Agricultural biomass and biowaste include crop residues and wood, food processing waste, animal manure, and algae. Crop residues and wood primarily contain lignocellulose, while animal and food processing waste contains lipids, protein, and usually small amounts of lignocellulose (except ruminant animal manure). In this section, the primary compounds and structure in these feedstocks will be summarized and the potential functions in hydro­thermal are explored, in the hope that it provides some bases to understand the interaction of carbohydrates with water (H+ or H * radicals).

Lignocellulose

Lignocellulosic compounds belong to the carbohydrates group of organic compounds. Carbohydrates are hydroxy aldehydes, hydroxy ketones, or substances derived from them. It is the principal substance that composes plants. The carbohydrates in swine manure come from both digested and undigested feed, and food processing waste contains carbohydrates food and lignocellulocic sources.

Glucose is one of the simplest monosaccharides. The isomers of glucose are shown in Figure 10.3. There are four chiral carbon atoms in the molecule. The carbon atoms at the end of the molecule do not hold four different groups and are not chiral. D-glucose forms a cyclic molecule by an addition reaction involving the carbonyl group and a hydroxyl group. The ring formation produces a new chiral center, and two isomers of D-glucose exist that differ in the orientation of the new OH group (Figure 10.4). Notice the a form of the OH group of the extreme right right-hand carbon atom is on the same side of the ring as the OH group of the adjacent carbon atom. In aqueous solution, a and в forms of the D-glucose exist in equilibrium, together with a low concentration of the open-chain form.

H

C==O

Figure 10.4. Ring forms of glucose.

Cellulose is a polysaccharide that only yields D-glucose upon hydrolysis. The number of D-glucose units in the molecular structures is estimated to be as high as several thousands. The D-glucose units of cellulose are linked in long chains in в combination shown in Figure 10.5. The structure is stabilized by hydrogen bonds between adjacent D-glucose units in the same strand. Cellulose occurs in fibrils brought about by hydrogen bonds among different strands.

Hemicelluloses are polysaccharides that are chemically related to cellulose, having back­bones of 1,4-P-linked major sugar units, and being morphologically strongly associated with cellulose in the plant cell walls as well as to lignin in lignified cell walls. These polysac­charides are generally heterogeneous, built up of different hexoses (C6-sugars) and pentoses (C5-sugars), sometimes in addition to uronic acids. They have a lower degree of polymer­ization than cellulose (100-200 units), are largely soluble in alkali, and also more easily hydrolyzed (Figure 10.6).

The chemical structure of lignin is more complex than cellulose and hemicellulose. It resembles a network of aromatic compounds linked together in a more random fashion. The structure varies depending on source. To illustrate, the structure of a possible lignin molecule is shown in Figure 10.7. Lignin has high carbon content typically more than 60% and about 30% oxygen. Although in smaller amounts than cellulose, lignin represents about half of the available combustible energy in naturally occurring sources (Glasser 1985). Thermal decom­position of lignin occurs above 280°C depending on the source of lignin (Chornet and Overend 1985).

Figure 10.8. General structure of amino acid compounds.