3% improvement in rankine cycle at exist plant 1

[davefitz]

report in Modern Power Systems , as in attached, is a remarkably simple improvement in the regenerative heating system of nearly all rankine cycle based steam plants that can improve net station heat rate by ( 1-3%).

Currently , most steam turbine extraction contains excess superheat, and passes thru a desuperheating zone of the feedwater heater prior to condesning in the feedwater heater. If , instead of this desuperheating zone, the Hp exhaust steam ( cold reheat ) is instead passed thru a "tuning turbine" and expanded to 0-10% liquid by wt then passed int othe LP heater shells, there is a heat rate improvement. ( This was discovered as an incidental aside of the EU AP700 project for a 700 C final steam temperature cycle )

<

http://www.modernpowersystems.com/story.asp?sectioncode=83&storyCode=2052406>

 

[MJCronin]

UUummmmm... theoretically sounds good.

But the piping/valving might get a little wierd and expensive

Wouldn't you have to have several "tuning turbines" for all of the different extraction pressures...?

 

[eyec]

not a turbine expert, but isnt that what the IP stage does or a topping turbine for two stage turbines?


Steven C
Senior Member
ThirdPartyInspections.com

 

[davefitz]

Should be simple to simulate on today's turbine simulation computer programs.

IP steam turbine extractions still have remnant superheat to the feedwater heater desuperheater zone- because the starting point for the IP turbine is hot reheat ( which is about 400 F hotter than the cold reheat). Using a "tuning turbine" that has cold reheat as the turbine influent , the steam to extractions will have zero superheat and likely have some liquid.

In the EU, most steam turbines use full arc throttling and operate the boiler at variable pressure, which retains high superheat at each extraction point over the load range- but in the US, the boilers operate at constant throttle pressure and the steam turbines use partial arc admission, so the extractions have much less superheat at part load. So I would not expect as much of an efficiency gain in the US at part load if used at existing installations.

 

[davefitz]

If I had to guess, this was discovered by using an "exergy analysis" of the cycle- in addition to the standard output of T,P,H, duty, power etc output for each process point and component in the cycle, the europeans now typically plot an exergy diagram to determine the primary culprits for increasing entropy, and then focus on the worst culprits for improving the cycle efficiency..

 

[eyec]

what about the addition of a topping turbine from the IP or LP stage? would this not be a more cost effective and easier engineering modification? especially with load cycling units?

i am looking more at this from a retrofit than new design/construction.



Steven C
Senior Member
ThirdPartyInspections.com

 

[stgrme]

The technologies, discussed in the article in Modern Power Systems, are not new, but a re-application and combination of previously proven technologies.

A number of power plants in the USA were built in the 1960s, which employed boiler feedwater pump drive turbines with extractions to some of the intermediate pressure feedwater heaters and an exhaust to a low pressure feedwater heater. In fact, a technical paper by R. L. Bannister at Siemens-Westinghouse (now, Siemens) several years ago suggested re-considering this configuration to improve cycle efficiency.

Note the wording in the sixth paragraph of the article, '...the "tuning turbine" is fed from the first cold reheat steam line.' This wording indicates a double reheat cycle, which was also employed at a number of US plants in the 1960s and 1970s. Double reheat cycles make a lot of sense when used in ultra-supercritical cycles.

Ultra-supercritical cycles is another old idea; only the name is new. Exelon Eddystone Unit 1 went into service in 1959 and still operates today in an ultra-supercritical, double reheat cycle with main steam pressure and temperature greater than 4500 psig and 1100 degrees-F, respectively.

Please note that the technologies from the article in Modern Power Systems would only be applicable to new turbines since turbine blade paths are custom designed to suit the thermal cycle.

 

[davefitz]

stgme:

you are right on most counts, but the new twist is in using exhaust from a "tuning turbine" in lieu of extractions from the IP/LP turbines , and this change alone contributing to an additonal 1-3% neat rate improvement. It probably has larger benefits on a 700 C / 1300 F cycle than at a standard 1005 F final steam temp cycle.

The other cycle changes you noted have been used, and offer additional cycle improvements.

The eddystone unit had originally used a higher steam temperature of perhaps 1300 F, but derated to 1100F to deal with high temp coal ash corrosion of the superheater and reheater, and probably other metallurgical issues. There may be perhaps 20 double RH units in the usa of the 1960-1975 vintage.


The EU project rehashes many of these imrovements, again for a 1300 F (700 C) final steam temp, using high nickel alloys and attempting to close-couple the boiler with the steam turbine to minimize the cost of the main steam leads.

 

[davefitz]

correction :just checked the orignal Eddystone desing case: 5000 psig at 1200 F HP main steam.

 

[rmw]

None of this makes much sense to me. First, while the desuperheating zones have a low heat transfer rate, the sensible temperature exchange is transferred to the feedwater which is an important part of the cycle. I read the article but did no further research. I didn't see enough information here to convince me that there was any merit to the concept. Secondly, if you are going to drop bleeds through "tuning turbines" and not through FWH's then why not just design the steam path (all the way through the condenser) to leave the steam in the main steam turbine steam path. That large machine would have to be much more efficient than a set of smaller "tuning" turbines.

I once had to do an economic analysis for taking the first point heater (last heater in the chain) out of service. Yes, there was a MW gain because of the increased steam flow through the main turbine (until the condenser ran out of surface) but there was a severe penalty due to the loss of temperature contribution of the first point heater (the point was cold RH) to the FW and the increased pressure drop though the RH.

I fail to see how this would contribute 1-3% to the heat rate on the basis of what I read.

rmw

 

[davefitz]

I cannot quanitatively prove the 1-3% claims without a license to use one of the cycle simulation programs, but I suppose others with licences could.

Certainly, if it was a cycle improvement that is easy to visualize without such programs , it would have been tried 40 yrs ago.

In the case of 700 C (1300 F) cycle, some of the extractions would be over 900 F, implying a temperature difference of over 400 F in the desuperheater zone- that specific increase in entropy due to a large DT during heat transfer may be a clue to benefits of alternately using saturated or wet steam for extractions ( sourced from the tuning turbine) - although from a global viewpoint there is no shortage of high temperature heat available when the furnace's flame temp is on the order of 2500 F.

Again, one may need to crank the numbers to confirm any benefit.

 

[rmw]

It is an interesting discussion.

The whole point of feedwater heating is to get the boiler feedwater temperature as close as possible to the boiling point before it enters the boiler all the while taking as little steam from the power generation steam flow path as possible. Every bit of steam that is taken out of a bleed point is steam that doesn't generate any KW.

This is typically done with FWH's and economizers and usually with those in combination in large utility boilers. That is why the highest pressure point heater is something usually on the order of cold reheat or the first few stages of the IP turbine. There isn't much economy in using once hot bleed steam that has been cooled off through a 'tuning' turbine. The water temperature produced wouldn't be close enough to boiler temperature to make it worthwhile. Besides, at that point, there is usually a plethora of low temp steam available; BFP turbine exhaust, low pressure bleeds, gland leakage steam, etc.

Also, I'd have to think for a while to remember all the reasons why, but colder water entering the economizer is problematic as well.

The closer you can get to the boiling point of the water to steam, the more efficient the boiler is. I fail to see how utilizing low pressure tuning turbine exhaust gets you there no matter what KW is produced by those devices.

I don't have a license any more or I would model it. I work with some who do so I may see if they have any interest in looking at it.

I've seen schemes that substituted exhaust heated FWH's for bleed steam FWH's and the numbers are there, but the flue gas cold end temperatures are tricky to maintain above the acid dew point. So it is not as if I am saying that bleed steam FWH's are the holy grail.

rmw

 

[stgrme]

I found a technical paper on the AD700 program from 2007, which might clear up some of the speculation in this thread. Please see the attached PDF file.

The cycle diagram on page 10 shows the highest pressure FWH fed from the first cold reheat, the next five heaters, including the deaerator, are fed from the "tuning turbine", and the three lowest pressure heaters are fed from the LP turbine. Also note that the "tuning turbine" drives the BFP in conjunction with a generator/motor.

I'm not sure if the cycle has changed much since this paer in 2007, but I suspect that only minor refinements have been incorporated.

Perhaps there is another way to look at the efficiency of using superheated steam in FWHs. Typical TTD for a FWH fed from the cold reheat on a single reheat plant, which has a small amount of superheat, is zero degrees-F. For the same plant, the TTD for the FWH fed from the first extraction in the IP turbine, which has a considerable amount of superheat, may be minus (-) three degrees-F. The TTD for FWHs fed from the LP turbine is typically five degrees-F. The difference from no superheat to considerable superheat only changes the TTD by about eight degrees. Therefore, you can see that the FW temperature leaving any FWH is more closely related to the saturation pressure in the heater than the amount of superheat in the extraction steam.

 

[davefitz]

stgrme:

That is a good paper. It indicates the cycle config has been patented by Elsam.

The TTD for a HP heater is relative to the sat temp at the heater shell pressure or fraction of extraction nozzle pressure, and a TTD of -3F does not imply that the extractioin superheat is only on the order of 5-10F. As per the tables in paper , the superheat at the extraction nozzle can be hundreds of degrees hotter than the heater water outlet temp.

 

[rmw]

I will try to read this paper more in depth over the weekend. Today I am too tired from the ravages of my day job.

At first glance, however, I see sixes. That is it is a rob peter to pay Paul situation. All the extraction except the very lowest pressure (below atmospheric most probably) are done by the T turbine instead of the main turbine. All I see this doing is reducing the size of the re-heater(s). That might have merit. On the other hand, this cycle has 7 heaters (not counting the deaerator) and those and their piping don't come cheap.

Schematically this is a nice deal but I would like to see a real heat balance with real numbers with realistic numbers.

The extraction points are just trade offs between doing this in the t-turbine - that's acceptable - and the main turbine. It might actually make the main turbine less complicated and help with the flow path. The trip down the expansion line from the throttle temp/pressure to the expansion line end point is the same regardless of which turbine it flows through. The slope of the line is merely a function of the efficiency of the machine.

One thing I will assert is that superheated steam is better utilized making HP than heating FW. So if this T-turbine permits the utilization of double reheat without having to waste a lot of that reheat at the bleed points to heat FW, then this cycle might have real merit. Thanks STGRME for the link.

As to the TTD as pertains to SH temp, the TTD is merely a function of the amount of surface area put into the heater to desuperheat the steam. Normally the purpose of the DSH zone is to get the extraction steam to condensing temperature (or some temperature right above condensing so as to prevent wet/dry cycling of the last pass(es) of the DSH zone), not to produce some value of TTD. The extraction point should be chosen so as to minimize the amount of SH in the extraction steam while allowing enough to prevent wet steam admission to the heaters. The LP heaters have to have special designs to prevent tube cutting at the inlet.

rmw

 

[davefitz]

rmw:

I agree , but there are other factors that impact the selection of extraction points. When using conventional tubesheet based heaters, there is a thermal stress limit to allowable tube side temperature gain on the water side, and this may typically be limtited to 80 F per heater at MCR- but if using a header type heater, then this limit no longer needs to be respected.

To some extent, the value of the T-turbine may be reduced due to (a) the expected trend toward parallel poweredcycles using biomass to produce the energy to heat feedwater and US installed capacity is mostly partial arc turbines with low values of extraction superheat at part load.

 

[stgrme]

Let's clear up some misconceptions. First, partial arc turbines do not have low values of superheat in the extractions at low loads. At low loads, the HP portion of the partial arc turbines are less efficient and the enthalpy at the HP exhaust actually rises. This higher enthalpy results in more, rather than less, superheat at the HP exhaust.

From the IP inlet to the LP exhaust, the temperature at any extraction is essentially constant over the load range if the inlet temperature to the IP turbine is maintained. If the IP inlet temperature falls off, extraction temperature will fall off by a similar amount. From the IP inlet to the LP exhaust, full arc admission and partial arc admission turbines act alike since there is no partial admission at the IP inlet.

Second, the technologies discussed in the Modern Power Systems article are not, I repeat are not, directly applicable to existing units because their blade paths were designed for extractions to FWHs. Application to any existing units would involve a complete retrofit to, or replacement of, the main turbine and the BFP drive turbine, plus extensive replacement of the FWHs and modifications to the BOP.

 

[davefitz]

stgme:
I beg to differ regarding the fall off in extration superheat. The higher enthalpy drop across the governor stage on constant pressure /partial arc units leads to a monotonically decreasing cold reheater steam temp as load drops, unless the unit has had its boiler converted to variable pressure ops. So the no 1 HP heater extraction superheat ( typically cold reheater / HP exhaust) drops off pretty quickly, and if the design hot reheater steam temp drops, similar loss of extraction superheat occurs for other extractions.

Methods to forestall the drop in HRH steam temp include high excess air operation at low loads, GR fan ( until the fan self destructs) , buner tilt and convective pass dampers on parellel pass units, but achieving design reheater steam temp is often limited to the load range 70-100% MCR load on many constant throttle pressure units.

I have no idea of the abilty to modify the IP/LP turbine to allow the use of a " tuning turbine", but replacement of older ( pre 1992 blades ) with modern blades ( designed with 3d-Cad and fabricated with NC lathes) may provide other efficiency or capacity improvements in parallel with the ( supposed) improvement of the tuning turbine.

And I am now oficially out of my field of expertise.

 

[stgrme]

My apologies davefitz, you are correct about the superheat at the HP exhaust falling off for a partial arc admission turbine. I actually had a different operating scenario in mind when I stated that the enthalpy at the HP exhaust rises at lower loads. I checked expansion lines for an older fossil unit and found about 25 to 30 degrees-F less superheat at lower loads.

Typically, the installation of modern blades will improve efficiency by 2% to 4% in each turbine section. However, the improvement in HP section efficiency lowers the cold reheat temperature and more heat input to the reheater is necessary to reach the original hot reheat temperature (usually, by adding more reheater surface).

As for modifications to the IP and LP blade paths to eliminate extractions, the flow areas must be increased, which will most likely result in longer stationary vanes and rotaing blades. Even if the longer vanes and blades can be accomodated in the turbine casing, increased bending stresses in the vane diaphragms and blade foils/fastenings may exceed allowables.

 

[rmw]

Re-reading the title and the first paragraph of the OP, the initial premise of this thread started out being that this novel concept would raise the efficiency of existing Rankine cycle plants by the stated percentages-or that is the way I understood it.

I think it has been well established by valued contributors to this thread that to modify an existing large steam turbine power plant's steam cycle by the addition of a T-turbine would require serious modifications to the main turbine's steam flow path, probably more extensive (and expensive) than physically (and financially) possible with a 20-30 year old machine short of complete rotor/diaphragm (spindle/dummy ring) change out which, while expensive and a major undertaking, are not totally unheard of. I think GE has some GER's on their site to this effect.

However, that effort, with the other required cycle modifications, such as to the power piping to the FWH's now served by extractions from the T-turbine rather than the main turbine's IP and LP sections as well as the FWH's themselves as they now have to be designed ground up with new steam and water conditions and flows make this project more daunting by the minute.

While I have warmed somewhat to the overall concept, my fuzzies will have to strictly be limited to projects that start with a clean sheet of paper as we used to say.

I do like what it does to the second point heater which in the normal (single RH at least) cycle is furnished steam by relatively high temperature superheated steam from the first extraction point of the IP turbine and which in my experience is a trouble spot in the FWH string. The DSH sections of that heater and occasionally the third point heater take a beating at those temperatures. The high temperatures place limitations on certain tube metallurgy's too.

Again, while taking at face value the thermodynamics of the steam flows through the T-turbine I do have to wonder why the t-turbine doesn't exhaust it's remaining steam to a LP heater in the main turbine's condenser neck rather than a separate condenser called a regenerative heater; but who am I to say?

Seriously, however, I do have to wonder what type of generator that it will take that will turn via the SSS clutch at the same speed as the turbine and BFP. Will that be a half or 3/4 pole machine since it is stated that the T-turbine and pump need to turn ~5K rpm for optimum stage efficiency purposes. So, oops, now we need a gear box somewhere in the T-turbine train; more expense, not to mention the cost and complexity of the additional generator itself.

I will also have to concede that by loading the T-generator when the main turbine is at partial loads, steam flows can be maintained through the T-turbine insuring that the FW temperature stays as high as possible, only limited by the temperature of the cold RH rather than the decay of temperatures throughout the entire main turbine steam path.

So, I still have to wonder what we have accomplished. Even with a clean sheet of paper design, the additional components required make me wonder if the capital expense will ever be recouped. While I can be convinced if I see the real numbers that prove it out, however, let's just say that the first of these plants will have to be located at Missouri P&L for me to believe it.

BUT, and that is a capital BUT, I still say that this has been an excellent peer to peer discussion of an interesting emerging technology. Thank you DAVEFITZ for bringing this to our attention. I'll be interested to see where this concept goes-if it goes, that is.

Weigh in if you have any more thoughts or comments to my thoughts.

rmw