SO₂-Catalyzed Steam Fractionation of Aspen Chips for Bioethanol Production:  Optimization of the Catalyst Impregnation

The pretreatment step has a key role in the enzymatic conversion of lignocellulosic biomass to bioethanol. Steam pulping of biomass has long been recognized as being effective in producing high biomass fractionation. In this step, the preliminary impregnation of biomass with acid catalysts, including SO₂, has been shown to further improve the hydrolytic effect and increase the digestibility of the fibers by enzymes. This work focused on developing an experimental setup for the SO₂ impregnation, enabling accurate control of the process variables. The final purpose was the assessment of a method to minimize the quantity of SO₂ used, and that could be applicable to continuous operations. Aspen chips were impregnated with the acid catalyst at room temperature in a stainless steel batch reactor, and the influence of the biomass humidity and contact time on SO₂ uptake was explored. After the catalyst adsorption, biomass was fed into the steam explosion batch reactor and steamed at 205 °C for 3 and 10 min to obtain slurries. The addition of catalyst (∼0.9% w/w raw material dry matter) reduced the degree of polymerization of the cellulose by 50%, on average. The highest yield of xylose (10.3 g/100 dry chips) was obtained via water extraction following a steam pretreatment of 3 min. The amount of monomer xylose was 80% of the total extracted dry matter. Simultaneous saccharification and fermentation (SSF) of the washed fibers yielded 37 g of glucose/100 g dry chips, 96% of which fermented to ethanol.