CCGT minimum load

[kkanel]

Does anyone have an idea on how low one may operate continuously a combined cycle plant- or rather how low such a plant may be designed to operate without by-passing steam? Data available report 40% of nominal load which is too high for the application I am examining
My current problem is that I am working on this 2 GTs + 1 ST configuration (fan assisted cooling tower) and I am trying to find out whether I may operate the plant with the ST at 20% its nominal load. That is, one GT running at approximately 40% load. This is serious off-design operation and I am particularly worried about steam cycle operation.
I run heat balance calculations and I get only warnings of steam piping loss increase, which may be countered by proper pipe design, but other than that not much else.

 

[ccfowler]

I'm curious as to why this is an issue of concern. With the dreadful part-load fuel consumption of these beasts that are so efficient at full load, who or why would one be operated at such light loads for an extended period of time?

 

[poetix99]

Because (unfortunately) it occassionally happens that the few people who actually try to provide for the future get burned, and have to downsize their entire operation. They find themselves wishing they'd purchased half the capacity in the first place.

This does sound like a very extreme circumstance.

I hope for you that it is temporary. I also hope that the actual efficiency of the ST is not a concern at this part load. Does it make any sense to inlet throttle the steam to the turbine? Depending upon the inlet valve gear for your ST, such a low part load could impose significant stresses on the blades exposed to the active steam nozzles of the first stage.

You ought to consult with the steam turbine OEM about "long term" operation at this part-load. It is a situation that causes (relatively) high blade loading, but it might be provided for in the original design. Don't let them try to take advantage of this and give you a sales pitch without some techincal info to back it up.

This does not address your apparent concerns about "steam by-passing", which I do not understand.

 

[ccfowler]

poetix99, perhaps I should have explained my question more. It was (and still is) intended as a serious question.

Since the beginning of the serious publicity and marketing of CCGT's, I've always expected that some buyers of these units would be blinded by their full load efficiency and would get hurt by the part load efficiency problem. (I often recall my astonishment/horror many years ago when I first learned that a simple cycle GT that I was dealing with required over 90% of its full load fuel consumption rate just to maintain a synchronous idle condition.)

My question relates to the simple economics of trying to run a GT at such low loads for an extended period of time. I'd expect that power could be purchased for much less than the fuel cost alone at such low loads. Thus the question of why operation is being considered for such a low loading? This represents such a painful waste of fuel and money that it seems some special circumstance must be involved. It does not seem that the steam is needed for some purpose other than feeding the ST.

 

[kkanel]

Actually the plant is expected to operate down to this regime less than 10% of the total time.
However if one does not carefully assess the plant performance at this level one could get seriously burned.
The steam bypassing is mentioned because at his operating level the plant operator may be forced to stop the ST and keep the GT running in simple cycle while by-passing the steam, which worsens a lot performance.
The ability to keep the ST running at very low loads for 3 or 4 hours at night theoretically improves plant economics.
What made me raise my eyebrows was ccfowler's comment on a GT's 90% of nominal fuel consumption to remain synchronised. I thought fuel consumption at his level very low despite the very high heat rate mainly because of very low output.

 

[ccfowler]

The GT I mentioned at synchronous idle was a nominal 20 MW range single shaft, simple cycle unit operating at synchronous speed but not connected to the grid while being operated for testing purposes. Its auxiliary loads (lube oil pumps, cooling coil fans, etc.) were being carried by an auxiliary diesel generator. Obviously, the compressor load was substantial. I had not previously paid serious attention to low load characteristics of GTs, so the observation of this seemingly high rate of fuel consumption really caught my attention.

In retrospect, it all seemed to make sense since the machine delivered the expected power output and heat rate when operated at full load. It promptly became apparent to me that such machines need to be operated either at full load with reasonably rapid ramping rates or they need to be kept on their turning gear. Indeed, the subject machine was never intended to be operated in any mode but full load for peaking duty.

Multiple shaft machines can be expected to deliver significantly better part-load fuel usage, but even so low-load operation is never likely to be economically attractive.

One issue that I would want to consider for low load operation of the ST would be the potential for water droplet erosion damage to the last stage blading. If such low load operation was not contemplated in the ST design, it may not include suitable provisions for adequate blade protection for extended low load operation.

 

[poetix99]

ccfowler, I took your question to be serious, and my reply was not meant in sarcasm.

I am aware of the part-load problems of CCGT/ST, but only in overview, having more experience only with STs.

In any case, from the steam turbine side, I think that if the steam conditions out of the HRSG are not outside of the "warranted" range of steam conditions (I am thinking specifically of high turbine inlet temperature), it remains only to have some confidence (via consultation with the OEM) about part load stress on the turbine's stage blading.

If this operating condition was indicated in the purchase specs it should be covered, especially if the equipment was purchased single-source.

Verify, rather than hope or trust that it was.

 

[ccfowler]

poetix99, I defer to your greater familiarity with STs regarding the potential ST exhaust condition problems. My background is primarily with auxiliary equipment and systems. I recall seeing some major damage on the last row blades of several STs that had been operated at reduced loads for extended periods.

kkanel, your study of this low load situation seems like an excellent idea to avoid needless damage. Along with this study, you may want to consider alternative operating plans. Specifically, if your plant has means to sell excess power, it may make sense to keep the unit loaded at either 1 GT at full load or both GTs at full load and sell the excess power at attractive rates.

When you consider all of the implications of avoiding load changes, low load operation, additional starts, etc. on maintenance costs, unit service life, etc., you may find some very interesting economic effects and opportunities that may not be immediately apparent without some very creative evaluation of the data. For example, it may be attractive to sell the excess power priced mainly on the basis of the relatively small additional amount of fuel needed while realizing a significant portion of the profit of these power sales in the form of avoided maintenance and unit life costs.

 

[kkanel]

Thanks ccfowler and poetix 99 for your precious comments on the particular question.
Actually the off-design problem of the particular project is more complex because there are many constraints on the expected operation profile.
This plant will sell power in an island with a relatively contained grid. The purchaser will be the grid operator and current sole power producer on the island. So the power that may be sold is very specific for each hour and is dispached on specific orders form the grid operator based primarily on economic assesments. However current production based on diesel engines is more economical.
The plant that suits best the operational profile with current practices is the reciprocating diesel engine running on heavy fuel oil which have better performance at off-design and higher temperatures. However for this particular project the very low emission limits have posed problems with the diesel engine solution since SCRs and scrubbers have to be uitilised. Hence the CCGT solution. However my feeling is that if this plant is not the first to dispatch to the grid and therefore operate at decent loads most of the time, then it is nonsense.
Finally my big dilemma is that since I have to quote a heatrate for operation as described in my first question I must know whether I may quote e.g 33% for cc operation or 22% for open cycle operation GT and simply forget operating the ST at those loads.

 

[ccfowler]

Since you are going to have to quote heat rates at low loads, you have my unlimited sympathy. Is the heat rate of the GT alone at 40% load as high as 22%? I'd be pleasantly surprised if it is really that high.

What fuel is available for the CCGTs? If they are to be operating on natural gas, then that fuel would also be available for dual fuel recips which would probably be cleaner than heavy oil fueled diesels yet have very good heat rates down to the 40% range. Since multiple dual fuel recips would be needed to reach the power output of the CCGT plant, it would be practical to run only some of them at lower loads while keeping most at higher load operation, and the combined heat rate would remain very high under all conditions.

I'll offer a good piece of advice that I got many years ago for evaluating the performance claims and ratings of equipment: The published data is always accurate, and it originates in the Engineering Department based on design and test data. The important point to remember is that the data is published by the Sales and Marketing Department in a manner to always put everything in the most favorable perspective. Always be sure that you really understand the full implications and limitations of the data. From my own experience, this seems to be almost universally true.

For your GTs, I'd want to know what fuel rate to expect while operating at synchronous idle conditions. This would be very important in putting low load fuel consumption estimates in a meaningful perspective.

Good luck!