POWER March 2014 - 43

THE FUTURE OF COAL-FIRED GENERATION
2. Unit F after MATS controls are installed. Source: ADA Carbon Solutions
Selective catalytic
reduction
Economizer
Stack gas
Dry sorbent
injection
Activated
carbon
injection
Electrostatic precipitator
Wet scrubber
other untested units. Depending on the degree
of control needed or desired upstream
of the scrubber, flue gas injection of FastPAC
Premium can be used in conjunction with
hydrated lime for control levels more than
70% (demonstrated at Unit F), or FastPAC
Premium-80 can be used without hydrated
lime injection for more than 50% capture of
mercury (demonstrated at Unit E).
Figure 3 shows the results achieved at these
Air
preheater
of the cases when evaluating historical mercury
CEMS data over a period of about two years.
Typical fuel for Unit F is a bituminous coal,
nominally 2.5% sulfur by weight. The future
configuration of Unit F is depicted in Figure 2.
With Unit F, two separate, temporary ACI
systems provided by ADA-ES Inc. and Nol-Tec
Systems Inc. were installed to accommodate
the full test range of PAC injection rates. Upstream
of PAC injection, an alkali sorbent injection
system provided by Nol-Tec was available
to inject hydrated lime. PAC and lime were then
removed along with the fly ash in the ESP.
Reliable, active MATS compliance on this
critical unit is a key component of Southern
Co.'s strategy. Compared to the baseline data,
Unit F would require a minimum of 11% additional
control to meet the MATS mercury
limit of 1.2lb/TBtu, and additional control to
maintain a margin on the compliance level.
In addition, Unit F's configuration is similar
to other Southern Co. units that could benefit
from the test data. The Southern Co. units
analyzed would benefit from varying levels
of control to guarantee MATS compliance,
up to 60% additional mercury control over
co-control (also known as native capture).
Upon reviewing the available CEMS data for
Unit F, Southern Co. developed a test program
to determine whether MATS compliance could
be reliably addressed using PAC either through
in-duct injection (ACI) or as a scrubber additive
(in conjunction with STEAG Energy Service's
technology and injection system).
At the time it was well understood that
Generation 1 and Generation 2 PACs had experienced
interference with mercury capture in
flue gas upstream of Unit F's scrubber in the
presence of SO3
levels up to 15 to 20 parts per
million (ppm). Empirical evidence has clearly
shown that the presence of SO3
greater than 5
ppm can inhibit mercury capture in conventional
Generation 1 PAC. Alkali sorbent injecMarch
2014 | POWER
tion such as hydrated lime, sodium bicarbonate,
trona, and the like can be used to mitigate the
SO3
with a higher-SO3
that Southern Co. was interested in evaluating.
Minimizing the alkali sorbent injection is advantageous
from both an economical perspective
and because it minimizes the potential impact on
ESP particulate matter capture performance.
SO3
-tolerant PAC such as ADA Carbon Solutions'
registered FastPAC Premium-80 can help
achieve this goal, obtaining good mercury capture
while minimizing both dry sorbent injection
(lime in this case) and PAC quantities.
Test results from Units F and E show there
are alternative options for mercury control
on these units, and also inform strategies for
3. In compliance. Here are results of
using FastPAC Premium and FastPAC Premium-80
injection in the ESP with and without
hydrated lime at Units E and F at about 15 to
20 ppm SO3. Source: Southern Co.
◆ FastPAC Premium
80
70
60
50
40
30
20
10
2
4
6
www.powermag.com
8
PAC injection ratio (lb/MMacf)
10
lime ▲ FastPAC Premium-80
80 + hydrated lime
FastPAC Premium + hydrated
FastPAC Premiumtwo
units. These control levels more than address
the incremental levels needed at Units A
through G for MATS compliance. An advantage
of controlling mercury at the ESP is that no
PAC is introduced to the scrubber, so gypsum
quality is unaffected. Also, the balance of the
complex, multi-phase chemistry in the scrubber-critical
to successful acid gas control-is
unaffected by additives and mercury loading.
Figure 4 shows the benefit of adding ACI
prior to PAC injection, and combining this
-tolerant PAC is one solution
with either an ESP or baghouse at Unit B, as
an example. For this unit, which experienced
baseline 30-day rolling averages greater than
the MATS limit 24% of the time, it is clear
that ACI upstream of the ESP provides compliance
with a significant operating margin,
and that a baghouse retrofit is not necessary
for mercury capture in all circumstances.
In separate testing, Southern Co. evaluated
an ADA Carbon Solutions PowerPAC
in collaboration with STEAG for mercury reemission
control into the flue gas desulfurization
(FGD) absorber tower. This testing was
performed without the injection of PAC and
alkali sorbent upstream of the scrubber. This
alternative solution was of interest to Southern
Co. for potentially providing more options for
solving the MATS compliance puzzle.
The carbon slurry was injected directly
into the forced oxidation wet limestone absorber
(FGD) with an oxidation reduction
potential (ORP) of 600-700 millivolt. The
baseline testing of the flue gas at full load
revealed an FGD inlet elemental Hg of less
than 0.5 lb/TBtu with the baseline stack Hg
emission of 1.7 lb/TBtu. The baseline testing
revealed that periods of Hg re-emission (conversion
of oxidized Hg back to elemental Hg)
were taking place in the FGD.
With the addition of PowerPAC in the FGD
with a concentration range of 200-300 ppm
(average carbon injection rate: 100 pounds per
hour), the total Hg stack emission was reduced
to 0.7 lb/TBtu while the FGD inlet elemental
Hg entering the FGD remained at less than 0.5
lb/TBtu. The dissolved phase Hg in the FGD
slurry decreased from 95% during baseline
to 4% during the carbon addition test, while
the solid phase Hg increased from 5% during
baseline to 96% during the carbon addition
test. Mercury partitioning from the liquid to
the solid phase of the FGD slurry corresponding
to PAC addition was the primary reason
41
Removal of vapor-phase Hg (%)
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POWER March 2014

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