Page 21 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-61 May 19, 2001 19:33

146 Biomass, Bioengineering of

economic growth, in fact, is a substitution of meat for As with the starch crops, most of these oilseed crops
grains in the human diet. Animals convert grain to meat produce one or more residues that are rich in lignocellu-
or milk with widely varying efficiencies, however. Fish, lose. For example, soybean straw is typically left in the
poultry, and swine are relatively efficient converters, while fields when the beans are harvested. Soybean hulls are
beef cattle are considerably less efficient. produced as “wastes” at the oilseed processing plant. In
Most grain crops produce a byproduct, or residue, that the United States, approximately 10 million tons per year
is primarily composed of cellulose, hemicellulose, and of these soybean hulls are produced as a byproduct of
lignin, called collectively lignocellulose. Thus very large soybean crushing operations.
tonnages of rice straw, corn straw (called corn stover), and
many other straws are produced as a low value (and low
4. Tree and Fiber Crops
cost) byproduct of grain production. Approximately 1 to
2 tons (dry weight) of these straws are produced per dry In contrast with the crops mentioned, essentially all of
ton of grain. Using this ”rule of thumb,” the total world- the wood harvested is destined for industrial uses, rather
wide production of just corn stover and rice straw is in the than food/feed uses. Production of wood for lumber in the
neighborhood of 1 billion tons per year. Taken together United States amounts to about 170 million tons per year
with sugar cane bagasse production, the total amount of while U.S. pulpwood production (destined for all kinds
corn stover, rice straw and bagasse produced each year is of paper uses) is about 90 million tons/year. A wide vari-
approximately 2 billion tons. ety of industrial chemicals such as turpentine, gums, fats,
Verylargequantitiesofotherstrawsandcropprocessing oils, and fatty acids are produced as byproducts of pulp
residues are also produced. Many such residues are pro- manufacture.
duced at centralized processing facilities. While some of Not all paper is derived from trees, however. Some
this residual plant matter should be left on the field, much grasses and crop residues such as kenaf and sugar cane
of it can be removed and used elsewhere without degrad- bagasse have been used or are being considered as
ing soil fertility. For example, rice straw is often burned fiber/paper crops. The giant reed kenaf, in particular, has
to clear the fields for the next crop. There is considerable very rapid growth rates and favorable agronomic char-
political pressure in the United States and elsewhere to acteristics. A major impediment to its introduction as an
eliminate or greatly reduce this practice of field burning. alternative newsprint crop seems to be the huge capital
investment required for a new pulp and paper plant.
The growing worldwide demand for paper products of
3. Plant Oil and Protein Crops
all kinds may limit the ability to use tree and pulpwood
There are many different plant oil crops including soy- crops for other industrial applications, given the value of
beans, palm, coconut, canola, sunflower, peanut, olive and long plant fibers in paper production. Even short rota-
others. The total worldwide production of fats and oils by tion woody crops (trees grown for energy use as if they
these crops exceeds 75 million tons per year, with an addi- were grasses), must cope with the demand for that land
tional 12 million tons per year or so of animal fats. (An oil and the long fibers grown on it for pulp and paper uses.
is simply a liquid fat.) Most plant oils go into human foods Typical pulp prices are in the neighborhood of $600 per
or animal feeds. However, there is a very long history of ton or $0.30 per pound, a high raw material cost hur-
also using and modifying plant oils for fuels, lubricants, dle indeed for commodity chemicals that are often tar-
soaps, paints and other industrial uses. Oils consist chem- geted to sell for less than $0.30 per pound. Some residues
ically of an alcohol (glycerol) to which are attached three from fiber crop production and processing may be avail-
long chain carboxylic acids of varying composition. Plant able at much lower cost and could perhaps be used for
oil composition varies widely with species and the com- chemical and fuel production. Typically these residues are
position strongly affects the industrial uses to which these burned to get rid of them and recover at least their energy
oils can be put. Therefore by modifying these oils, they value.
can potentially be tailored to desired applications. The most important fiber crop is cotton. Worldwide pro-
The other major product of oilseed crops is a high pro- duction of cotton in 1998 totaled about 91 million bales,
tein “meal,” usually produced by expelling or extracting each weighing about 480 lb. Given the high value textile
the oil from the seed. Total world production of high pro- uses of cotton, it is similarly unlikely that much cotton will
tein meals from oilseeds is approximately 180 million tons be devoted to other uses. However, there are many mil-
per year. The predominant oilseed meal is soybean meal lions of tons of wastes generated at cotton gins and mills
containing approximately 44% protein. While there are that might be used industrially if appropriate, low-cost,
some industrial uses for this meal, the bulk of it is fed to conversion technologies were available. Chemically, these
animals. tree and fiber crops and their residues are essentially all