The purpose of the research reported here is to explore how minor coal composition changes can affect oxy-coal flame ignition. Whereas previous work focused on the stability of coaxial turbulent diffusion flames for a single coal, we explore here how two coals of similar rank, fired under aerodynamically identical input conditions, may show very different flame stability characteristics. Since oxy-coal combustion allows oxygen contents in primary and secondary oxidant streams to be varied, flame stability characteristics were determined here by effects of partial pressure of oxygen (P-O2) in the primary stream (with differing preheat temperatures in the secondary stream) and effects of P-O2 in the secondary stream (with zero O-2 in the primary stream), on the flame stand-off distance. The two coals investigated were a Utah Skyline bituminous coal and an Illinois #6 bituminous coal. Flame stability was quantified by probability density functions (PDF) of the flame stand-off distance, which were determined using photoimaging techniques developed in previous work. The location of ignition is defined here as the onset of the luminous zone of the flame since that is what an operator in the field might use to control flame stability. At a 420 degrees C preheat temperature, the Illinois coal showed a more stable flame compared to that of the Utah coal. However, the flame stability of the Illinois coal was not significantly changed by increases in secondary oxidant inlet temperature, while that of the Utah coal was. Both coals were bituminous and had similar proximate and ultimate analyses. However, the variations in the results could be interpreted in the light of TGA analysis and solid state C-13 NMR analyses, since these suggested that the observed ignition differences between the two turbulent combusting coal jets might be explained through differences in the fundamental structure of these two coals, as well as differences in their pyrolysis behaviors. Thus, subtle differences in the fundamental chemistry of coals can play important roles in the stability of complicated turbulent diffusion flames, and this may have implications on coal ignition mechanisms in practical turbulent coal flames.