Al. Boehman et al., DYNAMIC STRESS FORMATION DURING CATALYTIC COMBUSTION OF METHANE IN CERAMIC MONOLITHS, Combustion science and technology, 122(1-6), 1997, pp. 257-303
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.