Thermal-structural stress assessment and optimization of high-temperature superheater bends in a 300 MW CFB boiler: A practical case study under deep peak-regulation operation

High-temperature superheater bends in circulating fluidized bed (CFB) boilers are subjected to coupled pressure–temperature variations under deep peak-regulation operation, which may induce local stress concentration and structural reliability risks. In this study, representative quasi-steady operating conditions were extracted from the peak-regulation process of a 300 MW CFB boiler, and local steady thermal-structural coupled models were established for high-temperature superheater bends. The thermal boundary conditions of the local models were determined by combining global and local thermal calculations based on practical operating data, while the support constraints were assigned according to the actual fixation arrangement. The results show that, within the investigated temperature variation range, the stress level of the bend structure is mainly associated with the working-fluid pressure, whereas temperature affects the material allowable stress and safety margin. During load reduction from 100% BMCR to 3% BMCR, the maximum stress of the original 90° single-bend structure decreased from 142.84 MPa to 81.46 MPa, with critical stress regions mainly located at the inlet support and bend section. Structural optimization indicated that increasing the single-bend angle could alleviate inlet stress concentration, although the overall stress reduction remained limited. By contrast, the optimized 45° double-bend configuration provided a more effective stress-relief mechanism, reducing the maximum stress from 142.84 MPa to 93.52 MPa under the high-load condition and decreasing the stress difference between representative operating conditions. A supplementary stress-range-based fatigue-damage indication further showed that the optimized structure reduced the relative fatigue-damage tendency by approximately 71.9%–94.8% when the fatigue exponent varied from 3 to 7. These findings provide a practical case-based reference for thermal-structural reliability assessment, structural optimization, and reduction in relative fatigue-damage tendency of high-temperature superheater bends in CFB boilers under flexible operating conditions.