Photo: Bioreactor using wood chips as a source for cellulosic ethanol production. USDA photo by Alice Welch via Wikimedia Commons.
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The vast majority of Canada’s fuel ethanol is currently derived from corn and wheat. The long history of industrial-scale sugar fermentation positioned this technology to hit the ground running. Corn ethanol has environmental benefits, but the diversion of a food crop for fuel production has been criticized Change may be coming for fuel ethanol. Scientific research and industrial innovation are “fueling” a shift away from grain feedstock toward non-food cellulosic feedstock such as wheat straw, corn stover (stalks and leaves), and forestry waste.
A Molecular Match-Up of Cellulose and Starch
Cellulose is the most abundant organic molecule on Earth, and it is made by plants to provide strength. Starch and cellulose are both polysaccharide polymers made of long chains of covalently bonded glucose units. Alpha-1,4-glycosidic bonds join glucose molecules to make starch, but cellulose uses beta-1,4-glycosidic bonds. This may sound like a small difference, but it has big consequences. Starch polymers are water-soluble, have many branches, are packaged in granules (blobs) and are broken down by amylase enzymes. Cellulose polymers are water-insoluble, unbranched chains that are packed very tightly together and are broken down by cellulase enzymes. The molecular difference between starch and cellulose is shown in Figure 1. Because cellulose chains pack tightly together, they are very strong - think about the strength of wood, cotton fibres, corncobs or wheat stalks.
In plants, cellulose is tightly woven with two other large biological polymers: hemicellulose and lignin. Hemicellulose is another branched polysaccharide, but it has a variable composition. It contains a mixture of 5-carbon sugars (pentoses: xylose and arabinose) and 6-carbon sugars (hexoses: mannose and galactose) with both alpha and beta linkages between them. Lignin, a complex polymer with a variable composition, binds plant cell walls and vessels together and is responsible for the tough, woody characteristics of tree trunks and straw.
The Biochemical and Thermochemical Paths from Cellulose to Ethanol
The ethanol that is produced from cellulosic material is exactly the same as the ethanol that is derived from grains. Chemically speaking, ethanol is ethanol. However, it is much more challenging to start with cellulose because it is entangled with hemicellulose and lignin. There are two mains ways to turn cellulosic feedstock into ethanol: biochemical conversion and thermochemical conversion.
Here are the main steps for biochemical conversion (also outlined in Figure 2A on page 3):
- Grinding: The feedstock is mechanically ground into small pieces.
- Pretreatment: The cellulose, hemicellulose and lignin are untangled using physical (steam explosion) or chemical (acid or alkali) treatments. Sometimes, this pretreatment releases free sugars from the hemicellulose polymer.
- Enzyme Treatment (Saccharification): Cellulase enzymes are added to catalyze the breakdown of the cellulose chains into glucose. Hemicellulase enzymes can also be used if the pretreatment doesn’t break the hemicellulose down. Fungi, and sometime bacteria, are the source of the cellulase and hemicellulase enzymes.
- Fermentation: Yeast cells metabolize the glucose into ethanol through fermentation, exactly as described for corn ethanol production. The various hemicellulose sugars can also be fermented to ethanol by different microorganisms.
- Distillation, Dehydration, Denaturation: The ethanol from the fermentation tanks is treated exactly as described for corn ethanol production.
- Lignin Utilization: Lignin can’t be fermented to produce ethanol, but it can be burned to generate heat and electricity to power the cellulosic ethanol biorefinery.
Here are the main steps for thermochemical conversion (also outlined in Figure 2B on next page):
- Feedstock Preparation: The feedstock is dried, and the cellulose, hemicellulose and lignin stay tangled together.
- Gasification: The dried feedstock is burned to generate a synthesis gas (syngas) composed mainly of carbon monoxide (CO) and hydrogen (H2). Gasification occurs at extremely high temperatures of 700-1000oC.
- Tar and Sulphur Conversion: Two byproducts of gasification, tar and sulphur, are chemically converted into syngas.
- Syngas Cleaning and Conditioning: Any remaining byproducts or contaminants are removed and the syngas is compressed.
- Catalytic Conversion to Ethanol: The compressed syngas is passed over a metal catalyst to convert the CO and H2 into ethanol (CH3CH2OH). Common metal catalysts are rhodium and cobalt. Some species of bacteria can act as microbial catalysts to convert syngas into ethanol.
The thermochemical process converts cellulose, hemicellulose and lignin to ethanol via syngas, so it has the largest ethanol yield from cellulosic feedstock. Syngas can also be converted to other alcohols (e.g., methanol) and hydrocarbons (e.g., gasoline).
Cellulosic Ethanol Beats Corn Ethanol in an Environmental Face Off
Cellulosic ethanol has the same environmental benefits as corn ethanol. It is a renewable, non-fossil fuel that reduces GHG emissions. A 2012 Canadian lifecycle analysis calculated that E100 corn ethanol reduced GHG emissions by 45% compared to gasoline. Cellulosic ethanol produced using biochemical conversion from corn stover came out even better, at a 68% reduction in GHG emissions (for E100). The best performer was cellulosic ethanol produced using thermochemical conversion from wood, at a whopping 93% reduction (for E100).
The same model was used to calculate the lifecycle energy balance of different first and second generation ethanol production technologies. Remember that a positive energy balance means more energy is obtained from a fuel than is used in its production. There are two different ways to examine energy balance. The first includes all sources of energy that are used during fuel production, including renewable energy. In this scenario, corn ethanol has a small positive total energy balance, yielding about 60% more energy than it takes to produce. In contrast, both biochemical conversion of corn stover and thermochemical conversion of wood have negative total energy balances. This is because both conversion processes are very energy intensive. However, recall that cellulosic biomass itself can provide energy when lignin is burned. When this renewable biomass energy is removed from the energy balance calculations, the fossil energy balance of corn stover is bigger, with a yield of 2.6 times the input energy. Most impressive is the fossil energy balance for thermochemical wood conversion, with a yield almost 50 times the input.
Using corn stover, wheat straw and forestry wastes as cellulosic ethanol feedstock avoids the thorny issue of using food grains for fuel production. One of the world leaders in biochemical conversion of agricultural waste is the Canadian company Iogen (http://www.iogen.ca). There are also dedicated cellulosic ethanol crops, such as grasses and fast-growing trees like poplars. Dedicated fuel crops sometimes use land that could be used for food crops, which is a variation on the same ‘food versus fuel’ issue. However, some energy crops can grow on ‘marginal’ land that is not nutrient-rich enough for food crops. One Canadian company called Enerkem has optimized their thermochemical technology to make cellulosic ethanol from municipal waste, turning garbage into fuel (http://enerkem.com/en/home.html). Worldwide, other waste products are being studied for their ethanol production potential.
Widespread commercialization of cellulosic ethanol production has been limited mostly by the enormous cost of building and running a cellulosic ethanol biorefinery. There is still some work to go to make it economically feasible.
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