Water Injection System in Gas Turbine

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Maint

what is water injection system in gas turbine? what is the purpose of system? how it is work?
 
Water injection is used to help control NOX emmissions. Water is mixed with the fuel as the fuel enters the combustion chambers.

There is usually a control valve placed in line with the water injection pump to control the amount of injection.
 
It helps lower the combustion temperature for greater efficiency.

There is a NOx steam inject which helps lower the Nox emissions but raises the CO at the same time
 
NOx formation is a function of flame temperature; reduce the flame temperature and the formation of NOx is reduced.

CO is or can be burned, so reducing flame temperature has somewhat the opposite effect on CO.

Water injection for NOx reduction is used to cool the combustion flame temperature to reduce the formation of NOx (nitric oxide). It's injected directly into the combustors of most GE-design heavy duty gas turbines, and on a rare few is mixed with liquid fuel before being introduced into the combustor(s).

BOTH water- and steam injection (which serves the same purpose: to cool the combustion flame temperature to reduce the formation of NOx) can lead to increased CO emissions if excessive injection occurs. There is a kind of "balancing" act which takes place when trying to control NOx emissions, whether it be by "wet" means (water- or steam injection) or dry means (Dry Low NOx, DLN, combustion systems) to find a spot where the NOx and CO are both lowest.

Some sites use water or steam injection to augment (increase) power output; some use it properly and others don't. The discussion above refers to the use of water injection for the reduction of NOx emissions.

When water (in liquid or gaseous form) is used for any purpose in a gas turbine, it must be treated just like water that is used on a typical boiler that produces steam for a steam turbine or some other process to reduce damage to the boiler components. Treating water is costly, and when water is injected into a gas turbine, it goes up the stack and is lost in the atmosphere. It cannot be recovered and condensed and reused as in most boiler applications. So, the money spent to treat that water goes "up the stack" along with the water/steam. And can't be recovered.

Injecting water or steam into a gas turbine combustor can be harmful. It increases the dynamics inside the combustor, and over time, increased dynamics causes premature wear on hot gas path parts. So, maintenance intervals are usually decreased (must happen on a more frequent basis) when water or steam is used for NOx reduction. Over-injecting water or steam can be seriously damaging.

And there is a limit to how much NOx reduction can be achieved with water or steam injection, much higher than is possible with Dry Low NOx combustion systems. For the same emissions reduction, less water injection is required than if steam injection is used. (The injected water flashes to steam, which reduces the flame temperature more than injecting higher temperature steam, which is already a vapor.)

Lastly, many GE-design heavy duty gas turbines with DLN combustion systems use water or steam injection to reduce NOx emissions when operating on liquid fuel.

I'm not sure about the statement regarding lowering temperature to increase efficiency....
 
Dear CSA Thank you,

i have some points please find the answer? we have Gas Turbine GE Nouvo Pingion Fram 5, but i am inconvenience with water injection system. how we know the NOX in combustion chamber that mean parameter nox, and i see PI&D there is two line with Hugh rate and other with Low rate. what is the different?

Is this system critical with turbine if we have some problem system (not working )
 
The only way to be certain what the NOx emissions are is to have a continuous emissions monitor that is properly maintained.

Wet low NOx injection is usually only used in order to lower emissions to a level defined by a permit issued to the site. Injecting water (or steam) is not cheap, in fact, it's downright expensive.

First, there must be a source of water, and usually that water must be purchased. The water must be treated to be boiler-quality water, and that is expensive. (In my experience, the water treatment plant of a combustion turbine equipped with wet low NOx is one of the weakest links in the plant, because it's usually built on the cheap and doesn't work very well without a lot of maintenance.)

Once injected to the combustor, that treated water (either in the form of steam or water) is exhausted to atmosphere, so it's not recuperable. That means, that a constant supply of boiler-quality water must be available for injection.

Lastly, injecting diluent (water or steam) into a combustor increases the dynamic pressure oscillations in the combustor, which increases the wear on the hot gas path parts (liners; seals; transition pieces; nozzles; etc.). So, no one would choose to inject water or steam unless it were not required in order to be able to build and operate the plant.

Yes, wet low NOx injection does have a slightly positive effect on the heat rate, but that still comes at a cost; there is no such thing as a free lunch in this world. Whether it's the treated water, the raw water, or the hot gas path parts, the performance increase is not free.

It is a well-accepted fact that NOx formation is proportional to fuel flow-rate, so the wet low NOx flow-rate reference is based on fuel flow-rate, usually kg (water)/sec-per-kg (fuel)/sec, or kg (water)/kg fuel. When GE-design heavy duty gas turbines using wet low NOx methods are commissioned, the emissions are monitored and the injection flow-rate (as a function of fuel flow-rate) is adjusted to make the emissions just slightly below permit level (usually also the same as the turbine packager guaranteed to the turbine purchaser).

Early emissions monitors were not fast enough to provide closed-loop feedback for controlling wet low NOx injection rates, so most regulatory agencies agreed to accept that, when demonstrated, a particular wet low NOx injection flow-rate for a particular fuel flow-rate that the emissions were under the limits of the permit being issued to the "generator" (emissions generator, not electrical generator) that would be the measure of compliance with the permit.

Now, if the regulatory agency came around and put their own sensor in the stack and found otherwise, well, they could cite and even fine the site for not meeting the permitted emissions limits. But, they usually don't do that. But, they have also required that sites have in-situ (on-site) emissions monitors that the sites are responsible for maintaining (so there's the cost of purchase, installation, and maintenance for these systems). And, if the emissions monitor isn't working, then usually the site is not allowed to run the turbine either.

I'm <b><i>SO</i></b> sorry to hear that you are inconvenienced by wet low NOx injection. You and just about every other similarly-equipped site have experienced inconveniences to some degree over time. The older systems which used motor-operated control valves and differential pressure regulating valves were susceptible to issues if not understood and maintained properly.

I'm not sure exactly what you're referring to on the P&ID, but I presume it's a "note" to the system designers, those who will be building the treated water supply systems and supplying the piping between the Water Injection Skid (with the pump(s) and control valve(s) and the turbine base about what the expected flow-rates (min and max) would be so they could design the piping with minimal pressure drop. (Most of the GE-design turbine packagers were not responsible for the interconnecting piping supply or design.)

In most parts of the world where plants are permitted to allow the production of emissions, the generators (emissions generators: the turbine(s)) can't run if the emissions control systems aren't running, including the emissions monitors. You will have to ask your plant management, -ownership, and/or legal representatives if you can operate the turbine--legally--without water injection.

The turbine would "prefer" to run without wet low NOx (lower combustor dynamic pressures), but a usual condition of the permit to generate (emissions) states that there are very limited conditions or periods when wet low NOx injection is not required.

But, hey! Go ahead! The turbine won't blow up if you run it without wet low NOx injection. In fact, the hot gas path parts will likely last longer due to the decreased dynamic pressures in the combustors. But, depending on where the plant is located and what the regulations and laws are there, you might go to jail for running it without wet low NOx injection. So you might delay your inconvenience, but you can't avoid it. (I would consider jail to be a <b>very</b> big inconvenience; I imagine you would, as well.)

You can minimize your inconvenience though, by learning and understanding how the system at your site works, and how to optimize it for the most trouble-free operation.
 
Do you have a CEMS (continuous emissions monitoring system)? You mentioned "High rate" and "Low rate." NOx analyzers for gas turbines usually have High rate (typically 0 to 200 ppmvd) for when water injection is off on liquid fuel, or below pre-mix mode on gas fuel), and Low rate (typically 0 to 20 ppmvd) when water injection is on or in pre-mix mode. (ppmvd = parts per million volumetric dry at 15% O2.)

The system is not critical to the operation of the gas turbine, but it is usually critical to the environmental authorities permitting you to operate the turbine.
 
Ok Good ,

I have some Question?

I see in the PID'S there are two line which injected to fuel nozzle, one line High Flow rate and another one Low Flow rate.

Does these line work together? or when you use low or high?
 
Not being able to see the P&ID for your system it's impossible to say for sure exactly how your system works. But, based on previous experience with similar systems the explanation below you should be able to use the P&ID and the sequencing or application code in the Speedtronic, as well as the system descriptions in the Instruction Manual provided with the unit, as well as the descriptions which might be in the Control Specification drawing, to work out exactly how your system works. My experience is with systems that have a single pump (or redundant pumps with only one pump operating at a time in a lead/lag situation), and with a single flow control valve that controls the total flow to the unit, and then there is a valve downstream of the control valve that can be opened to allow a portion of the flow to go through a second set of nozzles (the high flow nozzles).

You should be able to work backwards from these two lines on the P&ID to find a valve that will open to allow flow through the high flow lines when the sequencing in the Speedtronic senses the flow is greater than can be accommodated in the low flow lines/nozzles.

The presence of a high flow line indicates that the designers of the system felt that the required flow would exceed the ability of a single set of nozzles (the "low" flow nozzles), so they added the valve and piping and a second set of nozzles so that as the flow reached the upper limit of the low flow nozzles the valve would open and water would flow through both the low and high flow nozzles.

On the systems I have worked on with low- and high flow water injection manifolds the valve which opens to allow flow through the high flow piping/nozzles is a unique valve in that it needs to open and close slowly so as not to disturb the water injection flow or the turbine operation. Sometimes that valve is called a "shear" valve (I don't know the derivation of that term, just that that's what some people call it and how it's sometimes described in the documentation provided with the unit and system).
 
N

Namatimangan08

> what is water injection system in gas turbine? what is the purpose of system?
> how it is work?

It purpose is actually to lower the intake temperature.

GT is a constant volume displacement machine. For a given time it displaces air at a constant sweep volume. For the given volume of air, its mass is density dependent. Air density, in turn is temperature dependent. The lower the air temperature the denser it will be. This helps to improve both GT heat rate or thermal efficiency and its maximum capacity.

I think it obvious how higher air mass flow rate can improve GT output. Higher air mass flow rate allows the controller to supply more fuel even though high exhaust temperature limiter remains the same. More fuel input added means more output can be extracted by the expansion turbine.

It is more difficult to see how water injection can improve GT overall thermal efficiency. Such improvement can be proven via Carnot cycle efficiency relationship, i.e.<pre>

Carnot cycle efficiency = 1-T_cold/T_hot.

Assuming our GT is designed to operate at T_cold (intake air temp)=300K and T_hot (combustion temp)=1500K, then its Carnot cycle efficiency (CCE)is,

CCE = (1-T_cold/T_hot)*100
= 80%

Now let us assume T_c= 299 and T_hot remains the same. The new CCE becomes

CCE_new = (1-299/1500)*100

= 80.068%</pre>
CCE is the maximum theoretical efficiency limit that a heat engine can achieve. In the other words it is impossible to have a heat engine that has overall thermal efficiency that has higher than the value given by the CCE.

Don't get me wrong. It doesn't mean that we can make money for having water injection system. It depends on economic evaluation between the cost to reduce water temperature and saving in fuel cost due to efficiency improvement. What I know is the main objective for having water injection is to increase the maximum capacity of the GT.

The main motivation to get an additional "stretched output" via inlet air cooling of the GT is due to the fact that there are occasions that the incremental cost to produce 1MWh of electricity can be as high as USD 3000/MWh! The normal cost could be between says USD 30- 40/MWh. This is true if the incremental demand of 1MW has to be met by putting a 100MW GT into the system. In order to put a 100MW GT into the system, somebody have to pay for the cost to keep the GT compressor to rotate at the grid frequency. The power consumption could be as high as 60MW, although the actual incremental demand is only 1MW. By stretching the GT output the grid operators can reduce the frequency of having to add additional unit to meet this small incremental load demand.
 
You should clarify what type water injection. The type just described is also called evaporative cooling.

There are 2 other types, involving water injection into the combustors. One is done for power augmentation the other is done for NOx reduction. Both of these will increase power due to the increased mass flow through the turbine. The power augmentation type has the water injected downstream of the combustion zone. The NOx control type has the water injected into the combustion zone, which reduces the peak flame temperature and hence the NOx.
 
N

Namatimangan08

>Namatimangan08,
>
>Inlet air cooling is <b>NOT</b> the
>same as water for NOx emissions
>reduction.

Right. They are not similar. In our place we hear about inlet air cooling more often then NOx emission reduction. That is why my first impression when I read about water injection system, inlet air cooling will come across my mind.

Sorry about it.
 
in GE 9FA machines to increase the power from 75MW upwards Water Injection is a must. what could be the reason for this?. At base load if water injection tripping will unload the machine to 75MW. is it limiting the TIT?
 
You haven't provided enough information about why water is being injected into the turbine at your site. what fuels are being burned, what the emissions regulations are for your site, the type of combustors (DLN or conventional), etc.?

 
A GE 9FA is certainly capable of more than 75 MW without water injection UNLESS it is required for NOx control and you are not allowed to operate without it. At ISO conditions (sea level, 15 degC inlet air temperature) with DLN gas fuel system it should be capable of at least 200 MW.
 
sorry for the incomplete info. 75MW limitation is only for liquid fuel firing (DFO). emission regulation is NOx 20ppm and it's a DLN 2.0 combustor.

if water injection trips at Full load Turbine runs back to 75MW. my wild guess is it is to limit the TIT.
 
The runback is not to limit turbine inlet temperature (also known as firing temperature), but to limit peak flame temperature, which will limit NOx emissions. The requirement is not imposed by GE, but by whatever authority governs environmental impact at your site.
So, if you need to run at base load without water injection, you need to run on gas fuel. If the reason you are running on liquid fuel is that gas is not available, they you have a problem. You have to fix the water injection system.
 
C

conserveEnergy

For non-DLN combustion when firing natural gas, is there a preference for steam rather than water?

Thanks

> And there is a limit to how much NOx reduction can be achieved with water or steam injection,
> much higher than is possible with Dry Low NOx combustion systems. For the same emissions
> reduction, less water injection is required than if steam injection is used. (The injected
> water flashes to steam, which reduces the flame temperature more than injecting higher
> temperature steam, which is already a vapor.)

> Lastly, many GE-design heavy duty gas turbines with DLN combustion systems use water or
> steam injection to reduce NOx emissions when operating on liquid fuel.
 
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