what is the need of gas heating/temperature in DLN machines?

 

Gas fuel is heated for multiple reasons.

One is to ensure there is no condensation of liquids as the gas flows through valves and nozzles in the fuel system. It has been found that liquids in the gas fuel can lead to primary zone re-ignitions in DLN-I machines and flashbacks in DLN 2.n machines.

In my personal opinion (so, take this for what it's worth), if you can be certain the fuel will always enter the turbine fuel system (upstream of the SRV) with at least 50 deg F of superheat, the gas fuel heater is wasting energy to add more heat unnecessarily. But, that's <b>only</b> if you are 110% certain that the gas fuel will always have at least 50 deg F of superheat when it enters the turbine fuel system. Since a lot of sites don't know or can't be certain <b>over the life of the machine</b> that the fuel will always meet the specification, GE and their packagers have been providing gas fuel heaters with most DLN-I machines by default. So, if your gas meets the specification and you know when it might change, you do have some flexibility to change the operation of the gas fuel heater if you so choose.

Gas fuel is also heated in some machines to improve efficiency of the plant.

So it would be best if you have further questions on this topic to provide as much information about the unit(s) at your site and the characteristics of the fuel being burned.

You may also find some information about fuel heating in the Control Specification drawing provided with the Speedtronic turbine control system for your unit(s).

 

ours is a GE 9FA DLN 2+ machine. For premix operation we are raising the temperature of gas up to 165deg c. For maintaining the premix operation, gas temperature should be maintained between 105 to 180deg c. If the gas temperature is not within this range, machine will unload and premix operation lockout will come. My doubt is for what reason we have to maintain this temperature?

 

As CSA stated, it depends:

1. If the only fuel heater is the electric heater, then the reason is to insure sufficient superheat to avoid condensation. ANY liquids in the gas fuel are poison to the gas turbine. In addition to the reasons CSA stated, liquids burning downstream of the combustion zone can damage first stage nozzles and buckets.

2. If you have a performance fuel heater on a combined cycle application, then this heater improves (lowers) your heat rate. The performance heater heats the gas fuel using water from the outlet of the intermediate pressure economizer in the HRSG. When supplied, it may also be used to control the Wobbe index by adjusting the temperature setpoint based on specific gravity and LHV measurements from a gas analyzer.

 

ok ...its good...i got your point.. i am talking about the performance heater with IP economizer only. can you explain how it will affect the heat rate? because basically my discipline is elec. and working in operation thats y.....what is the need to maintain gas temperature in premix operation alone....

 

Unfortunately, I don't know the details on how this improves performance. My background is controls engineering. The systems engineering organization defined what we needed to do, but I don't recall the reason it helps performance. It may have to do with the DLN combustion system. There is a need to maintain the Wobbe index of the fuel within a certain range - the fuel nozzles are designed for a specified pressure drop based on fuel pressure, temperature, density and heating value. (Don't ask me what the physical meaning of the Wobbe index is, I don't know.)

I do know that using the IP economizer water as the heat source is more efficient than using an electric heater.

I am retired now, and don't have access to the documentation on this. If you can find GE's proposal for the project you might find your answers there - system descriptions are usually included. Also, at least if the control for the heater is in the Mark VI gas turbine control rather than the DCS, try looking at the Control Specification supplied for the gas turbine control.

 

Why does heating feed water improve performance of a boiler? Because it takes less fuel to heat the water to the same temperature.

I believe that's one of the reasons for heating gas fuel. Injecting relatively cold gas fuel into a combustor has to have something of a quenching effect on the combustion gas temperature, so it takes (marginally) more fuel to make the combustion gas temperature equal to the desired temperature. But when we're talking about the kinds of fuel flow-rates required for a Frame 9FA, that small amount can add up to a significant fuel savings over the period of many years. I think the heat rate increase is less than one percent, but, again, over many years that's a lot of money at high fuel flow-rates.

There is a group of gas turbines that uses exhaust heat to heat the air exiting the axial compressor before it enters the combustor. These machines are called 'regenerative' machines, and have higher performance (lower heat rates) than the same machines which don't heat the compressor discharge air before entering the combustor.

Lastly, there are machines that cool some portion of the air flowing through the axial compressor to improve performance. There are a lot of ways to improve the performance of various aspects of the gas turbine cycle, depending on lots of different factors and the availability of heating and cooling medium(s).

I believe that otised is exactly right about Wobbe indices of fuels. Once heated, some fuels (not all!) require different nozzle orifices than the same fuel at a lower temperature. So, to maintain the parameters within allowable limits that are so important to a DLN combustion system (NOx, CO, flame stability, and combustion dynamic pressure oscillations) some machines do not allow Premix Steady-state operation unless the fuel is heated to the proper temperature for nozzles it will be flowing through when operating in Premix Steady-state combustion mode.

Lastly, we aren't text-messaging each other using mobile phones. This is an engineering forum, and to communicate properly we need to use clear, concise, understandable and unambiguous terms. Not text-speak.