Hello everyone. I would love to get some insight on why IGVs closing on a gas turbine increases its exhaust temperature.

Reading manuals, posts on forums, and O&M manuals, I've gotten to this so far: Reducing the airflow through the axial compressor on a gas turbine changes the air-to-fuel ratio (reduces air), enriching the mixture. However, according to my research, rich mixtures tend to burn at lower temperatures.

Am I on the right track here? Or am I missing something? I am by no means a combustion specialist/engineer. I'm just trying to understand what's going on in our gas turbine for some fine HRSG steam temperature control fine-tuning.

Thanks in advance!

 

When you reduce the fuel flow into a gas turbine, its energy output decreases. In your case, the gas turbine is driving a generator synchronized to a grid, and thus the shaft speed will not decrease when the fuel flow is decreased. If your machine was simple cycle (no HRSG), then the IGVs would not move and the airflow through the gas turbine would not decrease significantly, but the reduced energy output would result in a decrease in exhaust temperature.
In combined cycle applications where there is an HRSG making steam, it is usually desirable to maintain the steam temperature, albeit at a reduced flow because suddenly decreasing the steam temperature can harm the steam turbine. So, in order to maintain the gas turbine exhaust temperature, the IGV's are closed down to reduce the airflow through the gas turbine and the HRSG. (The enthalpy of the air is maintained but the mass flow is decreased.) Hope this helps.

 

@malédiction,

There are essentially two ways a combustion turbine can be operated: Simple Cycle and Combined Cycle. In Simple Cycle mode the machine is controlled to produce power without regard to the temperature of the exhaust gases. In Simple Cycle mode the gas turbine exhaust is usually directed to the "sky" through the exhaust "stack." In this case the exhaust gases--at rated power output can be around 1000 deg F--and that heat is lost and gone forever.

In Combined Cycle mode--so named because the exhaust is usually directed into an HRSG (Heat Recovery Steam Generator--a fancy name for a boiler to turn water into steam) producing steam to drive a steam turbine to produce electricity or for some other process (such as paper-making or food processing (drying vegetables, etc.). In this case the temperature of the exhaust gases are important as the exhaust gas heat is being used for some process (producing more electricity or making paper or drying vegetables, etc.) so maximizing the exhaust heat is desirable at gas turbine loads below rated power output (called Part Load operating condition).

As a gas turbine is loaded after synchronization the IGVs of most modern machines will open at some point to increase the ability of the gas turbine to burn more fuel and produce optimum power output. Machines operating in Simple Cycle mode keep their IGVs closed until reaching some specific exhaust temperature and the begin opening the IGVs as the machine is just open the IGVs at some specific exhaust temperature as the machine is being loaded--that exhaust temperature having been determined to be helpful in controlling combustion dynamics inside the machine where the fuel is being burned. So, for example, the exhaust temperature of a Simple Cycle machine increases from synchronization until it reaches a temperature of 900 deg F (as an example) and as more fuel is added to the machine to increase its load the IGVs will start opening to maintain 900 deg F until they are fully open. As the machine is loaded further the exhaust temperature will increase up to its maximum for the machine and ambient conditions and at that point the machine is producing optimum power.

A gas turbine operating in Combined Cycle mode behaves similarly--the IGVs are held at minimum operating angle as the machine is initially loaded after synchronization and the exhaust temperature increases as fuel flow and load increase. However this time the IGVs are held closed longer as the machine is loaded to maximize exhaust temperature as fuel flow increases. And as the machine nears it rated output and exhaust temperature the IGVs are opened until they are fully open and the machine is at rated power output for the ambient and machine conditions.

Keeping the IGVs closed longer during loading in Combined Cycle mode actually reduces the thermal efficiency of the gas turbine--but the thermal efficiency of the entire plant (including the HRSG and whatever the steam is used for) actually increases because the temperature of the gas turbine exhaust gases is higher and produces more and hotter steam. (In a Simple Cycle application, there is no need to control exhaust temperature (except to minimize combustion dynamics at part load) and efficiency isn't usually important or necessary as most Simple Cycle machines operate at or very near rated power output when the IGVs are fully open. Many Combined Cycle machine are operated at Part Load for various reasons and the overall thermal efficiency of the plant is important.)

Remember that there is between 3 and 5 times the amount of air required to burn the fuel when a gas turbine is running--gas turbines are essentially mass flow machines. Fuel is used to heat that air (and increase it's pressure) in a gas turbine, so reducing the amount of air for a given amount of fuel essentially makes the combustion gases--and ultimately--the exhaust gases hotter. And in a Combined Cycle application that is important and over the life of the Combined Cycle plant can save millions in fuel costs.

The whole process is actually a little more nuanced than that, but that's the 5-cent version.

Machines with DLN (Dry Low NOx) combustion systems use the IGVs to control air flow through the combustors to help achieve and maintain very tight air/fuel mixtures to reduce the formation of pollutants (primarily NOx and CO). And, while on older machines there is no air/fuel monitoring many decades of empirical data and laboratory testing have determined how machine exhaust temperature is related to combustion "chemistry" so it's cheaper and as effective as using fuel/air monitoring equipment.

Hope this helps add to otised's excellent description.