POWER September 2012 - 79
THE FUTURE OF NATURAL GAS
4. Workers repair an Alstom GT24
gas turbine. Alstom says its global parts
chain is flexible enough to enable it to position
parts where they are most needed, including
North America, during this period of higher
gas unit dispatch. Courtesy: Alstom
fouling can result from ammonium bicarbonate
condensation on the catalyst. Once
this happens, the catalyst is less effective
at removing nitrous oxides. To help safeguard
the catalyst, minimum load limits are
around 50% to 60% of maximum capacity,
as much as double what the turndown otherwise
would be without SCR equipment.
As a final measure, Lefton recommends
to restart its pulverizers. The challenge is
to restart the pulverizer with sufficient coal
demand to prevent metal-on-metal rubbing
while guarding against jamming the equipment.
During startup, electrical cross-line
starts cause an inrush of current to electric
motors, adding to their wear and tear.
" If you start and stop the equipment, the
electric motor has so many cycles in it, " Carnes
said. He said motorized equipment failures
are up at the power plant.
Corrosion. A turned-down unit left unprotected
for as little as three days may be
susceptible to pitting and eventually cracking
in boiler and turbine components. The reason
is the combination of chlorides, water,
and oxygen. Baseload operation typically
prevents humidity from forming on unprotected
surfaces and reacting with oxygen and
chlorides that are present in feedwater, said
EPRI's Hesler. But a unit that is laid up and
left unprotected can allow these three elements
to combine, which can lead to pitting,
a precursor to cracking.
Safeguards include introducing nitrogen
to bottle up the system or dehumidifying
and dewatering critical components. EPRI
began a project in late 2011 with 10 utility
participants to test the effectiveness of adding
amines to feedwater. The idea is that,
like waxing a car, the amines will add a protective
layer to surfaces that will help keep
oxygen from reacting with moisture and
chlorines on critical surfaces. Hesler said
the pilot project will run through the end of
this year.
Emissions Controls. As a coal-fired
unit turns down and operates at low loads,
its heat rate goes up and its efficiency goes
down. " The key to survival is the ability to
operate safely and without negative effects
on the environmental control equipment, "
Hesler said. In particular, the coal unit's
economizer needs to maintain a high output
to keep the SCR operating. SCR catalyst
September 2012 | POWER
that plant operators know the cost of cycling
their unit and the likelihood that the
unit-should it come offline-will return to
service in a short period of time. " We encourage
all asset managers to evaluate the
cost of cycling their unit off and back on, "
keeping in mind that cycling adds to fatigue
on high-temperature, high-pressure components
that can lead to costly maintenance
outages. " Cycling can be very costly, " he
said, and expenses can range from additional
maintenance to higher heat rates to
fuel expenses, costly forced outages, and
increased personnel costs.
Gas Unit Maintenance Issues
Many of the gas-fired combined cycle units
that were added in the late 1990s during
the last natural gas boom are at last running
close to their potential, but with maintenance
issues of their own. Hesler said the biggest
problems encountered with combined cycle
units running more frequently result from
heat recovery steam generators (HRSGs),
which he said can exhibit thermal fatigue " on
a very large scale. " The problem stems from
what he said is an HRSG's " basic inability
to control and reduce the amount of water in
the system, " which can lead to distortions in
headers and header tube connections. A second
problem is the result of temperator spray
design, which is used to control HRSG steam
temperatures. If not controlled properly, a
temperator can dump too much water and
lead to quenching.
Major HRSG manufacturers met with
EPRI researchers in late June to discuss possible
improvements to temperators and drains
as well as other HRSG operational issues.
" The concern is that as combustion turbine
ramp capabilities improve, if HRSGs aren't
improving, then the plant as a whole can't
cycle effectively, " Hesler said.
He said combustion turbine manufacturers
are working to achieve fast unit starts to
improve dispatch flexibility, but those efforts
may be less effective if HRSGs aren't able to
respond. Hesler said that HRSG manufacturers
indicated at the June meeting a willingness
to produce more robust products if the
industry is ready to buy them.
Tenaska's Jonas said that more frequent
gas unit dispatch has resulted in more frequent
inspections, as recommended by original
www.powermag.com
equipment manufacturers (OEMs). " Planning
is critical as to when a major maintenance
outage might occur, " he said. Some OEMs
might need up to a year of lead time to make
sure they have parts and people available for a
major maintenance outage. Increased demand
for inspections as a result of higher dispatch
among gas plants becomes an exercise in anticipating
when required maintenance will occur,
he noted. He said some OEMs have been
caught off guard by the higher demand for
maintenance services, an observation echoed
by NV Energy's Geraghty.
Steve Hartman, general manager of thermal
engineering for GE, said that he has not
yet seen much of a difference in maintenance
strategies as gas-fired units run more frequently.
A unit's combustion system continues
to be the main driver in determining unit
availability, and no unique issues have arisen.
" Parts like to operate in baseload mode, "
he said. GE engineers have not had to develop
new technology to deal with increased
operations. The fleet has " hit its outages and
continues to operate well, " he said.
Charlie Athanasia, vice president of Alstom
Thermal Services, North America, said
his company's supply chain is in place to respond
to its customers' demand for inspection
and maintenance services. He said the
company is committed to deploy parts inventories
to meet demand for services, such as
maintenance work on the GT24 gas turbine
pictured in Figure 4.
Jonas said that running units more frequently
has reduced the incidence of HRSG
tube leaks at the company's gas-fired power
plants. He said that depending on the circumstances,
units can continue to operate for
some period of time with HRSG tube leaks
until plant operators find a convenient time
to bring a unit down for repairs.
Combined cycle units may have more
flexibility to operate for extended periods,
even with deficiencies such as steam leaks,
but there may be less flexibility with a combustion
turbine (CT). " In a steam turbine,
you generally don't expect rotating parts to
come apart, " Geraghty said. A CT, by contrast,
could be destroyed if a critical failure
occurred during operations. As a result, the
risk of running a CT beyond its recommended
service hours becomes significant.
The Human Element
The shifting fortunes of coal- and gas-fired
generation affect not just equipment and systems,
but people as well. Moving a coal plant
from baseload operation to cycling and even
unit layup can have an effect on a generating
unit's personnel.
" It's an emotional thing at a power plant
to go from the feeding and caring of a plant
79
http://www.powermag.com
POWER September 2012
Table of Contents for the Digital Edition of POWER September 2012
Contents
POWER September 2012 - Cover1
POWER September 2012 - Cover2
POWER September 2012 - Contents
POWER September 2012 - 2
POWER September 2012 - 3
POWER September 2012 - 4
POWER September 2012 - 5
POWER September 2012 - 6
POWER September 2012 - 7
POWER September 2012 - 8
POWER September 2012 - 9
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POWER September 2012 - Cover3
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