The rate process of H 2S emission during steam gasification of black liquor char

Abstract

The chemical recovery cycle, a key component of a kraft pulp mill, produces energy and regenerates the active pulping chemicals, NaOH and Na 2S, from the spent pulping liquor (called black liquor). The development of gasification-based recovery technology requires information on the rates of several interrelated chemical reactions, most importantly the gasification of carbon and sulfur in the char. The sulfur emission rate during steam gasification of black liquor char was studied as a function of temperature (600–700°C) and H 2O and H 2 concentrations. The major sulfur-containing gas generated is H 2S. Analyses of the experimental results obtained with a thermobalance system indicate that the H 2S concentration in a 0.5 mm high char bed is at thermodynamic equilibrium. Therefore, the rate of H 2S production is influenced by thermodynamics, the carbon gasification rate, gasification product distribution, and the mass transfer coefficient between the char bed and the surrounding gas phase. Based on thermodynamics, gasification kinetics, and a material balance, a simple expression for the H 2S emission rate is derived. Good agreement is obtained between the measured and predicted rates.