REDUCING HEAT RATES OF COAL-FIRED POWER PLANTS

Examples of Heat Rate Improvement Opportunities: 

Bituminous, Subbituminous and Lignite Coals 

Potential Heat Rate Reductions (%)

Lignite and PRB Bituminous 

BOILER AND FUEL MOISTURE 

• Optimize Combustion and Sootblowing 1 to 2 1 to 2 

• Upgrade Air Preheater Seals 0.5 0.5 

• (a) Pre-dry High Moisture Coals 2 to 4 (a) ----- 

 and Reduce Stack Temperature to 140°F 1.5 (a) 1.5 (a)

or 

 (b) Reduce Stack Temperature to 100°F 1.5 to 3.5 (b) 1.5 (b)

TURBINE CYCLE AND HEAT REJECTION 

SYSTEM 

 

• Install Advanced Steam Turbine Blading 

and Seals 2 to 3 2 to 3 

• Upgrade Steam Temperature Control 

Capabilities 1 1 

• Upgrade Cooling System Performance 1 to 3 1 to 3 

Note: Use either (a) or (b) above, but not both 

 Improving the heat rates of their coalfired generating units has become a 

priority for many power generation 

companies. Sharp increases in the cost 

of coal has led many in the industry to 

search for ways to reduce their annual 

fuel bills. In addition, some companies 

are placing increased emphasis on heat 

rate reductions in preparation for 

possible future CO2 regulations. 

Research projects at the ERC are 

developing new techniques for reducing 

heat rate as well as developing an 

information-base on heat rate reduction 

options for existing coal-fired power 

plants. 

 Edward Levy explains, “Heat rate 

improvement opportunities for existing 

units include reductions in heat rate due 

to process optimization, more aggressive 

maintenance practice and equipment 

design modifications. Opportunities 

exist in the boiler, turbine cycle and heat 

rejection system. The overall level of 

heat rate improvement which can be 

achieved varies with unit design, 

maintenance condition, operating 

conditions, and type of coal.” 

 The Energy Research Center has been 

working on research projects dealing 

with heat rate improvements in coal-fired 

power plants since the 1980’s. These 

projects cover a wide range of heat raterelated topics extending from optimization of combustion and sootblowing 

practice to development of methods to 

accurately measure changes in unit heat 

rate. Center researchers who have been 

involved in this area of research include 

Harun Bilirgen, Edward Levy, Carlos 

Romero and Nenad Sarunac. This article 

summarizes findings from some of these 

projects. 

Optimizing Combustion. The boiler is 

often a good place to begin to look for 

heat rate improvements. The operating 

conditions of a typical pulverized coal 

boiler can be controlled by adjusting fuel 

and air flow rates among burners, 

adjusting mixing patterns of coal and 

combustion air and adjusting economizer 

O2 level. Changes to these parameters 

affect quantities such as combustion 

efficiency, steam temperatures, slagging 

and fouling patterns and furnace heat 

absorption, which in many boilers have 

significant effects on unit heat rate. There 

are systematic procedures which can be 

used to identify the combinations of boiler 

control settings which minimize unit heat 

rate. Referred to as “Combustion 

Optimization” these procedures typically 

involve use of intelligent software to assist 

in the optimization process. The Energy 

Research Center has optimized 

combustion at over 25 coal-fired units at 

which achievable heat rate reductions in 

the 0.5 to 1.5% range were identified. 

Sootblowing Optimization. Slagging 

and fouling deposits from coal ash 

accumulation on heat exchanger tubes 

affect boiler heat absorption patterns, 

steam temperatures and unit heat rate. 

Sootblowing optimization is used to 

identify sootblowing strategies which 

prevent uncontrolled buildup of slag and 

soot deposits and minimize heat rate. 

The Center has developed test 

procedures to determine optimal 

sootblowing schedules and has also 

developed adaptive intelligent software 

to automate the sootblowing process. 

The Center’s recent sootblowing projects 

indicate potential heat rate improvements 

in the 0.25 to 1% range. 

Steam Temperature Control. One of 

the techniques used to prevent 

excessively high steam temperatures at 

the inlets to the HP and IP turbines is to 

spray liquid H2O into the steam. 

Referred to as attemperating spray, these 

liquid flows are taken from the turbine 

cycle and result in increased heat rate. 

Consequently, attemperating spray flow 

rates should be the lowest flow rates 

January 2009, Vol. 27, No. 1 
RESEARCHERS’ PROFILES 

• Dr. Harun Bilirgen is a Senior 

Research Scientist in the Energy 

Research Center and his research 

focuses on emissions control and 

performance improvement of coalfired power plants. 

• Dr. Edward Levy is Professor of 

Mechanical Engineering and 

Mechanics and Director of the 

Energy Research Center. His 

research deals with emissions 

control and performance 

improvement in coal-fired power 

plants.

• Dr. Carlos Romero is an Associate 

Director of the Energy Research 

Center. He is a specialist in 

combustion kinetics and emissions 

control. 

• Dr. Nenad Sarunac is Associate 

Director of the Energy Research 

Center. His research focuses on 

power plant heat rate improvement, 

emissions control and process 

optimization.

• Dr. Eugenio Schuster is an 

Assistant Professor of Mechanical 

Engineering and Mechanics. His 

research focuses on dynamic 

modeling and control of electric 

power generation systems.

• Mr. Zheng Yao is a Research 

Scientist in the Energy Research 

Center. His research deals with 

optimization of power plant 

operations.

For more information on optimizing 

combined boiler-SCR operations, 

please contact Carlos Romero at 

cerj@lehigh.edu or (610) 758-4092. 

For more information on heat rates of 

coal-fired power plants, please contact 

Edward Levy at ekl0@lehigh.edu or 

(610) 758-4090. 

needed to control steam temperatures to 

design levels. The table below shows 

data from a unit in which the steam 

temperatures were lower than desired, 

while both main steam and hot reheat 

steam attemperating sprays were in 

operation. This resulted in heat rate 

penalties due to low steam temperatures 

and the use of attemperation when it was 

not needed. The total heat rate penalty 

was 89 Btu/kWhr or approximately 

0.8%. An upgrade to the steam 

temperature controls and perhaps repair 

of leaking flow control valves would be 

needed to prevent this type of loss. 

Steam Turbine Maintenance. The 

performance of HP, IP and LP turbine 

stages deteriorates over time due to 

factors such as nozzle and blade erosion 

and seal leakage and periodic turbine 

outages are used to restore degraded 

turbine components to as-new condition. 

However, a new generation of aerodynamically improved turbine stage 

designs with nozzles and blades made 

from more erosion resistant materials 

have been made available by steam 

turbine vendors. For older units, these 

designs make it possible to produce 2 to 

3% more gross power than can be 

produced by the original steam turbines. 

Condenser Back Pressure. LP steam 

turbines are designed to operate with 

specific values of turbine back pressure. 

The turbine back pressure increases 

above the design value as the steam 

temperature in the condenser increases 

above the design value, which results in 

a reduction in MW produced and leads to 

increases in turbine cycle and unit heat 

rates. For units which reject heat to river 

water, increases in condenser pressure 

can occur due to factors such as an 

increase in river water temperature 

and/or condenser fouling. For units 

equipped with cooling towers, factors 

such as condenser fouling, maintenance 

related cooling tower performance 

deterioration, and increases in ambient 

temperature and humidity can all cause 

increases in back pressure. For full-load 

operation, increases in turbine cycle heat 

rate of more than 2% are typical for an 

increase in exhaust pressure of 2 inches 

Hg above design. It is not uncommon to 

find units operating with turbine back 

pressures approaching 5 inches Hg, 

which results in even larger heat rate 

penalties. 

Pre-dry High Moisture Coal and/or 

Reduce Stack Temperature. High fuel 

moisture levels found in low rank coals 

have several adverse impacts on the 

operation of a pulverized coal generating 

unit, for they can result in fuel handling 

problems and they affect heat rate, stack 

emissions and maintenance costs. ERC 

research has shown that use of power 

plant waste heat to reduce coal moisture 

before pulverizing the coal can provide 

heat rate and emissions benefits. The 

degree to which performance improves 

depends strongly on the degree of drying, 

with heat rate gains expected to be in the 2 

to 4% range. 

 Opportunities also exist to condense 

moisture from flue gas by reducing flue 

gas exit temperature. Captured sensible 

and latent heat can be used to improve 

unit heat rate through efficiency 

improvements both in the boiler and 

turbine cycle. 

Summary. The potential for improvement in unit heat rate for a given unit will 

depend on fuel type because of the added 

flexibility to reduce heat rate for units 

firing high moisture low-rank coals. The 

table on page 4 lists examples of 

opportunities to improve heat rate for 

units fired with bituminous, subbituminous and lignite coals. If improvements 

could be made in all possible areas, the net 

improvement in heat rate would approach 

10% for bituminous and 15% for PRB and 

lignite coals. While it would not be 

possible to take full advantage of all 

possible improvements on every coal-fired 

unit, the table nevertheless shows there is 

potential for making significant heat rate 

improvements to the fleet of coal-fired 

units. 

 Levy adds, “Center researchers are 

working on analyses of the heat raterelated factors which would affect the cost 

and performance of CO2 capture systems 

for controlling CO2 emissions from 

pulverized coal power plants. The initial 

results are quantifying the impacts of unit 

heat rate improvements on the physical 

size of back-end CO2 scrubbers and on the 

thermal efficiency penalties which would 

result from adding CO2 capture 

capabilities. We plan to estimate the 

impacts of heat rate improvements on the 

cost of carbon capture, once these 

analyses are complete.” ■

Example of Steam Temperature Control Impacts on Unit Heat Rate 

 Design Actual ΔHR (Btu/kWh)

TMS°F 1005 996 8 

TRHT°F 1000 985 20 

m (lb / h) MS,spray & 0 20,000 5 

m (lb / h) RHT,spray & 0 22,500 56

 TOTAL 89