Experiment and Grey Relational Analysis of CWS Spheres

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Energy & Fuels 2007, 21, 1924-1930

Experiment and Grey Relational Analysis of CWS Spheres Combustion in a Fluidized Bed Hui Wang,† Xiumin Jiang,*,† Jianguo Liu,† and Weigang Lin‡ School of Mechanical Engineering, Shanghai Jiao Tong UniVersity, Minhang District, Shanghai 200240, China, and The State Key Laboratory of Multiphase and Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China ReceiVed January 25, 2007. ReVised Manuscript ReceiVed April 29, 2007

In order to study the combustion of coal water slurry (CWS) in fluidized bed boilers, artificial CWS droplet spheres were used for simulation of the spheres formed from CWS droplets which fall from the furnace top to the bed. The artificial spheres were introduced to a bench-scale fluidized bed furnace. Quartz sand was used as the bed material. The influence of the operation conditions (e.g., bed temperature, superficial gas velocity, and bed height) on the combustion characteristics was investigated. The bed temperatures were varied at 650, 750, 850, and 950 °C. The gas velocities were in a range of fluidization numbers W (defined as U/Umf) of 3, 3.5, 4, and 4.5. The bed heights were varied 30, 50, 70, and 90 mm. The CWS spheres were taken out at five residence times (15, 30, 45, 60, and 75 s). The mass ratio of the residue fixed carbon to parent fixed carbon was calculated for studying the influential factors. Under the reference conditions, it is shown that the burnout time is less than 150 s. The grey relational analysis was used to study the degree of relative importance of the influential factors. The results showed that the influence of the bed height is the least, the fluidization number has the greatest influence in the early and later stages, and the bed temperature contributes most in the intermediate stages.

Introduction Coal is the major energy resource in China. In contrast, the resources of petroleum and natural gas are relatively scarce. Thus, China started to import oil in 1993, and the amount of imported crude oil exceeded 100 million tons in 2004. Increased demand and a limited reserve of petroleum make it urgent for China to seek alternatives to oil. Coal water slurry (CWS) is one such alternative, especially as a substitute of oil burned directly in boilers. CWS consists of 65-70% finely ground coal suspended in 30-35% water on a mass basis, together with 1% additives to improve its stability. In such a way, CWS behaves as a liquid fuel that can be transported, stored, and burned in a manner similar to heavy fuel oil. Thus, it is a good candidate as a substitute for oil. Among the combustion technologies, fluidized bed combustion is attractive for burning CWS due to its fuel flexibility and the possibility to achieve an efficient and clean operation.1 The previous investigations show that carbon fines are formed to a large extent by the attrition and fragmentation of coarse particles in a fluidized bed combustor.2 The attrition behavior of a number of alternative fuels was studied.3-6 Attrition and fragmentation appears to be far more severe for high volatile fuels than low volatile ones. * Corresponding author. Tel.: 86-21-3420-5681. Fax: 86-21-3420-5681. E-mail: [email protected]. † Shanghai Jiao Tong University. ‡ Chinese Academy of Sciences. (1) Scala, F.; Chirone, R. Fluidized Bed Combustion of Alternative Solid Fuels. Exp. Therm. Fluid Sci. 2004, 28, 691-699. (2) Gulyurtlu, I.; Reforco, A.; Cabrita, I. Fluidized Combustion of Corkwaste. Proceedings of the 11th International Conference on Fluidized Bed Combustion; ASME: New York, 1991; pp 1421-1424. (3) Arena, U.; Chirone, R.; Salatino, P. The Fate of Fixed Carbon during the Fludized-Bed Combustion of a Coal and Two Waste-Derived Fuels. Proc. Combust. Inst. 1996, 26, 3243-3251.

This propensity indicates the importance of the devolatilization period, during which either highly porous, friable chars or a multitude of fragments of fines were formed in the combustion process.1 Miccio et al. have performed a series of works on CWS injection to study the interaction of fuel-water slurries (FWS) with a hot fluidized bed under combustion7,8 and inert conditions.8,9 Studies on the fragmentation of coal particles in a fluidized bed show that the main reason for the fragmentation is the primary fragmentation, and that the release of the volatile matter can drastically influence the degree of fragmentation of coal particles.10 Due to its high water content and the evaporation of water, in addition to devolatilization and combustion, the fragmentation of CWS in fluidized bed combustion becomes even more complicated. The basic mechanism of influential factors on the fragmentation is not yet clear, partly due to the limitation of (4) Salatino, P.; Scala, F.; Chirone, R. Fluidized Bed Combustion of a Biomass Char: The Influence of Carbon Attrition and Fines PostCombustion on Fixed Carbon Conversion. Proc. Combust. Inst. 1998, 27, 3103-3110. (5) Scala, F.; Salatino, P.; Chirone, R. Fluidized Bed Combustion of a Biomass Char (Robinia pseudoacacia). Energy Fuels 2000, 14 (4), 781790. (6) Scala, F.; Chirone, R.; Salatino, P. Fluidized Bed Combustion of Tyre Derived Fuel. Exp. Therm. Fluid Sci. 2003, 27 (4), 465-471. (7) Miccio, M.; Arena, U.; Massimilla, L.; Maresca, A.; DeMichele, G. Combustion of Fuel-Water Slurries Injected in a Fluidized Bed. AIChE J. 1989, 35 (12), 2040-2042. (8) Miccio, M.; Massimilla, L. Fragmentation and Attrition of Carbonaceous Particles Generated from the Fluidized Bed Combustion of FuelWater Slurries. Powder Technol. 1991, 65 (1-3), 335-342. (9) Miccio, F.; Miccio, M.; Okasha, F. Formation Rates of Characteristic Carbon Phases during Fuel-Water Slurry Injection in a Hot Fluidized Bed. Powder Technol. 1997, 91 (3), 237-251. (10) Zang, H.; Cen, K.; Yan, J.; Ni, M. The Fragmentation of Coal Particles during the Coal Combustion in a Fluidized Bed. Fuel 2002, 81 (14), 1835-1840.

10.1021/ef070042+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/14/2007

CWS Spheres Combustion in a Fluidized Bed

Energy & Fuels, Vol. 21, No. 4, 2007 1925

Table 1. Proximate and Ultimate Analysis of CWS proximate analysis(as received, %w)

ultimate analysis(as received, %w)

moisture Mar/%

ash Aar/%

volatiles Var/%

net heat value Qar,net/(kJ/kg)

carbon Car

hydrogen Har

oxygen Oar

nitrogen Nar

sulfur Sar

32.9

5.64

43.758

18 877

50.57

3.27

6.13

0.93

0.56

Table 2. Size Distribution of Quartz Sand as Bed Material size range/mm mass percent/%

0.88-1.6 7.5

1.6-2.5 47.5

2.5-3.01 35

3.01-4 10

the available measuring methods. For this case, grey relational analysis is an option to analyze the effect of operation parameters on the fragmentation and eventually the combustion characteristics without detailed knowledge of the process. Grey relational analysis is a part of grey system theory, which is suitable for solving the complicated inter-relationships between multiple factors and variables.11 Grey relational analysis provides a useful tool to deal with the problems of limited and superficial ruleless data processing, for searching primary relationships among the influential factors and determining important factors that significantly affect the defined objectives.12 According to the complexity of fluidized bed combustion, grey relational analysis is an appropriate method to analyze the influence of operational parameters on the combustion performance. Experimental investigations of attrition and fragmentation and with respect to the variation of sphere shape in the progress of the combustion of CWS in a fluidized bed is very limited, and application of the grey relational analysis for analysis of the influence of operational parameters is not found in the literature. The objective of this work is to simulate the combustion behavior in a fluidized bed and to study the influential degree of operational parameters, trying to find the sequence of degrees. Experimental Section Materials. The CWS used in this study is obtained from Shandong province, China. The parent coal is Datong bituminous coal. The characteristics of the CWS are shown in Table 1. Quartz sand is used as the bed material. Its size distribution on a mass basis is shown in Table 2. The mass average diameter is 1.629 mm. Miccio et al.7-9 found from FWS-injecting combustion that carbon is allocated in the fluidized bed as three phases: an A phase, made of carbon aggregates usually larger than 1 mm that may include inert bed particles, an S phase formed by carbon fines (less than 30 µm) deposited on individual sand particles, and an F phase made of carbon fines (less than 400 µm). Being a part of CWS fluidization-suspension combustion technology, CWS is granulated to 3∼10 mm drops in the industrial fluidized bed boiler, which is bigger than that of injection. To make the study of the interaction of CWS with the bed material easier, the experiments in this paper begin with CWS spheres being put into the fluidized bed, and the granulating and falling processes before it were simulated by preparing CWS spheres with a Muffle furnace. The preparation procedure is described as follows: A Muffle furnace was heated to a preset temperature. Various preset temperatures were applied in the preparation, which are 650, 750, 850, and 950 °C. When the preset temperature was reached, a stainless steel container, which is filled with quartz sand, was inserted into the Muffle furnace. As soon as the temperature in the container approached that in the furnace, the container was taken (11) Mora´n, J.; Granada, E.; Mı´guez, J. L.; Porteiro, J. Use of Grey Relational Analysis to Assess and Optimize Small Biomass Boilers. Fuel Process. Technol. 2006, 87 (2), 123-127. (12) Han, X.; Jiang, X.; Liu, J.; Wang, H. Grey Relational Analysis of N2O Emission from Oil Shale-Fired Circulating Fluidized Bed. Oil Shale 2006, 23 (2), 99-109.

Table 3. Proximate Analysis of Fresh CWS Sphere of Four Different Temperatures temperature (°C)

moisture (%)

volatiles (%)

fixed carbon (%)

ash (%)

650 750 850 950

2.35 2.22 1.69 1.58

33.04 32.04 31.38 29.79

55.85 56.97 58.30 59.21

8.76 8.77 8.63 9.42

Table 4. Scheme of Experiment factor

variable range

bed temperature, °C fluidization number W bed height (unexpanded) H/mm residence time, s

650, 750, 850, 950 3, 3.5, 4, 4.5 30, 50, 70, 90 15, 30, 45, 60, 75

out of the furnace and 10 CWS drops were distributed onto the hot quartz sand. The process was controlled in such a way that the CWS drops formed smooth spheres with a diameter range of 8.5-9 mm with no sand in them. The photograph of CWS spheres is shown in Figure 1. Then, they were put into a desiccation device until they were cooled to the ambient temperature. These steps are used to simulate the process of CWS drops dripping from the furnace top to the hot bed material and releasing their water content partly, and maybe some volatile matter with it. A proximate analysis of fresh CWS spheres is shown in Table 3. Apparatus. The combustion experiments were carried out in a bench-scale fluidized bed combustor, which is schematically shown in Figure 2, and its photograph is shown in Figure 3. The setup consists of five parts: an air preheater, a dense bed with an inner diameter of 51 mm and a height of 200 mm, a freeboard with an inner diameter of 83 mm and a height of 615 mm, a cyclone, and a bag-type gas-solid separator. Air is provided by an air compressor controlled by a pressure regulator and a rotameter. Three independently controlled heating elements are used for the air preheater, dense bed, and dilute sections. The combustor is insulated by refractory materials. The air preheater with a maximum power of 5 kW can heat up the air to 800-950 °C. With the help of the other two heating elements, the air temperature in the furnace can keep a steady value up to 950 °C. The distributor plate is rectangleshaped, made of four layers of stainless steel mesh screen with 200 acceptably sized net openings per square inch. Procedures. The range of parameters in the experiments is listed in Table 4. The procedure of the combustion experiments of the CWS spheres is described as follows: (1) Select 10 prepared CWS spheres, and measure their mass and volume. (2) Feed the quartz sand into the furnace via a feeding entrance (10) of the benchscale fluidized bed shown in Figure 2. Introduce the set air flow, and then heat up the combustor to the desired temperature. As soon as the set temperature is reached, suspend the air. Then, the CWS spheres, prepared at the same temperature as the set temperature, are introduced to the bed quickly via the feeding entrance. Then, the air is fully open again, and the vacuum pump is started immediately. No sooner than the desired residence time is reached, the air flow is switched to a high-purity nitrogen flow. Then, loosen the long bolts fastening the flanges at the distributor to a large enough space, and drag the distributor plate out with all of the quartz sand and CWS spheres on it. (3) All the CWS spheres and fragmental parts were picked up and put into crucibles with lids for further cooling down. The mass, volume and proximate analysis of the samples were measured for calculation of the mass ratio of residual carbon to the carbon content of the introduced fresh CWS spheres.

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Table 5. Experimental Conditions of CWS Spheres in the Fluidized Bed and Some Results mass ratios of residual carbon to the carbon content of fresh CWS sphere % number

bed temperature °C

W

H mm

air flow flux Nm3/h

15 s

30 s

45 s

60 s

75 s

1 2 3 4 5 6 7 8 9 10

650 750 850 950 850 850 850 850 850 850

4 4 4 4 3 3.5 4.5 4 4 4

90 90 90 90 90 90 90 30 50 70

10.98 10.36 9.83 9.40 7.38 8.60 11.06 9.83 9.83 9.83

81.3854 82.5135 80.2046 72.9562 82.3485 76.0677 79.4209 71.8634 75.7318 84.9739

74.3590 68.0003 54.4606 56.4975 68.5568 67.9677 62.0968 59.1932 64.6032 63.0200

59.9699 60.9441 38.8541 38.5006 53.8556 47.0965 42.7936 45.6955 40.4211 49.5809

45.1721 44.3897 20.4214 22.7805 40.2179 39.8408 26.0958 35.7995 39.1679 40.8762

28.5277 26.3692 18.8426 13.5309 34.9150 25.1435 21.0796 31.5033 35.0269 26.9277

A bed temperature of 850 °C, a fluidization number of 4, and a static bed height of 90 mm were treated as reference conditions to design the single-factor-changed experiment scheme of 10 conditions in Table 5. For each condition, CWS spheres are measured five times at 15 s intervals, and 10 fresh ones are used each time. The carbon content of fresh CWS spheres fed into the furnace in every experiment is 55.85∼59.21%. The calculated mass ratios are shown in Table 5, too.

Results and Discussion From the short residence time experiments of 75 s, breakage and gas-phase combustion are the main phenomena. But from the experimental results, some spheres broke and others did not. Primary fragmentation may be considered to be instantaneous as soon as the fuel particles are introduced into the fluidized bed.13 The broken ones split apart as a consequence of thermal stresses and internal overpressures induced by evaporation and devolatilization; the others did not break obviously, maybe because they have a lower residual water and volatile matter content or a not big enough thermal stress, so the effect of the caking properties of their parent coal and additive plays a major role. As the residence time becomes longer, all spheres undergo a coupled process of primary fragmentation upon evaporation and devolatilization, char particle attrition by abrasion, and secondary and percolative fragmentations. A gas-solid reaction occurs, and the burning behavior of the original CWS spheres and their fragments in the bench-scale fluidized bed shows a layer-by-layer inward flaking off combustion. As a result, the CWS spheres taken out are covered by a thin ash layer, which has a gray color and is easily removed. The kernels are hard chars, and some are black hollow spheres with a porous honeycomb structure. Some particles with a diameter of about 1.6 mm are stuck to each other and form agglomerates, but the ash layer bonding particles is weak and easily taken apart. It may be a result of one CWS sphere which broke severely, with only weak bridges left to keep connecting. When the bridges are burned out, the ash layers will be left there, and an agglomerate will be formed. Another explanation is that some parts of different spheres could be stuck to each other. The fragmentations floating and burning on the surface of the bed material may meet and wander at a dead zone or bad fluidized zone. They will burn together and stick to each other. Because experimentation shows that the agglomerates are usually of a spherical shape, the latter explanation is closer to the truth. Or the parts are residual spheres which are subject to attrition by mechanical stresses induced by collisions and surface wear with the bed material or with the internals of the furnace. So they are not necessarily the fragments of CWS spheres. The volume (13) Scala, F.; Chirone, R.; Salatino, P. Combustion and Attrition of Biomass Chars in a Fluidized Bed. Energy Fuels 2006, 20 (1), 91-102.

ratios of the residual sphere to the original sphere at various conditions as a function of the residence time are shown in Figures 4-6, and the change of mass ratios is shown in Figures 7-9. From Figures 4-9, the results of single-factor experiments, changes of volume ratio and mass ratio are most prominent when the bed temperature is 850 °C, the fluidization number is 4, and the bed height is 90 mm. It proves that the conditions taken as a reference are the ones for which CWS spheres have the fastest combustion rate. To quantify the relative contribution of the bed temperature, fluidization number (defined as U/Umf, a ratio of superficial gas velocity to critical fluidization velocity), and bed height to the combustion of CWS spheres, a proximate analysis of the residual CWS spheres retrieved from each experiment was performed. The mass fractions of residual carbon to the residual sphere, that is, the carbon content, are presented in Figures 10-12. The mass fractions of residual carbon to the fresh CWS sphere were calculated and are shown in Table 5. Figure 10 shows that the change of carbon contents at 650, 750, and 850 °C with time is insignificant in a range of 74.8-80.95%. Figure 11 shows a similar trend with variation of the fluidization number. The results indicate that the combustion behavior of CWS spheres is a layer-by-layer inward flaking off combustion because the carbon content of the residual sphere remains nearly constant. From the experimental results, with a decrease of the spheres’ volume with the actions of attrition, fragmentation, and combustion, the char becomes smaller and the ash layer thickness shows a tendency of increasing a little. As an explanation, in the early stages of the experiment, volatile matter releases from the porous structure formed by vapor and, in full contact with oxygen on the spheres’ surface, carries out a kinetic-controlled homogeneous combustion. With time proceeding, because of the fragmentation induced by thermal stress and attrition of the bed material to the outer ash layer, the char can keep in contact with ambient oxygen. But with the emission of volatile matter ending gradually, combustion of the CWS spheres will change from kinetic-controlled to diffusion-controlled heterogeneous combustion. In later stages, the residual ash layer cannot drop off from char in time, so a thicker ash layer is accumulated. When the char burns out, an ash clump will be left. Figure 12 shows that the carbon content fluctuates greatly when the bed height changes. And the experiment using a 90 mm bed height results in the least one, which shows in that condition the CWS spheres keep a relatively good layer-by-layer inward flakingoff combustion. And if the combustion state is good enough, the char of the residual sphere should have a nearly steady carbon content. The effect of the residence time is further studied with the residence time being extended to 150 s. The results are shown

CWS Spheres Combustion in a Fluidized Bed

Energy & Fuels, Vol. 21, No. 4, 2007 1927

Figure 1. Photograph of fresh CWS spheres. Figure 4. Volume ratio vs time curve of all bed temperatures under W ) 4 and H ) 90.

Figure 5. Volume ratio vs time curve of all fluidization numbers under 850 °C and H ) 90.

Figure 2. Schematic diagram of experimental system.

Figure 6. Volume ratio vs time curve of all bed heights under 850 °C and W ) 4.

Figure 7. Mass ratio vs time curve of all bed temperatures under W ) 4 and H ) 90.

Figure 3. Photograph of experimental system.

in Figures 13-16. Figure 13 indicates that the volume of the sphere decreases rapidly in a short time. As the increased residence time, the decrease rate becomes lower. The relation between the volume and residence time is fitted with an exponential function as

Y ) 0.9201 exp(-x/30.9394) + 0.05029

(1)

where Y is the volume ratio of the residual sphere to the original sphere, and x is the residence time. From the mass loss curve in Figure 14, the slope coefficient before 75 s is bigger than that after 75 s. Considered with Figures 15 and 16, the curves indicate that the mass loss due to fixed carbon combustion before 75 s is relatively slight because

Figure 8. Mass ratio vs time curve of all fluidization numbers under 850 °C and H ) 90.

the carbon content in that range is relatively high, and the ash shell is ground off at the same speed that the carbon burns because the carbon content and ash content keep nearly constant. So, most of the mass loss before 75 s could only be water content and volatile matter. About 75 s later, the combustion

1928 Energy & Fuels, Vol. 21, No. 4, 2007

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Figure 9. Mass ratio vs time curve of all bed heights under 850 °C and W ) 4. Figure 13. Volume ratio curve of the combustion and residence time experiment under reference conditions.

Figure 10. Carbon percentage vs time curve of all bed temperatures under W ) 4 and H ) 90.

Figure 14. Mass ratio curve of the combustion and residence time experiment under reference conditions.

Figure 11. Carbon percentage vs time curve of all fluidization numbers under 850 °C and H ) 90.

Figure 15. Carbon content curve of the combustion and residence time experiment under reference conditions.

Figure 12. Carbon percentage vs time curve of all bed heights under 850 °C and W ) 4.

of fixed carbon enhanced, and the carbon content of residual CWS spheres was lost quickly. But, an increase in ash content demonstrates that attrition of the bed material has a lesser impact on the CWS spheres, see Figure 14. The char was still covered by an ash shell which even got to a climax at 135 s. Then, the rest of the fixed carbon continued to burn, and the ash shell broke under the effect of the bed material and changed into fine particles that could be blown out from the furnace. There was no residue that could be retrieved at 150 s, so in this experiment, the residence time of CWS spheres in the furnace is less than 150 s. Grey Relational Analysis of the Factors Influencing Combustion Performance. Grey relational analysis provides a useful tool to deal with the problems of limited and superficial ruleless data processing. The analysis procedure was described in detail in previous papers.12,14 Grey relational analysis is defined as a quantity analysis of developing trend in various

Figure 16. Ash content curve of the combustion and residence time experiment under reference conditions.

systems, and the calculated relational extent is proportional to the similarity of developing trends; that is, the more similar are the developing trends, the greater is the relational extent. The (14) Wang, H.; Jiang, X.; Liu, J.; Yuan, D. Attrition Experiment and Grey Relational Analysis of Quartzite Particles as Medium Material in fLuidized Bed. Huagong Xuebao (Chin. Ed.) 2006, 57 (5), 1133-1137.

CWS Spheres Combustion in a Fluidized Bed

Energy & Fuels, Vol. 21, No. 4, 2007 1929

Table 6. Results of Grey Relational Analysis grey relational number residence time s

bed temperature (X1)

fluidization number (X2)

unexpanded bed height (X3)

sequence

15 30 45 60 75

0.77146 0.72923 0.65317 0.61349 0.66757

0.78583 0.70501 0.63396 0.59266 0.69252

0.59147 0.63200 0.60039 0.57788 0.58508

X 2 > X1 > X3 X 1 > X2 > X3 X 1 > X2 > X3 X 1 > X2 > X3 X 2 > X1 > X3

way to compare the relational extent among factors is called the relational coefficient or relational grade method. And the magnitude order of the relational grade implies the influence degree order of corresponding factors. The influence of operational parameters on the attrition, fragmentation, and combustion of CWS spheres at a short residence time of 75 s in a fluidized bed was analyzed by a grey relational analysis method. Each of the residual fixed carbon to carbon content of the fresh CWS spheres mass ratios were respectively affirmed after five residence times as a reference sequence, and the bed temperature (°C), fluidization number, and bed height were affirmed as comparison sequences. An equalization approach was used in processing the data mentioned in Table 5 to obtain a dimensionless eigenvector matrix. The index for distinguishability was set to 0.5, and grey relational sequences considering the influence the three operational parameters of bed temperature, fluidization number, and bed height at five residence times had on the ratio of fixed carbon were obtained, see Table 6. From the order, the degree of the effect of comparison sequences on the reference sequence can be worked out. The order in Table 6 demonstrates the degree of influence of each operational parameter on the ratio of fixed carbon. Instances of 15 and 75 s show an order of X2 > X1 > X3, and the others are X1 > X2 > X3, which indicates that the bed height has the least effect on the ratio, the fluidization number has the greatest influence in early and later stages, and the bed temperature contributes most in the intermediate stages. Discussion of the Results by Grey Relational Analysis. As soon as CWS spheres are fed into the furnace, the temperature of the surface will approach the bed temperature quickly. A large temperature gradient along the radius exists. Thus, the effect of bed temperature on the CWS spheres in the initial stages is mainly the result of thermal stress caused by the large temperature gradient. Although releasing some of the water content when CWS spheres are prepared leads to breakage, fragmentation, and a porous structure and makes it easier for heat to enter into the spheres, vigorous releasing of the residual moisture and volatile matter effectively prevents oxygen from reaching the particles’ surface and reacting with the char, so it turns out to be haphazard for heat to enter. In other words, the higher the bed temperature is, the quicker water is released and CWS spheres go through pyrolysis and the bigger the temperature difference between the inner and outer spheres is, which results in more intensive breakage and fragmentation and benefits burning out the CWS spheres. When CWS spheres are fed into the bed, the furnace temperature drops down a little over about 12-15 s; then, it increases steadily again, which indicates that CWS spheres are heated up by the bed material and begin to burn; then, the heat generated can make up for the heat carried away by fluidization air. The residual water content affects the furnace temperature a little eventually. It is evidence that the tremendous thermal storage capacity of a fluidized bed can make sure fuels burn

quickly once they are fed into the bed, and their combustion stability is affected little. That makes the fluidized bed a good choice to burn high-moisture fuels such as CWS. In the experimental range of this work, the residual spheres are relatively light; the drag force from the air and collision force of the bed material make a great deal of sense.15 The larger the fluidization number is, the better the fluidization quality is. Bed material particles and CWS spheres mix better and come into contact more often. Because the maximum heat transfer from collision increases with the diameter of the coal particle and the fluidization velocity,16 the intensive contact between the bed material and CWS spheres results in a high degree of attrition and fragmentation of the CWS spheres, and it will be easier for chars of CWS spheres to come into contact with the hot flue and burn. In the 15 s instance, water vaporization plays a big part. Because carbon combustion will not start until water evaporation is over, some of the bed temperature effect is reduced and the fluidization number effect is made prominent. It appears that the fluidization number has a greater influence on the carbon ratio than the bed temperature does at 15 s. When time proceeds, water evaporation is over, and the CWS spheres obtain a steady layer-by-layer, inward flaking-off combustion. Attrition, fragmentation, and combustion of the CWS spheres reach a dynamic equilibrium. With the superficial ash layer flaking off, fixed carbon starts to burn, and the effect of the bed temperature becomes more prominent. This is because the higher the bed temperature is, the faster the chemical reaction is. The temperature gradient of the porous char decreased quickly, and its internal combustion accelerated. So, the bed temperature plays a bigger role than fluidization mixing in the combustion of fixed carbon in residual CWS spheres. From volume curves in Figures 4-6, the decrease of the volume slows down with time, and the experimental results show that the residual CWS spheres retrieved have a slightly thicker ash layer. It illustrates that, in later stages, the ash shell produced by the combustion of fixed carbon plays a haphazard role in furthering combustion. For a superficial ash shell, enhancing fluidization will be an effective way to destroy it and help heat and oxygen get into the inner core of CWS spheres. During this time, the bed temperature will play a lesser role than the fluidization number. It appears that the influence of the fluidization number is greater than that of the bed temperature again, see Table 6. Conclusion The burning behavior of CWS spheres in a fluidized bed is a layer-by-layer, inward flaking-off combustion. The resulting surface is a grey ash shell easily removed, and the kernel is hard char. The carbon and ash contents of the residual spheres change little over 75 s; thereafter, the carbon content decreases quickly and the ash content does the opposite. The CWS spheres can finish their combustion in a furnace in less than 150 s according to the combustion and residence time experiment. Because of the attrition effect of the bed material, the residual CWS spheres retrieved are covered by a thin layer of ash, which will turn thicker with time. Fragmentation of the CWS spheres is easily observed mainly because the large particle diameter (15) Wu, J.; Zhang, Y.; Li, Q. Visual Research on Big Particle Behavior in Dense-Phase Zone of Circulating Fluidized Bed [J]. Proc. Chin. Soc. Electric. Eng. 2006, 26 (4), 41-45 (in Chinese). (16) Zhou, H.; Lu, J. Coal combustion in a fluidized bed with DEMLES simulation [J]. Proc. Chin. Soc. Electric. Eng. 2004, 24 (12), 212217 (in Chinese).

1930 Energy & Fuels, Vol. 21, No. 4, 2007

results in a large amount of thermal stress. And, the porous structure formed as a result of water evaporation and volatile matter emission weakens the strength of the CWS spheres, too. The influence of operational parameters (bed temperature, fluidization number, and bed height) on the ratio of fixed carbon is analyzed by using a grey relational analysis approach. The

Wang et al.

order of the grey relational gradient shows that the influence of the bed height is the least, the fluidization number has the greatest influence in the early and later stages, and the bed temperature contributes most in the intermediate stages. EF070042+