A combined experimental and theoretical study on laboratory-scale comminution of coal and biomass blends

The work presented in this paper was part of a larger study on comminution of coal/biomass blends for cofiring applications in pulverised fuel coal-fired boilers. A series of laboratory-scale ball milling experiments were conducted as part of this investigation to study the milling characteristics of bituminous coal and hardwood chips blends. A mass-balance based size reduction model was also employed to gain a greater understanding of the co-milling behaviour of coal/biomass blends. Milling properties of the blends were studied for blending ratios (wt.% of biomass in the blend) of 0 wt.%, 3 wt.%, 5 wt.%, 10 wt.%, 50 wt.% and 100 wt.%. Results suggest that for blending ratios of up to 50 wt.% the output particle size distribution of the blend is the arithmetic average of the particle size distributions of the parent materials. Analysis of particles images also revealed that the two-dimensional “Roundness” shape factors for biomass and coal particles were distinctly different with mean values about 0.25 and 0.8, respectively. So too, the three-dimensional “sphericity” shape factors of the two materials were found to differ significantly. Values of 0.58 (biomass) and 0.86 (coal) were obtained from a correlation between the particle Roundness and sphericity.

The results of the present study indicate that “Roundness” is the most suitable shape factor for differentiating coal and biomass particles from each other. As shown in the following figure the Roundness for coal particles are greater than 0.5 while biomass particles exhibit a wider distribution of Roundness with 94% of particles showing values between 0.0 and 0.6.Figure optionsDownload as PowerPoint slideHighlights
► Comminution of biomass is very likely governed by the abrasion fracture mechanism.
► Roundness of coal and biomass were found to be 0.8 and 0.25, respectively.
► Sphericity of coal and biomass were found to be 0.86 and 0.58, respectively.
► Milling characteristics of blends correlate with the properties of parent materials.
► Milling characteristics of blends is a linear function of blending ratio.