Combined Cycle Power Plant - GT and ST power output

[FelipeSC]

I'm trying to design a project of a combined cycle power plant with the two-on-one arranje, (2 GT, 2 HRSG and 1 ST).

Well, i got the results from the Aalborg (HRSG manufacturer) engineer.. he didn't show me the calculations, of course, but he returned the amount of steam that hrsg can deliver...

So, the Gas turbine is:

Gas Turbine Siemens SGT-300:
Power Output: 7.9 MW(e)
Fuel: Natural Gas
Frequency: 60 Hz
Electrical Eficiency: 31.2%
Heat Rate: 11,532 kJ/kWh
Rotation: 14,010 rpm
Gas Flow: 29.8 kg/s
Temperature: 537 °C

With that GT, the Aalborg HRSG can deliver 14.5 ton/h of steam with 25 bar pressure and 225 °C, and water on 105 °C.

With that information, i searched a way to estimate the power output of a steam turbine to be attached to this hrsg. I got from a Siemens presentation, that the preliminar perfomance calculation for industrial steam turbines is given by:

Power output (kw) = Steam Flow (kg/s) x Steam entalpy (kj/kg) x efficiencies (%), where

turbine efficiency (average) = 50-60% single-stage
75-89% multi-stage
reductor efficiency (approx) = 98.5%
generator efficiency (approx) = 98%
entalpy is taken from steam tables using the pressure of the steam used in the hrsg

So it means that ONE hrsg can run a steam turbine with:

power = 14.5 ton/h x 2802.2 kj/kg x 0.75 x 0.985 x 0.98

power = 8.17 MW

In the two-on-one arranje, there are 2 hrsg, the steam flow will be 29 ton/h and the power output of the gas turbine will be 16.34 MW.

Two Gas Turbine with total 15.8 MW can run a Steam turbine with 16.34 MW? Is that possible, or absurd?

I thought that combined cycle power plants usually have their steam turbines with approx. 1/3 of the total power, for example, 2 GT with 100 MW and 1 ST with 100 MW. The ST will have half power of the GT.

Can anyone please help me before i fall into despair???

Thank you very much..

Felipe

 

[rmw]

Go play with this site and check your conditons.

http://www.dresser-rand.com/steam/calc/main.asp

Also, a valued member of this forum H. Katmar has a freeware. Googld Katmar software and look for his "turbine" program. I don't remember if it goes as big as your needs or not, but I have used it many times.

rmw

 

[davefitz]

I think you either need to obtain access to a turbine cycle simulation program or you should study further on calculating steam turbine output.

The equation in the posted question is not correct. Instead of enthalpy, you need to use enthalpy difference. The enthalpy difference is that between the inlet throttle conditions minus the turbine exhaust condition under the assumption of isentropic ( ie constant entropy) expansion down to the exhuast pressure.

 

[FelipeSC]

rmw:

Great Site, i got great results with that tool, thank you very much!
-----------------------------------------------

davefitz:
I think you're right, i'm just using the entalpy in that formula, i checked and it is the difference, as you said.
The technical data of the steam turbine is:

Siemens SST-100
Power output: up to 8.5 MW
inlet pressure up to 65 bar
inlet temperature up to 480 C
rotational speed up to 7500 rpm
exhaust pressure (back pressure) up to 10 bar
exhaust pressure (condensing) up to 1 bar
exhaust area 0.22m2/2.4 sq ft

Well, the hrsg is generating steam at 25 bar. The entalpy, according to the steam table, is 2802.2 kJ/kg, but i should use the difference right? Can you help me on what difference should I use?

thanks again, a lot
and sorry for my ignorance =)

Felipe

 

[rmw]

Nothign to apologize for. We are all ignorant. The only question is; about what and to what degree?

rmw

 

[CH5OH]

the enthalpy on the exhaust of the turbine should be taken into the equation:
power=flow x (enthalpy inlet - enthalpy outlet) x efficiency

 

[FelipeSC]

the enthalpy inlet is 2802.2 kJ/kg, taken from the Saturated Steam Table...
My question is, the outlet steam is not saturated anymore, what table should I use? I know that the outlet is 1 bar (condensing), but I don't know what enthalpy I should use!

Plz help! =)

Felipe

 

[rmw]

Are you sure about the 1 bar condensing pressure, and is it 1 bar abs. or 1 bar gage? Either one is a rather high pressure for a condensing turbine unless you are using the condensing steam to make the 105C hot water you mentioned in the OP.

Most power plant turbines condense into a deep vacuum where 0.1-0.15 bara would be more the norm.

Please explain what is going on at the bottom end of your cycle.

And, for a outlet enthalpy, to get there quickly I would use a Mollier diagram starting at the steam inlet condition and follow an expansion line with the slope between 75-89% (I'd try to narrow this down somewhat) down to the condensing pressure what ever that is.

There are other ways to get there, but that would be the quickest for me.

rmw

 

[FelipeSC]

the exhaust pressure (condensing) is up to 1 bar, according to the Siemens Steam Turbines Brochure..

Here's the deal, this is a combined cycle power plant, with the two-on-one arrange, 2 GT, each one with a HRSG, and 1 ST

The GT is:
Gas Turbine Siemens SGT-300:
Power Output: 7.9 MW(e)
Fuel: Natural Gas
Frequency: 60 Hz
Electrical Eficiency: 31.2%
Heat Rate: 11,532 kJ/kWh
Rotation: 14,010 rpm
Gas Flow: 29.8 kg/s
Temperature: 537 °C

With this GT, the HRSG can produce, according to Aalborg Industries, 14.5 ton/h of steam.

The Steam Turbine is:
Siemens SST-100
Power output: up to 8.5 MW
inlet pressure up to 65 bar
inlet temperature up to 480 C
rotational speed up to 7500 rpm
exhaust pressure (back pressure) up to 10 bar
exhaust pressure (condensing) up to 1 bar
exhaust area 0.22m2/2.4 sq ft

With all these information, i tryied to estimate the power output of the Steam Turbine to be attached to the 2 HRSG... with the website

http://www.dresser-rand.com/steam/calc/main.asp

you suggested me and the software Katmar, I simulated and the results point to approx. 4 MW Steam Turbine, wich is an acceptable result, I guess.

By the technical information of the Siemens Steam turbine, I assumed that the outled pressure is 1 bar (condensing)...

Is it too much?

Thanks a lot!

Felipe

 

[FelipeSC]

And the water mentioned 105 C enters the HRSG, turns into steam, go to the Steam Turbine, after that to a condenser and is pumped back to the HRSG, and the cycle goes on and on...

 

[rmw]

OK, that makes sense. Your normal condensing pressure will be into a deep vacuum which will be determined by your heat sink conditions; cooling tower water temperature, seawater temperature, river water temperature, lake, pond, dry cooler, etc - whatever your heat sink is.

You have to decide which of the above (or any other) is the most probable fit for your situation and go from there. Let's assume you are going to cool your condenser with sea water that is 18C year round. That would dictate a condenser pressure based on the design of your condenser, but for rough purposes like you are doing, let's say that that would give you a back pressure of 4" hg (you can convert that to bara).

That would be the exhaust pressure of your turbine and the value you would use in the Dresser tool or Katmar's software. So your steam would expand from 25 bar, 480C to 4" hg, and (read the enthalpy from a mollier diagram).


That right there should give you a little more MW out of your steamer.

I am going to say that in a situation like yours, I would be looking for the steamer to be producing in the 6-7 MW range. I ran the tool with a back pressure of 0.15 bara and an inlet of 25 bara, 480C, 3k rpm and it showed 5.2 MW, so my estimate was close.

You have the option of duct firing your HRSG's (if you design your plant that way) in order to be able to get all the possible power available out of your ST. But, understand that that hurts your overall cycle efficiency, but hey, on some days when power is selling high, it makes economic sense to duct fire. Plenty of people make that choice every day.

rmw

 

[FelipeSC]

rmw:

I guess you're right. Probably the heat sink conditions will count on cooling tower water temperature or riverwater temperature, since there's a river near the region. The commom outlet pressure of condensing steam turbines is usually low, so I ran both the Dresser and Katmar tools with the real data I got:
Inlet pressure 25 bar, 225C (saturation temperature), inlet steam flow 36 ton/h (18 from each hrsg), exhaust pressure 0.05 bar, turbine efficiency 75% (worst case), and it returned approx 6.7 MW and 6.8 MW. I attached the image below!

Are the input data i ran the simulations with correct? I mean, is it ok to assume 0.05 bar on the outlet pressure, given the heat sink conditions cooling tower water temperature or riverwater temperature?

thanks again rmw, you're being vey helpful!

 

[rmw]

Felipe,

There is a problem with a couple of your numbers. First, 0.05 bara is a little too low, unless, of course you are located up near where they just had the winter Olympics. Your condenser pressure is going to be about a 5c approach to your cooling water temperature and judging by the Portuguese in your document title, you are in either Brazil or Portugal neither of which have year around conditions to consistently have cooling water that cold. Anything north of the Great Lakes of the USA, yes, maybe so. That is why I used 18C water. If you have a good source of 16 degree water, that would be low for cooling tower water if you are in Brazil.


Also, something is wrong with your steam temperature number. You have CT's with exhaust gas temps of 537C and a steam turbine capable of taking steam at 480C but your steam temperature is only 225C, barely above saturation. Realistically, any HRSG worth its salt would be good for steam at 15-25c below CT exhaust gas temperature. Steam turbines want to run on superheated steam for a variety of reasons, too many to go into here, and that is what HRSG's are designed to deliver.

Double check that number and post back.

rmw

 

[racookpe1978]

Call Siemens.

I'm sure a salesman would be "happy" to give you advice. Love. Comfort. Sales material. 8<)

1 Meg is unusually small. I'm more used to seeing 200, 300 and 400 Meg Gas turbines, often with 2x HRSG's driving one ST for + percent.

 

[Bilsko]

I don't want to hijack this thread, but my questions are pretty similar to Felipe's, so hopefully I can draw out a bit more help from the forum.

I'm running a performance/finanical assessment of an onsite setup for a college campus in the Mid-Atlantic. Our assessment is trying to determine the best configuration of gas turbine plus backpressure or condensing/extraction steam turbine. The turbine is sized just about at their baseload, so in the winter its fine to send all the thermal to steam for their steam loop, during the summer, the hourly steam load is lower than the turbine output, so it would be nice to divert some of the steam to generate electricity. So my first question is on the technical possibility of this configuration: Can we have a system that sends 150psig/366F steam directly from the HRSG to their loop during winter, and then runs 500psig/550F steam from the HRSG through a backpressure turbine, with 150psig/366F steam at the outlet to the loop during shoulder and summer hours? If the steam load is low enough during the summer, is it possible to operate the backpressure turbine in condensing mode to extract more kW and then send the condensate back to the HRSG economizer? If this type of setup is possible, then I have some more detailed questions:

We're assuming a Taurus 70 for the gas turbine:
kWe: 7500
MMBTU thermal: 46.2 (212.3k lbs/hr exahaust @ 913F)

I'm assuming that for the steam turbine (in backpressure mode) we'll want 500psig/550F steam so we can have a pretty large delta between the inlet and outlet. Their steam loop expects 150psig/366F steam.
For the HRSG to produce steam at that level, I should be able to expect about 27.5 mlbs of steam with an enthalpy of 1265BTU/lb. (I've used the tools available here:

http://users6.jabry.com/vganapathy/temprofa.asp

and here:

http://www.hrsgdesign.com/steamprop.htm)

So I know how many BTUs of thermal I have at the inlet: (1265BTU*27.5mlbs) and I assume that there is no loss of flow through the turbine so I'm sending 27.5Mmlbs*1197BTU (150psig/366F steam) on to the steam loop.

That means that I have 1.85MMBTU to play with for electricity generation. I've seen efficiency figures all over the map (both isentropic and power generation efficiencies) and am not sure what to use.
*What is a good middle of the road efficiency figure to use for how that 1.85MMBTU gets converted to electricity? 70%?* I've seen performance curves for backpressure systems that show everything from 10kW/mlb to 25kW/mlb. If I use the 70% efficiency figure in the calculation above, I end up with around 360kW of output, so 13kW/mlb - seems to be kind of low.

If I'm running the system in condensing/extraction mode, then I assume that the Delta I need to be calculating is between the enthalpy of the inlet steam (1265BTU) and the condensate that is going back to the HRSG - water at around 240F - is that correct? Can I expect the same generating efficiency?

Sorry for the flood of questions - my grasp of the engineering and thermodynamics concepts is rudimentary at best. I'm still trying to find a good primer on the properties of steam and the operating pressures that these systems require - if anyone has any recommendations, I'm all ears.


As a side note - it looks like the calculator over at the Dresser-Rand site is down for maintenance -- hoping it comes back online soon.

 

[MisterDonut]

Bilsko,

Designing a HRSG to operate at two different pressures is no trivial matter. The specific volume difference for sturated steam from 150 to 500 psig is on the order of 3 times.
Bypassing the superheater is potentially problematic, as running it "dry" will result in the tubes being at the same temperature as the turbine exhaust, potentially ~1100ºF.
Not ot say this is impossible, but it should be approached carefully, and will pobably be expensive.
Another approach would be to size the electrical output of the combustion turbine generator and the steam turbine generator to be base-loaded. Thus, all HRSG steam would go through the steam turbine. Additional steam for the campus heating loop could be generated by package boielrs operating at 150 psig.

 

[Bilsko]

MisterDonut,
Thanks for the reply - good to know that we shouldn't expect multiple, seasonal output profiles form a single HRSG.

There are a few reasons why we're trying to avoid electric-only output from the turbines and package boilers for steam:

-In this particular case, the thermal is just a bit more valuable than the electricity, so it be best if we can utilize the turbine's thermal output for steam and then take the marginal electricity generation as a bonus.

-Notching down the size on the gas turbine to anything much lower than the 7.5MW of the Taurus means we take a pretty significant hit on the electrical efficiencies. So sizing a smaller gas turbine plus steam turbine just to meet baseload may not give us all the efficiency benefits we might expect from the backpressure setup.

-The customer already has existing boilers with enough capacity to meet load. However, from a reliability standpoint, our approach with the backpressure setup would be to rely on the existing boilers only for a) steam to supplement the HRSG's winter output to meet load, and b)as backup steam generation for a customer with reliability needs.

 

[racookpe1978]

I might be backwards on this approach, but about sizing the pair (Taurus and HRSG) so the electric output is near optimum but slightly under "perfect" size for wintertime electric loads. Then plan on the "excess superheated steam" in winter bleeding down directly LP heating steam in winter (?) to reduce the need for all of your existing boilers.

in winter, you won't need as much electric power.

Then, in summer, when AC electric load is high, you reduce the need for expensive peak priced summertime electric loads with the bigger generator, but you have enough steam to power the generator.

 

[Bilsko]

racookpe1978 -
I don't think that's the backwards approach at all - in fact that's precisely what we're proposing.

The gas turbine alone (7500kW nominal, maybe up to 8400 on really cold days) is sufficient to meet most of their winter/shoulder baseload.

From the calculations that I've done so far (see my post with questions above), it seems to me that we shouldn't expect more than 300 or 400kW of additional output from the backpressure turbine (if its delivering the full flow of steam to their loop at 150psi)

The second half of my questions above are predicated on the assumption that we are going with a backpressure turbine (instead of a condensing unit)...and just trying to understand if I have reasonable assumptions for the power output calculations and efficiencies

 

[rmw]

My head is kind of swimming with all the details. Is this an existing turbine or are you proposing putting in a turbine or changing to a different turbine?

Combination back pressure / condensing turbines are not that uncommon. When extraction steam is desired, the bulk of the steam is sent to process and when the process doesn't need steam, the steam is diverted to the condenser.

However, in this type of turbine, there always has to be some small amount of steam going to the condenser to prevent overheating of the stages between the last extraction point and the condenser.

rmw