In this paper, problems relating to the Approach to Adiabatic Saturation Temperature (AAST) and the semi-dry Flue Gas Desulfurization (FGD) system operation reliability have been studied. The first part of the paper is to clarify some confusion with regard to AAST. The correlations and differences among 'Adiabatic Saturation Temperature (t(as))', 'Wet Bulb Temperature (t(w))' and 'Dew Point Temperature (t(d))' have been compared and analyzed. A humidity chart has been specially made to describe the humidification process in a semi-dry FGD reactor and to analyze the significance of the parameter, AAST, in details. It can be concluded that the process in the semi-dry FGD reactor is not an,ideal adiabatic humidification process, and the parameter of the Approach to Dew Point Temperature instead of the Approach to Wet Bulb Temperature can play a similar important role as AAST in the semi-dry FGD process. The second part of this paper discusses the sensor fouling and corrosion problems during measurement in the present control of the amount of water sprayed. A new control scheme with its modeling mechanism based on the flue gas humidity chart has been developed in this paper. The required control parameter, AAST, can be obtained through a soft sensor method. (C) 2004 Elsevier Ltd. All rights reserved.