The structure of a luminous, laminar, methane-air premixed flame has been investigated using gas chromatograph-mass spectrometer (GC-MS) ; a luminous flame is realized owing to energy recirculation by radiation transferred from the luminous flame to a porous medium at the upstream side of the combustion space. The results show that in the early stage of reaction the methane breaks down to C2H2 through a pathway similar to that of the conventional methane-oxygen premixed flame, i.e., CH4→CH3→C2H6→C2H4→C2H2 Many species, such as C2H2, C4H2 and C6H6, appear in the upstream region of the luminous zone ; these species play an important role in soot nucleation. Furthermore, when the concentration of these species exceeds a critical point, that is, for an equivalence ratio larger than 1.8, a luminous flame is observed.
