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Each power plant has a load gradient characteristic, describing to what extent it can change the output of its generators in a given timeframe. Reading the literature, I know that, for example, a gas turbine power plant has a vastly higher load gradient than a one that uses coal as its primary energy.

However, I cannot find a description which parts of the plant contribute the most to the load gradient. I can imagine that a gas-powered plant can change its output faster because there's no mill involved as in a coal power plant, but where does its gradient exactly come from?

More specifically: What constitutes the load gradient of

  • a gas turbuine power plant,
  • a Lignite (brown coal) power plant
  • a bituminous coal power plant
  • a nuclear power plant?

And why is there a difference between lignite and bituminous coal power plants?

I'd very happy to read up on the details if one could name a book. I've read several books that describe how these plants work, but fail to mention what creates or limits their load gradient.

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A gas turbine plant basically uses jet engines optimized for shaft power instead of exhaust push to drive the generators. These can be controlled quickly because there is little stored energy. You reduce the fuel flow rate, and the turbine pretty much has to slow down quickly, within a few seconds, taking tens of seconds to reach the new steady state. The combustion chamber is small, and the little heat stored in its walls and the turbine blades is small compared to the overall power flowing thru the device.

A large coal or oil fired plant has a large boiler with water/steam pipes running thru it. These things take some time to heat up and cool down. Then there is also a lag from less heat into the boiler before the less steam makes its way to the turbine. Pressures and flow rates also have to be adjusted so that the liquid to gas phase change continues to happen in the right region of the pipes in the boiler. Simply shutting off the fuel to the boiler abruptly can possibly cause damage. All this takes more time than for a jet engine to slow down or speed up due to a change in its fuel flow.

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    $\begingroup$ Thank you for the explanation! Now I know what to look out for. I still don't get the differences between the two coal types: Why have stone-coal power plants a higher gradient than brown-coal power plants? Is the energy higher density of stone-coal vs. brown-coal the cause? $\endgroup$ – Technaton May 27 '15 at 21:23
  • $\begingroup$ I'm not sure about the distinction between stone coal and brown coal, so don't know. The one coal plant (now decommissioned) that I got to see the workings of pre-processed the coal into a fine powder that was injected into the boiler along with air, sortof like a gas/fuel mixture. If stone coal means burning the chunks of coal directly, obviously the fire will continue for a significant time after you stop adding coal. $\endgroup$ – Olin Lathrop May 28 '15 at 12:43
  • $\begingroup$ From your comment I just found out that "stone coal" is a false friend translation from German. Bituminous coal is the correct expression (I'll edit my original question accordingly). "Brown coal" is also called Lignite. Sorry for the confusion. I can see why a power plant using bituminous coal can provide a higher output fast (there's simply more Watts in a ton of bituminous coal), but why can it also decrease its output faster? Or is it just the lower of the two gradient values that is taken for the overall load gradient? $\endgroup$ – Technaton May 28 '15 at 13:41

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