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sensible energy calculation of flue gas
2

sensible energy calculation of flue gas

sensible energy calculation of flue gas

(OP)
Hi,

I need to calculate the sensible energy content of a flue gas with the following composition and temperature:
2% CO, 27% CO2, 2% O2, 4% H2 and the balance is N2.
The gas temperature is 650 deg C and the volume is 145000nM^3/h.

Can somebody tell me how to calculate the sensible energy content? What formulas do i use? I calculated the mass flow but how do i calculate the hu?

Thank you in advance

RE: sensible energy calculation of flue gas

For each constituent you listed:
Q = M * Cp * dT

M=mass flow rate of the constituent

Cp=specific heat for the constituent...how accurate do
you need your calc to be?  This will change slightly with temperature unless you are planning to condense any constituent.  Use an average over the dT range unless more accuracy is required or the dT is very large

dT=temp change
 

RE: sensible energy calculation of flue gas

(OP)
seasar,

Thank you for this information. I was worried that i have to use the equations recommended by NASA Glenn Coefficients /TP-201-210959/Rev1 to calculate the enthalpy.

RE: sensible energy calculation of flue gas

(OP)
Seasar,
well, it should be accurate since we want to calculate the energy balance of an electric arc furnace.
dT is about 300 - 700 deg C at the point we measure the gas flow, temperature and composition.
Any additional thoughts?

Thank you in advance for your valuable imput!

RE: sensible energy calculation of flue gas

(OP)
Thank you for this information. You were very helpful!

RE: sensible energy calculation of flue gas


19421945,

Your value of 487.289 kJ/Kg is too low by more than 200 kJ/kg. Please re-check and let me know of your findings.

Consider that oxygen alone (being the smallest contributor) has some 627 kJ/kg.

Am I wrong smile ?

RE: sensible energy calculation of flue gas

(OP)
Hu is the chemical energy content of an off gas and is calculated only with components that generate energy such as CO, H2 and CH4.

I believe strongly that this is correct.

I now try to calculate the sensible energy of an electric arc furnace When both are shown as a chart, the operator of an electric arc furnacea
sees on time that he is loosing valuable energy due to the fact that CO, CH4 and H2 are leaving
He might consider to use more oxygen for post combustion in the arc furnace or less CH4- and C input.  

RE: sensible energy calculation of flue gas


19421945,

Taking zero enthalpy at normal conditions (0oC, 1 atm. abs.) from the literature I get the following results using published values:


          kJ/kg       mass %    mol %    component
          -----        ------         ------    -----------
          901.1        1.78            2.0       CO
          686.8      38.02           27.0       CO2
          631.5       2.04             2.0        O2
          703.4      57.90           65.0        N2
         9473.7       0.26            4.0        H2

          722.0      100.0          100.0    total

 

RE: sensible energy calculation of flue gas

(OP)
Thanks again, but now i am more confused: can you detail how you got to the mass%, kJ/kg for O2? How did you get to 722 and what dimmension is it?

I divided 2/32 to get to the molar mass of O2 at this example of 0.0625. Doing this for all components, i get a total molar mass of 5.0533, and 1.237 for the volumetric mass of O2. Is that wrong?
Thanks for being patient!

RE: sensible energy calculation of flue gas


• The mass % can be obtained multiplying, not dividing, for example, as follows:

           mol%       MW      mass     mass %  
                                    (3)=
            (1)        (2)       (1)x(2)     (4)
    
CO           2.0       28         56        1.8  
CO2        27.0       44       1188      37.9
O2           2.0       32          64       2.0
N2         65.0       28       1820      58.0
H2           4.0        2            8        0.3

total     100.0      31.36     3136     100.0

The average MW = 31.36, or better 31.4, is obtained by dividing the total of column (3) by the total of column (1).

• The individual kJ/kg for each component at 650oC from the literature, then the total kJ/kg is the result of:

          mass %   kJ/kg        kJ
            (4)      (5)   (6)=(4)x(5)/100  

CO          1.8     901         16
CO2        37.9    687       260
O2          2.0     631         13
N2         58.0     703        408
H2          0.3    9474         28

total     100.0                725

• The difference from previous estimated figures, results from rounding them up, which is justified since the information from the literature may have an error of about ± 5%
         

RE: sensible energy calculation of flue gas

(OP)
Thank you very much.
I am now 7 years retired but still interested in reading literature. I found some articles which talked about chemical and sensible energy and i wanted to understand how that is beeing calculated.

Thank you for your assistance and explanations.

best regards,

MH

RE: sensible energy calculation of flue gas

(OP)
On more question, if i may:

can you please tell me the title of the literature that has the information regarding the kJ/kg for the gas components? Is this literature available on the internet?
I would like to get the data for H2O too.

Thanks again,

MH

 

RE: sensible energy calculation of flue gas


Regretfully, I cannot help you on the source of data. I took it from a relatively "old" book on fired heaters expressed in kcal/kg. As I told you most of the data can be found on the NIST webbook I mentioned above.  

As for water vapor the same source gives, kcal/kg (assuming zero at "normal" -as above- conditions) at 1 atm. abs. as follows:

        oC           kcal/kg

        100             44.5
        200             90.1
        300            136
        400            185
        500            235
        600            286
        700            339
        800            394
        900            451
       1000            509
       1250            660
       1500            819
       1750            985
       2000           1155

BTW, myself, I'm 11 years in retirement.
 

RE: sensible energy calculation of flue gas

(OP)
Thank you,

it is good to keep learning and keep the brain working even in retirement.

Thanks again,

MH

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