MASTER'S THESIS Control Structure Evaluation for a BFB Bark Boiler

This thesis goes into the task of analysing a bubbling fluidized bed (BFB) bark boiler and the structure of its automatic control system. The boiler being discussed is located at Billerud Karlsborg AB, although the results are to be able to be used on any BFB bark boiler. The main objective is to try to answer the following question: Exists there any changes to be made to the current structure of the control system that will add any significant improvements in the operation of the boiler, and if so what are those changes? As no experiments were able to be carried out on the boiler, the analysis were done with measurements from a normal day of running. This lead to rather inaccurate results in the modeling of the boiler, especially concerning the modeling of the bed temperature. The control structure evaluation was nevertheless carried out, as also weaker process models can point on improvements to be made. RGA analysis made it clear that with the current process knowledge, the decentralized control structure currently in use were the only decent option. HIIA analysis, although with uncertain results because of the modeling work, indicated on the possibility that the oxygen content controller could be complemented with a feed-forward signal from the bark and basic air flow input.