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Measurement of temperature profiles of gas-solid two-phase flow at different heights in commercial-scale circulating fluidized bed (CFB) boilers was carried out. Experimental results showed that the thickness of thermal boundary layer was generally independent of the distance from the air distributor, except when close to the riser outlet. Through analysis of flow and combustion characteristics in the riser, it was found that the main reasons for the phenomena were: 1) the hydrodynamic boundary layer was thinner than the thermal layer and hardly changed along the CFB boiler height, and 2) both radial and axial mass and heat exchanges were strong in the CFB boiler. Numerical simulation of gas flow in the outlet zone confirmed that the distribution of the thermal boundary layer was dominated by the flow field characteristics.