Demonstration of Lignite Fly Ash for Stabilizing Soil Surfaces
Progress Report

Scott Birchall, Waste Management Specialist
Carrington Research Extension Center, North Dakota State University

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.

Ash treatments

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 durable surface.

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

Laboratory experiments 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 plants:

Utility Company

Power Station

Location

Great River Energy

Coal Creek Station

Underwood, ND

Otter Tail Power Co.

Coyote Station

Beulah, ND

Otter Tail Power Co.

Hoot Lake Station

Fergus Falls, MN

Great River Energy

Stanton Station

Stanton, ND

Preliminary results from this laboratory testing indicate that soil stabilized with fly ash and compacted has improved strength over compacted soil with out amendment:

Sample evaluated

Moisture Content – Range (%)

Dry Density – Range (lbs/cu.ft.)

Strength – Range (psi)

Soil

9 - 12

113 - 120

50 - 66

Fly ash (25%)/soil mixture

8 - 12

105 - 117

94 – 227
(after 14 days)

Fly ash (18%)/soil mixture

9 - 12

102 - 119

173 – 285
(after 14 days)

In June 2000, Hoot Lake fly ash was placed in four pens (numbers 9 through 12) in the Bison feeding research facility.  The various treatments were:

Pen

Ash addition (%)

Mixing

Compaction

9

12

Disc

Tractor

10

12

Disc

Rubber-tire compactor

11

16

Disc + Rototiller

Rubber-tire compactor

12

22

Disc + Rototiller

Tractor

Following placement, 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 approximately 8.5%.

Future work

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.

Project participants

Many groups are participating in this demonstration effort.  Financial support has been provided by Great River Energy, Otter Tail Power Company, the North Dakota Industrial Commission, the North Dakota State Board of Agricultural Research and Education, and the U.S. Department of Energy.  The technical project team includes professional staff from the University of North Dakota Energy & Environmental Research Center, the North Dakota State University Carrington Research Extension Center, and En-Rock, Inc.


NDSU Vice President,
Dean and Director for Agricultural Affairs
NDSU Extension Service ND Agricultural
Experiment Station
NDSU College of Agriculture NDSU College of Human Development and Education