Impact of loop seal structure on gas solid flow in a CFB system

Abstract

In a circulating fluidized bed (CFB) boiler, loop seal is one of the most commonly used non-mechanical valves. When the solids are conveyed horizontally from the supply chamber to the recycle chamber through the horizontal section, the length of the horizontal section will cause much flow resistance, which not only influences the fan burden but also the pressure and mass balance between the furnace and the external loop.In this paper, a new structure loop seal with a slope passage connecting the supply and recycle chambers (named N-type) is described. The flow characteristic of quartz sand (Geldart-B particles) in the new loop seal was studied by comparison with the traditional loop seal (named U-type).The experimental data show that the passage design connecting the two chambers of a loop seal has an obvious effect on the gas solid flow not only in the loop seal, but also in other components in the whole loop. For the N-type loop seal, the minimum aeration rate required for the onset of solid flow is equal to the minimum fluidization velocity of the bed materials, much less than with the U-type loop seal. The solid recycling capability of the N-type loop seal is better than of U-type one at low aeration rate, but this status will be reversed at high aeration rate. A small amount of vertical aeration into the bottom of the standpipe can further improve the solid recycling capability of the N-type loop seal, by reducing the inter-particle friction and build up a higher pressure drop gradient in the standpipe. At the same time, less air bypassing upward into the cyclone through the standpipe is found for the N-type loop seal at high level of aeration rate, while, at the same condition the U-type suffers severe gas bypassing with a fluidized state in the standpipe. At last, the pressure drop of the new type loop seal is much less than the pressure drop of the traditional one with the same solid flow rate, which provides an advantage for reducing fan power and sustaining more solids in the boiler furnace.