Recovery Boiler Modeling: An Improved Char Burning Model Including Sulfate Reduction and Carbon Removal

Abstract

This paper describes an improved model of char burning during black liquor combustion that is capable of predicting net rates of sulfate reduction to sulfide as well as carbon burnup rates. Enhancements include a proper treatment of CO2 and H2O gasification, reactions between oxygen and combustibles in the boundary layer, and integration of sulfate reduction and sulfide reoxidation into the char burning process. Simulations using the model show that for typical recovery boiler conditions, char burning behavior is independent of oxygen concentration up to the point of carbon depletion. Under these conditions, H2O and CO2 gasification reactions are primarily responsible for carbon removal. The H2 and CO coming from the gasifying particle consume oxygen in the boundary layer and help protect against sulfide reoxidation. After carbon depletion, sulfide reoxidation occurs at a rate determined by oxygen mass transfer. The process variables having the biggest effect on char burning behaviour are initial black liquor drop diameter and temperature. There is a direct tie between char burnout times and the amount of sulfate reduction. Increasing drop size increases char burnout times and the extent of reduction. Increasing temperature gives shorter char burnout times but higher reduction. At a given temperature, any variable that shortens the char burnout time will result in proportionately less reduction. There are some indications that the model underpredicts reduction rates. There remains a need for experimental data on sulfate reduction kinetics under typical char burning conditions.