Biofuels in Perspective

Sustainable Production of Cellulosic Feedstock 29
concentrations. The Chicago Climate Exchange (CCX), a voluntary greenhouse gas trading
market, allows farmers to sell the climate benefits of their cropping practice to industries
wishing to offset their emissions. The Exchange offers farmers a conservative 0.5 metric ton
per acre ‘exchange soil offset’ credit for no-till operations, which translates to roughly $1
per acre at current CCX credit prices of $1.65 to $2.00 per metric ton CO2. The Iowa Farm
Bureau and other regional growers associations have organized farmers to collectively sell
their credits on the CCX market.
In European markets, where mandatory limits on greenhouse gas emissions exist, carbon
credit prices have ranged between $10 and $30 per metric ton CO2, suggesting that if
mandatory greenhouse gas emissions limits are established in the United States, benefits to
farmers of no-till adoption could exceed $10 per acre, further driving the transition to no-till.
2.11 Pretreatment
In addition to the challenges of sustainable production, collection and transport, cellulosic
biomass presents a unique problem in that, unlike traditional starch feedstocks, it is not
readily hydrolyzed into usable sugars. Cellulosic biomass must first be ‘pre-treated’ to
make the sugars available to chemical or biological hydrolysis.
Due to nature’s barriers, cellulose in biomass feedstock is ‘recalcitrant’ to degrade,
ensheathed in protective layers of lignin and hemicellulose (see Figure 2.12).
The cell walls are intermeshed with carbohydrate and lignin polymers and other minor
constituents. The major components are cellulose, hemicellulose, and lignin. These
polymers have different properties, reacting in different ways to thermal, chemical, and
biological processing.
Historically thermal and chemical processes overcame this recalcitrance by sheer energy
input and severe chemical treatment. Typical processes included gasification, pyrolysis,
Fischer-Tropsch and concentrated sulfuric acid for conversion to fuels and chemicals. 26–33
These processes developed when supplies of fossil fuels were disrupted during World
Wars I and II or fuel imports embargoed under UN sanctions. Mechanical preprocessing
is simple – pelletizing for thermoprocessing or commutation for concentrated acid treatment.
For thermo processing, mechanical compaction increases energy density and eases
bulk handling. Particle size reduction-mechanical grinding increases the surface area for
improved acid processing.
Figure 2.12 Lignocellulosic biomass cell wall.
Source: National Renewable Energy Laboratory.
Lignin
Hemicelluloses and
other polysaccharides
Cellulose microfibrils
Matrix polymers