Catalytic Combustion

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3.2.2
Catalytic Combustion
Dr. Lance Smith
Dr. Shahrokh Etemad
Dr. Hasan Karim
Dr. William C. Pfefferle
Gas Turbine Group, Precision
Combustion, Inc.
410 Sackett Point Road, CT
06473
phone: (203) 287-3700 x217
email: setemad@precision-
combustion.com
255 255
3.2.2-1 Introduction
The earliest work on what is now termed catalytic combustion was conducted
by Pfefferle at Engelhard Corporation in the 1970s and introduced the use of both
catalytic and non-catalytic combustion reactions in a temperature range amenable
to both1
. The original-type catalytic combustor is a ceramic honeycomb monolith
containing catalytically-coated parallel channels and placed within a combustion
chamber2. In this original-type catalytic combustor, surface reactions release heat and
reactive intermediates into the boundary layer above the surface, eventually inducing
gas-phase (non-catalytic) reactions. As a consequence, combustor operation can be at
lean limits well beyond those feasible without the infl uence of a catalyst, and pollutant
emissions can be extremely low. Early work on systems of this type were conducted
at Engelhard, Acurex, Westinghouse, NASA, the Air Force, and elsewhere3
.
Active interest in catalytic combustion for power generation increased during
the early 1990s as it became clear that continued pressure for reduced emissions
could not be met simply by re-design of conventional combustors. A new approach
of partial conversion in the catalyst bed and the use of metal catalyst substrates to
circumvent thermal shock issues, revived catalytic combustion for power generation.
Metal-substrate type catalyst beds were thus employed for catalytic combustion with
increasing success during the 1990s, demonstrating the low NOx potential of catalytic
combustion for gas turbine applications4
.
Ultimately, two very different systems emerged during this period: a fuel-lean
catalyst system developed by Catalytica, Inc. and a fuel-rich catalyst system developed
by Precision Combustion, Inc5
. Engine tests of these two systems are described,
respectively, in Yee et al. and Smith et al.
6 . These systems are also described in greater
detail in Sections 3.2.2.1.1 and 3.2.2.1.2 of this Handbook.
3.2.2-2 Role of Catalysis in Combustion
In broad terms, a catalyst is used to promote a desired chemical reaction.
Catalysts fi nd a wide range of applications in the production of energy and power, but
for combustion turbines there are three basic classes of reactions that one may desire
to promote: fuel preparation such as reforming prior to combustion, fuel oxidation
with heat release, and pollutant destruction. “Catalytic combustion” normally refers
to fuel oxidation with heat release, particularly when the catalyst is placed inside an
engine and within the combustor casing. We restrict our discussion here to catalytic
combustion and exclude other catalytic processes such as fuel reforming or exhaust-
gas cleanup.
In simple terms, the presence of a combustion catalyst enables complete
combustion at lower temperatures than otherwise possible. This fact can be used
for multiple benefi ts, but the primary motivation for low temperature combustion is
reduced NO
x emissions and/or increased combustor turndown. In particular, most
non-catalytic combustors operate with peak fl ame temperatures higher than 1525°C
(2780°F) to ensure adequate fl ame stability and margin from blowout. As is well
known, NOx emissions even for perfectly premixed fuel-air fl ames at 1525°C (2780°
F) can exceed the 3 ppm threshold (at 15% O2) targeted for many new power plants7
.
Catalytic combustors, however, can operate stably with fl ame temperatures far below
1525°C (2780°F), offering both reduced NOx emissions and improved combustor
turndown.
3.2.2-3 Catalyst Materials for Combustion
Applications
By defi nition a catalyst promotes a chemical reaction, such as fuel with
oxygen, but is itself neither consumed nor produced by the reaction. Precious
metal catalysts are useful in promoting combustion reactions, and it is desirable to
preserve such valuable catalysts by fi xing them to a stationary, solid surface known
as a substrate. The reactants, fuel and air, react on contact with the catalyst surface