Gas Turbine Gas Fuel Composition Performance Correction Using Wobbe Index

Abstract

Gas turbine thermal performance is dependent on many external conditions, including fuel gas composition. Measured performance must be corrected to specified reference conditions prior to comparison against performance specifications. A performance correction for fuel composition is thus required. One current method of correction commonly used is to characterize fuel composition effects as a function of heating value and hydrogen to carbon ratio. This method has been used in the past within a limited range of fuel composition variation around the expected composition, yielding relatively small correction factors on the order of ±0.1%. With industry trends suggesting continued exposure of gas turbines to a broader range of fuels such as liquefied natural gas and synthesized low BTU fuel, the corresponding performance effects will be much larger. As a result, a more comprehensive correction methodology is required to encompass a broader range of fuel constituents encountered. Analytical studies have been completed with the aid of thermodynamic models to identify the extent to which the Wobbe index can be used to correlate the response of gas turbine performance parameters to fuel gas composition. Results suggest that improved performance test accuracy can be achieved by using the Wobbe index compared with the aforementioned conventional fuel characteristics. This proposed method remains compliant with intent of internationally accepted test codes such as ASME PTC-22, ASME PTC-46, and ISO 2314.