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Homework Help: Combustion question

  1. Jun 16, 2016 #1
    Hi there,

    I was wondering if you could kindly help me in one of the question. its a tiny question..

    the question is
    1. A fuel gas consists of 75% butane (C4H10), 10% propane (C3H8) and 15% butene (C4H8) by volume. It is to be fed to the combustion chamber in 10% excess air at 25ºC, where it is completely burnt to carbon dioxide and water. The flue gases produced are to be used to generate 5 bar steam from water at 90ºC

    If 5% of the heat available for steam production is lost to the atmosphere, determine the amount of steam raised per hour when the total flow of flue gsaes is 1400 kmol /h

    Data: Net calorific value (MJ m–3) at 25ºC of: Butane (C4H10) = 111.7 MJ m–3 Butene (C4H8) = 105.2 MJ m–3 Propane (C3H8) = 85.8 MJ m–3 Air is 21% oxygen, 79% nitrogen by volume and 23.3% oxygen and 76.7% nitrogen by mass. Atomic mass of C = 12, O = 16, N=14 and H = 1.

    now the solution i'm meant to do is :
    find the total heat required to heat water and we do it in three steps
    1. find the sensible heat 90-100C --- the equation is m x cp x (temp difference) im not sure what tthe flow rate is and is the water m,cp ?
    2. Latent heat - 100-152C
    3.Sensible heat- not sure the temp difference
    then we add all them to find the total and then 5% is lost so total heat/ 0.95 to find the heat supplied and at the end we are meant to divide the mass available by heat required.

    does my method seems alright?
    i will be highly glad if you could kindly help me in this.
  2. jcsd
  3. Jun 16, 2016 #2
    What are the three chemical reaction equations? How many moles of oxygen are required stoichiometrically per mole of each fuel gas species? How many moles/hr of each species are flowing in the fuel gas? How many moles/hr of oxygen are required stoichiometrically. How many moles/hr of oxygen are required if there is a 10 % excess. How many moles/hr of nitrogen accompany this amount of oxygen?
  4. Jun 17, 2016 #3
    three equations are :
    c4H10 + o2 --> co2 + h20
    c3H8 + o2 ---> co2 + h20
    c4h8 + 6o2 --> co2 + h20

    0.628moles will be present in the flue gas.

    the flue gas in total contains 40.9moles

    according to the reaction stoichiometry for component :
    0.75moles c4h10 will require = 4.88moles of o2, 3.75 h20, 3.0 co2
    0.10moles c3h8 will require = 0.5 o2, 0.4 h20, 0.3 co2
    0.15 moles c4h8 will require = 0.90o2, 0.6 h20, 0.6 co2
    in total 6.28moles,theres 10% excess so actual amount = 6.91moles

    nitrogen - 25.974mol
  5. Jun 17, 2016 #4
    please check the above info
  6. Jun 17, 2016 #5
    Let's start over. What are the three PROPERLY BALANCED chemical reaction equations?
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