Gas flow regimes in fluidized-bed combustors

Abstract

The effects of pressure and temperature on the gas flow regimes in the dense beds of fluidized-bed combustors were reviewed and investigated. Diagrams were developed to identify the gas flow regimes in both bubble and particulate phases, since these significantly affect the combustion processes occurring within the bed. In commercial pressurized fluidized-bed combustion, bubbles were effectively cloudless and no gas backmixing or slugging occurred: so the gas flow in these beds could be modeled by assuming two phases (bubble and particulate) with plug flow through each phase. For industrial-scale atmospheric fluidized-bed combustion (AFBC), different gas flow regimes occurred in the bubbling bed and circulating bed technologies. The bubbles in bubbling AFBC were cloudless with no gas backmixing or slugging, so the gas flow in these beds could also be modeled by assuming two phases (bubble and particulate) with plug flow through each phase. However, the dense bed formed at the base of the riser in industrial-scale circulating fluidized-bed combustion operating at atmospheric pressure contained clouded bubbles and experienced significant gas backmixing, so the gas flow should be modeled by assuming three phases (bubble, cloud, and emulsion) with perfect mixing in each phase, providing the primary air fluidizing velocity is below the exploding bubble regime.