Typical organic components in representative, mature biomass species are shown in Table 3.9 along with the corresponding ash contents. With few exceptions, the order of abundance of the major organic components in whole — plant samples of terrestrial biomass is celluloses, hemicelluloses, lignins, and proteins. Aquatic biomass does not appear to follow this trend. The cellulosic components are often much lower in concentration than the hemicelluloses as illustrated by the data for water hyacinth. Other carbohydrates and deriva­tives are dominant in species such as giant brown kelp to almost complete exclusion of the celluloses. The hemicelluloses and lignins have not been found in this species.

Biomass often undergoes compositional changes, some of which can be subtle or pronounced, during growth and sometimes after harvesting depend­ing on age of the biomass and environmental factors. An example of this phenomenon is the gradual decrease in sugar content and the gradual increase in hydrocarbon content during the maturation of E. lathyris (Ayerbe et ai, 1984). Another phenomenon that is quite common during biomass growth is the nonuniform distribution of organic components in various plant parts. For example, the hydrocarbon content in the leaves of E. lathyris is more than twice the amount in the stems (Sachs et al, 1981). All of these factors must be considered in some detail when biomass is utilized for production of cer­tain organic compounds or as a feedstock for conversion to fuels and energy products.

Alpha cellulose or cellulose as it is more generally known, is the chief structural element and a major constituent of many biomass species. In trees, cellulose is generally about 40 to 50% of the dry weight. As a general rule, the major organic components on a moisture and ash-free basis in woody biomass are about 50 wt % cellulosics, 25 wt % hemicelluloses, and 25 wt % lignins. However, cellulose is not always the dominant component in the carbohydrate fraction of biomass. As just mentioned and as shown in Table 3.9, it is one of the minor components in giant brown kelp. Mannitol, a hexahydric alcohol that can be formed by reduction of the aldehyde group of D-glucose to a methylol group, and alginic acid, a polymer of mannuronic and glucuronic acids, are the major carbohydrates.

The lipid and protein fractions of plant biomass are normally much less on a percentage basis than the carbohydrate components. The lipids are usually present at the lowest concentration, while the protein fraction is somewhat

higher, but still lower than the carbohydrate fraction. Crude protein values can be approximated by multiplying the organic nitrogen analyses by 6.25. This factor is used because the average weight percentage of nitrogen in pure dry protein is about 16%, although the protein content of each biomass species can best be determined by amino acid assay. The calculated crude protein values of the dry biomass species in Table 3.9 range from a low of about 0 wt % for pine wood to a high of about 30 wt % for Kentucky bluegrass. For grasses, the protein content is strongly dependent on the growing procedures used before harvest, particularly the fertilization methods. Some biomass spe­cies such as the legumes, however, fix nitrogen from the ambient atmosphere and often contain high protein concentrations.

It is apparent from the data in Table 3.5 that the sulfur content of virgin and waste biomass ranges from very low to about 1 wt % for primary biosolids. This sulfur level is similar to the sulfur content of high-sulfur Illinois bitumi­nous coal. Woody biomass generally contains very little sulfur.