Steam-Powered Turbines at Coal Power Stations

Look up "latent heat".

Your reply was better. Sometimes I try to be educational, especially if a question shows initial knowledge or effort; and I try to some extent to work around language barriers. Other times I may be more abrupt. In this case, I should have said a little more about energy transport with-vs-without change of state.

Part of the same answer, for those who do not recognise it, is the fact that once steam is condensed back into water its high density (compared with gas/steam) allows it to be fed back into the heating part of the cycle, and against the prevailing head of pressure, relatively easily and efficiently from a hardware point of view and with minimum energy.

The properties of water that makes it ideal is

- Heat capacity (highest of all solids and liquids except NH₃)
- Latent heat of fusion (highest except NH₃)
- Latent heat of evaporation (highest of all substances)
- Thermal expansion (in the first section we showed that the temperature of maximum density decreases with increasing salinity)

- Conduction of heat - Highest of all liquids
- Transparency (large absorption of radiant energy)

Only I wonder why the steam and water was in use even before scientists decided that it is the best .

...why the steam and water was in use even before scientists decided that it is the best .

¿Because it´s everywhere?

Also not to mention the abundance of coal in the USA.

oilcan13

Simply put, using the post-combustion exhaust (CO2 and other products of combustion) will gum up everything (stuff will condense on turbine blades, heat-transfer surfaces in the condenser, etc) and will corrode anything that's not made of re$i$tant, high-performance alloy. The reason for the latter: CO2 will combine with air moisture (water vapour in the combustion air) to form carbonic acid (H2CO3), which will eat away metal surfaces. That would be ESPECIALLY the case back in the 1800s when thermal power was developed). So a non-post-combustion fluid has to be used to thrust on the turbine.

The second reason, as CoffeeBean has said is that water's everywhere, to which I'll add a third: water absorbs a LOT of heat when it vapourizes, which makes it ideal to use.

(Note: this property is useful in many ways, including firefighting ... fires can be put out in three ways: removing fuel (or fuel running out), cutting off the oxidizer supply (usually air, but could be other substances such as chlorine gas, which will burn iron), and dropping the temperature. Water quenches many types of fires by absorbing of lot of heat when its thrown onto a blaze and its vapourizes. For example, water thrown by a plane onto a forest fire will quench the fire where it falls not because it wets the wood or forms a barrier of water drops on the wood; it quenches the fire by dropping its temperature too low for there to be combustion. When the water all evaporates after a while, the wood will relight if there's fire nearby).

Also, water condenses at ambient temperatures and atmospheric pressure here on Earth, so water can be recycled in a closed loop back to a boiler (minus losses due to blowdown, which is needed to clear mineral contaminants that accumulate in the water when make-up water's brought in).

And so the three keys: water's everywhere; it absorbs a lot of heat when it vapourizes; it can be vapourized at fairly low temperature (100 C, 212 F at 1 atm.) for low-pressure applications, AND it can be condensed back to liquid at lowpressure and ambient temperature. The latter's as important as the others: not being able to condense the water by dumping its heat to atmosphere or to condenser-cooling water would mean that the spent water vapour, once exhausted out of the turbine, would have to be vented to atmosphere, which would greatly increase water consumption.

Other fluids can also be used (organic fluids such as propane, isopentane, etc., are used in ORCS, 'Organic Rankine Cycles'. Same idea, except that organic fluids are used instead of water. They're not as readily available as water, though, so they're not used nearly as much.

Cheers! DZ

"why not use heated CO2 , nitrogen or helium or any other gas to drive the turbine ..."

Heating just those gases would not yield higher pressure pressure. What pressure rise would you expect by heating Nitrogen in a closed container/boiler? So the right comparison is heating 1 liter of liquid Nitrogen and 1 liter of water, which would you prefer?

On the overall steam should be cheaper and easily (water) available than those gases.

Besides, there is another reason:

Water to flue gas heat transfer is much more efficient than any gas to gas thus resulting in lower heat transfer area and equipment. Boiler size would be huge if you want to extract energy to gas instead of water.

using the post-combustion exhaust (CO2 and other products of combustion) will gum up everything (stuff will condense on turbine blades, heat-transfer surfaces in the condenser, etc)

Gas Turbines uses only this, only the exhaust heat is used to drive the steam turbines in CCTPs (wherever sufficient exhaust is available) As do the simple diesel/ petrol engines, of course minus the turbine for these.

Also in case the steam temperature falls below the critical point it is extremely damaging as well as corrossive to the metals.

As Coffebean said, it is just by chance (due to availability and also may be during boiling beans cooking) the energy was seen and the manking thoght why not use it. It is just coincidence that it proved to be the optimum as on now.

May be in future better chemicals will arrive with more heat capacity and latent heat, making the system more efficient.

400:1 volumetric expansion ratio from water to supercritical steam.

water is cheap and reuseable.

1:400, fine...And what would be expansion ratio of CO2, nitrogen or helium from their respective liquid state to dry or supersaturated gas?

This is a good question, but please remain aware of the temperatures required to place these in the liquid state.

One other factor which on the scale of utilization in a power-station is the safety factor. Catastrophic failure of any pressure system will have casualties in the immediate confines, but in the case of steam/water that would be far less damaging to the site than some of the gasses mentioned. A CO2 (or any other toxic gas) escape could kill ever thing in the proximity of the event, not to mention combustible gasses.

I would still prefer modern nuke power over coal any day of the week. If you have not lived in the proximity of large coal fired stations you may be aware of just how negative an influence they have on the environment and everybody/thing in it. (Sorry - off topic)

As mentioned above, the biggest factor - if you do the sums you will see the answer clearly. From a total energy requirements to generate the electricity from coal the sum is not all that pretty. Water/steam comes out best. However most leave off the costs post production, i.e. rehab of environment, no I do know most agencies enforce the pathetic level of mine-site rehab, it's everything within a 200 mile radius that gets messed up with acid rain, dust and sulphur pollution and cost of health issues to man and beast. You would not want to feed the lungs of a cow from next to a power station (and cowses doesn't smoke ciggarrettessses) to your neighbour's barking dog. But don't worry they use those banged up lungs in your hot-dogs and polony.....

This might interest some of you on this subject. Of all the ideas that wouldn't fly today this has to be it.

http://www.dself.dsl.pipex.com/MUSEUM/POWER/mercury/mercury.htm

thnx everyone for your comments they are really apprecieted