Occurrence of Multiple Steady States in the Catalytic Combustion of Propane


This paper reports experimental results on the existence of multiple steady states in a catalytic combustor for various inlet temperatures (Tin), velocities (Vin), and equivalence ratios (φ in). The catalyst was comprised of binary transition metal oxides (CO₃O₄, Cr₂O₃) supported on alumina wash-coated ceramic honeycomb substrates. Only oxidation of lean propane-air mixtures (equivalence ratios <0.2) were studied.

Multiple steady states were detected only over a narrow range of inlet temperatures (600-670°K). For temperatures outside this range only one steady state exists, while for temperatures in this range at least two steady states, corresponding to heating up and cooling down, exist. In a few cases, when these two steady states were widely separated, one and sometimes two additional steady states (intermediate steady states) were detected. These intermediate steady states, in general, lie close to the heating up or cooling down steady states.

For a fixed set of inlet conditions, different steady states yield different axial temperature profiles, and, usually, different exit conversions. However, if Tin is sufficiently high, both steady states yield complete conversion and consequently nearly identical outlet temperatures and gas compositions. Under these circumstances the presence of two steady states cannot be easily detected by measurements made only at the exit of the combustor but can be detected by measuring the axial temperature profile (internal hysteresis). At lower Tin, first the lower and then the upper, steady states yield incomplete conversions at the exit and therefore give rise to different exit conditions. It is at this stage that hysteresis starts to be detected at the combustor exit.

Transition between the heating up (lower) and cooling down (upper) steady states could be achieved by varying Tin over a cycle or using relatively large perturbations in φIn(Δφ > 0.02). In a few cases these transitions were also accomplished using large perturbations in Vin. Transition between the intermediate steady states and the upper/lower steady states were achieved by using small perturbations in φin(Δφ - 0.004).

The effect of multiple steady states on NOx emission was small. However this is primarily because: (1) the φin used were low yielding low outlet temperatures and consequently low thermal NOx (<4 ppm), and (2) the fuel used did not contain any nitrogenous compounds. Emission of CO and unburnt hydrocarbons were measured in the cases of incomplete combustion.


Mechanical and Aerospace Engineering

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