Hemicelluloses problems

We have seen that lignin is a highly branched amorphous pol3rmer containing an extremely complex array of structures and that when in situ in the wood, it is a graft polymer with cellulose and hemicellulose to boot. Characterization of lignins thus becomes a problem. 

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

The steam explosion process is a recent development in wood processing (1,2). Much attention has been paid to this process from the viewpoint of total wood utilization. Cellulose and hemicellulose from this process can be converted into sugars of commercial value by enzymatic methods (3). However, the conversion of lignin from this process (steam explosion lignin) into useful materials continues to present difficulties. Preparation of adhesives from it is considered to be a feasible way to solve this problem. 

A major problem in the commercialization of this potential is the inherent resistance of lignocellulosic materials toward conversion to fermentable sugars (4). To improve the efficiency of enzymatic hydrolysis, a pretreatment step is necessary to make the cellulose fraction accessible to cellulase enzymes. Delignification, removal of hemicellulose, and decreasing the crystallinity of cellulose produce more accessible surface area for cellulase enzymes to react with cellulose (5). 

This process was developed for cellulose-rich raw materials since high concentration of the acid may cause degradation of the pentoses in hemicellulose to furfural derivatives. The ethanol yield is usually about 35%. However, corrosion problems and the need for higher capital investment and dangers associated with the recovery of the concentrated acid make this method less attractive. Furthermore, during combustion of lignin that is contaminated with hydrochloric acid there is some risk of dioxin emissions. 

The other noticeable product of this era was a steady stream of publications, mainly to the Journal of the Chemical Society, but also to the Biochemical Journal, to Chemistry and Industry, and later, to Carbohydrate Research. The atmosphere of this period is conveyed in several review lectures given by Hirst for example, the fourteenth Pedler lecture to the Chemical Society (1955) on Some Problems in the Chemistry of the Hemicelluloses, the Presidential Addresses to the Chemical Society (1957 and 1958) on Some Aspects ofthe Chemistry of the Fructosans and Polysaccharides of the Marine Algae, respectively, on Plant Gums, at the IVth International Congress of Biochemistry, Vienna (1958), and the Bakerian Lecture to the Royal Society (1959) on Molecular Structure in the Polysaccharide Group. These lectures were delivered with a quiet authority, and the published manuscripts show meticulous attention to detail. 

A problem but little discussed in the hterature is the possible association of polysaccharide molecules of various kinds with noncarbohydrate molecules. The association can be random, and a direct fractionation would then be impossible. Thus, it has frequently been observed in the authors laboratory that hemicelluloses containing lignin could not be separated into the pure components, but, after further dehgnification, they could be readily fractionated. It is possible that similar comphcations may also occur with other types of polysaccharide, for example, protein- or lipide-polysac-charide complexes. In this connection, it should be noted that the solution obtained on dehgnification of some materials may contain polysaccharide material of particular interest. For instance, the main part of the galactan present in spruce compression wood can be recovered from such a solution. ... 

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