Effects of operational conditions on auto-catalyzed and sulfuric-acid-catalyzed hydrothermal pretreatment of sugarcane bagasse at different severity factor

Bagasse, a major by-product of sugarcane-processing industries, has potential to play a significant role as feedstock for production of cellulosic ethanol, platform chemicals, and bio-based commodities. Pretreatment is essential for efficient processing of lignocellulosic feedstocks by biochemical conversion. In this work, auto catalyzed (A-HTP) and dilute sulfuric-acid-catalyzed (SA-HTP) hydrothermal pretreatment of sugarcane bagasse was investigated, setting the temperature (175-205 degrees C) and the time (4-51 min) in such a way that the severity factor (SF) was always maintained at one of three predetermined values (2.8, 3.8, and 4.8). The investigation covered the effects of different operational pretreatment conditions on (i) the formation of sugars and water-soluble bioconversion inhibitors, including newly discovered inhibitors such as formaldehyde and pbenzoquinone, in the pretreatment liquid, (ii) the chemical composition and recovery of constituents in the solid phase, as determined using two-step treatment with sulfuric acid, Py-GC/MS, and solid-state NMR, (iii) pseudo lignin formation, (iv) furan aldehydes in condensates from the gas phase, (v) enzymatic digestibility of pretreated solids, (vi) enzyme inhibition by pretreatment liquids, and (vii) fermentability of pretreatment liquids using Saccharomyces cerevisiae yeast. Glucose and xylose were the predominant sugars in pretreatment liquids from SAHTP and A-HTP, respectively. For A-HTP, the enzymatic digestibility of the pretreated solids was proportional to the SF, while for SA-HTP no clear trend was observed. The best enzymatic digestibility (above 80%) was achieved for A-HTP performed at SF 4.8. The highest total yields of glucose and xylose, the predominant sugars, were achieved for A-HTP at SF 3.8 and temperatures of 190 degrees C and 205 degrees C. The fermentability of the pretreatment liquids by Saccharomyces cerevisiae was lower for SA-HTP than for A-HTP. The investigation suggests that hydrothermal pretreatment of sugarcane bagasse can be performed with good results without addition of sulfuric acid, but that the conditions must be just harsh enough to almost quantitatively solubilize the hemicelluloses