Catalytic Combustion in Large Frame Industrial Gas Turbines

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3.2.2.2
Catalytic Combustion in
Large Frame Industrial
Gas Turbines
Walter Ray Laster
Siemens Power Generation
Corporation
4400 Alafaya Trail MC Q3 -042
Orlando, FL 32826
phone: (407) 736-5796
email: walter.laster@siemens.com
285 285
3.2.2.2-1 Introduction
Large frame gas turbine engines employ three different types of
combustion systems: diffusion fl ame, lean premixed combustion and catalytic
combustion. In diffusion fl ame combustors the fuel and air are injected separately
into the combustion zone where they mix and react. Because of the nature of
the design, these combustion systems tend to have fl ame temperatures that are
typical of stoichiometric combustion and therefore produce high NOx emissions.
Obtaining reasonable emissions from a diffusion fl ame combustion system generally
requires the injection of diluents into the combustion section to lower the fl ame
temperature, typically either water or steam. At current F-class fi ring temperatures
these systems can produce NOx emissions in the range of 25 ppm NOx. In the
lean premixed combustion system, the fuel and air are allowed to premix upstream
of the fl ame zone. This results in a signifi cantly lower fl ame temperature than
the standard diffusion fl ame combustor resulting in lower NOx emissions without
the need to inject water or steam. The limitation on low emissions from the lean
premixed combustion systems is the combustion instabilities which occur as the
lean fl ammability limit of the mixture is approached. These instabilities can lead
to large pressure fl uctuation in the combustion chamber. At F class temperatures
the lean premixed combustion system can obtain NOx emissions in the range
of 7-9 ppm. The catalytic combustion system shows promise to achieve lower
emissions because the combustion instabilities at the lean fl ammability limit are
no longer a limiting factor. Although catalytic combustion systems have not yet
been employed in large industrial gas turbines, results from current development
are encouraging and emissions in the range of 2-3 ppm are achievable.
3.2.2.2-2 Catalytic Combustion Design
The major development effort for catalytic combustion in large frame gas
turbine engines was initiated as part of the DOE ATS program1
. The goal of the
ATS program was the development of a high–effi ciency, high-fi ring temperature
engine (>1700 K) with NOx emissions less than 10 ppm for lean premixed systems
and 5 ppm for the catalytic system. On this program the basic design of the catalytic
combustor for a large industrial gas turbine was developed. Since this program,
considerable progress has been made on the design.
At the high fi ring temperatures of a typical gas turbine engine, it is not
possible to design a pure catalytic approach where all of the fuel is reacted in the
catalyst section. In the current design philosophy a hybrid catalytic two stage
system is employed where the catalyst stage is followed with a homogeneous
burnout region. Generally these systems will react 20-40% of the fuel in the
catalytic stage. By reacting a portion of the fuel in the catalyst the stability of the
fl ame in the homogeneous burnout zone is signifi cantly improved. The hybrid
catalytic combustion systems that have been investigated for large gas turbine
engines are the lean catalytic lean burn (LCL) design and the rich catalytic lean
burn (RCL) design.
Figure 1 shows the basic concept of the LCL design. In this design all
of the fuel and air are premixed upstream and enter the catalyst section under fuel
lean conditions. At the end of the catalyst section any fuel not reacted is burned out
in a homogeneous reaction zone. To insure proper catalyst activity, this concept
requires an inlet temperature of fuel air mixture to the catalyst of approximately
500 C. Since this temperature is higher than the compressor exit temperature of
a typical gas turbine engine, a preburner will be necessary to achieve the desired
catalyst inlet conditions. Operation of the catalyst in the lean region requires very
close control of the air fuel ratio in the vicinity of the catalyst to avoid high reaction
rates and excessive catalyst temperatures. The lean combustion concept has been
pursued by Catalytica in their patented Xonon technology. This technology has
been commercially operated on a small scale in the Kawasaki 1.5 MW engine.
On large frame engines this technology has been studied by General Electric and
Siemens Westinghouse.