negative index in product of combustion

Hi everyone,

Im tryin to simulate combustion in combustion chamber. the fuel is gas pyrolysis (ch4, c2h6, h2, and co) it enter the domain from the bottom of the chamber. i also have air input which comes into chamber from holes at the side of chamber.

i use 3 interfaces :
-heat transfer in fluid
-turbulent k-e
-transport in diluted species

the problem is, concentration result seems so unlogic because there are some spot where the reactant increase and also only a little spot the product exist. and the product is decrease.

hopefully anyopne could help me to get out from this problem.

thank you