Cellulosic Ethanol
Many supporters of renewable biofuels see cellulosic ethanol as our best hope in the next few decades. The attraction lies in the sheer volume of potential feedstocks. Cellulosic ethanol (or butanol) does not refer to a distinct end product. Ethanol made from cellulosic materials is the same as ethanol from corn or sugar cane. The distinction lies in the feedstocks. Cellulosic biofuels can be made from practically any organic material. The U.S. Department of Energy (DOE) Biomass Program lists the following categories of cellulosic feedstocks:[i]
● agricultural residues (leftover material from crops, such as the stalks, leaves, and husks of corn plants)
● forestry wastes (chips and sawdust from lumber mills, dead trees, and tree branches)
● municipal solid waste (household garbage and paper products)
● food processing and other industrial wastes (black liquor, a paper manufacturing by-product)
● energy crops (fast-growing trees and grasses) developed just for this purpose
Diverse feedstocks will allow an expansion in ethanol output. The challenge is in liberating plant sugars from the grip of cellulose, hemicellulose, and lignin. The following descriptions of these complex polymers are from the U.S. DOE:
● Cellulose is the most common form of carbon in biomass, accounting for
40%–60% by weight of the biomass, depending on the biomass source. It is a
complex sugar polymer, or polysaccharide, made from the six-carbon sugar,
glucose. Its crystalline structure makes it resistant to hydrolysis, the
chemical reaction that releases simple, fermentable sugars from a
polysaccharide.
● Hemicellulose is also a major source of carbon in biomass,
at levels of between 20% and 40% by weight. It is a complex polysaccharide made
from a variety of five and six-carbon sugars. It is relatively easy to hydrolyze
into simple sugars but the sugars are difficult to ferment to ethanol.
● Lignin is a complex polymer, which provides structural integrity in
plants. It makes up 10% to 24% by weight of biomass. It remains as residual
material after the sugars in the biomass have been converted to ethanol. It
contains a lot of energy and can be burned to produce steam and electricity for
the biomass-to-ethanol process.[ii]
See chapter 9 of Sustainable Ethanol for the following addtional topics:
- Commercializing Cellulosic Production
- Cellulosic Conversion Technologies
- Biochemical Methods
- Thermochemical Methods
- Biogas as a Transportation Fuel
- Waste & Coproduct Feedstocks
- Dedicated Cellulosic Energy Crops
- A Sticky Coproduct
- Harvest and Transportation of Feedstocks
- The Pyrolysis Route to Cellulosic Ethanol
- Regional Biomass Processing Centers
- Pipeline Transportation of Corn Stover Silage
- Economics of Cellulosic Ethanol
- How Much Ethanol Can We Make?
- Figure 9-1: The Biochemical Cellulosic Production Process
- Figure 9-2: Annual Biomass Potential from U.S. Forests and Agriculture
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Notes
[i]. U.S. DOE Energy Efficiency and Renewable Energy Biomass Program, Information Resources, 2007, http://www1.eere.energy.gov/biomass/abcs_biofuels.html.
[ii]. Ibid.
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