Comparison of steam-assisted versus microwave-assisted treatments for the fractionation of sweet sorghum bagasse

Abstract

The fractionation of lignocellulosic materials can be applied to get each of the components (cellulose, hemicellulose and lignin) of biomass in its maximum purity and yield. The individual component can be further processed to high-value products such as fuels and biomaterials at existing or newly developing biorefineries. The steam-assisted and microwave (MW)-assisted processes were used to fractionate sweet sorghum bagasse into hemicellulosic sugars, cellulose-rich and high-density residue and solid lignin. The treatment temperature evaluated for the fractionation process was 121 °C for 30 to 120 min. The substrate was autohydrolysed to extract the hemicellulose, and the residue was delignified using lime solution to obtain cellulose-rich residue. The lignin and lime from the liquor obtained after the lime treatment of the autohydrolysed sweet sorghum bagasse (SSB) was precipitated using carbon dioxide gas. Under optimum conditions, the steam-assisted autohydrolysis extracted 72.69 (±0.08) % by weight of the hemicelluloses while the MW-assisted autohydrolysis extracted 70.83 (±0.49) % of the hemicelluloses from the sweet sorghum bagasse. The steam-assisted lime treatment resulted in 69.67 (±1.26) % of the lignin extraction the MW-assisted lime treatment resulted in 68.27 (±1.19) % of the lignin extraction from the corresponding autohydrolysed sweet sorghum bagasse samples. The CO2 treatment precipitated 58.85 (±3.2) % of the lignin dissolved in the yellow liquor of the steam-assisted process while 60.26 (±2.11) % of the dissolved lignin was recovered from the yellow liquor of the MW-assisted process. The two methods have not exhibited significant differences in overall recovery of the solids, hemicellulose extraction, delignification, residual concentration of cellulose and ash or in the recovery of lignin and lime. The difference was significant (p value <0.05) only in the concentration of total reducing sugars in the hydrolysate and the yellow liquor. The MW-assisted process increased the total crystallinity index (TCI) of the cellulose in the treated SSB and also increased the concentration of guaiacyl lignin content in the recovered lignin which was thermally more stable than the lignin produced in the steam-assisted process.