Tutorials Manual

Chemkin 4.1.1
Chapter 3: Catalytic Processes
Figure 3-1
Turbine Flow Capacity
The basic rule for minimizing NO x emissions from a gas turbine combustor is to keep
the gas temperature low by operating the combustor under ultra-lean conditions.
However, if the fuel-air ratio gets too low, the combustor will run into flame stability
problems. We decide to work around the flame stability issue with the implementation
of a catalytic combustor. A catalytic combustor produces essentially no NO x and can
convert very lean fuel-air mixtures at relatively low temperatures. There are some
disadvantages of a catalytic combustor, though. For instance, the honeycomb
monolith introduces a large pressure drop and the thermal mass of the honeycomb
material can slow down the catalytic combustor's response to changes in operating
conditions. The precious metals used as catalysts are usually very expensive (Pt
$870/oz, Pd $230/oz) and, to prolong the lifetime of the catalyst, the maximum
operating temperature is much lower for catalytic combustors than for homogeneous
(gas-phase only) combustors. To raise the catalytic combustor exit gas temperature
(< 1200 K) to the desired TRIT (~1475 K), a second stage combustor must be added
and it has to be a homogeneous combustor. We expect that almost all NO x emission
from this two-stage combustor system will come from the homogeneous combustor.
Fortunately, the gas mixture entering the homogeneous combustor is already at an
elevated temperature, we can try to push the fuel-air ratio as lean as possible to
minimize NO x generation without getting into flame stability issues. If the exit
temperature of the second combustor becomes too high for the turbine rotor, a third
stage can be added to cool the gas down with excess air.
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