POWER October 2016 - 33

COAL
Table 1. One man's trash is another man's treasure. With less than half of the coal ash produced in the U.S. being beneficially
used in 2014 (2015 data won't be available until November), there is plenty of room for improvement. All amounts are tons. Source: American
Coal Ash Association
Flue gas
Fly ash
Total coal combustion
products (CCPs) produced
Total CCPs used
Use to production rate
Bottom
ash
Boiler
slag
50,422,238 12,478,705 2,694,056
23,181,723 6,063,028 1,706,621
46%
49%
concrete products, and grout; and gypsum
panel products.
Mining Applications. In many parts of
the U.S., abandoned coal and other mines
have caused serious water pollution problems.
Acidic water forms in mines through
the chemical reaction of surface water and
shallow subsurface water with rocks that
contain sulfur-bearing minerals. Heavy metals
can be leached from rocks that come in
contact with the acid. The highly toxic acid
mine drainage (AMD) often migrates into
nearby waterways, contaminating them.
A technical report titled " Use of Coal
Combustion Products in Mine-Filling Applications:
A Review of Available Literature
and Case Studies " was prepared several years
ago by Ishwar Murarka and Jim Erickson,
with partial funding provided by the U.S.
Department of Energy and the ACAA. The
researchers found that several physical and
chemical properties of CCPs make them beneficial
in mining reclamation projects.
The report says that because many CCPs
are alkaline materials, they can be used to
neutralize acidic groundwater or to inhibit the
production of acid. It also notes that " placement
of CCPs into deep mines can provide
structural support to abate subsidence, and
placement of CCPs in surface mines or other
open pits can aid in restoring mined land to
beneficial use. " It suggests that using CCPs
as mine backfill may be a good way to limit
the effects of AMD.
Concrete,
Concrete Products,
and
Grout. Most readers are probably aware that
fly ash is frequently used in concrete mixes.
What may not be widely known, however,
is that fly ash alone has very little or no cementitious
value.
Portland cement-which, incidentally,
gets its name from the Isle of Portland off the
British coast-is most often used as the primary
cementitious material in concrete. But
that doesn't mean that fly ash is just filler;
it's actually a very important supplementary
material that increases the concrete's strength
and durability.
Jimmy Knowles, vice president of research
and market development for The
SEFA Group, explained that fly ash is a
October 2016 | POWER
63%
desulfurization
(FGD) gypsum
34,123,820
16,750,990
49%
FGD material
wet scrubbers
12,596,231
1,163,434
9%
pozzolan. A pozzolan-which gets its name
from a volcanic ash mined near Pozzuoli,
Italy-reacts with calcium hydroxide (CH)
in the presence of water to form compounds
that possess cementitious properties. What
makes that so important is that portland cement
mixed with water forms calcium silicate
hydrate (CSH) paste structure and CH.
CSH is what gives concrete its strength,
while CH is the weak link in hardened cement.
Therefore, adding a pozzolan, such as
fly ash, consumes the CH and forms more
CSH, which makes the concrete stronger
and more durable.
" It's widely known in the concrete industry,
if there were no ash, the concrete industry
would have to invent something like it to get
the same high level of strength and durability
expected from today's concrete, " Knowles
said.
Gypsum Panel Products. Flue gas desulfurization
(FGD) scrubbers are commonplace
at coal-fired power plants these days
to control emissions. Scrubbers utilize highcalcium
sorbents, such as lime or limestone,
to absorb sulfur and other elements from flue
gases. One of the by-products of the process
is calcium sulfate, or synthetic gypsum,
FGD material
dry scrubbers
275,999
22%
FGD
other
1,255,775
344,551
0%
Fluidized bed
combustion
ash
Totals
15,768,766 129,684,142
13,285,766 62,427,561
84%
48%
which has the same composition as natural
gypsum.
Although not technically ash, synthetic
gypsum is managed and regulated as a CCP.
One of the most common uses for the material
is in the manufacturing of wallboard (or
drywall). According to Adams, more than
half of the wallboard manufactured in the
U.S. utilizes FGD gypsum from coal-fired
power plants.
" Only about three or four years ago, about
33% of the wallboard in the U.S. was made
with FGD gypsum, " Adams said, " so they've
had about a 50% increase in the use of that
material to meet the demand for wallboard. "
Other Common CCP Uses. Gypsum
can be used for more than just wallboard. In
agricultural areas, the use of synthetic gypsum
is growing rapidly. It can be applied as
a soil amendment to farm fields, using conventional
spreading equipment (Figure 2).
The gypsum not only provides nutrients for
plants, such as calcium and sulfur, but it also
helps reduce fertilizer and insecticide runoff.
Gypsum is also used in the manufacture of
portland cement.
Boiler slag-molten ash that is quenched
with water and collected at the base of wet1.
Giving 110%. We Energies has been able to exceed 100% utilization by developing new
ways to use old, previously landfilled coal combustion products (CP). Courtesy: We Energies
■ CP produced (tons) ■ CP utilized (tons)
1,200,000
1,200,000
1,000,000
800,000
600,000
400,000
200,000
1,000,000
800,000
600,000
400,000
200,000
Year
www.powermag.com
33
Combustion products (tons)
Combustion Products (Tons)
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015

http://www.powermag.com

POWER October 2016

Table of Contents for the Digital Edition of POWER October 2016