Bubbling bed combustor

Figure 29. Comparison of dimensionless power spectra of differential pressure fluctuations. Double probe across levels 2 and 3 x/L = 0.0, coal burning bubbling bed combustor. Full set of scaling laws with iron grit in cold bed hot bed material in cold bed violates scaling laws. (From Nicastro and Glicksman, 1984.)...

Ackeskog et al. (1993) made the first heat transfer measurements in a scale model of a pressurized bubbling bed combustor. These results shed light on the influence of particle size, density and pressure levels on the fundamental mechanism of heat transfer, e.g., the increased importance of the gas convective component with increased pressure. 

Figure 45. Comparison of heat transfer coefficient measured in 20 MW bubbling bed combustor vs prediction from MIT cold test. (From Glicksman et al, 1987)...

Figure 47. Wear rates for in-line tubes, simulation of 20 MW bubbling bed combustor. (From Glicksman et at., 1987b.)...

Simulating Bubbling Bed Combustors Using Two-Fluid Models... 

The earliest scaling studies were directed at atmospheric bubbling bed combustors. To date, a rich variety of questions have been addressed. 

Bubbling bed combustors conduct almost all of their combustion and, simultaneously, considerable heat transfer within the relatively dense bubbling bed. The freeboard is primarily used to complete CO and volatiles combustion and to disengage solids from the raw combustion gas before the gas reaches the convective heat transfer surface. Elutriated particles that are captured by cyclones may be recycled, at relatively low temperature, to the fluidized bed combustor. 

The elevated pressure of PFBC (1000-1500 kPa) means that even the bubbling bed combustor cross-sectional area is reasonable for shop fabrication at relatively large capacities. 

Ammonia injection near the bottom of bubbling bed combustors can lead to increases in emissions (Minchener and Kelsall, 1990 Hoke et al., 1980). The effect is attributed to the high oxygen concentration due to the close proximity of the primary air distribution which can lead to oxidation of ammonia. Studies have revealed that the optimum ammonia injection location is either in the upper furnace area or in the cyclone (Hoke et al., 1980 Minchener and Kelsall, 1990 Hiltunen and Tang, 1988 Shimizu et al., 1990). Injection locations anywhere else either increases emissions or creates unacceptable ammonia slip into the atmosphere. Experimental studies indicated that under high concentrations of char and limestone, emissions can increase with ammonia injection (Shimizu et al., 1990). The effect is attributed to the catalytic effect of char and limestone on ammonia oxidation. Using a kinetic model for formation, Johnsson (1989) has shown the importance of the ammonia oxidation in the presence of limestone and char catalyst. 

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