Gas turbine, liquid fuel

[MortenA]

Gents, hope to get some insights to the following design proposal:

- We are looking at a field development with excess NGL's and would like to use these (some of it) for fuel
- We will have an NGL splitter, a reboiled absorber
- We will have diesel back-up so that we dont stop the turbines if the NGLs are out (duel fuel type gas turbine)
- I was thinking: Is there a need for a special NGL buffer vessel or will the reboiler serve this purpose?

 

[ccfowler]

There is nothing particularly unusual about using a mix of gaseous and liquid fuels in older industrial GTs that were designed for such operation. (I'm not so sure about newer, high efficiency GTs.) The burners and control systems need to have been designed to handle both types of fuels simultaneously. Most likely, there may be some modest efficiency variations along with the differing fuel mixes, and satisfying newer emission regulations could get to be more than a bit interesting. In general, the heavier the liquid fuels, the more likely there will be nastier problems. The more the liquids and gases vary in their composition, the more flexibility will be required of the burner and control systems. Most likely, there may be some variation in maximum load capability depending on the fuel mix. Fuel cleanliness will always be important for both liquids and gases.

Based on experience with a couple of troublesome units, I would want to be assured of a very extensive array of individual exhaust temperature measuring devices (thermocouples or RTD's) around the entire exhaust annulus to assure truly uniform distribution of the fueling and firing to avoid damaging hot spots. Averaging of the individual measuring points is fine for control purposes, but the individual points must be monitored continuously to assure proper operation.

At the very least, this should be a very interesting project. Since part of the purpose is to make good use of otherwise "waste" products, this should be carefully included in all performance and emission considerations. It may change all of the rules and criteria for the project.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[MortenA]

@ccfowlwe,

Thanks for the advice! If theres anybody out there with experience from a similar setup ii would still like to hear about the buffer vessel issue (if the reboiler is sufficient).

Best regards, Morten

 

[ScottyUK]

I used to work on tri-fuelled machines burning natural gas, propane or naphtha. The latter was a rather variable brew from a neighbouring facility and was troublesome as a fuel source. cswilson is spot on about temperature monitoring - we used to see uneven burn patterns and tended to suffer spread and deviation trips on naphtha where they were fairly uncommon burning gaseous fuels. Naphtha used to place heavier demands on the NOx suppression steam too - what a DLN burner would make of it I'm not sure.

What is the supply temperature from the reboiler, and how stable is it? Fuel pre-heating isn't unusual but needs to be reasonably stable. Provided you can guarantee sufficient suction head at the HP fuel pump inlet and maintain it under shifting load conditions I don't see a problem - or are you considering not using an HP fuel injection pump and delivering high pressure fuel direct from process?

 

[MortenA]

@SCottyUK,

As i said its a development project and we are considering various means of getting rid of the excess NGL.

The simulation currently says something in the 20-30ºC range. We have good experience from using dual fuel gas/diesel turbines but none wrt NGL/diesel. Most likely the pressure in the reboiler will be high enough to avoid a transfer pumps.

Thanks for the advice.

Best regards, Morten