*2.5. Pretreatment Conditions*

Wheat straw; sunflower peels; bagasse; and aspen, larch, and pine sawdust were fine milled using an AGO-2 laboratory ball mill (<20 µm). Sugar beet pulp were pretreated in a double-screw extruder at 182 ◦C, 30 atm, for 50 s.

Aspen wood was milled (300 µm sieve) using an IM450 industrial impeller mill (SE TechPribor, Shchyokino, Russia). Milled aspen wood was pretreated in different ways: with diluted (0.9–12.7%) sulfuric acid at 120–180 ◦C for 15 to 180 min, with 0.18–0.54% sulfuric acid solution in ethanol and butanol, and with diluted (0.15–4.8%) nitric acid at 100–160 ◦C and elevated pressure for 60 min. The diluted acid pretreatment was conducted in a 100 mL pressurized steel cylinder. A cylinder with milled aspen wood and an acid solution with a solid-to-liquid ratio of 1:5 was placed in a temperature-controlled oil bath. Elevated pressure was created by nitrogen injection. By the end of the pretreatment process, the reactors were cooled in cold water, then the slurry was filtered and the remaining solids were washed with water or alcohol–water mixture.
