A method of introducing oxygen into a boiler system for combustion with a prepared fuel, the boiler system including a furnace comprising a combustion chamber having an outside wall with an intake region, at least one burner, a prepared fuel delivery means connected to the intake region, and a plurality of air stream delivery lines connected to the intake region adjacent and at least partly surrounding the prepared fuel delivery means, to provide a stream of flue gas, the method comprising
- (a) providing a plurality of swirl-inducing elements to the air stream delivery lines;
- (b) providing a plurality of oxygen delivery jets adjacent the intake region and substantially surrounding the air stream delivery lines;
- (c) igniting the fuel to create a flame having a flame envelope;
- (d) delivering a stream of oxygen through the jets tangentially to the flame envelope such that
- (i) the stream of oxygen substantially shapes and stabilizes a swirling flow field for the flame;
- and
- (ii) the oxygen concentration in the flame envelope gradually increases towards an outer periphery of the flame envelope.