A combustion system is provided for an oxy-combustion furnace. The combustion system includes at least one windbox mountable on the oxy-combustion furnace and having at least one main firing location. At least one primary inlet is positioned in the at least one main firing location for conveying fuel and the first oxidant into the oxy-combustion furnace. At least one secondary inlet is positioned in the at least one main firing location for conveying the second oxidant into the oxy-combustion furnace. The at least one secondary inlet is angularly offset from the at least one primary inlet.

A method of controlling the operation of an oxy-fired boiler includes combusting a fuel that comprises oil heavy residues in a boiler, the oil heavy residues including hydrocarbon molecules having a number average molecular weight from approximately 200 to approximately 3000 grams per mole, discharging flue gas from the boiler, recycling a portion of the flue gas to the boiler, combining a first oxidant stream with the recycled flue gas to form a combined stream, splitting the combined stream into a plurality of independent split streams, introducing each independent split stream at a different elevation of the boiler, and controlling independently a parameter of each of the independent split streams to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler by adding a second oxidant stream to each respective independent split stream to form respective independent oxygen enriched split streams.