Sir,
At the outset I salute to Control.com for running this site.
I am working in Operations of a Gas Turbine plant,we have 2 nos GE Mark V machines 20 MW capacity, Naphth is the fuel.
Query:

1. I like to know the effect of Water injection, used for controlling NOx,on the efficiency of the GT.

2. I recently read in an article that spray of water into the air intake increases efficiency by 10%, is this true?

What is the technical explanation?

 

Yes; we're all thankful for control.com's existence--and to all who are contributing to the growth of the GE-design heavy-duty gas turbine control "community" on this forum.

If you haven't already done so, use the search feature of control.com to look for turbine- and turbine control-related posts; the search feature here is among the best of any forum on the Internet--one of the reasons is that it can accept multiple words, which many sites' search features absolutely CHOKE on. A lot of topics have been covered--and the text of every thread is included in the search, not just the titles or summaries (or so it seems). (For disclosure purposes, this author does not work for, and is not associated with, control.com or any of its affiliated entities.)

[NOTE: To those who read these turbine- and turbine control-related posts, this author is going to stop responding so quickly to posts in the hopes that others will start contributing more of their their experiences, and will just add to or clarify replies as necessary. With the growth of this community, there should be more people posting replies, and this might be one way to encourage others to post their experiences and knowledge by not responding so quickly. So, don't be surprised if replies from markvguy aren't as quick as they have been in the past, or as voluminous. Keep the questions, comments, and replies coming!]

1) Injecting water into a combustor has the effect of cooling the combustion gas temperature. If one is operating a unit at Part Load with Water Injection turned off--and WITHOUT Pre-Selected Load Control enabled--and then enables Water Injection, the power output of the unit will decrease. Yes, the mass flow through the unit will increase due to the water being injected, but because the fuel flow-rate is being held relatively constant during Part Load operation the energy required to vaporize the water into steam is NOT available for "extraction" by the turbine section, so the power output decreases.

To this author's mind, that causes efficiency to decrease--for several reasons. First, less power output is being produced for the same fuel flow when Water Injection is enabled. Second, the water to be injected must be boiler-quality water, which means it must be treated--and that requires energy (and money for chemicals or RO elements or whatever method is being employed to treat the water to remove salts and minerals). Third, the water which is injected is non-recuperable, which means it can't be "recovered" because it goes up the stack and disappears into the environment--requiring more treated water. (This author is NOT a mathematician, and doesn't enjoy spreadsheet analyses of efficiency and heat rate and so on. So, the technical definition of efficiency is left to others--but water injection for NOx reduction doesn't seem to increase efficiency to this author. This author is more concerned about proper operation of systems designed by others to control and protect GE-design heavy-duty gas turbines, and leaves the spreadsheet analyses and efficiency determinations and calculations to others.)

Now, when the turbine is operating without Water Injection at Base Load, on CPD-biased (Compressor Pressure - Discharge) exhaust temperature control, and Water Injection is turned on, the power output of the unit increases--BUT SO DOES THE FUEL FLOW-RATE!!! The same thing happens when Water Injection is enabled while operating at Base Load--the combustion gas temperature decreases. BUT, exhaust temperature control will increase the fuel to maintain firing temperature and increased fuel flow means higher power output. So, contrary to popular myth, one doesn't get something for nothing when injecting water into a turbine--even at Base Load. And, there's still the cost of the non-recuperable water....

2) There are many different systems which can be used to "spray" water into the compressor inlet air flow of a combustion turbine. Most act to cool the air, making it more dense; denser air results in a higher mass flow-rate and the ability to burn more fuel--which means higher power output for the same ambient temperature. Most of these systems only work at ambients above approximately 65 deg F, and most require or should use demineralized water ("boiler quality") so there's the cost of the water and of treating the water.

Most of these systems are also only effective in ambients and/or locales which have "low" relative humidity--so that the water can be "efficiently" evaporated. There is one system which is AMAZING to watch--it injects what, to this author's mind, are massive amounts of water into the inlet ductwork. There is SO much water that what doesn't get drawn into the inlet is recirculated to the pump's suction for re-admission to the inlet. Watching it through a Plexiglases window on the inlet duct, it looks like a front-loading washing machine in agitation mode! Personally, it seems risky--but the economic benefits must outweigh the risks--at least on someone's spreadsheet!

One such vendor of combustion turbine inlet spray/vaporization technology is Mee Industries (http://www.meefog.com/turbine/gas-turbine.html). They have some pretty good "white papers" on their equipment, and the technology in general. They have lots of charts and graphs and figures on their site which should be of interest to the "bean counters." Other manufacturers and vendors can be found using any Internet search engine (Google, Dogpile, etc.).

markvguy

 

The explanation by "Mark V guy" was very clear. Still a part of my second question unanswered, i.e. whether 10% improvement in efficiency is true. Any inputs on this will be appreciated.

 

Presuming proper ambient conditions (low humidity and elevated ambients), it would be possible to achieve approximately a 10% increase in POWER output by using some kind of evaporative cooling or fogging scheme.

However, when the air density increases the fuel flow will increase AT BASE LOAD.

Read the white papers on the site which was sited. They provide all kinds of numbers!

markvguy

 

Using water or steam injection is a cost-effective means of reducing gas turbine emissions and increasing power output. For peaking gas turbines, steam is generally not available, and water injection is the ideal method.

Whether trying to meet new emissions requirements or to increase power output on an existing turbine to get optomized improvement you need to get support from someone who can provide expertise to provide injection systems that are ready to install and complete, from control system modifications to fuel nozzles and combustion hardware.

Since emissions reduction depends on turbine type and many other factors – you need to discuss the specific machine details with an expert for detailed information on water flow and emissions data for specific unit types.

System Components

The design varies based on machine and customer need but the basic components can include some or all of the following:

Water Injection Pump Skid
Water Injection Control Skid
Instrumentation
Water Injection Manifold
Water Injection Rings

Key to the injection system since it will be used on a mature unit includes:

The systems are assembled from standard industrial components for fast delivery and low cost maintenance. Fast-acting three-way water-actuated water stop valves and water control valves are selected to meet stringent design requirements for handling de-ionized water.Components are intelligently packaged to minimize size without sacrificing maintainability and that the systems can be installed in existing heated space minimizing the amount of heat tracing and new civil works.

I might be able to help define this if you come back to me directly.

 

From the Mee Industries, Inc., website, up to a 5% improvement in heat rate is achievable with their inlet cooling system--which means efficiency is improved.

A 10% improvement in efficiency by evaporating water in a combustion turbine inlet seems a little over-stated--however, spreadsheets can be made to "display" a lot of different analyses depending on who's manipulating the data and how, and who's interpreting the data and how....

markvguy

 

Cooling gas turbine inlet air increase both turbine output and fuel efficiency (heat rate is reduced).

Example: Cooling the inlet air of GE 7EA turbine by 21°F (11.7°C) will cause the turbine to produce about 6.5 megawatts (MW) of additional power (a boost of about 9%). The heat rate (BTU/kW) is reduced by about 2%, i.e. 2% less fuel is burned for each kW of power produced.

Additional power can be produced by adding more water to the inlet air than can evaporate with the given ambient conditions -- called "overspray" or "wet compression." This additional water is carried by the airflow into the compressor where it evaporates to produce an inter-cooling effect.

Example: Spraying water at the rate of 2% of the air mass flow (about 100 gal/min for the GE 7EA turbine) will produce an additional 12 MW, for a total boost of 18.5 MW. Fuel economy will be improved by about 4.7% over the non-fogged case.

Mee Industries has about 650 fog system installations on gas turbines, including many installations in tropical and offshore areas where ambient humidity is very high. We can provide very accurate (guaranteed) estimates of improvements in gas turbine output and efficiency for a specific site. This is done using meteorological data and gas turbine modeling software (see http://www.thermoflow.com/).

As Markvguy mentioned, fuel savings are offset by demin water costs. Usually about half of the fuel savings is consumed by the cost of water. Turbines with higher pressure ratios respond better than the turbine in the above example.

I'm happy to answer specific questions. I can be reached by email at thomas at meefog dot com.

Regards,

Thomas Mee
CEO/Mee Industries Inc.

 

I have another question relating to steam and water injection into the combustion chamber.

I notice from the GE file "Uprate Options for the
MS6001 Heavy Duty Gas Turbine" that while both these methods can be used to increase output and reduce NOx, steam injection improves efficiency, while water injection is reduces efficiency. Why is this?

 

It takes less water (mass flow-rate) than steam for the same emissions reduction. This is because the water is more effective at quenching the flame temperature than steam. The steam already has a certain amount of heat, which the water doesn't and must absorb from the flame ball as it quenches the flame and flashes to steam. This heat is taken away from the process, which is probably the major reason.

So, it's my opinion that the reduction in efficiency is because of the heat which is taken from the combustion process to flash the water to steam (which must happen because of the temperature in the combustor where the water is sprayed). On the other hand, since it takes a higher mass flow-rate of steam to achieve the same amount of emissions reduction and less energy is given up to heat the water the overall efficiency is increased.

Mass flow-rates probably don't have all that much to do with it, but the difference is noteworthy nonetheless.

 

Q: What would be required to retrofit an existing Frame 6 GE machine with water injection for NOx control? Does anyone know of any companies that specialise in this type of work?

 

GE, of course.

Wood Group.

TTS.

To name a few.