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Use of Supercritical Water Could Cut Costs for Ethanol

A startup says it can make sugar for biofuel from wood chips at a fraction of the normal cost.

Renmatix, a startup based in Kennesaw, Georgia, is using water at high pressure and temperature to transform wood chips into sugar, which can then be fermented to make biofuels and other chemicals. The company says the process can produce sugar for the same price as making it from sugarcane, which has led to profitable biofuels production in Brazil.

Super converter: A bag of ground-up wood chips feeds a system that converts the wood to sugar using supercritical water at this Renmatix demonstration plant in Georgia.

Renmatix is addressing the most difficult step in producing ethanol from abundant cellulosic materials such as wood chips, instead of from corn or sugar crops. Once the sugar is made, the same technology employed in a conventional corn or sugarcane ethanol plant can be used to produce ethanol.

So far, Renmatix has only demonstrated the technology on a small scale, using a facility that can process three tons of wood chips a day. As with all advanced biofuels companies, one of the biggest challenges will be convincing investors to hand over the money needed to build a larger commercial facility to prove the venture is commercially viable. The U.S. Environmental Protection Agency has been forced to waive requirements for cellulosic ethanol because commercial plants for converting cellulosic material to ethanol haven’t yet been built. By lowering the cost of producing sugar from cellulosic materials, Renmatix hopes to at last break this logjam.

Researchers and companies have tried many methods of turning cellulose into sugar. Some involve breaking the biomass down using acids or specially tailored enzymes. Others involve using heat and pressure to turn biomass into hydrogen and carbon monoxide, which can be converted to biofuel using inorganic catalysts. Each method has drawbacks: enzymes are expensive; acids are toxic. Both processes are slow, and they require expensive equipment. The processes that use high heat and inorganic catalysts also have relatively low yields of the desired fuels.

Instead of using enzymes or acids, Renmatix employs supercritical water—water at very high temperatures and pressures. Under these conditions, cellulose will dissolve and very quickly break down into sugar molecules. The reactions take seconds, compared to days for some other processes. Because of the high speed of the reaction, a relatively small amount of equipment can produce a large amount of sugar, keeping capital costs down. Smaller equipment could also make it possible to distribute the production of biofuels, thereby decreasing the cost of transporting biomass.

Sugar supply: A worker at Renmatix pumps sugar produced by the company’s new supercritical water process into a storage container.

However, working with supercritical water comes with challenges. The materials that can be used with supercritical water are limited—it will dissolve glass, for example. The extremely fast reactions also make it difficult to ensure that the chemistry doesn’t go too far and produce undesirable by-products. In past attempts, the supercritical water has dehydrated some of the sugar produced, resulting in compounds that can poison the yeast used to convert sugar to ethanol. Typically, the process also yields a relatively small amount of sugar from a given amount of biomass.

Fred Moesler, Renmatix’s vice president of process technology, says the company has overcome these problems. The company hasn’t said how it does this, but Gary Aurand, a research scientist at the University of Iowa who is familiar with the company from its early days (when it was known as Sriya Innovations), suggests the company may be using supercritical water in only part of its process.

Turning biomass into sugar using supercritical water involves first grinding biomass into small particles, then dissolving cellulose in water. Without dissolving it, only the cellulose molecules at the surface of the particles will be broken down, resulting in low sugar production. After the cellulose is dissolved, further exposure to high temperatures and pressure will break the cellulose molecules down into sugar.

Aurand says that water only needs to be supercritical for the dissolving step. If Renmatix could engineer a system to move the dissolved material into an area of lower temperature and pressure, it could slow down the process of breaking down the cellulose into sugar, preventing the formation of the unwanted compounds.

All Renmatix has said is that it uses two steps to break down cellulose and a similar material, hemicellulose. Breaking down cellulose produces glucose, the sugar that yeast can readily use to produce ethanol. Breaking down hemicelluloses produces another sugar called xylose, which doesn’t work with conventional fermentation, but which can be used in some advanced biofuels and biochemicals processes. The economics of the process will depend on the market for xylose.

Renmatix has raised some of the money for a plant capable of producing 100,000 tons of sugar per year—large enough to show that the process has commercial potential, it says. But the company is still working to obtain the loans needed to go forward. In the past, using supercritical water to process biomass has been seen as uneconomical, so it may prove difficult to get banks to sign on. “Little is known about the technology,” says Andy Aden, manager of biorefinery analysis at the National Renewable Energy Laboratory in Golden, Colorado. Based on prior calculations, he says, “it is likely to be expensive.”

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