Ignition-extinction phenomenon

Experiments with the system H2-02 were performed also by Horak et al. (21-23) who observed pronounced ignition-extinction phenomena. They were able to construct a reliable mathematical model based on the heat and mass balances describing the gas-to-solid heat and mass transfer. Their general conclusion is that the multiplicity phenomena may be explained in terms of thermokinetic theory. However, on the other hand, because of the high thermal capacity of the pellet, the oscillations cannot be described by this mechanism (72). Obviously we should examine a more detailed kinetic mechanism to be able to analyze successfully this phenomenon (25). 

The observed phenomenon is the so-called ignition-extinction-behaviour. A close examination of Figure 4-11 will explain this terminology. If one follows the curve of possible steady-state operating points, starting at low reference temperature values, a point is reached at which ... 

A similar behaviour is found for the case of an increase in the ammonia contraits at the inlet of the converter. In fact, when the molar fiaction of ammonia in the feed stream increases, the equilibrium shifts to the reactants and the heat generation rate decreases. If the reactor is being operated under open loop, an extinction phenomenon rqipears due to the autothermal operation of the converter (Fig. 11). Under closed loop operation, the control action leads to a decrease in the cold by-pass fraction (Fig. 12). As a result, the reactor remains at the upper branch of the curve shown in Fig. 2 (ignited steady-state) and the outlet conversion drops slightly. 

The gas velocity affects heat and mass transfer between the particles and flowing gas as well as the axial dispersion and heat conduction phenomena. For a reactor operating near at the extinction boundary, an increase of inlet velocity results in a sudden decrease of exit conversion. Sometimes this effect is called the blow-out phenomenon (Fig. 17). On the other hand, for a very active catalyst a decrease of inlet velocity leads to an ignited upper steady state. 

This point is called ignition point. A small increase of the reference temperature will lead to a rapid stabilization in a new operating point at a very high stationary process temperature. If, on the other hand, the reference temperature is slowly decreased, another operating point is reached at which condition Equ.(4-96) is again valid. This is the so-called extinction point. A further decrease in reference temperature will lead to a rapid stabilization, but this time in an operating point at a very low steady state process temperature. To make the understanding of this phenomenon somewhat easier it is... 

If extinction occurs when the gas phase combustion energy density falls below a minimum (critical) value, analogous to the phenomenon that governs ignition, then equations 38 and 48 provide a criterion for flame extinction in a flammability test. After the sample ignites and the flame is removed sample... 

These constitute a richly varied and complex field, but the interaction between reaction rate and heat release has enough in common with the homogeneous cases for the mathematical models exploited there to give clear insights into the old phenomenon of ignition and to predict or explain a new phenomenon viz. extinction. 

---