[Hydrolysis processes of fast-growing tree species wood] | Hydrolýzne procesy dreva rýchlorastúcich drevín

The pretreatment of lignocellulosics, destined to the production of technically important raw material (pulp, bioethanol etc.) is particularly important part of the relevant technological processes. The goal of the pretreatment is to remove the hemicellulosic fraction and water-soluble lignin, to reduce cellulose crystalinity, and to increase the porosity of the raw-materials. In the present time different methods of lignocellulosics pretreatment are used. Their selection is influenced by a technological method, a variety of used raw-material (one-year plants, wood, lignocellulosic industry wastes) and also by a final product. This work deals with the influence of various water prehydrolysis conditions (autohydrolysis) on the chemical composition of wood species as an aspen (Populus tremula L.), a willow (Salix alba L.), an alder (Alnus Glutinosa L. Gaertn) and a birch (Betula Verrucosa Ehrh.). During the autohydrolysis of wood at the temperature range of 140-200 deg C and the time range of 30-240 minutes, substances of various characters are released into hydrolysates. Nascent organic acids (e.g. acetic acid), which were created by the scission of functional group and degrading reactions of released monosaccharides, moved pH values of solutions into the acid ranges. Catalytic effects of these substances on hydrolysis processes, which occured in wood and hydrolysates, emerged also in the chemical composition of hydrolysates. The carbohydrate portion and aromatic part of wood -lignin, were liable to the influence of autohydrolysis which caused depolymerisation and destruction reactions. The carbohydrate portion of wood mainly hemicellulosic was released in the form of monomeric and low-molecular forms of polysaccharides (oligomers) into the solution. In the odtained hydrolysates there were present all saccharides (pentoses and hexoses) which are typical for beech wood (D-glucose, D-galactose, D-mannose, D-xylose, L-arabinose, D-ribose and L-ramnose). The concentration of carbohydrates in hydrolysates was decreased with the increasing temperature of prehydrolysis at 200 deg C and extending time of boiling. This concentration gradient was accompanied by the growth of degradation reactions of carbohydrates. In hydrolysates there are present low-molecular volatile substances which are mostly toxic. 2-Furaldehyde and acetic acid were created by destruction reactions of monosaccharides. These substances act inhibitivly when the following treatment of hydrolysates e.g. for enzymatic production of bioethanol. Therefore it is important to check their concentration in solution. The aromatic portion of wood -lignin, undergoes degradation reaction, too. This was proved by the presence of methanol in the obtained hydrolysates, which is generated when scission of methoxycal group of lignin cleavage. It is very important to know the amount of carbohydrates in hydrolysates, which are destined for enzymatic fermentation. The maximum yield of released carbohydrates from wood, after hydrolysis of glycoside bonds by 4 % H2S04 (w/w), depended on the operation conditions (temperature, time), both species and the age of wood. The maximum concentration of carbohydrates in hydrolysates decreased in the following sequence: birch less than aspen less than alder less than willow. The necessary temperature for releasing of the maximum amount of hemicellulosic portion from alder and birch is the water temperature 180 deg C (60 min); 160 deg C for aspen (120 min) and willow (240 min). The result of this experiment proves that the hydrolysates from birch and aspen are the most suitable for the production of bioethanol.