Hi.

I have few questions about how gas turbine works at part load when DLN I combustion system is employed. This is Frame 9 gas turbine - PG 9171E

Last week a had an appointment with peoples who work in gas turbine combined cycle power plant. They use Pre-select load control while operating at part load on Droop Speed Control. The position of inlet guide vanes is automatically modulated to control fuel/air ratio in combustor.

As I said in my latest post I am trying to develop a mathematical model of this gas turbine. Now I am thinking how to simulate a behavior of unit at part load when IGV are modulated to control combustion process (amount of air supplied to DLN I to avoid flame instability) and Pre-select load control is enabled and active (not IGV temperature control). I found here:

http://www.control.com/thread/1403944262

a following sentence (second post):

Two last things. First, if the turbine you are working on has DLN (Dry Low NOx) combustors, IGV exhaust temperature control is not operator-selectable; it's always on.

Questions are:

1.How gas turbine operates at part load (with DLN combustor)? IGV are modulated to control fuel/air ratio in DLN I or to maximize gas exhaust temperature? I am really confused about it. What happens with speed control?

2.If at part load amount of fuel is controlled by speed control (in this case Pre-select load control) how a position of IGV and exhaust temperature are changing while load is changing?

3.Can I assume that at part load unit operates using IGV exhaust temperature control? If this is true, actual exhaust gas temperature TTXM equals TTRX (for particular CPD)?

Best regards
Marcin

 

The 7FA+e DLN combustion system does indeed have Operator-Controlled exhaust temperature--the temp can be set from 700 deg F to whatever the max exhaust temp of the GT has--depending on the mfg of the HRSG the GTs are exhausting into.

The GTs at part load here would be controlled by the steam demands of the turbine accepting steam from the HRSGs.

 

Marcin,

I am sure that CSA will have a reply for you that will contain a longer and more detailed explanation, but I will try to give you a short version that attempts to answer part of your question.

Please note that the reply that Sqgoat gave discusses a GE 7FA machine which uses a completely different type of combustion system, and in my opinion does not directly apply to your question.

Your PG9171E uses the DLN 1 style of combustion. This system operates in a non-premixed (diffusion burn) flame during startup and low load conditions. This style of combustion results in high Nox output, but stable flame. Once turbine load is high enough, the unit will transfer into "pre-mixed" or low emission mode.

During the time the unit is operating in diffusion mode the control of the air/fuel ratio is not as critical for controlling flame stability and emissions. When operating in this low load, diffusion burn mode, it is possible to select a mode where the IGV's can be opened to a higher angle to reduce the exhaust temperature. Thereby attempting to control the exhaust temperature for HRSG requirements. But note there are limitations to this type of operating. If turbine load is too low, then trying to heat up the exhaust may be limited due to the minimum requirements of the IGV angle for compressor protection. My point is there is a limited area that the unit can be operated in for "Exhaust temperature control".

Once the unit load is raised sufficiently and the unit transfers into "pre-mixed" or low emission mode, then IGV angle and Inlet Bleed Heat, if equipped, is used to control air flow into the combustor for proper air/fuel ratio, and control the temperature of exhaust gas as it enters the 1st stage turbine nozzle. The first stage nozzle temperature is the limiting factor of the machine and is a calculated number based on measured exhaust temperature and calculated airflow.

Simply and quickly put. The machines IGV's can be used to control exhaust temperature within certain criteria at low loads when operating in diffusion burn mode.

Once the unit load is high enough to transfer into pre-mixed mode then exhaust temperature is a result of controlled airflow into the combustor to for emissions, flame stability, and compressor protection.

The other part of your question regarding speed control, also the short answer.

The control system will admit fuel to match the load being requested, since once the unit is tied to the grid, the actual speed of the machine is tied to grid frequency. In this case basically the IGV's are following the machine as it changes fuel input to control load output. As more fuel is admitted, exhaust temperature will rise, and the IGV's will open as needed. As fuel is reduced the exhaust temperature will fall, and the control system will respond by slowly closing the IGV's to reduce airflow and bring exhaust temperature back up.

This is a VERY simple answer to your complex question, but I hope it helps some.

Lastly just an FYI as to the 7FA machines with DLN2.6 or newer combustion hardware. These machines use fuel nozzles that are all basically "pre-mixed", they do not rely on moving flame from the primary zone to the secondary zone that the DLN1 technology uses to "pre-mix" part of its fuel.

 

Drop me a note to [email protected] and I can send you a document on DLN1 which will help.