Electricity from air at 250¦C

[Lalbatros]

Hello, I am going back to the basics with my question:

- let us assume we have flue gas at 250°,
- let us assume its composition is nearly that of air,
- let us assume the cold source is at 25°C,
- let us finally assume this stream carries an enthalpy-power of 10 MW (calculated from 250°C to 25°C)

I have these back-to-the-basics questions:

- what is the maximum electrical power that could be produced from this stream, how would you calculate it,
- what are the practical efficiencies that could be achieved by using a (simple) steam turbine

I am not asking for an homework, I am too old for that.
I ask because I (probably) made a mistake answering the first simple question and therefore I would like to see what your answers are.
The second question is of practical interrest for me today.

Thanks anyway,

Michel

 

[sailoday28]

Start with the Carnot Cycle.

 

[btrueblood]

For the steam turbine, look at the Rankine cycle. But you might want to look at a Stirling cycle engine also.

 

[dcasto]

First, you need to calculate how much steam you could make.

10 MW is 34,000,000 BTU/hr (sorry for englih units). It takes about 1100 BTU to make steam, so 30,000 pounds per hour of steam at 300 psig and 475 F.

Going through a steam turbine, you'd have to decide what exit pressure. Just pick atmosheric. The efficency could be 50% so calculate the ethalphy of steam in and at exit and subtract. Then cut that in half. I ended up with 1300 HP or 1 MW. If you went to a 100 deg F condensing turbine at 60%, you could double the output to 2 MW.

 

[Lalbatros]

sailoday28, btrueblood, dcasto,

To start with, what about the maximum possible efficiency (and power)?
By "maximum possible efficiency", I mean the maximum efficiency according to the second principle.
How would you calculate it?

Thanks

 

[sailoday28]

Between the hot temp and cold temp, there is no cycle more efficent than the Carnot cycle.
(Th-Tc)/Th *100 % efficiency

Temperatures are absolute.
Regards

 

[Lalbatros]

Thanks sailoday28, but I still have a problem.

My hot stream of air is not what we might call a "hot source", since the temperature of this stream can not be constant in the process.
Therefore, I don't see how we could implement a Carnot cycle in this case.
Therefore too, I think the Carnot efficiency is not the maximum efficiency that might be realized with this stream.
I believe that the Carnot efficiency over-estimates quite a lot the potential mechanical efficiency.
When the temperature of my stream drops, the conversion can only become less efficient.
Taking that into account, and assuming constant specific heat, I come to the following expression for the maximum efficiency:

eff = 1 - T0/(T1-T0)Log(T1/T0)

Numerically, this is much lower than the Carnot efficiency.

My reason for starting a thread here was to discuss this with you.
I am not a specialist in power generation, but from time to time I have to consider the production of power from waste heat. Today, the economic conditions are giving a much better chance to such cogeneration projects. Therefore, I was considering cogeneration for a new plant in my industry. In doing so I calculated the potential power production assuming an efficiency between 15% and 20%. By curiosity I also calculated the Carnot efficiency. And this is when I came to this question.

Did I make a mistake somewhere?
Do you agree with my comments?

I am also still interrested in practical values for the efficiencies for simple steam turbines. Is there a table or database somewhere on the web where I could find some applications similar to the one I am considering?

Thnaks

 

[HEC]

Lalbatros,
Just going back to your original post, if you are considering a standard steam turbine i.e. water your air outlet temperature will need to be much higher than 25C. In fact it is more likely to be closer to 80C therfore teh effective temperature drop will be 250 to 80C, already a 75% decrease in heat recovery. For practical purposes you should consider Sterling Cycle heat recovery engines as these will operate down to the lower temperatures you are looking at.

Mark Hutton

 

[Lalbatros]

Thanks HEC.

I was also looking for the Organic Rankine Cycle, as there are industrial references (

http://www.stowa-selectedtechnologi...nkine.cycle.for.electricity.generation.html).

Would you know of some database of practical performances with some basic descriptions ?

 

[moltenmetal]

I guess you have no other use for low-pressure steam than making electricity with it? That would be my first choice with a low-grade heatsource like this: make some "free" low pressure steam and use it to displace fossil fuel or electric-supplied low-grade heat sources elsewhere in the process.

 

[Lalbatros]

moltenmetal,

Indeed, this stream is nearly at atmospheric pressure.
Something like 500000 Nm³/h at 250°C.
I checked the temperature calculations and found it may be even lower.
The payback is never excellent for this kind of project, but I hoped the economics had improved recently. However, it remains that the physical parameters are still the most significant.

 

[dcasto]

now you want practicle solutions. First off, going below 200 C on the flue gas will start condensation of acids that will destroy the equipment, below 100C and the water will condense adding even more problems.

An organic Rankine cycle is the best bet for success. Pentane is in the range of materials.

 

[dcasto]

now you want practicle solutions. First off, going below 200 C on the flue gas will start condensation of acids that will destroy the equipment, below 100C and the water will condense adding even more problems.

An organic Rankine cycle is the best bet for success. Pentane is in the range of materials. google Ormat.

 

[Lalbatros]

Useful remarks dcasto.

But these flue gas will be deSox-ed.
So maybe it is not impossible?

 

[chicopee]

You still have carbonic acid from carbon dioxide and water vapor mixture.

 

[Lalbatros]

chicopee,

I have often calculated the SO2 acid dew point.
I never did the same for CO2.
Would you have a reference for CO2/H2O acid dew point?

I should also note that the stream will be rather dusty.