(72c) Heat Transfer in a Pressurized Fluidized Bed with Continuous Addition of Fines

Oxygen-fired pressurized fluidized bed combustor technology with downstream carbon capture and sequestration is considered a key approach to clean coal combustion. In such technology, the fluidized bed combustor operates at elevated pressures and houses an in-bed heat exchanger tube bundle. The goal of this work was to investigate the impact of pressure and presence of fine particles (i.e., surrogate for pulverised fuel) on the overall tube-to-bed heat transfer coefficient. Experiments were conducted in a pilot-scale fluidized bed with an inner diameter of 0.15 m under cold flow conditions. A tube bundle consisted of five horizontal staggered rows was completely submerged in the bed (simulating the combustor in-bed heat exchanger tubes). One of the tubes was replaced by a 900 W heating cartridge housed in a hollowed copper rod. Five thermocouples distributed at 45º intervals along the copper rod circumference measured the surface temperature and ensured that local effects were included. Through supplying DC power and monitoring bed and copper rod surface temperature, the overall convective heat transfer coefficient was determined. The bed material was large glass beads of 1.0 mm in diameter while the fines were glass beads of 60 µm in diameter and thus susceptible to entrainment. The fine particles were continuously fed to the fluidized bed over a designed period of time, and then captured downstream by a filter system. Fluidization was conducted at 101, 600 and 1200 kPa with excess gas velocities (U_g - U_mf) of 0.21, 0.29 and 0.51 m/s. Fine particle feed rates were 0, 9.5 and 14.4 kg/h. The results showed that average heat transfer coefficients matched the correlation developed by Molerus et al. (1995) within a 5% difference across all conditions when fines were not present. At atmospheric conditions, where slug flow regime occurred, the maximum heat transfer coefficient was at the bottom of the rod, while it moved to the side of the rod at high pressures where the bubbling regime occurred. Overall, the heat transfer coefficient doubled when pressure was increased from 101 to 1200 kPa. The addition of fine particles decreased the average overall heat transfer coefficient by 3-9%, but there was no effect on the angular profile across the tube surface. The fines likely reduced the turbulence adjacent to the tube surface negatively impacting heat transfer via gas convection, which is the primary mechanism.

References

Molerus, O., and Mattmann, W. (1992). Heat transfer mechanisms in gas fluidized beds. Part 3: Heat transfer in circulating fluidzed beds. Chemical Engineering & Technology, 15 (5), 291-294.