A method and a system for optimization of parameters for a recovery boiler in a pulp mill are disclosed. An exemplary method is based on use of a first principle mathematical model to estimate parameters that are otherwise not measurable, to accurately control the performance of the recovery boiler. In addition, a method is disclosed to use estimated parameters of a recovery boiler to control and stabilize the processes downstream of the recovery boiler. A system is also disclosed to carry out a method for control and optimization of performance and operational parameters of a recovery boiler.