defining chemical reactions

tang xin

Hello all: I am simulating with the multiple-species-Model in FLUENT the combustion of a(CH4+CO)mixture with air.I'd like to how to choose the chemical reactions. Can you tell me ? 1)If I choose the CH4-air-2-step recations. Whether I also need to define the combustion reaction of CO or not? 2)Or I can only define CH4-air-1-step reaction, then add species CO to the mixture, and define the CO reaction.

Thanks a lot in advance.