Airblast atomization of micronized coal slurries using a twin-fluid jet atomizer

Abstract

Airblast atomization of micronized coal water slurries was carried out at low atomizing air pressures (< 270k Pa) using twin-fluid jet atomizers of various distributor designs. Drop size and size distribution were measured using the laser diffraction technique. It was found that the atomized drop sizes of micronized coal water slurries substantially decrease as the atomizing air pressure exceeds a threshold value. In addition, the atomized drop size, represented by the mass median diameter scaled to a characteristic atomizer length ( MMD L c ), can be described by wave mechanism based models in terms of three non-dimensional groups, the slurry to air mass ratio ( s A ), the Weber number (We) and the Ohnesorge number (Z): MMD L c = (1 + S A )(x 2We −x 1 + x 3Z jx 1 ) where j = 1 and 2, We equals the ratio of aerodynamic force to surface tension and Z represents the viscous effect. The linear dependency on ( 1 + S A ) is based on momentum balance and energy considerations. The three parameters ( x i , i = 1−3) are determined by the best least-squares fit of the equation to the experimental results using the iterative generalized inverse method. Excellent agreement with coefficients of correlation of 0.97–0.98 has been obtained. The presence of coal particles causes a broader drop size distribution curve compared to pure viscous liquids. Nevertheless, the effects of coal volume fraction and liquid composition on slurry atomization can be accounted for by their effects on slurry rheology. Whereas the slurry pseudoplasticity plays a significant role in slurry atomization, the high shear slurry viscosity based on the free stream slurry velocity during atomization dictates the atomized drop sizes. The effects of distributor design on airblast atomization can be accounted for by its effects on the Weber number and the air mass flow rate.