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Currently, the severe fluctuation of steam temperature in double-reheat (DR) ultra-supercritical (USC) units has become a major problem that constrains their efficient and flexible operation, wherein the deviation of coal calorific value from the design value is one of the major reasons for this issue. However, the effect of coal calorific value variation on the steam temperature characteristics during the flexible operation of DR USC units remains unclear. A dynamic model of a 1000 MW DR USC unit is developed with primary principles and validated with real operation data. The effect of the coal calorific value variation on the steam temperature characteristics is revealed. Results show that as the calorific value of the coal increases, the main steam temperature can always maintain at the design value with attempering water adjustments, while the two reheat steam temperatures decrease. The steady-state deviations of the reheat steam temperatures are positively correlated with the magnitude of the coal calorific value variation, where the deviation of the secondary reheat steam temperature is larger. Meanwhile, as the load increases, the steady-state deviations of reheat steam temperatures increase, but the response time decreases. Finally, the effect of calorific value variation on the boiler efficiency is studied. As the coal calorific value increases, the boiler efficiency firstly rises caused by the decrease in the loss of the flue gas and then falls caused by the limited furnace temperature increase. This study provides guidance on the DR USC boiler operation under coal variation. • A dynamic model of a 1000 MW double-reheat ultra-supercritical unit is developed. • The effect of the coal calorific value variation on the steam temperature characteristics is revealed. • The effect of calorific value variation on the boiler efficiency is revealed. • This study provides guidance on the double-reheat ultra-supercritical boiler operation under coal variation. [ABSTRACT FROM AUTHOR]