the Absorption Coefficient
 

please
how to set the Absorption Coefficient and other Coefficients
in the DO model for the combustion of methan-air

I'm not sure, go ask your question at www.ansysforum.com in the fluent and reaction section. They have some experienced member that might be able to help you out. ;)

i don't know your case, but if you're planning to solve h2o and co2 you can use weighted sum of gray gases model. i've tried and validated my results with experiments for many cases.