Page 294 - Synthetic Fuels Handbook

280 CHAPTER NINE

The greatest potential for ethanol production from biomass, however, lies in enzymatic
hydrolysis of cellulose. The enzyme cellulase, now used in the textile industry to stone
wash denim and in detergents, simply replaces the sulfuric acid in the hydrolysis step. The
cellulase can be used at lower temperatures, 30 to 50°C, which reduces the degradation
of the sugar. In addition, process improvements now allow simultaneous saccharification
and fermentation (SSF). In the SSF process, cellulase and fermenting yeast are combined,
so that as sugars are produced, the fermentative organisms convert them to ethanol in the
same step.
Once the hydrolysis of the cellulose is achieved, the resulting sugars must be fermented
to produce ethanol. In addition to glucose, hydrolysis produces other six-carbon sugars
from cellulose and five-carbon sugars from hemicellulose that are not readily fermented to
ethanol by naturally occurring organisms. They can be converted to ethanol by genetically
engineered yeasts that are currently available, but the ethanol yields are not sufficient to
make the process economically attractive. It also remains to be seen whether the yeasts can
be made hardy enough for production of ethanol on a commercial scale.
A large variety of feedstocks is currently available for producing ethanol from cellulosic
biomass. The materials being considered can be categorized as agricultural waste, forest
residue, and energy crops. Agricultural waste available for ethanol conversion includes
crop residues such as wheat straw, corn stover (leaves, stalks, and cobs), rice straw, and
bagasse (sugar cane waste). Forestry waste includes underutilized wood and logging resi-
dues; rough, rotten, and salvable dead wood; and excess saplings and small trees. Energy
crops, developed and grown specifically for fuel, include fast-growing trees, shrubs, and
grasses such as hybrid poplars, willows, and switchgrass.
Although the choice of feedstock for ethanol conversion is largely a cost issue, feed-
stock selection has also focused on environmental issues. Materials normally targeted for
disposal include forest thinnings collected as part of an effort to improve forest health, and
certain agricultural residues, such as rice straw. Although forest residues are not large in
volume, they represent an opportunity to decrease the fire hazard associated with the dead
wood present in many forests. Small quantities of forest thinnings can be collected at rela-
tively low cost, but collection costs rise rapidly as quantities increase.
Agricultural residues, in particular corn stover, represent a tremendous resource base for
biomass ethanol production. Agricultural residues, in the long-term, would be the sources
of biomass that could support substantial growth of the ethanol industry. At conversion
yields of around 60 to 100 gal/dry ton, the available corn stover inventory would be suf-
ficient to support 7 to 12 billion gallons of ethanol production per year.
The cost of agricultural residues is not nearly as sensitive to supply as is the cost of for-
est residues, although the availability of corn stover could be affected by a poor crop year.
The relatively low rise in cost as a function of feedstock use is due to the relatively high
density of material available that does not involve competition for farmland. In addition, the
feedstock is located in the corn-processing belt, an area that has an established infrastruc-
ture for collecting and transporting agricultural materials. It is also located near existing
grain ethanol plants, which could be expanded to produce ethanol from stover. Initially,
locally available labor and residue collection equipment might have to be supplemented
with labor and equipment brought in from other locations for residue harvesting and storage
operations, if the plants involved are of sufficient scale. Eventually, however, when the local
collection infrastructure has been built up, costs would come down.
Dedicated energy crops such as switchgrass, hybrid willow, and hybrid poplar are
another long-term feedstock option. Switchgrass is grown on a 10-year crop rotation basis,
and harvest can begin in the first year in some locations and the second year in others.
Willows require a 22-year rotation, with the first harvest in the fourth year and subsequent
harvests every 3 years thereafter. Hybrid poplar requires 6 years to reach harvest age in the
Pacific Northwest, 8 years in the Southeast, Southern Plains, and South Central regions,