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Why do coal power stations run by creating steam rather than from the expansion due to combustion?

I understand that in order to generate electricity in a coal power plant, steam is usually produced by burning coal, and using the heat to boil water. The steam then moves through a turbine which generates the electricity. However, in an internal combustion, a similar thing occurs except that the expansion of air due to combustion directly causes movement of the pistons or turbines. So why do coal power stations not bypass the steam generation stage, and generate movement of the turbines directly from the combustion of coal? It seems less inefficient to require this intermediate step.

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In a word: erosion. The efficient burning of coal is a very dirty process - to achieve the highest temperatures you use a "fluidized bed", that is, inject air into finely powdered coal. High temperatures = high efficiency. But you get lots of burning particles carried in the hot air stream. Keeping the flow velocity down you can let them turn harmlessly into ash; but if you want to turn a turbine you need them to move at high velocity and they would simply destroy your turbine blades in no time.

A second consideration: work done to "compress" the working fluid. In a gas turbine you compress air, add fuel, then combust and expand. When you use the steam cycle, you have to add just a little water to the high pressure side and by boiling it you convert it to a much higher volume - yielding a very high effective compression ratio.

Both of these are important.

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  • $\begingroup$ The 'compression' reason is a red herring. Gas fired power stations use gas turbines just fine, together with a steam cycle. A theoretical combined cycle coal power plant would do the same thing. Erosion and soot buildup is (AFAIK) the only reason. $\endgroup$ – JanKanis Sep 14 at 15:04
  • $\begingroup$ @JanKanis You may be right. But if you wanted to inject water into a primary cycle (with coal particles) you would create a very corrosive mixture (sulfuric / sulfurous acid).Most natural gas burns cleanly, so combined cycle becomes feasible. $\endgroup$ – Floris Sep 14 at 15:08
  • $\begingroup$ True about the corrosion (which would also be a problem without injecting extra water). But combined cycle gas plants don't mix water and combustion gases, rather the exhaust from the gas turbines is used to raise steam in a regular steam boiler. $\endgroup$ – JanKanis Sep 14 at 15:19
  • $\begingroup$ @JanKanis I was referring to injection as for example described here: engineering.purdue.edu/~propulsi/propulsion/jets/basics/… $\endgroup$ – Floris Sep 14 at 19:19
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I think it has to do with the burning process of coal. It doesn't explode but rather it burns slowly and for a very long time. It wouldn't generate enough thrust at once and then there would be the problems of how to get the coal into the combustion engine, since it is a solid.

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1-the moment of steam is faster than combustionable energy. (because of pressure) 2-direct combustion supply will harm the turbine. Similarly principal use in nuclear power plant.

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This has been tried, although the turbines did not power electric generators but train locomotives. Gas turbine trains fueled by coal and by heavy fuel oil have been tried, but both had problems with soot buildup and turbine blade erosion from soot and ash particles. Running a gas turbine on any fuel that does not burn cleanly but that has solid particles in its exhaust is akin to pointing a sandblaster at the turbine, so while possible, it isn't economical.

Impurities can be removed from liquid fuels (and at some point the fuel oil turbine locomotive changed to such a cleaner oil, according to the wikipedia page), but not from solid coal (at least not economically). And natural gas does not contain any solid impurities.

I feel like the incomplete combustion resulting in soot should be solvable (make sure enough oxygen is available, the particles are small enough, and have enough time to combust). But that still leaves the ash (which is the result of impurities).

There is some research into having gas turbines powered by coal, but those involve gasifying the coal first. The additional gasification step makes this an unattractive option from a cost and efficiency point of view, but it offers better emissions and the possibility of CO$_2$ capture and sequestration.

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I suspect the reason that with steam a higher pressure can be generated and that the turbines operator more efficiently when fed with high pressure gases from a steam boiler than low pressure gases directly from coal. Another probable reason is that using gases directly from coal combustion would likely result in a buildup of carbon deposits on the turbine blades over time, which would not be a good thing (e.g., deposits might throw off the balance of the blades and affect gas flow over the blades).

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