8/3/2019 F04NL.vogt Pages.nov 12-Final

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FOCUS on CFD

For Power Generation

FOCUS on CFD

For Power Generation

s2 overviewFuture Power Generation

Challenges

s3 combinedcycle powerReducing Gas Turbine

Emissions

s4 renewable powerA Powerful Wind of Change

The Wind in Spain

Aerating Water in the

Summertime

s8 nuclear powerFast Breeder Meltdown

CFD for Advanced NuclearReactor Design

s11 fossil fuel powerCoal Gasification for Future

Power Generation

Low Emissions Bluff-body

Burner

Newsletter Supplement

8/3/2019 F04NL.vogt Pages.nov 12-Final

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Fluent Newsfall 2004 S3

combined cycle power

Reducing Gas Turbine EmissionsBy Keith C. Kaufman, Vogt Power International Inc., Louisville, Kentucky

Since 1923, the Turlock Irrigation

District (TID) has been providing

electricity to customers with a cur-

rent customer base of more than 84,000

accounts in California's Stanislaus and

Merced counties. TIDs generation

resources include large and small-scale

hydro-electric power plants and two nat-

ural 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 equip-

ment, upstream of an existing heatrecovery steam generator (HRSG).

The newly installed CO oxidation cat-

alyst and selective catalytic reduction

(SCR) modules are designed to react

with large volumes of gas to eliminate

CO and NOx contaminants. An ammo-

nia injection grid (AIG) upstream of the

SCR catalyst provides ammonia to com-

plete 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 cat-

alysts to the entrance of the HRSG. In the

Almond Power unit, a collector/diffuser

spool redirects exhaust gas from the tur-

bine into an expanding inlet duct that in

turn directs gas into the catalyst modules

and HRSG. The FLUENT analysis con-

firmed what VPI engineers had anticipat-

ed: the gas velocity from the turbine was

unevenly distributed across the surfaces

of the catalyst, so only a portion of thecatalyst material was engaged.

An analysis of the existing equip-

ment 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 large-

ly non-uniform at the plane of the AIG,

which is positioned just downstream of

the CO modules. The underperfor-

mance 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 distribu-

tion grid that provides an improved flow

distribution entering the CO modules,

AIG and SCR. Installed just upstream of

the CO catalyst modules, the grid redis-

tributes 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 gas-

side pressure drop that would decreasethe 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 suffi-

cient redistribution of flow top-to-bot-

tom across the unit, allows for uncertain-

ty 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 distribu-

tion 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 emis-

sions systems now outperform the regu-

latory requirements.

The original designconfiguration

GT collector/diffuser

CO catalyst modules

SCR catalyst modules

Pathlines illustrate the flow inthe original configuration

Velocity contours on the AIG before (left)and after (right) the addition of thedistribution grid

The addition of a distribution grid(black) improved the flow uniformity

on the catalyst module surfaces