This paper reports results of experimental and numerical studies of NOx formation in a 30 kWth lab-scale bubbling fluidized bed running in oxy-fuel mode. The numerical model is based on the GRI-mech 3.0 mechanism to compute the kinetics of homogeneous reactions of volatiles and char combustion products and takes into account the flue gas recirculation. The impact of the oxygen excess and of the fluidized bed temperature was examined. Both the numerical simulations and the experiments shown a significant correlation of NO_X formation with the excess of oxygen, where a higher oxygen concentration enhances the fuel-bound nitrogen oxidation to NO_X. It was also found that there is no correlation of the NOx formation and resulting emissions with the fluidized bed temperature in temperature range typical for bubbling fluidized bed combustors (840–960 °C), but the numerical simulations showed an increased NO_X concentration when the temperature raised more (up to 1360 °C). The agreement of experimental and numerical results shows that the numerical model can provide useful insight into the mechanism of NO_X formation. © 2018 Elsevier Ltd