I am calculating a combustion chamber using the partially premixed combustion model with a methane-air mixture. While iterating, the residual informations for "premixc" always stays at 0.0000e+00. Is that normal or am I doing something wrong?

zero residual generally means it is not calculated by the solver although it is being monitored. generally, even if a small quantity is produced, residual will be zero (large or small).

Err, read last sentence as---,residual will be NON-zero (large or small).