POWER October 2013 - 49

COAL-TO-GAS CONVERSIONS
Natural Gas Has the Advantage
Natural gas combustion in general has several advantages when
compared to coal:
■ Loading and duty cycles of the coal-related equipment are reduced,
typically proportional to the overall change in natural
gas heat input.
■ Regulated emissions tend to decrease proportionally to the change
in natural gas heat input, as long as the gas burner design is sound
and combustion controls are able to accommodate dual-fuel use.
■ Slagging and fouling in the boiler can be greatly reduced with
even modest levels of natural gas cofiring, provided that the
gas burner placement does not overheat a specific region of the
steam generator while the unit is still burning coal.
■ As the heat input level of natural gas increases, excess air requirements
will decrease, which will reduce sensible heat losses
and reduce both air and flue gas fan power requirements.
■ Coal mill loading and duty cycles are decreased, which can be
especially helpful for coal-fired units that are currently milllimited
or that have frequent mill maintenance outages. The
number of spare mills will increase as the natural gas fuel heat
input increases, leading to more opportunities for plant staff to
perform preventative maintenance and for operators to avoid
mill derates during unplanned outages.
■ Erosion and corrosion will decrease throughout the unit as the
natural gas heat input increases, most significantly within the
steam generator itself. Mill, ash-handling systems, flue gas
ductwork, and emissions equipment wear will be reduced, as
will corrosion from sulfur, chlorine, and alkali metals.
■ As the natural gas heat input increases, the duty cycles of
ily favoring natural gas. Conversely, American
Electric Power (AEP) president and CEO
Nick Akins said during an April 2013 report to
shareholders that the price of natural gas has
risen to levels where " you'll see the pendulum
swing back toward coal-fired generation. " At
the time, AEP was operating its coal plants at
a 65% capacity factor and its gas-fired plants
in the mid-30% range-almost a complete reversal
from 2012.
Power plant emissions are also dropping.
Using 2005 as a baseline, the Environmental
emissions equipment will decrease. With natural gas cofiring,
electrostatic precipitators fields may be de-energized, scrubber
recycle pumps or even entire scrubber absorber towers may be
turned off, and selective catalytic reduction system catalyst life
will be extended. Coal and flue gas stream additive use may be
reduced as well.
■ Bottom ash, fly ash, pyrites and mill rejects, scrubber sludge or
gypsum, and many other waste products typical of coal combustion
will be reduced.
Natural gas combustion also poses several problems when used in
a cofiring application:
■ Due to the high hydrogen content of natural gas (roughly 25%
by mass), latent heat losses resulting from the production of
water during the combustion process can be much higher than
for all but the wettest coals.
■ Differences in the flame temperature, gas mass flow, soot and
ash reflectivity, and slag levels on the boiler waterwalls can
result in heat transfer imbalances throughout the steam generator.
At gas heat input levels below 50%, these heat transfer
imbalances can typically be managed via operations efforts. At
higher gas heat input levels, more costly measures are often
called for, including heat transfer surface modifications, special
coatings on furnace waterwall tubes, or pronounced use of flue
gas recirculation to stabilize furnace temperatures.
■ Supply restrictions limit the maximum level of natural gas used,
and seasonal restrictions may be put in place to prevent the
plant from jeopardizing the gas supply for home heating use.
Integrity Project reports that the changing fuel
mix reduced CO2
emissions from power generating
plants by 13.1% by the end of 2012.
However, as fuel prices rose over the past year,
the Energy Information Administration projected
an increase of 8.7% in coal-fired generation
in 2013, although CO2
emissions are
projected to stay below pre-2005 levels.
The operating advantage will go to utilities
with a diversified fleet that are able to
switch between coal and gas as the market
price of fuels seesaws, particularly during
Table 1. Change in furnace and air heater temperature as a function of
the ratio of coal and gas fuel use in a typical steam plant. If natural gas supply
prices fall below the delivered cost of coal, a unit can reduce its busbar generation costs by
burning more natural gas and proportionally reducing its coal burn rate-or vice versa. Source:
Black & Veatch
Temperature profile
Main steam outlet (F)
Reheat steam outlet (F)
Furnace vertical exit (F)
Economizer gas outlet (F)
Air heater gas outlet (F)
October 2013 | POWER
100% Coal
0% Gas
995
999
2,050
730
332
75% Coal
25% Gas
986
981
2,042
710
326
50% Coal
50% Gas
962
985
2,001
680
303
25% Coal
75% Gas
946
927
1,986
659
288
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0% Coal
100% Gas
922
897
1,910
642
272
periods of flat electricity growth such as that
experienced over the past few years.
Four Practical Options
Utility owners of coal-fired power stations
that wish to balance their exposure to coalfired
generation with additional natural
gas-fired generation have several options to
consider. The four most practical options are
cofiring coal and gas in the same boiler, converting
the coal-fired boiler to gas-only operation,
repowering the coal plant with natural
gas-fired combustion turbines, or replacing
the coal plant with a combined cycle plant.
Cofiring Gas and Coal. Cofiring is the
lowest-risk option for substituting gas use for
coal. Cofiring is defined as burning two different
fuels simultaneously to produce heat
in the steam generator, and it is typically
implemented with natural gas or fuel oil at
coal-fired power stations (Table 1).
Natural gas cofiring has many advantages
that make it attractive (see sidebar). The first
is that many power stations already use natural
gas as a startup fuel, so a gas fuel supply
infrastructure to the steam generator is already
in place. Some power stations currently oper49

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POWER October 2013

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