Des Moines test-drives asphalt made from biomass.

PROJECT: Recreational trail extension paved with plant-based asphalt binder

COST: $420,000 from an Iowa Department of Economic Development grant PARTNERS: Iowa State University, Iowa DOT, Asphalt Paving Association of Iowa, and Elder Construction Inc.

DEVELOPERS: Iowa State University's Asphalt Materials and Pavements Program and Avello Bioenergy Inc.

BLENDING: Bituminous Materials & Supply

CONTRACTOR: Grimes Asphalt and Paving Corp.

THE LATEST: Eighteen months ago we reported that Chris Williams, associate professor of civil engineering at Iowa State University, had learned that fast pyrolysis (see sidebar at right) could produce a viable asphalt binder.

The material allows for an asphalt mix temperature as much as 104° F lower than normal, lowering emissions and fuel consumption at asphalt plants — and likely translating into lower market prices.

On Oct. 6, the “bioasphalt” was proven to be viable in large-scale production. Approximately 3% of the binder was terminally blended at Bituminous Materials' facility in Tama, Iowa, with a performance grade (PG) 58-28 asphalt yielding a PG64-22 binder. The combined binder was trucked to Grimes Asphalt and Paving for production in a 50-ton/hour batch plant and trucked to Des Moines for placement on a 10-foot-wide, 4,500-foot extension of the Waveland Bike Trail in northwest Des Moines. The city's parks and recreation department oversees 40 miles of trails.

“We're starting next by replacing 10% of asphalt, then we'll move to an asphalt extender with 25% biomass, and finally full replacement,” Williams says. “We're looking at the 2011 construction season to have a couple of additional demonstration projects with Avello Bioenergy's larger 2.5-ton/day facility coming on line in early 2012 at Iowa State's Biocentury Research Farm west of Ames, Iowa, to allow for larger demonstration projects as well the use of greater percentages of bioasphalt.” Avello plans to commercialize the product.

The terminal blending as well as the production and construction of the asphalt mix demonstrated that bioasphalt can be used in conjunction with existing asphalt industry processes. The product could eventually carry 40-ton vehicles.

The lab trials have shown that bioasphalt can be used to partially (and possibly even completely) replace asphalt derived from crude petroleum. ASTM and AASHTO test methods for grading asphalt binders has shown higher percentages of bioasphalt can partially or fully replace asphalt derived from crude petroleum.

The ISU team plans to enhance the low temperature properties of the full replacement bioasphalt. Additional mix performance testing will include rutting, thermal cracking, and moisture sensitivity.

The viscoelastic properties of bioasphalt are similar to crude derived asphalt, and at larger percentages of replacement will allow for the reduction in production and placement temperatures similar to warm-mix asphalt.

Of particular interest as binders are nonfood biorenewable resources such as crop residues, wood waste, or fiber crops like switchgrass. These nonfood resources are composed primarily of lignocellulose, a natural composite material consisting of cellulose fibers embedded in a matrix of lignin, a phenolic-based polymer with many attributes attractive as asphalt binder.

Since large quantities of biorenewable materials from different agricultural and forest coproduct sources are available, there is technical and economic potential for using them to produce biobinders.

What's a ‘biorenewable'?

Biological material that includes carbohydrates, lipids, proteins, and lignins. One of the simplest ways to separate lignin from plant fibers, known as fast pyrolysis, involves rapidly heating the material in the absence of air to partially decompose the lignin, allowing it to escape as “pyrolytic lignin.” A process developed at Iowa State University allows pyrolytic lignin to be recovered as “bioasphalt,” closely resembling conventional asphalt in its physical appearance and properties.