Pulp and paper mills represent a major platform for the use of abundant, renewable forest-based biomass as raw material. The pulping processes produce a large amount of black liquor solids, which is currently burnt in a conventional Tomlinson recovery boiler for recovery of energy and inorganic chemicals. This combustion technology can recover chemicals with good efficiency, and steam and power can be produced for the mills. However, Black Liquor Gasification (BLG) can be used to substitute for the combustion process for potential higher energy efficiency, lower greenhouse gas emissions, and more safety. With BLG technology, current pulp and paper mills can be extended into future biorefineries. In this work, a thermodynamic equilibrium model using Gibbs free energy minimization approach and the software FactSage are utilized to analyze the thermodynamic equilibrium constraints of the complex multiple phase reactions and the effects of different operating conditions during black liquor gasification. The modeling results can help better understand the black liquor gasification process and be useful in process modeling and analysis of the future BLG-based biorefinery.