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Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

(OP)
Hello all,

For those that haven't seen them, I've previously started two threads seeking information and viewpoints on the above subject; here are the links to them...

Can 3 GT's Supply 1 HRSG? http://www.eng-tips.com/viewthread.cfm?qid=341551" \l "
thread391-341551: Can 3 GT's Supply 1 HRSG?

and

What's Better: VFAC-to-AC & Back or DC-to-AC & Back? http://www.eng-tips.com/viewthread.cfm?qid=342485" \l "
thread237-342485: What's Better: VFAC-to-AC & Back or DC-to-AC & Back?

There is already a response from waross within one of the threads [I'm still learning how Eng-Tips works, which is why I didn't start just one thread about it all in this forum; sorry about that].

I appreciate that Bill called it the way he sees it; nevertheless I'm hoping that there may be a more sanguine assessment of the concept out there...

Thanks for listening.

CR

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

Sorry for the "Tough love." I appreciate and respect your attitude.
Note to self. Don't post before first coffee in the morning.
I don't know if this will work but:
You mentioned the issues with running a large multiple expansion engine with a low load. As I understand your information the first stage tends to do all the work with the possibility of condensate carryover into the following stages. Can this be mitigated by using a lower pressure steam? I am thinking a small amount of steam may be bled off from the combined cycle to a heat exchanger to generate enough low pressure steam to drive the reciprocating engine with a light load. An exchanger as a steam generator will allow you to keep the oily recip' condensate separate from the clean steam turbine condensate.
PS Welcome to this forum.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

(OP)
Hi again Bill,

No apologies necessary for the 'tough love.'

You also wrote: 'Welcome to this forum.' To which I respond: Thank you! Plus, maybe I finally found the right home for all the questions I've been asking... ;o)

Avoiding problems by using LP steam? Hmmm...I'm afraid it's my turn to be pessimistic...

Here are the issues I can see:

The capital cost of gas turbines and medium-pressure HRSG's was already piling up; having high-pressure HRSG's and an HP steam turbine PLUS a heat exchanger PLUS all the additional piping...now it's MY mind that's boggling!

Based on the way powerplants are registered and licensed in Ontario, adding a steam turbine to the powerplant mix pushes the staff qualification requirement up to the next level, adding significantly to the anticipated ongoing operating cost.

Shoe-horning in all of the required extra equipment would use up too much of the available space within the holds.

Would not the law of diminishing returns kick in at this point? Since the frictional load of the engine would be fixed by its size, its overall efficiency at lighter loads would be permanently and inherently poorer, wouldn't it?

Would a lower-pressure heat exchanger be that much more tolerant of oil than an LP HRSG? Correct me if I'm wrong, but would not the same problems be encountered in both a low-pressure HRSG and a low-pressure heat exchanger?

Questions, questions...

CR

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

I'm not a steam guy but I have been thrown into the deep end once or twice.
I understood that the issue with oil was with the turbine rather than the HRSG.
I don't anticipate using a lot of steam, and not high pressure steam, but using steam rather than turning gear. Now you have a working reciprocating engine driven by steam. The question was, will dropping the steam pressure even out the expansion across all stages instead of all the work being done by the first stage?

I did not visualize a high pressure turbine, I am looking for a best approach to running the reciprocating engine realistically and economically.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

(OP)
Hi again Bill,

Gotcha this time...I think.

My understanding of oil is that if it gets into any heated pressurized water vessel it will attack the metal in the vicinity of the waterline; as to how the rate of attack relates to temperature, I don't know.

Whether the presence of oil in the steam supply to a turbine would be injurious to it, I also don't know...

As far as the expansion thing: you're quite correct that for a given load, at a lower steam pressure later cylinder cutoff would be required, which would indeed even out the amount of work performed across the cylinders of a multiple-expansion steam engine.

As a result, I'm thinking you may be onto something; perhaps having a single 2 MW gas turbine with a tail-end HRSG producing steam at, say, 100 psi to run the engine at rated speed of 120 RPM but at an output of, say, 1 MW might be doable...

There would still be the issue of internal lube for the engine, but there are ways of dealing with that...

Thanks for the input, for your patience, and for not holding back.

Carl

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

As I see it, the main issue is demonstrating the multiple expansion engine running on steam power. Driving a load and doing useful work ois good but the main issue is demonstrating the engine under steam power. To that end either a relatively small load or even no load.
1 MW will require quite a bit of steam.
The heat exchanger/steam generator may be quite small for no load steam. Using an exchanger will isolate the oily condensate from the large HRSGs.
I thought possibly a de-superheater and a once through system but then realized that the cost of make-up water would probably rule ths out.
However there may still be a place for a de-superheater.
How about a heat exchanger/steam generator running at a suitable pressure to drive any auxiliary equipment associated with the reciprocating engine. Then a de-superheater to supply lower pressure steam to the recip'?
This may be the minimum required in terms of space and dollars to turn the old engine on steam power. (The last de-superheater that I worked near was pretty small and cheap considering the capacity.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

(OP)
All good points.

My thinking in going with a 1 MW output from the recip is:

a] it would be somewhere near its efficiency envelope at that loading for that pressure,

b] the costs of staffing with operators would have to be paid no matter what loading is applied to the recip, so maximizing its earning power via electricity sales would be quite favourable for the balance sheet. It will take further research, but in the scenario you're suggesting it might be possible that operating with just one gas turbine/HRSG combo sized to supply the necessary steam would actually decrement the qualification requirements so that staffing costs would be reduced from the route I'm considering.

Note that there is no existing operating steam plant, therefore no existing or anticipated need for superheated steam, therefore no requirement for desuperheating; saturated steam would likely serve all expected needs.

I'd have to do some digging to see if the auxiliaries were originally operated using main steam at ~220 psi or if they were all fed using aux steam at ~100 psi; I haven't chased all those pipes yet...in the latter case everything would probably all be fed at the same pressure, due to the dictates of a and b above.

CR

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

When steam pressure is dropped through a restriction it becomes superheated. A de-superheater then adds water to saturate the steam and increase the volume. It is basically a length of pipe with a steam nozzle in, a steam nozzle out and a water nozzle. Add two control valves and pressure and temperature instrumentation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

(OP)
Hi Bill,

That's one type; the water supplied must however be very pure or scale may develop around the water injection point.

It is also possible to not completely desuperheat, in other words to employ the same device as an attemperator by re-converting only a portion of the enthalpic content of the superheated steam, not that there's a huge degree of superheat in 'flashed' S/H steam anyway...a benefit of doing this is that the slight remaining S/H eliminates any concerns about maintaining the proper dryness fraction, which can be important in some applications.

A much older type of desuperheater incorporated a U-shaped heat exchanger situated within the steam drum water space; steam from the superheater outlet was routed through this tube, giving up almost all of its superheat in the process. This was of great benefit in plants where both saturated and superheated steam were used; steam pressure would have to be raised quite gingerly, with superheater outlet drains open, to preclude subjecting the superheater to excessive temperatures. As soon as there was enough steam available to drive some of the auxiliary plant [which generally operated on saturated steam] the S/H drains could be closed and the warmup rate increased somewhat since there would now be a greater steam flow through the superheater to protect it.

Unfortunately this design of desuperheater suffered from high and/or fluctuating delta T at its inlet, which led to thermal cycling and metal fatigue, hence it fell out of favour. At least that's what my old engineering books tell me... ;o)

CR

RE: Concept Study: [Very!] Small-Scale Combined-Cycle Power Plant, Marine Steam Engine As Prime Mover

I suspect that the best solution may be to design the combined cycle power plant with little regard to the reciprocating engine. Then investigate the possibility of diverting a small amount of steam to run the reciprocating engine.
The issue of whether to do useful work and how much work, or just using steam rather than turning gear to rotate the old engine may be dependent on the amount of steam that may be diverted from the combined cycle plant without negatively impacting the main revenue stream.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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