Acid hydrolysis procedure

The acid hydrolysis pretreatment used to extract hemicelluloses was performed in a 2 L stainless steel reactor from Parr Instruments Co. (Moline, IL, USA). The liquid/soil ratio was kept constant at 8 kg_water kg_{raw material}^–1 (o.d.b.). The independent variables of the extraction process were temperature (130, 150 or 170 °C) and acid (H₂SO₄) concentration (0.5, 1.25 or 2%). Based on existing recommendations [26], the reaction time was 60 min in all tests. Once extracted, the liquid fraction with hemicelluloses were separated from solid residue by filtration and washed in water, the solid being air-dried and weighed to calculate the yield of the process.

The solid residue from the acid hydrolysis treatment was characterized for yield and the amount of hemicelluloses (xylose, arabinose and acetic acid) extracted under identical conditions for the three raw materials. The residue was also characterized for calorific power according to standards CEN/TS 14918:2005 E (“Solid biofuels method for the determination of calorific value”) [68] and UNE164001 EX [69]. A Parr 6300 automatic Isoperibol calorimeter, a CGA 540 connector, 99.5% pure oxygen and a maximum pressure of 2500 psig were used for this purpose.