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Ash To Dirt

Minnesota biomass-energy plants are producing more than 100,000 tons of ash per year that could have fertilizer value, but the ash has to be granulated before it can be applied to fields. AURI is helping test an ash-densification system, pictured above, where enters a processor as a fine powder then exits as granules, which are being tested for the nutrient value in corn test plots.

Minneapolis , Minn. — Alan Doering and several curious onlookers watch in anticipation as thousands of BB-sized granules drop down a sleeve into holding barrels. Ash entering as a fine powder exits as irregular-shaped grains about an eighth of an inch across — the potential makings of commercial fertilizer.

“Ash has value as fertilizer, but without finding a way to granulate or pellet it, the ash is so light and powdery that it’s difficult to land apply or blend with commercial fertilizer,” says Doering, who heads AURI’s coproduct utilization program in Waseca. “We are producing more than 100 thousand tons of ash per year in Minnesota from facilities that produce energy from biomass. The supply is there, but we need to identify ways to more effectively handle it.” That may have just happened.

Ash from three combustion technologies: gasification, direct combustion and a fluid-bed reactor were collected and tested in a granulation process developed by Bepex International in Minneapolis. The process, used to densify and blend powdered materials, successfully bound two of the three ash types, forming granules with a 50 to 70 pound-per-cubic foot density, similar to commercial fertilizer. Char ash from gasification didn’t respond as well.

The densification tests were part of a multi-phase project to evaluate the nutrient value of biomass- derived ash and ways to improve handling. Using a relatively low-cost coproduct like ash could also temper the sharp commercial-fertilizer cost increases of the past several years.

Ash nutrients vary with feedstock and combustion method. In general, agricultural ash contains about 200 to 300 pounds of phosphorus (P) and potassium (K) per ton, says Jeff Vetsch University of Minnesota soil scientist. Ash also contains important micronutrients such as sulfur and zinc, but little nitrogen.

Vetsch is leading research at the Southern Research and Outreach Center in Waseca where the three ash types have been applied to several corn test plots. Two application rates are being evaluated, equivalent to 50 and 100 pounds-per acre of phosphate. Corn’s ash response will be compared to conventional fertilizers. Besides comparing plant growth and grain yields, corn-tissue samples will be analyzed for P and K concentration and uptake. Soil samples will be taken before, during and after the growing season.

“We are trying to help these biomass plants identify real world applications for utilizing their ash in crop production,” Doering adds. “Once this information is available, it will be up to the plants to pursue whatever makes the most sense for them.”

Field trial results are expected in the spring 2009.