sensible energy calculation of flue gas 2

[19421945]

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

 

[seasar]

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

 

[19421945]

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.

 

[19421945]

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!

 

[25362]

Enthalpies for most of the gases are given in

http://webbook.nist.gov/chemistry

 

[19421945]

Thank you for this information. You were very helpful!

 

[25362]

You may also find iuseful information on:

http://www.hrsgdesign.com/page6_2.htm

 

[19421945]

Thank you for this information.

Can you please check attached spread sheet and tell me that i am on the right track?
Once i have calculated the Q for each gas component a just add them up and get Q for the gas?

 

[25362]

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 ?

 

[19421945]

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.

 

[25362]

19421945,

Taking zero enthalpy at normal conditions (0^oC, 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

 

[19421945]

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!

 

[25362]

[•] 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 650^oC 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%

 

[19421945]

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

 

[19421945]

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

 

[25362]

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.

 

[19421945]

Thank you,

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

Thanks again,

MH