Dec 2011 - Parsons Brinckerhoff

Using Water to Clean the Air
Minimizing air pollution was a
priority for owner TransCanada
at the Coolidge Generating
Station. The plant runs on
natural gas, which is the cleanest-burning
of the traditional fuel sources. Nevertheless,
all combustion creates nitrogen oxides
(NO X
) when the nitrogen and oxygen molecules
in the air break apart. The higher
the flame temperature, the more NO X
produced. Flame temperatures in turbine
combustion chambers are extremely high—
around 1,100 C (2,000 F)—making NO X
the
primary air pollutant of concern for most
power plants.
Coolidge, like many modern plants,
uses turbine water injection technology to
reduce NO X
air emissions. Water is mixed
with the fuel as it is injected into the combustion
chamber to lower the temperature
of the flame, which in turn lowers the
amount of NO X
produced. Further, because
the water adds mass to the power turbine,
more power is produced for each unit of
fuel burned. That translates to fewer emissions
per MW of electricity produced.
Water injection technology is part of
the General Electric turbine design, but
Parsons Brinckerhoff was responsible for
engineering the supporting systems. “Water
injected into the turbines must be ultrapure
and demineralized. We designed an
on-site well water supply and a water treatment
facility that works by reverse osmosis
to purify the water. The water piping is
stainless steel to prevent corrosion,” says
Project Manager Colin McRae.
Water injection reduces NO X
emissions
by a factor of about 10 at Coolidge bringing
levels down to 25 parts per million (ppm).
To achieve further reductions, Parsons
Brinckerhoff implemented a selective catalytic
reduction system—similar to the catalytic
converter in a car—to convert NO X
to water
vapor and nitrogen. It reduces stack emissions
to below 2.5 ppm, exceeding environmental
permitting requirements. “The air permit
for this plant only requires a tons-per-year
limit on air pollutants, not the typical (stricter)
parts-per-million limit, but TransCanada wanted
to invest in low emissions,” says McRae.
“Minimizing air pollution is the right thing to
do, and also avoids possible costly retrofitting
in the future if local regulations change.”
At Coolidge Generating Station, routine maintenance is performed on the General Electric LM6000 combustion turbine, an “aero-derivative” turbine—a modified
aircraft engine capable of the fast start-up needed for peaking plants.
and solar power onto the grid because
they can quickly come up to speed on a
still, cloudy day to make up for the fluctuations
in power availability from renewable
sources,” says McRae.
Built for Extremes
The design was developed according to technical
and performance specifications established
by TransCanada. “For example, the
plant design criteria included operation in
ambient temperatures up to 122 F [50 C]—but
it was up to us to determine how,” McRae
explains. “The lubricating oil systems on
peaking turbines are often air-cooled, but
at an ambient air temperature of 122 F, air
cooling won’t work. Therefore, we had
to use a chilled water cooling system. A
cooling tower would have been the typical
solution; however, TransCanada wanted to
minimize the amount of plant wastewater
generated. We designed a closed chilled
water system using mechanical refrigeration—unusual
for a power plant.”
It does get cold on occasion in Phoenix,
so Coolidge also had to be able to handle
freezing temperatures. “If the water vapor in
the air entering the turbines freezes, serious
turbine damage would result,” McRae says.
To prevent the turbines from icing, the team
designed a large electrically heated hot water
closed-loop circulating system to heat the
turbine air intakes.
Coolidge was also required to be a
“zero liquid discharge” plant, meaning all
wastewater goes to lined evaporation ponds
on site rather than to the local sewer system.
Minimizing the amount of wastewater generated
by the plant’s cooling systems made
it possible to design smaller ponds on the
40-hectare (100-acre) site.
Benefits of Tight Teamwork
Several factors contributed to the team’s ability
to complete the project ahead of schedule
and under budget. McRae notes that most
of the Parsons Brinckerhoff team members
have worked together on numerous power
projects and most are located in the firm’s
San Francisco office, which made coordination
and communication easy and efficient.
Further, Parsons Brinckerhoff and TIC have a
long history of successful partnership. “That
proven teamwork and trust in our capabilities
appealed to TransCanada. In turn, the team
received the go-ahead to begin engineering
early—while the final plant permits were
being obtained—which was a great advantage
on this project,” McRae says.
Cost savings and better solutions also
came from the nature of the contract. “On
a joint venture, all partners share in the risk
and profit. It helps keep the designer and
the builder working collaboratively to come
up with better ideas that will save money in
the long run,” says McRae. “That’s good for
the joint venture partners, the owner, and the
utility’s customers, who want reliable electricity
at a fair price.” n
Colin McRae
4 • Notes
Notes • 5