Production of biodegradable film, bioethanol, and pulp with integrated forest product biorefinery using wheat straw and corn stalks

Depending on the production method, traditional paper mills often burn the black liquor for energy. The black liquor undesirably includes some soluble hemicelluloses, which has lower heating value compared to lignin. Therefore, the extraction of hemicelluloses before the pulping process is an alternative for pulp and paper mills and the extracted hemicelluloses could be converted to higher economic value by producing biofuels, biopolymers, paper additives, other chemicals etc. However, the amount of pre-extracted hemicelluloses before pulping will be regulated to maintain pulp yield and paper quality while comparing with untreated pulp. Wheat straw and corn stalks have a significant amount of carbohydrate contents (70-75% w/w), and thus both were examined in this study in terms of the integrated biorefinery concept. The aim of this study was to pre-extract hemicelluloses from wheat straw and corn stalks by treating them with hot water, alkali (NaOH), and modified alkali (NaBH4+NaOH) at varying temperatures and chemical concentrations. The pre-extracted solid materials were utilized to produce papermaking pulps. In addition, the liquid portions (pre-extracted liquids) consisting mostly of xylan were utilized to produce bioethanol and biodegradable films. Furthermore, to overcome the restrains in hot water pre-extraction, due to "sticky lignin" formation in hot water pre-extraction, formic acid (FA) reinforced autohydrolysis was examined for wheat straw at different chemical concentrations. The obtained results were statistically analyzed by ANOVA to identify significant differences and the differences between groups were determined by the Duncan test. For wheat straw, the hot water (135 and 150 °C), alkali (16.7% NaOH at 50 °C), and modified alkali (16.7% NaOH+0.1% NaBH4 at 50 °C) pre-extractions removed 16.5%, 41.5%, 33.6% and 33.9% of the xylan from the straw structure, respectively. Ethanol produced from the removed xylan for the hot water (135 and150 °C), alkali (16.7% NaOH at 50 °C), and modified alkali (16.7% NaOH+0.1% NaBH4 at 50 °C) pre-extractions were yielded up to 7.79%, 16.9% 6.81%, and 4.22% (g/100 g soluble material) ethanol, respectively. In addition, good-quality biodegradable films were produced when some gluten and nanocellulose was added to the extracted xylan. The hot water pre-extracted pulp yields and the paper produced from this pulp's physical and mechanical properties were comparable to those of the corresponding conventional soda (control) pulp. For corn stalks, the results showed that the hot water (150 °C) and alkali (26.7% NaOH at 50 °C) pre-extractions removed 44.8% and 35.6% of the xylan from the stalk structure. When bioethanol was produced from the dissolved liquid, the hot water (150 °C) and alkali (26.7% NaOH at 50 °C) pre-extractions gave 14.7% and 7.66% (g/100g soluble material) ethanol yield. Similar to wheat straw, when gluten and nanocellulose were added to the xylan, good-quality biodegradable films were produced. Additionally, the pulps produced from the hot water pre-extracted solid fractions were comparable in yield and pulp properties to the control soda pulp. It should be mentioned that modified alkali pre-extraction preserved some glucan and dissolved more lignin from the structure of wheat straw and corn stalks. A cost efficient pretreatment technology is required for competitive sugar production at industrial implementation. Therefore, autohydrolysis would be a cost-effective process because only water/steam is needed in this application. Unfortunately, in this pretreatment technique, "sticky lignin" precipitates are formed in the hydrolysate that leads to severe plugging in the hydrolysis reactor and downstream equipment in a continuous process. In this concept, wheat straw was pretreated at temperature of 150 °C for 100 min at a liquor-to-straw ratio (L/S) of 6 L/kg (oven dry) straw. Different FA concentrations, ranging from 0 to 15 g/L, were applied during autohyrolysis. The results showed excellent sugar and lignin mass balances for pretreatment were obtained, and precipitated lignin in the hydrolysate decreased from 0.94 g/100g straw at autohydrolysis conditions (no FA) to 0.31 g/100g straw at 15 g/L of FA. The average molecular weight of precipitated lignin decreased from 1970 g/mole for autohydrolysis to 710 g/mole at 15 g/L of FA. The monomeric sugar yield during pretreatment improved dramatically with increasing FA concentration relative to autohydrolysis. Without FA the sugar yield in the hydrolysate was only 3.83 g/100g. However at a FA concentration of 15 g/L, the dissolved sugar yield increased to 23.5 g/100g. The results showed that FA reinforced autohydrolysis not only minimize the formation of "sticky lignin" but also significantly increased dissolved sugar yield.