## REDUCING HEAT RATES OF COAL-FIRED POWER PLANTS

Examples of Heat Rate Im provement 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 bothImproving 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 CO 2 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, tu rbine 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. Theseprojects 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

O 2 level. Changes to these parameters affect quantities such as combustion efficiency, steam temp eratures, 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 H 2 O 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. 1RESEARCHERS’ PROFILES

• Dr. Harun Bilirgen is a Senior

Research Scientist in the Energy

Research Center and his research

focuses on emissions control andperformance 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 pr essure 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 pres sure. 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 ex pected 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 forunits 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 areworking on analyses of the heat raterelated factors which would affect the cost and performance of CO 2 capture systems for controlling CO 2 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 CO 2 scrubbers and on the thermal efficiency penalties which would result from adding CO 2 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 Δ HR (Btu/kWh)

T MS ° F 1005 996 8

T RHT ° F 1000 985 20 

## ( ) h / lb m spray

, MS & 0 20,000 5 

## ( ) h / lb m spray

, RHT & 0 22,500 56

TOTAL 89