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Researchers began heating biomass without oxygen to 932° F for a couple seconds at a time. The process produces gas that's used to fuel the processing operation, a solid called char that's used as fertilizer, and a liquid called bio-oil. About 30% of the biomass is fractionated into usable bio-oil; the rest, which isn't as viscous, is used in applications that include biodiesel and pharmaceuticals.
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Iowa State University engineering professor Chris Williams estimates that a biorefinery that converts 600 to 800 tons of biomass per day would cost $30 million. Photos: Iowa State University

It may expensive and on the market only in Europe, but a plant based binder is coming to North America. With an increased focus on sustainability, the timing's fortuitous.

In 2004, looking to capitalize on the growing interest in pavement materials made from renewable resources, French road builder Colas SA patented Végécol, a binder made from sunflower oil.

At about the same time, Chris Williams made a discovery in Ames, Iowa. Talking with a family friend, the associate professor of civil engineering at Iowa State University learned about fast pyrolysis: the process of generating energy by heating a condensed substance so quickly that it quickly decomposes. He began experimenting with various types of biomass available in central Iowa.

“My original hypothesis was that this could be used as a modifier, but now we think we can replace asphalt with this stuff entirely,” says Williams, who estimates the binder's cost at $250 to $280/liquid ton. This summer, his team will replace 10% of the asphalt binder on a local bicycle path with their plant-based version.

“Our research shows that we can do 100% replacement, but we're moving slowly,” he says. “We'll move to an asphalt extender with 25% biomass and finally full replacement.”

REDUCING RELIANCE ON OIL

Plant-based binders could bring a dramatic reduction in energy consumption. The material reduces mix temperatures by as much as 104° F. Although bicycle paths aren't highways, Williams will be able to see how the binder performs during the freeze-thaw cycles and exposure to sunlight. “It's not load-related, but there are other performance issues we can measure,” he says.

Colas laboratory tests show that Végécol is highly resistant to rutting and fatigue and adheres to aggregate as well as asphalt binder. Since 2005 it has been used in warm mixes applied to rural roads in France and Germany, and the company says it's performed as well as asphalt binder. It's also been used in European hot-in-place recycling projects.

The product is transparent, so it can be colored, says Christine Deneuvillers, laboratory manager in the research and development department at Colas SA. That makes it attractive to urban planners concerned about heat islands, where temperatures are 2° to 10° F higher than rural areas. Eliminating black roads would lower peak energy demand and air pollution.

Shell Bitumen, which also has been experimenting with plant-based binders, conducted two highway trials in Norway in 2007. It concluded that there are no differences in the way the product holds up against initial traffic loads compared to asphalt binders, and in 2008 it released its version of a plant-based binder — Floraphalte — designed mainly for use on bicycle paths and sidewalks.