Wet Limestone - gypsum flue gas desulfurization system (FGD system) is currently dominant in the domestic coal-fired power plant FGD system. FGD is designed to chemically remove sulfur dioxide in the flue gas leaving the flue gas will be purified. pH value as a measure of gypsum slurry pH, is an important technical parameters of the desulfurization process, at the same time as an environmental indicators relate with chemical reaction. Utilization of limestone, desulfurization efficiency and quality of gypsum slurry are closely related with the control of pH value. We usually control the pH value by controlling the mixing amount of limestone (CaCO3). Increasing the pH value can increase the amount of limestone, and vice versa reduction. On operation, the stability of pH value must be maintained. Increasing the pH value will increase the alkalinity of system and desulfurization efficiency, but the utilization of limestone will be reduced, and accelerate the formation of scale, ultimately affect the quality of gypsum. Reducing the pH value will increase the acidity of the system, improve the utilization of limestone, accelerate precipitation of gypsum crystals, but it will also accelerate the corrosion of the system, make the system is unstable, and the desulfurization efficiency will be greatly reduced. Therefore, the control of slurry pH value should seek the best value to meet the requirements under the premise of the desulfurization rate and generally the pH is controlled in the best range of 5.0-5.8.%湿法石灰石-石膏烟气脱硫系统（即FGD系统）目前在国内外燃煤电站FGD系统中占绝对优势。FGD的目的是用化学方法除去烟气中的二氧化硫而使烟气得以净化。pH值作为石膏浆液酸碱度的度量，是脱硫工序中一项重要的技术参数，同时可作为一项与化学反应相关的环境指标。石灰石利用率、脱硫效率和石膏浆液品质等与pH值的控制密切相关。通常我们通过控制石灰石（CaCO3）掺加量来控制pH值，要提高pH值，可增加石灰石添加量，反之则减量。运行中，必须维持pH值的稳定性。pH值升高会提高系统碱度和脱硫效率，但石灰石利用率会相应降低，并加速垢的生成，最终影响石膏品质。降低pH值会提高系统酸度，同时提高石灰石利用率，并加速石膏晶体析出，但会加速系统腐蚀，造成系统运行不稳定，脱硫效率也会大大降低。因此，浆液pH值的控制应在达到要求脱硫率的前提下谋求最佳值。一般控制pH在5.0-5.8为最佳范围。
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