Effects of pressure on flow regimes transition velocities and bubble properties in a pilot-scale pressurised circulating fluidised bed

Abstract

Pressurised fluidised beds have been widely used in industry, e.g., for coal gasification. However, gas-solids flow dynamics at high pressure, closely relevant to fluidised beds performance, is not yet well understood. For this purpose, electrical capacitance tomography (ECT) and pressure fluctuation measurements were carried out in a pilot-scale pressurised circulating fluidised bed (PCFB), operated at pressures ranging from 0.2 MPa to 1.3 MPa and gas velocities from 0.5 m/s to 2.65 m/s, to investigate the effects of pressure on gas-solids flow characteristics. The critical flow regimes transition velocities and bubble properties were examined by ECT images along with time and frequency domain analysis. The results indicate that a wide range of flow regimes including bubbling, slugging, turbulent and fast fluidisation, can be identified with distinct transition velocities in the pressurised fluidised bed. With an increase in pressure, the minimum fluidisation velocity and transition velocities between different flow regimes decrease for particles under investigation, i.e., quartz sand with mean diameter of 352 µm. With an increase in pressure, a more homogeneous flow structure with smaller bubbles and lower bubble rise velocities takes place compared with that at ambient pressure condition. The novelty of this research lies in first time demonstration of ECT measurements for high-pressure hydrodynamics over a wide range of flow regimes. The minimum fluidisation velocity can be derived without de-fluidising a PCFB, and bubble rise velocity can be obtained by cross-correlation of dual-plane ECT signals.