Regulation of Fuels and Fuel Additives: Renewable Fuel Standard ...

To produce ethanol from cellulosic feedstocks, pretreatment is necessary to
hydrolyze cellulosic and hemicellulosic polymers and break down the lignin sheath. In
so doing, the structure of the cellulosic feedstock is opened to allow efficient and
effective enzyme hydrolysis of the cellulose/hemicellulose to glucose and xylose. The
central problem is that the β-linked saccharide polymers in the cellulose/hemicellulose
structure prevent the microbial fermentation reaction. By comparison, when corn kernels
are used as feedstock, fermentation of the starch produced from the corn kernels which
have α-linked saccharide polymers takes place much more readily. An acid hydrolysis
process was developed to pretreat cellulosic feedstocks (through hydrolysis which breaks
up the β-links), but it continues to be prohibitively expensive for producing ethanol.
Some technologies that are being developed may solve some of the problems
associated with production of ethanol from cellulosic sources. Specifically, one problem
with cellulosic feedstocks is that the hydrolysis reactions produce both glucose, a sixcarbon
sugar, and xylose, a five-carbon sugar (pentose sugar, C5H10O5; sometimes called
“wood sugar”). Early conversion technology required different microbes to ferment
each sugar. Recent research has developed better cellulose hydrolysis enzymes and
ethanol-fermenting organisms. Now, glucose and xylose can be co-fermented—hence,
the present-day terminology: weak-acid enzymatic hydrolysis and co-fermentation. In
addition, several research groups, using recently developed genome modifying
technology, have been able to produce a variety of new or modified enzymes and
microbes that show promise for use in a process known as weak-acid, enzymaticprehydrolysis.
Cellulosic biomass can come from a variety of sources. Because the conversion
of cellulosic biomass to ethanol has not yet been commercially demonstrated, we cannot
say at this time which feedstocks are superior to others. In particular, there is only one
cellulosic ethanol plant in North America (Iogen, Ottawa, Ontario, Canada). To the best
of our knowledge, the technology that Iogen employs is not yet fully developed or
optimized. Generally, the industry seems to be moving toward a process that uses dilute
acid enzymatic prehydrolysis with simultaneous saccharification (enzymatic) and cofermentation.
3. Renewable Fuel Distribution System Capability
Ethanol and biodiesel blended fuels are not shipped by petroleum product
pipeline due to operational issues and additional cost factors. Hence, a separate
distribution system is needed for ethanol and biodiesel up to the point where they are
blended into petroleum-based fuel as it is loaded into tank trucks for delivery to retail and
fleet operators. In cases where ethanol and biodiesel are produced within 200 miles of a
terminal, trucking is often the preferred means of distribution. For longer shipping
distances, the preferred method of bringing renewable fuels to terminals is by rail and
barge.
- 132 -