POWER November 2014 - 36

HEAT RATE IMPROVEMENT
Improve Plant Efficiency and
Reduce CO2 Emissions When
Firing High-Moisture Coals
Improving efficiency at existing coal-fired power plants anywhere in the world
is a path of no regret: A more efficient power plant uses less coal, has lower
emissions, and experiences lower variable costs. Great River Energy's
Coal Creek Station has demonstrated that by recycling low-grade waste
heat with DryFining, it can reduce emissions, including CO2, and improve
plant performance when burning wet coal.
Nenad Sarunac, Mark Ness, and Charles Bullinger
M
bon dioxide (CO2
aking efficiency improvements at
existing power plants is an effective
and economic way of reducing car)
emissions. For a typical
pulverized coal-fired plant, a 1% increase in
net unit efficiency results in a 2.7% reduction
in CO2
emissions, at constant power
output. To put this efficiency increase into
perspective, the U.S. Environmental Protection
Agency in one of its options for meeting
statewide power sector greenhouse gas emission
reduction goals in the proposed Clean
Power Plan, targets an average 6% improvement
in net unit heat rate of coal-fired generating
units, which is approximately a 2.7%
improvement in net unit efficiency for a coal
fleet with an average heat rate of 10,000 Btu/
kWh. That is a formidable goal for the coalfired
power industry.
Many Heat Rate Improvement
Options
The power industry does have options. A
compendium of potential heat rate improvement
and emission reduction options is summarized
in Table 1. However, the degree of
possible heat rate improvement largely depends
on the type of coal burned. The most
desirable heat rate improvement option for
power plants burning high-moisture coals is
to pursue coal-drying technologies using recovered
low-grade heat from flue gas. (For
more information concerning power plant
heat-recovery technologies, see " Power 101:
Flue Gas Heat Recovery in Power Plants, "
Parts I, II, and III, in the POWER archives at
www.powermag.com.)
Low-rank, high-moisture coals constitute
about 50% of the U.S. and world coal
reserves. Given the abundance of these low36
cost
coals, the use of high-moisture coal for
power generation is common and growing.
In the U.S. alone, 279 power facilities burn
high-moisture coals, such as lignite and Powder
River Basin subbituminous coal. These
plants produce nearly a third of the coal-fired
electric generation in the U.S., according to
the Department of Energy.
When high-moisture coals are burned in
utility boilers, about 7% of the fuel heat input
is used to evaporate and superheat fuel moisture
that leaves with the flue gas, with most
of this loss associated with the latent heat
of evaporation. Furthermore, high-moisture,
low-heating value coals result in higher fuel
and flue gas flow rates, higher auxiliary power
use, higher net unit heat rate, and higher
mill, coal pipe, and burner maintenance compared
to bituminous coals. Conversely, a reduction
in coal moisture content by thermal
drying improves boiler and unit efficiency,
plant operation, and economics-all while
reducing CO2
emissions. However, many of
the coal-drying thermal processes developed
thus far are either mechanically complex or
require costly primary energy or steam to
remove moisture from the coal. This significantly
increases fuel-processing cost, which
is the main barrier to industry acceptance.
Coal Creek Selects FBD
Technology
A novel low-temperature coal-drying and
-cleaning process employing a moving bed
Table 1. Many heat rate improvement options. In general, unit heat rate can
be improved by improving boiler combustion efficiency and turbine cycle efficiency, and by
reducing auxiliary power use. Note that heat rate improvements are generally not additive and
depend on many site-specific factors. Source: Increasing Efficiency and Reducing Emissions of
Existing Pulverized Coal-Fired Units, ICCI Project Number 07-1/5.1A-1
Improvement option
Thermal drying of high-moisture
coals
Improvements to steam turbine
cycle
Improvements to heat rejection
system
Improvements to sensors and
controls
Balancing of coal flow
Combustion optimization
Sootblowing optimization
www.powermag.com
Heat rate improvement
(%)
0.6 to 5.9
Use of heat recovered from flue gas 1.2 to 3.6
2 to 4.5
0.5 to 3
Improvements to boiler auxiliaries 0.2 to 1
up to 1.5
up to 1
1 to 2
1 to 2
Comment
Depends on coal type
Depends on utilization of recovered heat
Depends on unit load
Site-specific
Depends on unit age
Site-specific
Site-specific
Site-specific
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POWER November 2014

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