POWER August 2013 - 20

Improve Plant Heat Rate with Feedwater
Heater Control
Meaningful, yet often hidden thermal performance losses
occur in feedwater heaters. These controllable losses frequently
occur because of poor-performing level controls. In
this article, three issues related to feedwater heater performance
are discussed. First, the large cost of a minute
change in plant thermodynamic performance is quantified.
Next, the physical difficulties of accurate level control are
presented, as well as background on the important role that
feedwater heaters play within the plant steam cycle. Finally,
two short case studies are presented that quantify the cost
savings that accrued to plants that successfully updated
their feedwater heater level controls.
Cost of Heat Rate Deviation
Heat rate is a measurement used in the energy industry to calculate
how efficiently a power plant uses the heat content of
the fuel, expressed as the Btu of fuel heat required to produce
a kilowatt-hour of electricity in units of Btu/kWh. The heat rate
of just the steam turbine portion of the conventional power
plant may also be calculated by dividing the energy content of
the steam entering the steam turbine by the electricity generated.
The turbine cycle heat rate represents the combined performance
of the turbine, condenser, feedwater heaters, and feed
pumps. The unit heat rate is found by dividing the steam turbine
heat rate by the boiler efficiency.
If the plant operated without any losses, all of the fuel
heat energy would be converted into electricity and the unit
heat rate would be 3,412 Btu/kWh (the conversion factor
between Btu and kWh at 100% efficiency). Although never
achievable, the goal of the performance engineer is to find
ways to continuously reduce the plant's heat rate, because a
lower heat rate (higher efficiency) reflects more cost-efficient
operation of the plant.
Calculating the annual fuel cost associated with slight deviations
from the plant's target heat rate is always enlightening
because the impact is much more profound than expected. For
example, if a plant's target heat rate is 12,000 Btu/kWh and the
actual is a mere 0.1% higher than target or 12,011 Btu/kWh,
what is the increase in annual fuel cost?
The change in annual fuel cost ($/year) for a 1 Btu/kWh deviation
in heat rate can be calculated simply as HRD/BE x FC x
CF x UGC x T, where:
HRD = heat rate deviation (Btu/kWh)
BE = boiler efficiency
FC = fuel cost ($/million Btu)
CF = plant capacity factor
UGC = unit gross capacity (kW)
T = 8,760 hours per year
If we assume for a typical 500-MW coal-fired plant that BE =
88%, the fuel cost is $2.01/MMBtu, CF = 85%, we will calculate
the change in annual fuel cost as 1/0.88 x 2.01/1,000,000 x
0.85 x 500,000 x 8,760 = $8,504/year. A very small 11 Btu/kWh
deviation means that the fuel cost will increase $93,540 per year
(see sidebar).
20
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POWER | August 2013
Feedwater heater train
2. Three-step process. A standard high-pressure feedwater
heater has three sections: desuperheating, condensing, and the drain
cooler or subcooling. Low-pressure heaters are similar in design, although
they often can eliminate the desuperheating zone. This method
of feedwater heating is far more efficient than relying on burning
more fuel to bring water up to temperature. Source: Magnetrol International
General
Guidelines for Heat Rate
Small changes in heat rate produce large changes in a plant's
annual fuel bill. Additionally, small improvements in heat rate
are often very economic investments. Consider the following:
■ A 1% improvement (reduction in heat rate) = $500,00 in annual
savings for a 500-MW plant.
■ A 5 degree F reduction in final feedwater temperature increases
heat rate by 11.2 Btu/kWh, resulting in an average
increase in annual fuel cost of $59,230, for the typical 500MW
plant.
■ The average U.S. coal-fired plant has a heat rate of ~12,000
Btu/kWh (based on the higher heating value of coal) with a
range of 9,000 to 12,000 Btu/kWh.
■ The modern gas-fired combined cycle plant has a net heat
rate of ~7,000 Btu/kWh, the best in the business (based on
the lower heating value of natural gas).
■ Every plant has many opportunities to improve its heat rate.
1. Efficient water heating. The feedwater heater uses steam
extracted from the steam turbine to heat the boiler's incoming water.
The net effect of this process is improved boiler efficiency and a lower
plant heat rate. A typical steam plant will have six or seven feedwater
heaters. Source: Magnetrol International
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POWER August 2013

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