Combustor, fluidized-bed

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

Cobalt aHoys may find appHcation ia a fluidized-bed process for the direct combustion of coal (qv). CoCrAlY-coated Haynes 188 has proven to be one of the most resistant materials to a fireside corrosion process encountered ia tubes coimected the fluidized-bed combustor to a steam turbiae. 

FluidiZed-Bed Combustion. Fluidized-bed combustors are able to bum coal particles effectively in the range of 1.5 mm to 6 mm in size, which are floating in place in an expanded bed (40). Coal and limestone for SO2 capture can be fed to the combustion zone, and ash can be removed from it, by pneumatic transfer. Very Htfle precombustion processing is needed to prepare either the coal or the sorbent for entry into the furnace (41). 

In the 1970s commercial fluidized-bed combustors were limited to the atmospheric, bubbling-bed system, called the atmospheric fluidized-bed combustor (AFBC). In the late 1970s the circulating fluidized combustor (CFG) was introduced commercially, and in the 1980s the new commercial unit was the pressurized fluidized-bed combustor (PFBC). 

Another of the six CEBC technologies, the multisohd fluidized-bed combustor (MSEBC), has been under development by BatteUe Memorial Institute since 1974 (48). In an MSEBC a CEBC is superimposed on an AEBC in the combustor section. An early 15 MWt commercial version of MSEBC was designed and constmcted by Stmthers Thermo-Flood Corp. for Conoco. 

PressurizedFIuidized-Bed Combustors. By 1983 the pressurized fluidized-bed combustor (PFBC) had been demonstrated to have capacities up to 80 MWt (49). PFBCs operate at pressures of up to 1500 kPa (220 psi) and fluidization velocities of 1—2 m/s. Compared to an AFBC of the same capacity, a PFBC is smaller, exhibits higher combustion efficiencies with less elutfiation of fine particles, and utilizes dolomite, CaCO MgCO, rather than limestone to capture SO2. 

Some of the advantages of fluidized beds include flexibiUty in fuel use, easy removal of SO2, reduced NO production due to relatively low combustion temperatures, simplified operation due to reduced slagging, and finally lower costs in meeting environmental regulations compared to the conventional coal burning technologies. Consequently, fluidized-bed combustors are currently under intensive development and industrial size units (up to 150 MW) are commercially available (Fig. 10). 

Table 2. Operating Conditions for an Atmospheric Pressure Fluidized-Bed Combustor ...

Beyond the ATS program, the DOE is looking at several new initiatives to work on -with industry. One, Vision 21, aims to virtually eliminate environmental concerns associated with coal and fossil systems while achieving 60 percent efficiency for coal-based plants, 75 percent efficiency for gas-based plants, and 85 percent for coproduction facilities. Two additional fossil cycles have been proposed that can achieve 60 percent efficiency. One incorporates a gasifier and solid oxide fuel into a combined cycle the other adds a pyrolyzer with a pressurized fluidized bed combustor. Also under consideration is the development of a flexible midsize gas turbine. This initiative would reduce the gap between the utility-size turbines and industrial turbines that occurred during the DOE ATS program. 

In some scaled up fluidized bed combustors, the lower combustion zone has been divided into two separate subsections, sometimes referred to as a pant leg design, to provide better mixing of fuel and sorbent in a smaller effective cross section and reduce the potential maldistribution problems in the scaled up plant. 

Figu re 23. Exact and simplified models of a pressurized fluidized bed combustor. 

Table 1 gives the values of design and operating parameters of a scale model fluidized with air at ambient conditions which simulates the dynamics of an atmospheric fluidized bed combustor operating at 850°C. Fortunately, the linear dimensions of the model are much smaller, roughly one quarter those of the combustor. The particle density in the model must be much higher than the particle density in the combustor to maintain a constant value of the gas-to-solid density ratio. Note that the superficial velocity of the model differs from that of the combustor along with the spatial and temporal variables. 

Fitzgerald et al. (1984) measured pressure fluctuations in an atmospheric fluidized bed combustor and a quarter-scale cold model. The full set of scaling parameters was matched between the beds. The autocorrelation function of the pressure fluctuations was similar for the two beds but not within the 95% confidence levels they had anticipated. The amplitude of the autocorrelation function for the hot combustor was significantly lower than that for the cold model. Also, the experimentally determined time-scaling factor differed from the theoretical value by 24%. They suggested that the differences could be due to electrostatic effects. Particle sphericity and size distribution were not discussed failure to match these could also have influenced the hydrodynamic similarity of the two beds. Bed pressure fluctuations were measured using a single pressure point which, as discussed previously, may not accurately represent the local hydrodynamics within the bed. Similar results were... 

Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

Figure 39. Model of 20 MW bubbling fluidized bed combustor showing tube arrangement. (From Jones and Glicksman, 1986.)...

Figure 54. Solid fraction profile comparison between pressurized circulating fluidized bed combustor and one-half size scale model based on simplified scaling law. (Glicksman et al., 1995.)...

Most of the simulation effort has been applied to fluidized bed combustors which use relatively large size particles. Simulation can also be used for other fluidization processes in the petrochemical industry. Research should be undertaken to identify the proper scaling parameters for beds fluidized with smaller particles. Similar simulations may also apply to components such as cyclones. 

Ake, T. R., and Glicksman, L. R., Scale Model and Full Scale Test Results of a Circulating Fluidized Bed Combustor, Proc. 1988 Seminar on Fluidized Bed Comb. Technol. for Utility Appl., EPRI, 1-24-1 (1989)... 

Almstedt, A. E., and Zakkay, V., An Investigation of Fluidized-bed Scaling-capacitance Probe Measurements in a Pressurized Fluidized-bed Combustor and a Cold Model Bed, Chem. Eng. Sci., 45(4) 1071 (1990)... 

Farrell, P. A., Hydrodynamic Scaling and Solids Mixing in Pressurized Fluidized Bed Combustors, Ph.D. Thesis, Massachusetts Institute of Technology (1996)... 

Fitzgerald, T. J., Bushnell, D., Crane, S., and Shieh, Y., Testing of Cold Scaled Bed Modeling for Fluidized-bed Combustors, Powder Technol., 38 107 (1984)... 

Glicksman, L. R., Yule, T., Dymess, A., and Carson, R., Scaling the Hydrodynamics of Fluidized Bed Combustors with Cold Models ... 

Horio, M., Scaling Laws of Circulating Fluidized Beds, Workshop on Materials Issues in Circulating Fluidized-Bed Combustors, EPRI Report 65-6747, (12—1)—(12—14) (1990)... 

---