Applying Advanced Digital Control to the Recovery Boilers

Recovery boiler is an expensive and critical subprocess in the sulphate pulp mill. It has partly two contradictory functions; it is a chemical reactor and a steam generator. The recovery boiler converts the used chemicals to a reusable form and utilizes the heat produced by the burning process for steam production. In addition to this the environmental load must be considered by minimizing dust and gaseous emissions. The dynamics and computer control of recovery boilers has been studied since the beginning of 70´s. The example model of recovery boiler described in this paper is based on mill experiments and it is an overall discrete state space model, which is updatable with the process computer. The model includes all the process variables affecting on the emissions of the recovery of chemicals and on the steam production. The main part of the paper deals with the design of multivariable controllers for the recovery boiler using the above model. The design in based on the linear quadratic control theory. The different multivariable controllers include the multivariable P, PI or PID feedback control, the feedforward control from the measurable disturbances and from the set points. Some simulation results of the controllers will be given. The simulated results and some comparisons between different structures of the controller will also be included. Finally a comparison between the conventional digital algorithms i.e. the digital algorithms as applied in industry to the control of recovery boilers (mainly SISO plus some MISO) and the new strategy will be given.