| A modular steady state recovery furnace model has been developed to predict the reduction ratio, temperature distribution, pressure drop and dust loading throughout the furnace. The combustion zones in the furnace are represented by an adiabatic free energy minimization calculation. Kinetic modifications are made on reduction ratio, carbon fraction burned and sulfur loss. Convective and radiant heat transfer details are used throughout the fire box and each individual tube section. The model is a powerful tool to study the effect of changes in operating conditions, such as firing rate, liquor firing temperature, feed composition, heating value and firing with high solids content.; Model validation is based on data collected from an actual mill recovery furnace. Special measuring equipment and data logging computers provide a detailed and reliable data base for validation. The mill data shows that liquor firing rate has a strong influence on the gas temperature profile. Sulfate reduction appears to be controlled by equilibrium at high flow rates of primary air and controlled by kinetics at low flow rates.; Both the carryover and fuming mechanisms are important in fireside deposits because the enrichment of chloride and potassium in dust deposits can not be explained by either mechanism alone. Potassium contamination alone in the black liquor feed is not as critical as chloride to tube plugging. It is best to control the chloride level in the virgin liquor to below 0.8 wt% so that the Na(,2)Cl(,2) concentration in fireside deposits is below 7 mole%. Several alternatives are suggested to reduce the sodium chloride concentration in the dust deposits. Low primary air to convert the NaCl(s) in the dust to HCl(g) through the reaction with SO(,2) is one way to raise the sticky dust temperature and reduce deposits in the boiler tube bank. |
