Analysis of ground rice straw with a hydro-textural approach

The lignocellulosic material contained in agricultural residues like straws, represents a resource with many points of biomolecular interest but their extraction is subject to a succession of treatments highly energy-consuming and generating effluents. Due to the strength of the supramolecular structure of the lignocellulosic matrix at various levels, it is difficult to improve the efficiency of separation processes for these materials. The utilization of biomass constituents often requires pretreatment, which is a crucial step in the separation into constituents. The rice straw must firstly be fractionated by grinding, which can also require pretreatment to improve efficiency. The question arises as to how best to identify and model the mechanisms involved in the grinding process with or without pretreatment. The aim of this study was to complete the characterization of the grinding process using a hydro-textural approach applied to biopowders to help with identification of the mechanisms involved. Experimental trials were conducted with rice straw through several milling steps, which led to decreasing particle sizes. The physical properties (density, cohesion, coefficient of friction, ability to flow) were characterized for some of the obtained powders, which were then described with a hydro-textural diagram. The results reveal that the breakdown process led to a “loss of porosity” regardless of the size of the powder particles. The fragmentation seemed to be located in the “zones of stronger porosities”. These structures containing residual water, decreased weakly with grinding. Assuming that the water was located within cells of the straw (intracellular water), given the power law, which correlates compactness to median diameter (d50), we introduced a characteristic size, which corresponds to the physical limit of optimal grinding. Its value, calculated from the model, is nearly equal to the cell wall thickness: limϕ→1d50=ecell.~2μm. © 2016 Elsevier B.V.