Extension of gaseous fuel flexibility for an industrial gas turbine engine is reported in this paper. A Siemens SGT-400 engine with 13.4 MW rating with hybrid combustion system configuration was string tested to operate on and switch between three different gaseous fuels with temperature corrected Wobbe Index varying between 45 MJ/m(3), 38 MJ/m(3) and 30 MJ/m(3). The alteration of fuel heating value was achieved by injection or withdrawal of N-2 into or from the fuel system. The results show that NOx decreases for fuels with lower heating value at the same engine output power and most importantly the engine can maintain stable operation on, and switching between these three different fuels with heating values changeover rate faster than 10% per minute without shutdown or change in load condition. The effect of gaseous fuels with different compositions and heating values on turbine entry temperature, NOx emission and combustion was further investigated in a high pressure combustion testing rig and it is found that the peak temperature and temperature gradient of turbine entry temperature are reduced by application of lower heating value of fuels. It is also noted that for the fuels having the same heating value different diluent, either CO2 or N-2, has insignificant effect on hot gas temperature profile, however, different diluent does have impact on NOx emission and NOx emissions is lower by up to 5 ppm if CO2 is selected as diluent compared to N-2 as diluent. The physical and chemical mechanisms for NOx reduction for gas turbine engines operating on lower heating value fuels have been identified: improving fuel and air mixing by stronger fuel to air momentum ratio and enhanced turbulence level, slowing chemical reaction by decreasing laminar flame speed and reduction of thermal NOx due to decrease in adiabatic flame temperature and flame shifted to downstream.