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Published online 7 January 2008 | Nature | doi:10.1038/news.2008.415
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Prairie grass energy boost studied in the field
Using switchgrass as a biofuel yields five times more energy than is used to grow it.
Switchgrass, a prairie grass that sways around the borders of many US fields, offers 540% more energy than the energy sown into it, research has shown. The renewable fuel should be seriously considered as a low-greenhouse-gas, high-energy biofuel source, the researchers say.
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At 7100kg/ha they found in the study, and their assumed 0.38L/kg, it would 632,755 mi^2 to replace fuel 2005's Vehicle Miles Traveled (2.7 trillion) assuming the 2020 CAFE standards of 35 MPG. (Of course in 2020, VMT will be much higher.) In contrast, it would take only 3,607 mi^2 of Concentrated Solar Power to fuel the same VMT in 300 Wh/mi battery electric vehicles (such vehicles were designed and produced in the 1990s--not new technology). 632,755 mi^2 vs. 3,607 mi^2: which would you choose?
Would it be feasible to harvest prairie grasses in general as fuel for power generators? The University of Iowa has been able to replace some of the coal they use in their conventional power plant with pelletized sawdust from a building products factory (Pella Windows and Doors, actually). Harvesting prairie plants for this would not require as much processing - just grind it up and pack it into pellets. This might end up depleting the soil, unless some of the mineral nutrients could be recovered from the ash. PArt of the goal for this would be to restore the prairie ecosystem in a modified fashion. Leaving strips of standing plants at harvest time would help with soil erosion and maintaining habitat for animals. Richard.
So what? It's really more important to consider the deprivation of food from the wildlife that eat switchgrass, some of whom we can and should eat as hunted wild game which is better for our health; and the sudden taking of switchgrass from the prairie ecosystem in an entirely abnormal way. There is no alternate Earth where we can leave the switchgrass to function as part of a healthy ecosystem. We can't have it both ways on the same plot of land.
Even less processing is necessary for burning biomass. Pelletizing is not needed in many industrial boilers. At Great Northern Paper in the 1970s, they ran the wet tree bark through a high-pressure auger-grinder that pressed most of the water out, then blew it directly into the combustion chambers along with the heavy fuel oil that they had been using in the power plant. The bark produced the same steam-electric output with less sulfur and metal emissions than with oil alone. The bark resembled the fine, fiberous mulch sold at garden centers - and they sold some of it locally for landscaping. A nearby sawmill also burned their wood waste in a power plant. The sawdust was burned without processing. Scraps were pulverized in a "wood-hog", which I think was just a hammer mill, for ease of handling by automated machinery. Some of the wood-hog's output resembled pellets in shape and size, but not texture, as they were just tumbled until they were small enough to fall through the screen. Erosion is not a big problem on the switchgrass plantation described by Vogel or the diverse switchgrass prairie described by Tilman et al in AAAS Science 2006-12-08, as a tremendous rootmass remains after each harvest. Much greater rootmass than a cornfield. Both studies expect to plow only once in ten years or more, as yield is lower in young stands of these perennial crops. The roots and their breakdown products will bind the soil in place even when it is disturbed by harvest traffic or tillage. While I have not personally plowed a tallgrass prairie, I have plowed an old wild pasture on Monarda-Burham-clay-silt-loam. Six months later the snowmelt runoff from the furrows was crystal clear, but from the potato field next to it was like coffee with cream. While wildlife may miss the winter cover provided by our CRP program acreage, they will still fatten on the fresh and tender new growth in spring and summer on the new CRP-switchgrass fields - and still go over to lap up the spilled grain in nearby cornfields during hunting season.
"Would it be feasible to harvest prairie grasses in general as fuel for power generators?" Sure. Tilman proposed exactly this in his December 2006 Science paper and estimated the resulting electricity at 18.1GJ/ha. To replace the 2868 TWh of fossils we burned for electricity in 2006 would then take 2.2 million mi^2. Unfortunately, the U.S. has only 1.1 million mi^2 of prairie, and it would be a crime to use more than 20% of it. In contrast, 3,607 mi^2 of desert is much more modest. Of course, only the U.S. West has excellent concentrated solar power potential, so biomass might be used in the East somewhat, but the enormous land requirement means it will be a niche energy source.
I agree that the use of grasses may be more useful pelletized than as a biofuel as a short-term solution. Corn alcohol is a massive misuese of resources and is already causing a rippling effect among our upstate NY dairy farmers who are being forced to sell out because of rising feed costs. For the long term (I estimate in 20 years) there is a technology that is rapidly progressing which will vitiate the need for liquid fuels. This is the ultracapacitor, or as one group is calling it the "bacitor" - for battery-capacitor. In the last 5 years the capacity of these cells has already increased by 4 orders of magnitude. The first use will be in the auto industry but eventually a homeowner will receive a year's supply of electricity via truck from those parts of the country (or world) that can produce it most cheaply. The liquid fuel industry will eventually give way to the packaged electricity industry. Thus, pelletized fuel is a more viable short-term solution that the massive liquid fuel plants required to make a dent in our fossil fuel consumption.
To continue this dialog - Harvesting swichgrass or diverse prairie biomass for energy would neither replace all of the fuel used for transportation nor all of the coal for generating electricity. But it could be a part of the solution as well as addressing another problem. In response to Jean SmilingCoyote, a significant effort to plant prairie grasses would be adding to that ecosystem, not taking it away. If you have travelled around the so-called prairie states you know how little of that habitat there is left. If farmers could make a living off prairie grasses this could lead to a huge expansion of that ecosystem just when the creatures suffering from climate change need it most. And, based on what Frederic Lewin says, such a field need only be partially cut each season, leaving food and cover for the animals that live there.