Catalytic ignition

Beyond the catalytic ignition point there is a rapid increase in catalytic performance with small increases in temperature. A measure of catalyst performance has been the temperature at which 50% conversion of reactant is achieved. For carbon monoxide this is often referred to as CO. The catalyst light-off property is important for exhaust emission control because the catalyst light-off must occur rehably every time the engine is started, even after extreme in-use engine operating conditions. 

Chattopadhyay, S., Veser, G., Detailed simulations of catalytic ond non-catalytic ignition during H2-oxidation in a micro-channel reactor isothermal case, in Proceedings of the ChemConn-2001, pp. 1-6 (December 2001), Chennai,... 

Metallic rhodium prepared by heating its compounds in hydrogen must be allowed to cool in an inert atmosphere to prevent catalytic ignition of the sorbed hydrogen on exposure to air. 

To further ensure there will be no contribution of transport within the washcoat, calculations done based on a method of transient analysis for catalytic ignition by Balakotaiah should be done. Fig. 15 shows the results of the calculation in comparison to where washcoat diffusion occurs. By maintaining a nominal washcoat thickness of 7 micron, the catalyst will operate in the kinetic regime for all reaction conditions considered. 

In steady state, for surfaces where the chemistry is only catalytic, the Stefan velocity is zero because there is no net mass exchange. However, it should be noted that there may be net mass exchange during a transient process such as a catalytic ignition [323] as coverage on the surface may vary. 

As an illustration, consider the stagnation flow over a catalytic surface during an ignition event. The inlet flow is steady, but the surface temperature increases as power through the platinum-foil surface increases. At a certain temperature the catalytic ignition occurs very rapidly. The flow configuration and conditions, which are taken from Deutschmann [101], are u n — 8 cm/s, Tln = 300 K, with an inlet mixture of 3% CH4, 3% O2, and 94% N2. The inlet-to-surface separation is L = 5 cm, and the surface Pt sites are initially covered... 

Fig. 17.15 The top panel shows the transient surface-state composition during catalytic ignition on a long time scale. The lower panel shows the transient response of the Stefan velocity and the pressure-curvature eigenvalue on a very short time scale during the ignition transient. The zero point for the abscissa scales is arbitrary.

In addition to the catalytic-ignition problem, this approach has been successfully implemented on opposed-flow strained-flame simulations with the inlet flow oscillating at high frequency [193]. It has also been used to model transient chemical-vapor deposition processes where the inlet flow is varies under a real-time control algorithm [324]. Although it is unlikely that a practical process-control system would be designed to induce extremely fast transients, it is important that the simulation remain stable to any potential controller command. 

O. Deutschmann, R. Schmidt, F. Behrendt, and J. Wamatz. Numerical Modeling of Catalytic Ignition. Proc. Combust. Inst., 26 1747-1754,1996. 

Catalytic Ignition(Rocketry). A method of igniting the main propints in a rocket combustion chamber by utilizing a solid or a liq catalyst to initiate chem decompn or breakdown of one of the proplnts and thereby to generate ignition temp and pressure Ref Rocket Encycl(1959), 73... 

The initial state, corresponding to catalytic ignition, was given in the form of an adiabatically burned layer of thickness x0 before the initial explosive mixture with temperature 8j. The boundary conditions assume an adiabatic wall or symmetric development of the process in both directions from the... 

Mechanism of Catalytic Ignition and Combustion of Solid Rocket Propellants , Bimonthly Report No 1, NYU, Contract No DA-28-017-AMC-2222(A) (1965) 8) Ibid, Bimonthly Report... 

Air and Hydrogen. Pd catalysts prepared on high surface-area supports cause catalytic ignition of H2/air or solvent/air mixtures2,3 Pd on charcoal, when thoroughly dry, is extremely pyrophoric.4... 

Slettenhaar, B., Zevenbergen, J. F., Pasman, H. J., Maree, A. G. M., and Moerel, J. L. P. A., Study on catalytic ignition of HNF-based non-toxic monopropellants in 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville, AL, July 2003 (AIAA 2003-4920). 

A catalytic combustor is basically a lean-prenux combustor, in which the combustion is stabilized by a catalytic surface Hence, the expression catalytically ignited thermal combustion or catathermal combustion is also used [15] The catalyst stabilizes the combustion at low temperatures, which broadens the window in which both CO and NO are sufficiently low cf. Fig. 3. The next section briefly discusses the prominent features of catalytic combustion. 

Fig. 3 Heat generated and removed at the inlet of a monolith combustor vs. temperature, calculated from Eqs. (3) and (4) for the conditions presented in Table 1. The straight lines represent the heat transfer curves in the absence of radiation losses. When the inlet gas temperature is 280 C, Eq. (5) is satisfied for three values of 297 C, 371 C, and 1326 C. As the temperature of the inlet gas is increased, the two lower intersection points approach each other and eventually both points merge at = 335 C when the inlet gas temperature is 292°C. This is referred to as the catalytic ignition or light-off temperature. A further increase in the inlet gas temperature results in a situation where there is only one intersection point. (View this art in color at www.dekker.com.)...

Throughout this paper, the catalytic ignition behaviour will be discussed in terms of ignition temperature vs a corrected equivalence ratio. While the equivalence ratio is usually defined in the combustion literature as the ratio of the current fuel/air ratio divided by the fuel/air ratio at the stoichiometric composition for total oxidation to H2O and CO2. we prefer to depict the data vs a modified equivalence ratio wUich we define as >/(l 4>). This modification has the advantage that it puts equal weight on the fuel lean and fuel rich sides of the ignition curve, i.e. while the usual 4> maps fuel lean mixtures on a scale from 0 to 1 and fuel rich mixtures on a scale from 1 to infinity, the modified ratio maps... 

Figure 2 Ignition curve for methane/air mixtures on a platinum foil catalyst. Shown is the catalytic ignition temperature vs equivalence ratio. Sot all experimental data points are shown.)...

Since our intent was to study catalytic ignition, we abandoned the assumption by Hickman and Schmidt (199.3b) that oxygen adsorbes non-dissociatively onto different adsorption sites than the other reactants. This assumption, while valid at the high temperatures during steady state operation of the reactor, will break down at the comparatively low temperatures of surface ignition processes. Thus, the free surface sites Ojrec obey the algebraic equation ... 

The temperature profile of combustor for catalytically ignited premixed combustion Figure 13 Ignitlon/extinction catalyst performance at... 

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