Cellulose forms the main part of cell walls in plants or parts of plants. Cellulosic ethanol employs agricultural "waste," such as corn cobs, wheat straw, sugar-cane stems and leaves, and wood chips, as well as crops not grown on prime agricultural land but specifically planted for biofuel use such as switch grass and woody vegetation. It is often touted as ethanol's "second generation," lacking the problems of indirect land use and environmental exhaustion, and representing the future of biofuels. Indeed, cellulosic ethanol uses "non-edible crops, reducing potential competition between food and fuel" (UN 13). Beyond agricultural residue, cellulose proponents hope to create an environmentally rich mixture of nonfood vegetation: "Biofuel visionaries picture a resurgence of deep-rooted perennial prairie grasses like switchgrass or buffalo grass, sequestering carbon in the soil, providing wildlife habitat and erosion control, and supplying a bounty of homegrown fuel" (Bourne). To those concerned about climate change, the carbon sequestration is particularly appealing, as growing these plants would actually keep CO2 out of the atmosphere. Of course these benefits would only last until the vegetation is harvested, but other vegetation planted elsewhere would do the job, in a continuous cycle of harvesting and replanting.
Yet cellulose is extremely difficult to break down into fuel form. Plant defenses resist industrial use: "Plants are tough. They have evolved for billions of years to avoid destruction. First-generation biofuels are made from the simple sugar molecules in corn and wheat, but cellulose is held together by a strong, glue-like substance called lignin," which defends against weather and disease (Castaldo). The defenses are even more ornate; "plants have evolved an extremely complex array of structural and chemical devices to protect them from external assault, including epidermal tissue, vascular bundles, thick wall tissues, and molecular arrays of microfibrils and polymers" (Morrow). Therefore, breaking cellulose "down into its component sugars is still far more difficult and expensive than doing the same to starch" (Pearce).
Given these obstacles, cellulosic ethanol has long resisted the optimistic predictions of its proponents. The old joke in the field is that cellulose production is always five years away, although given recent breakthroughs, "it actually now might be three years away," says Greene. Still, a July 2008 United Nations and Organization for Economic Cooperation and Development report "dismissed cellulosic fuel, stating it is not expected to be produced on a commercial basis in the next decade" (Castaldo).
The two basic technologies for converting cellulosic biomass to liquid fuel are enzyme-enhanced fermentation and Fischer-Tropsch synthesis (UN 13). Fischer-Tropsch was "invented to fuel Germany's World War I effort, to reassemble these simple molecules into long-chain hydrocarbons, suitable for burning in internal combustion engines" (Engineer 2007). Unfortunately the energy input is high in this process.
An alternative is using enzymes to break down cellulose. To aid in this, microoganisms are being engineered. One company "has developed genetically engineered bacteria that speed up the initial steps of the process to break down wood chips and other lignin material into cellulose, and it has engineered enzymes that convert the cellulose into sugars" (Scott et al). Scientists are hard at work trying to create "the ideal organism" that "would do it all--break down cellulose like a bacterium, ferment sugar like a yeast, tolerate high concentrations of ethanol, and devote most of its metabolic resources to producing just ethanol" (Shreeve). Another approach is "genetically engineering tropical fungus to make enzymes that will eat straw" (Pearce). DNA research is also working on plant manipulation "to radically redesign plant-cell walls to make them more receptive to cellulosic conversion" (Morrow). Such plants might be more susceptible to disease, however.
A completely different approach is using solid biomass not for transportation but to replace energy currently generated from coal and other fossil fuels. One study "found that compressing switchgrass into pellets and burning them for heat and power generation resulted in nearly three times the greenhouse-gas reductions of using switchgrass for liquid biofuels" (Castaldo). Most recent research, however, has gone into cellose as a transportation fuel.
Other technical problems remain. One is that "biomass is variable. Wood is very different from straw, and even more different from palm oil waste, for example" (Engineer 2007). Handling these different materials requires differences in the ethanol conversion process, adding technological difficulty and expense. Environmentalists also object to using agricultural waste for ethanol since such "waste" returns nutients to the soil and protects against wind erosion.
Cellusolsic wholesale costs remain at $2 to $3 a gallon versus $1.56 for corn ethanol (Davidson). Of course prices vary greatly over time, and better technology will result in less expensive cellulosic ethanol. Research continues apace; "In the US alone there are 30 projects in the pipeline to develop cellulosic ethanol 'biorefineries'" (Pearce). However no full-scale commercial enterprise yet makes cellulosic ethanol. The closest is the company Iogen, which operates the largest and most successful cellulosic demonstration plant in Ottawa, Canada.
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List of Visuals
- Making Fuel from Plants
The Philadelphia Inquirer
2009 Knight-Ridder/Tribune News Service, Taken from Proquest's eLibrary
- How Cellulosic Ethanol is Made