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Harvesting Liquid Fuels from Trees

Posted by | January 24, 2009

The goal of growing more fuel to meet our energy needs looks increasingly realistic. Those of us who heat our houses with wood, of course, have been relying on trees for energy for a long time. But the more interesting task ahead may be to use trees for liquid fuels that can power vehicles.

There are two basic approaches that researchers are studying that could lead from trees to liquid fuels for transportation.

As high school biology teachers like to explain, the woody parts of plants are mainly made of two materials, cellulose and lignin. I think of cellulose as the tough stuff in plants, and because trees are large and strong, they each contain a substantial amount of cellulose.

For a simple rock-head like myself, cellulose is made of “hard nuggets” that are derived from sugar. One way to make liquid fuel out of cellulose is to use microbes that break down the “nuggets” to the sugars within them – material that then can be fermented into ethanol and burned as fuel. There are details yet to be worked out, but I think you’ll soon see the day that genetically modified microbes will be used to convert cellulose to ethanol in roughly economic terms.

Lignin is the other material in trees. If you’re reading this column in a newspaper, you’re holding a lot of lignin in your hands. Lignin turns yellow with time when it’s exposed to air – a feature of old newspapers you’ve doubtless noticed.

Lignin gets in the way of processing cellulose for biofuel. Lignin and cellulose are both natural parts of the cells in trees, and we cannot simply eliminate lignin in a pine tree. But the day may come when we use biotechnology to manipulate the lignin-cellulose partnership to suit our purposes when the tree is harvested.

This is just one example of the fact that biotechnology – the art of manipulating the living world at the level of the gene and the cell – is the quiet revolution that’s going on all around us. Biotechnology is not as obvious to consumers as the internet or cell phones, but biotech is reshaping the fundamentals of our world at a pace that will only increase.

Trees could also yield biofuels in quite a different way. Think for a moment of school lessons about rubber trees in the tropics. Many of us who live in northern climates vaguely remember being taught about rubber trees, but we don’t know that other tropical trees produce sap that’s similar to kerosene and diesel fuel.

That sounds odd, I know, but it’s just a fact some tropical trees naturally produce liquid hydrocarbon fuel. What’s not to like about that?

The best “diesel tree” produces about 10 gallons per year of biodiesel fuel. And that’s led some farmers in the tropical part of Australia to plant 20,000 of the diesel trees for later tapping.

Now, mind you, the quantities produced by diesel trees are small compared to what us geologists rip out of the Earth in oil fields. Ten gallons per year is not exactly a flood of biblical proportions. Still, it’s liquid fuel harvested directly from trees – quite a feat.

Here in the continental U.S. we don’t have the climate for diesel trees. But some of our trees, like the Western red cedar, do have significant amounts of heartwood chemicals in them of interest for our energy concerns. Using biotechnology once again, we may be able to “improve” such trees from the point of view of biofuel production.

Professor Norman Lewis of Washington State University recently gave me an overview of this whole realm of bioengineering. Everything from the lignin and cellulose divisions in plants to the possible hydrocarbon content of tree heartwoods is up for research these days.

“No matter how you look at our climate and energy situation,” Lewis said, “there’s a role for biofuels in some form. And I think in the next ten years you’ll see some major advances.”

If we can find ways to better utilize Earth’s largest plants to produce more fuel, even us hard-core geologists will become tree-huggers.