POWER March 2014 - 47

THE FUTURE OF COAL-FIRED GENERATION
this plant online. Starting in spring 2012,
LG&E planned to spend approximately $1.3
billion to modernize the FGD systems and
install fabric filter baghouses for increased
particulate and mercury control on all units at
the plant. This construction project is under
way and will continue through 2015. And in
November 2012, LG&E officials announced
that, as part of the $1.3 billion, they would
be spending approximately $940 million on
clean coal technology at the station. Mike
Kirkland, general manager of Mill Creek
Station, told POWER that would include
replacing existing scrubbers with new ones,
installing new baghouses, and replacing exhaust
stacks.
Mill Creek burns approximately 4 million
tons of high-sulfur coal annually, primarily
sourced from the Illinois Basin. Kenny Tapp,
senior by-products coordinator for LG&E
and KU Services Co., noted that over 60%
of the plant's fly ash is used in the manufacturing
of cement and concrete; the economic
value of the fly ash utilization in concrete is
estimated to be in excess of $5,000,000 to
the regional manufacturers of concrete- and
cement-based products. In addition, the plant
realizes significant savings on landfill capacity
and associated costs, though neither the
plant nor Charah would release detailed data
on these savings.
The plant has had wet scrubbers and a FGD
slurry processing plant on its property since
1978, and its processing plant can dewater up
to 1,800 tons of gypsum per day for use in the
manufacturing of cement, drywall, or other
uses. Now that gypsum has expanded utilization
opportunities as fertilizer. This additional
use can consume 200,000 plus tons per year of
the total gypsum annual production.
From Flue Gas to Gypsum
The sulfur-scrubbing process at a coal-fired
power plant typically involves grinding highcalcium
limestone to powder and then mixing
it with water to form a lime slurry. The lime
slurry is then sprayed into a contact chamber,
2. Conversion site. The Charah product
manufacturing facility sits on the Louisville
Gas and Electric Co.'s Mill Creek Generating
Station property in southwest Jefferson
County, Ky. Courtesy: Charah Inc.
Sulfur's Role in Agriculture
A key component of agriculture production
in the U.S. has been the proper deployment
of various types of fertilizers.
Historically, the primary fertilizers have
been nitrogen (N), phosphorus (P), and
potassium (K). High-efficiency farming
requires that particular attention be
focused on secondary nutrients, which
include calcium (Ca), magnesium (Mg),
and sulfur (S). Sulfur has become more
important to high production and is often
referred to as the " fourth major nutrient. "
Each of the secondary nutrients
is essential for high-intensity farming
activities. Though required in smaller
quantities than NPK, they are essential
for plant growth.
As a nutrient, sulfur is needed in significant
quantities by many crops that utilize
approximately the same amount of sulfur
as they do phosphorus. A typical crop,
such as corn or soybeans, can extract and
remove from the soil 12 to 20 pounds
of sulfur per acre (Table 1). The sulfate
ion (SO4
) is the form of sulfur absorbed
by most plants. Replenishment of sulfur
is crucial to maintain high production on
each acre. Typical sources of sulfur include
organic matter, ammonium sulfate, gypsum,
zinc sulfate, and elemental sulfur.
Table 1. Typical nutrient uptake. Source: Charah Inc.
Typical nutrient uptake
Crop
Corn
Soybeans
Wheat
Alfalfa
Yield
200 bu/ac
60 bu/ac
80 bu/ac
6 ton/ac
Notes: ac = acre, bu = bushels.
where it combines with boiler exhaust gases
and the sulfur reacts with the lime to become
chemically bound.
Scrubbers come in two types: wet and dry.
In wet scrubbers, the ratio of lime slurry is
greater and a slurry by-product is produced.
In dry scrubbers, the ratio of slurry to hot
exhaust gases is controlled, to dry the lime
slurry and result in a dry product. Charah
has developed a process to beneficially use
the wet scrubber slurry dewatered gypsum to
manufacture a sulfur and calcium fertilizer.
Wet scrubbers capture sulfur from all
four units at Mill Creek. The lime and sulfur
slurry is aerated to create calcium sulfate,
dewatered to produce high-quality gypsum,
and then processed to make fertilizer at the
adjacent Charah facility (Figure 2).
Mill Creek produces approximately
600,000 to 800,000 tons per year of calcium
sulfate gypsum. The gypsum products are
stockpiled onsite, and Charah manages the
gypsum on behalf of Mill Creek.
From Gypsum to Fertilizer
The Mill Creek gypsum typically has higher
purity than natural gypsum because it has
less inert impurities. Mill Creek gypsum is
90+% pure calcium sulfate. Charah utilizes
this calcium sulfate gypsum to manufacture
a patent-pending fertilizer named " SUL4RPLUS
product " that can be used to replenish
March 2014 | POWER
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the sulfur and calcium in farm soils, turf, and
specialty crops (see sidebar). As Danny Gray,
executive vice president of Charah, explained,
this process essentially closes the cycle loop
for the sulfur that once was returned to farm
fields with rainfall, but now is removed by
the power plant emissions control equipment
before discharging the cleaned exhaust gases
into the atmosphere.
The Charah plant accepts the gypsum
when it discharges from the existing Mill
Creek dewatering facility onto a new conveyor
that moves it directly into the Charah
plant. That gypsum serves as the feed stock
for the processing steps that include pelletizing
to create the granular SUL4R-PLUS
product. Although synthetic gypsum has
previously been used as a soil amendment,
Charah says it is the first to pelletize the byproduct,
which makes application easier for
the farmer.
That granular product is stored inside the
Charah warehouse until it is transported to
customers. Custom truck loading is done inside
the warehouse facility. Charah also has
barge-loading capability, as well as onsite
railcar-loading capacity to meet customers'
logistics needs. Because the Kentucky plant
is located near the Ohio River, Charah can
reach distant markets by barge at economical
rates.
The sulfur level of SUL4R-PLUS prod45
Nitrogen
(lb/ac) Phosphate (lb/ac) Sulfur (lb/ac)
150
240
92
225
85
48
44
60
15
12
7
30
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POWER March 2014

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