“GT STRUDL and the SCR Reactor Project, Units 1 & 2, Plant Bowen, Cartersville,
Georgia”
by
Scott Lange
Southern Company Services
Atlanta, GA
Abstract
Selective Catalytic Reduction (SCR) is a method to remove
NOx compounds from flue gas as it exits the boiler in a coal-burning power
plant. As the flue gas enters the SCR reactor, ammonia is injected
into the flow stream. The ammonia reacts with the NOx in the presence
of the catalyst at a minimum temperature of 650F producing water vapor
as well as nitrogen in safer forms. In order to comply with the Clean
Air Act, Southern Company decided to install SCR boxes at several power
plants, starting with Plant Bowen in Cartersville, Georgia. The design
work began in January 1999 while construction began in June of 1999 and
is currently ongoing.
Relocation of C & D Precipitator Duct and Duct Support
Before the SCR was installed, flue gas from the boiler returned
to the air preheater. During times of high ozone levels, that gas
would need to be diverted through the SCR before returning to the air preheater.
Installing a damper to divert the gas required relocating the existing
precipitator duct and associated support steel. The gradual changes
to the duct arrangement as each piece of duct was relocated necessitated
many different construction load combinations as well as different structural
framing systems, each of which was checked with a GT STRUDL model of the
duct support structure. The model showed that while extensive modifications
to the support towers was necessary, it could still be accomplished without
tearing the existing structure down and starting from scratch. Also,
the duct itself had to have a very irregular shape in order to fit in the
small space available. Designing such an irregular piece of duct
by hand would have been quite difficult, so a GT STRUDL model was used
for it as well.
Design of SCR Support Structure
The two fully loaded SCR boxes would weigh approximately
2.7 million pounds each, and had to be supported at a height of roughly
150 feet off the ground. The steel supporting these massive boxes
had to negotiate an incredible maze of interferences. A GT STRUDL
model of the entire support structure was put together and used to investigate
the options. After considering several different possibilities, the
arrangement arrived at consisted of fourteen columns connected by a series
of trusses spanning between them and supporting the SCR boxes. The
columns were positioned to avoid as much of the existing structures and
equipment as possible, but some existing features were moved, and one of
the support columns actually penetrated existing ductwork. The final
geometry file comprised nearly 10,000 members. A separate lode file
contained 100 independent loads, 260 ACI factored load combinations, and
260 AISC load combinations. The best computer that ran the model
was a 975Mhz dual processor Pentium with 1 Gb of RAM. On that computer,
a stiffness analysis took nine hours and a complete code check took eighty
hours.
Design of Support Structure Foundations
The fourteen columns of the support structure came to rest
on twelve foundations (two foundations supported two columns each.)
Each foundation was comprised of a number of caisson foundations between
twenty and twenty-five feet in length, tied together with a roughly five-foot
deep pile cap. A separate GT STRUDL model was made of each foundation,
and the reactions at the bases of the fourteen columns on the support structure
model were applied to the foundations. Some of the foundations were
relatively straight forward (column in the middle of a pile cap spanning
three caissons arranged in an equilateral triangle.) Others were
more complicated, notably the two dual-column foundations, each of which
had to straddle a 36” high-pressure water line. GT STRUDL allowed
us to continually check the foundations for the new reactions each time
the design of the support structure was modified.
Education
B.S. Civil Engineering, Georgia
Institute of Technology, 1998