Abstract: A method and system for supplying compressed air to a process plant using a combustor-turbine unit directly coupled to a bull gear meshing with pinions on which are mounted gas compression stages and expansion stages. Some stages compress a stream of air supplied to the combustor-turbine unit for combustion and to the process plant. Other stages expand or compress other gas streams directed to the combustor-turbine unit or to external applications. Direct energy transfers and intercooling and aftercooling after compression stages enhance the efficiency of the system.