Environmental Engineering Reference
In-Depth Information
and grasses to alcohol. However, the high cost of feedstock and the cost of production have placed
barriers for being widely accepted.
Assuming corn produces 80 gal of ethanol per ton, a 50-million gallon corn-fed facility would
require 625,000 tons of feedstock annually, harvested at the rate of 4.29 tons/acre national average
yield or the equivalent of 145,690 growing acres (7344 acres corn/day). Paulownia grown specifically
for cellulosic ethanol production would be planted on a grid of 8- by 8-ft spacing or 680 plants/acre,
and unlike the practices used for forestry (growing for lumber) the plants would be manipulated for
multiple sprouts (coppicing). Approximately 8-10 sprouts emerge from each plant. The Paulownia
can be harvested each year or left to grow for 2- to 3-year harvest cycles. Once harvested, the
Paulownia will regenerate from the stump and the cycle is repeated. Another distinct advantage of
tree-type cellulosic feedstock is that one can harvest year-round and supply fiber to the production
facility on a daily basis. Corn and row crops must be harvested when the crop is ready because they
are seasonal. The latter poses numerous problems—not only the large-scale harvesting, but also the
transportation, drying, and storage of hundreds of thousands of tons of fiber, all delivered in one
month to be used over the year.
We have evaluated Paulownia wood and found the composition to be 14.0% extractive, 50.55%
cellulose, 21.36% lignin, 0.49% ashes, and 13.6% hemicellulose (N. Joshee et al. unpublished research).
This material is an attractive candidate for thermochemical conversion to fuels and energy through
pyrolysis or gasification. The lignin content may provide the opportunity to extract this lignin for
higher-value uses before thermochemical conversion. The resulting syngas will also be lower in tars.
Plant design, bioprocess engineering, and biomass processing strategies are intimately linked.
Plants have evolved complex mechanisms for resisting assault on their structural sugars (wall
polymers) from the microbial and animal kingdom. Cell-wall polymer organization and interactions
are formidable barriers to access by depolymerizing enzymes and must be deconstructed in the
pretreatment step to obtain adequate rates of release and sugar yields.
27.5.5 E
conomic
J
uStification
A biofuel industry would create jobs and ensure growing energy supplies to support national and
global prosperity. In 2004, the ethanol industry created 147,000 jobs in all sectors of the economy
and provided more than $2 billion of additional tax revenue (RFA 2005). Growing energy crops
and harvesting agricultural residuals are projected to increase the value of farm crops, potentially
eliminating the need for some agricultural subsidies. Finally, cellulosic ethanol provides positive
environmental benefits in the form of reductions in greenhouse gas emissions and air pollution.
Another advantage of tree-type cellulosic feedstock is that one can harvest year-round and supply
fiber to the production facility on a daily basis. According to the estimates of our collaborator, WPI
(Lenox, GA), Paulownia producing 80 gal of ethanol per dry ton for a 50-million-gallon-per-year
facility would require 1712 tens of fiber per day or the equivalent of 50 acres of fiber per day to
be harvested. It would require 18,250 acres of Paulownia to sustain the supply to the facility year-
round. Excluding the cost of land, Paulownia grown on land contiguous or close to the production
facility could be supplied for less than $40.00 per dry ton, one-third of the cost predicted for corn.
The argument in favor of cellulosic ethanol as a replacement for gasoline is compelling. Cellulosic
ethanol will reduce dependence on imported oil, increase energy security, and reduce the trade
deficit for many nations. The benefit for rural economies will be in the form of increased incomes
and jobs. The use of food grains for ethanol production has caused a steep rise in the price of corn-
and wheat-based products and the related dairy, meat, and cattle feed industries.
27.6 PauloWnIa BIotechnoloGy
The wide use of forest-tree products and the progressive deterioration of natural forests mean that
foresters can no longer rely on the exploitation of existing forests. Extensive accelerated breeding
