Fuel-Rich Catalytic Combustion

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3.2.2.1
Fuel-Rich 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
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3.2.2.1-1 Introduction
Currently, gas turbines operating on natural gas offer the lowest
achievable NOx emissions without exhaust-gas aftertreatment, as compared
to other fuels. Commercially, this has been achieved through the use of lean-
premixed combustion systems, allowing NOx emissions below 9 ppm (at 15%
O2) to be guaranteed for natural gas operation, and emissions near 5 ppm to be
demonstrated1. Lower emissions are needed, however, and are possible through
the use of catalytic combustion2
.
Because natural gas has historically offered the greatest potential for
low emissions, catalytic combustor development has until recently focused
largely on methane and natural gas operation. As a result, the unique properties
of methane have led to a number of development issues and design strategies,
generally related to the behavior of Pd-based catalysts used for methane
oxidation.
In particular, for methane oxidation under fuel-lean conditions, only
Pd-based catalysts among the PGM group are currently practical, because
only they offer acceptable activity, lightoff temperature, and resistance to
volatilization3. Unfortunately Pd-PdO catalyst morphology and its reactions
with methane are complex, and lead to complex behaviors such as deactivation
at high temperature (above about 750°C / 1380°F), hysteresis in reaction rate
over heating and cooling cycles4, and oscillations in activity and temperature5
.
In addition, lightoff and extinction temperatures are well above 300°C (570°F)
for fuel-lean reaction on Pd-based catalysts, thus requiring the use of a preburner
in many engine applications6
.
Fuel-rich operation of the catalyst circumvents many of these issues
and provides signifi cant catalyst advantages, including a wider choice of
catalyst type (non-Pd catalysts are active to methane under fuel-rich conditions),
improved catalyst durability (non-oxidizing catalyst environment), and low
catalyst lightoff and extinction temperatures. Catalyst extinction temperature is
particularly low, and is generally less than 200°C (400°F) for the precious-metal
catalysts used in the work reported here (that is, once the catalyst has been lit
off, the catalyst remains lit at inlet air temperatures less than 200°C / 400°F), and
a preburner is generally not required. A more complete discussion of fuel-rich
versus fuel-lean catalyst behavior for methane oxidation is given by Lyubovsky
et al.7
.
In addition to catalyst material challenges, commercial acceptance of
catalytic combustion by gas turbine manufacturers and by power generators has
been slowed by the need for durable substrate materials. Of particular concern
is the need for catalyst substrates which are resistant to thermal gradients and
thermal shock8. Metal substrates best fi ll this need, but their temperature must
be limited to less than 950°C (1750°F) to assure suffi cient material strength and
long life. Downstream of the catalyst, combustion temperatures greater than
about 1100°C (2000°F) are required for gas-phase reactions to complete the
burnout of fuel and CO in a reasonable residence time (on the order of 10 ms).
Thus, only a portion of the fuel can be reacted on the catalyst.
A major challenge, then, is to limit the extent of reaction within the
catalyst bed such that excessive heat does not damage the catalyst or substrate,
yet release suffi cient heat that downstream gas-phase combustion is stabilized
under ultra-low emission conditions. For systems which lean-premix fuel and
air upstream of the catalyst, the degree of reaction can be limited by chemical
reaction rate upon the catalyst, or by channeling within the reactor such that only
a limited fraction of the fuel contacts the catalyst. In all cases, however, it is
imperative that uncontrolled gas-phase reactions do not occur within the catalyst-
bed, since this implies a loss of reaction limitation and ultimate over-temperature
and failure of the catalyst bed. Preventing such gas-phase reactions is especially
challenging in applications to advanced, high-fi ring temperature turbines, where
fuel/air ratios in the catalyst-bed can be well within the fl ammability limits.