POWER October 2012 - 36

TOP PLANTS
1. Fighting flue gas pollutants. Management at the 440-MW Merrimack Station recently
installed a single scrubber system to cut mercury and sulfur dioxide emissions. The flue
gas desulfurization absorber vessel, which is made of alloy 2205 stainless steel, is shown with
its flue gas inlet connection. When this photo was taken, the vacuum filters had been set in
place and the building's steel, stair tower, and elevator were being erected. Courtesy: URS
built in the 1960s and are not designed to operate
with negative pressure excursions. The
project required additional new booster fans for
each unit with fan discharges tied together in a
common plenum leading to the absorber inlet.
Unit 1 incorporated an absorber bypass to
atmosphere using an original Unit 2 stack to aid
in providing an open flow path for boiler venting
and to allow limited single-unit operation
independent of FGD system outages. Both units
have booster fan recirculation ducts that allow
a portion of the flue gas to bypass the absorber
vessel when the recirculation dampers are
opened, effectively flattening the fan operating
curve and assisting with fan control during independent
unit startup and low-load operation.
Wastewater Treatment System Design
Air Project achieved substantial completion
on Mar. 30, 2012. Initial emission performance
results indicate a reduction of 97% to
98% for mercury emissions and 96% to 98%
for sulfur dioxide emissions. Further testing
is planned, including additional performance
tests and state testing (Figure 1).
Major subcontractors for the Clean Air Project
were Sterling Boiler (FGD system), Northern
Peabody (wastewater treatment system),
Merrill Iron & Steel/AZCO (duct and steel
components), AZCO (mechanical work), and
E.S. Boulos (electrical work).
Other contractors included:
■ Engineering and design of secondary wastewater
treatment system (WWTS) to eliminate
effluent discharge: Burns & McDonnell
■ Engineering and design of a potential adjustment
protection system to mitigate A2205
absorber vessel corrosion: Sargent & Lundy
■ FGD technology and erection: Siemens Environmental
■
WWTS, including physical/chemical process
with enhanced mercury and arsenic filtration
technology: Siemens Water Technology
■ Limestone and gypsum handling and storage
equipment and erection: Dearborn Mid-West
Conveyor
■ Chimney equipment and erection: Hamon
Custodis
■ Distributed control system: Emerson
■ Wastewater treatment system, including
distillation and evaporation process:
Aquatech
Under normal operating conditions, the
Merrimack Station employs more than 100
36
people. This number, however, is supplemented
with contractors during planned outages.
Hitchko said that nine new positions
were added to the station staff due to this
Clean Air Project-eight physical worker
positions and one engineering position.
Putting Safety First
Working at an operating plant poses numerous
safety challenges. Therefore, safety was
a priority at the earliest stages of the project.
Hitchko explained, " URS effectively incorporated
constructability review input and 'design
for safety' protocol into the design process,
helping to contribute to a safe construction
site without impacting plant operations. This
was especially true for the ductwork and steel
erection, where crane access was limited and
the weights of duct sections needed to be coordinated
with the construction work plan. "
One of the design criteria that resulted from
the design review process was the use of walkways
on top of all of the ductwork with stair
access to all platforms, which provided a safe
working environment for plant operations and
maintenance personnel. The project achieved
over 1.6 million man hours worked without a
lost-time accident.
Innovative Design Features
The Clean Air Project included a number of
technologies customized for the Merrimack
Station.
Two-into-One Absorber. The project configuration
includes a single absorber vessel and
FGD system to serve the two-unit plant, and
each unit operates independently. The boilers
are positive pressure, cyclone burner design,
www.powermag.com
Evolution. The WWTS consists of an innovative
physical-chemical treatment system for
FGD wastewater, using well-established technology
that has the benefit of two decades of
worldwide operation. Following the physicalchemical
treatment system, the FGD wastewater
is directed through a treatment subsystem
that is specially designed to address the FGD
wastewater generated by Merrimack Station's
FGD system. This enhanced treatment system
polishes the treated FGD wastewater to achieve
additional removal of mercury, arsenic, and
other metals, as well as further reduction in total
suspended solids concentrations. Using proven
wastewater polishing techniques, this subsystem
employs filters and two different types of
adsorbent media. This enhanced system was
added to meet state effluent limits.
Hitchko explained that due to the U.S. Environmental
Protection Agency's reluctance to
provide National Pollutant Discharge Elimination
System (NPDES) modifications to accommodate
the state-reviewed and approved
scrubber liquid effluent, the project team had to
add a secondary WWTS to the project scope in
early 2011. He said, " This equipment reduces
the system effluent to zero and thus does not
need any NPDES approvals. The recently issued
new draft NPDES permit for Merrimack
Station is in a comment/review period, which
will extend the date for a final NPDES permit
even further into the future. "
Acoustical Modeling Design Impact. In
addition to ductwork and limited accessibility
issues, the design was further challenged when
acoustical modeling resulted in the need to add
an acoustical enclosure around the booster fans.
Because the enclosure was added subsequent to
the fan/ductwork design phase, it created additional
design and construction challenges. The
large enclosure ensures that the project's acoustic
design goals are achieved and that the project
will have negligible acoustic impact on the station's
neighbors. ■
-Angela Neville, JD is POWER's
senior editor.
POWER | October 2012

http://www.powermag.com

POWER October 2012

Table of Contents for the Digital Edition of POWER October 2012