An Experimental Investigation Into Powder Dispersion Process In The Heat-exchange Pipe Of A Suspension Preheater

The dispersion process of bulk solids in gas phase is a key factor which controls momentum and energy transfer in gas-solid two-phase flows. For further decreasing system pressure drop, the calorific consumption and improving thermal efficiency, the paper presents the research works of the experimental investigation into powder dispersion process in the heat-exchange pipe of a suspension preheater of laboratory scale.Based on the research on mechanism of powder dispersion in gas streams, the author has carried out an experiment project of powder dispersion process in the heat-exchange pipe of the suspension preheater. The morphology and dispersion distance of powder in the gas streams has been observed with high speed photographic instrument. The composition of the pressure drop across the pipe and the powder concentration distribution along the pipe also has been measured. Meanwhile the effect of the three dispersion devices on the powder dispersion process and the pressure loss along the pipe has been discussed.The results indicated that powder dispersion process included six procedures: accelerating motion downward, beening sheered and spalled by gas step by step, appearancing cracks, firstly cleavaging, secondly beening sheered and spalled, finally completely breaking off. When the average diameter of the powder conglomeration exceeds 2mm, breaking off is the chief process. When less than 2mm, the conglomeration is sheered and spalled by gas to cloud fog phase, which could follow with the gas streams. A general conclusion is that the diameter of the particles is more fine, the agglomeration in the powder is more remarkably.The pressure drop over the whole heat-exchange pipe was separately measured in three sections: section of solids accelerating upwards, section of solids uniformly moving and the pipe bend. Measurement indicates that most pressure drop (over 60%) existed in the acceleration section, taking a key effect on decreasing pressure drop in the pipe.Particle concentrations at 7 levels along the axial direction of the heat-exchange pipe were measured with the superficial gas velocity ranging from 12 to 21m/s and different solids loading. The results indicated that the particle concentrations are distributed non-uniformly in radial direction, with lower particle concentrations occurred in riser center and higher near the pipe wall. From the pipe bottom to the top, in the near wall region, particle falling downward was recognized, while around riser center line the particle concentrations varied insignificantly. With increasing solids loading, the particle concentration increases in whole heat-exchange pipe, the dispersion distance of the powder increases and as well as the radial fluctuation of particle concentrations.The results indicated that all of the three tested dispersion devices with different geometrical configuration have remarkably effects in decreasing system pressure drop. The relative effect of type 2 in lowing pressure drop is superior to type 3 which is superior to type 1. Application of dispersion device of type 2# resulted in pressure drop reduction about 30%. Also the three devices could disperse the powder conglomeration completely and make the powder distribution across the pipe section more uniformly with the same order. When the phenomenon of material collapse occurred in the conventional heat-exchange pipe with high solids loading, it still works well in the pipe with dispersion devices, where powder materials been dispersed completely, In a word, the type 2# dispersion device has the best effect.