Currently, a need to balance power grids and compensate the variable electricity demand caused by a large share of electricity produced from renewable energy sources is observed. In countries where the coal-fired power plants are the main energy sources, the operation in the flexible regime is required. The flexible operation is characterized by frequent load changes, work outside the nominal operating conditions, and the more frequently start-ups, shutdowns, and partial load changes. Circulating Fluidized Bed combustion technology gives boiler and plant operators greater flexibility in burning a wide range of coal and other fuels, keeping high efficiency and reduced pollution. The unit load changes are realized by changing the amount of produced steam and load of the boiler. Paper presents the results of numerical modeling of superheater operation in Circulating Fluidized Bed (CFB) boiler. The superheater is characterized by the complex shape of tube cross-section. During the superheater calculations, the temperature distribution of the material in complex cross-section need to be determined. To perform numerical analysis and calculate the superheater operation in the transient state, the mathematical model of superheater was created. The use parameters dependent on temperature and pressure allowed to develop a universal superheater model. To verify the numerical model, the temperature distributions at steady state were compared with the results obtained using CFD software, confirming good results accuracy. Finally, the numerical analysis of the transient operation of steam superheater was performed. The calculation was carried out for three different partial loads and for changing steam temperature at the superheater inlet. Steam and tubes material temperature changes during transient flexible boiler operation were presented. The developed model can be used to perform calculations in the full range of boiler load, both during steady and transient boiler operation. Equations describing the physical processes occurring inside the superheater makes the model universal for a given type of a boiler. (C) 2019 Elsevier Ltd. All rights reserved.