A combustion system comprises a combustion chamber and a flue gas duct located downstream of the combustion chamber to receive combustion products from the combustion chamber, the combustion chamber has a base and an enclosing wall connecting the base to the flue gas duct. The combustion chamber is provided with a plurality of combustion devices, the combustion devices being configured to deliver fuel and gas and/or air into the combustion chamber so as to generate a fireball within a combustion zone in the combustion chamber, the combustion devices further being configured such that the generated fireball rotates about an axis extending between the base and the flue duct. A secondary gas and/or air nozzle is provided in the base of the combustion chamber, at a location upstream of the combustion zone, the nozzle being for delivering gas and/or air into the combustion zone in a direction towards the flue duct.