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We operate a fleet of 4 GE Frame 6B GTs with MkV controls currently running in open cycle.

Recently, the fuel (natural gas) supplier switched to a different source with the new fuel having a lower C1 (methane) % and a higher CO2 content (~7% comapred to <2% in the previous fuel).

Although the GTs seem to operate normally and GE confirmed that the new fuel specs are acceptable, we noticed that the heat rates have increased quite significantly (from the normal 11500 kJ/kWh to ~13000 kJ/kWh).

The GTs were not retuned when the new fuel was introduced.

We suspect that the gas spec has changed significantly as to require the combustion to be retuned.

My questions are:
1. Is this assumption true?
2. What needs to be done to confirm that the GTs are not running at the optimum FAR.
3. What needs to be done to correct this.

Thanks,

Khairil

It seems that you are assuming that the calorific value of the fuel has remained the same. A fuel with a lower methane content and a higher CO2 content will have a lower calorific value. So more fuel will have to be pumped in the gas turbine to obtain the same shaft power output. However if you assume that calorific value of the fuel has remained the same, then your calculations will indicate that the gas turbine has degraded (higher heat rate). Do you have the calorific value of the new fuel?

I assume you have adjusted your heat rate calculations for the the lower heating value for your new fuel. I also assume you have compared your current heat rate to design heat rate parameters. A good measure for the need for tuning changes are the NOx and CO outputs from the turbine. Very high CO outputs are a sign of incomplete combustion and a need for tuning.