It is not uncommon for gas turbines to be required to operate on fuel gases with high inert gas contents. The inert gas is usually carbon dioxide (CO2) or nitrogen (N2), and either occurs naturally in the gas or is present as a result of tertiary Enhanced Oil Recovery processes (CO2 or N2 injection)
While the primary concern of using high inert gas fuels is in the ability to combust the fuel gas, consideration needs to be given to other áreas of gas turbine design and performance on which high inert gases may have an impact.
Assuming the combustion issues are resolved, the low calorific value of a high inert gas fuel means that more fuel needs to be fed into the gas turbine to achieve the design power output. This additional fuel flow means that modifications must be made to the fuel supply system and fuel injectors, while the additional fuel flow also increases the mass flow across the turbine stages, affecting gas turbine performance. Starting the gas turbine with ‘weak’ fuel gases is an area of special significance; whilst it is possible in some case to ignite and operate on such fuels, the transient operation may result in ‘lean blow-out’. Alternative fuels, such as good quality kerosene, may not be a viable solution for Oil & Gas applications so gas enrichment to a fuel quality point where satisfactory transient operation is known to occur will be demonstrated.
This paper looks at all the effects of high inert fuel gases on all aspects of gas turbine performance and design, looking at the necessary changes from a unit operating on a pipeline quality natural gas, and the operational limitations, such as compressor surge margin and heat transfer on cooled blades, and why these limitations vary from gas turbine model to model. There are occasions where fuels contain so much inert species that some pre-treatment is required to make these more suitable for gas turbine use, and a brief introduction to some of these methods is provided.