The kinetic analysis of the treatment of black liquor by a thermal process can be performed in the conventional overall kinetic approach. However, while the detailed reaction mechanism remains unknown, the experimental analysis of black liquor treatment by submerged plasma demonstrates that the operating conditions (catalyst, plasma gas, temperature...) modify the kinetics and render this approach inefficient. The phenomenological approach based on the severity factor developed for the kinetic analysis of lignocellulose decomposition, is then proposed as an alternative to the conventional approach. The 3-parameters model is validated for decomposition of formic acid and for sulfur oxidation.