Page 240 - Synthetic Fuels Handbook

226 CHAPTER EIGHT

The sugars produced by these crops are easily fermented by Saccharomyces cerevisiae.
The sucrose produced by sugarcane, sugar beet, and sweet sorghum can be fermented
directly after squeezing them from the crop. Corn traps its carbohydrate largely in the form
of starch which must first be converted into glucose through saccharification with glu-
coamylase. The residues left over after removing fermentable sugars can also be utilized.
In some cases they end up as animal feeds, but many agricultural residues can be converted
into additional fermentable sugars through saccharification with cellulases and hemicellu-
lases. The hemicellulose sugars are not fermentable by S. cerevisiae, and must be converted
to ethanol by pentose fermenting yeasts or genetically engineered organisms.
Bioenergy crops include fast-growing trees such as hybrid poplar, black locust, willow,
and silver maple in addition to annual crops such as corn, sweet sorghum, and perennial
grasses such as switch grass.
Briefly, switch grass is a thin-stemmed, warm season, perennial grass that has shown
high potential as a high yielding crop that can be readily grown in areas that are also suitable
for crop production. In fact, there are many perennial crops (grass and tree species) that
show high potential for production of cost-competitive cellulosic biomass. Switch grass
can be viewed as a surrogate for many perennial energy crops when estimating biomass
supply and availability.
Many other crops are possible and the optimal crop will vary with growing season and
other environmental factors. Most fast-growing woody and annual crops are high in hemi-
cellulose sugars such as xylose.
Corn stalks and wheat straws are the two agricultural residues produced in the largest
quantities. However, many other residues such as potato and beet waste may be prevalent in
some regions. In addition to quantity it is necessary to consider density and water content
(which may restrict the feasibility of transportation) and seasonality which may restrict
the ability of the conversion plant to operate on a year-round basis. Facilities designed to
use seasonal crops will need adequate storage space and should also be flexible enough to
accommodate alternative feedstocks such as wood residues or other wastes in order to oper-
ate year-around. Some agricultural residues need to be left in the field in order to increase
tilth (the state of aggregation of soil and its condition for supporting plant growth and to
reduce erosion) but some residues such as corncobs can be removed and converted without
much difficultly.
Similar to herbaceous crops, straw usually has lower moisture content than woody bio-
mass. Conversely, it has a lower calorific value, bulk density, ash melting point and higher
content of ash, and problematic inorganic component such as chlorine, potassium, and
sulfur, which cause corrosion and pollution. The last two drawbacks can be relatively easily
overcome by leaving straw on the field for a while. In such a way rainfall “washes” it
naturally from a large part of potassium and chlorine. Alternatively, fresh straw can be
directly shipped to the gasification plant, where it is washed by dedicated facilities at mod-
erate temperatures (50–60°C). Due to washing, the initially low moisture content of straw
becomes higher in both cases and hence a mandatory drying is applied afterward. In both
cases also the content of corrosive components is reduced, but not completely taken out.
In order to decrease handling costs, straw and dedicated herbaceous energy crops are usu-
ally baled before being shipped to the gasification plant. The weight and the size of bales
depend on the baling equipment and on the requirements of the gasification plant (Van Loo
and Koppejan, 2003; NSCA, 2004).
Softwood residues are generally in high demand as feedstocks for paper production,
but hardwood timber residues have less demand and fewer competing uses. In the past, as
much as 50 percent of the tree was left on site at the time of harvest. Whole tree harvest
systems for pulp chips recover a much larger fraction of the wood. Wood harvests for timber
production often generates residues which may be left on the site or recovered for pulp pro-
duction. Economics of wood recovery depend greatly on accessibility and local demand.