Components of coal ash

• Detrital or authigenic minerals
• 95% of the mineral matter present in coal is composed of clay, pyrite and calcite
• Clay minerals make up 60–80% of the total mineral content of coal
• Type of clay mineral is dependent on environment Kaolinite with fresh water influence
• Illite with overlying marine rocks

Coal combustion

• Finely crushed coal is fed into a combustion chamber where it ignites
• Most of the carbon is burned, the volatiles vaporize and the boiler tubes, filled with water are heated to form steam to drive the turbines
• The inorganic noncombustible material forms coal combustion byproducts

Coal combustion products

• Fly ash (flue gas stream)
• Boiler slag (coats boiler tubes)
• Bottom ash (sand size material+boiler slag)
• FGD sludge (SO2 scrubber)
• Depending on the type of coal, burners, and boiler, 65-85% of the inorganic material is fly ash

SEM of Fly Ash From Coronado generating station, St. Johns, Arizona.

Fly ash characteristics

• Molten minerals such as clay, quartz, and feldspar, solidify in the moving air stream, giving approximately 60% of the fly ash particles a spherical shape.

Definition of pozzolanic material

• Siliceous or siliceous and aluminous materials Possess little or no cementitious value
• Finely divided form in the presence of moisture Reacts with Ca(OH)2 in cement at ordinary temperatures Forms cementitious compounds
• Fly Ash is an artificial pozzolan

ASTM C 618 Specifications

 

Use of fly ash in concrete

• Pozzolanic properties reduce need for cement
• Spheres act like ball bearings, increasing workability
• Fills in voids with cementitious material and acts as filler, reducing total surface area to be covered with cement
• Retards heat of hydration; important for large concrete pours (dams, Hibernia oil platform)
• Increased strength with age
• Alkali-silica reactions-”sacrificial silica”

Other fly ash uses

• Road base–substitute for aggregate
• Structural fill–high shear strength, good compaction
• Waste stabilization–solidify into inert mass
• Soil modifier–Aids in compaction, soil density, controls swell potential
• Backfill–Can be poured in place, less time consuming

Fly ash from Western U.S. coal

• Class C fly ash from subbituminous and lignite coal
  -- low ash, Tertiary age coals from fluvial, raised swamp environments
• Class F fly ash from bituminous and subbituminous coal
  -- medium to high-ash coals,
  Late Cretaceous age developed in interdeltaic, nearshore environments

View tables of Class C and Class F Fly Ash

LOI content

• > 6% does not meet ASTM C 618 standards can not be used for concrete
• Color California market Air Entrainment Admixtures-frost resistance easily decomposed organic matter detrimental
• Low NOx Burners
  

View table of Class C and F Fly Ash %Usage and %LOI

Summary-plants surveyed

• Fly ash produced–8.766 million mt
• 3.831 million mt (44%) used
• -- Class F-51%, Class C-42%
• Concrete or concrete products-1.509 million mt, cement-0.4 million mt
• Other major uses–Backfill-662 thousand mt, stabilization-182 thousand mt, and road base-109 thousand mt

View pie graphs of Class C and F Fly Ash Usage (thousands of mt)

Western U.S. usage

Class C	Class F
42% produced is used-791 thousand mt	51% produced is used-2.3 million mt
246 thousand mt for concrete products	1.3 million mt for concrete products
more diversity in uses	Backfill 2nd largest application
cementing properties are a factor in many of the applications	Cenospheres-minor use

Factors influencing western U.S. market

• Type and quality of fly ash
  -- FGD contamination
  -- Calcium content
  -- LOI content

• Distance to market areas
  -- Rail vs truck transport (quantity, distance to market)
  -- Storage facilities

Conclusions

• Class F and Class C fly ash are produced from a variety of coal sources in the Western U.S.
• FGD contamination has a significant impact on the fly ash that meets specifications
• Sparsely populated areas, weather, and available aggregate influence % used
• Distance to market important, but quality and type of fly ash are major factors in marketing- making fly ash in some cases a specialty mineral