Advanced biorefinery concepts related to non-wood feedstocks

Abstract

Agricultural residues, such as wheat straw (Triticum aestivum), okra stalk (Abelmoschus esculentus), and giant miscanthus (Miscanthus × giganteus, a hybrid of M. sinensis and M. sacchariflorus) were investigated to assess their possible consumption for integrated lignocellulosic biorefining. The efficient fractionation and recovery of all important chemical components (cellulose, hemicelluloses, and lignin) of such feedstocks are a prerequisite for realistic biorefinery concepts. Water is one of the most eco-friendly solvents with the highest potential for industrial use, and it is also suitable for full-scale biorefinery purposes. For example, under pressure at elevated temperatures over 100 °C, water can degrade and dissolve a significant portion of hemicelluloses from a lignocellulosic biomass.

This study included the preparation of various hydrolysates from wheat straw, okra stalk, and miscanthus stalk under different temperatures (140 °C and 150 °C) and P-factors of 50 (at 140 °C for 60 min or at 150 °C for 25 min) and 200 (at 140 °C for 240 min or at 150 °C for 100 min), referred to herein as hot-water pretreatment, preceding sulfur-free alkaline soda-anthraquinone (AQ) pulping; to prepare the chemical fiber, NaOH charges of 15%, 20%, and 15% on oven- dried feedstock for wheat straw, okra stalk, and miscanthus, respectively were used. An AQ charge of 0.05% on oven-dried feedstock was used in each experiment. In this hot-water pretreatment stage, the most substantial mass removals of 12%, 23%, and 10% of the initial wheat straw, okra stalk, and miscanthus stalk were obtained, respectively, at 150 °C, with a treatment time of 100 min. After the pre-treatment the hydrolysates obtained were characterized in relation to the amount of various carbohydrates, volatile acids (acetic and formic acids), and furans, as well as the pH.

It was noted that the hot-water-pre-treatment step also facilitated the subsequent pulping phase, i.e., pulp yields of 57% (145 °C, 15 min, and kappa number 18), 41% (165 °C, 180 min, and kappa number 32), and 62% (165 °C, 60 min, and kappa number 23) were achieved for wheat straw, okra stalk, and miscanthus stalk, respectively, pretreated at 150 °C with a P-factor of 200 or P200. Pulps, mainly containing cellulose, were separated from the cooking black liquors, which were then analyzed in detail; the primary emphasis was given to hemicellulose-derived aliphatic carboxylic acids: volatile acids and low-molar-mass non-volatile acids (hydroxy carboxylic acids). In all cases, the contents and molar-mass distributions of dissolved lignin (4,160 - 8,730 g/mol) in black liquors were also determined. The overall amount of aliphatic carboxylic acids decreased in order: okra/150°C/P200, 39 g/L > miscanthus/150°C/P200, 31 g/L > wheat/150°C/P200, 15 g/L. In the case of the hot-water pretreated feedstocks, fewer volatile acids and more nonvolatile acids (especially monocarboxylic acids) were detected. In conclusion, the results clearly indicate that the three non-wood materials studied were all have potential for this type of biorefining approach.