What is the difference between wet and dry lime-based FGD systems?
Wet and dry systems are distinguished mainly by the type of by-product produced immediately after capture of sulfur dioxide. In dry lime FGD, the product of reaction of lime with sulfur dioxide is a dry powder. In wet lime FGD, the reaction product is a liquid. In dry FGD, milk of lime is sprayed directly into hot flue gas containing sulfur dioxide. Sulfur dioxide reacts directly with lime, and the heat of the flue gas evaporates water in the milk of lime, leaving a dry by-product. The by-product is removed from the gas using either fabric filters in a so-called baghouse or in an electrostatic precipitator. In wet lime FGD, sulfur dioxide is removed from flue gas using a two-step chemical process, compared with dry FGD which is a one-step process flue gas containing sulfur dioxide flows into the side of and up through a cylindrical vessel called an absorber (see figure). A liquid mixture of water, products of previous reaction of sulfur dioxide and lime, including an alkaline salt (mag
