1. A control system for controlling operation of 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, said plurality of sensors including a plurality of oxygen sensors spatially separated in a boiler of the power generating unit, wherein said plurality of oxygen sensors provide an oxygen profile; a combustion optimization system for determining optimal setpoint values for a first set of manipulated variables associated with a combustion process of the fossil fuel fired power generating unit, said combustion optimization system controlling the oxygen profile by determining an individual deviation of each of the plurality of oxygen sensors from an overall average oxygen value determined from the plurality of oxygen sensors; an oxygen optimization system for determining optimal setpoint values for a second set of manipulated variables associated with air flow for the combustion process, wherein said combustion optimization system and said oxygen optimization system each includes: (a) a model for predicting values of controlled variables associated with operation of the power generating unit, the model comprising: a plurality of inputs for receiving input values associated with manipulated variables and disturbance variables of the power generating unit, and a plurality of outputs for providing a plurality of output values indicative of predicted values for the controlled variables; and (b) an optimizer for determining the 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 of the manipulated variables, wherein said optimizer determines the optimal setpoint values in accordance with at least one goal and the at least one constraint associated with operation of the power generating unit; and control means for controlling operation of the power generating unit using optimal setpoint values for the first and second set of manipulated variables as respectively determined by the combustion optimization system and the oxygen optimization system.