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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. © 2007 Reaction Design 92 RD0411-C20-000-001
Chapter 3: Catalytic Processes <strong>Tutorials</strong> <strong>Manual</strong> 3.1.1.2 Problem Setup The CHEMKIN project file for this tutorial problem is called reactor_network__two_stage_catalytic_combustor.ckprj located in the default sample directory. We are going to use methane as the main fuel in our micro-turbine combustor system, so we choose GRI Mech 3.0, as described in section Section 2.8.2, to handle the gas phase combustion and, for the catalytic combustion, the surface reaction mechanism developed by Deutschmann et al. 28 for methane oxidation on platinum catalyst, as described in section Section 3.3.1, is employed. The chemistry set, reactor_network__two_stage_catalytic_combustor.cks, which includes the gas phase (GRI methane oxidation mechanism) and surface (Deutschmann’s methane/platinum catalytic oxidation mechanism) reactions can be found in the working directory samples\reactor_network\ two_stage_catalytic_combustor. Since we will use only the perfectly stirred reactor (PSR) and plug flow reactor (PFR) in our model, we do not need any transport data. Before building a model for our combustor system, we need to find out all the important parameters of the system. The total mass flow rate of our sub-scale microturbine system has to match the gas-turbine design point, which, in our case, is 980 g/sec. Since this designed flow rate is too large to be handled by a single combustor, we divide the flow evenly into 6 identical combustor units in parallel and merge them before entering the gas turbine. We set the mass flow rate of each catalytic combustor to 127 g/sec so that there is about 21.5% (or 35 g/sec) excess air per combustor for liner cooling and downstream dilution. Methane and compressed air are pre-mixed before entering the catalytic combustor. We keep the fuel-air mixture very lean so it will not ignite before reaching the catalytic combustor. The gas temperature and pressure at the inlet of the catalytic combustor are 715 K and 3.75 atm, respectively. The inlet gas temperature is higher than that of a homogeneous combustor. We have to use a higher inlet gas temperature to ensure a light-off (or surface ignition) on the catalyst surface. Note that we could lower the inlet temperature if a palladium-based catalyst were used or if a small amount of hydrogen were added to the fuel. However, at this point, we consider methane as the only fuel component. The catalytic combustor is 10 cm in diameter and 10 cm long. It consists of a metal outer liner providing structure support and a honeycomb monolith core. The catalyst is coated on all internal surfaces of the monolith. The cell density of the honeycomb monolith we selected is 400 cpsi (cells per square-inch) and the cell wall thickness is 0.18 mm. The pressure drop across the monolith is determined to be 0.064 psi/cm for the given flow rate. 5.2 grams of platinum are wash-coated onto the internal surfaces 28. Deutschmann et al., Proceedings of Combustion Institute, 26:1747-1754 (1996). RD0411-C20-000-001 93 © 2007 Reaction Design
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August 3, 2007 1:53 am Release 4.1.
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Chemkin 4.1.1 Contents Table of Con
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Chemkin 4.1.1 Tables List of Tables
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Tutorials Manual 4.1.1 Figures List
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List of Figures Tutorials Manual 2-
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Chemkin 4.1.1 1 1 Introduction The
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Chemkin 4.1.1 2 2 Combustion in Gas
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Chapter 2: Combustion in Gas-phase
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Tutorials Manual 4.1.1 Index Index
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