of Lignite Fly Ash for Stabilizing Soil Surfaces
Waste Management Specialist
A new research project at the Carrington Research Extension Center is using coal combustion by-products from power utilities in North Dakota and Minnesota in an attempt to improve feedlot conditions. Research from other states such as Texas, Iowa, Pennsylvania and Ohio suggests that coal combustion byproducts (CCBs) can be used to form a stronger, more durable surface in feedlot pens than the original soil would allow. The Carrington project will measure the impact of several different ash treatments on pen conditions (surface density and compressive strength, maintenance requirements etc) where bison will be maintained in a new nutrition research facility. The project will monitor ground water and surface runoff for environmental impacts and also investigate if ash treatments can limit the amount of leaching of nutrients under feeding areas.
Types of ash
Most coal-fired power plants produce a lightweight, fine ash called fly ash and a coarser ash called bottom ash. It is important to note however, that the type of ash formed, and hence it’s properties and potential uses, can vary depending both on the source of the coal and the type of power plant.
Fly ash is made up of fine, spherical particles and moves through the coal combustion system with the exhaust gases before being screened out of the air stream for utilization or disposal. The volume of fly ash produced at a typical plant is usually greater than the bottom ash produced. Fly ash can often be used in construction and engineering applications such as concrete and concrete products, engineered fills, road-building applications, and grouts.
Fly ash is a pozzolan. That means that it “sets up” when mixed with a source of calcium and water. Some fly ash is cementitious. A cementitious material sets up when just water is added to it. Most fly ash in North Dakota and the surrounding states is both pozzolanic and cementitious – these materials are called reactive fly ashes. Such properties make these fly ashes useful in any application where cementing action is desirable.
Bottom ash is agglomerated ash particles that are too large to be carried away in the flue gases and impinge on the boiler walls or fall through open grates to an ash hopper at the bottom of the boiler. Bottom ash is typically gray to black in color, is quite angular, and has a porous surface structure. Bottom ash looks like a blend of sand and coarse aggregate and can be used as an aggregate.
While bottom ash does not set up like fly ash, it can be used to improve wet weather access in those areas that are not subject to frequent mechanical scraping, for example, feed roads and alleyways.
This project will is using three broad categories of ash treatment in and around the feedlot:
1. Soil stabilization of entire pen surfaces.
Soil stabilization is a means of improving the properties of soil so that it performs better as a feedlot surface. Soil stabilization can make a soil stronger, more durable, and less permeable. Soil stabilization can be accomplished by mechanically compacting the soil or by the addition of materials like cement, lime or fly ash. Fly ash is well suited for soil stabilization as the results are comparable to those achieved through use of cement or lime and fly ash is generally more economical. Treating the pen surface with fly ash will improve its ability to withstand saturated conditions and help prevent deep areas of mud from forming.
2. Fly ash/bottom ash mixtures for feed bunk and waterer aprons.
Because most fly ash in
our region is cementitious, it is proposed to use a mixture of fly ash and
bottom ash to produce a concrete-like surface for feeding and watering areas of
the feedlot. It is proposed that
this “stiff” mixture of fly ash, bottom ash, and water will be mixed prior
to placement. After the mixture is
placed at the feedlot, it will be leveled and compacted to provide a strong
3. Bottom ash surfacing on feed roads.
Bottom ash will be used
to surface driveways and feed roads the same way as other aggregates.
Bottom ash provides good drainage in this type of application because of
the gradation of the material.
Progress to date
have been completed to evaluate the performance of fly ash-soil mixtures for
stabilized soil feedlot surfaces. The
fly ash that will be used in this project will come from four coal-fired power
from this laboratory testing indicate that soil stabilized with fly ash and
compacted has improved strength over compacted soil with out amendment:
In June 2000, Hoot Lake
fly ash was placed in four pens (numbers 9 through 12) in the Bison feeding
research facility. The various
nuclear density testing indicated compaction was achieved at 89% to 96% of
maximum and that moisture levels ranged from 7.5% to 9.5%.
The higher compaction levels were associated with moisture contents of
Pens 13 through 16 will
receive additions of fly ash from the Stanton Station in July.
The ash addition rates, mixing techniques, and compaction techniques will
be similar to those used in the hoot Lake ash placement.
Additional pens demonstrating Coal Creek Station and Coyote Station fly
ash stabilized soils will be completed later in the year.
Also during 2000, placement activities are planned for the experimental
concrete-like mix of fly ash, bottom ash, and water. Laboratory work is underway to develop suitable mix designs
for this material.
In 2001, the project
team plans to demonstrate ash use at up to three private feedlot operations at
various locations throughout North Dakota.