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By Keith C. Kaufman, Vogt Power International Inc., Louisville, Kentucky
View the pdf of this Supplement
Since 1923, the Turlock Irrigation
District (TID) has been providing
electricity to customers, with a current
customer base of more than 84,000
accounts in California’s Stanislaus and
Merced counties. TID’s generation
resources include large and small-scale
hydro-electric power plants and two natural
gas-fired turbine generating plants.
A General Electric LM6000 engine was
recently installed to upgrade TID’s
Almond Power Plant. The higher exhaust
temperature of the LM-6000 gas turbine
necessitated the replacement of both the
carbon monoxide (CO) and nitrogen
oxide (NOx) emissions control equipment,
upstream of an existing heat
recovery steam generator (HRSG).
The original design
configuration
Pathlines illustrate the flow in
the original configuration
The addition of a distribution grid
(black) improved the flow uniformity
on the catalyst module surfaces
The newly installed CO oxidation catalyst
and selective catalytic reduction
(SCR) modules are designed to react
with large volumes of gas to eliminate
CO and NOx contaminants. An ammonia
injection grid (AIG) upstream of the
SCR catalyst provides ammonia to complete
the NOx reduction reaction. TID
found, however, that the newly installed
emissions systems were not performing
at expected levels, so TID turned to Vogt
Power International Inc. (VPI), a Babcock
Power Inc. company, to help correct the
problem. Using FLUENT, VPI engineers
modeled the exhaust system from the
gas turbine through both emissions catalysts
to the entrance of the HRSG. In the
Almond Power unit, a collector/diffuser
spool redirects exhaust gas from the turbine
into an expanding inlet duct that in
turn directs gas into the catalyst modules
and HRSG. The FLUENT analysis confirmed
what VPI engineers had anticipated:
the gas velocity from the turbine was
unevenly distributed across the surfaces
of the catalyst, so only a portion of the
catalyst material was engaged.
Velocity contours on the AIG before (left)
and after (right) the addition of the
distribution grid
An analysis of the existing equipment
showed a highly non-uniform
velocity profile at the entrance to the
CO modules. The gas exiting from the
collector/diffuser was strongly biased to
the bottom and sides of the duct, with
significant regions of backflow in some
sections of the inlet expansion. While
the CO modules acted to straighten the
flow somewhat, the flow was still largely
non-uniform at the plane of the AIG,
which is positioned just downstream of the CO modules. The underperformance
of the SCR was shown to be due
to non-uniform mixing of the ammonia
and exhaust streams following the AIG,
and poor gas distribution at the SCR
catalyst itself.
Using CFD as an evaluation tool, VPI
engineers designed a two-zone distribution
grid that provides an improved flow
distribution entering the CO modules,
AIG and SCR. Installed just upstream of
the CO catalyst modules, the grid redistributes
the flow using angled perforated
plates that allow the passage of varying
amounts of air in different regions of the
plane, while limiting an increase in gasside
pressure drop that would decrease
the gas turbine efficiency. The engineers
used CFD to adjust the design of each
grid sector, and to evaluate its influence
on each component downstream. The
resulting two-zone design provides sufficient
redistribution of flow top-to-bottom
across the unit, allows for uncertainty
in the gas turbine exhaust profile and
variation with gas turbine load, while
minimizing pressure drop across the
grid. The CFD results show more than a
20% improvement in velocity distribution
at the CO modules and AIG plane.
With the revised design, more than 90%
of the flow upstream of the SCR falls
within +/-15% of the average velocity at
this location. Physical testing after the
grid was installed verified these results.
Most importantly, the field test values
after installation confirm that the emissions
systems now outperform the regulatory
requirements.
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