gas cycle, rankine cycle, organic rankine cycle, kalina cycle, ...

[Lalbatros]

Hello,

I have been asked an advice about waste heat to power systems (say wh2p).
The available waste heat to be considered is a (dirty) gas in a temperature range between 250°C and 450°C.
The useful power that could be generated would range between 2MW and 10MW.

I know some examples based on steam turbines, either the rankine cycle or the organic rankine cycle.
I have read quite a lot about that, but I not a specialist of this topic.
I am interrested by answers to some very naïve questions like these:

* Apparently the back pressure in wh2p steam turbines is above athmospheric pressure. Is that right? And what about the temperature at the turbine outlet?
* If this is right, why is that so? Going to lower pressure could increase the efficiency, isn't it? Is the cost too high? Why? Is there any other drawback?
* I have never seen any application based on a gas cycle? Why?
* What is the advantage of a steam-based cycle conmpared to a gas-cycle in such circumstances?
* A gas cycle could be build with a compressor, and heat exchanger and a gas turbine. I think this is called "indirect cycle" as compared to a gas cycle based on a combustion chamber. Have such devices been used? If no, why?
* Naïvely, I think that the lowest temperature in a cycle is the most important parameter. Is that true?
* From all the cycles mentioned in the title, I think that none have any special advantage or disadvantage, as far as the lowest temperature in the cycle is concerned. What do you think?
* The overall efficiency of a wh2p plant results from the heat-to-power efficiency as well as from the heat exchanger efficiency. This exchanger is needed to bring heat from the waste stream to power stream. Therefore, focusing of the cycle efficiency is not enough and could even lead to wrong conclusions.
* How can the "organic rankine cycle" (orc) and the "kalina cycle" (kc) be compared?
(orc is based on a pure fluid with low boiling point, kc is based on an ammoniac-water mixture with a variable boiling point)
* In other words, when comparing orc and kc performance, how to avoid comparing apples and pears? Is it only possible to draw general comparisons?
* As far as the cycles efficiency are involved, I would guess that orc and kc have the same efficiency if the boiling point is the same. Do you agree?
* Is the Carnot efficiency based on max and min temperatures practically relevant to compare the orc and kc cycles? Or would there be obvious big differences between orc and kc that would make their cycle efficiency obviously and significantly different?
* The different fluids (pure organic versus ammoniac/water mixture) could make a difference on the heat exchanger side. Is it possible to draw a general conclusions, or is it necessary to study specific cases?
* For sure, the kc has one additional degree of freedom: the mixture level. Therefore, it should be no surprise that an optimisation would be possible, leading to an advantage over the ocr cycle. What do you think about that?
* Does that mean that designing a kalina cycle implies optimizing the cycle (the mixture) for the given waste heat stream in order to get a real advantage over another cycle?
* Would that also mean that any deviation from the optimum (either in implementation or in operation) could just vanish the benefit of the kc?
* Would there be a tendency to complicate and over-design a kalina cycle and impact the cost negatively?
* If the simplest version of the kc is chosen, what are the degree of freedom to optimize the overall efficieny? I see one parameter: the ammoniac/water mixture on the colde side that determines the lowest cycle temperature. Would there be a second or a third parameter?
* If there is only one parameter to optimize in the simplest kalina cycle, why would the kalina efficiency be (always) better that the orc efficiency for the same temperatures?
* The advantages of the orc and kc decrease as the hot side temperature increases? Do you agree? Would you have some data about that?
* Would have some data or ideas about the costs involved?
* When does it make sense to consider orc?
* When does it make sense to consider kc?

* Last but not least, any comment about wh2p is of interrest. Specially comment that could help a non-specialist to sort out what is important and what is not.

With my thanks and best regards,

Michel

 

[davefitz]

not enough questions listed to ensure a response.

 

[CH5OH]

* Apparently the back pressure in wh2p steam turbines is above athmospheric pressure. Is that right? And what about the temperature at the turbine outlet?* If this is right, why is that so? Going to lower pressure could increase the efficiency, isn't it? Is the cost too high? Why? Is there any other drawback?

correct, however you rather have steam leaking out your shaft seal to atmosphere than air sucking in (labyrinth seals)

* I have never seen any application based on a gas cycle? Why?* What is the advantage of a steam-based cycle conmpared to a gas-cycle in such circumstances?

having a fluidum (what drives your turbine) change phase (going to liquid in condencer and back to vapour in boiler)
increases the amount of energy it carries dramatically
(compare energy required to boil one kg of water and energy required to increase steam temperature)
this translates in smaller size heat exchangers (compared to gas cycle)also instead of a compressor you use a pump with much smaller dimension for the same kW installation

* A gas cycle could be build with a compressor, and heat exchanger and a gas turbine. I think this is called "indirect cycle" as compared to a gas cycle based on a combustion chamber. Have such devices been used? If no, why?

also called ericcson cycle
term gas turbine is used where you have internal combustion

(orc is based on a pure fluid with low boiling point, kc is based on an ammoniac-water mixture with a variable boiling point)

all depends on your waste stream as a heat source
is the temperature high enough to boil the fluidum?
hence application of fluidum with lower boiling temperature than water (ORC)

 

[Lalbatros]

Thanks for your answers CH5OH !

The waste heat streams I consider are dirty gas with a temperatures between 250°C and 450°C.

The benefit of organic rankine cycle for low temperatures waste heat stream is quite understandable. However, it is the same rankine cycle, and the advantage is technological. As you explained it is easier to cycle on a liquid phase than on a low pressure or vacuum phase!

I can also understand some potential benefit of the kalina cycle, simply because of an additional degree of freedom available for optimization.

However, the many papers I have read on this topic were not clear about where the real benefits would come from. In the steam versus gas example, the benefit is in the design and not in the thermodynamic efficiency.

I would be interrested by an objective comparison of three optimum designs based of the three candidate cycles (steam, orc, kr) for a given waste heat stream (say combustion gas at 350°C for example). By "objective" I mean that "arguments in favor or against" are avoides and replaced by flowsheets and numbers for process and for economics. By "objective" I also imply that each of the three possibilities have been used in an optumum way.

Would you have seen such a comparison?

 

[CH5OH]

all right lets talk basicsnot exact science just rule of thumbs)
-for every cycle a thermal efficiency of about 33% can be obtained, meaning if you have 100kW available as a heat source, 33kW can be converted into rotating energy.
-if you want to obtain a higher efficiency, you need to add
cycles.
1 cycle=efficiency 0,33 installation cost:X
2 cycles=efficiency 0,55 installation cost:2X
3 cycles=efficiency 0,7 installation cost:3X
in the real world you probably end up for a dual cycle
efficiency around 0,43 (gasturbine+steamturbine,for example)
you find constraints on two matters:
1.(most important)your budget
2. at the end, you need to dump 67% of heat remaining after last cycle into the atmosphere (condenser/cooler)in order to close the cycle.Therefore you are limited to about 50degC (watercooled) or 60degC (aircooled)at your last cycle.
For the fluidum used to drive the turbine take following limitations into consideration:
*boiling point:your heat source needs to have a temperature a lot higher than that point,preferably above critical temperature.
*critical temperature:limits the use of a fluidum above that temperature (for water around 42bar,470degC on the top of my head),hence the application of a first cycle with mercury,second cycle with water or first cycle gasturbine,second cycle with water)
-to maximize the efficiency of a cycle, you want to get close to the critical temperature in your heater/boiler
*percentage of liquid in turbine:as a turbine is built to be driven by gas (superheated steam), it doesn't like to get liquid on its blades.therefore the heater/boiler consists out of two parts:
boiler: to convert water to wet steam
superheater: to convert wet steam into dry steam (superheated steam).As the steam expand in the turbine,its pressure and temperature drops.If the temperature is alowed to reach the boiling temperature, the amount of water in the steam increases (see molier diagram)which would eventually damage the turbine, hence the application of multiple stage steam turbines with intermediate reheaters.
so there you have the operating envelope in which water can be used as fluidum.A high efficiency installation would be:
feed pump,boiler,superheater=steam @ 42bar,470degC
watercooled vacuum condenser (steam @ inlet 70degC)
keep in mind,if you loose the vacuum, you end up damaging your turbine (as explained above)

so if the the heat source is at a higher temperature,use a fluidum with a higher boiling temperature (like mercury)
if the heat source is at a lower temperature, use a fluidum with a lower boiling temperature (like butane).However nothing comes as cheap,enveronmental friendly and safe as water.(butane used as fluidum in a boiler is kinda not a good idea

ammonia-water cycle:
ammonia boils around -33degC,water boils at 100degC atmospheric coditions,however due to the anhydrous character of ammonia (it kinda likes water),ammonia/water mixture is stable (the ammonia doesn't boil out of the water) at room temperature (10% ammonia) and is commercially available (degreaser liquid)
hence the advantages:
on the heater/boiler side:
-the temperature at which the ammonia "divorces from the water" (boiling temperature) can be changed by changing the mixing ratio.
-once the ammonia boils out of the water, it becomes superheated, so no need for a superheater
on the cooler/condenser side:
as the boiling temperature is far lower than the temperature of the heat sink (water/air cooled), the ammonia stays superheated whilst in the turbine)

so it all comes down to a cost/benefit calculation, while keeping technology constraints, environmetal and safety issues in mind.good luck!

 

[davefitz]

ch5oh:

I'm not so sure regarding your comments on the critical point. There are many supercritical simple cycle ( Rankine) plants in the world, operating at steam turbine inlet conditions of 3500 psig/ 1005 F ( 241 bar/ 540 C), and the latest designs are much higher conditions of 310 bar/ 621 C. The working fluid's critical point offers no barrier to cycle inlet conditions.

For the specific max cycle temp of 450 C, One could configure a CO2 or hydrocarbon or binary fluid based cycle to the max practical temp limited by practical sizing of the heat exchanger for example, but a water based cycle is more likely due to 120 + yrs experience with that working fluid at that cycle temp range.

For the lower cycle temp range of 250 C, a hydrocarbon or binary fluid may be used for the working fluid, as commonly used in geothermal applications or as was proposed for combustion turbine combined cycles where an aeroderivative gas turbine is used with a low exhaust temp of 250 C ( GE/ Kalina cycle).

 

[eadwine]

The most efficient cycle is the pure Carnot Cycle with two isothermal cycles and two adiabatic cycles. Your solution lies in finding a cycle that most closely approximates the Carnot cycle (best efficiency) and the most cost effective heat engine to approximate that Carnot cycle.

As for the details of working this out, I will leave this to you.

 

[CH5OH]

davefitz,
indeed I had to find an old textbook (it has been a while I took classes on thermodynamics)
but you're right critical temperature isn't a concern at turbine inlet
further critical temp steam =374degC
anhydrous means without water (eg dry ammoia) and was a bit wrong used in that context
apologies
(damn my knowledge getting rusty and is due for an upgrade)
lol