POWER December 2014 - 52

Coal Combustion Residuals
In fact, in some instances the pond can
become an integral part of site remediation
through its conversion to a wetlands
treatment system. In conditions where
the pond bottom elevation intercepts the
groundwater, closure of the pond may provide
opportunities for the construction of
intersecting extraction, barrier, or reaction
systems.
Constructability Concerns
Coal ash presents some unique challenges as
a construction material potentially affecting
site safety and constructability. Coal ash itself
is classified as nonhazardous. However,
because unintended health effects always depend
upon the duration of exposure and the
concentration of the chemical constituents
present in coal ash (that is, crystalline silica
and heavy metal content), appropriate health
and safety planning should be done prior to
construction activities.
Respiratory exposure of site workers to
coal ash may need to be mitigated by means
Guidebook: Focus on Gas
This PDF guidebook features gas-related articles originally published in POWER
magazine. Included are full charts, photographs, graphs and step-by-step
instructions for quick and handy reference.
Topics include shale gas development outlook and challenges, how shale gas can help an
economy, Europe embracing shale, the natural gas trade, utility options for leveraging natural
gas, plus much more.
This guidebook contains information on the follow topics:
       
             
           
              

       
       
In little more than a decade, the discovery of economic methods to extract natural gas from
shale has exploded traditional assumptions about the world's energy future and reversed
trends across the power business. But the shift has only just be-gun, and more big changes are
on the way.
   
     
        
     
       
      
For many utilities there is a razor-thin difference in the price of electricity generated by
natural gas and by coal. Slight changes in fuel price can therefore produce dramatic swings
in production costs, creating market opportunities for utilities with both gas- and coal-fi red
assets and assured fuel supplies. Utilities considering new gas-fi red assets have several
options.
       
             
           
              
      
     
              
            
             
      
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of dust control or respirators. Additionally,
the potential for dermal contact with the
material should be considered. The pH of
coal ash will vary from alkaline to acidic,
depending on the source of the coal and facility
operations.
Because of its grain-size distribution,
some ash deposits demonstrate thixotropic
behavior. The ash materials appear to be
competent, solid materials that maintain a
steep " angle of repose " as they are excavated.
However, after being subjected to vibration,
such as during transport in the bed of
a truck, the physical properties change dramatically
as the ash liquefies and becomes
a fluid that is impossible to stack, grade, or
cover (Figure 3).
Constructability will be influenced by the
ability to safely excavate and stack ash in a
stable configuration. Equipment operating
on a weak, unstable surface can also become
bogged down and disabled, or even engulfed.
Therefore, stability is an important consideration
for excavation safety, equipment access,
material stacking and handling, and ash
fill construction (Figure 4).
Stability of ash solids within a pond
will be controlled by the shear strength
of the ash materials. This shear strength
is influenced by factors that include particle
size distribution, moisture content,
and density. Weaker ash materials tend to
be finely graded, saturated, and loosely
compacted, whereas ash that's coarse, dry,
and dense is stronger. Ash material that
demonstrates high shear strength can be
stacked at steeper slopes than materials
with a low strength. Steeper slopes allow a
given volume of ash to be stacked within a
smaller footprint.
There is a significant economic benefit
for designing a disposal cell with a smaller
footprint for higher-strength ash containment
because construction and long-term
maintenance costs are lower.
Site Configuration Constraints
Usually, the " as-built " conditions do not
match the originally designed configuration.
If the pond has been periodically dredged or
excavated, it may be deeper than the design
assumed. Through progressive excavation,
the perimeter berms may eventually be narrower
or steeper than originally designed.
Material excavation, hauling, and placement
activities typically represent the majority of
project costs associated with ash pond closure.
If the actual shape is even a foot or
two deeper than assumed, expect the costs
to increase dramatically, especially with
larger ponds.
Production costs increase when the cycle
time of hauling vehicles is lengthened, where
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POWER December 2014

Table of Contents for the Digital Edition of POWER December 2014

Contents
POWER December 2014 - Cover1
POWER December 2014 - Cover2
POWER December 2014 - Contents
POWER December 2014 - 2
POWER December 2014 - 3
POWER December 2014 - 4
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