Ignition curve

Evaluation of catalyst for oxidation of pollutants usually involves an Ignition Curve determination. This is a slightly overused expression, because only heat generation is evaluated, not heat removal. For a true ignition curve representation, heat removal evaluation would also be required. 

From the heat generation alone the maximum tolerable temperature difference between catalyst and gas can be evaluated, as will be shown in a later chapter. This is never done in pollution control catalyst testing. Due to the simple conditions at very low concentration, the Ignition Curve can be evaluated for first order kinetics. 

These equations hold if an Ignition Curve test consists of measuring conversion (X) as the unique function of temperature (T). This is done by a series of short, steady-state experiments at various temperature levels. Since this is done in a tubular, isothermal reactor at very low concentration of pollutant, the first order kinetic applies. In this case, results should be listed as pairs of corresponding X and T values. (The first order approximation was not needed in the previous ethylene oxide example, because reaction rates were measured directly as the total function of temperature, whereas all other concentrations changed with the temperature.) The example is from Appendix A, in Berty (1997). In the Ignition Curve measurement a graph is made to plot the temperature needed for the conversion achieved. 

Basis Lafayette Experiments of 7/15/93, Figure 2, Ignition Curve, Reactor 2, MMG catalyst 3, 57.1 PPM TCE, GHSV of 10800... 

Figure 7-46. Ignitibility curve and limits of flammability for methane-air mixtures at atmospheric pressure and 26°C. By permission, U.S. Bureau of Mines, Bulletin 627 [43].

Paulis, M Gandia, LM Gil, A Sambeth, J Odriozola, JA Montes, M. Influence of the surface adsorption-desorption processes on the ignition curves of volatile organic compounds (VOCs) complete oxidation over supported catalysts, Appl. Catal, B Environmental, 2000, Volume 26, Issue 1, 37-46. 

The evolutionary paths of the central density and temperature are plotted by the solid lines in Figure 1 for cases A-C. The dashed lines show the paths for the same accretion rates as for cases A-C but with the NCO reaction switched off. The dotted lines denote the ignition curves for the 3a and NCO reactions. Note that the NCO reaction dominates over the 3a reaction to heat up the core. The inclusion of the NCO reaction leads to the ignition of the helium flash at considerably lower density. 

The evolutionary track followed by the center and the temperature profiles against density are shown by the solid lines in Figure 2. The numerals attached to the lines denote the time before the onset of the flash in units of 10 yr. The center does not reach the NCO ignition curve before the 3a reaction ignites at the site of the maximum temperature. The NCO reaction does not change the existing evolutionary models, as was pointed out by Spulak (1980), except that a considerable amount of 80 is produced in the central region. 

Fig. 7.6 Ignition curves obtained from heat integration of hot spots from the library shown in Fig. 7.7 (at 50 and 150°C).

The output power P = 0.25 UNL Isc can be achieved by electronic stabilization (dashed line in the left side diagram) with U = 0.5 UNL and I = 0.5 Isc, defining the intrinsic safety of the circuit (the minimum ignition curves cannot be applied as a reference in this case, however, due to the nonlinearity of a stabilized power source). 

With this spark test apparatus, the reference curves (minimum ignition curves) of Figs 6.162 to 6.167 (showing price ratio ex-protected to non-ex-protected cage induction motors versus rated power types of protection -(E) Ex e IIT3 (E) Ex d IIC T4 and (E) Ex d IIB T4) have been established with reference gas-air mixtures according to Table 6.38. 

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.)...

Low-temperature ignition in hydrocarbons proceeds through pathways less dependent on free H radicals. Nevertheless, releasing active radicals by dissociation of the passive radical HO2 can also significantly shift ignition curves similarly to the case of explosion of H2-... 

Each atmosphere needs a minimum quantum ofpower to cause ignition. The plot of voltage and current poin, which provides the minimum quantum of power, is called the ignition curve as shown. Power Pis a product of current I and voltage V. So, P=V I. Therefore when voltage drops, the maximum current requirement increases. Dotted tine determines the / limit... 

Table 4.1 highlights the activity of some recently reported catalysts. Typically, catalytic activity has been characterized by monitoring the rise in conversion as a function of tenperature for a particular Cl-VOC in some given test condition. A characteristic curve referred to as the light-off or ignition curve is obtained. T o and 90... 

Most assessments of activity, selectivity and deactivation are based on the comparison of ignition curves under the same reaction conditions. The temperature at which... 

The weak effect of hydrogen additives on liquid kerosene atomized in oxygen has been proved by direct measurements [73]. The data from those measurements are summarized in Fig. 6.20. The value bands 1,2, 3 denote the atomized propellant RG self-ignition in shock-compressed oxygen at various pressure levels. Curve 4 denotes the spray C10H22 self-ignition. Curves 5 and 6 and band 7 were obtained in experiments with 7.5% H2 + 92.5% O2 and 15% H2 + 85% O2 mixtures. Curves 8,9 are calculated data for 7.5% H2 + 92.5% O2 mixture at 1 MPa and 4 MPa pressures respectively. 

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