Acid impregnation and steam explosion of corn stover in batch processes

The synergistic effect of pre-impregnation by sulphuric acid and steam explosion has been investigated. Sugar recovery by water extraction and cellulose digestibility by enzymes have been considered. The acid diffusion inside a representative particle, having thickness of 0.7 mm, has been modelled on the basis of available models and diffusion coefficients. The experiments of acid impregnation have pointed out a plateau of the acid uptake after 10 min corresponding to an acid adsorption of 22 g/kg of dry stover. An experiment of acid desorption from the pre-impregnated biomass particles to a new bulk of pure water (reverse diffusion), has confirmed that only a small fraction of the uptaken acid remained free, while most has been consumed by the substrate. Nine conditions have been tested for the steam explosion treatment selecting the temperature of 180, 190, 200 °C and sulphuric acid loadings of 0, 1.5, 3 wt.%. The maximum sugar recovery by water extraction is produced by a SE treatment of 190 °C for 5 min and an acid loading of 1.5 wt.%; at these conditions 25% of the sugars in the feedstock can be recovered as monomers or oligomers. The acid has greatly improved the sugar solubility, e.g. at 180 °C the sugar recovery has been only 1.5% without acid, while by using 3 wt.% of acid the sugar recovery has increased to 16.8%. Also the cellulose digestibility has been improved by the acid pre-impregnation, after 48 h of digestion the yield of glucose reached 93% of the theoretical by using the substrate that was pre-impregnated with 3 wt.% of acid an treated at 190 °C. The high acid loading has also been required to achieve the best recovery of glucose (85% of the initial glucan) as sum of water extraction and 48 h hydrolysis.