in which the reactions are enabled only in the specified list of
cellZones.  e.g. in constant/combustionProperties

combustionModel zoneCombustion<psiChemistryCombustion>;

active  true;

zoneCombustionCoeffs
{
    zones (catalyst);
}

and in constant/zoneCombustionProperties

combustionModel laminar<psiChemistryCombustion>;

active  true;

laminarCoeffs
{}