I was using the laminar flow because my flow rate is such that the Reynolds number is very low… so I selected it from the flow perspective, I did not know that I am supposed to specify turbulent flow for the sake of the reaction even when the Reynolds number is in the low 50.

I tried the suggested solutions but they did not really change the issues I was facing so I decided to move to the 2-step reaction. The results look sensible, the residuals have converged and there is a temperature rise in the channel.

There is one last issue I’m trying to understand though: 

In this simulation I am using a low methane concentration (from a volume fraction of 0.003, I get a mass fraction of 0.001667 which I use as input). From the mass flow rate input and the LHV, I should get a heat of reaction of 0.729 W and a dT of about 20 K but I am getting 2.559 W with dT of almost 70 K.

The simulation seems to be self consistent in the sense that when I use the 2.55 W to calculate dT, I indeed end up with 70 K. But I am not sure where the additional heat from 0.729 to 2.55 W comes from. Is there something that I am missing? Is there a way to get the actual mass of ch4 in the flow domain instead of the fraction? Please find attached Fluent input/results as well as my calculation check.