The steady-state model of a molten carbonate fuel cell (MCFC) energy plant can be formulated with differing degrees of detail and implemented within any of the frameworks available today, but it is difficult to obtain an efficient, lean, robust simulation tool also capable of providing sensitivities. If such a tool were available, there would be much to gain in applying it on-line to support the operation of these plants. In this paper we present a new system simulation tool for MCFC energy plants, based on a simplified model and implemented with the general purpose C++ process simulation library LIBPF. We have tested our approach for an application example based on a reference hybrid 1 MW(el) MCFC-gas turbine energy plant. With the new tool it proved possible to analyse off-line the degrees of freedom when all operational constraints are considered, and to study the system part-load behaviour. The tool is also suitable for integration in an on-line application. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.