The estimation of acid dew point temperature (T-adp) of flue gas, which is mainly determined by SO3 concentration and steam concentration (C-H2O) in flue gas, is crucial for coal-fired boiler system design. In this work, the heterogeneous and homogeneous effects for the conversion of SO2 to SO3 in the oxy-fuel flue gas have been examined. An updated formula for T-adp estimation of flue gas was proposed, which is: T-adp = 186 + 20lgC(H2O) + 26lg(C-SO2* (127.65C(SO2)*(-0.585) +lg(Aar x alpha fly/C-SO2*)(1/2) x (15.191 + 0.071Fe + 0.044Al-0.058Ca-0.117Mg) x 10(-2))) The updated formula shows that the main effects of the T-adp were the steam concentration, initial SO2 concentration (C-SO2, ppm), ash content of raw coal (A(ar), %), fly ash proportion of the inherent coal ash (alpha(fly), %), main mineral contents of coal ash (Fe, Al, Ca, Mg, g/kg). The formula was then fully validated by a large amount of data obtained from laboratory-scale experiments, pilot-scale experiments and industrial scale coal-fired power plants. The results showed that, the heterogeneous effect for the conversion of SO2 to SO3 contributed to 133 to 20.86% of the total SO3 formed, smaller than the homogenous effect. Furthermore, the influence of the heterogeneous effect for oxy-fuel process was 1.2-1.6 times larger than that for the conventional air-fired process. The accuracy of the updated formula was proven much higher than the conventional empirical formula for air-firing combustion. The relative deviation of the calculation for oxy-fuel combustion by the updated formula was <7.8%. (C) 2016 Elsevier B.V. All rights reserved.