1. A system for controlling operation of a fossil fuel fired steam generating unit, the system comprising: a model of the steam generating unit embodied in a computer readable medium, said model predicting mercury emissions of the steam generating unit based upon a plurality of input values that are current values for manipulated variables and disturbance variables associated with current operating conditions of the steam generating unit, and for outputting at least one output value indicative of mercury emitted by said steam generating unit in accordance with the input current values of the manipulated variables and the disturbance variables; and a computer control system for receiving the at least one output value from the model and determining optimal setpoint values for manipulated variables using the at least one output value of the model, and for controlling operation of the steam generating unit in accordance with the determined optimal setpoint values for the manipulated variables, said manipulated variables affecting mercury oxidation by influencing one or more of the following factors for oxidation of mercury: proximity of compounds reacting with mercury, residence time of mercury within a reaction temperature zone, and the presence of surface structures as they relate to conversion reactions, wherein said computer control system comprises: an optimizer for determining the optimal setpoint values for the manipulated variables to obtain a desired level of mercury emissions of the steam generating unit, said optimizer receiving said at least one output value from said model to determine the optimal setpoint values for the manipulated variables, wherein said optimizer determines a respective optimal setpoint value for each said manipulated variable by accessing said model to minimize a cost value of a cost function that is a mathematical representation of one or more goals, while observing a plurality of constraints, said cost value affected by the value for each said manipulated variable, said cost function including a predicted value indicative of elemental mercury emissions of the steam generating unit and a value associated with carbon in ash (CIA), said cost function including a term that decreases as the level of mercury oxidation increases, wherein said computer control system repeatedly provides the model with current values of the manipulated and disturbance variables associated with current operating conditions of the steam generating unit, in order to determine updated optimal setpoint values from the at least one output value of the model.