A non-stoichiometric equilibrium model based on free energy minimization is developed to predict the performance of gasifiers. The model considers five elements and 44 species in both the gas and solid phases. The gas: composition and heating values vary primarily with temperature and the relative abundance of key elements, especially carbon, hydrogen and oxygen. Pressure only influences the result significantly over a limited temperature range. The model predicts the onset of formation of solid carbon where the gas composition becomes insensitive to additional carbon. The carbon formation boundary is plotted in C-H-O ternary diagrams as a function of temperature and pressure. When the experimental carbon conversion is introduced, the kinetically modified equilibrium model gives good predictions of the gas composition from an air-blown pressurized circulating fluidized bed gasifier for two coals. The role of water, including both fuel moisture and steam injection, is examined based on a water balance on the feed and product gas to evaluate the steam demand. (C) 2000 Elsevier Science Ltd. All rights reserved.