A modified gas turbine system is provided that adapts an advanced pressurized fluidized bed combustor cycle for use with existing combustion turbine technology. To accommodate current combustion turbine technology, the air extracted from the compressor discharge is limited, e.g. to about 20% of the total. The additional air required for the APFBC cycle is provided by an auxiliary source of compressed air. The compressor discharge air that is not extracted is used for combustion of the fuel gas produced by the carbonizer and also provides internal cooling as required by conventional combustion turbine design. A separate expander turbine is used to recover energy from the vitiated air from the PFBC. Energy from the vitiated air may further be recovered by directing the expander exhaust to an HRSG.