DYNAMIC STRESS FORMATION DURING CATALYTIC COMBUSTION OF METHANE IN CERAMIC MONOLITHS

Catalytic combustion of methane can generate thermal gradients that ar e large enough to shatter monoliths during several transient operating modes. This paper presents simulations From a 2-D transient numercial model of the transport and surface chemistry in a passage in a cataly tic monolith, including convection and diffusion in the gas phase and heterogeneous reactions on the monolith walls. Rates of surface methan e oxidation are based on reaction kinetics for a supported PdO catalys t assigned from differential reactor measurements. A thermal stress an alysis package uses the predicted temperature profiles to calculate st ress formation profiles in the monolith. The model neglects homogeneou s reactions, but nevertheless describes the essential features of tran sient operating modes that generate the largest thermal stresses. Simu lations are reported for inlet methane concentrations from 3 to 5 vol% in air and inlet temperatures from 300 to 500 degrees C. Results illu strate dynamic stress formation during combustor warm-up, cool-down an d cycling of the inlet methane concentration, including cases with the rmal stresses as high as 630 MPa, which exceed the fracture strength o f typical monolith materials such as mullite and cordierite. The highe st thermal stresses form perpendicular to the flow direction during wa rm-up transients, and would tend to crack the monolith walls along the ir axes.