Computational fluid dynamics (CFD) coupled with a detailed description of the gas phase chemistry is used to find a firing mode that minimizes emissions of nitrogen oxide. The studied combustor is fired with product gas from a waste derived fuel gasifier. First, three experiments were carried out with varying air supply and air ratio in order to gain insights into the nitrogen chemistry inside the burner. Then, CFD modelling is carried out using a mechanism considering 21 species and 54 reactions. Based on the obtained insight additional CFD calculations are performed to find a firing mode with lower NOx emission. In the optimal firing mode enough air was introduced in the upper part of the burner to obtain conditions favourable for NOx reduction through reactions between NH3 and NO. The rest of the air was supplied in the lower part of the burner using low jet velocities. If too little air was introduced in the upper part of the burner, NO was formed at the lower air level. If too much air was supplied, NO was formed in the upper part of the burner. The CFD calculations also indicate that the generally low NOx yield in this device is closely connected to the low temperature associated with combustion of low calorific gases.