IBH Valve Operating Mode at Part Load

F

Thread Starter

fhabbio

Hello everybody,

forgive my poor english, I'm an italian student and need of your help.
I'm not sure I understood working conditions of IBH valve.
Correct me if I'm wrong plz...

IBH valve allows a re-circulation of inlet air flow. This is bleeded from one of the last steps of compressor. So this hotter mass flow rate avoids ice formation and protects the compressor by stall.

Is it?

Now, I have a very HUGE doubt.
In addition to these functions, in CCGT power plant, IBH often is used to optimize operating modes at part load.

I understand that opening IBH valve increases air inlet temperature, so the temperature of flue gases exiting gas turbine raises as well as HP steam generation rate in HRSG.

But how is it possible?

Indeed I think that if the air inlet temperature raises, its mass flow rate decreases and at the same way exhaust gas flow decreases. Can higher temperature of flue gases leads to increase HP stream generation although exhaust gas flow decreases?

I hope I make myself clear enough.
Thx for reading...

If you can, suggest me some reference about IBH operating mode at partial load.
 
fhabbio,

Inlet Bleed Heating, at least as GE applies it for their heavy duty gas turbines, is used to protect the axial compressor when the IGVs (Inlet Guide Vanes) are closed below their normal minimum operating angle. The IGVs are closed below their normal minimum operating angle in order to allow the turbine combustors to remain in the lowest NOx formation mode over as wide a range of operation (load) as possible. For example, many GE-design heavy duty gas turbines can only operate in the lowest NOx emissions mode possible from approximately 80% of rated load to 100% of rated load. Using Inlet Bleed Heat and allowing the IGVs to close below their normal minimum operating angle allows the load range to increase to approximately 40%-100% while remaining in low NOx emissions mode. This can be a great economic benefit to many owner/operators. BUT, it's only possible when the IGVs can be closed below normal minimum operating angle.

On older turbines with non-DLN combustors it was common to open the IGVs during loading to maximum operating angle to keep the heat rate of the turbine as high as possible. This had the effect of making the gas turbine exhaust temperature relatively cool during part load operation--because the IGVs were open for a large portion of the load range.

Then, it was discovered that the gas turbine exhaust could be used to produce steam by flowing it into a waster heat recovery boiler, or an HRSG (Heat Recovery Steam Generator). And, then it discovered that by closing the IGVs as much as possible during part load operation that the gas turbine exhaust temperature would be higher than it would otherwise be which increases the steam production. Now, this has the disadvantage of worsening the heat rate of the gas turbine, BUT it increases the overall heat rate of the plant--which is good.

Then when DLN combustors came around, it was necessary to use the IGVs to more tightly control air flow through the turbine by limiting the air flow at part load. And this is done by closing the IGVs. And this has the effect of raising the gas turbine exhaust temperature--which is okay when the gas turbine exhaust is being used to produce steam.

AND, when its necessary or desirable to increase the load range of a DLN combustor-equipped unit while remaining in low NOx emissions mode it's necessary to close the IGVs below their normal minimum operating angle--which has the effect of raising the exhaust temperature. And, to close those IGVs below their normal minimum operating angle it's necessary to use IBH to protect the axial compressor.

IBH, by extracting some of the axial compressor discharge air flow and recirculating it back to the axial compressor inlet does two things. First, it reduces the air flow into the turbine (combustors and exhaust), which has the effect of slightly elevating the gas turbine exhaust temperature even further, but not by a lot. Second, it increases the axial compressor inlet temperature which is the primary desired effect because it protects the axial compressor. (Increasing the axial compressor inlet temperature reduces the density of the air flowing through the compressor--which is how the axial compressor is protected when the IGVs are closed below normal minimum operating angle.)

So, it's not really the IBH that increases gas turbine exhaust temperature. It's using the IGVs to limit air flow into the DLN combustors that causes the gas turbine exhaust temperature to be higher than it would otherwise be (if the IGVs were left to operate at their original design control scheme--which was to open them to their maximum operating angle earlier in the loading process). And, then to allow DLN combustor-equipped units to operate at low loads in the low NOx emissions mode it is necessary to close the IGVs below their normal minimum operating angle.

It's really all about the IGV angle--and how that affects the gas turbine exhaust. Closing the IGVs to maximize exhaust temperature helps to improve overall plant thermal efficiency (even though it slightly reduces gas turbine thermal efficiency). And when the IGVs have to be closed below their normal minimum operating angle it's necessary to use IBH to protect the axial compressor.

Hope this helps! Keep studying!

The prevention of ice on the inlet guide vanes when they are at their maximum operating angle is accomplished using much the same hardware (exactly the same in some installations). BUT, GE calls it Inlet Air Heating (not Inlet BLEED Heat), or Anti-Icing Control. And, because it reduces the air flow through the unit it has the effect of raising the exhaust temperature. EXCEPT that it's usually only used when the IGVs are fully open and the unit is already producing high exhaust temperature. BUT the undesirable effect of Inlet Air Heating/Anti-Icing Control is that it reduces the power output of the unit--which most owner-operators just absolutely hate!

So, it's really important to understand what the axial compressor discharge extraction air/re-circulation scheme is being used for--and when.

Again, hope this helps!
 
fhabbio,

Inlet Bleed Heating, at least as GE applies it for their heavy duty gas turbines, is used to protect the axial compressor when the IGVs (Inlet Guide Vanes) are closed below their normal minimum operating angle. The IGVs are closed below their normal minimum operating angle in order to allow the turbine combustors to remain in the lowest NOx formation mode over as wide a range of operation (load) as possible. For example, many GE-design heavy duty gas turbines can only operate in the lowest NOx emissions mode possible from approximately 80% of rated load to 100% of rated load. Using Inlet Bleed Heat and allowing the IGVs to close below their normal minimum operating angle allows the load range to increase to approximately 40%-100% while remaining in low NOx emissions mode. This can be a great economic benefit to many owner/operators. BUT, it's only possible when the IGVs can be closed below normal minimum operating angle.

On older turbines with non-DLN combustors it was common to open the IGVs during loading to maximum operating angle to keep the heat rate of the turbine as high as possible. This had the effect of making the gas turbine exhaust temperature relatively cool during part load operation--because the IGVs were open for a large portion of the load range.

Then, it was discovered that the gas turbine exhaust could be used to produce steam by flowing it into a waster heat recovery boiler, or an HRSG (Heat Recovery Steam Generator). And, then it discovered that by closing the IGVs as much as possible during part load operation that the gas turbine exhaust temperature would be higher than it would otherwise be which increases the steam production. Now, this has the disadvantage of worsening the heat rate of the gas turbine, BUT it increases the overall heat rate of the plant--which is good.

Then when DLN combustors came around, it was necessary to use the IGVs to more tightly control air flow through the turbine by limiting the air flow at part load. And this is done by closing the IGVs. And this has the effect of raising the gas turbine exhaust temperature--which is okay when the gas turbine exhaust is being used to produce steam.

AND, when its necessary or desirable to increase the load range of a DLN combustor-equipped unit while remaining in low NOx emissions mode it's necessary to close the IGVs below their normal minimum operating angle--which has the effect of raising the exhaust temperature. And, to close those IGVs below their normal minimum operating angle it's necessary to use IBH to protect the axial compressor.
*Question 1)
If IGV operating angle is decreased, why became exhaust temperature increase?

IBH, by extracting some of the axial compressor discharge air flow and recirculating it back to the axial compressor inlet does two things. First, it reduces the air flow into the turbine (combustors and exhaust), which has the effect of slightly elevating the gas turbine exhaust temperature even further, but not by a lot. Second, it increases the axial compressor inlet temperature which is the primary desired effect because it protects the axial compressor. (Increasing the axial compressor inlet temperature reduces the density of the air flowing through the compressor--which is how the axial compressor is protected when the IGVs are closed below normal minimum operating angle.)

So, it's not really the IBH that increases gas turbine exhaust temperature. It's using the IGVs to limit air flow into the DLN combustors that causes the gas turbine exhaust temperature to be higher than it would otherwise be (if the IGVs were left to operate at their original design control scheme--which was to open them to their maximum operating angle earlier in the loading process). And, then to allow DLN combustor-equipped units to operate at low loads in the low NOx emissions mode it is necessary to close the IGVs below their normal minimum operating angle.

** Question 2)

You mentioned that IGV operating angle should be controlled to close direction in order to satisfy low NOx emission at part load operation.
Why? Could you explain what is the relationship between IGV opening angle and NOx emission?

It's really all about the IGV angle--and how that affects the gas turbine exhaust. Closing the IGVs to maximize exhaust temperature helps to improve overall plant thermal efficiency (even though it slightly reduces gas turbine thermal efficiency). And when the IGVs have to be closed below their normal minimum operating angle it's necessary to use IBH to protect the axial compressor.

Hope this helps! Keep studying!

The prevention of ice on the inlet guide vanes when they are at their maximum operating angle is accomplished using much the same hardware (exactly the same in some installations). BUT, GE calls it Inlet Air Heating (not Inlet BLEED Heat), or Anti-Icing Control. And, because it reduces the air flow through the unit it has the effect of raising the exhaust temperature. EXCEPT that it's usually only used when the IGVs are fully open and the unit is already producing high exhaust temperature. BUT the undesirable effect of Inlet Air Heating/Anti-Icing Control is that it reduces the power output of the unit--which most owner-operators just absolutely hate!

So, it's really important to understand what the axial compressor discharge extraction air/re-circulation scheme is being used for--and when.

Again, hope this helps!

Hello,
This information above is very interesting because I wonder the relationship between NOx emission and IBH system.
I would like to ask some questions as BLUE letter. Please refer to above.
It would be highly appreciate if you reply to my queries above.
 
Question 1: Reducing the air flow through the machine by closing the IGVs while maintaining the same fuel flow-rate will cause the exhaust temperature to increase.

Question 2: The DLN-I combustor burns fuel in Premix Mode Steady State mode in an extremely lean fuel-air mixture, and it can only do so in a pretty small load range at the higher end of the load range. To reduce load, one has to reduce fuel, which because of the design of the DLN combustor will eventually lean out the Premix flame so much it will be extinguished unless the IGV angle is reduced to reduce the air flow through the machine and into the combustor. So, the minimum operating angle for most GE-design heavy duty gas turbines using DLN-I combustors is 57 DGA (DeGrees Angle), and while the IGVs can be closed to angles less than 57 DGA that can cause problems for the axial compressor.

SOOO, the designers came up with a way to recirculate a portion of the axial compressor discharge air to the inlet of the compressor when the IGV angle is less than 57 DGA using a poorly-named method called Inlet Bleed Heat. Yes, it raises the temperature of the axial compressor inlet air a couple of degrees, but it's MAIN PURPOSE is to protect the axial compressor when the IGVs are closed below 57 DGA. And, to remain in Premix Stead State combustion mode (the lowest NOx emissions mode) at loads lower than would otherwise be possible if the IGVs couldn't be closed below 57 DGA it is necessary to have a method of protecting the axial compressor. IBH is that method. Yes; closing the IGVs and restricting air flow also causes the exhaust temperature to increase (for the same fuel flow-rate), but since fuel flow-rate is changing during loading and unloading anyway it all works well together to allow the unit get into low emissions mode earlier than would otherwise be possible (by closing the IGVs below 57 DGA--which requires IBH to protect the axial compressor), and also allows the unit to remain in the lowest emissions mode at lower loads than would otherwise be possible if the IGVs couldn't be closed below 57 DGA.

IBH is about protecting the axial compressor when operating at rated speed when the IGVs must be closed to less than 57 DGA (the design minimum operating angle for GE-design heavy duty gas turbines using DLN-I combustors). It's not about anything more than protecting the compressor. And the compressor must be protected when air flow through it is restricted which it must be when it's necessary to be in the lowest emissions mode at low loads--loads lower than would otherwise be possible if the IGVs couldn't be closed below 57 DGA.

Hope this helps!

IBH is an extremely poor name for the function it provides. It (the name) confuses a lot of people.
 
Sincerely appricate for detail and kind reply!

My essential question was the reason why IBH is neccessary at part load with DLN combustor.
Thanks to your explanation, I think I understand and summarize as follows.
If there is incorrect informaton, please let me know.

For part load operation equipped DLN combustor, both reducing fuel and IGV angle are required.
If only fuel is reduced without IGV close, it can cause flame to be extinguished because originally DLN itself is operated
under lean fuel-air mixture condition and besides, the ratio of air to fuel will be more increase when fuel is reduced.
Thus, IGV also should be controlled to reduce inlet air flow rate. The lower load(MW) the unit produce, the more IGV close.
But, there is limited allowable range of IGV angle. It is important that IGV angle should be maintained over design minimum operating angle, otherwise it can affcet axial compressor such as rotating stall or surge.
Therefore, in order to protect the compressor below IGV minimum angle , IBH is neccessary.

In addition, I would like to ask two more questions.
1. For example, base load is 100MW and the part load at IGV design minimum angle is 80MW.
In case IBH system is not installed, if the operator input 60MW, I think DLN is operated at another mode not Premix
Stead State combustion mode (the lowest NOx emissions mode) because surge can occur if it is operated at Premix
Stead State mode. Is it correct? It not, please let me know.

2. Second is about standard combustor with De-NOx steam injection to reduce NOx emission.
I wonder if standard(diffuser type) combustor with steam injection has same IGV design minimum angle with DLN case.
Actually, our turbine is GE frame 5 with steam injection and can be operated at any load even though it does not have IBH. So, I guess there is difference between them.
If do you hava an idea why part load of standard combustor case is no problem, could you let me know?

Thank you again for warm response.
 
H.J Kim,

You've got it pretty straight.

1. You're correct. Typically, without IBH the unit can't get into Premix Steady State during loading and will transfer out of Premix Steady State (to Lean-Lean combustion mode) at approximately 80% of rated load. And, as you correctly noted, that's because the IGVs on a machine without IBH won't typically close to less than 57 DGA, so in order to have the most stable Premix flame possible (because it's inherently unstable to begin with (a characteristic of a very lean fuel/air mixture at just about any load/fuel flow) the load range (fuel flow range, really, into the combustor's "head end" (the primary combustion zone where premix combustion takes place)) is pretty limited--from approximately 80-100% is it. Yes, the IGVs could be closed to less than 57 DGA without IBH, BUT the risk of axial compressor damage without something to protect the compressor (IBH in GE's case) would be very high, uncomfortably high. I don't know if it's actually stall or surge; I know their technically different, but I think in many cases there can't be one without the other--at some point in the transient mode just before failure.

2. The design minimum IGV angle at rated speed for most GE-design axial compressors is 57 DGA (I'm not referring to F- or FA or 7F.0n machines--they are their own beast and when it comes to standards for them, they are all over the place so let's not (anyone) try to define a standard for them). I'm therefore referring to Frame 5, Frame 6B, Frame 7E/EA, and Frame 9E GE-design heavy duty gas turbine axial compressors with modulated IGVs. The DLN-I machines use, basically, the "standard" axial compressor design--meaning the typical minimum IGV operating angle at rated speed is 57 DGA, which is the same for the conventional (non-DLN-I; diffusion flame) combustor-equipped machines.

When Wet Low NOx is used (be it water- or steam injection--for limiting/reducing NOx emissions) for a machine with conventional combustors the IGV control is the standard IGV control. That is, it's either simple cycle (no waste heat recovery), or combined cycle (waste heat recovery)--whether Wet Low NOx is used or not. Similar to IGV control for DLN-I machines, IGV control in combined cycle mode is used primarily to maximize gas turbine exhaust temperature (to make as much steam as possible at loads less than rated/Base Load, when the IGVs are, by definition, always at maximum operating angle and the exhaust temperature is what it is).

Diffusion flame is MUCH more stable at any fuel flow-rate of a machine with conventional combustors, so modulating air flow isn't going to even approach causing flame instability. In a machine with conventional combustors and a single fuel nozzle, the flame "ball" is large and the fuel/air mixture in the combustor (which has only one combustion zone, instead of two as in the DLN-I combustor designs) is much richer. This means the flame is also much hotter--which is what creates more NOx (high temperature combustion). So, Wet Low NOx (Water- or Steam Injection) dilutes the flame temperature, which reduces the NOx formation.

There is usually a minimum fuel flow-rate for Wet Low NOx injection--and that's because below a certain fuel flow-rate the water flow-rate required to reduce emissions is so high that it causes very high pressure pulsations in the combustor because it's actually causing flame instability (the water flow-rate--NOT the air flow-rate). So, Wet Low NOx injection doesn't usually start until the fuel flow-rate is above about 30% or so of rated fuel flow during loaded operation. And, again, that's because the water would put out the flame--not the air flow. The fuel/air mixture is sufficiently rich it won't be extinguished by closing or keeping the IGVs closed, but the sheer amount of water required would extinguish the flame.

Hope this helps! IBH isn't required for conventinal combustor-equipped machines because the fuel/air mixture at all load ranges is sufficiently rich to prevent flame blow-out by limiting air flow. But, while you may not have realized it, the Frame 5 Wet Low NOx injection probably doesn't start immediately after the generator breaker closes (or even before the generator breaker closes). Meaning the NOx emissions are higher than when Wet Low NOx injection is running. In some parts of the world, there is a legal requirement to load a machine with conventional combustors to at least the minimum point at which Wet Low NOx injection is enabled in order to reduce NOx emissions as quickly as possible during starting and loading. Properly tuned, Wet Low NOx will maintain a fairly stable NOx output from shortly after Wet Low NOx injection starts all the way up to Base Load, and then back down to when Wet Low NOx injection is stopped.

IBH is required in order to get into Premix Steady State combustion mode as soon as possible when loading the machine, and to stay in Premix Steady State as long as possible (to loads as low as possible) when unloading the machine. Typically, IBH will allow the unit to reach Premix Steady State at loads around 40% of rate, and to remain in Premix Steady State when unloading from Base Load to as low as approximately 40% of rated. But, to do that--IBH is necessary to protect the axial compressor, because it's necessary to close the IGVs below 57 DGA--which is the typical design minimum IGV operating angle at rated speed. And, that's because the fuel/air mixture in the Primary combustion zone of the DLN-I combustor (which is where most Premix combustion occurs) is very lean to begin with, and if fuel is reduced below a certain point without also reducing the air flow (which can only be done by closing the IGVs) the Premix combustion flame will go out, and the unit will usually lose load VERY quickly, and may even trip. And if it doesn't, well, that means that a LOT of raw, unburnt fuel is entering the secondary combustion zone of the DLN-I combustor, and could also enter the gas turbine exhaust, where it might collect and if ignited, could result in an explosion. And, if the fuel all burns in the secondary combustion zone (which is mostly diffusion flame) then the flame temperatures can cause the combustion liner to collapse--and that's also catastrophic.

DLN-I flame is a big balancing act. To reduce flame temperature without a diluent (water- or steam injection), it is necessary to have a very lean fuel/air mixture. And that fuel/air mixture is not very stable at the air flow-rates required to keep the flame temperature low. And, to complicate matters, the window of emissions reduction is also kind of small. When it's working, DLN-I works well; when it's not, well, it can be difficult to troubleshoot (mostly because the typically-supplied instrumentation is sparse and minimal) and if it's not working well and the unit is allowed to continue to run the resulting damage can be pretty bad.
 
Thank you very much!!
I have tried to find information about IBH valve's function at part load, however, I could not get a satisfying information before touching with you. Thanks to your professional and warm reply, eventually I had a good answer. Appreciate again!
 
H.J Kim,

I think I kind of mis-spoke about IGV operation in DLN-I part load operation. The IGVs aren't really used to maximize exhaust temperature--they are held closed to minimize the air flow into/through the unit at part load, which in turn maximizes the exhaust temperature (up to the maximum allowable exhaust temperature limit, TTRX). They can't be held closed any more than when TTXM (the actual exhaust temperature) reaches the allowable exhaust temperature limit (TTRX).

With IBH, the IGVs are closed to something less than the minimum design angle to further reduce air flow into/through the unit at part load. This allows the unit to transition into Pre-mix Steady State earlier during loading and to stay in Pre-mix Steady State longer when unloading. BUT, the IGVs are never closed to an angle that would make the actual exhaust temperature (TTXM) greater than the maximum allowable exhaust temperature (TTRX). NEVER. Not in units with conventional combustors; not in units with DLN-I combustors (whether or not they have IBH).

Hope this helps! It is really kind of difficult to explain, but in reality, it's all about not exceeding the maximum allowable exhaust temperature--no matter the type of combustor or whether IBH is being used.
 
Really thank you!
It helps me understand further about new system(DLN Combustor, IBH Valve) and IGV operation.
I feel its information is limited on web, so your detail explanation is very very useful for me.
 
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