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Thread: question stoichiometric flue gas volume

  1. #1 question stoichiometric flue gas volume 
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    Hi everyone,

    I'm new here and this is my first thread. I'm a performance engineer in a coal powerplant.

    For performance calculations of our boiler I wish to use the NEN-EN 12952 norm.

    I understand how the formule for the stoichiometric dry air quantity is derived this is formule 8.3-58:
    It's say:
    mu_Aod = 11.5122yC + 34,2974yH + 4.3129yS - 4.3212yO
    The result is the stoichiometric dry air quantity in kg air/kg solid fuel
    yi = different composition of the solid fuel: carbon, hydrogen,...

    But I don't understand how the formule 8.3-60 can be derived?

    It's say:
    Vgod = 8.8930yC + 20.9724yH + 3.3190yS-2.6424yO + 0.7997yN
    The result is the stoichiometric flue gas volume (STP) quantity in m≥flue gas/kg solid fuel

    Can somebody explane how I can get to the different coŽfficients? 8.8930; 20.9724;...

    Thanks in advance!

    Kind regards


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  3. #2  
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    It has been a while since you posted this, so this answer is a bit late.

    I can't reproduce the coefficients exactly, but I can explain why the coefficient for the hydrogen is so low. It is because the flue gas is being calculated on a dry basis. So the hydrogen in the fuel only serves to increase the volume of nitrogen and it does not add volume in terms of steam.

    So by my calculation, that would give coefficients of 8.93,21.18,3.35 for C, H and S respectively. That's close to the mark, but not quite there. There are probably some other assumptions being made that I am not aware of. I'll turn it over in my mind, or perhaps this might inspire someone else to have a go.


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  4. #3  
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    After some more thinking, I thought that perhaps since they are quoting 4 decimal places, they might be using very accurate atomic weights and molar volumes or something. But that doesn't seem to explain it.

    Take the nitrogen calculation, for example.


    moles of nitrogen = yN/14.0067
    moles of nitrogen gas = yN/28.0134
    volume of nitrogen gas = 22.414*yN/28.0134 = .8001yN
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  5. #4  
    Bullshit Intolerant PhDemon's Avatar
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    Could the small differences be from not assuming the gases are ideal and using the van der Waals equation of state



    rather than PV=RT

    to calculate molar volume for each gas?
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  6. #5  
    Bullshit Intolerant PhDemon's Avatar
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    I actually looked up the values for a and b for Nitrogen to test this. (Sad I know but it was procrastination, I should have been working on something else!).

    a = 0.137 Pa m3 mol-1
    b = 38.7 cm3 mol-1

    These give a molar volume of 22.380 (rather than 22.414) so the number comes out as 0.7989 a bit closer but still not exact (but I can't vouch for the accuracies of the "a" and "b" values used...
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  7. #6  
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    Good effort. That equation is nasty. Needs an iterative solution. I had a go with sulphur dioxide (don't blame me, it is the spell checker) using excel and got a molar volume of 22.76 l. I don't believe it, but in any case that only makes things worse I think. Sure can gobble up some time

    I have been using Van der Waals work in another field today so thought he deserved a picture. But I can't get the image posting function here to work. I suppose I could put it on photobucket somewhere, but if there are tools here then they should work. I've had my bandwidth shaved to dialup levels so too slow to investigate further.
    Last edited by Warron; January 26th, 2014 at 10:37 AM.
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  8. #7  
    Bullshit Intolerant PhDemon's Avatar
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    I think the photo upload no longer works (it seemed to happen after a server upgrade and I remember one of the mods dsaying ther was no space allocated for uploads now) so photobucket etc. or copying the URL from a website are the only options.

    Here's one:
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  9. #8  
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    Thanks for that. Smart fellow he was. Has rather an intense look about him.

    I was just reading the OP again and can recall doing this sort of calculation for a coal fired power plant, albeit a small one when I was working in an alumina plant in my 20s. There was a factor in the calculation to allow for something called 'C-grits' IIRC. That was unburned carbon that got caught in the flue system. Probably in electrostatic precipitators I think. I remember this stuff pretty clearly as I attempted to take a sample of it from one of the conical hoppers in which it accumulated. As I inserted the sampler, the disturbance caused the entire contents of the hopper to fluidise and flow all over me as a very black cloud. I ended up looking like one of the Black and White Minstrels.

    This was my introduction to a hopper flow phenomenon known as 'flushing'.
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