Superfine comminution characteristics of low-rank coal pyrolysis semicokes and evolution of fragmentation fractal dimension

Abstract

• The millimeter size comminution process conforms to the Steacy model. • The scale-depended fractal model could describe the superfine comminution process. • Pore structure and micro-defects have a significant impact on the comminution process. • With size decrease, the comminution mechanism shifts from shattering to abrasion. Based on the real demand of co-combustion of low-rank coal and its semicokes in pulverized coal furnace, Shenhua sub-bituminous coal (SH) and Ximeng lignite (XM) and their 700 °C semi-coke were selected to study their superfine comminution characteristics. With the increase of input energy, the comminution result of SH semi-coke is better than that of SH raw coal. And the comminution result of XM semicokes is always better than its raw coal. The fragmentation fractal analysis showed that as the input energy increases, the fragmentation fractal dimension (Df) of the two coals increased, approaching the limit fractal dimension of its material, similar to the Steacy model's 2.565. With excessive comminution energy input, both SH and XM samples showed that the higher the semicokes content, the better the comminution result. The size-dependent fractal model analysis showed that as the size of the breakage product gradually decreases, its Df decreases from about 3 to about 2 and the dominant comminution mechanism changes from overall shattering to surface abrasion. For semicokes with larger particle size contain more micro-cracks, these defects inside are easier to expand when excessive energy is input. Thus, semicokes samples would exhibit brittleness. When the size is reduced to a few microns, the raw coal and semicokes particles contain fewer defects and higher strength, resulting in a change in the comminution mechanism from overall shattering to surface abrasion.