Electrostatic characteristics in a large-scale triple-bed circulating fluidized bed system for coal gasification

Abstract

Electrostatics charge generation by triboelectrification has significant implications for the proper design and operation of a circulating fluidized bed. In this study, electrostatics in the fully developed regions of both the riser and downer of a large-scale triple-bed combined circulating fluidized bed was characterized in terms of the equivalent currents over the cross section of the developed region. The average equivalent currents and solids holdup were measured under different superficial velocities in the riser, downer and gas-sealing bed. It was found that in the fast fluidization regime in the riser, the negative equivalent currents were comparable with the positive equivalent currents due to the typical core-annulus flow pattern. With increasing superficial velocities in the riser or in the gas-sealing bed, the flow pattern would approach dilute phase transport or dense suspension upflow regime. Thus, the positive equivalent currents became dominant because the backflow of sand particles were greatly suppressed. Some dominant frequencies for the equivalent currents in the riser were almost identical regardless of the magnitude of superficial velocities in the riser and the gas-sealing bed, indicating that they were determined by the inherent characteristics of electrostatics and/or signal noise, and were not affected by gas-solids flow behaviors. The frequencies in the downer were focused on the low value range, and the dominant frequencies were nearly zero. Both the solids mass flux and solids holdup had significant influence on the average equivalent currents. With increasing superficial velocities in the gas-sealing bed, the average equivalent currents first increased and then approached a constant. This led to the same trend in variation of solids mass flux, as well as solids holdup. With increasing air superficial velocities in the riser, the average equivalent currents in the riser increased, while the average equivalent currents decreased in the downer with increasing air superficial velocities in the downer. © 2012 Elsevier Ltd.