Black liquor constitutes a huge energy potential. In order to improve the efficiency of a pulp mill, this study is focussed on borate autocausticizing, which has proved to work efficiently in recovery boilers. The leading idea is to complete an overloaded recovery boiler with a booster gasifier. In this configuration, the black liquor is gasified with air at low overpressure. Results regarding conventional black liquor gasification are close to the reality and very promising. Regarding black liquor gasification with borate, lack of data for orthoborate, like the Gibbs free energy, did not enable good results. The model so far is a good starting point for black liquor gasification studies, and needs to be improved as soon as new data on borates will be available