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Bottom ash boosts poor soil.

Heavy with fruit, the limbs of the trees almost touch the ground. Big, luscious-looking apples, deep yellow streaked with red, cover trees in two orchard rows.

"Don't pick from those trees!" admonishes Ronald Korcak. "We need to get yield data from them."

The picture-perfect fruit in this particular section of the orchards at the Beltsville Agricultural Research Center hang from Gala apple trees that are part of ongoing soil experiments.

Korcak, research leader of the ARS Fruit Laboratory in Beltsville, Maryland, is using industrial byproducts in three ways: as a limestone substitute, to increase calcium levels in both soil and plants, and as a gypsum-containing soil amendment.

In addition to benefiting soil and plants, agricultural use of these industrial byproducts also provides for environmentally sound disposal.

In 1980, Korcak applied fluidized bed bottom ash, a byproduct of coal-fired power generation, to the surface of the soil under the apple trees.

This ash is the residue from burning groundup limestone and coal together in power plants that generate electricity. Sulfur in the coal reacts with calcium from the limestone, forming gypsum and calcium oxide in a process called fluidized-bed combustion (FBC). [See Agricultural Research, April 1989, p. 9.] FBC bottom ash is a dry, alkaline byproduct high in acid neutralizers and gypsum.

At traditional power plants, limestone is not mixed with coal in the combustion process, and the ash does not contain gypsum.

Korcak estimates that coal-burning utilities generate over 88 million tons of waste annually. By the year 2000, this amount will increase significantly. Bottom ash from the FBC process accounts for only a small percentage of this. "However," says Korcak, "FBC and other clean-coal technologies should result in reduced emissions and the potential for decreased acid rain."

Having a texture similar to that of sand, FBC bottom ash modifies the structure of clay soil. When applied in small amounts, it improves aeration, increases water infiltration rates, and makes cultivation easier.

"We applied 50 tons of FBC bottom ash per acre to study how it affects the soil and then on trees and fruit," Korcak explains. This was a one-time, surface application. Not only was the resulting fruit attractive and plentiful, but the trees were vigorous and healthy.

The fruit showed no nutritional problems from the ash, he reports, and calcium and pH levels in the soil increased significantly. Although the ash remained on the surface, the pH increased down to a depth of 8 feet.

"The ash particles fused together to form a porous, cementlike cap on the soil surface that prevented weed growth for about 4 years after application," Korcak says. "And over 6 years, cumulative yields were increased in three of the four apple varieties in the FBC plots."

Although the cap on the soil surface is crusty, it is porous enough to allow water to seep through.

Gypsum from FBC bottom ash nourishes crops by adding calcium and sulfur to the soil. The higher calcium levels lead to better quality fruit and more disease-resistant trees, he says.

"We've had good results growing sudangrass with bottom ash in greenhouse tests," says K. Dale Ritchey. "We've seen a substantial improvement in rooting depth, indicating that with this soil additive, plants can better withstand drought by going deeper into the soil for moisture."

A soil scientist at the ARS Appalachian Soil and Water Conservation Research Laboratory in Beckley, West Virginia, Richey is using other coal-combustion byproducts from flue gas desulfurization, called scrubber sludges, as soil additives.

Ritchey and colleagues at Beckley are testing these byproducts as amendments to the acidic, hilly soils of the Appalachian region.

In addition to apples, Korcak has used bottom ash on tomatoes grown in raised beds. He surface-applied the ash at about 95 tons per acre.

Compared with bare soil and two other commercial surface mulches--black plastic and rolled newspaper--tomato yields and fruit firmness were equal or superior to any other treatment.

"Analysis of tomatoes and plant leaves revealed higher calcium levels than with other treatments," Korcak reports. "Studies continue to test for any adverse from the ash."

He is now evaluating this method in larger scale field tests.

The Industry Perspective

John Pizzella, superintendent of fuels byproducts for the Potomac Electric Power Company (PEPCO), says bottom ash is different from other industrial byproducts that may pose a threat to the environment.

"The Environmental Protection Agency does not consider bottom ash a hazardous waste," he says.

PEPCO, which supplies power to more that 650,000 customers in the Washington, D.C., metropolitan area, generates about 175,000 tons of bottom ash a year. This is about 25 percent of the waste byproduct PEPCO produces by the coal combustion process. The remainder consists of fly ash and other residues.

How does PEPCO store bottom ash? "We store some of it at designated ash sites and use some in onsite construction projects," Pizzella says.

"We also supply it to plant nurseries and turf farms. A lot is used to make concrete blocks and for ice control on highways. Horse farm operators like bottom ash because it stays in place better than gravel and is easier to walk on," he continues. "But, we'd like to see more bottom ash used in agriculture. And we do have stockpiles available."

Onsite storage costs range from $10 to $45 per ton, according to the American Coal Ash Association (ACAA). The association represents coal-burning utilities and suppliers of related goods and services in the United States.

They expect this figure to double--and perhaps even triple--over the next several years. Some power companies do not have onsite storage space available and must dispose of their waste through offsite landfills. Offsite costs can range from $35 to $150 per ton, which takes into account transportation and tipping fees at the sites.

And it's becoming more and more difficult to find landfill space.

Making More Use of Bottom Ash

Soil scientist Arthur Peterson, University of Wisconsin, has mixed bottom ash in soils planted with corn for about 8 years and attests to its improving the physical conditions of the Wisconsin soil.

"Our heavy clay soil responded well to bottom ash. We worked it into the soil to a depth of about 8 inches and found that it increased rainwater infiltration considerably," he says.

The success rate of the waste as a soil amendment, Peterson notes, depends on the type of coal from which the product comes and the type of soil it is being applied to.

The ACAA confirms the importance of these two factors. Robyne Fillmore, communications director for the association, says that the bottom ash from some types of coal may have better properties for soil use than that from other types.

"The industry produced close to 14 million tons of bottom ash in 1990. Of this, it used only about 5 million tons, primarily in construction and for snow and ice control," Fillmore says. "The remaining 8.5 million tons were either disposed of in landfills or stored by individual plants. It would certainly make a big difference to coal-burning utilities if agriculture could use more bottom ash."

However, there can be a downside to using certain byproducts from coalfired industrial plants, cautions W. Doral Kemper, ARS national program leader for soil management, based at Beltsville, Maryland. "Although most of the elements in the byproducts are benign, beneficial, and even essential to crop production, heavy metals and some other compounds present can be toxic at excessive concentrations.

"We're accumulating information that will help prescribe appropriate levels of land applications. But we definitely need more research on how elements in the ash interact with different soils and plants."

Meanwhile, Korcak is doing some forward-thinking about the increasing amounts of bottom ash that are piling up.

"One idea we're looking at is what we call byproduct recycling. We could incorporate bottom ash with materials like leaves and yard debris and try this mix in the soil. We're exploring the practically of designing new soils, a science we call pedotechnology," he says.
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Title Annotation:fluidized bed bottom ash
Author:Stanley, Doris
Publication:Agricultural Research
Date:Apr 1, 1993
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