The heat balance of gas turbine combustors is used for determining the average Combustor Exit Temperature CET. It is important for designing the hot parts in this area. Direct measurements of the CET is difficult or impossible and therefore it is typically evaluated based on a 1-D cycle calculation, in which the combustor receives compressed air and fuel and it discharges the hot combustion gas at the temperature CET.

In the classic approach the fuel heat received in the combustor is evaluated based on the lower heating value LHV of the fuel and after the complete combustion the mixture of excess air and combustion products leaves the combustor at the temperature CET, which is calculated based on its specific enthalpy function.

So far so simple but this is tricky. The reaction energy is not the LHV but the higher heating value HHV, which includes additionally the discharged energy for condensing the combustion water at ambient temperature. The total heat comes into the flue-gas in the combustor, which is designed for a combustion efficiency of typically 99%+. There is no significant downstream reaction known, which could add the missing difference of HHV-LHV. In GT based power stations condensation is mostly avoided by sufficiently high stack temperature. But condensation it is often visible in the wake of high flying aircrafts. For methane as a fuel the HHV is around 11% higher than the LHV and the derived CET for a given fuel mass flow rate therefore becomes considerably higher, typically in the order of 100K. The consequence of this is a disturbing underestimation of the real average CET when using the LHV.

The author has never got a convincing evidence for the current practice with the LHV. The intention of this paper is an attempt to understand this practice historically.

Up to now gas turbine catalogues indicate performance data based on burning pure methane. This may have its historic roots in the statement that methane (only Methane, not higher hydrocarbons) burns with oxygen without a specific volume change. This simplified the cycle calculation in the sense that combustion could be modelled by adding the LHV to a mixture of air and methane and by calculating the expansion of this mixture in the turbine correspondingly. At ambient temperature this fuel-air mixture is still gaseous and therefore the heat balance of the GT matches exactly with the LHV because there is no condensation issue.

Another feature of the air compensated the CET mistake partly when using the LHV. It is the effect of dissociation. This increases the specific heat and therefore reduces the calculated CET. In the older time the used specific heat function of air did not include the dissociation effect while nowadays it is frequently included.

In this paper the good match of a cycle calculation considering the HHV and dissociation with published OEM data will be demonstrated. Indeed this method contradicts existing standards and practices and a further discussion considering the evidence shown below is welcome.


How Is a Correct GT Combustor Heat Balance Established?

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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