POWER November 2014 - 30

HEAT RATE IMPROVEMENT
Coal-Fired Power Plant Heat Rate
Improvement Options, Part 1
Even without regulatory considerations, there are good reasons for virtually
every coal-fired power plant to improve its heat rate. The Electric Power
Research Institute (EPRI) has looked at dozens of methods for improving
heat rates and evaluated their applicability and costs. Part 1 of this twopart
series explains the basics of heat rate improvements and the range of
possible options explored in recent EPRI reports.
Sam Korellis
T
he heat rate of a coal-fired power plant
represents the amount of heat, typically
in Btus, needed to generate 1 kilowatthour
(kWh) of electricity. Accordingly, typical
units for heat rate are Btu/kWh.
Heat rate is the heat energy input per unit
of electrical energy output, or fuel consumption
rate for specific levels of power plant
output. Heat rate is also the inverse of plant
efficiency. In this sense, it is comparable to a
golf score: Lower is better.
For a given power plant, heat rate depends
on the plant's design, its operating conditions,
and its level of electric power output.
In theory, 3,412 Btu of thermal energy is
equivalent to 1 kWh of electric energy. For
existing coal-fired power plants, heat rates
are typically in the range of 9,000 Btu/kWh
to 11,000 Btu/kWh. A plant with the U.S. industry
average heat rate of 10,300 Btu/kWh
is operating with an overall plant efficiency
of about 33% (3,412/0.33 = 10,339).
All heat rates discussed in this article and
referenced EPRI reports have been determined
based on net generation. Plant net generation
accounts for the auxiliary power consumption
required to operate the machinery in the
plant. Using net station or unit output as the
denominator helps maintain a holistic view
of plant performance and permits inclusion
of the effect of all modifications, including
emissions controls that change auxiliary power
consumption. Net heat rate permits better
comparisons of units using steam-driven components
to those using electrical motors, as the
steam used to drive large components is typically
less expensive than electricity, but it robs
the steam turbine of some capacity.
Benefits of Lowering Heat Rate
The heat content of coal is in the range of
8,000 Btu/lb to 12,000 Btu/lb. Coal costs
$1.5/MMBtu to $2/MMBtu, or about $30/
ton. A typical coal plant consumes 6,000 tons
per day. For a coal-fired plant, fuel is by far
30
the largest expense item, representing about
55% to 75% of total plant expenses. Reducing
a power plant's heat rate can significantly
lower fuel consumption and thus lower its
costs as well, directly benefitting power producers
and their customers (Figure 1). For
example, at a typical 500-MW plant operating
at 80% capacity factor and firing $2.00/
MMBtu bituminous coal, a 1% heat rate reduction
will save about $700,000 in annual
fuel costs (500,000 kW x 10,200 Btu/kW/hr
x 365 days/yr x 24 hr/day x 80% x 1% x $2/
MMBtu = ~$700,000).
Heat rate improvement is also the first
obvious step to reduce carbon dioxide (CO2
)
and all other emissions. It is commercially
proven and is the most cost-effective and
immediately available control process for
lowering CO2
. The 1% heat rate reduction
described above also corresponds to a 1%
reduction in CO2
emissions-about 40,000
tons/year-something that could amount to
significant savings if new regulations permit
trading of CO2
CO2
credits or impose a " fee " on
emissions.
Even assuming the eventual implementation
of carbon capture and storage technologies,
optimizing heat rate will still make sense
as a first line of CO2
reduction and could be
a complementary activity with other control
options. Heat rate reductions will also result
in decreases in other emissions, such as nitrogen
oxides (NOx
), sulfur dioxide (SO2
),
particulates, and mercury, which can help
plants meet other compliance requirements.
Even for a constant emission rate in pounds
per million Btu, an improvement in heat rate
will result in fewer Btus fired, and thus fewer
total pounds of a given pollutant produced. In
some cases, the benefit of emissions reduction
may exceed the value of fuel savings.
Historical Heat Rates
Unfortunately, since the mid-1960s, the average
heat rate of fossil-fueled electric power
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plants in the United States has gradually increased.
Several factors have contributed to
this slow degradation in unit performance.
One early reason was the introduction of
nuclear generating units to provide an increasing
share of baseload generation and
the anticipation of a large expanding nuclear
construction program over the next several
decades. With these low-cost generating
units forecast to provide a large fraction of
the baseload capacity, utilities devoted less
attention to the maintenance and upkeep of
their older fossil stations in anticipation of
their retirement in the 1970s or 1980s.
This trend was exacerbated as nuclear
construction costs escalated, reducing the
funds available for maintaining fossil station
performance, as well as diverting the attention
of utility upper management from the
operation of these stations. For those utilities
that brought nuclear units online, many of the
fossil plants that formerly constituted their
system's baseload capacity were changed to
cycling duty. The thermal inefficiencies associated
with startups, shutdowns, and swings
in load, as well as extended periods of operation
at less than full power, resulted in increased
heat rates for these units. Generating
units are designed and built to achieve their
best heat rates when operated in steady state
at full load.
In addition, environmental regulations
forced many utilities to retrofit energy-consuming
pollution control equipment such as
flue gas desulfurization (FGD) systems. The
key deleterious effects caused by the addition
of emission controls were the increase
in auxiliary power consumption and the decrease
in boiler efficiency. This adverse trend
started many decades ago with the required
addition of electrostatic precipitators (ESPs)
to remove particulate matter from the flue
gas prior to exhausting it out the stack. Those
new ESPs created a pressure drop, forcing
the fans to work harder and increased the
POWER | November 2014
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POWER November 2014

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