Ind. Eng. Chem. Res. 2004, 43, 5763-5769
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Effect of Particle Coating on Fluidized-Bed Heat Transfer Jerry Sjo1 sten,† Mohammad R. Golriz,*,† Anders Nordin,‡ and John R. Grace§ Department of Applied Physics and Electronics and Energy Technology and Thermal Process Chemistry, Umea˚ University, SE-90187 Umea˚ , Sweden, and Department of Chemical & Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
Heat-transfer coefficients from a fluidized suspension to immersed stainless steel spheres and an aluminum cylinder were studied in a laboratory-scale bubbling fluidized bed using coated and uncoated silica sand particles from a 90-MWth circulating fluidized bed (CFB), as well as a 30-MWth bubbling fluidized bed (BFB). The experiments were carried out for gas-velocity-tominimum-fluidization-velocity ratios (u/umf) from 1 to 3.5 and mean particle diameters of 350 and 635 µm. The heat-transfer coefficients for the coated particles were lower than those for the uncoated particles in all cases, with the difference between uncoated and coated sand particles being more significant for the CFB material than for the BFB sand. 1. Introduction Fluidization has been used for decades and has been applied to many different technical processes.1-4 One of the most important applications is fluidized-bed combustion, which offers many advantages such as flexible use of fuels, effective combustion, and good possibilities for control. In large-scale power plants, fluidized beds have been used successfully for combustion of biomass. Biomass fuels are becoming more common in modern heat and power plants. Biomass is a renewable source of energy with lower environmental impacts than traditional fossil fuels.5 However, the combustion of biomass in fluidized-bed boilers is associated with various problems. For example, ash-forming elements may have a considerable negative impact on the performance of the boiler. These elements can cause ash deposits on heat-transfer surfaces, reducing the heattransfer rate.6 They may also form coatings on the bed material particles (usually sand), and if their melting temperature is exceeded, the particles start to agglomerate. If the coating and agglomeration processes proceed, the whole bed may agglomerate, resulting in defluidization of the bed and failure of the boiler. Agglomeration is, therefore, a severe problem, which can be both laborious and costly. To prevent defluidization due to agglomeration, it may be necessary to change the bed material frequently.7 Researchers have carried out extensive work to determine and predict heat transfer in bubbling as well as circulating fluidized beds (CFBs). Studies have been carried out with single particles (fuel particles) or spherical surfaces immersed in fluidized beds (see, e.g., refs 8-10), to find empirical models for the prediction of heat-transfer coefficients (see, e.g., refs 11 and 12), and to investigate heat transfer to cylindrical bodies (furnace walls and superheater tubes; see, e.g., refs 13 and 14). * To whom correspondence should be addressed. Fax: +46 90 786 64 69. E-mail:
[email protected]. † Department of Applied Physics and Electronics, Umea˚ University. ‡ Energy Technology and Thermal Process Chemistry, Umeå University. § University of British Columbia.
Figure 1. Schematic of the experimental setup: (1) mass flowmeter and control unit; (2) plenum chamber; (3) distributor plate; (4) fluidized bed; (5) scale; (6) thermocouple; (7) measurement probe with a handle; (8) pressure taps; (9) manometer; (10) thermocouple; (11) exhaust air openings; (12) data acquisition system.
In a number of fluidized-bed processes including biomass combustion, coking, and polymerization, the properties of particles change with time as a result of coating of their surfaces. Despite all of the research on heat transfer in fluidized beds, very little has been done to investigate the influence of the coating of bed particles on the heat-transfer coefficient. This paper considers the influence of the coating of sand particles on the convective/conductive heat-transfer coefficients of immersed surfaces in bubbling fluidized beds (BFBs). 2. Experimental Materials and Apparatus 2.1. Experimental Unit. The experiments were carried out in a cylindrical Plexiglas column of inner diameter 89.6 mm and height 361 mm. A schematic is shown in Figure 1. The distributor (3) was a 1-mm-thick steel plate with 47 perforations of diameter 1 mm on a
10.1021/ie034317u CCC: $27.50 © 2004 American Chemical Society Published on Web 07/24/2004
5764 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004
12-mm square pitch, giving a total open area ratio of 0.6% to ensure a uniform distribution of fluidizing gas from the plenum chamber (2). The column was equipped with pressure taps (8) for pressure measurements at different levels. Three pressure taps at 27, 60, and 79 mm above the distributor, together with one of three taps above the bed, were used to determine pressure gradients. A thermocouple 39 mm above the distributor plate, on the axis of the column, measured the bed temperature. To measure expanded bed heights, the column was also equipped with a millimeter scale (5). Bed heights were also determined from pressure gradients across the bed, as described by Werther.15 A copper wire was installed vertically along the column wall, from the distributor to a height of 263 mm, and connected to Earth in order to reduce the effects of electrostatics. The four exhaust air openings were equipped with filters to prevent particle egress. The fluidizing gas was dry (0.99. The temperature dependence of the thermophysical properties of the three experimental test probes was taken into account, with the specific heat and the thermal conductivity of the stainless steel balls obtained from Touloukian and Buyco19 and Touloukian et al.,20 respectively. The specific heat (Cp) of aluminum alloy 6262 was calculated21 with data of weight fractions taken from ASM International22 and Cp of the constituents from Incropera and DeWitt.23 When the measurements were evaluated, Cp was taken at the average
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Figure 6. Mean heat-transfer coefficient vs velocity ratio, u/umf, for uncoated and coated CFB sand particles determined with a 10-mm spherical test probe.
Figure 9. Mean heat-transfer coefficients vs velocity ratio, u/umf, for uncoated and coated BFB sand particles determined with a 10-mm spherical test probe.
Figure 7. Mean heat-transfer coefficient vs velocity ratio, u/umf, for uncoated and coated CFB sand particles determined with a 15-mm spherical test probe.
Figure 10. Mean heat-transfer coefficient vs velocity ratio, u/umf, for uncoated and coated BFB sand particles determined with a 15-mm spherical test probe.
Figure 8. Mean heat-transfer coefficient vs velocity ratio, u/umf, for uncoated and coated CFB sand particles determined with an aluminum cylindrical probe.
Figure 11. Mean heat-transfer coefficient vs velocity ratio, u/umf, for uncoated and coated BFB sand particles determined with an aluminum cylindrical probe.
probe temperature during the evaluation period (Figure 5). The thermal conductivities of the spherical test probes were also taken at the average probe temperature, while the thermal conductivity of the aluminum test probe was taken at 20 °C.22 When the measurements were evaluated, the heattransfer coefficients were corrected in all cases for heat conduction along the handles, by approximating these as fins of infinite length. The correction typically amounted to approximately 10%. The validity of the correction method was confirmed for the 10-mm test probe cooling in air at room temperature.
4. Results and Discussion In the following, all heat-transfer coefficients in Figures 6-11 were corrected for heat conduction along the handles. The error bars in the figures give the estimated 99% confidence intervals (assuming normally distributed measurement data). The results for the CFB sands are presented in Figures 6-8. Note that significantly higher heattransfer coefficients were achieved with the uncoated particles for all three probes, at least for u > 1.5umf. At lower gas velocities, the scatter is such that it is not
5768 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 Table 4. Fuel Characteristics for Different Boiler, Ash-Forming Elements7 commercial boiler asha SiO2b Al2O3b CaOb Fe2O3b K2Ob MgOb Na2Ob P2O5b Sb Clb
90-MWth CFB boiler
30-MWth BFB boiler
1.04 7.75 2.92 41.4 1.57 9.06 5.27 0.091 4.31 1.33
