Standards

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Education & Training

Publishing

Conferences & Exhibits

Feedwater Heater Control

System for Improved Rankine

Cycle Power Plant Efficiency

Don Labbe, Invensys Operations

Management

2

Presenter

• Don Labbe is a Consulting Control Engineer for Invensys

Operations Management.

• He received a bachelor’s degree in Nuclear Engineering from

UMass Lowell and a Master’s degree from MIT.

• Don is a registered professional engineer in the state of

Massachusetts.

• Don has published 45+ papers and articles and has 4 patents and 2

patents pending.

• He is an ISA Fellow and is a recipient of the ISA E.G. Bailey Award

and the ISA POWID Achievement Award.

• He currently serves as the ISA POWID Director.

Outline

• Rankine Cycle Review

• Boiler Energy Distribution

• High Pressure Feedwater Heater Control

• Conclusions

3

Basic Rankine Cycle (Wikipedia)

4

Basic Rankine Cycle T-S diagram

(Wikipedia)

5

Regenerative Rankine Cycle with Reheat

6

Regenerative with Reheat Rankine Cycle T-S

Diagram (Wikipedia)

7

Boiler Energy Distribution for Drum Type

Power Plants

• Prior to plant construction the cycle is analyzed combining information from

the turbine, boiler and subsystems to design the boiler, turbine and

configuration of feedwater heaters– The objective of the boiler is to heat the feedwater to generate the design superheated steam

flow at the design pressure and temperature

• Drum style steam power plants have fixed surface areas for each function of

the steam process– economizer for water preheating

– evaporator for steam generation

– superheater for superheating steam and reheater for reheating steam

• These surface areas are designed based on assumptions for fuel and

feedwater temperature– The turbine extraction points determine the feedwater heater temperature

• Modifications to a boiler section surface area or changes in fuel quality can

result in an energy distribution imbalance and potentially an inability to attain

design steam temperatures under all operating conditions

8

Steam Temperature Controls

• Boiler typically includes one or more steam

temperature control systems, such as– Superheat and reheat sprays

– Gas path dampers

– Burner tilts

– Burners-in-service

• Sometimes these are inadequate to attain design

superheat steam temperatures due to

– Fuel quality changes

– Boiler design changes

– Adverse consequence of temperature controls on emissions

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High Pressure Feed Water Heater Control for

Steam Temperature Control

• Boiler feedwater temperature impacts superheat

steam temperature (Drum boilers)– Predicted 16ºF superheat increase per 10ºF cooler feedwater

– Predicted increase in reheat steam flow

• Apply high pressure feed water heater control as part of

a multivariable boiler control

– Efficiency gains from higher steam temperatures and boiler

efficiency more than offset efficiency loss from cooler feedwater

temperature

– Increase load generation capability, both short term and long

term

– Lower emissions

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Feedwater Heater Control Test 1: Effect on

Superheat and Turbine Control

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Confirmation Test 1: Partial closure of the 1st point

feedwater heater extraction isolation valves

• Forced ~ 50 psi drop across extraction valves of

high pressure feedwater heater

• Confirmed the expected drop in feedwater outlet

temperature

• Confirmed predicted increase in superheater

steam temperature

• Confirmed increase in reheat steam flow

• Confirmed reduction in turbine throttle valve

position for same load generation

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Test 2: HP Heaters In-service to Out-of-

service

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HP Heaters

in Service

HP Heaters Out-

of- Service

Performance Comparison: HP Heaters In-

service to Out-of-service

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Test 2: Full isolation of the B side high

pressure feedwater heaters

• Confirmed drop in economizer inlet temperature

• Lower burner tilts

• Lower NOx emissions

• Lower furnace exit gas temperatures

• Lower reheat spray flow

• Increase in reheat steam flow

• Reduced main steam flow and turbine throttle

valve position

• Increased load generation margin

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Test Results

• Control of 1st point heater extraction steam can be

applied to

– increase superheat steam temperature,

– lower burner tilts,

– lower furnace exit gas temperatures,

– lower NOx and

– compensate for the adverse impact of higher BTU coal, an overly

clean furnace, and placing the lower coal mill in service

• Tests confirmed that peak load generation capability can

be significantly increased through the modulation of the

extraction steam to the 1st point feedwater heater with

little, if any, consequence on the unit heat rate

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Conclusions

• By controlling the high pressure feedwater heater outlet

temperature an additional element of control is provided

to attain design steam temperatures.

• The potential improvement in cycle efficiency through

achieving design steam temperatures and increasing

boiler efficiency more than offsets the efficiency loss due

to cooler feedwater

• Feedwater heater outlet temperature control can be a

vital component of a multivariable boiler and steam

temperature control system

• An additional benefit is increased MW generation and

the potential for reduced NOx emissions.

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Standards

Certification

Education & Training

Publishing

Conferences & Exhibits

Feedwater Heater Control

System for Improved Rankine

Cycle Power Plant Efficiency

Don Labbe, Invensys Operations

Management