Gas Turbine Gas Fuel Composition Performance Correction Using Wobbe Index

Gas turbine thermal performance is dependent on many external conditions, including fuel gas composition. Variations in composition cause changes in output and heat consumption during operation. Measured performance must be corrected to specified reference conditions prior to comparison against performance specifications. The fuel composition is one such condition for which performance corrections are required. The methodology of fuel composition corrections can take various forms. 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%. Industry trends suggest that gas turbines will continue to be exposed to broader ranges of gas constituents, and the corresponding performance effects will be much larger. For example, liquefied natural gas, synthesized low BTU fuel, and bio fuels are becoming more common, with associated performance effects of +/− 0.5% or greater. As a result of these trends, performance test results will bear a greater dependency on fuel composition corrections. Hence, a more comprehensive correction methodology is required to encompass a broader range of fuel constituents encountered. Combustion system behavior, specifically emissions and flame stability, is also influenced by variations in fuel gas composition. The power generation industry uses Wobbe Index as an indicator of fuel composition. Wobbe Index relates the heating value of the fuel to its density. High variations in Wobbe Index can cause operability issues including combustion dynamics and increased emissions. A new method for performance corrections using Wobbe Index as the correlating fuel parameter has been considered. 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 the gas turbine performance parameters to fuel gas composition. Results of the study presented in this paper suggest that improved performance test accuracy can be achieved by using Wobbe Index as a performance correction parameter, instead of the aforementioned conventional fuel characteristics. Furthermore, a relationship between this method’s accuracy and CO₂ content of fuel is established such that an additional correction yields results with even better accuracy. This proposed method remains compliant with intent of internationally accepted test codes such as ASME PTC-22, ASME PTC-46, and ISO 2314.