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Questions about super critical boilers

  1. Jul 23, 2012 #1
    I have a couple questions on supercritical boiler systems with reference to pulverized coal power plants.

    My first question concerns what exactly a supercritical fluid is. I realize that the phrase 'supercritical' means above the critical point where there is no clear distinction between liquid and gas, or the mixture is homogenous. I think part of my problem is visualizing what exactly such a homogenous fluid is, or how it behaves if that makes any sense. How does something actually transition from sub-critical to supercritical? i.e. to go from liquid to gas, you boil. Is there a process something goes through to become a critical fluid or does it just sort of happen?

    Moving on, how does supercritical boiler operation compare to a sub-critical plant? My understanding is that (ideally) the fluid is pressurized above about 22MPa by the feedwater pumps, and remains there until it goes through the turbine. Of course, between those two points we go through the boiler. But in this case, we aren't boiling anything, right? Because there's no phase change to go through. We are just adding heat energy to the supercritical boiler. One of my sticking points is that I have been told supercritical plants use a seperator instead of a steam drum. Of course, a steam drum isn't needed because there's no steam to seperate from liquid, there's just this supercritical fluid. So what is the purpose of a seperator if there is nothing to seperate?

    Furthermore, is the reason why supercritical plants are more efficient than sub-critical due to the fact that there is no phase change to suck up energy?

    Sorry for the long post, but I have some questions here I'd like to clear up. Thanks for your help!
     
  2. jcsd
  3. Jul 23, 2012 #2
    Another question I thought of. I've read that the water in a supercritical system needs to be more pure than that of a subcritical system, why is that?
     
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