Subcritical Water and Chemical Pretreatments of Cotton Stalk for the Production of Ethanol

Abstract

The objective of this study was to explore the potential of sodium hydroxide, sodium percarbonate and subcritical water as pretreatments for cotton stalk to aid ethanol production. Sodium hydroxide was tested at concentrations of 0, 4 and 8 % (w/v) and sodium percarbonate was tested at concentrations of 0, 1, 2, 4 and 8 % (w/v). Cotton Stalks were pretreated with the chemicals by autoclaving 10% solids slurry at 121 degrees C and 15psi for 30, 60 or 90 minutes. Both chemical pretreatments used a factorial experimental design where each time was run with each concentration. NaOH degraded more lignin with the maximum being 63.4% at 4% concentration and 30-minute treatment time. The maximum lignin degradation by Na-percarbonate was 42.0% at 8% concentration and 90-minute treatment time. Higher concentrations of both chemicals degraded more lignin. Based on the HPLC carbohydrate analysis, NaOH produced significantly higher xylan solubilization than Na-percarbonate. The maximum solubilizations for NaOH and Na-percarbonate, respectively, were 82.7% with 8% concentration and 90-minute treatment time and 59.0% with 2% concentration and 30-minute treatment time. Higher concentrations of NaOH produced higher xylan solubilizations but the solubilization values did not change significantly (p>0.05) for concentrations above 0% Na-percarbonate. Subcritical water was tested at temperatures of 230, 275 and 320 degrees C, holding times of 2, 6 and 10 minutes and ground particles sizes of 1⁄8, 3⁄16 and 1⁄4 inches using a response surface model experimental design. Whole and smashed cotton stalks were also pretreated with subcritical water for each time-temperature combination. Enzymatic hydrolysis was performed on the three ground sample combinations showing the highest lignin degradation and the three with the highest percent total sugars. Lignin analysis was done on all pretreated samples and sugars were analyzed using DNS assay for the subcritical water pretreatments and HPLC for the chemical pretreatments. The highest percent total sugar of 46.3% was found for 230 degrees C, 10 minutes and 3/16 inches and the highest percent lignin reduction of 36.7% was found for 320 degrees C, 2 minutes and 3⁄16 inch particle size. This suggests that lower temperatures produce more total sugars and higher temperatures produce higher lignin degradation. Response surface models were developed for percent total sugars and percent lignin reduction as a function of time, temperature and ground particle size with R-squared values of .6048 and .5112, respectively. The maximum percent total sugar for the whole cotton stalks was 44.8% for the 230 degrees C, 10 minutes combination and for the smashed stalks it was 22.6% for the 320 degrees C, 2 minutes combination. Except in one case, smashing the stalks did not increase the percent total sugars and did not increase the lignin reduction in any cases when compared to the whole stalks. Also, the ground stalks did not show any significant increase in lignin degradation when compared to whole and smashed but there was an increase in the percent total sugars for two of the whole sample sets and four of the smashed sample set. Pure lignin and cellulose samples were pretreated with subcritical water revealing that interaction between the components of lignocellulosic biomass affects the effectiveness of subcritical water pretreatment.