# The whys of Combined Cycle power plants

1. Feb 13, 2012

### Charles123

Why cant coal be the fuel for combined cycle power plants?
Why is the succession of Brayton cycle and the Rankine cycle the most common design solution? Why using two Brayton cycles in series, using the lower temperature heat to drive a second gas turbine be a bad idea? Why would efficiency be lower (I am thinking thats the case)?
Thank you

2. Feb 14, 2012

### jim hardy

""Why can't coal be the fuel for combined cycle power plants?""

If you had a jet engine that ran on pulverized coal you could do it.

I think Pratt-Whitney did some research into blades for such an engine but i dont know how far that project went.

Your thermodynamic questions i will defer to someone better qualified.

old jim

3. Feb 14, 2012

### Pkruse

I'm thinking of all the crap that would precipitate out on the blades & clog the cooling holes.

4. Feb 14, 2012

### Pkruse

Two b-cycle in series is common. Just add more disks onto a single turbine.

5. Feb 14, 2012

### Pkruse

Now that I'm at a real key board, I can add more detail. Texting from a phone should be left for the kids who like that stuff. :-)

The idea is to continue to extract energy from the exhaust until what remains is either not technically or ecconomically viable to extract some more. In theory, you could therefore continue adding disks to the turbine until you had gas coming out at ambient temperature. But technical constraints in any device impose a certain minimum temperature be low which you don't want to go. This is normally defined as the temperature at which certain undesireable exhaust components begin to either condense or precipitate. So you end up with exhaust coming out of the gas turbine that is still fairly hot, but not hot enough to add another turbine disk to extract more energy. So you redirect that gas through another device where the minimum required temperature is lower, like maybe the production of steam to drive a steam turbine. Or if someone near by needs heat for space heating or some industrial process, you can send the energy that way without turning it into electricity.

6. Feb 15, 2012

### Charles123

Jim Hardy, thank you for your answer. Please pardon my possibly stupid question but I still do not understand why you could not use coal, since what would drive the turbine would be hot exhaust gases, not the coal particles. Can you please develop your answer?
Pkruse, also thank you for your answer. From what I understood there are power plants where you use turbines in series followed by a Rankine cycle. Is this right?

7. Feb 15, 2012

### AlephZero

In a conventional jet engine design, all the products of combustion go through the turbines, since that is the only exit from the combustion chamber.

So either you have to make a turbine which can survive being "sandblasted" with very hot abrasive particles (i.e. unburnt coal ash), or design a way to filter out the particles from the hot gas and extract them some other way. Neither option is very practical. Of course ash is filtered out of the gas from combustion in coal-fired power stations, but that is done after the gas has cooled

Sand ingestion into jet engines (e.g. in the middle east) is a serious issue, made worse by the fact that sand can melt in the combustion process and finish up as a film of solid glass deposited on metal turbine blades, which has a big effect on the blade cooling. It may require a shorter interval between inspections and overhauls, increasing operating costs.

There is a separate issue with the fuel supply chain. For a relatively small CC plant, supplying gas fuel by a pipeline is a much simpler operation than delivering coal by road or rail, storing a sufficient quantity on site, pulverising it before use, etc. There are jet-engine-based gas pumping systems that work completely unattended for 6 or 9 months at a time in places like Alaska. It would be hard to devise a coal fired system with that little human interacton required to operate it.

8. Feb 15, 2012

### jim hardy

They came up with blades that would stand the abuse.
Last i heard of the project was they had significant troubles getting the coal dust to behave itself in the "burner cans" . It wanted to cake up.

as i said i dont know exactly what became of the project but i seem to remember it never went commercial..
It was not one that i worked on. Back in early 80's a friend and i were exchanging work anecdotes (over beer and barbecue of course) and that's how i heard of it.

Sorry i dont have a better answer for you.
And those are great questions.

old jim

9. Feb 15, 2012

### Charles123

So what would be the methodologies for improving efficiency in coal power plants, since them seem reduced to the use of Rankine cycles?

10. Feb 15, 2012

### Charles123

Also, I was assisting a discussion other day in television that from what I understood concluded that the use of cogeneration in fuel oil power plants (we have some in Portugal) is a very inefficient processed, a lie, as they put it. Why would that be the case?

11. Feb 16, 2012

### Pkruse

It is good to see so many knowledgeable people posting to this interesting thread. Most of what I would have said has already been posted, but I do have a little more concerning gas turbines and their tolerance for dirty gas.

I put some queries out today to some of my more experienced friends in the gas turbine industry. Between them, they have a couple of hundred years of experience designing gas turbines. While I also work in that industry, I consider them to be my teachers and mentors. I was surprised to learn that a few test rigs have been built to test out various coal-fired combustor ideas. But none of them were judged worthy of funding for any sort of full scale development project, and none of them ever hit the market as working systems. That really does not surprise anyone who is familiar with turbine design.

Turbine blades are very susceptible to damage by impurities in the hot gas. About a dozen of these impurities are a particular problem, and coal contains most of them. Modern blades are probably more susceptible to damage than in the past because of the very sophisticated but fragile coatings we put onto them, in order to tolerate the extremely high temperatures that have made modern so much more efficient than in the past. Many turbines are running at temperatures 200 degrees C above the melting point of the blades, so they must be protected very well with modern coatings, and they must be cooled.

As for a ground turbine, as in a power plant, if you are going to install it within 60 miles of the ocean, you must make some design changes to it because of the trace amounts of salt in the atmosphere. (These changes reduce the efficiency a little.) Navy turbines also have this problem. With systems as sensitive as that, I have a hard time understanding why anyone ever thought it was worth building those test rigs for coal combustors. Some politician probably gave them some money, so they felt compelled to spend it to prove what they already knew. (My opinion.)

I will type another response to address efficiency.

12. Feb 16, 2012

### Pkruse

Gas turbines are more efficient than steam turbines for one reason and one reason only: They run at higher temperatures. The laws of thermodynamics limit you to lower efficiencies in any engine that runs at lower temperatures. I do not know why steam turbines run at such low temperatures. When I asked my thermodynamics professor why, he simple said that the steam turbines already run at the limits of their materials. I suspect that since with water and steam higher temperatures also mean higher pressures, that pressures are more of a limiting factor in steam design, whereas temperatures are more of a limit in gas turbine design.

To answer the other question: Nobody is going to put two gas turbines in series with each other to produce power. That is not what I meant. I meant that you essentially accomplish the same thing by adding more disks to one turbine. That is done all the time. But the exhaust from that single turbine may very well still have enough heat to drive a steam boiler and turbine in a combined cycle system. That makes for an extremely high overall efficiency. I’ve heard claims as high as 70%, but I tend to doubt any numbers higher than 65% for the total efficiency of a combined cycle system.

The terms “combined cycle” and “co-generation” are sometimes used to mean the same thing. But it is my understanding that they are similar but different. I defined “combined cycle” in my last paragraph. It is my understanding that “co-generation” takes the waste heat from the turbine or any other type of engine driving an electrical generator, and uses it for something other than making electricity. It may be used for space heating in colder climates, or it may be used in some industrial process. These can be very efficient, also.

I was a small child 50 years ago when I first heard of co-generation plants in Europe. They built them a long time before anyone else decided it might be a good idea to do the same. If the units in Portugal are “inefficient,” then I suspect that is only because they are old and therefore less efficient than the ones built in more modern times.

13. Feb 16, 2012

### Charles123

You are right in the definitions of cogeneration and combined cycle, I should have written combined cycle, the aim is an increased efficiency in the production of electricity, not heat as a product. What I meant about the inefficiency of these Portuguese power plants was specific to the ones running on fuel oil. What I understand is that our gas combined cycle power plants have normal efficiencies. So my question was if there is something in the process of electricity generation from burning fuel oil that makes combined cycle a useless resource?
About the efficiency and its increase in coal power plants are you aware of some methods for its increase, once combined cycle is not a possibility?
Regards

14. Feb 16, 2012

### jim hardy

""The laws of thermodynamics limit you to lower efficiencies in any engine that runs at lower temperatures. I do not know why steam turbines run at such low temperatures. When I asked my thermodynamics professor why, he simple said that the steam turbines already run at the limits of their materials. I suspect that since with water and steam higher temperatures also mean higher pressures, that pressures are more of a limiting factor in steam design, whereas temperatures are more of a limit in gas turbine design.
""

Water above about 3206 psi no longer changes phase when heated so there might be no use in higher pressure than that, i'll defer to a thermodynamicist there. Plenty of "supercritical" boilers have been built to take advantage of that property of water.
B&W literally wrote the book and its title is simply "Steam its generation and use" , peruse thrift shops and used bookstores for it.
Here's their 'brag sheet' on supercritical boiler, looks like 1100F is still about highest steam temperature used.
http://www.babcock.com/products/boilers/swup_specs.html [Broken]

and their book
http://www.babcock.com/library/steam.html [Broken]
sometimes it shows up on Ebay. It's been in print since 1880's, my oldest one is ca 1920.
http://www.ebay.com/itm/HTF-1963-ST...t=Antiquarian_Collectible&hash=item20c28508bb

In my day temperature was limiting factor - not of the turbine blades but the boiler steel. We dont think of water as a solvent but at high temperature and pressure any impurities in it attack steel. Our chrome-moly superheater tubes were okay at 1,000degreesF and 2400 psi , mid 1960's vintage. The company had trouble with some replacement tubes made overseas - after a short while in service they developed cracks and split. They went back to US made tubes.

Nuke plants are limited to slightly over 500 degreeF, maybe 800psi steam because of reactor materials. Except the General Atomic one at Ft St Vrain (near Lyons Colorado) which would have made 1000F 2400psi steam had it ever run. It used helium instead of water to transfer heat from reactor to boilers.

Probably there's newer designs out there.

One of my old employers' fossil plants is interesting, it uses a hundred acres or so of solar reflectors to preheat the water on its way to the boiler,
will inquire as to the efficiency gained that way.

But the general answer to your question is (as i'm sure you know) - higher inlet temperature, lower exhaust temperature as in any Carnot engine.

I'll make a plug here for solar water heating - for each BTU in your morning shower, ~two were discarded as waste heat from a power plant.
Best thing we could do for CO2 emissions is put an old fashioned flat panel heater on every roof. And that's a pretty easy DIY.

old jim

Last edited by a moderator: May 5, 2017
15. Feb 17, 2012

### Pkruse

Combined cycle is always more efficient than the same plant would be without it. It is not a wasted resource.

I do not have any details as to why these fuel oil plants would be less efficient, but that is certainly possible. Perhaps they are steam plants whereas the gas fired plants are gas turbines. As we have been talking about here, steam will be less efficient than gas turbine. But rest assured that these oil fired plants would be less efficient than they currently are if they were not combined cycle.

Also keep in mind that smaller gas turbines are less efficient than larger ones. Gas turbines do not scale down well. So if they are gas and not steam, then perhaps they are smaller ones.

16. Feb 17, 2012

### Pkruse

“About the efficiency and its increase in coal power plants are you aware of some methods for its increase, once combined cycle is not a possibility?”

Back when I was in school, my professors would point out that it is not fair to speak of efficiency to the exclusion of all other factors that are considered when people build a plant to burn coal. Back then, coal was by far the cheapest way to go after all else was considered. It cost less to put a kilowatt-hour into the power grid with coal than with any other option, so who cares if it is less efficient from a purely thermodynamic perspective. It was more efficient from a dollar perspective, and in the end that was all that counted. I think in Europe, coal is still your cheapest option.

But that was a long time ago and today we look negatively upon coal only because it has the largest negative impact on the environment of all the fuel options. None the less, I believe that in the future we will return to burning coal massively, but only after we learn to do so cleanly. Several ideas as how to do that are currently being funded massively, and I believe that at least one or two will result in practical, cost effective, and clean ways of burning coal. I won’t speak to details for fear of accidently spilling proprietary secrets, which would get me fired. But the research scientists doing this work have published massively in journals and other forums that cater to their particular area of specialties, so you can find it if you look for it.

One of the lines of research falls under the heading of “Carbon Sequestration.” There are many ideas of how that could be done, and many of them have been funded massively for research. So that would be a good place to start your Google search. It now seems reasonable to burn any fuel and have zero emissions at a reasonable cost. We just need to work out the details, and many people are fully engaged at doing just that.

17. Feb 17, 2012

### Pkruse

Jim: Thanks for giving my brain the kick in the butt it needed. I knew all that, but was not thinking about it. Some of that old stuff is coming back. I've seen discussion on this forum as how to design a Helium system, and the guy is talking about temperatures a whole lot hotter than that. His work might have something to do with what General Atomics is bragging about on their web site.

Another type of system that is currently in the press as they build a few test plants is using water at those very high temperatures to break down pretty much any waste, hazardous or not, into simple gas that can power a conventional plant. That makes for some interesting contemplation, too. I wonder what sort of materials they are using to contain it.

18. Feb 17, 2012

### Charles123

I have a question about your first reply, can you elaborate on the issue of downsizing in gas turbines and its associated decrease in efficiency?
About coal, I am familiar with those cleaner ways to use it, like the example you used of carbon sequestration. As well as with the fact the it holds huge interest for its price, availability and for its wide geographical distribution as a geological resource. My question was purely related to thermodynamic efficiency, how much electric energy can I generate with a x amount of fuel. Any ideas of what its being done or can be done in the near future?
Regards

19. Feb 18, 2012

### Pkruse

If we could build steam engines to run at the same temperatures as gas turbines, then their efficiencies would be about the same. That would mean using an inert gas, or at least a gas that is not terribly reactive at these temperatures. Jim suggested Helium in one of the notes above. That would be ideal. But all proposals to use anything other than water and steam are a whole lot more expensive than water or steam. That is why the industry has chosen to operate as it does.

As for scaling down a GT, that is a complicated issue. One of the causes of inefficiency in GTs is leaks past the clearances between moving parts. So we keep the clearances as small as possible. This is complicated by the fact that the parts get bigger under stress or when they get hot, so we always have some clearance. We just try to make that clearance as small as possible so as to leak as little as possible through it. That clearance cannot be scaled down when you make smaller GTs, because it is always as small as we can make it. In big GTs, the effect of a leak is a small percentage of the total gas flow. In a small GT, the percentage is bigger and therefore has a bigger effect on the inefficiency of the machine.

20. Feb 18, 2012

### Charles123

Thank you for the answers! This was very elucidating.
Regards