h Oil-Shale-Fired

gas-solid two-phase flow of the dilute phase has high particle concentration and turbulent motion, which has an important effect on the combustion, he...
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Ind. Eng. Chem. Res. 2006, 45, 4329-4334

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GENERAL RESEARCH Flow Structure and Combustion Characteristic of 65 t/h Oil-Shale-Fired Circulating Fluidized Bed Riser. 1. Dense Phase Xiumin Jiang, Xiangxin Han,* Zhigang Cui, and Lijun Yu Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China

In this paper, an industrial hot experiment was done for studying the flow structure and combustion characteristic of a 65 t/h oil-shale-fired circulating fluidized bed (CFB) dense phase. The operational characteristics of this boiler and the particle properties of crushed oil shale and circulating ash were obtained. The particle-size distribution and carbon content of solid particles within the dense phase were investigated along the horizontal direction under different boiler loads. The experimental results may be used as a reference for modeling the flow structure, combustion, heat transfer, and abrasion of the oil-shale-fired CFB dense phase. At the same time, the experimental data obtained lay a foundation for the scale-up of oil-shale-fired CFB boilers. Introduction The combustion technology for oil shale mainly involves pulverized fuel furnaces, bubbling fluidized beds, and circulating fluidized beds (CFBs). Because of its very low pollution emissions and good adaptability to low-grade fossil fuels, the oil-shale-fired CFB combustion technology has been widely accepted as the most economic and cleanest of all the oil-shalefired utilization modes.1,2 The study of complex gas-solid flows encountered in a CFB riser requires a precise and detailed analysis of the local behaviors of both phases. Depending on the solid particle concentration in the flue gas, the CFB riser is divided into two flow regimes along the riser height: (a) dense phase below the secondary air level and (b) dilute phase above the secondary air level. The dense phase is similar to the fluidized regime of a bubbling fluidized bed in some characteristics: (1) the gassolid two-phase motion is very intense; (2) a portion of fine particles are entrained into the dilute phase by the flue gas flow; and (3) some of coarse particles are cast into the dilute phase from the bed surface, due to the breakage of air bubbles. The gas-solid two-phase flow of the dilute phase has high particle concentration and turbulent motion, which has an important effect on the combustion, heat transfer, attrition, and design of the CFB riser. Many researchers have studied the flow structure and concentration distribution of CFB risers using laboratoryscale cold testing blocks.3-5 Some mathematical models have been developed to characterize and quantify the gas-solid flow structure and combustion characteristic of CFB risers.6-9 However, because of the complexity of many characteristics, such as flow structure, combustion characteristic and heat transfer, it is very difficult to describe them accurately, and the industrial test data about them are inadequate as well. Thus far the hot industrial test studies of the flow structure and combustion characteristic of oil-shale-fired CFB risers have never been reported. The authors have previously reported the flow structure and combustion characteristic of a 65 t/h oil-shale-fired CFB dilute * Corresponding author. Tel: +86-21-54742835. Fax: +86-2154742835. E-mail: [email protected].

Figure 1. Schematic diagram of a 65t/h oil-shale-fired CFB boiler. All dimensions are in mm. (1) Fuel feeder, (2) furnace, (3) superheater, (4) evaporation tube bundle, (5) cyclone, (6) ash hopper, (7) economizer, (8) loop seal, and (9) air preheater. T1∼T6: Temperature measurement positions.

phase under hot operation.10 The objective of this work is to study the flow structure and combustion characteristic of the 65 t/h oil-shale-fired CFB dense phase under hot operation. Experimental Section Experimental Setup. In this paper, the experimental setup is a 65 t/h oil-shale-fired CFB boiler which has been introduced by Han et al.10 Figure 1 illustrates the schematic diagram of the CFB boiler. Along the flow direction of hot flue gas, the boiler in turn contains a furnace with membrane waterwalls, a superheater, an evaporation tube bundle, two cyclones at moderate temperature, an economizer, and an air preheater. The cyclones, ash hoppers, loop seals, and standpipes are applied

10.1021/ie060005q CCC: $33.50 © 2006 American Chemical Society Published on Web 05/16/2006

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Figure 2. Sampling positions of solid particles within the dense phase. All dimensions are in mm.

Figure 3. Three-dimensional maximum scaling relation of crushed oil shale. Table 1. Proximate and Ultimate Analysis of Oil Shale proximate analysis (ara) moisture volatile matter ash fixed carbon net calorific value a

14.86 wt % 25.03 wt % 54.78 wt % 5.33 wt % 8033 kJ‚kg-1

ultimate analysis, wt % (ara) C H O N S

21.14 2.58 5.16 0.48 1.0

ar: received basis.

Table 2. Physical Properties of Crushed Oil Shale Particles into the Dense Phase range (mm)

mass fraction (%)

10

20 16 15 17 18 14

Other Properties equivalent diameter of particle, dp (mm) apparent density of particle, Fs (kg/m3) bulk density of packed bed, Fbulk (kg/m3) minimum fluidizing velocity, Umf (m/s) terminal velocity, Ut (m/s) voidage at minimum fluidization, mf

2.69 2100 1100 1.571 19.44 0.57

in the 65 t/h CFB boiler, to form a fly ash circulating loop together with the riser. An ash cooler is adopted to retrieve the heat of hot slags discharged from the bottom of the CFB furnace. The furnace height is 17 m, and the cross-section area is 23 m2 in the dilute phase and gradually decreases upstream to 10.47 m2 at the air distributor to achieve a high flow velocity of flue gas, which can make bed material particles be fluidized

efficiently. In addition, the expansion of the riser cross section from 10.47 to 23 m2 also promotes the internal solids circulation along the membrane walls, since the solids will decelerate as they pass from the zone of narrower cross section to larger cross section, which has a positive effect on solids-to-wall heat transfer coefficients. The pressure and temperature of main steam is 5.29 MPa and 450 °C, respectively. The furnace temperature is kept in the range of 850-900 °C. The air velocity at the nozzle hole of air caps is 58.9 m/s, the fluidizing velocity under hot operation is 4.5-6.5 m/s, and the circulating ratio is 6. As used herein, the term “circulating ratio” is a ratio of circulating ash to feed fuel. In addition, there are four sampling positions of solid particles for this experiment along the same horizontal level above the air distributor. The vertical distance between the horizontal level and the air distributor is 420 mm, just as shown in Figure 2. Oil Shale. The fuel of the 65 t/h boiler is oil shale obtained from Huadian, China. Table 1 gives its proximate and ultimate analysis. The physical properties of crushed oil shale particles dropped into the dense phase are shown in Table 2. Figure 3 gives the three-dimensional maximum scaling relation of crushed oil shale particles, showing that crushed oil shale particle has a platy structure. Due to the platy structure, the windward plane of oil shale particles will sharply change after being dropped into the dense phase: the fluidization status is steady when the maximum projection plane is windward; however, if the minimum projection plane is windward, these particles cannot be fluidized and will sink down. In particular, the phenomenon is more evident near the boundary wall of the furnace. To avoid the channel flow resulting from the platy structure, the fluidizing velocity under hot operation is increased to 4.5-6.5 m/s. Experimental Procedure. In general, the flow structure and combustion characteristic of the CFB dense phase are mainly dependent on the operational parameters of the boiler and the characteristics of oil shale particles and circulating ash. For further studies of the flow structure and combustion characteristic of the 65 t/h CFB dense phase, six boiler loads (30%, 50%, 70%, 100%, 100%, and 110% without secondary air) were determined in advance without secondary air. Under each boiler load, the operational parameters were measured and the solid particles within the dense phase were sampled and analyzed. Results and Discussion Operational Parameters. Figure 4 shows the quantitative relationship of operational parameters to boiler loads. The primary air ratio is 0.6-0.64 for the former five boiler loads, but the secondary air ratio is 0 for the sixth boiler load. In the boiler load of 50%-110%, the bed temperature of the dense phase is maintained in the range of 850-900 °C for the following reasons: (1) Although N2O concentration in the flue gas is high at such bed temperature,11 the thermal NOx concentration can greatly decrease. (2) The deformation temperature, softening temperature, and fusion temperature of Huadian oil shale are 1077, 1153, and 1220 °C, respectively. Thus, oil shale ash cannot agglomerate at such bed temperature and may be used as a good raw building material.12-14 (3) At such bed temperatures, carbonates of oil shale may calcine to CaO, which can react with SO2, producing CaSO4. The ash analysis of oil shale shows that CaO content within oil shale is 8.1 wt %; based on the ultimate analysis of oil shale, the sulfur content is 1.0 wt %. Consequently, its molar Ca/S ratio is up to 4.63, which is sufficient to reduce the SO2

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Figure 4. Quantitative relationship of operational parameters to boiler loads.

Figure 5. Temperature distribution over the fly ash circulating loop under six boiler loads.

concentration in the flue gas to acceptable levels without additional limestone dropped into the combustor. Figure 5 shows the temperature distribution over the fly ash circulating loop, and the temperature measurement positions 1-6 are indicated in Figure 1. Figure 6 presents the particlesize distribution of the circulating ash collected from the loop seal under different boiler loads. According to Figure 6, the

Figure 6. Particle-size distribution of circulating ash within the loop seal under different boiler loads

particle-size distribution of the circulating ash within the loop seal is wide. With a wide size range of particles, the fine solids often help improve the fluidization characteristic of the coarser solids within the loop seal. Even at a very low fluidizing velocity in the recycle chamber of the loop seal there is a high circulating rate.15 The carbon content of the circulating ash from the loop seal is presented in Figure 7, which shows that the average

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Figure 7. Carbon mass content distribution of circulating ash within the loop seal under different boiler loads.

carbon content of the circulating ash is less than 0.8% under different boiler loads. Figures 4-7 give important environmental conditions under which the flow structure and combustion characteristic of the dense phase are studied as follows. Particle Characteristics of Solid Particles within the Dense Phase. When a CFB boiler is in hot operation, the particle-size distribution of solid particles within the dense phase is greatly different from that of fuels fed into the furnace, due to fragmentation and abrasion. Consequently, it is more significant for the design and operation of the CFB boiler to directly study the particle-size distribution of solid particles within the dense phase. Figure 8 gives the particle-size distribution of solid particles within the dense phase, based on sieve measurements. It was mentioned above that a crushed oil shale particle has a platy structure. The particle size being reported in Figure 8 is the width of solid particles. (1) By comparing the particle-size distribution of solid particles within the dense phase with that of crushed oil shale particles dropped into the dense phase, it can be seen that the

mass content of d < 1 mm particles largely increases from 20 to 35 wt % and that of d > 10 mm particles evidently decreases from 14 to 5 wt %, due to the attrition and combustion of oil shale particles. The mass content of other particles also changessmore or less. (2) The difference between four sampling positions is not marked in the particle-size distribution under the same boiler load on the whole, which shows that most of the bed material particles can be uniformly fluidized. (3) In CFB boilers, the fluidizing velocity, bed temperature, and circulating times of solid particles change with the boiler load, which results in the particle-size distribution of the solid particles within the dense phase changing accordingly with the boiler load. On the basis of Figure 8, the mass content of fine particles under a high boiler load is less than under a low boiler load, because an increase in the fluidizing velocity with the boiler load causes more fine particles to be entrained into the dilute phase. Carbon Content Distribution of Solid Particles within the Dense Phase. The carbon content distribution of solid particles within the dense phase is related to bed temperature, fluidizing velocity, fluidization quality, terminal velocity of solid particle, combustion portion, feed fuel position, uniformity of air distribution, etc. Figure 9 shows the carbon content of solid particles obtained from the four sampling positions under 50% and 100% boiler loads. Figure 10 represents the carbon content of solid particles collected from the sampling positions 1 and 3 under different boiler loads. On the basis of Figures 9 and 10, the following characteristics are described: (1) From Figure 11, the separation efficiency of the cyclone of the 65 t/h CFB boiler is relatively low for 0-0.1 mm particles, and most of these particles will pass through the cyclone into the back pass, which results in a short residence

Figure 8. Particle-size distribution of solid particles within the dense phase under different boiler loads.

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Figure 9. Carbon mass content of solid particles from different sampling positions.

Figure 10. Carbon mass content of solid particles under different boiler loads.

allows a large amount of solid particles to participate in circulating combustion. (5) Although the carbon content of particles from the same sampling position decreases with increased boiler loads, the difference between the four sampling points is small in the carbon content of particles under the same boiler load, which shows that the cross-mixing, combustion, and fluidization quality of the 65 t/h CFB dense phase are good. Conclusions

Figure 11. Separation efficiency of the cyclone.

time for them in the furnace. So, whether under high or low boiler load, the carbon content of 0-0.1 mm particles is always high. (2) Although 0.1-1 mm particles can be entrained away from the furnace into the cyclone by gas, they can be effectively captured by the cyclones and be returned to the furnace to be involved in the circulating combustion. The residence time of these particles is thus very long. So, the carbon mass content of them is very low. (3) The high ash content of oil shale makes the mass transfer resistance of the ash layer of oil shale particles increase gradually with particle size, which lengthens the burn-out time of the fixed carbon. So, the carbon content of d > 1 mm particles increases with particle size. (4) In the same sampling position, the carbon content of particles decreases with increased boiler load. The causes are that (a) combustion characteristics, fluidization quality, mixing degree of gas-solid, and heat transfer under high boiler loads are better than those under low boiler load and (b) an increase in the circulating time of solid particles with greater boiler load

In this paper, the flow structure and combustion characteristic of a 65 t/h oil-shale-fired CFB dense phase were studied under six boiler loads. The operational characteristics of the boiler and the particle properties of crushed oil shale and circulating ash were investigated. The particle-size distribution and carbon content of solid particles within the dense phase were obtained along the horizontal direction under different boiler loads. The following conclusions can be drawn from the experimental results. (1) This work proves that burning oil shale in circulating fluidized beds is a perspective use mode for potential oil shale resource. (2) Oil shale has some characteristics, such as platy structure, high ash content, and volatiles being the main combustible matter of oil shale, which are relevant to the design and operation of oil-shale-fired CFB boilers. The 65 t/h CFB boiler has taken effective measures to deal with these characteristics, which are helpful for the scale-up of oil-shale-fired CFB boilers. (3) The particle-size distribution and carbon content of solid particles within the dense phase may be used as a reference for modeling the flow structure and combustion of the dense phase, because the experimental data have been obtained in an industrial hot oil-shale-fired CFB boiler.

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Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 50476021). Literature Cited (1) Jiang, X.; Liu, D.; Chen, H.; Zheng, C.; Qin, Y. Experimental Investigation on Oil Shale Circulating Fluidized Bed Boiler. Oil Shale 2001, 18, 73. (2) Paist, A. New Epoch in Estonian Oil Shale Combustion Technology. Oil Shale 2004, 21, 181. (3) Van den Moortel, T.; Azario, E.; Santini, R.; Tadrist, L. Experimental Analysis of the Gas-Particle Flow in a Circulating Fluidized Bed Using a Phase Doppler Particle Analyzer. Chem. Eng. Sci. 1998, 53, 1883. (4) Guo, Q.; Werther, J. Flow Behaviors in a Circulating Fluidized Bed with Various Bubble Cap Distributors. Ind. Eng. Chem. Res. 2004, 43, 1756. (5) Zhou, H.; Lu, J.; Lin, L. Turbulence Structure of the Solid Phase in Transition Region of a Circulating Fluidized Bed. Chem. Eng. Sci. 2000, 55, 839. (6) Pugsley, T. S.; Berruti, F. Predictive Hydrodynamic Model for Circulating Fluidized Bed Risers. Powder Technol. 1996, 89, 57. (7) Andrews IV, A. T.; Loezos, P. N.; Sundaresan, S. Coarse-Grid Simulation of Gas-Particle Flows in Vertical Risers. Ind. Eng. Chem. Res. 2005, 44, 6022. (8) Adanez, J.; Gayan, P.; Grasa, G.; De Diego, L. F.; Armesto, L.; Cabanillas, A. Circulating Fluidized Bed Combustion in the Turbulent

Regime: Modelling of Carbon Combustion Efficiency and Sulphur Retention. Fuel 2001, 80, 1405. (9) Hyre, M. R.; Glicksman, L. R. Axial and Lateral Solids Distribution Modeling in the Upper Region of Circulating Fluidized Beds. Powder Technol. 2000, 110, 98. (10) Han, X.; Jiang, X.; Cui, Z. Flow Structure and Combustion Characteristic of 65t/h Oil Shale-fired Circulating Fluidized Bed Riser 2: Dilute Phase. Chem. Eng. Sci. 2006, 61, 2533. (11) Han, X.; Jiang, X.; Liu, J.; Wang, H. Grey Relational Analysis of N2O Emission from Oil Shale-fired Circulating Fluidized Bed. Oil Shale 2006, in press. (12) Baum, H.; Bentur, A.; Soroka, I. Properties and Structure of Oil Shale Ash Pastes: 2. Mechanical Properties and Structure. Cem. Concr. Res. 1985, 15, 391. (13) Baum, H.; Soroka, I.; Bentur, A. Properties and Structure of Oil Shale Ash Pastes: 1. Composition and Physical Features. Cem. Concr. Res. 1985, 15, 303. (14) Ish-shalom, M.; Bentur, A.; Grinberg, T. Cementing Properties of Oil Shale Ash: 1. Effect of Burning Method and Temperature. Cem. Concr. Res. 1980, 10, 799. (15) Cheng, L.; Basu, P. Effect of Pressure on Loop Seal Operation for a Pressurized Circulating Fluidized Bed. Powder Technol. 1999, 103, 203.

ReceiVed for reView January 3, 2006 ReVised manuscript receiVed April 2, 2006 Accepted April 19, 2006 IE060005Q