NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Flexible Coal: An Example 

Evolution from Baseload to 

Peaking Plant

“Innovations in Flexible 

Generation” Webinar

Jaquelin Cochran, Ph.D.

May 29, 2014

NREL/PR-6A20-62125
2

What are the impacts of cycling coal and gas?

0% wind 

and solar

16.5% wind 

and 16.5% 

solar energy 

penetration

Generation dispatch for 

challenging spring week in 

the U.S. portion of WECC

Source: WWSIS Phase 1 

(2010)
3

Coal ramping increases with wind & solar

Biggest cycling change is that coal units are ramped 10 times more in the 33% 

cases compared to the 0% case

0% wind/solar

13% wind/solar 

33% HiWind

33% HiMix

33% HiSolar

0% wind/solar

13% wind/solar 

33% HiWind

33% HiMix

33% HiSolar

Source: WWSIS Phase 2 (2013)

http://www.nrel.gov/electricity/transmission/western_wind.html

Starts, in capacity (GW), over one year

Number of ramps over one year
4

Emissions impacts of cycling are relatively small

Source: WWSIS Phase 2 (2013)

http://www.nrel.gov/electricity/transmission/western_wind.html
5

From a system perspective...cycling costs are relatively small

33% Wind/Solar Scenarios

Source: WWSIS Phase 2 (2013)

http://www.nrel.gov/electricity/transmission/western_wind.html
6

Costs can be significant from generator perspective

Fossil plant O&M increases by $0.5-1.3/MWh over no RE case

Source: WWSIS Phase 2 (2013)

http://www.nrel.gov/electricity/transmission/western_wind.html
7

Case study: Flexible Coal

• Coal plant that cycles on and 

off, up to twice daily

• Capital modifications critical

But primary savings came 

from changes to operating 

procedures

• Flexibility comes at a cost—

but costs can be minimized 

with strategic modifications 

and maintenance

How baseload plants can evolve to serve other system needs

Flexibility that helps increase 

RE penetration levels

Source: Cochran, Lew, and Kumar (2013)

http://www.nrel.gov/docs/fy14osti/60575.pdf
8

Case study: attributes of flexibility at the plant

• Start up and shut down same day

o Even twice daily (5-10am then 4-8pm)

• Load follow and run at minimum generation 

levels

o 480 MW net capacity units running at 90 MW net

o Down to 60 MW net (up to 6 hours) with gas support

• Provided automatic generation control

• Operated at sliding pressure (increases efficiency 

and flexibility at part load)
9

Case study: from baseload to peaking plant

• 1970’s: Commissioned

• Intended to run at 80% annual CF

• Mid-80’s: Nuclear came in lower 

on the dispatch stack resulting in 

cycling of coal (50% annual CF)

• 1980s: Extensive research on 2- shifting; modifications 

implemented

• 2000s: competitive market; 

initially operated at full output

• YET...significant forced outages 

due to latent damage from 1990s 

cycling

• Competitive marketincentive to 

operate flexibly (2 shift, min gen)

Photo from iStock 72283000
10

Experiences with cycling

Average number of 

starts over life of 

unit

Cold start 523

Warm start 422

Hot start 814

Total 1,759

Year EFOR [%]

2002 14

2003 20

2004 33

2005 25.5

2006 22

2007-2009 16-17

2010 9

2012 20.5

Starts Recent Forced Outage Rates

Plant operators responded to changing market conditions. Decisions to modify the 

plant, replace parts, and lower EFOR were evaluated piecemeal, based on profit 

potential.
11

Cycling impacts

- Boiler tube failures

- Cracking in dissimilar metal welds

- Tube cracks in condenser

• Cracking in generator rotors

• Stresses from changing pressure, e.g., 

turbine shells

• Wear & tear on cycling-only auxiliary 

equipment

• Oxidation from exposure to air on 

startup and draining

• Corrosion caused by O2 entering 

system and changes to water quality 

(falling pH)

• Condensation from cooling 

corrosion, water leakage, increased 

need for drainage

Foreign object damage in turbine fins 

(e.g., from oxides dislodging) 

• Thermal fatigue, e.g., from cold feedwater entering boiler on 

startup, steam heating up, materials heating up at different rates

Photo: Cochran, Lew, and Kumar (2013)

http://www.nrel.gov/docs/fy14osti/60575.pdf
12

Modifications to minimize cycling impacts

Operating procedural changes

• Natural cooling

• Temperature monitoring of economizer inlet 

headers, turbine parts, etc.

• Changes to layup procedures

• Pressure part management

• Changes to boiler operating procedures

• Water chemistry maintenance

• Breaker maintenance

• Gap review with best practices

~90% of plant savings (post physical modifications) came from 

changes to operating procedures
13

Physical changes

• Boiler, examples:

o Added metal overlay to water walls to minimize oxidation

o Replaced dissimilar metal welds

o Strategically replaced corner tubes

• Pulverizers – converted water deluge system to steam inert

• Turbines – added drains, upgraded lubrication system

• Rotors – insulated key parts to reduce rotor cracking from rubbing

• Condenser – sacrificially plugged tubes at top of condenser due to low loads

Note that plant had horizontal, not pendant, boiler designs and automated drains

Graphic: Lefton 2013

Modifications to minimize cycling impacts
14

Can my coal plant do this?

• Physical distinctions: boiler design is horizontal, 

improving drainage

• Operating distinctions: Much higher EFOR rates than 

most operators are comfortable with; tradeoff 

between maintenance costs and EFOR

• Regulatory distinctions: Can run plant without SCR up 

until SCR minimum generation level

• TVA is cycling coal because of cheap gas and lighter 

loads. e.g., 1420 MW Kingston plant:

o More starts

o More ramps

o Increased $ for preventive and corrective maintenance
15

Conclusions

• Flexibility in plant operations is an increasingly 

desired service as RE penetrations increase

• Coal plant in this case study was modified to 

access this flexibility

• Key to success: changing operational practices; 

tolerance for higher EFOR rates

• Success also due to plant-specific factors, e.g., 

better design for drainage

• Coal can be operated flexibly—at a cost—but 

these costs can be minimized through rigorous 

inspection and training programs
16

Recently published analyses

Market Evolution: Wholesale Electricity Market Design for 

21st Century Power Systems

Reviews the international suite of wholesale power market designs in use and under 

consideration to ensure adequacy, security, and flexibility in a landscape of significant variable 

renewable energy. [http://www.nrel.gov/docs/fy14osti/57477.pdf]

Integrating Variable Renewable Energy in Electric Power Markets: Best 

Practices from International Experience

Documents diverse approaches to integration of variable RE among 6 countries—Australia 

(South Australia), Denmark, Germany, Ireland, Spain, and the US (Colorado and Texas)—and 

summarizes policy best practices. [www.nrel.gov/docs/fy12osti/53732.pdf]

Flexible Coal: Evolution from Baseload to Peaking Plant 

This case study reviews how power plants intended to run at baseload can evolve to serve other 

system needs. The CGS case illustrates the types of changes that may occur in global power 

systems, especially those with legacy plants. [http://www.nrel.gov/docs/fy14osti/60575.pdf]

Flexibility in 21st Century Power Systems

Flexibility of operation—the ability of a power system to respond to change in demand and 

supply—is a characteristic of all power systems. Flexibility is especially prized in twenty-first 

century power systems, with higher levels of grid-connected variable renewable energy (primarily, 

wind and solar). [http://www.nrel.gov/docs/fy14osti/61721.pdf]
Thank you!

Jaquelin.Cochran@nrel.gov