POWER August 2014 - 55

COAL COMBUSTION RESIDUALS
Addressing Multiple New EPA Rules
The proposed U.S. Environmental Protection Agency (EPA) Coal
Combustion Residuals (CCR) regulation targets the location, design,
management, and closure of coal ash impoundments, with
an emphasis on controlling fugitive dust and protecting groundwater.
(See " Be Prepared for Coal Ash Regulations " in the March
issue of POWER, available at powermag.com.) The proposed Effluent
Limitation Guidelines (ELG) set numerical standards for
wastewater discharges from power plants and prohibit discharge
of coal ash transport water.
Coal-fired plants are affected by these two rules more than any
other type of power plant. The 80 or so U.S. coal plants that currently
operate traditional wet slurry ash management systems and
that are not likely to be shut down will be required to change to
alternative ash management systems to comply with the new rules.
For most options presented in the ELG, " dry handling " is required
for fly ash. All this really means is that transport water cannot
be discharged. While dry handling conforms to the proposed
rule, it tends to be expensive. In addition, water is likely to seep
through dry-ash impoundments quite readily, which results in high
leachate volume and increased risks to groundwater.
the physical and chemical properties of the
solid particles and is automatically controlled
to maintain optimum viscosity and low-energy
transport. Defining this optimum ratio is
the key to producing a mixture that contains
just enough water for the chemical processes
and mineral transformations. The process
starts as a wet system (though with much less
water than conventional slurry systems) but
ends as a dry system, with the additional benefit
of the favorable properties of the cured
end-product: low hydraulic conductivity and
nondusting nature.
The dense slurry is transported to the disposal
facility via pipelines with relatively
small diameters, in which erosion is minimized
due to particles kept in suspension in
the viscous slurry.
The landfill is divided into cells-at least
three, but optimally four. Discharge is always
to one cell at a time. When one cell is filled
up, the discharge stops and starts at another
cell while earthworks and elevation work is
carried out at the cell that is full.
These cells are large areas with several
discharge points, some 70 to 75 meters from
each other, that are alternated on a roughly
weekly basis. Shifting the discharge points
also ensures that the impoundment is filled
evenly. When discharged, the fresh slurry
spreads in a thin layer over the solid deposit,
which does not subject the dikes to hydraulic
stress (Figure 2).
After disposal, the slurry solidifies-
typically within 24 to 72 hours-and cures
to form a solid, compact matrix in about 60
to 90 days. The solidification process is not
August 2014 | POWER
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55
As explained in this article, the Circumix Dense Slurry System
(DSS) produces a dense and solid product and requires little to
no discharge of transport water. The volumes of both leachates
and excess water (if any) are orders of magnitude less than in
existing wet ash disposal (lean slurry) applications, and properly
engineered landfills are capable of collecting/capturing them all
(in trenches around the landfill area) so that these leachates are
all returned in a closed system to the power plant, where they can
be reused. The process also inhibits water infiltration, eliminates
the potential for future spills due to liquefaction, and exhibits
superior leach performance.
The EPA defines best available technology (BAT) as proven technology
that is characterized by low complexity, low life-cycle cost,
the ability to utilize existing infrastructure, energy efficiency, and
conformity with effluent standards. Circumix meets these BAT
standards and is an economic ash management alternative that is
protective of the environment. For these reasons, NAES Corp. and
GEA EGI entered into an agreement that names NAES as the exclusive
source of Circumix technology in North America. NAES is initiating
testing programs at multiple plants throughout the U.S.
disturbed by fresh slurry being discharged
over the previous layer. In fact, the little
excess water that may seep down from the
fresh layer to the previous one may well
feed the long-term (60- to 90-day or more)
mineral transformations. The hardened
slurry also significantly reduces the risk of
releasing fugitive dust, compared with dryhandling
systems.
There is very little free water, as most of
the process water is either consumed in the
hydration process or physically entrapped in
the cured product. The little excess quickly
evaporates. Because the hydraulic conductivity
of the solidified slurry is low (measured
values are 1 × 10-4 to 1 × 10-10 cm/
sec), the risk of groundwater pollution is
very slight.
Heavy metal dissolution is minimized
owing to the enhanced metal sequestration
properties of the material. Pollutants (metals)
are not chemically bound in the end product;
rather, the solid matrix structure retains the
pollutants and prevents them from being dissolved
by not allowing water (such as rainwater)
to penetrate the deposited material.
2. Slurry layers. After disposal, the slurry spreads in a thin layer and hardens within 24 to
72 hours. Courtesy: GEA EGI

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POWER August 2014

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