Influence Of Fluidization Velocity Research Articles

Study of the Bed Agglomeration in the Straw-fired Fluidized Bed

In this article, the bed agglomeration in the straw-fired fluidized bed was studied. The influence of fluidization velocity and bed temperature on the fluidization time was investigated. The effect of running parameters on the fluidization recovery, after the defluidization had occurred, was studied. Results showed that the fluidization time decreased with the increase of the bed temperature or with the decrease of the fluidization velocity. Fluidization can be recovered only by means of decreasing the bed temperature. The molten materials in the agglomerates were formed by the fuel particles. An agglomeration mechanism was proposed according to experimental findings.

Erosion of horizontal tubes in a pressurized fluidized bed — Influence of pressure, fluidization velocity and tube-bank geometry

Measurements of local tube erosion were carried out in a cold pressurized bed with horizontal tubes. The influence of fluidization velocity, pressure and circumferential position was studied at different locations within tube banks for three different tube-bank geometries. The erosion results were correlated with the hydrodynamic properties of the bed obtained in a previous investigation under the same operating conditions. At high pressures, the erosion decreases with increasing pressure. Preliminary results from heat transfer measurements in the bed show a significant increase of the bed-to-tube heat transfer coefficient with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels. The erosion tests were carried out using target tubes coated with a thin layer of stearin. This coating wears rapidly and, thus, an exposure time of 1 h per operating condition was sufficient to obtain an accurately measurable erosion. This exposure time compares favourably with those reported in most other investigations using more erosion-resistant tube materials. The bed has a cross-section of 0.2m × 0.3 m, and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. Three different tube-bank geometries were used, one with a fairly dense pitch and two with more sparse configurations. The bed material was silica sand with a mean particle diameter of 0.7 mm and a shape factor of approximately 0.8. The erosion results presented here are generally in good agreement with results reported for real tubes under hot conditions, both from atmospheric and pressurized fluidized bed combustors.

Local instantaneous and time-averaged heat transfer in a pressurized fluidized bed with horizontal tubes: Influence of pressure, fluidization velocity and tube-bank geometry

Measurements of local instantaneous bed-to-tube heat transfer were carried out in a cold pressurized bed with horizontal tubes. The influence of fluidization velocity and pressure was studied at different circumferential positions around a horizontal tube within a tube bank for three different tube-bank geometries. The signal from a heat transfer sensor was compared with capacitance probe signals sampled simultaneously in an adjacent position. The capacitance probe registers the alternating passage of bubbles and dense phase at the probe, and the respective contributions to the local instantaneous heat transfer from the gas and particle convection were thus identified by comparison with the heat transfer signal. The local time-averaged heat transfer coefficients at the different circumferential positions were determined from the instantaneous heat transfer signals, for the different operating conditions, as were the spatial-averaged heat transfer coefficient for the tube. The time-averaged heat transfer results were correlated with the hydrodynamic properties of the bed obtained in a previous investigation under the same operating conditions, and a strong coupling between the local mean bubble frequency and the local time-averaged heat transfer coefficient was obtained. The heat transfer results show a significant increase of the bed-to-tube heat transfer coefficient with increasing pressure, while results from a previous erosion study in the same bed show that, at high pressures, the tube erosion decreases with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels. The heat flux was measured using flush-mounted sensors on a heated copper cylinder replacing one of the tubes in the horizontal tube banks. The bed has a cross-section of 0.2m × 0.3 m, and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. Of the three tube-bank geometries used, one had a fairly dense pitch and two had more sparse configurations. The bed material was silica sand with a mean particle diameter of 0.7 mm.

Experimental study of sawdust and coal particle mixing in sand or catalyst fluidized beds

AbstractThis paper presents an experimental study in order to establish mixing and segregation conditions of wood sawdust and coal particles in a fluidized bed of sand or alumina particles. Experiments were performed at room temperature under atmospheric pressure. Simple equipment was used to divide the bed into layers and useful results could be obtained in a short period of time. The influence of fluidization velocity, time of an experiment, size and concentration of sawdust or coal particles was investigated.Uniformity of mixing was found to increase with an increase in the fluidization velocity. For u/umf less than 2.5, strong segregation occurred between sawdust and sand particles.Determination of good mixing conditions will be useful in gasification studies of sawdust and coal.