Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier

A non-stoichiometric equilibrium model based on free energy minimization is developed to predict the performance of gasifiers. The model considers fiv e elements and 44 species in both the gas and solid phases. The gas: compos ition and heating values vary primarily with temperature and the relative a bundance 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 com position becomes insensitive to additional carbon. The carbon formation bou ndary is plotted in C-H-O ternary diagrams as a function of temperature and pressure. When the experimental carbon conversion is introduced, the kinet ically modified equilibrium model gives good predictions of the gas composi tion from an air-blown pressurized circulating fluidized bed gasifier for t wo coals. The role of water, including both fuel moisture and steam injecti on, is examined based on a water balance on the feed and product gas to eva luate the steam demand. (C) 2000 Elsevier Science Ltd. All rights reserved.