1. A model for predicting mercury emissions produced by a fossil fuel fired steam generating unit, the model comprising: a plurality of inputs for receiving input values for manipulated variables and disturbance variables associated with the steam generating unit; and at least one output for outputting at least one output value indicative of mercury emitted by said steam generating unit in accordance with the input values for the manipulated variables and the disturbance variables, wherein said model communicates said at least one output value to control means for controlling operation of said steam generating unit in accordance therewith.