1. A control system for controlling oxygen in flue gas produced by a boiler for a fossil fuel fired power generating unit, said control system comprising: means for receiving data signals from a plurality of sensors for sensing parameters associated with operation of the power generating unit; a model for predicting oxygen in the flue gas produced by the boiler, the model comprising: a plurality of inputs for receiving input values associated with manipulated variables and disturbance variables associated with the power generating unit, and one or more outputs for providing output values associated with controlled variables, at least one output providing a first output value indicative of predicted oxygen in the flue gas in accordance with the input values; an optimizer for determining optimal setpoint values for each of the manipulated variables by accessing said model to minimize a cost value of a cost function while observing at least one constraint associated with operation of the power generating unit, said cost value affected by the value for each said manipulated variable, wherein said optimizer determines the optimal setpoint values in accordance with at least one goal and the at least one constraint, said optimal setpoint values producing optimal air flow into the boiler; and means for controlling operation of the power generating unit using the optimal setpoint values for the manipulated variables.