Modelling black liquor combustion with the constrained Gibbs energy method

A simulation model based on multiphase thermodynamics has been developed for the combustion of black liquor in a recovery boiler. The model provides a quantitative description of the observed super-equilibrium of excess sodium, potassium, sulphur, and chlorine in recoveryboiler fumes and the respective effects of super-equilibrium on the condensed phases. The methodology is based on the constrained free energy (CFE) technique, which is an extension of the conventional Gibbs free-energy minimization method used to estimate kinetically controlled and local equilibria of multiphase chemical systems. This work extends the Na-K-S-Cl-C-O-H-N system used for ash-chemistry studies by introducing additional constraints on the volatility of Na, K, S, and Cl. The methodology presented here enables simultaneous estimation of chemical reactions and heat of combustion in the recovery boiler.