A novel ultra-supercritical circulating fluidized bed (CFB) boiler technology based on recycling the high-temperature internal solids in the dense-phase zone to heat final superheater and reheater was investigated in this paper. The merit of this technology was that it can guarantee the superheated and reheated steam outlet temperatures especially under the low load condition. The flow characteristics of internal solids circulation was investigated on a cold test rig using the circulating solids from the Baima's 600 MW supercritical CFB boiler and the valid internal solids circulation rate was measured by optical fiber probes. The effects of fluidizing air velocity, partition height and static main bed height on the valid internal solids circulation rate were studied. Moreover, the valid internal solids circulation rate of thermal state was measured on the Baima's 600 MW supercritical CFB boiler. By the cold and thermal tests, an empirical formula which could predict the valid internal solids circulation rate was summarized and refined. In addition, the heat exchange ability of internal solids circulation was analyzed and the needed internal solids circulation rate of actual boiler was calculated. Besides, practicable schemes of this technology were proposed in the end of the paper. Based on the above analysis, it was proved that the novel technology using the internal solids circulation was feasible to guarantee the temperatures of final super-heated and reheated steam in an ultra-supercriticalCFB boiler. (C) 2018 Elsevier B.V. All rights reserved.