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Biofuel Economics: Will Pure Switchgrass Stands Be Required for Cellulosic Ethanol?

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Cole Gustafson, NDSU Extension Service Biofuels Economist Cole Gustafson, NDSU Extension Service Biofuels Economist
Whether or not pure stands of any perennial grass will be required depends on which conversion technology ends up being the most efficient and economical for cellulosic ethanol production.

By Cole Gustafson, Biofuels Economist

NDSU Extension Service

This week I am devoting the entire column to answering a question. Here’s the background. I attended NDSU’s Central Grassland Research Extension Center field day on June 26.

During the event, Paul Nyren, center director, discussed his current switchgrass research and field trials. He commented that it is extremely difficult to raise a pure stand of switchgrass. He indicated that selective herbicides to remove other weeds and unwanted grasses were unavailable. Moreover, he explained that mixtures of grasses actually resulted in stronger, more robust production. At the end of his presentation, Paul said, “If we need pure stands of switchgrass for cellulosic ethanol, I don’t know if that is even feasible.”

Therefore, the question I would like to answer this week is: Will pure stands of switchgrass or mixtures of various forages be needed for cellulosic ethanols?

I have several responses to this question. First, even though switchgrass often is touted for ethanol production, there are a wide variety of other grasses that have equal potential for cellulosic ethanol production. Wheatgrass is just one example of other perennials that the Central Grasslands REC is evaluating. Based on preliminary research, it is likely that other native grasses that are more adapted to the northern Plains offer greater production protential.

Whether or not pure stands of any perennial grass will be required depends on which conversion technology ends up being the most efficient and economical for cellulosic ethanol production. In general, two vastly different systems are emerging. One is a chemical process, while the second is thermal.

With the chemical process, the perennial grass is preprocessed to remove foreign matter, such as rocks and dirt, and then chopped. Next, it is soaked in one or more enzymes to break down the cellulose and hemicellulose walls of the grasses into sugars. These sugars later are fermented into ethanol.

At the moment, scientists have identified a number of enzymes that efficiently break down grass feedstocks. However, none of the enzymatic processes are economically viable yet due to the high cost and quantity of the required enzymes. The cellulosic industry doesn’t expect these to be commercially viable for at least five years. In addition, it appears that each variety of grass will require a unique enzyme or “cocktail” of enzymes. Therefore, if chemical or enzymatic conversion technologies eventually become the technology of choice, pure stands of perennial grasses most likely will be required for cellulosic ethanol production because of the specialized enzymes needed.

Thermal processes, such as gasification, also are undergoing rapid development and look equally promising. Gasification is a very flexible process that essentially burns the feedstock in an oxygen-free environment to produce synthetic gas, or syn-gas. Less preprocessing is required because the perennial grasses don’t have to be chopped as much, if at all. Moreover, any combination of a single grass or mixture of grasses can be gasified. Therefore, a pure stand of switchgrass, mixtures of other grasses or even stands with large weed infestations could be used because they readily support combustion.

Other thermal processes, such as fluid bed technology, also are being explored. These technologies are appealing because grass feedstocks don’t necessarily even have to be dried down. Because it is so unique, I am going to discuss fluid bed technology in a separate future column.

Regardless of whether a chemical or thermal cellulosic ethanol process ends up being technologically superior, the greater challenge facing the industry is economic production, densification, transportation and preprocessing of grass feedstocks.


NDSU Agriculture Communication

Source:Cole Gustafson, (701) 231-7096, cole.gustafson@ndsu.edu
Editor:Rich Mattern, (701) 231-6136, richard.mattern@ndsu.edu
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