Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 198

Biomass is typically composed of 75 to 90% by weight sugar species, the other 10 to 25 wt% being mainly lignin. The energy in biomass is the chemical energy associated with the carbon and hydrogen atoms contained in oxidizable organic compounds which are the source of the carbon and hydrogen is carbon dioxide and water. The conversion by plants of carbon dioxide and water to a combustible organic form occurs by the process of photosynthesis in which solar energy and chlorophyll are the important players.

Chlorophyll, present in the cells of green plants, absorbs solar energy and makes it available for the photosynthesis, which may be represented by the simplified chemical reaction:

The oxidizable organic materials that are produced by photosynthesis and which determine the properties of the plant matter of relevance to biomass energy utilization are carbohydrates and lignin.

All of the carbohydrates present are saccharides (i.e., sugars) or polymers of sugars (i.e., polysaccharides) that fall into three types: (i) starch, (ii) cellulose, and (iii) hemicellulose. The simple sugars include glucose, fructose, and the like, while the polymeric sugars such as cellulose and starch can be readily broken down to their constituent monomers by hydrolysis, preparatory to conversion to ethanol or other chemicals.

Starch is a granular polysaccharide which accumulates in the storage tissues of plants such as seeds, tubers, roots, and stem pith. It is an important constituent of corn, potato, rice, and tapioca. Starch consists of 10 to 20% amylose, which is water soluble, and 80 to 90% amylopectin, which is insoluble. Both the constituents of starch are polymers of glucose, with amylose linked in chain structures, while amylopectin is a highly branched structure. Starch is not as chemically resistant as cellulose, and can be readily hydrolyzed by dilute acids and enzymes to fermentable sugars.

Hemicelluloses are polysaccharides that occur in association with cellulose. They are chemically different from cellulose, are amorphous, and have much lower molecular weight mass. While cellulose is built from the single sugar glucose, most hemicelluloses contain two to four different sugars as building blocks. Glucose is a component of some hemicelluloses, although xylose is a dominant sugar in hardwood hemicellulose, and mannose is important in softwood hemicellulose. Unlike the other sugars described so far, xylose contains only 5 carbon atoms and is a pentose.

The fraction of the cellulose containing xylose polymers is often referred to as pentosan. Hemicellulose is more soluble than cellulose, is dissolved by dilute alkaline solutions, and can be relatively readily hydrolyzed to fermentable sugars.

In contrast, lignin is a complex structure containing aromatic groups and is less readily degraded. Although lignocellulose is one of the cheapest and most abundant forms of biomass, it is difficult to convert this relatively unreactive material into sugars. Among other factors, the walls of lignocellulose are composed of lignin, which must be broken down in order to render the cellulose and hemicellulose accessible to acid hydrolysis. For this reason, many programs focused on ethanol production from biomass are based almost entirely on the fermentation of sugars derived from the starch in corn grain.

Lignin is the final major constituent of plant material important to biomass processing, and it is a complex chemical compound that is most commonly derived from wood and is an integral part of the cell walls of plants. The chemical structure of lignin is unknown and, at best, can only be represented by a hypothetical formula, the veracity of which is questionable. In fact, lignin is one of most abundant organic compounds on earth after cellulose and chitin.

By way of clarification, chitin (C₈H₁₃O₅N))_n is a long-chain polymeric polysaccharide of ß-glucose that forms a hard, semitransparent material found throughout the natural world. Chitin is the main component of the cell walls of fungi and is also a major component of the exoskeletons of arthropods, such as the crustaceans (e.g., crab, lobster, and shrimp), and the insects (e.g., ants, beetles, and butterflies), and of the beaks of cephalopods (e.g., squids and octopuses).

Like hemicellulose, lignin is amorphous and more soluble than cellulose. It may be removed from wood by steaming or by dissolving in hot aqueous or aqueous bisulfite solution. Lignin resists hydrolysis and is resistant to microbial degradation.

Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components and is covalently linked to hemicellulose. Lignin also forms covalent bonds to polysaccharides and thereby cross-links different plant polysaccharides. It confers mechanical strength to the cell wall (stabilizing the mature cell wall) and therefore the entire plant.

See also: Hemicellulose, Lignin, Starch.