurs with

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the heat treatment time of 10-30 min. In addition, the temperature of char particles is generally higher than the surrounding gas temperature, which might further enhance the deactivation. Char deactivation nearly does not occur in “fresh” particles because of their short residence time in the CFB loop. Therefore, the lowreactivity char particles in the fly ash are “old”, probably originated from large coal particles. Recycling the ash from an electrostatic precipitator (ESP) to burnout “fresh” particles can improve the overall combustion efficiency,13 while, for the low-reactivity char particles, even if they are fed back to the furnace, their burnout will not increase much. It can be deduced that if the fraction of the large coal particles are decreased to a certain extent, the inert fine char in the fly ash would be reduced, which will promote the char burnout. Cyclone Performance. As discussed above, the residence time of char particles in CFB loop directly influences the reactivity of the char and its burnout, so most people believe the cyclone is the key to improve carbon burnout. At a lower combustion temperature, the reaction rate is lower than that in PC boilers. The burnout of char particles requires longer residence time. The cyclone efficiency controls the particle residence time in the CFB loop, especially that of the fine particles. The majority of the fly ashes have the size of less than 100 µm, in agreement with the founding in ref 14. The cut size of the large cyclones used in CFB boilers is (13) Tao, S. Discussion on Combustion Efficiency in CFB Boilers. Sichuan Power Technol. 1998, 6, 46-49 (in Chinese). (14) Dallas, W.; Thorpe, I. A. An Update of Operating Experiences Burning Petroleum Coke in a Utility Scale CFB the NIS-CO Cogeneration Project. In Proceedings of the 14th International Conference on Fluidized Bed Combustion, Vancouver, British Columbia; Preto, F. D. S., Ed.; American Society for Mechanical Engineers: Fairfield, NJ, 1997; pp 57-70.

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normally about 30-50 µm. It means that 50% of these sized particles flee out the cyclone in each circle. This is one reason the carbon content is highest in the size range of 30-50 µm fly ash, shown in Figure 5. It is easily understood that if increasing 1% collection efficiency for the fine char particles, they will have more opportunity to stay in the CFB loop longer time, which will increase the burnout of the fine chars except for the deactivated chars. So the cyclone efficiency, especially the cut size, will greatly influence the carbon content in the fly ash. Conclusions The higher carbon content in the fly ash directly decreases the combustion efficiency of CFB boilers. The carbon content in the fly ash depends on the coal rank strongly. The coal index, defined by the ratio of the volatile content to the coal heat value, is a useful parameter for analyzing the char burnout. Bad gas-solid mixing in the furnace is another important reason for the higher carbon content in the fly ash. The secondary air should have enough momentum to penetrate deeply into the furnace to avoid the lean oxygen core region. Some chars in the fly ash are deactivated during combustion of large coal particles and has very low carbon reactivity. The generated fine inert char particles by attrition of large coal particles could not easily burn out even with the fly ash recirculation. The fraction of large coal particles in feeding coals should be reduced during fuel preparation process. Acknowledgment. Financial support by National Natural Science Foundation of China (Grant No. 50406002) is greatly acknowledged. EF049678G