The changing natural world at our doorsteps | Illustration and text by Patterson Clark
May 15, 2012
Silver maple: a better source for ethanol
As a spring storm advances, winds plow through the branches of a silver maple, rolling over the leaves to reveal their silvery-white undersides.
The flash of silver, for which the tree was named, could also be signaling its potential wealth as a resource for biofuels made from cellulose, the fibrous carbohydrate molecule found in plants.
Ethanol made from corn starches is currently cheaper to make than cellulose ethanol. But it yields only 26 percent more energy than is used in its production. Cellulose ethanol yields 80 percent more energy than is used to make it, with net-contribution of greenhouse gases.
Silver maple is an ideal source of cellulose. It can be planted in a wide variety of sites. The tree sprouts profusely after being cut, "is a fast-growing species and is relatively pest-resistant," says Eric Holzmueller, from the Southern Illinois University Department of Forestry. "In order to produce the maximum amount of biomass over time," says Holzmueller, "the species would be planted in a plantation, in rows like corn."
Silver maple trees spaced about 11 feet apart can, after three years, yield 6.3 dry tons of biomass per acre annually, potentially producing more than 500 gallons of ethanol.
Saplings, ground into pellets, can be sent to a biofuels plant, where enzymes from microorganisms break the cellulose into sugars, which are fermented to produce ethanol.
Silver maple might help lead us into clean energy independence, but it is a lousy tree for landscaping. Its roots can penetrate and clog water systems, and although its rapid growth is desirable for making shade, the wood is weak, brittle and prone to rotting, causing many limbs
and twigs to shed from mature trees after heavy snowfall and during windy storms.
SOURCES:Department of Energy, National Plant Diagnostic Network, Oak Ridge National Laboratory, Genera Energy, Microbiology and Molecular Biology Reviews