The main impact of firing a coal or a blend with poor flame stability performance would be inability to maintain a stable flame at low load without the need for auxiliary fuel support. A flame will be inherently unstable if the energy release from the volatile matter combustion is insufficient to raise the char particle temperature to a level at which char combustion is self sustaining under conditions of heat loss (including radiation to and from the flame) and mixing intensity (including hot gas recirculation) for a given burner/boiler arrangement.

The pilot scale flame standoff data for the unblended and blended coals at different loads is given in figure below.  This figure shows that the results at the lower load were best ranked by the curve for proximate volatile matter content, while the results for the higher load were best ranked by the curve for high temperature volatile yield. The exception is coal 79 where even at the lower load there was sufficient heating of the coal by the hot refractory within the furnace to cause heterogeneous ignition of the char.  See blending for further information on blending of thermal coals.

The ranking of the flame stability performance of coals fired in advanced Low NOx burners is best done by determining the heating value of the volatiles released at the temperature within the flame envelope created by these burners.