The influence of external clay additive and inherent minerals on the ignition of a Xinjiang lignite and its volatile flame propagation in air versus oxy-fuel combustion have been clarified in this work, through the use of a flat-flame burner reactor (FFBR) coupled with in-situ optical diagnosis tools. As has been confirmed, ignition of the lignite studied in this paper was initiated by homogeneous oxidation of a tarry volatile cloud. The removal of HCl-soluble metals shifted coal devolatilization toward higher temperatures in air and 21% O-2 in CO2. The mixing of external clay with coal had little effect on the ignition time. However, it enhanced the decomposition of volatiles, leading to a larger volatile cloud shielding on the particle surface. The oxygen fraction in the bulk gas was found to be most influential. Increasing the oxygen fraction to 30% eliminated all of the discrepencies between raw lignite, acid-washed lignite, and a mixture of raw lignite and clay.