The influence of the air combustion mixing into the main flow, operating conditions and fuel type on the level of NO emission in the in-line low-NO x calciners was investigated. The analysis was carried out using a heterogeneous mathematical model based on non-isothermal diffusion-reaction models for char combustion and limestone calcination. The mixing in the oxidizing zone was modeled using a modified Zwietering approach, which supposes that only oxygen or both oxygen and nitrogen are gradually mixed into the sub-stoichiometric suspension leaving the reducing zone. The analysis shows that the difference in NO emission due to the mixing rate of the oxygen into the main flow is the consequence of the different local oxygen concentration in the oxidizing zone and different char-N oxidation rates. The change of limestone feed position in the oxidizing zone modifies greatly the temperature in the region up to the feed level and thereby the NO emission. In the case of bituminous coal, changing the limestone feed ratio in the reducing zone corresponds to an insignificant modification of the NO concentration. The increase of the air combustion temperature has a negative influence on NO emission.