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International audience; Mechanical size reduction is considered as a primordial step of current and future lignocellulosic biorefinery. In this sense, it is of high interest to understand who are the biochemical and structural features of the lignocellulosic biomass, which affect the Specific Energy Requirement (SER), and in consequence the cost of mechanical size reduction processes. First, it was shown that the initial moisture content of the lignocellulosic biomass affect the SER and the final particle size distribution. The highest the moisture content gives raise the highest SER. Then, at fixed initial moisture content (≈7% DW), structural and biochemical features of lignocellulosic biomass that can affect the SER were determined. It was noticed that both arabinose/xylose ratio and accessible surface area lead to increasing the SER. On the contrary, the content of cellulose, lignin, crystallinity and p-coumaric acids links were found to have a positive effect on the reduction of the SER.

To enhance the methane production from ensiled sorghum forage and wheat straw, thermal, alkaline and thermo-alkaline pre-treatments were performed in batch mode. Alkaline pretreatment was conducted at 40 degrees C for 24 h with the addition of 1% and 10% gNaOH/gTS; thermal and thermo-alkaline pre-treatments at 100 degrees C, and 160 C for 30 min, with and without the addition of NaOH solutions at the above dosages. All the pre-treatments tested led to a solubilization of the organic matter, with a maximum concentration (around 30-40% for both substrates) obtained at 40 and 100 C with 10% NaOH. Furthermore, a reduction in the fibrous fractions was observed for both substrates. The highest lignin reduction, compared to untreated samples, was found at 100 degrees C with 10% NaOH dosage (53% and 72% for wheat straw and sorghum, respectively). Under this pre-treatment condition a high hemicelluloses reduction yield was also found (63% for both substrates). The highest increase in methane yield (up to 32%), compared to the untreated substrate was observed at 40 degrees C with 10% NaOH for sorghum. As for wheat straw, significant increases in methane yield were observed at 40 degrees C with 10% NaOH (43%) and at 100 degrees C with 1-10% NaOH (48% and 67%, respectively). According to the results of the preliminary economic analysis, the pretreatments seemed to be sustainable for both substrates, especially for wheat straw, due to a higher methane yield increase than that of sorghum.