Government regulations to reduce pollutants and increasing environmental awareness in the power generation industry have encouraged coal power plants to begin firing biomass in their boilers. Biomass generally consists of larger particles which produce longer flames than coal for a given burner. The length of the flame is important in fixed-volume boilers because of its influence on heat transfer, corrosion, deposition, and pollutant formation. Many pulverized fuel burners employ a series of co-annular tubes with various flows of fuel and air to produce a stabilized flame. A variable swirl burner with three co-annular tubes, each of variable diameter, has been used to collect flame length data for nearly 400 different operating conditions of varying swirl, fuel type, air flow rate, enhanced oxygen flow rate and oxygen addition location. A model based on the length required to mix fuel and air to a stoichiometric mixture was developed. Inputs to the model are the flow rates of fuel, air, and oxygen, swirl vane position and burner geometries. The model was exercised by changing flow rates and burner tube diameters one at a time while holding all others constant. Physical explanations for trends produced were given.The model also requires two constants, one of which is solved for given a case without swirl, and the other is found by fitting experimental data. The constants found in this study appear to be accurate exclusive to the BYU burner. Thus burner designers will need to obtain minimal amounts of data to predict constants for their reactor and then employ the model to predict flame length trends. The resulting correlation predicts 90% of the flame lengths to be within 20% of the measured value. The correlation provides insights into the expected impact of burner flow rates and geometry changes on flame length which impacts particle burnout, NOx formation and heat transfer.