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Feature Article

Cellulosic ethanol is generally being sold on the fact that it can be produced from any kind of plant-based material, especially the currently unwanted cellulose-rich stalks and stems of many food crops. But the elephant in the room with this argument is that there will not be nearly enough of this kind of plant waste for cellulosic ethanol to make a major contribution to fuel supplies.

For this to happen, plants will need to be grown specifically as cellulosic ethanol feedstocks, and in huge quantities. So the question then becomes which plants make the most effective and convenient cellulosic ethanol feedstocks. In the US at least, an increasing number of plant scientists are coming around to the idea that the answer is grass.

For, on the face of it, tall grasses such as switchgrass and indiangrass seem to tick all the boxes. First off, these grasses are native to the US and so have evolved a natural resistance to many of the local pests and diseases. They are also fairly hardy, able to grow in poor quality soils that would defeat many food crops.

Unlike the food crops used as ethanol feedstocks, such as corn (maize) or sugar cane, these grasses grow all year round, which means that they can generate a lot of biomass. While, as with lawn grass, they can be cut rather than harvested, leaving behind healthy roots able to sprout more shoots.

But there are still many questions that need to be answered, such as which particular varieties of grass make the best feedstock and what specific planting and management techniques should be adopted. Fortunately, a number of studies are currently being conducted to try to find these answers.

For example, Cornell University’s College of Agriculture and Life Sciences is growing a range of different warm- and cool-season grasses, including 12 varieties of switchgrass, indiangrass and tall wheatgrass, on 80 acres spread over New York state. The idea is to find which of these varieties grow best in the state, especially through its often rather harsh winters.

Meanwhile, the Oklahoma Bioenergy Center (OBC), a collaboration between the University of Oklahoma, Oklahoma State University and the Samuel Roberts Noble Foundation, a plant science research institute, is throwing its weight behind switchgrass.

This prairie grass is the front-runner in the race to become top cellulosic ethanol feedstock, as it is able to grow up to 12 feet high and produce 10–15 tonnes of biomass per hectare each year. Furthermore, a recent study by researchers from the University of Nebraska found that ethanol derived from switchgrass can generate 540% more energy than used it to produce it (see BioFPR, 2008, 2(2), p95).

The OBC is currently conducting the largest every field study of switchgrass, which involves growing the grass on 1,000 acres in Oklahoma. As well as assessing the effectiveness of various different growing practices, the study will also supply switchgrass to a new cellulosic ethanol biorefinery being built nearby by the Spanish biofuel producer Abengoa Bioenergy.

Other cellulosic ethanol producers are also beginning to frolic in the grass. At the end of October, DuPont Danisco Cellulosic Ethanol, a joint venture between the US chemical giant DuPont and the Danish food manufacturer Danisco, announced that it had started building a biorefinery in Tennessee that would use switchgrass as its main feedstock.

Nevertheless, some US researchers are wondering whether foreign grass varieties may make even more effective feedstocks. Back in July, plant scientists from the University of Illinois, led by Stephen Long, published the results of a field trial in which they compared switchgrass with miscanthus, which is being grown as an energy crop in Europe. They found that miscanthus was able to produce an average of 30 tonnes of biomass per hectare each year, compared to 10 tonnes per hectare for switchgrass.

But at even at this level of biomass production, the amount of land required for cellulosic ethanol to make a major contribution to fuel supplies is enormous, According to Long and his team, miscanthus would need to be grown on 12 million hectares, or over 9% of US arable land, to produce enough ethanol to replace one-fifth of current US gasoline use.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.

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