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Most power plants work by heating water in order to create steam that is then used to turn turbines which then generates electricity. As a side effect of generating this heat, there is usually some sort of exhaust such as smoke in coal plants and water vapor in nuclear fission plants. My question is simple: why don't we utilize smoke, water vapor, and other such exhaust to turn additional turbines to generate more electricity?

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    $\begingroup$ short answer - they typically do in any modern plant, and the stuff that comes out, while still "hot" relative to human touch, already used most of its energy content relative to its combustion temperature. That is already used most of the energy which could be extracted efficiently into mechanical/electrical work. The theoretical efficiency is a function of absolute temperature ratio (vs the "outside"), so the last 20% of energy content is least efficient (ie require larger plant) to extract. Finally, in some parts of the world, the last remaining heat is used for central hot water heating. $\endgroup$
    – Pete W
    Commented Feb 10 at 14:36
  • $\begingroup$ As @PeteW says, this already happens in newish (built in the last 25 years or so) power stations. The magic phrase to find out more about it is "combined cycle power plant". But really, no-one should be building new fossil-fuel power plants at this stage, and in that sense, the technology is at least semi-obsolete. $\endgroup$ Commented Feb 11 at 14:43
  • $\begingroup$ @DanielHatton Huh??? China is planning to commission about one a week forever. $\endgroup$
    – Phil Sweet
    Commented Feb 13 at 21:51
  • $\begingroup$ @PhilSweet I said no-one should be, not no-one is. $\endgroup$ Commented Feb 13 at 21:55
  • $\begingroup$ There is zero available energy in smoke and coolant vapor because both are at atmospheric pressure and have zero kinetic energy by the time you can see them. $\endgroup$
    – Phil Sweet
    Commented Feb 13 at 21:57

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Let's have a quick look at the process of spinning turbines with hot gas (in this case, steam) to produce shaft work.

The hot gas is prepared in such a way as to maximize its pressure (there's your effort variable) and its mass flow rate (there's the flow variable) because power is their product.

The objective of the turbine is to extract work from the fast-flowing, high pressure gas. In the process, that flow is sent through a large number of individual turbine wheels each of which pulls an increment of power from the flow, which progressively cools, expands and slows down more and more as it traverses the wheels.

There comes a point where adding more wheels (which get progressively larger and larger as the flow slows, expands and cools) does not extract enough additional work from the flow to justify the added expense- and by that point the steam itself is on the verge of condensing to water inside the turbine, something the turbine cannot gracefully manage. So at that point the now "spent" steam is exhausted from the turbine, condensed the rest of the way back to hot water by pulling out the remaining heat content (see below), and then pumped back into the boiler to turn it back into high energy steam once again.

Can we pull more work out of the exhausted steam? Certainly; it can be used to preheat the fresh feed water which is added from time to time to the steam flow, or to heat the office buildings attached to the power plant, or to run air conditioning systems for those offices in the summer. These are called topping cycles and are commonly used to get the very last smidge of useful work out of the circulating water/steam/water/steam cycle.

Now then, how did we make the steam to begin with? We burned coal, oil, or natural gas to boil the water, producing high energy steam flow and an exhaust flow of hot combustion products. These are not wasted- they are used to preheat the incoming combustion air, recycling as much exhaust heat as possible back into the boiler so it does not go to waste.

What is the practical limit to that recycling process? Since the combustion gases contain water vapor, and since we are pulling heat out of that vapor, there comes a point where the vapor condenses back into hot water inside the heat exchanger which mixes with nitrogen and sulfur oxides in the exhaust to form hot and highly corrosive acids there, which eat away at their insides. This sets a practical limit on how much heat can be exchanged out of the stack gases before the acids have a chance to form and destroy the heat exchangers.

So you see, neither the residual heat in the exhaust steam exiting the turbine nor the residual heat going up the boiler's exhaust stack are lost. To the greatest amount practicable, they are beneficially used to help run the power plant and thereby increase its overall thermodynamic efficiency.

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Coal powered electricity generating plants don't use smoke to generate electricity - the temperature of the smoke is too low.

Coal powered electricity generating plants, gas powered electricity generating plants and nuclear electricity generating plants use heat to turn water into steam which then turns turbines.

Nuclear plants generate heat via the fission of radio nuclides whereas coal plants generate heat by burning coal and gas plants generate heat by burning natural gas (generally methane).

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  • $\begingroup$ I explain what you just said in my post. My question is: why isn't the exhaust (smoke, water vapor) used to ALSO generate electricity? $\endgroup$ Commented Feb 10 at 5:37
  • $\begingroup$ @HomegrownPotatoes Fred just told you that there is not enough energy in smoke $\endgroup$
    – jsotola
    Commented Feb 10 at 6:35
  • $\begingroup$ Sure, there isn't enough heat in smoke to turn water into steam. But couldn't smoke act as an additional way to turn turbines? Smoke and water vapor rises, why can't this energy be used to turn turbines? If the answer is simply there isn't enough kinetic energy to turn the turbines, Fred did not include that in his answer. $\endgroup$ Commented Feb 10 at 6:49
  • $\begingroup$ @HomegrownPotatoes, any attempt to turn turbines with the exhaust will cause back pressure which will oppose the forced air infeed required to burn the fuel in the first place. This will choke the boiler. $\endgroup$
    – Transistor
    Commented Feb 10 at 18:11
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You must leave enough energy in the exhaust to run the exhaust system. If your exhaust is not hot, it will not rise through the chimney.

On simple systems, you also have to leave enough energy in the exhaust to draw inlet air into the air inlet.

Any system for extracting energy has to be designed for the specific temperature, volume, and speed of the steam or fire gasses. Systems often contain multiple stages to extract energy. Turbine fans have different blade geometries as the steam or fire gasses pass through the stacked fans of a turbine.

Eventually you get to the point where adding another fan, or another stage, just isn't worth it: the extra weight, the extra cost, the physical size, the erosion by water droplets, the added inlet stages (supercharger, turbo charger, draft fans), the added outlet stages (catalytic converter, flue scrubbing, exhaust fan), aren't justified by the value of the remaining energy in the water vapour or flue gasses.

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