I'm trying to model a methane-air combustion process (turbulent)in a burner. This model consist in an orifice from which methane flows, a venturi where the methane air mixture is created (primary air inlet) and a series of orifices with secondary air mixing and combustion. I have tried many combustion models (EDC, FRC and combined EDC/FRC) but none of them gives a meaningful result (depending on the model there is no ignition or, on th contrary, the mixture ingnites inside the venturi and not outside the oulet orifices). Does anyone have some good advice?

Starting with the EDC: you should asure that the constant EDM B has value larger than zero, otherwise you will have reaction in any place where fuel and air are mixed. In this case reaction rate of the EDC is larger than zero only if there products in the mixture. Consequently you have to initialize with some products somewhere in your domain.

EDM B less than zero is only reasonable for diffusion flames.