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Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Cost-Benefit Analysis of 

Flexibility Retrofits for Coal and 

Gas-Fueled Power Plants

August 2012 – December 2013

S. Venkataraman, G. Jordan, and M. O’Connor 

GE Energy

Schenectady, New York 

N. Kumar and S. Lefton

Intertek AIM

Sunnyvale, California

D. Lew, G. Brinkman, D. Palchak, and 

J. Cochran

National Renewable Energy Laboratory (NREL)

Golden, Colorado

NREL Technical Monitors: Debra Lew and Kara Clark

Subcontract Report

NREL/SR-6A20-60862

December 2013
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Cost-Benefit Analysis of 

Flexibility Retrofits for Coal 

and Gas-Fueled Power Plants

August 2012 – December 2013

S. Venkataraman, G. Jordan, and 

M. O’Connor 

GE Energy

Schenectady, New York

N. Kumar and S. Lefton 

Intertek AIM

Sunnyvale, California

D. Lew, G. Brinkman, D. Palchak, and 

J. Cochran 

National Renewable Energy Laboratory (NREL)

Golden, Colorado

NREL Technical Monitors: Debra Lew and Kara Clark

Prepared under Subcontract No. KAFT-1-11349-00

Subcontract Report

NREL/SR-6A20-60862

December 2013
This publication received minimal editorial review at NREL.

NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

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trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.

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Foreword and Legal Notices

This document was prepared by General Electric International, Inc. It is submitted to the 

National Renewable Energy Laboratory (NREL). Some of the findings in this study are

published in a separate brochure, NREL/BR-6A20-60575, entitled Flexible Coal: 

Evolution from Baseload to Peaking Plant. Technical and commercial questions and any 

correspondence concerning this document should be referred to:

Sundar Venkataraman, Director

Energy Consulting

GE Energy Management

3150 S. 48th Street

Phoenix, AZ 85040

Phone: (518) 225-2265

E-mail: sundar.venkataraman@ge.com
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This report was prepared by General Electric International, Inc. (GE) as an account of 

work sponsored by NREL. Neither NREL, nor GE, nor any person acting on behalf of 

either:

1. Makes any warranty or representation, expressed or implied, with respect to the 

use of any information contained in this report, or that the use of any information, 

apparatus, method, or process disclosed in the report may not infringe upon

privately owned rights

2. Assumes any liabilities with respect to the use of or for damage resulting from the 

use of any information, apparatus, method, or process disclosed in this report.

Copyright © 2012 GE Energy. All rights reserved.
v

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Table of Contents

Introduction..................................................................................................................................................1

Operational Flexibility of Thermal Power Plants...................................................................................2

Flexibility Retrofits for Gas-Fueled Simple and Combined Cycle GT Power Plants.............................3

Operating Boundaries of a GT .........................................................................................................3

Startup ..............................................................................................................................................4

Operational Flexibility Retrofits for GTs................................................................................................7

Flexibility Retrofits for Coal-Fueled Steam Turbine Power Plants.......................................................10

Operating Boundaries of Coal Units.....................................................................................................10

Startup ............................................................................................................................................11

Operational Flexibility Retrofits for Coal Plants..................................................................................13

Flexibility Improvements from Operating Procedures...................................................................18

Study System for Retrofit Analysis .........................................................................................................19

Load Assumptions ................................................................................................................................19

Generator Assumptions.........................................................................................................................19

GT Technology...............................................................................................................................20

Steam Turbine Technology ............................................................................................................22

Renewable Generation Assumptions....................................................................................................24

Fuel Price Assumptions........................................................................................................................24

Cost-Benefit Analysis ...............................................................................................................................25

Methodology.........................................................................................................................................25

Gas-Fueled Power Plant Retrofits.........................................................................................................25

Coal-Fueled Power Plant Retrofits .......................................................................................................32

Conclusions ...............................................................................................................................................38

Appendix A. GE Operational Flexibility Solutions .................................................................................40

GE OpFlex Startup Agility Solutions...................................................................................................40

GT 40

Steam Turbine ................................................................................................................................40

HRSG .............................................................................................................................................41

GE OpFlex Combustor Operability Solutions......................................................................................41

Grid Stability..................................................................................................................................41

Automated DLN Tuning ................................................................................................................41

GE OpFlex Load Flexibility Solutions.................................................................................................42

Output.............................................................................................................................................42

Responsiveness...............................................................................................................................42

Turndown .......................................................................................................................................42

Efficiency .......................................................................................................................................42

GE OpFlex System Reliability Solutions .............................................................................................43

Fuels Reliability .............................................................................................................................43

System Reliability ..........................................................................................................................43

Diagnostics/Productivity ................................................................................................................43

Summary...............................................................................................................................................44

Appendix B. Additional Information on Coal Retrofits..........................................................................45

 
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List of Figures

Figure 1. Capacity mix for the study systems...................................................................................... vii

Figure 2. Contemporary power plant mission profile: spending less time at full load ...........................3

Figure 3. Thermal generators in the RMPP system ..............................................................................19

Figure 4. Thermal generation mix in the RMPP case...........................................................................20

Figure 5. Thermal generation mix in the Modified-RMPP case...........................................................20

Figure 6. Combined cycle GT technology mix for the RMPP system..................................................21

Figure 7. Simple cycle GT technology mix for the RMPP system.......................................................22

Figure 8. Coal-fired generation technology mix for the WECC system...............................................23

Figure 9. Coal-fired generation technology mix for the RMPP system................................................23

Figure 10. Coal-fired generation technology mix for the RMPP system..............................................24

Figure 11. Operational impact on combined cycle plants for various renewable penetrations ............26

Figure 12. Dispatch of combined cycle Unit A, with and without turndown retrofit...........................29

Figure 13. Dispatch of combined cycle Unit B, with and without turndown retrofit ...........................30

Figure 14. Dispatch of combined cycle Unit C with and without turndown retrofit ............................30

Figure 15. Operational impact on coal steam plants for various renewable penetrations.....................32

Figure 16. Coal steam plants, hours at mingen for Case 4....................................................................33

Figure 17. Mingen profile at Coal Unit D, before and after retrofit .....................................................35

Figure 18. Coal Unit D, revenue and costs with and without retrofit ...................................................36

List of Tables

Table 1. Performance and Cost Information for Turndown Improvements............................................9

Table 2. Performance and Cost Information for Startup Improvements.................................................9

Table 3. Performance and Cost Information for Ramp Rate Improvements ..........................................9

Table 4. Performance and Cost Information for Boiler Retrofits.........................................................15

Table 5. Performance and Cost Information for Coal Mill Retrofits....................................................15

Table 6. Performance and Cost Information for Emissions Control Retrofits......................................16

Table 7. Performance and Cost Information for BOP Retrofits............................................................16

Table 8. Performance and Cost Information for Turbine Retrofits.......................................................17

Table 9. Performance and Cost Information for Chemistry-related Improvements .............................17

Table 10. Frame Type, Make, and Model of GTs.................................................................................21

Table 11. Renewable Generation Assumptions....................................................................................24

Table 12. Impact of Combined Cycle Retrofits on Production Cost ....................................................27

Table 13. Net Revenue Impact of Combined Cycle Retrofits for Unit A.............................................27

Table 14. Net Revenue Impact of Combined Cycle Retrofits for Unit B.............................................28

Table 15. Net Revenue Impact of Combined Cycle Retrofits for Unit C.............................................28

Table 16. Annual Generation With and Without Retrofits for Case 2, Modified-RMPP 

System............................................................................................................................................28

Table 17. Impact of Combined Cycle Retrofits on Combined Cycle and GT Starts for Case 2, 

Modified-RMPP System ................................................................................................................29

Table 18. Net Revenue Impact of Coal Plant Retrofits for Unit D.......................................................34

Table 19. Net Revenue Impact of Coal Plant Retrofits for Unit E .......................................................34

Table 20. Net Revenue Impact of Coal Plant Retrofits for Unit F........................................................34

Table 21. Annual Generation with and Without Retrofits for Case 4, RMPP System .........................35

Table 22. Hours at Mingen Before and After Coal Retrofits................................................................36
vii 

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Executive Summary 

In the Electric Power industry, there is tremendous interest in investigating mitigation 

measures for lowering cycling-related costs of thermal power plants. One approach is to 

retrofit a few existing units for improved operational flexibility (i.e., capability to 

turndown lower, start and stop faster, and ramp faster between load set-points). While 

having this capability on some power plants might result in them cycling more, it will 

reduce the cycling on the remaining power plants in the system resulting in system-level 

savings. Having this operational flexibility will also help reduce the curtailment of wind 

and solar generation at higher renewable penetration levels. This report presents the 

findings of a study that examined the costs and benefits of retrofitting gas-fueled simple 

and combined cycle and coal-fueled steam turbine power plants for lower turndown, 

faster ramping, and faster starting capability. (Please also see the NREL brochure, BR6A20-60575, entitled Flexible Coal: Evolution from Baseload to Peaking Plant). 

First, the performance and cost information for a few operational flexibility retrofits 

available in the market for gas turbine (GT) and coal-fueled power plants was gathered. 

Next, a cost-benefit analysis was performed by running Plexos simulations with and 

without the retrofits on the selected power plants using the cost information. For this 

study, the units in the Rocky Mountain Power Pool (RMPP) system were studied for the 

year 2020. Since this system is predominantly coal-based, a “Modified-RMPP system” in 

which some of the coal units were converted to GTs was also studied. This study was 

performed under four different renewable penetration assumptions for the RMPP and 

Modified-RMPP systems. 

Figure 1. Capacity mix for the study systems

From the two simulations, the changes in system-level production costs, as well as 

changes to the retrofitted power plants revenues (i.e., the benefits) were determined. The 

results were then analyzed under two scenarios. Under the first scenario, the study system 

was assumed to be a vertically integrated utility under a cost-based return structure. In 
viii

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this case, the system-level production cost savings due to the retrofits were compared 

against the additional revenue required to cover the capital (and operating costs, if any) 

associated with the retrofits. A fixed charge rate (FCR) of 16% was used to convert the 

capital cost into annual revenue requirement. If the production cost savings were higher 

than the annual revenue requirements, the investment was deemed to have an economic 

merit.

Under the second scenario, the study system was treated as a deregulated market with 

merchant generation. The same results as those used in the regulated scenario were used 

in this analysis. However, in this case, for each generator that was retrofitted, the change 

in net revenue was compared against the cost of capital associated with the retrofit. The 

Net Present Value (NPV) associated with the cash outflow (capital investment) and cash 

inflows (annual net revenue increase/decrease) for a 20-year period were calculated. If 

the NPV was positive, the investment was deemed to have an economic merit.

The cost-benefit of the coal-fueled and gas-fueled power plants were studied separately in 

order to isolate their impacts. Although retrofits are available to improve turndown, ramp 

rate, and start up performance, the retrofits that primarily targeted turndown performance 

were studied since lower turndown capability was found to have the most beneficial 

impact on a system. Additionally, since it’s neither feasible nor economical to retrofit all 

units for lower turndown, only a subset of units (around 25% of the capacity) was 

targeted for the retrofitting. 

Study results show that at a system level, retrofits that improve the turndown levels of 

gas-fueled and coal-fueled power plants have a net-benefit to the system. The systemlevel net-benefit was determined by comparing the system production cost savings with 

the capital and operating costs associated with the retrofits. It should be noted that 

operational flexibility retrofits may not always result in a net-benefit to the system. For 

example, operational flexibility retrofits for gas-fueled, gas turbine power plants may not 

be of much value in a coal-dominated system with low coal prices. Conversely, coal plant 

retrofits may not be of much value in a gas-dominated system with low gas prices.

The results also showed that while there might be system-level net-benefits, there may or 

may not be a benefit at plant level. The plant-level benefit for a power plant that is 

retrofitted is its potential increase in net revenues. The study results showed that in a gasdominated system, only one out of the three retrofitted power plants showed an increase 

in net revenue after the installation of the retrofit. Plant-level revenue impacts are 

particularly important in deregulated regions where generators make their retrofit 

decisions based on the potential for increased profits. Policymakers need to explore 

mechanisms to incentivize installation of retrofits so that they benefit systems and plant 

owners.
1

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Introduction

High penetrations of wind and solar power plants can induce on/off cycling and ramping 

of fossil-fueled generators. This can lead to wear-and-tear costs and changes in emissions

for fossil-fueled generators. Phase 2 of the Western Wind and Solar Integration Study 

(WWSIS-2) determined these costs and emissions and simulated grid operations to 

investigate the full impact of wind and solar on the fossil-fueled fleet. The study 

examined three high penetration scenarios with a nominal 33% wind/solar penetration 

across the U.S. portion of the Western Interconnection, resulting in 26% nominal 

penetration across the entire Western Interconnection.

The results of the study showed that wind and solar generation induces additional annual 

cycling costs of $35-157M. This same generation also displaces about $7B annually in 

fuel costs. The increase in cycling cost for the average fossil-fueled plant is $0.47 to 

$1.28 per MWh of fossil-fueled generation.

1 The cycling costs, although small from a 

system perspective when compared to the savings in fuel cost, are still substantial costs

for the owners of these power plants. There is tremendous interest in the Electric Power 

industry—not only to quantify the cycling costs—but also to improve the cycling 

performance of the thermal units and to investigate mitigation measures for lowering

cycling-related costs.

One approach for potentially mitigating the cycling costs is through operational 

strategies. This could involve the curtailment of renewable generation under certain 

conditions, shutting down expensive cycling units during certain periods, and/or alternate 

unit commitment strategies that take the cycling-related costs and emissions into account.

An alternative approach is to retrofit existing units for improved operational flexibility

(i.e., capability to turndown lower, start and stop faster, and ramp faster between load setpoints). While having this capability on some units might result in them cycling more, it 

will reduce the cycling on the remaining units in the system, achieving system-level 

savings. Having this operational flexibility will also help reduce the curtailment of wind 

and solar generation at higher renewable penetration levels.

Some of the retrofit strategies that were explored in this study are as given below.

i. Retrofitting a certain percentage of the cycling fleet (primarily gas-fired combined 

and simple cycle power plants) with flex technology that allows:

a. Lower turndown capability while adhering to emission limits

b. Faster ramping capability

c. Faster startup capability.

 

1 “Western Wind and Solar Integration Study.” (2013). National Renewable Energy Laboratory (NREL). 

Accessed September 2013: http://www.nrel.gov/electricity/transmission/western_wind.html. 
2

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ii. Retrofitting a certain percentage of the base load (primarily, coal and oil-fired 

steam turbine generators) fleet to improve flexibility such as:

a. Combustion system optimization

b. Coal mill design changes.

This report presents the findings of a study that examined the costs, as well as the

benefits, of the retrofit strategies discussed above. The report is organized as follows:

Section 2 of the report discusses what operational flexibility means. Sections 3 and 4 

discuss the flexibility retrofits available for gas-fueled simple/combined cycle gas turbine

(GT) power plants and coal-fueled steam turbine power plants, respectively. Section 5 

presents the details of the study system that was used in this study and Section 6 

discusses the results of the cost-benefit analysis. Finally, Section 7 presents the 

conclusions of the study.

Operational Flexibility of Thermal Power Plants

As the penetration of renewable generation increases, the remaining generation fleet 

needs to be more flexible. The thermal power plants in the system will be required to 

start and stop more often, ramp faster between load set-points, and turndown to their 

minimum generation (frequently known as mingen or turndown) levels more frequently

and stay at those levels longer. A prior NREL Study2 showed that the number of cold 

starts for thermal units increase by nearly 40% in a year, going from a system with no 

renewable generation to one with a 30% penetration of renewable generation for the 

WestConnect system. In the same study, the number of turndowns or large ramps for 

thermal power plants (i.e., the number of times a unit ramps by more than 10% from one 

hour to the next) more than double for the same case. It was also observed from this study 

that the thermal power plants spent significantly more time at their mingen or turndown 

levels.

The operating profile of a coal or gas steam and GT power plant under a high renewable 

penetration scenario is shown in Figure 2. In this figure, the generation from the power 

plant is shown on the y-axis.

 

2 Jordan, G.; Venkataraman, S. (2012). Analysis of Cycling Costs in Western Wind and Solar Integration 

Study. Golden, CO: National Renewable Energy Laboratory. NREL/SR-5500-54864.
3

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Figure 2. Contemporary power plant mission profile: spending less time at full load

There are several retrofit solutions available from Original Equipment Manufacturers 

(OEMs) which can make the thermal power plants more flexible over their entire mission 

profiles. In particular, the flexibility attributes that are of primary importance for 

integrating renewable generation are as follows:

• Lower turndown capability

• Faster ramping capability

• Faster and less expensive starts.

The next sections discuss the flexibility retrofits for gas-fueled simple and combined

cycle GT power plants and coal-fueled steam turbine power plants.

Flexibility Retrofits for Gas-Fueled Simple and Combined Cycle 

GT Power Plants

This section discusses the operational flexibility retrofits that are available for GT power 

plants. Before discussing the retrofit solutions that are available to improve the 

operational flexibility of GTs, the reasons why these operational boundaries exist are also 

presented. Section 3.1 discusses the operational boundaries of a GT and the possible 

approaches to extend the boundaries. Section 3.2 discusses some of the specific 

operational flexibility retrofits solutions that are available from OEMs.

Operating Boundaries of a GT

The operating range of a GT is restricted by equipment design limitations coupled with 

externally imposed operating requirements.

Equipment design limitations are primarily driven by parts-life durability. More 

specifically, GTs are expected to be able to deliver a specified number of operating hours 

and startups between overhauls while maintaining a stated level of performance. The 

operating range of a GT may be increased further; however, there are mechanisms (such 

as peak firing) which have a tendency to increase variable operation and maintenance 

(O&M) costs. In some cases, these mechanisms may have an adverse impact on other 

parameters, such as heat rate. As a result, it is important to balance the benefits 

associated with increased operating boundary against the potential detriments to other 

characteristics for a given mechanism.
4

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The externally imposed operating requirements may be imposed from either a regulatory 

perspective (i.e., emissions thresholds) or by the ambient conditions (i.e., outside 

temperature/humidity/pressure, fuel temperature/pressure/quality, grid frequency 

variability, etc.). In most cases, the operating range of a GT can be extended if the 

controllable subset of externally imposed requirements can be relaxed. For example, if 

the required emissions rate at lower loads is relaxed, the mingen (i.e., turndown) load 

level could likely be improved. Correspondingly, improvements in fuel quality can drive 

more aggressive emissions and performance guarantees. However, this may not be a 

parameter that is adjustable at a given location.

The following sections will further discuss key operating boundaries that are of 

contemporary interest and the driving force behind most operability enhancements: 

startup (time, emissions, and variation), ramp rate, and turndown. While the 

enhancements discussed in the following sections represent those adopted by GE, other 

OEMs may have similar approaches.

Startup

Flexibility-related improvements to GT/CC plant startup typically center on reductions in 

four key areas: (1) startup time, (2) startup emissions, (3) startup fuel cost, and (4) startup 

variation.

Startup Time 

The typical GT startup involves several phases, such as a ramp-up from turning gear to 

purge speed, a purge cycle at 20-30% speed to expel volatile gases from the exhaust and 

Heat Recovery Steam Generator (HRSG), a light-off process, ramp-up to synchronization 

(i.e., breaker-closure) at full-speed / no-load (FSNL), and finally, a load ramp to the 

desired operating condition. For this discussion, the emphasis will be placed on heavyduty GTs, which historically have yielded startup times of about 20-30 minutes for 

simple cycle operation. In combined cycle, the startup times vary as function of the 

thermal state of the bottoming cycle equipment (i.e., hot, warm, cold) at the desired time 

startup is to be initiated. The range for a typical combined cycle is on the order of 60-75 

minutes (hot) – 4 hours for a cold-start. In the absence of a well-integrated digital control 

system (DCS) to assist with the startup, and more manual intervention, combined cycle 

power plant startup times can be significantly longer.

For simple-cycle applications, the solutions have focused on items such as:

• Purge credit: reducing or eliminating the purge3 cycle, which saves time by 

allowing the purge to be completed at a more conducive time independent of the 

startup cycle. Specifically, this technique employs a purge on shut-down (or 

dedicated offline purge cycle) followed by the placement of an inert gas between 

the fuel supply and combustion system which prevents downstream leakage of 

fuel gases into the exhaust.

 

3 Purge is the procedure of motoring the GT prior to introducing fuel so that all combustibles are removed 

from exhaust and HRSG.
5

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• Ignition “light-off” procedures: these procedures4 have been optimized to allow 

higher-speed ignition which save time by avoiding a coast-down from purge 

speed to light-off speed. In some cases, “fire-on-the-fly” technologies have 

allowed the hold-time prior to light-off to be eliminated completely. 

• Clearance management: the limiting aspect for the acceleration and load-ramps 

is typically driven by compressor and turbine tip clearances, respectively. Due to 

the significant difference in the thermal-mass between the rotor and casing 

materials, the growth rate between the rotor and case pose the challenge of 

preventing “tip rubs” during transient events such as startup. To maintain high 

steady-state operating efficiencies, it’s important for a GT to have tight tip 

clearances; however, this typically requires a more delicate and repeatable startup 

procedure. Improved analytical techniques, advanced controls systems, case and 

rotor temperature management methods, and enhanced materials technologies are 

all key aspects that have been deployed to aid in tip clearance management and 

allow more rapid startups.

On the whole, these types of approaches have allowed simple GT start times to be 

reduced by more than 50-60%.

For combined cycle applications, startup times are typically not limited by the GT. 

Instead, combined cycle startup times are limited by a series of hold-points that range 

from Steam Turbine (ST) temperature matching requirements to manage stresses and rubrisk, Heat Recovery Steam Generator (HRSG) stresses, and the risk of producing more 

steam via GT exhaust flow than the system is able to handle. Different techniques have 

been deployed by the various OEMs to improve the ability to start combined cycles more 

rapidly. These approaches have typically focused on either modification to the HRSG or 

steam bypasses around the HRSG to the condenser well, which help to de-couple the GT 

from the bottoming cycle during startup. Due to durability issues, OEMs do not typically 

pursue the use of bypass dampers for heavy-duty combined cycle applications. Further, 

most of the bottoming cycle advancements in startup time are available in new unit 

applications only due to the significant design integration that is required to achieve such 

benefits. Nonetheless, these procedures and equipment packages can allow combined

cycle plants to reduce startup times by as much as 50%.

Startup emissions: in general, the faster startups will be accompanied by reduced 

emissions. This is driven by the fact that the combustion system is optimized for 

operation over the range from 50-100% load (for most heavy-duty products). As a result 

of the faster startup, the GT is more rapidly brought into an operating range that is 

coincident with its low-emissions design point. Reductions in NOx and CO on the order 

of 50% or more can be achieved with startup improvements. 

 

4 Before the GT engine can run on its own power, the engine must be accelerated by an external source, 

such as a battery, to provide sufficient airflow to the combustor for ignition, typically referred to in the 

industry as light-off.
6

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Startup fuel cost: for simple cycle units, the startup fuel cost can be reduced by 50-60% 

as a result of the faster startups. However, for combined cycle units, the potential for an 

increase in the fuel cost during startup is possible. This is driven by the fact that the GT 

is decoupled from the load ramp of the ST (still limited by stress and rub risks). So, 

while the GT is able to achieve full load more rapidly and operate very efficiently at low 

emissions, the overall plant will be operating at simple-cycle-like efficiencies until the ST 

output is available to dilute the variable cost. There are retrofits available which improve 

the ST responsiveness during startup. For the combined- cycle, there is certainly an 

upside in the rapid startup because the GT is able to produce substantially higher energy 

early on in the startup, allowing increased revenue capture.

Startup variation: additional emphasis has been placed on improving the repeatability of 

startup times of GT and CC power plants through the automation of startup procedures, 

equipment permissives, sensor technologies, etc. Such actions allow operators to initiate 

their startups at the most optimal time, reducing risk of having to buy replacement power 

in the energy spot market if they are late to meet dispatch commitment. 

Often related, the cost of cycling a unit is the minimum uptime and downtime constraints

of the asset. Typically, the minimum uptimes and downtimes are driven by a desire to 

minimize thermal stresses in the casing and rotor components coupled. Further, the risk 

of a clearance rub-out due to the mismatch in the growth characteristics of the rotor and 

casing components is often the limiting factor in establishing a minimum down-time setpoint.

Ramp Rate

There are several different aspects which are important to consider when characterizing 

ramp rates. For example, most GTs have both a nominal and emergency load rate. For 

primary (governor) response, the GT load rate can be substantially higher. However, in 

the context of flexibility, the discussion is usually focused on the nominal load rate 

because it will be leveraged on a more continual basis. Ramping may be required to 

support the regulation ancillary service which sends “output raise/lower” pulses every 3-6 

seconds and has the potential change sign (+/-) between pulses. Use of the emergency 

load rates for regulation service would add stress to the hardware and substantially 

elevate maintenance costs. 

To support ramping, the control system and hardware need to be nimble. Two of the key 

items which limit the ramping capability are: (1) combustion processes and (2) hardware 

stress. When ramping, the combustion system is operating in a transient condition. For 

gaseous (compressible) fuel units, (particularly Dry Low NOx (DLN) pre-mixed 

combustion systems), the fuel-air ratios during steady-state operation are incredibly low. 

Sustaining the combustion process during transients is even a greater challenge. During 

ramping events, both fuel flow and airflow are fluctuating simultaneously. Maintaining 

adequate blow-out margin (i.e., avoiding a fuel-air ratio that is too low) and remaining in 

emissions compliance (i.e., proper fuel-air mixing and not running too rich of a fuel-air 

ratio) is an even greater challenge. Further, the ramping can elevate combustion 

dynamics, which induces hardware-damaging pressure pulses. All of this needs to be 
7

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managed properly to ensure reliable operation. The rapid increase/decrease of firing 

temperature can further add stress to the hardware during ramping events.

Turndown

Many combined cycle plants built in the U.S. during the early 2000s were built for 

baseload operation and not continuous operation at part load. The smaller HRSG 

attemperation systems in the plants cannot supply enough spray to handle the high 

exhaust temperature and low flow conditions during part load. This plant limitation can 

greatly impact overall GT turndown, allowing only a turndown of 70% versus 40-50% 

normally. With the desire to increase the penetration of renewables, more emphasis has 

been placed on the ability for GTs to operate at lower load levels while maintaining 

emissions compliance (i.e., turndown capability). The deeper turndown capability allows 

GTs to avoid cycling costs (both startup fuel and variable O&M penalties), but also 

provides opportunities for the GTs to garner ancillary payments for services such as 

regulation and spinning reserve.

There are several challenges with improving the ability for a GT to reduce its emissionscompliant minimum load set-point. Typically, the threshold is driven by CO compliance. 

As load is reduced, the combustion temperatures come down as well. While NOx is 

typically reduced at low loads, the reduced temperatures are unfavorable for CO and the 

emissions increase. There are several techniques that can be used to improve the 

turndown capability; most of these seek to increase combustion temperatures which 

reducing combustor airflow. Options such as a bypassing compressor discharge air 

around the combustion system; these have been investigated by multiple OEMs and are 

increasingly available on new units and retrofit offerings. Unfortunately, most of these 

techniques detract from the efficiency at the minimum load level as they introduce a 

thermodynamic penalty to compensate for an emissions constraint.

The next section discusses specific operational flexibility retrofits that are available from 

OEMs such as GE for improving turndown, startup, and ramp rate performance.

Operational Flexibility Retrofits for GTs

This section discusses the advanced controls solutions offered by one OEM for 

improving the operational flexibility of GT units. Additional details regarding these

solutions are included in Appendix A. While the retrofits discussed are based on one 

OEM’s offerings, similar retrofit solutions may be available from other vendors.

The operational flexibility solutions from this OEM are categorized by the segment of the 

mission profile that they impact. For example, the solutions distinctly target four 

segments: (1) startup, (2) full load, (3) part-load, and (4) minimum load. The respective 

solutions and corresponding applications for three relevant segments are described in 

more detail below.

The startup-related operational flexibility solution targets a fast, reliable, and repeatable 

startup profile. Specifically, the underlying applications target reductions in startup time 

and corresponding startup variability along with reduced emissions. For example, the 

OEM’s solution employs a “purge credit” system which moves the startup purge to the 
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prior shutdown, plus faster acceleration and loading rates to achieve near baseload output 

in 10 minutes in simple cycle applications. The startup operational flexibility solution 

results in a 50%-60% reduction in startup time combined with 50%-70% reduction in 

startup emissions.

The part-load-related operational flexibility solution focuses on the part-load regime 

offering improved responsiveness, efficiency, customer-controlled emissions, elevated 

loading rates, and automated combustion system tuning. For example, the OEM’s

solution enables load ramping up to twice the normal rate, such that the full mingen to 

baseload range can be covered in less than five minutes. As much as a 50% improvement 

on a MW/min basis can be obtained through this solution.

The minimum load-related operational flexibility solution enables lower emissions and 

promotes minimum fuel use at the turndown condition. The associated applications 

reduce overall fuel consumption, yield reduced emissions, and facilitate improved 

dispatch priority. For example, the OEM’s solution extends low emissions operation to 

lower load levels, improving the economics to remain online overnight and avoid 

shutdown and startup costs. It also extends the available load range for operation, 

improving dispatch flexibility and enabling greater participation in regulating reserve 

markets.

The operational flexibility solutions discussed above are primarily focused on improving 

the performance of GTs through changes in the control software. For a combined cycle 

power plant, there may be other avenues for improvement in operational flexibility. 

These include changes in the design of the Heat Recovery Steam Generator (HRSG), 

steam turbine, and the Balance of Plant (BOP). Some of the newer combined cycle 

plants achieve flexibility by breaking the link between the GT and steam turbine cycle.

However, the focus of this study was on the GT retrofit solutions.

Table 1 through Table 3 show the typical performance improvements that can be 

expected, as well as the associated costs from retrofit solutions that target turndown, 

startup, and ramp rate performance, respectively. These tables show the performance 

improvements for both simple and combined cycle power plants by GT frame type (B, E,

and F). A discussion of the different GT frames is included in Section 5.2. This data was 

obtained by analyzing the retrofit solutions that addressed 18 operational characteristics 

such as startup time, emission reduction, and turndown improvements for nearly two 

dozen GT models. Since a retrofit solution may have multiple benefits (for example, a 

retrofit solution that improves the startup time may also have a secondary benefit of 

improving the ramp rate performance), the cost associated with a specific benefit (startup 

time, in this case) was obtained by apportioning the total cost of the solution to all the 

benefits from that solution (i.e., startup time, ramp rate). This was done so that the costbenefit of improving turndown, ramp rate, and startup performance can be studied 

separately. Therefore, the cost ranges shown in the tables are not commercial prices, but 

representative prices to be used in the study. It should be noted that in reality there may 

not be a retrofit solution (and corresponding price) that targets a single operational 

characteristic such as turndown.
9 

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In the tables below, the expected performance improvement is shown as a percentage of 

existing default performance values. For example, with the ramp rate retrofit, a 100% 

increase (doubling) of ramp rate over existing ramp rate can be expected as shown in 

Tables 1through 3.5

Table 1. Performance and Cost Information for Turndown Improvements 

Table 2. Performance and Cost Information for Startup Improvements 

Table 3. Performance and Cost Information for Ramp Rate Improvements 

 

5

 Note: The cost ranges shown in the tables are not commercial prices, but representative prices to be used 

in the study. 

B Frame E Frame F Frame B Frame E Frame F Frame

Targeted Turndown Improvement 5% 5% 10% 5% 5% 10%

Price Range 120K-180K 160K-240K 400K-600K 120K-180K 160K-240K 400K-600K

Simple Cycle Combined Cycle

Combined Cycle

B Frame E Frame F Frame F Frame

NOX Emission Improvement 54% 54%

CO Emission Improvement 50% 50%

CO2 Emission Improvement 66% 66% 69%

Startup Fuel Improvement 66% 66% 31%

Startup Time Improvement 50% 50% 59% 59%

Price Range 240K-360K 240K-360K 800K-1200K 1,320K-1,980K

Simple Cycle

Simple Cycle Combined Cycle

F Frame F Frame

Ramp Rate Improvement 100% 100%

Price Range 400K-600K 400K-600K
10

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Flexibility Retrofits for Coal-Fueled Steam Turbine

Power Plants

This section discusses the operational flexibility retrofits that are available for coal-fueled

power plants. Similar to the GT units, there are operational boundaries on the coal-fired 

generation. Moreover, most of the coal generation was built for baseload operation and 

few units have been designed for cycling. Section 4.1 discusses the operational 

boundaries of a coal-fired unit and the possible approaches to extend the boundaries. 

Section 4.2 discusses some of the specific operational flexibility retrofit solutions that are 

available to asset owners.

Operating Boundaries of Coal Units

Providing generation flexibility requires lower and efficient part loads, faster ramping 

times, and short startup times. Most existing coal steam plants were not designed 

specifically for this flexible operation, but were instead intended to provide steady

baseload generation. The operating range of a coal unit is restricted by equipment design 

limitations coupled with externally imposed operating requirements. 

Coal (as well as gas) steam unit operating boundaries are typically related to fuel 

handling, the boiler, the steam turbine, the controls, and the BOP equipment design 

limitations. While, operational as well as design modifications can improve the 

operational flexibility of the units, a number of factors can in fact have an adverse effect 

on plant economics. For example, at lower load operation, the unit heat rate efficiency is 

lost. Heat rate deterioration can be caused due to any number of factors, including steam 

flow, efficiency of the turbine steam path, steam conditions, and the performance and 

operation of the BOP components. However, running the coal units at lower loads for 

extended hours can in fact lower component damage (creep) due to lower temperature. 

As a result, it is important to balance the benefits associated with increased operating 

boundary against the potential detriments to other characteristics for a given mechanism.

On the other hand, externally imposed operating requirements may be imposed from 

either a regulatory perspective (i.e., emissions thresholds) or by the ambient conditions 

(i.e., outside temperature/humidity/pressure, fuel temperature/ pressure/quality, grid 

frequency variability, etc.). Similar to GTs, the operating range of a coal plant can be 

extended if the controllable subset of externally imposed requirements can be relaxed. 

For example, if the required emissions rate at lower loads is relaxed, the mingen (i.e.,

turndown) load level could likely be improved. Several coal-fired units have gas igniters. 

At startups as well as low loads, these units can switch to gas for MW generation, thus 

reducing emissions. Correspondingly, improvements in fuel quality can drive more 

aggressive emissions and performance guarantees; however, as in the case of GTs, this 

may not be a parameter that is adjustable at a given location.

The following sections will provide further discussion on key operating boundaries that 

are of contemporary interest, and correspondingly, the driving force behind most 

operability enhancements: startup (time, emissions, and variation), ramp rate, and 

turndown. 
11

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Startup

Flexibility-related improvements to coal plant startup typically occur through reductions 

in four key areas: (1) startup time, (2) startup fuel cost, (3) startup emissions, and (4) 

startup variation.

Startup Time 

A typical small or large subcritical coal unit can take several hours for a startup. Coal unit 

startup has additional complexity than, for example, simple cycle combustion turbines. 

With a time-based classification, the shutdown/ startup event is classified by the number 

of hours the unit has been off-load prior to the startup. For medium sized units a 

commonly used time classification is:

< 8 hours off-load = hot start

8 to 48 hours off-load = warm start

> 48 hours off load = cold start.

In most cases, for large units these time values will be higher. Traditionally, the turbine 

casing temperature decay from a hot to ambient condition is used to determine the start 

types. Unfortunately, these time-based and high pressure steam turbine casing metal 

temperature-based startup classifications do not adequately classify the startup type with 

respect to the boiler’s thick-section pressure parts or other boiler components, which 

generally cool down much faster than the high pressure steam turbine casing. A hot or 

warm start on the turbine might be a cold start on the boiler. In general, a cold start is 

more damaging than a warm or hot start due to the excessive temperature differentials on 

the components.

It should also be noted that the cooling behavior of the boiler and drum can vary widely, 

depending on the shutdown procedure and how the procedure affects the preservation of 

drum pressure and furnace gas temperature, and the circulation of fluid into the drum 

either from the feedwater/economizer or throughout the waterwall circuits. In general, 

maintenance shutdowns tend to be more rapid than dispatch-related shutdowns—unless 

the operators have grown accustomed to only using a single shutdown procedure.

Purge time, fuel/air burner sequence, timing, flow, drain sequence, valve opening 

sequence, and a bypass system as well as attemperation can help to reduce startup times. 

Additionally, design retrofits such as auxiliary boilers can help keep components hot, and 

therefore reduce startup times. However, there are both positive factors which can reduce 

the damage to boiler components and potential pitfalls with each of the operational or 

design modifications to reduce start times.

Startup Fuel Cost: Unlike simple cycle gas units, coal steam units have slightly lower 

flexibility in reducing startup fuel costs. However, certain procedures such as coal drying 

can reduce pulverizer startup costs. For example, Ontario Power Generation was 

designed for low-sulfur bituminous coal, but switched to Powder River Basin (PRB) coal, 
12

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resulting in reduced heat available to the pulverizers.

6 Plant management decided to 

install natural gas-fired duct heaters to raise the Primary Air. While using gas on startups 

can become expensive depending on prevailing natural gas price, in today’s scenario 

there may be marginal benefits to startup fuel costs.

Startup Emissions

Using gas igniters on startup can help units reduce emissions on startups. In general, the 

faster startups will be accompanied by reduced emissions. However, fuel quality on coal 

units particularly impacts startup emissions. Due to the inherent moisture in subbituminous and lignite coals, all else being equal to a bituminous coal-fired boiler is more 

efficient than a corresponding boiler burning sub-bituminous or lignite coal. 

Ramp Rate

There are several different aspects which are important to consider when characterizing 

ramp rates. Coal units can have ramp rates of 1MW/min to 10 or 12 MW/min depending 

on size and control technology at the operator’s disposal. 

Several factors impact ramp rates on steam units, including the fuel quality variation,

which can have a significant impact on ramping capability. Fuel quality variation directly 

corresponds to temperature variations, thus making MW change more difficult. The 

control of boiler parameters is more challenging with ramping, often resulting in 

increasing variability of waterwall outlet temperatures. While control systems allow 

operators to vary fuel to air ratio and control rate of change of energy, hardware such as 

the pulverizers play an important role in improving ramp rates. 

Further, the ramping can elevate combustion dynamics which induces hardware 

damaging pressure pulses. All of this needs to be managed properly to ensure reliable 

operation. The rapid increase/decrease of firing temperature can further add stress to the 

hardware during ramping events.

Turndown

With the desire to increase the penetration of renewables, more emphasis has been placed 

on coal units’ ability to operate at lower load levels. With market deregulation, the 

traditional dispatch philosophy was focused on short term benefits of reduced fuel costs. 

However, several utilities and asset owners are now considering the impact of cycling on 

maintenance and life of the plant. Low load operation affects the boiler combustion 

process. The primary constraint on coal units, especially pulverized coal units, is the 

turndown of pulverizers and risk of a unit trip with minimum mills in service without 

support fire--from gas or other sources. Without gas support, most units are limited to a 

low load of no less than about 40-50% of the rated capacity of the unit. These limits vary 

from site to site, and are constrained by design of the asset and by external factors such as 

emissions regulations.

 

6 “Coal Plant O&M: Coal Drying Reduces Pulverizer Start-up Costs." (2007). Electric Power, 2007. 

Douglas J. Smith, IEng, Contributing Editor. Accessed 2012: http://www.powermag.com/coal-plant-omcoal-drying-reduces-pulverizer-start-up-costs/. 
13

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Modern control systems, along with sliding pressure procedures or variable pressure 

operation, can assist asset owners with turndowns without significant efficiency or wear 

and tear cost penalties. Sliding pressure offers advantages over throttle control during 

startup as well as part load operation. Units where the control system is limited to fixedpressure operation should consider retrofitting the plant to sliding-pressure operation.

While there may be efficiency losses with low load operation on the boiler, a recent case 

study showed that the turbine can sometimes be a limiting factor. At low loads, turbine 

blade fluttering is possible, which can again limit the turndown on the unit.

The next section discusses specific operational flexibility retrofits for improving

turndown, startup, and ramp rate performance.

Operational Flexibility Retrofits for Coal Plants

As in the case of GTs, the operational flexibility solutions can be categorized by the 

segment of the mission profile that they impact--(1) startup/shutdown, (2) ramp rates, and

(3) part-load and (4) minimum load. The respective solutions and corresponding 

applications are described in more detail below.

The startup operational flexibility retrofits target reductions in startup time and 

corresponding startup variability, along with reduced emissions. However, startup 

retrofits have a larger impact on startup times, with a smaller impact on emissions. Some 

of the retrofits considered in this study can positively impact startup times by 50%. In 

most cases these retrofits also benefit shutdown on units. Turbine, BOP retrofits, and 

improved controls provide the most benefit and can sometimes improve startup/shutdown 

times by 100%. In terms of the emissions equipment, heated precipitators can improve 

both part load operations and startups.

The part-load operational flexibility focuses on the part-load regime, offering improved 

responsiveness in terms of ramp rates, efficiency, and elevated loading rates. The 

minimum load operational flexibility solution enables lower emissions and promotes 

minimum fuel use at the turndown condition. In fact, allowing coal-fired units to avoid 

increased on/off starts by lowering their minimum loads will reduce overall wear and tear 

costs and damage. Larger units have typically limited their low load operation to 40% of 

their capacity. Most of the constraints are boiler and mill stability-related. Low load gas 

igniters alone can provide a significant impact to low load operation and reduce 

minimum loads to 25% of maximum rating or lower. 

Tables 4 – 9 show the performance and cost information for retrofitting different systems 

in the coal steam units for improved turndown, startup, and ramp rate performance. 

Additional information about these retrofits can be found in Appendix B, as well as the 

separately published brochure entitled Flexible Coal: Evolution from Baseload to 

Peaking Plant (NREL/BR-6A20-60575). The estimates for expected benefit from various 

flexible operation characteristics provided in the tables are generic. Almost always, these 

costs will vary based on plant design and location as well as financial and contractual 

agreements. In these tables, the cost estimates are shown for small subcritical, large 
14

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subcritical, and supercritical coal-fueled power plants. The expected benefits are the end 

states that can be achieved with the retrofit.

7

The tables provide cost estimates for several hardware retrofits that may provide benefits 

to more than one aspect of flexible operation. In most situations, keeping components hot 

can significantly reduce the thermal fatigue damage associated with cyclic operation. 

Occasionally, a single retrofit option may only impact one flexible operation mode. For 

example, turbine electric heating blankets provide a significant improvement to start/stop 

cycling, but may not benefit other flexible operating modes measurably.

 

7 Please note that the impact of the retrofits solutions for GTs is presented differently in Section 3.2. For 

GTs, the additional improvements (not the end state) that can be achieved by retrofit solutions are shown in 

Tables 3-1 to 3-3.
15

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Table 4. Performance and Cost Information for Boiler Retrofits

Table 5. Performance and Cost Information for Coal Mill Retrofits

Retrofit Options

Small Sub 

Critical Coal 

200MW

 Large 

Subcritical Coal 

500MW 

Supercritical 

Coal 750MW Ramp Rate Turndown 

Startup/ 

Shutdown

Improved and automated boiler drains $ 3.00 $ 5.00 $ 5.00 50% 50%

Steam flow redistribution and metallurgy improvements in in SH/RH $ 2.50 $ 5.00 $ 7.00 33% 33% 33%

Steam coil air heater to pre warm boiler and airheater $ 0.50 $ 1.00 $ 2.00 33% 33% 33%

Gas bypass to keep air heater warm $ 0.70 $ 1.50 $ 3.00 50% 50%

Improved APH basket life when cycling in or through the wet flue gas 

temperature region by installing traveling APH blowers to remove 

deposits prior to cycling down in load $ 0.75 $ 1.00 $ 1.00 50% 50%

Improved APH basket life with improved materials when cycling in or 

through the wet flue gas temperature region $ 1.20 $ 2.00 $ 2.00 50% 50%

Improved selected expansion joints. This is not a complete 

replacement of all expansion joints. $ 1.50 $ 2.00 $ 3.00 100%

Add steam cooled enclosure min flow protection for balanced flow 

with blow down or dump to LP turbine $ 0.30 $ 0.50 $ - 50% 50%

Cost to Install in Millions Expected Benefit:

Retrofit Options

Small Sub 

Critical Coal 

200MW

 Large 

Subcritical Coal 

500MW 

Supercritical 

Coal 750MW Ramp Rate Turndown 

Startup/ 

Shutdown

Improved flame proving equipment for burners $ 0.50 $ 1.00 $ 1.50 33% 33% 33%

Low load gas ignitors to allow min generation on gas fuel only $ 2.00 $ 3.00 $ 4.00 100%

Dual Fuel burners – use NG over coal ( Add NG to all burners). Gas 

supply is not included $ 10.00 $ 12.00 $ 16.00 33% 33% 33%

New feeders with gravimetric type feeder with improved weighing of 

coal feed to mill $ 3.60 $ 7.20 $ 10.00 33% 33% 33%

Automatic pressure control on roll and race to adjust the grinding 

pressure of the coal mill. $ 3.00 $ 5.00 $ 7.00 33% 33% 33%

Cost to Install in Millions Expected Benefit:
16

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Table 6. Performance and Cost Information for Emissions Control Retrofits

Table 7. Performance and Cost Information for BOP Retrofits

Retrofit Options

Small Sub 

Critical Coal 

200MW

 Large 

Subcritical Coal 

500MW 

Supercritical 

Coal 750MW Ramp Rate Turndown 

Startup/ 

Shutdown

Heated precipitator hoppers $ 0.50 $ 1.00 $ 1.80 50% 50%

New dry fly ash transport $ 2.00 $ 3.00 $ 4.00 50% 50%

Cost to Install in Millions Expected Benefit:

Retrofit Options

Small Sub 

Critical Coal 

200MW

 Large 

Subcritical Coal 

500MW 

Supercritical 

Coal 750MW Ramp Rate Turndown 

Startup/ 

Shutdown

Small package boiler, electric boiler with air modeling and permit $ 3.20 $ 4.00 $ 6.00 50% 50%

Motor driven boiler feed pump with startup to min load capability $ 4.00 $ 6.00 $ 8.00 50% 50%

Improved vibration sensing and monitoring of all rotating equipment $ 1.50 $ 2.50 $ 3.50 33% 33% 33%

Speed Controlled motors on ID and FD fans using variable frequency 

drives $ 5.00 $ 7.50 $ 7.50 33% 33% 33%

Transformer monitoring gas analysis and temperature and emergency 

oil removal if arching occurs $ 0.57 $ 1.50 $ 2.25 33% 33% 33%

Cost to Install in Millions Expected Benefit:
17

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Table 8. Performance and Cost Information for Turbine Retrofits

Table 9. Performance and Cost Information for Chemistry-related Improvements

Retrofit Options

Small Sub 

Critical Coal 

200MW

 Large 

Subcritical Coal 

500MW 

Supercritical 

Coal 750MW Ramp Rate Turndown 

Startup/ 

Shutdown

Turbine electric heating blankets $ 1.00 $ 2.00 $ 2.50 100%

Convert 3 rpm to 40 rpm turning gear motor to prevent blade 

attachment wear and generator loose parts $ 0.75 $ 1.50 $ 2.00 100%

New valve operators Full arc emission/sliding pressure $ 1.00 $ 1.80 $ 4.00 33% 33% 33%

Turbine drains to the condenser hot well including main steam drains 

and before seat warmup drains should be routed to the hotwell with a 

sparger discharge below the normal water level. $ 0.25 $ 0.50 $ 0.75 100%

Turbine water Induction prevention Upgrades including MOVs on 

extractions and inspection of heater drains and extraction piping and 

bellows in condenser $ 0.50 $ 0.75 $ 1.00 33% 33% 33%

Condenser tube shielding $ 0.50 $ 0.75 $ 1.00 50% 50%

Cost to Install in Millions Expected Benefit:

Retrofit Options

Small Sub 

Critical Coal 

200MW

 Large 

Subcritical Coal 

500MW 

Supercritical 

Coal 750MW Ramp Rate Turndown 

Startup/ 

Shutdown

Nitrogen Blanketing of condensate storage tank, Boiler, turbine $ 1.00 $ 2.00 $ 3.00 100%

Condensate polishing system for rapid water chemistry cleanup when 

cycling $ 1.25 $ 2.00 $ 3.00 50% 50%

Larger condensate storage tanks $ 1.50 $ 3.00 $ 4.00 100%

Add steam cooled enclosure min flow protection for balanced flow 

with blow down or dump to LP turbine $ 0.30 $ 0.50 $ - 50% 50%

Cost to Install in Millions Expected Benefit:
18

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Flexibility Improvements from Operating Procedures

Finally, it must be noted that significant improvement to cycling operation can be 

achieved through operational changes alone. Units with increased cycling operation 

should typically increase the frequency of scheduled inspections or replacements for the 

components susceptible to damage. While the increased frequency of inspections can add 

costs to operate, the benefit of increased reliability will help asset owners mitigate the 

risk associated with cycling units. Some other examples of operating procedure changes 

include:

• Pre-warming/DA system, pegging steam on the Low Pressure (LP) heater or the 

top LP heater using boiler feed pump recirculation with a temperature control 

system. These can be low cost improvements for mingen and startup benefits.

• Boiler optimization using revised procedures for cycling the plant, which can 

have major improvements to ramp rates and mingen operation as well as start/stop 

cycling.

• Enforcing operating guidelines for layup procedures. Depending on the plant 

offline period, the dry or wet layup procedures should be defined. For example,

for a quick shutdown (hot start, < 24 hours), the boiler, deaerator can have a wet 

layup with nitrogen and a vacuum on the turbine to prevent damage.

• Modifying and following cycle chemistry guideline limits during plant startup, 

shutdown, and layup. 

• Improving Selective Catalytic Reduction (SCR) inlet temperatures to mitigate low 

load operation issues at the SCR.

• Operator training to reduce and monitor damaging trends while operating in 

cycling mode.

A related case study on the capital and operational changes made to a coal power plant to 

enable it to go from a baseload unit to a super-peaking unit is included in the brochure 

entitled Flexible Coal: Evolution from Baseload to Peaking Plant (NREL/BR-6A20-

60575).
19 

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Study System for Retrofit Analysis 

The impact of the retrofits on the production cost of the system, as well as the revenues 

earned by units was studied using the Plexos simulation model. For this study, the units 

in the RMPP system were studied for the year 2020. Figure 3 shows the major thermal 

units in this system. Since this system is predominantly coal-based, a “Modified-RMPP 

system” in which some of the coal units were converted to GTs, was also studied. This 

section gives the major assumptions for the RMPP and Modified-RMPP systems. 

Figure 3. Thermal generators in the RMPP system 

Load Assumptions 

The peak load and energy totals for the RMPP region for the year 2020 are 13,661 MW 

and 80,000 GWh, respectively. 

Generator Assumptions 

The RMPP system consists of 10,383 MW of thermal power plants, 1,050 MW of hydro, 

and 560 MW of pumped storage power plants and nearly 168 MW of smaller power 
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plants. Out of the 10,383 MW of thermal power plants, more than half are coal-fueled 

steam power plants. The breakdown of coal, gas-fueled simple and combined cycle plants 

is given in Figure 4. This system, which has a high installed base of coal plants, may not 

be representative of other parts of the WECC system. Therefore, a Modified-RMPP 

system where nearly 2,200 MW of coal plants were converted to combined cycle power 

plants was developed for this study. The breakdown of coal, gas-fueled simple and 

combined cycle plants for the Modified-RMPP system is given in Figure 5.

Figure 4. Thermal generation mix in the RMPP case 

Figure 5. Thermal generation mix in the Modified-RMPP case

GT Technology

Since the available GT retrofits depend on the technology, a survey of the different GT 

technology (frame type) in WECC was conducted. GTs were classified as Aero, B, E, or 

F frame types, based on vintage and technology. Table 10 shows a few examples of GTs 

by make and model for each frame type.
21 

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Table 10. Frame Type, Make, and Model of GTs

In the RMPP system, there are approximately 4,000 MW of gas-fueled combined cycle 

plants, including 750 MW of generic expansion units added to maintain reserve margin. 

The mix of the various frame types for the RMPP system is shown in Figure 6. A large 

portion of the existing combined cycle capacity, including the new expansion units, is F 

frame. The coal units that were converted to combined cycle units in the ModifiedRMPP system were also assumed to be F frame. In general, there are plenty of combined 

cycle units in the RMPP and Modified-RMPP system that are capable of being retrofitted 

for operational flexibility.8

 

Figure 6. Combined cycle GT technology mix for the RMPP system

In the RMPP system, there are approximately 3,000 MW of gas-fueled simple cycle GT 

plants, including nearly 680 MW of generic expansion units added to maintain reserve 

margin. The mix of the various simple cycle GT frame types for the RMPP system is 

shown in Figure 7. A large portion of the existing simple cycle capacity, including the 

 

8

 In reality, there may be other factors that determine whether a combined cycle plant can be retrofitted for 

operational flexibility. These include considerations such as the age of the unit and controls, condition of 

the unit, impact on the BOP, etc. 

Frame Type Manufacturer Make and Model

Aero GE LMS100, GE LM6000, GE LM 5000, P&W FT4

B GE MS6001B

E GE MS7001E, Siemens V84.2, Westinghouse W251

F GE MS7001FA, Siemens V84.3, Westinghouse W501F

Other GE MS5001, Westinghouse W191
22

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new expansion units, is made up of aero derivative GTs. There is also a significant 

installed capacity of F frame simple cycle GTs, as shown in the figure.

Figure 7. Simple cycle GT technology mix for the RMPP system 

Steam Turbine Technology

Coal-fired generation technology can vary from one location to another. Boiler bottom 

and firing type / coal-fired power plants use one of five basic coal utilization processes:

• Stoker-fired 

• Pulverized coal (PC) 

• Cyclone-fired 

• Fluidized-bed combustion (FBC) 

• Coal gasification (IGCC).

To keep this analysis generic, we categorized the coal units based on size and steam 

pressure:

• Small Subcritical Coal (<300 MW)

• Large Subcritical Coal (>300 MW)

• Supercritical Coal.

Figure 8 shows the number of coal-fueled generation (some 39,000 MW) in the WECC 

systems. The mix of coal-fueled generation in the RMPP system is shown in Figure 9.
23 

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Figure 8. Coal-fired generation technology mix for the WECC system 

Figure 9. Coal-fired generation technology mix for the RMPP system

A comparison of both the RMPP and WECC systems in terms of percent capacity is 

shown in Figure 10. Evidently the systems are not very different in terms of MW capacity 

even though the number of units is different. 

Large 

Subcritical Coal 

(>300 MW), 76

Small 

Subcritical Coal 

(<300 MW), 55

Supercritical 

Coal, 3
24 

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Figure 10. Coal-fired generation technology mix for the RMPP system

Renewable Generation Assumptions 

This study was performed under four different renewable penetration assumptions for the 

RMPP and Modified-RMPP systems. The capacity and energy from wind and solar 

plants for each scenario is given in Table 11. The renewable energy penetrations in the 

four scenarios are 16%, 27%, 35% and 44%, respectively. 

Table 11. Renewable Generation Assumptions

Fuel Price Assumptions 

The fuel price assumptions are very important for this study since it determines which 

units (i.e., coal-fueled or gas-fueled) will be marginal, and which units will cycle. The 

annual average fuel price assumptions used in this study for the year 2020 are given 

below. There will be a slight difference in gas prices depending on the location of the 

power plant. 

Coal: 1.42 $/MMBTU 

Gas: 4.10 $/MMBTU 

Oil: 21.87 $/MMBTU. 

scenario Cap (MW) Energy(GWh) Cap (MW) Energy(GWh)

RE_101_Base 2,958 1,834 15,752 10,705 

RE_103_Base 5,074 3,168 26,019 18,097 

RE_105_Base 6,794 4,262 36,138 23,761 

RE_109_Base 9,062 5,261 41,434 29,892 

Solar Wind
25

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Cost-Benefit Analysis

This section discusses the results of the cost-benefit analysis of the flexibility retrofits. 

Section 6.1 discusses the methodology that was employed. Sections 6.2 and 6.3 discuss

the results of the cost-benefit analysis for gas-fueled and coal-fueled power plants,

respectively.

Methodology

The cost-benefit analysis was performed by running Plexos simulations with and without 

the retrofits on the selected power plants. From the two simulations, the changes in 

system-level production costs, as well as changes to the retrofitted power plants revenues 

(i.e., the benefits), were determined. The results were then analyzed under two scenarios. 

Under the first scenario, the study system was assumed to be a vertically integrated utility 

under a cost-based return structure. In this case, the system-level production cost savings 

due to the retrofits was compared against the additional revenue required to cover the 

capital (and operating costs, if any) associated with the retrofits. An FCR of 16% was 

used to convert the capital cost into annual revenue requirement. If the production cost 

savings were higher than the annual revenue requirements, the investment was deemed to 

have an economic merit.

Under the second scenario, the study system was treated as a deregulated market with 

merchant generation. The same results as those used in the regulated scenario were used 

in this analysis. However, in this case, for each generator that was retrofitted, the change 

in net revenue9 was compared against the cost of capital associated with the retrofit. The 

NPV associated with the cash outflows (capital investment) and cash inflows (annual net 

revenue increase/decrease) for a 20-year period were calculated. If the NPV was 

positive, then the investment was deemed to have an economic merit.

The cost-benefit of the coal-fueled and gas-fueled power plants were studied separately in 

order to isolate their impacts. Although retrofits are available to improve turndown, ramp 

rate, and startup performance, the retrofits that primarily targeted turndown performance 

were studied since lower turndown capability was found to have the most beneficial 

impact on the system. 

Additionally, since it’s neither feasible nor economic to retrofit all units for lower 

turndown, only a subset of units (around 25% of the capacity) was targeted for the 

retrofitting. Sections 5.2.1 and 5.2.2 demonstrate that more than 25% of the gas- and 

coal-fueled power plants are capable of being retrofitted based on their technologies.

Gas-Fueled Power Plant Retrofits

The economic impact of retrofits installed on gas-fueled power plants was studied using 

the Modified-RMPP system. This system has approximately 6,300 MW of gas-fueled 

combined cycle plants and 3,000 MW of gas-fueled simple-cycle plants, as shown in 

 

9 It was assumed that the revenues from the energy market were the only source of revenue for the 

generation owner. No (increases in) capacity payments were assumed.
26 

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Figure 5. Figure 11 shows the cumulative energy, average number of starts, average 

number of ramps, and average hours at mingen for the combined cycle plants for the four 

renewable penetration cases. As renewable penetration increases, combined cycle plants 

go from intermediate load operation to cycling to finally being displaced. Case 2 (26.7% 

renewable penetration) was chosen for further analysis since it showed maximum cycling 

of combined cycle plants. 

Figure 11. Operational impact on combined cycle plants for various renewable 

penetrations

About one-third of the combined cycle plant capacity (1,575 MW) was targeted for 

retrofitting. The best candidates for retrofitting are the units that produced most energy at 

their mingen levels in the Case 2 simulation. Using this criterion, three combined cycle 

power plants (3 out of 27) that totaled to 1,600 MW were chosen for retrofits. As shown 

in Section 5.2.1, the capacity of the combined cycle plants that are capable of being 

retrofitted far exceeds 1,600 MW. 

Table 12 shows the system-level production cost of the Modified-RMPP system with 

and without the three plants retrofitted for lower turndown. This table shows that the 

savings in annual production cost is roughly $2M. Table 13, Table 14, and Table 15 

show the annual energy, revenues, and variable costs for the three combined cycle plants 

with and without the retrofits. The regulation bid cost10 is an additional cost to the system 

that seeks to capture the operational costs (variable O&M, decreased efficiency, etc.) of 

providing regulation reserves. 

 

10 For a further explanation of how this cost was calculated for the RMPP system refer to Hummon, et al. 

(2013), http://www.nrel.gov/docs/fy13osti/58491.pdf.
27

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As observed in Table 12, the decrease in system-level production cost due to the retrofits 

is a little over $2M/year. The capital cost associated with the retrofits is $1.5M assuming 

the middle of the range cost estimate from Table 1. No additional operating costs are 

associated with these retrofits. Using an FCR of 16%, the annual revenue requirement

associated with the investment of $1.5M is calculated to be $240,000.

11 This annual 

revenue requirement is well below the savings in annual production cost, indicating that 

the retrofits have a net-benefit to the system. Therefore, in a regulated, verticallyintegrated utility environment, the investment in these retrofits does have some merit.

Table 12. Impact of Combined Cycle Retrofits on Production Cost 

Table 13. Net Revenue Impact of Combined Cycle Retrofits for Unit A

 

11 Annual Revenue Requirement = FCR*Capital cost. The FCR is the rate that, when applied to the capital 

cost, gives the amount that is equal to the return on investment expected by shareholders, return of 

investment (depreciation), and taxes.

Modified RMPP System - 

Case 2 (27% Penetration)

Modified RMPP System - 

Case 2 (27% Penetration) - 

with retrofits 

Difference

Fuel Cost ($) 1,217,685,234 1,216,455,256 1,229,978

VO&M Cost ($) 116,978,206 116,706,891 271,316

Start & Shutdown Cost ($) 54,149,325 53,565,142 584,183

Regulation Bid Cost ($) 6,248,313 6,244,994 3,318

Total Generation Cost ($) 1,395,061,078 1,392,972,283 2,088,795

UNIT A Modified RMPP System - 

Case 2 (27% Penetration)

Modified RMPP System - 

Case 2 (27% Penetration) - 

with retrofits 

Difference

Generation 1768351 1584755 (183,596)

Revenue 62,296,853 56,439,900 (5,856,953)

Fuel cost 53,621,511 46,939,490 (6,682,021)

Start & shutdown cost 766,638 653,062 (113,576)

VOM cost 5,322,738 4,770,113 (552,626)

Net Revenue 2,585,966 4,077,236 1,491,270
28

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Table 14. Net Revenue Impact of Combined Cycle Retrofits for Unit B

Table 15. Net Revenue Impact of Combined Cycle Retrofits for Unit C

Most of the savings come from a decrease in fuel costs and startup and shutdown costs,

as shown in Table 12. When the three combined cycle plants are retrofitted for lower 

turndown, they impact the operations of the remaining generators in the system. Table 16 

shows the annual generation for coal, combined cycle, and simple cycle GT power plants 

with and without the retrofits on the three combined cycle plants. With the retrofits, the 

three combined cycle units are able to turndown lower, which in turn increases the 

generation from the remaining combined cycle, as well as coal power plants. The 

generation from simple cycle GTs decreases since combined cycle units remain online for 

more hours with the retrofits. Overall, this has an impact of lowering the fuel costs of the 

system, as shown in Table 12.

Table 16. Annual Generation With and Without Retrofits for Case 2, Modified-RMPP 

System

UNIT B Modified RMPP System - 

Case 2 (27% Penetration)

Modified RMPP System - 

Case 2 (27% Penetration) - 

with retrofits 

Difference

Generation 2,728,741 2,697,415 (31,326)

Revenue 95,717,885 94,027,914 (1,689,971)

Fuel cost 86,545,750 85,819,230 (726,520)

Start & shutdown cost 1,998,486 1,903,320 (95,166)

VOM cost 3,001,615 2,967,157 (34,458)

Net Revenue 4,172,034 3,338,207 (833,827)

UNIT C Modified RMPP System - 

Case 2 (27% Penetration)

Modified RMPP System - 

Case 2 (27% Penetration) - 

with retrofits 

Difference

Generation 3,090,459 3,182,496 92,037

Revenue 108,461,274 111,015,123 2,553,848

Fuel cost 100,387,668 103,463,892 3,076,224

Start & shutdown cost 5,091,912 5,021,191 (70,721)

VOM cost 3,399,504 3,500,746 101,242

Net Revenue (417,810) (970,707) (552,897)

Without Retrofits With Retrofits Delta

Coal 29,462,018 29,480,195 18,177

Combined Cycle GT 22,753,833 22,774,524 20,691

Simple Cycle GT 948,251 913,177 (35,075)

Annual Generation (MWh)
29 

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A portion of the savings in production cost is also attributable to the savings in startup 

and shutdown costs. Table 17 shows the cumulative combined cycle and simple cycle 

GT starts for the two cases. As observed from the table, there are fewer simple cycle GT 

starts with the retrofits. 

Table 17. Impact of Combined Cycle Retrofits on Combined Cycle and GT Starts for Case 

2, Modified-RMPP System 

Table 13, Table 14, and Table 15 show the annual energy, revenues, and variable costs 

for the three combined cycle plants (A, B, and C), with and without the retrofits. Figure 

12 to Figure 14 show the dispatch of the combined cycle units with and without the 

turndown retrofit. 

Figure 12. Dispatch of combined cycle Unit A, with and without turndown retrofit 

Without Retrofits With Retrofits Delta

Combined Cycle GT 1,300 1,288 12

Simple Cycle GT 7,084 6,824 260

Number of Starts
30 

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Figure 13. Dispatch of combined cycle Unit B, with and without turndown retrofit 

Figure 14. Dispatch of combined cycle Unit C with and without turndown retrofit 

For combined cycle Unit A, the annual generation decreases with the addition of lower 

turndown capability. This is because Unit A is online during most hours with our without 

the turndown retrofit. With the retrofit, the unit operates at a lower dispatch level during 

off-peak hours. The fuel and VOM costs of operation decrease corresponding to the 

generation. However, the decrease in revenue is lower since the unit operates at a lower 

dispatch level during uneconomical hours. The net revenue is higher in the case with 

retrofits since the decrease in operating costs outweighs the decrease in revenues. 

With an annual revenue increase of nearly $1.5M, as shown in Table 13, the NPV (using 

a rate of 12%) of the investment of $500,000 is nearly $11,000,000 if the net revenue is 

assumed to increase at a constant 3% for 20 years. In a deregulated market environment, 

where generation owners make their investments based on returns, this investment does 

seem to have some merits. 
31

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For combined cycle Unit B, the annual generation decreases with the addition of lower 

turndown capability as observed for unit A. However, for this unit the net revenue 

decreases with the addition of retrofits. The decrease in revenue is more than the 

decrease in costs for this unit since the unit was in the money even during off-peak hours.

For combined cycle Unit C, the annual generation increases with the addition of lower 

turndown capability. This is because this unit cycled (starts and stops) frequently before 

the addition of the turndown retrofit (marginal unit). With the addition of the turndown 

retrofit, the unit stayed online more often, as shown in the figure (there are many 

instances when the blue dot (dispatch with retrofit) is above zero when the red line 

(dispatch without retrofit) is at zero). However, the increase in generation does not 

translate to an increase in net revenue because the additional generation comes from offpeak hours when the energy prices are lower.

From these examples it is clear that, while there may be a system-level benefit from a few 

units turning down deeper, there may or may not be a benefit at a unit-level. The next 

section discusses the turndown retrofits for coal-fueled power plants.
32 

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Coal-Fueled Power Plant Retrofits 

The economic impact of retrofits installed on coal-fueled power plants was studied using 

the RMPP system. This system has approximately 5,700 MW of coal-fueled steam 

turbine power plants, as shown in Figure 4 in Section 5. Figure 15 shows the cumulative 

energy, average number of starts, average number of ramps, and average hours at mingen 

for the coal power plants for the four renewable penetration cases. To determine the costbenefit of retrofitting coal units, the scenario with 44% renewable penetration (Case 4) 

was selected. This scenario entails a significant increase in the number of hours at 

mingen for the units. 

Figure 15. Operational impact on coal steam plants for various renewable penetrations 

Figure 16 shows the number of hours at mingen for all the coal units in Case 4. The three 

units chosen for retrofits (units D, E, and F) are highlighted in red. Generally, larger units 

that have significant hours of mingen operation are good candidates for turndown 

retrofits; however, the location of the units on the grid is a driving force in determining 

the operations of the units. 

Adequate consideration was given to the design and overall cost effectiveness of retrofits 

to coal units. For example, cost effective units with tilting burners-low and tangential 

firing were considered in this analysis. These units have dynamic control of steam 

generation through adjusting the position of the fireball within the boiler. Relatively new 

units are likely to have digital controls for faster ramping and cycling. A turbine bypass 

valve system is the most crucial technology, allowing for quicker run, down, and 

synchronization times. However, the back fit minimum cost estimate is almost $10-15M 

for this retrofit and may not be cost effective for every site. 
33

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Figure 16. Coal steam plants, hours at mingen for Case 4

Note: Red dots represent the retrofit units.

Table 17 shows the production cost of the RMPP system with 44% renewable 

penetration, with and without three coal plants that were retrofitted for lower mingen. 

The three coal units represented about 24% of the RMPP coal capacity. This table shows 

that the savings in annual production cost is roughly $13M. Table 18, Table 19, and 

Table 20 show the annual energy, revenues, and variable costs for the three coal plants 

with and without the retrofits.

Table 17. Impact of Coal Retrofits on Production Cost 

0

200

400

600

800

1,000

 - 1,000 2,000 3,000 4,000 5,000

MW Capacity

Hours at Min. Generation

Case 4 - 44% Renewable, Hours at Minimum Load 

Note: Coal retrofits units are highlighted in red

 RMPP System - Case 4 

(44% Penetration)

RMPP System - Case 4 

(44% Penetration) - 

with retrofits 

Difference

Fuel Cost ($) 679,659,987 668,431,932 11,228,056

VO&M Cost ($) 106,289,366 105,653,808 635,559

Start & Shutdown Cost ($) 57,087,248 55,012,198 2,075,050

Regulation Bid Cost ($) 10,199,155 10,406,849 (207,694)

Total Generation Cost ($) 853,235,756 839,504,786 13,730,970
34

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Table 18. Net Revenue Impact of Coal Plant Retrofits for Unit D

Table 19. Net Revenue Impact of Coal Plant Retrofits for Unit E

Table 20. Net Revenue Impact of Coal Plant Retrofits for Unit F

As observed in Table 17, the decrease in system-level production cost due to the retrofits 

is approximately $13M/year. The capital cost associated with the retrofits is $9 M 

($3M/unit). No additional operating costs are associated with these retrofits. Using an

FCR of 16%, the annual revenue requirement associated with the investment of $9M is 

calculated to be $1.44M. This annual revenue requirement is well below the savings in 

annual production cost, indicating that the retrofits have a net-benefit to the system. 

Therefore, in a regulated, vertically-integrated utility environment, the investment in 

these retrofits does have some merit.

UNIT D RMPP System - Case 4 

(44% Penetration)

RMPP System - Case 4 

(44% Penetration) - 

with retrofits 

Difference

Generation 1,762,285 1,713,801 (48,483)

Revenue 71,845,868 59,095,988 (12,749,880)

Fuel cost 41,037,978 19,408,547 (21,629,431)

Start & shutdown cost 2,273,568 2,178,836 (94,732)

VOM cost 8,239,498 6,125,135 (2,114,363)

Net Revenue 20,294,825 31,383,471 11,088,646

UNIT E RMPP System - Case 4 

(44% Penetration)

RMPP System - Case 4 

(44% Penetration) - 

with retrofits 

Difference

Generation 2,737,375 2,034,928 (702,446)

Revenue 48,124,867 49,470,982 1,346,115

Fuel cost 29,303,798 28,360,530 (943,268)

Start & shutdown cost 1,894,644 1,543,784 (350,860)

VOM cost 5,304,477 5,158,542 (145,935)

Net Revenue 11,621,948 14,408,126 2,786,177

UNIT F RMPP System - Case 4 

(44% Penetration)

RMPP System - Case 4 

(44% Penetration) - 

with retrofits 

Difference

Generation 2,683,113 2,559,704 (123,410)

Revenue 70,079,947 70,580,248 500,302

Fuel cost 39,753,736 37,810,581 (1,943,155)

Start & shutdown cost 1,961,141 2,046,408 85,267

VOM cost 8,076,172 7,704,709 (371,463)

Net Revenue 20,288,899 23,018,551 2,729,652
35 

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The savings in production cost come primarily from the savings in fuel costs. When the 

coal units are retrofitted for lower turndown, these units operate at lower dispatch levels, 

as shown in Figure 17 (for Coal Unit D), which allows the remaining coal and combined 

cycle plants to operate at a higher, more efficient dispatch. It also decreases the need for 

simple cycle GTs, and hence, the startup costs associated with their operation. Finally, 

the retrofits also result in less curtailment of renewable (primarily wind) generation, as 

shown in Table 21. However, the curtailment is not significant in either case. It 

decreases from 1.43% in the case without coal retrofits to 1.22% in the case with the 

retrofits. 

Table 21. Annual Generation with and Without Retrofits for Case 4, RMPP System 

Figure 17. Mingen profile at Coal Unit D, before and after retrofit

Table 18, Table 19, and Table 20 show the annual energy, revenues, and variable costs 

for the three coal plants with and without the retrofits. Figure 18 through Figure show 

the same information found in the tables. As in the case of combined cycle units, all 

three retrofitted coal units produce less energy, as shown in the tables. However, unlike 

the combined cycle units, the net revenue increases for all three units with the retrofits. 

Without Retrofits With Retrofits Delta

Coal 34,588,108 34,343,870 (244,239)

Combined Cycle GT 4,236,303 4,434,668 198,365

Simple Cycle GT 598,703 563,308 (35,395)

Wind+Solar 13,459,718 13,485,862 26,144

Annual Generation (MWh)

0 200 400 600

MW

0 2000 4000 6000 8000

Hours

Before After

COAL UNIT D
36 

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This is because of the reduction in fuel costs12 is much more significant when compared 

to the reduction in revenues. In fact, the revenues increase for Units E and F even with 

lower production. This is because the coals units are able to operate at a lower level 

during off-peak periods when they are not in the money. The off-peak prices are 

particularly low in this case, which has nearly 44% renewable penetration. Table 22 

shows that the number of hours of operation at mingen for the three coal units is 

significantly higher after the units are retrofitted than before. 

Table 22. Hours at Mingen Before and After Coal Retrofits 

Figure 18. Coal Unit D, revenue and costs with and without retrofit

 

12 The fuel costs in the tables do not account for the use of gas. However, we believe that with one mill 

operation and gas support, the total cost at the plants is a reasonable approximation at current natural gas 

price and the benefit from lower emissions. 

Hours @ Min. Gen

Without Retrofit With Retrofit % Change % Change (Before/After)

Coal-D 2,034,928 2,737,375 -26% 56%

Coal-E 1,713,801 1,762,285 -3% 59%

Coal-F 2,559,704 2,683,113 -5% 65%

Generation (MWh)
37 

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Figure 19. Coal Unit E, revenue and costs with and without retrofit 

Figure 20. Coal Unit F, revenue and costs with and without retrofit 

 
38

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Conclusions

The research performed as a part of this study show that retrofits for improving 

operational flexibility are available for both gas-fueled simple and combined cycle power 

plants, as well as coal-fueled steam turbine power plants. Retrofitting some power plants 

for flexible operations will no doubt provide a benefit to the system. This work indicated 

that the benefits are commensurate with the costs of installing and operating retrofits.

Study results show that at a system level, retrofits that improve the turndown levels of 

gas-fueled and coal-fueled power plants have a net-benefit to the system. The systemlevel net-benefit was determined by comparing the system production cost savings with 

the capital and operating costs associated with the retrofits. It should be pointed out that 

operational flexibility retrofits may not always result in a net-benefit to the system. For 

example, operational flexibility retrofits for gas-fueled, gas turbine power plants may not 

be of much value in a coal-dominated system with low coal prices. Conversely, coal 

plant retrofits may not be of much value in a gas-dominated system with low gas prices.

The results also showed that while there might be system-level net-benefits, there may or 

may not be a benefit at plant level. The plant-level benefit for a power plant that is 

retrofitted is its potential increase in net revenues. The study results showed that in a gasdominated system, only one out of the three retrofitted power plants showed an increase 

in net revenue after the installation of the retrofit. Plant-level revenue impacts are 

particularly important in deregulated regions where generators make their retrofit 

decisions based on the potential for increased profits. Policymakers need to explore 

mechanisms to incentivize plant owners to install retrofits that benefit the system.

While this exploratory study shows the value of operation flexibility retrofits to the 

system, especially under high renewable penetration conditions, additional work in the 

following areas are required:

• As a part of this study, an evaluation of power plants that could be retrofitted was 

performed based on the frame type for GT generators and pressure rating for coal 

units. There may be other factors that dictate whether a power plant is capable of 

being retrofitted for operational flexibility or not. Additional studies are required 

to determine what portion of the fleet could be retrofitted.

• This study determined that there is value in retrofitting both gas-fueled and coalfueled power plants for lower turndown capability. Further work is required to 

determine the value of faster ramp rates and faster and less expensive starts to the 

system.

• This study analyzed the impact of retrofitting approximately 25% of the gasfueled and coal-fueled generating capacity. Further analysis is required to 

determine the optimal level of flexible capability in the system.

• This study focused on operational flexibility available primarily through GT

controls. In a combined cycle application, further improvements can be made to 

the HRSG and steam turbine to make the plant more flexible. While information 
39

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on some of these retrofits was gathered, they were not examined as a part of this 

study. Future study of these retrofits is recommended.

• This study analyzed the impacts of operational flexibility under a set of fuel price 

assumptions. Any change in fuel prices, particularly lower gas prices, may 

increase the value of operational flexibility retrofits for GT generators. Future 

studies should analyze the impact of retrofits under multiple fuel price scenarios 

to increase the robustness of the conclusions.

• Finally, this study focused on the costs versus benefits of emission retrofits. 

While these retrofits are designed for emissions compliance, further analysis of 

the emissions impact of retrofits for gas-fueled and coal-fueled power plants is 

recommended.
40

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Appendix A. GE Operational Flexibility Solutions

The solutions included in the series of GE OpFlex packages improve the ability for 

GT/CC facilities to:

• Deliver power quickly in response to changing grid demands

• Overcome equipment limitations that prevent power plants from capitalizing on 

emerging market opportunities

• Eliminate slow, inefficient startups and their associated costs

• Stay online more cost effectively

• Meet more demand within existing markets

• Generate revenue through ancillary markets

• Reduce emissions “events” and potentially costly compliance penalties that can 

result

• Expand the operating window of a power plant.

GE OpFlex Startup Agility Solutions

OpFlex Startup Agility Solutions enable fast, reliable and repeatable startup profiles. 

Specifically, the underlying applications target reductions in startup time and 

corresponding startup variability along with reduced emissions and reduced fuel 

consumption. Solutions in the suite include:

GT

• Fast-start: employs a “purge credit” system which moves the startup purge to the 

prior shutdown, plus faster acceleration and loading rates to achieve near baseload 

output in 10 minutes. This enables participation in Non-Spinning Reserve 

Ancillary Services markets.

• Startup Fuel Heating: relaxes fuel temperature permissives that can cause holds 

in loading, enabling higher loads to be achieved in shorter times, and reducing 

time spent in inefficient, high emissions operating modes.

• Startup NOx: introduces a new combustion mode which reduces visible “yellow 

plume” and total NOx emissions during the startup sequence.

Steam Turbine

• Steam Turbine Agility: automates steam turbine startups using best-in-class 

sequencing and model-based control to enable faster and more consistent, 

repeatable steam turbine startups in order to achieve the best balance between 

rotor life consumption and startup speed. Faster steam turbine startups result in 

less GT operation at low loads in inefficient, high emissions operating modes.
41

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at www.nrel.gov/publications.

HRSG

• Advanced SCR Ammonia Control: utilizes feedback models to enable SCR 

operation at lower exhaust temperatures while ensuring minimal ammonia slip, 

thus significantly reducing NOx emissions during startup. 

• Advanced HRSG Attemperator Control: uses feed forward control loops to 

proactively adjust HRSG attemperator flows during GT startup and load changes, 

thus reducing CC plant instability and trip risk. Combined cycle starts are faster 

and more consistent, system reliability is improved, and the plant can be 

confidently operated at higher steam temperatures--closer to entitlement--to 

enable increased plant efficiency and output.

GE OpFlex Combustor Operability Solutions

OpFlex Combustor Operability Solutions employ advanced technologies to enable robust 

turbine operation during weather, fuel, and grid frequency variations to improve fuel 

flexibility, avoid trips, and extend operating intervals.

The applications which are included in this suite are:

Grid Stability

• Enhanced Transient Stability: employs multiple technologies on a Model-Based 

Control (MBC) software platform to improve robustness to grid frequency 

transients and meet future grid code requirements to ensure a stable power grid. 

Modern sensor fault detection, isolation, and accommodation (FDIA) schemes 

enable continued operation in conditions where traditional control would have 

results in a trip, thus improving overall availability and reliability.

Automated DLN Tuning

• Ambient Select: provides basic automated DLN tuning capability through

enabling the turbine to automatically choose from among several pre-programmed 

tuning fuel split schedules based on ambient temperature, reducing the need for 

seasonal tuning for emissions compliance.

• AutoTune LT: provides advanced automated DLN tuning capability through 

continuous fuel split schedule biasing as ambient conditions change and as turbine 

hardware and performance degrades over time, reducing the need for tuning at 

any time for emissions compliance. 

• AutoTune DX: provides GE’s most robust automated DLN combustor tuning 

solution by combining MBC technology and detailed, field validated combustion 

models with combustion dynamics feedback. Combustor health is monitored and 

tuned continuously, enabling increased gas fuel composition flexibility, avoidance 

of seasonal tuning for emissions compliance, and expanded capability to handle, 

large rapid transients. 
42

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

GE OpFlex Load Flexibility Solutions

OpFlex Load Flexibility Solutions focus on offerings to enable emissions-compliant load 

range expansion, efficiency, responsiveness, and customization. Solutions enable higher 

full load and peak load output, deeper turndown, and improved efficiency. The 

applications included in this suite are:

Output

• Variable Airflow: utilizes advanced combustor fuel scheduling to enable flexible 

operation at higher maximum IGV settings to provide increased output while 

maintaining emissions compliance, or at lower settings to provide improved 

combined cycle efficiency.

• Variable Peak Fire: provides the capability to variably overfire the GT for 

increased output when economic conditions justify the increased maintenance 

cost and increased emissions. This option includes functionality to increase 

output as much as possible while automatically maintaining emissions 

compliance.

• Cold-Day Performance: leverages OpFlex AutoTune DX to improve combustor 

operability in cold weather, thus allowing higher firing temperatures and 

significantly higher output in cold conditions while maintaining emissions 

compliance.

Responsiveness

• Fast Ramp: enables load ramping at up to twice the normal rate, such that the full 

minimum- load-to-baseload range can be covered in less than five minutes, 

enabling increased participation in regulating reserve markets.

• Grid Code Package: provides multiple custom software packages to ensure 

compliance with country-specific grid codes worldwide.

Turndown

• Extended Turndown: extends low emissions operation to lower load levels, 

enabling reduced fuel consumption at minimum loads and improving the 

economics to remain online overnight and avoid shutdown and startup costs. This 

also extends the available load range for operation, improving dispatch flexibility 

and enabling greater participation in regulating reserve markets.

Efficiency

• Variable Inlet Bleed Heat: replaces conservative anti-icing protection logic with a 

model-based control approach to reduce inefficient Inlet Bleed Heat use, 

particularly in warm weather, to provide significant improvements in part load 

efficiency.
43

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

GE OpFlex System Reliability Solutions

OpFlex System Reliability Solutions are a suite of enhancements focused on better 

enabling reliable, cost effective operation by reducing system trips, improving the 

recovery process, and reducing downtime. Solutions include:

Fuels Reliability

• HFO Availability Package: utilizes a rapid cooldown, automated turbine wash 

cycle, and MBC to improve availability of turbines burning heavy fuel oil (HFO), 

which are subject to rapid performance degradation. 

• Liquid Fuel Operability Package: provides a collection of control software 

enhancements that help improve operation on liquid fuels, and in particular, 

ensure successful transfers between liquid and gas fuels and vice versa.

System Reliability

• AutoRecover: enables B/E-class DLN1 combustors to quickly and automatically 

return to low emissions premix operation following external transients which can 

cause the combustor to enter high emissions, high maintenance factor operation

• Reliability Software Package: provides a collection of control software and 

sensor enhancements that help improve overall turbine operational reliability, 

leveraging the latest fleet experience and new unit design standards. 

Diagnostics/Productivity

• System Diagnostics Package: provides a collection of control software 

enhancements that improve operator capability to more quickly and efficiently 

diagnose system issues, restore operation, and reduce overall downtime. 

• Startup Productivity Package: provides a collection of control software 

enhancements that help improve the overall startup process to ensure on-time, 

reliable plant startup capability.

• Operational Productivity Package: provides a collection of control software 

enhancements that help improve operator productivity when executing various 

systems tests and procedures, particularly those not performed very frequently.
44

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Summary

The respective improvements in the key performance and flexibility attributes that can be 

obtained with these packages can be substantial. Some examples of the potential 

improvements to the GT capabilities are:

• Output: Approximately +3-5%

• Heat Rate: Approximately -0.2 to -0.4%

• Turndown Load Level: Additional 5-10% load reduction (in emissions 

compliance)

• Startup Emissions: ~50-70% reductions in NOx, CO, and CO2

• Startup Fuel Consumption:~70% heat consumption improvement

• Startup Time: More than 50-60% reduction in startup time

• Availability & Reliability: ~0.4% and ~0.2% respectively

• Ramp Rate: As much as a 100% improvement on a MW/min 

basis
45

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Appendix B. Additional Information on Coal Retrofits

Retrofit Description (What is the retrofit?)

Improved and automated boiler drains Implementation of advanced sensors and controls to automate 

drains.

Steam flow redistribution and metallurgy improvements in SH/RH Reduce local variations in superheater/reheater temperature by 

changing steam flow patterns and improve corrosion 

resistance/strength with different materials.

Steam coil air heater to pre warm boiler and air heater Recover waste steam heat by condensing steam to warm boiler 

components. Also, condensing steam may be used in an air 

preheater (APH) to warm air prior to the regenerative air heater.

Gas bypass to keep air heater warm Bleed gas from GT to warm air heater components.

Improved APH basket life when cycling in or through the wet flue 

gas temperature region by installing traveling APH blowers to 

remove deposits prior to cycling down in load

As the air preheater (APH) cools, flue gas may reach the acid dew 

point. By using a blower(s) to remove deposits from the external 

surface of APH prior to cooling, the amount of condensed acid is 

minimized.

Improved APH basket life with improved materials when cycling in 

or through the wet flue gas temperature region

Upgrade APH materials which pass through the acid dew point 

transition temperature to improve corrosion resistance.

Improved selected expansion joints. This is not a complete 

replacement of all expansion joints. 

Maintain system pressure and/or reduce pressure drop by replacing 

selected critical and/or degraded expansion joints. 

Add steam cooled enclosure min flow protection for balanced flow 

with blow down or dump to LP turbine 

During blow down or (high-pressure) turbine bypass operations, 

ensure steam flow by adding a heat transfer surface (or verifying 

sufficient area) to generate saturated steam.

Improved flame proving equipment for burners Upgrade flame proving equipment to verify the igniter is releasing 

adequate ignition energy over the desired area, which ensures 

complete combustion.

Low load gas igniters to allow min generation on gas fuel only Install new igniters to allow low load operation on gas fuel only.

Dual Fuel burners – use NG over coal ( Add NG to all burners). 

Gas supply is not included

Install new burners to allow low load operation on gas fuel only and 

to ensure complete combustion of coal at higher loads.

New feeders with gravimetric type feeder with improved weighing 

of coal feed to mill

Improve control of fuel mass flow rate by installing or converting to 

a gravimetric feeder.

Automatic pressure control on roll and race to adjust the grinding 

pressure of the coal mill.

Implement automatic grinding pressure adjustment in response to 

fuel grindability and desired degree of pulverization, which helps 

ensure complete fuel combustion.
46

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Heated precipitator hoppers Reduce clogging in fly ash collection hoppers due to flue gas 

condensation and/or freezing, especially within hopper throat.

New dry fly ash transport Install new system to extract fly ash from stack gases and 

sequester them for disposal and/or recycling.

Small package boiler, electric boiler with air modeling and permit Provide auxiliary/process steam while plant is shut down by using 

an electric boiler drawing from hotel or offsite power sources.

Motor driven boiler feed pump with startup to min load capability Allow cold startup by enabling feed water flow without the use of 

process/auxiliary boiler steam required for a turbine-driven feed 

water pump. The motor-driven boiler feed pump shuts down once 

load increases above minimal levels.

Improved vibration sensing and monitoring of all rotating 

equipment

Install vibration sensors and recording equipment to continuously 

monitor rotating equipment.

Speed Controlled motors on ID and FD fans using variable 

frequency drives 

Use motor speed rather than inlet vanes/dampers to regulate air 

flow from forced draft (FD) and induced draft (ID) air supply fans.

Transformer monitoring gas analysis and temperature and 

emergency oil removal if arcing occurs 

Install transformer monitoring and automated drain system. Oil is 

drained to minimize fire hazard in the event of a transformer fault.

Turbine electric heating blankets Maintain turbine temperature to facilitate cycling operations and 

warm re-starts.

Convert 3 rpm to 40 rpm turning gear motor to prevent blade 

attachment wear and generator loose parts

Increase turbine rotation speed while unloaded to maintain rotor 

balance. This helps prolong rotor life and prevents premature 

sagging of attached parts.

New valve operators Full arc admission/sliding pressure Install new main steam control/governor valves to allow equal 

amounts of steam along the full circumference of the turbine (“full 

arc” admission). Sliding pressure operation facilitates partial load 

operation by varying steam pressure to the turbine.

Turbine drains to the condenser hot well including main steam 

drains and before seat warmup drains should be routed to the 

hotwell with a sparger discharge below the normal water level.

Reroute drains so that sparger discharge is below condenser water 

level, which facilitates condensation and reduces condenser 

component wear, as the discharge is buffered by liquid rather than 

discharging directly onto tubes and structures.

Turbine water Induction prevention Upgrades including MOVs on 

extractions and inspection of heater drains and extraction piping 

and bellows in condenser

Reduce steam turbine damage due to ingestion of water. Deploy 

motor-operated valves (MOVs) on and conduct inspections of drain 

and extraction lines to identify potential sources of liquid water 

which may be entrained in steam flow.

Condenser tube shielding Provide physical shielding so that steam turbine exhaust does not 

impinge directly on condenser tubes. This will reduce condenser 

tube wear.

Nitrogen Blanketing of condensate storage tank, Boiler, turbine Place inert gas blanket in certain components to reduce corrosion.
47

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at www.nrel.gov/publications.

Condensate polishing system for rapid water chemistry cleanup 

when cycling

Provide mechanical and ion exchange filters to clean up 

condensate while plant is on-line. Size adequately to allow 

operation under varying loads.

Larger condensate storage tanks Install appropriately-sized condensate storage tanks to allow for 

increased storage of condensate when operating under low load 

conditions.