Method of maintaining a fuel Wobbe index in an IGCC installation

A method of reducing a modified Wobbe index of a fuel stream fed to diffusion combustors of a gas turbine to be used in an IGCC installation in which a nitrogen stream is fed into head ends of the combustors to lower NOx produced by combustion within the combustors and the modified Wobbe index of the fuel stream has been increased in an amount that is greater than at least about 10 percent of a design Wobbe index for the gas turbine due to a retrofit of the IGCC installation involving conversion of carbon monoxide within the fuel stream to hydrogen and carbon dioxide by a water gas shift reaction and removal of the carbon dioxide prior to the introduction of the fuel stream into the combustors, said method comprising:

• diverting the nitrogen stream to the fuel stream so that less nitrogen is introduced into the head end of the combustors after the retrofit by dividing said nitrogen stream into first and second subsidiary streams;
• the first subsidiary stream being fed into the head end of the combustor;
• the second subsidiary stream being combined with the fuel stream prior to the introduction of the fuel stream into the gas turbine combustor;
• and
• the second subsidiary stream having a subsidiary flow rate that is selected to at least partially lower the modified Wobbe index of the fuel stream after mixing thereof with the second subsidiary stream to within about 10 percent or less of the design modified Wobbe index.