Aeroderivative Gas Turbines

LM2500® to LM2500+DLEGas Turbine Combined CyclePlant Repowering

Authors:

Michael T. McCarrickGE Energy

Kenneth MacKenzie P.Eng.City of Medicine Hat

AbstractThe province of Alberta, Canada developed annual emission

intensity limits for the power generation sector in order to

preserve air quality by reducing emissions of sulfur dioxide (SOx),

nitrogen dioxide (NOx), and primary particulate matter. Effective

January 1, 2006, all new gas-fired power plants were required to

meet Annual Emission Intensity Limits for NOx. In the Spring of

2006, the City of Medicine Hat (COMH) LM2500 gas turbine units

were due for their 50,000-hour major overhaul cycle, and COMH

was faced with this new NOx emission requirement, along with

an increasing power load demand for the city. Working closely

with GE Energy’s Aero Services group, COMH decided to upgrade

the two LM2500 gas turbines to two LM2500+DLE gas turbines to

meet the new emissions requirements and boost power output

to support the growing needs of the city.

This paper will describe the engineering design challenges to

modify the existing LM2500 gas turbine package to accept the

longer and more powerful LM2500+DLE gas turbine, and the

results of the evaluations of the additional mass flow impact on

steam production and combined cycle plant performance.

This paper will also detail the upgrade execution outages and

commissioning experience of the LM2500+DLE gas turbines at

the COMH plant. The actual operational performance results will

be outlined to show the approximately 30% increase in gas

turbine power output, 5% decrease in gas turbine heat rate, and

the annual reduction in NOx emissions of up to 900 tons per year.

Background and Description of Original Plant ConfigurationThe City of Medicine Hat (COMH) Electric Utility is a municipally

owned utility in Alberta, Canada that operates a natural gas-fired

combined cycle power plant with a 209 MW ISO capacity.

The plant began in 1910 with a single-cylinder natural gas-fired

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engine driving a generator. Over the intervening years, the plant

grew into a strictly Rankine-cycle steam plant and finally into

today’s configuration, a fully combined cycle plant.

The present configuration of the plant is shown in Figure 1. Units

10 and 11, the subject of this paper, were originally installed in

1990 as refurbished General Electric (GE) Frame 5M gas turbines

operating in simple cycle with a heat rate of 13,900 BTU/kWe-Hr

(LHV). A summary of past and present unit configurations is

shown in Table 1.

In 1993, single-pressure HRSGs were constructed for both units to

improve plant efficiency and increase plant capacity, achieving a

combined cycle heat rate of 8,880 BTU/kWe-Hr (LHV) with

a corresponding steam turbine power production of 10.1 MWe.

Figure 2 shows the heat balance for the 1993 configuration.

In 1998 the Frame 5M units were replaced with GE’s LM2500-PE

model aeroderivative gas turbines in an attempt to improve

reliability and further improve efficiency. The LM2500 is derived

from the CF6 family of aircraft engines used on a variety of

commercial aircraft and is a hot-end drive, two-shaft gas

generator with a free power turbine. The LM2500-PE features

a Single Annular Combustor (SAC) and a 6-stage power turbine.

The ISO exhaust flow of the LM2500-PE engines, at 499,000 lb/hr

was significantly lower than the previous Frame 5 ISO engine

exhaust rate of 716,000 lbs/hr. Thus, the steam production from

the respective Unit 10 and 11 HRSGs decreased to 6.1 MWe as

measured in steam turbine electric power output. However, the

combined cycle heat rate improved to 7,500 BTU/kWe-Hr (LHV)

as a result of the turbine replacements. Figure 3 shows the heat

balance for the 1998 configuration with the LM2500-PE.

Table 1 – Units 10 and 11 Performance Summaries (at 15C 60%RH 666m el.)

Simple CycleHeat Rate LHV BTU/MWe-Hr)

Gas Turbine Generator Power (MWe)

HRSG HP Steam(lb/hr)

Steam Turbine(MWe)

Combined Power (MWe)

Combined CycleHeat Rate LHV BTU/MWe-Hr)

Frame 5M 13.90 17.9 91,000 10.1 28.0 8.9

LM2500PE 9.74 20.4 55,000 6.1 26.5 7.50

LM2500PR 8.88 27.0 62,000 6.9 33.9 7.08

LM2500 to LM2500+DLE Gas Turbine Combined Cycle Plant RepoweringMichael T. McCarrick

GE Energy

Kenneth MacKenzie P.Eng.

City of Medicine Hat

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Figure 2

Figure 1

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LM2500 Gas Turbine History and Evolution to the LM2500+/+G4In the early 1970s, the LM2500 gas turbine was derived from

the CF6-6 flight engine, which to date has accumulated over

300,000,000 flight hours on a variety of commercial aircraft.

The LM2500 utilized a 16-stage compressor section with inlet guide

vanes and 6-stages of variable stator vanes with a 2-stage

high-pressure turbine (HPT) section exhausting into a 6-stage free

power turbine. The original design had twin-shank HPT blades and

an ISO power rating of 17.9 MW with 35.8% simple cycle thermal

efficiency. In 1992 a single-shank HPT blade was introduced that

allowed for a higher firing temperature while maintaining the

expected 25,000-hour hot section life on natural gas. The ISO power

rating was correspondingly increased to 23.8 MW with a 37.5%

simple cycle thermal efficiency.

In 1997, the LM2500 was upgraded to the LM2500+ when a zero

stage blisk was added to the front end of the compressor section

to provide a total of 17 stages of compression and boost the

compression ratio from 20:1 to 23:1. Some material changes in

the HP turbine section and enlarging of the free power turbine

flow function raised the ISO power rating to 31.3 MW with 39.5%

simple cycle efficiency. The major differences between the

LM2500 and the LM2500+ gas turbines are shown in Figure 4.

The LM2500+G4 gas turbine, the most recent uprate to the

LM2500 family, was introduced in 2005. The LM2500+G4

provides approximately a 10% power increase over the LM2500+

model and was considered for this use in this project, however,

the electric generator could not handle the LM2500+G4 output

across the full range of ambient conditions, and the LM2500+

was selected for the COMH Repowering Project.

As of this writing, the LM2500/+/+G4 models have over

51,000,000 operating hours in power generation, marine,

cogeneration, and mechanical drive applications.

Figure 3

Figure 4 – LM2500 vs. LM2500+ Cross-Sectional View (SAC Versions Shown)

Figure 5 – DLE (Triple Annular Combustor) vs. Single Annular Combustor (SAC)

was introduced to achieve 25 ppmvd NOx emissions without

water or steam injection. The DLE combustor uses 30 premixers

comprised of 75 staged injectors and a 4-passage compressor

diffuser. The LM2500+DLE gas turbine has over 2,400,000

operating hours, and can achieve 25 ppmvd NOx and 25 ppmvd

CO from 75% to 100% load. The SAC and DLE Combustors are

compared in Figure 5 below.

Standard Annular Combustor (SAC) vs. Dry Low Emissions (DLE) TechnologyThe original LM2500 gas turbines operated with a standard

annular combustor that utilized a single compressor diffuser pas-

sage and a single row of 30 fuel nozzles, and NOx emissions

abatement was accomplished with water or steam injection to

suppress the firing temperature and reduce the formation of

NOx. In 1995, a Dry Low Emissions (DLE) combustion technology

Dry Low Emissions Single Annular Combustor

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Repowering Drivers and BenefitsThe original LM2500-PE gas turbines were scheduled to achieve

their 50,000-hour major overhaul cycle in 2006, and the COMH

was faced with four options:

1. Rebuild the existing LM2500 SAC engines at the depot

to achieve Zero Time.

2. Replace existing LM2500 SAC engines with identical new

LM2500 SAC units.

3. Replace the LM2500 SAC engines with new LM2500 DLE

engines.

4. Increase the capacity and reduce emissions by replacing

the LM2500 SAC engines with LM2500+ DLE engines.

To comply with emissions reductions commitments made to

regulatory agencies, and to increase plant capacity with a

marginally improved heat rate, it was decided to pursue Option

4, replacement of the engines with the LM2500+ DLE engines,

known formally as the LM2500-PR model.

1. Emissions ReductionsIn prior years, the COMH had made commitments to the

regulator, Alberta Environment (AENV) that would see NOx

abatement technologies implemented on Units 10 and 11 prior

to renewal of the environmental permit in 2009.

The LM2500-PE units had historically and consistently produced

NOx emissions of approximately 170 ppmvd, equivalent to a

mass emission of 125 pounds per hour of NOx.

Under the AENV’s 2006 Emission Intensity Limits for NOx, new

units in the 20 to 60 MW range (that range covers the LM2500

models) would be required to achieve an intensity of less than 0.4

kg/MW-Hr.

As shown in Table 2, the original LM2500-PE units produced NOx

emission intensities well in excess of the 2006 Limits. NOx

abatement equipment such as Selective Catalytic Reduction

(SCR) or water/steam injection would have been necessary to

achieve the 2006 limits using the LM2500 SAC engine.

The LM2500-PR was predicted, and has proven capable in

meeting the 2006 limits.

2. Increased Power OutputThe COMH electrical load in the 21st century has been growing

at an average of 2% per year, with corresponding increases in

air-conditioning load peaks during the summer months.

Replacing the LM2500-PE with a similar engine would obviously

do nothing to increase plant capacity to meet this increasing

load. On the other hand, the LM2500-PR offered a modest

7.4 MWe increase in combined cycle capacity per unit,

well within the regulatory limits set on the amount of electrical

generation the City may hold.

3. Improved Heat RateThe LM2500-PR features a higher compression ratio (23:1) than

the LM2500-PE (20:1), and consequentially is a more efficient

turbine. The guaranteed simple cycle heat rate for the

LM2500-PR was 9,238 Btu/kWe·Hr LHV versus the guarantee rate

for the LM2500-PE at 9,743 Btu/kWe·Hr LHV, a 5% improvement.

Technical Challengers Encountered in LM2500 to LM2500+DLE Repowering1. Accommodation of Longer EngineBy far the biggest challenge in the project was altering the

turbine compartments to fit the longer engines. As shown in

Figures 6 and 7, the plenum wall was moved 13-13/16” and the

main structural base was extended a proportionate amount.

The front engine mounts were changed from the top-hung

"horseshoe" frame to a link style system supported from the

subbase, which also required extension.

The original turbine compartment crane was removed and

replaced with a higher capacity unit due to the increased weight

of the PR engine. The engine-lifting beam required extensive

modifications as a result of reduced headroom from the

modifications.

2. Capacity of GeneratorThe generator supplied in 1998 with the original LM2500-SAC

package is a Brush Electric Machines Ltd. Type BDAX-7-167ESS

rated at 35,412 KVA at a 0.85 Power Factor.

On cold winter days, the LM2500-PR turbine is capable of

producing 30.1 MW with a corresponding turbine inlet temperature

of 32 degrees Fahrenheit. This power output matches exactly the

Brush generator capability at the 0.85 Power Factor and thus no

changes to the generator were required. The mechanical

coupling connecting the turbine to generator was found to be

inadequate for the higher mechanical power transmission duty

of the PR turbine and was replaced.Table 2 – NOx Intensities Comparison at Site Conditions

NOx conc.(ppvmd)(ref.15% O2)

NOx massemission(kg/Hr)

Combined HP Steam(lb/hr)

Steam Turbine(MWe)

LM2500-PE 170.00 57.0 91,000 10.1

LM2500-PR 21.60 10.0 55,000 6.1

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3. Fuel SystemThe original LM2500-PE engines required a natural gas fuel

supply delivered at 385 psig and 100 Degrees F at the skid base.

The new LM2500-PR engines require a significantly higher inlet

pressure of 520 psig.

A review of the entire gas system was conducted to ensure that

the gas delivery system piping and vessels were capable of the

increased operating pressure. This review revealed that the ANSI

Class 300 flanges and piping were indeed capable of the

increased pressure, but that the stainless steel filter vessels

would need alteration of nozzle reinforcement before being

recertified for the higher design pressure. A local pressure vessel

shop performed this alteration during the package conversion

activities.

4. Control SystemThe existing Woodward Netcon control system was removed and

replaced with a MicroNet Plus digital controller. This required new

control cubicles to be built and installed next to the existing

Turbine Control Panels (TCP). The field wiring between the MTTB

and the TCP was installed prior to the outage, and then

terminated once the outage began.

5. Inlet Air FiltrationAs a result of the Stage 0 Blisk incorporated in the LM2500-PR,

the airflow through the filter house to the engine increased from

499,000 lb/hr to 612,000 lb/hr. Consideration was given to

increasing the size of the filter house, but the penalty in power

output created by the additional inlet pressure drop was

predicted to be only a few hundred kilowatts.

Figure 6 – LM2500 vs. LM2500+ Turbine Compartment – Plan

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Figure 7 – LM2500 vs. LM2500+ Turbine Compartment – Elevation

Outage SummaryThe upgrades of Units 10 and 11 were planned as back-to-back

outages of 30 days each. Unit 11 was the first outage executed,

and took 14 days longer than expected due to fit-up, alignment,

and clearance issues. However, the lessons learned on the first

conversion expedited the second, as its duration was 30 days per

the plan.

Performance TestingBoth new gas turbines were tested in general accordance with

ASME PTC-22, "Gas Turbine Power Plants."

Table 3 summarizes corrected power output and heat rate of

the two LM2500+DLE 6-Stage (PR) units. The guaranteed power

output is 26,903 kW. The guaranteed heat rate is 9,248 Btu/kW-

hr (9,757 kJ/kW-hr). The tested units yielded corrected power

output and heat rate better than guaranteed values.

The actual combined cycle unit heat balance for Units 10 and 11

is shown in Figure 8. The combined cycle heat rates for Units 10

and 11 improved from 7.5 to 7.1 BTU/MWe-Hr, an improvement

of 5.6%. Figure 9 is compiled from plant data and contrasts the

respective combined cycle heat rates with those of the sister

unit, an LM6000-PD.

Table 3 – Performance Test Summary

CorrectedHeat RateLHV (BTU/MWe-Hr)

Gas TurbineCorrectedPower(MWe)

CorrectedPower Margin %

CorrectedHeat RateMargin %

AverageLHV,Btu/lb

Unit 10 8.865 27.1 0.75 4.1 19,998

Unit 11 8.904 27.0 0.25 3.7 20,049

Average 8.885 27.0 0.5 3.9 20,024

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ConclusionIn general, the two units have been performing very well and

have delivered a 30% increase in power output, and over

a 5% reduction in combined cycle heat rate.

A few issues remain outstanding at time of writing:

• Turbine compartment temperatures have been high during

the hot summer months, and ventilation air modifications are

planned for the fall of 2007.

• The power turbine thrust balance pressure is in alarm on both

units, and both the cause and solution are under investigation

by GE.

Unit availability and reliability for the first two quarters of 2007

have been very high at 98.7% and 99.9% respectively.

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Figure 8

Figure 9 – Comparison of Heat Rates and Load

References1. Alberta Regulation No. 33/2006, Environmental Protection

and Enhancement Act, Emissions Trading Regulation.

2. Figures 6 and 7 provided courtesy of Fern Engineering of

Pocasset, Massachusetts. Fern provided engineering design

services for the package modifications, and have provided

similar services on other Gas Turbine projects.

3. GER 4250 “The LM2500+G4 Aeroderivative Gas Turbine for

Marine and Industrial Applications” by Gilbert H. Badeer,

GE Energy September 2005.

www. ge-aero.com

LM2500 is a trademark of General Electric Company.Copyright © 2012, General Electric Company. All rights reserved.

GEA18640A (10/2012)