Abstract: Pyrolysis is recognized as one of the primary technological pathways for the recycling of end-of-life wind turbine blades, and its by-product, pyrolysis oil, is characterized by a relatively high lower heating value and considerable potential as an alternative fuel. Swirl combustion is regarded as an effective approach for achieving efficient combustion of blade-derived pyrolysis oil, whereas the influence of swirl intensity on combustion behavior and NO_x formation characteristics remains insufficiently understood. To address this issue, numerical simulations are performed to systematically investigate the effects of different swirl numbers （S = 0.50, 0.71, and 1.02） on the flow field structure, combustion efficiency, and NO_x formation during the swirl combustion of pyrolysis oil. It is demonstrated that, with increasing swirl number, the dominant flow pattern in the combustor is gradually transformed from an axial jet-dominated regime to a central recirculation-dominated regime, and significant changes are induced in the spatial distribution of the temperature field. A non-monotonic response of combustion efficiency to swirl intensity is observed. At S = 0.71, optimal coupling between the stability of the central recirculation zone and turbulent mixing intensity is achieved, and combustion efficiency and hydrocarbon removal efficiency are determined to be 71.14% and 94.59%, respectively. Furthermore, under swirl combustion conditions, NO_x is found to be primarily generated in the shear reaction zone between the outer edge of the central recirculation region and the main jet. A pronounced positive correlation is revealed between NO_x mass concentration and combustion efficiency, and the highest outlet NO_x mass concentration is detected at S = 0.71. It is indicated that regulating swirl intensity alone is insufficient to simultaneously achieve high combustion efficiency and low NO_x emissions. Instead, integrated strategies such as staged combustion and flue gas recirculation are required for coordinated optimization. Theoretical insights and design guidance are provided for the swirl combustion organization of pyrolysis oil derived from end-of-life wind turbine blades, as well as other high-viscosity, high-nitrogen liquid fuels.