POWER October 2013 - 68

Emissions
Reducing SCR Fly Ash
Accumulation with Improved
Reactor Inlet Airflow
Poor airflow around its selective catalytic reactor (SCR) was causing heavy fly ash
accumulation and pluggage at Kansas City Power & Light's 815-MW La Cygne
Unit 1. Flow modeling and redesign of the SCR hood led to dramatically improved
conditions and reduced maintenance costs.
Scott Hiedeman, Reid Thomas, Dale Pfaff, and Diane Fischer
T
he Kansas City Power & Light
(KCP&L) La Cygne Generating Station
provides 1,532 MW of peak power
from its site south of Kansas City. La Cygne
Unit 1 is an 815-MW Babcock & Wilcox
cyclone boiler with overfire air and selective
catalytic reduction (SCR) nitrogen oxide
(NOx
flow control baffles, corrections to the sonic
horn, covering or removal of horizontal
vane and beam surfaces, and enlarging the
honeycomb ceramic catalyst pitch. These
minor repairs were not successful, because
they were not addressing the root cause of
poor flow distribution.
) controls. Unit 1 burns a blend of
90% Powder River Basin (PRB) and 10% local
Missouri coal.
The unit's SCR was installed in 2007 with
a 3 + 1 catalyst layer configuration, which
consists of three initial layers of catalyst
with one spare layer for future use. In the
fall of 2012, plant staff vacuumed 2,900,000
pounds of fly ash from the SCR after a 17month
operating cycle. This was twice the
typical amount of fly ash removed in previous
outages because the unit had operated
for an extended time with a low demand load
factor. However, the catalyst pluggage of approximately
50% was typical of previous operating
cycles.
The SCR consists of two reactor casings
separated by a common division wall. Flue
gas enters the SCR North Side B and South
Side A of the SCR hood to flow down
through these separated casings. Fly ash
would accumulate 5 to 6 feet deep near the
SCR north to south reactor casing division
wall (Figure 1). The fly ash accumulation
caused high ash removal costs, high catalyst
replacement costs, high catalyst pressure
drop and fan power costs, and high
ammonia slip and ammonia reagent costs.
In addition, cleaning the catalyst became a
critical path activity during outages. The
fly ash accumulation also caused unit capacity
derates in combination with other
draft losses.
Many minor changes were attempted
through the first five years of SCR operation.
These included installation of missing
68
Poor Flow and Many Restrictions
To keep PRB fly ash moving through the
catalyst, flue gas flow must be of uniform
velocity, vertical, and without recirculation.
In 2010, KCP&L La Cygne initiated computational
fluid dynamics (CFD) flow modeling
with Fuel Tech to determine the cause of fly
ash accumulation and correct flow variances
to the catalyst. The CFD analysis shown in
Figure 2 indicated problem areas, including
the following:
■ SCR inlet duct north-south contraction
pushed flow to the lower four SCR hood
turning vanes.
■ Trusses and large gusset plates in the SCR
inlet duct and hood restricted flow to the
upper four SCR hood turning vanes.
■ The turning vanes and perforated plate on
top of the original egg crate flow-straightening
grid created flow recirculation zones
that caused fly ash dropout. The turning
vanes also collected ash on horizontal
surfaces because of low flow, and these
ash piles sloughed off during forced draft
(FD) fan startups.
■ The egg crate structural support steel created
flow disturbances that could not be
corrected in the short distance to the first
catalyst layer.
The flue gas recirculated behind the SCR
hood turning vanes, but the fly ash did not
follow this twisting route and instead fell
out on the catalyst layers. Because of these
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POWER | October 2013
issues, KCP&L decided that the existing
flow-straightening devices needed to be removed
and replaced with a new design that
addressed the findings of the flow model.
The Redesign
Modification of the SCR hood was originally
planned for the fall of 2013. However, low
demand load factors on KCP&L and Westar
systems in 2012, and the high price of
continuing catalyst ash pluggage, led to the
decision to move the project up for a 2012
1. A big problem. Poor airflow was causing
heavy fly ash accumulation in the SCR.
Here, mounds of ash have built up near the
reactor casing division wall. Courtesy: KCP&L
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POWER October 2013

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