The transformation of lignocellulosic biomass into biofuels represents an interesting and sustainable alternative to fossil fuel for the near future. However, one still faces some major challenges for the technology to be fully realized including feedstock costs, novel pretreatment processes, production, transportation, and environmental impact of the full chain. The development of new technologies focused to increase the efficiency of cellulose conversion to biofuels determines successful implementation. Mechanical fractionation is an essential step in order to increase final carbohydrate output, appropriate particle sizes and densification, enzymatic accessibility, and bioconversion affectivity without the production of toxic side streams. In this review article, we surveyed a substantial amount of previous work in mechanical fractionation or pretreatments of a variety of lignocellulosic biomasses; these include numerous milling schemes and extrusions, and their impacts on the physical and physicochemical properties of the lignocellulosic matrix (crystallinity, surface area, particle size, etc). We have also compared results with other pure chemical and physicochemical pretreatments in order to show the new aspects and advantages/disadvantages of such an approach. Last, but not least, the effect of mechanical treatment and physical properties on enzymatic hydrolysis and bioconversion has been discussed, with potentially interesting dry lignocellulosic biorefinery schemes proposed.

Additive manufacturing or 3D printing has the potential to displace some of the current manufacturing techniques and is particularly attractive if local renewable waste resources can be used. In this study, rice husk and wood powders were compounded in polylactic acid (PLA) by twin screw extrusion to produce filaments for fused-deposition modelling 3D printing. The biomasses were characterized in terms of physical features (e.g. particle size, density) and chemical compositions (e.g. solid state nuclear magnetic resonance, ash content). The two biomasses were found to have a different impact on the rheological behaviour of the compounds and the extrusion process overall stability. When comparing the complex viscosity of neat PLA to the biomass/PLA compounds, the integration of wood powder increased the complex viscosity of the compound, whereas the integration of rice husk powder decreased it. This significant difference in rheological behaviour was attributed to the higher specific surface area (and chemical reactivity) of the rice husk particles and the presence of silica in rice husks compared to the wood powder. Colour variations were also observed. Despite the biomass filler and rheological behaviour differences, the mechanical properties of the 3D printed samples were similar and predominantly affected by the printing direction.

The comminution of lignocellulosic biomass is a key operation for many applications as bio-based materials, bio-energy or green chemistry. The grinder used can have a significant impact on the properties of the ground powders, of those of the end-products and on the energy consumption. Since several years, the milling of lignocellulosic biomass has been the subject of numerous studies most often focused on specific materials and/or applications but there is still a lack of generic knowledge about the relation between the histological structure of the raw materials, the milling technologies and the physical and chemical properties of the powders. This review aims to point out the main process parameters and plant raw material properties that influence the milling operation and their consequences on the properties of ground powders and on the energy consumption during the comminution.

Lignocellulosic feedstocks present a growing interest in many industrial processes as they are an ecological alternative to petroleum-based products. Generally, the size of plant raw materials needs to be reduced by milling step(s), to increase density, facilitate transport and storage, and to increase reactivity. However, this unit operation can prove to be important in term of investments, functioning costs, and energy consumption if the process is not fully adapted to the histological structure of the plant material, possibly challenging the profitability of the whole chain of the biomass conversion. In this paper, the different technologies that can be used for the milling of lignocellulosic biomass were reviewed and different avenues are suggested to improve the milling performances thanks to thermal pretreatments. Based on examples on wheat straw milling, the main points to take into consideration in the choice of a milling technologies have been highlighted in regards to the specifications of ground powder. A specific focus on the hazards associated to the milling and the manipulation of fine biomass particles is also realized at the end of the paper from the perspective of industrial applications.