GE10 15 ppm combustor0 pages

Yesterday’s ideas are today’s hardware
GE OIL & GAS MEETING SHOWCASES THE NEW 15 PPM COMBUSTOR
ERIC JEFFS AND
KALYAN KALYANARAMAN

N

ew Technology Frontiers was the title
of GE Energy Oil & Gas customer
meeting held in Florence, Italy, from
Jan. 31-Feb. 1. The focus was on the
technological challenges in developing oil and
gas resources in remote parts of the world.
“The challenges include production from nontraditional resources, drastically increased
unit train size to leverage economies of scale,
and operation with respect for the environment,” said Claudi Santiago, president of GE
Energy’s Oil & Gas group.
Last year’s meeting was mainly about new
technologies for Liquified Natural Gas (LNG)
and Gas-To-Liquids production, and the
growing pressures on gas turbine operators to
meet environmental standards set a decade
earlier for the electric power industry. This
year GE announced the commercialization of
the 12 MW GE 10-2 gas turbine fitted with a
new dry low NOx combustor. The twin-shaft
turbine will be used predominantly as a
mechanical drive. The new combustor will be
adapted at the end of 2005 to the single-shaft
GE 10-1 turbine used for generator drive.
For the GE 10-2 fitted with the new combustor, GE is guaranteeing the following
emission limits for a load range of 50 to 100%
of baseload in a wide ambient temperature
range from -20 to +100 °F:
• 15 ppmvd (15% O2) for NOx
• 25 ppmvd (15% O2) for CO
• 15 ppmvd for unburned hydrocarbons
The NOx limit for the old K-1 combustor
was 25 ppmvd. And GE is offering the new
combustor as a retrofit on existing PGT 10/A
and GE 10-2 machines so that these units can
meet new and future emission regulations.

reducing the time the fuel-air mixture spends
in the combustion primary zone (called the
residence time). But all these actions increase
CO and unburned hydrocarbons, and therefore designers try to achieve a balance in the
levels of these pollutants.
To promote efficient combustion at “lean”
conditions, the fuel is “premixed” with air to
produce an intimate mixture before it enters
the combustion chamber. This also helps to
avoid local hot spots that may produce thermal NOx. But this NOx reduction technique
increases the chances of flame blow-out and
combustion pressure dynamics, particularly at
part-load and transient conditions. The premixed flames then need stabilizing solutions
in the combustor.
Around the world, new gas turbine installations are normally guaranteed at 25 ppm
NOx. However, power generating plants generally function at constant frequency and
hence constant speed, and at a steady output as
determined by the system dispatcher.
To achieve low-NOx operation over the
wider operating range of the mechanical
drive, GE has retained in the new combustor
the variable geometry construction that was
there in the older version.
A valve in the air path to the combustor
controls the fuel-air mixture by regulating the
air flow to the primary zone. At high load,
large quantities of air are employed to minimize NOx formation; at low loads the prima-

ry air flow is partially blanked off. This optimizes the fuel/air ratio and reduces the velocity to give high combustion efficiency and low
CO and unburned hydrocarbons.
The air valve has two concentric rings
with slots in the walls (Figure 1). The air flow
out to the cooling path is controlled by the relative rotation of the rings which makes the
slots overlap by a variable amount. Air which
is not used for combustion is passed into the
space between the inner liner and the outer
wall of the can, to reduce metal temperatures
and cool the combustion products before they
enter the turbine.
In the old K-1 combustor, the main premixed fuel was stabilized by pilot fuel that
came in through 32 holes distributed circumferentially on the combustor throat. The nonpremixed pilot flames were diffusive, which
means that the pilot fuel first came in contact
with the air only inside the combustion chamber. These flames have hot spots and produce
more thermal NOx.
GE engineers estimated that about 90% of
the total NOx in the old K-1 came from the
diffusive pilot flame. The engineers added
four premixed pilot burners and stabilized the
pilot flame with a lean and diffusive sub-pilot
flame (Figure 2). The result is that only a
small fraction burns in a non-premixed mode
as sub-pilot.
The GE 10-2 has a single combustor which
can be mounted either vertically or horizontally.

Figure 1: The air valve is formed by two
perforated rings which rotate relative to each
other to open and close the air path

Figure 2: Blue indicates the premixed pilot
flame; green refers to the diffusive sub-pilot;
yellow shows the premixed main flame

Reducing NOx
A significant part of the NOx in modern gas
turbines is produced when nitrogen and oxygen in the combustion air react with each
other above temperatures of 1,450 ºC.
Designers of combustors try to lower this
“thermal NOx” by adding more air in the primary combustion zone (lean combustion),
thus lowering the flame temperature, and
10 Turbomachinery International • January/February 2005

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