I'm working as an intern at a factory that produces refractory bricks, mainly doing measurements on a tunnel kiln they use for firing the bricks. The bricks are heated with several natural gas burners in a firing zone. To determine the required air for both stoichiometric and excess-air burning ([itex]\lambda[/itex]=1,1 and 1,2), I've done some basic combustion calculations we used to do back at the University. However, I'd like to check whether I've made correct assumptions and/or if I've simplified the problem too much.(adsbygoogle = window.adsbygoogle || []).push({});

The composition of the used natural gas is roughly the following (with mole-%):

and hence [itex]M_{NG} = \Sigma x_{i} M_{i}[/itex] = 17,822 g/moleCode (Text):

Methane CH4 89,51

Ethane C2H6 5,8

Propane C3H8 2,25

Butane i-C4H10 & n-C4H10 0,9

Pentane i-C5H12 & n-C5H12 0,21

Hexane C6H14 0,06

Carbon dioxide CO2 0,85

Nitrogen N2 0,42

I've assumed that the reactants combust completely and that both CO2 and N2 do not react. I've also assumed the following combustion reactions (is this oversimplifying things?):

1) CH4 + 2 O2 -> CO2 + 2 H2O

2) 2 C2H6 + 7 O2 -> 4 CO2 + 6 H2O

3) C3H8 + 5 O2 -> 3 CO2 + 4 H2O

4) 2 C4H10 + 13 O2 -> 10 H2O + 8 CO2

5) C5H12 + 8 O2 -> 5 CO2 + 6 H2O

6) 2 C6H14 + 19 O2 -> 14 H2O + 12 CO2

While considering, for instance, combusting 1 mole NG I've then calculated (with the reactions above) the required amount of O2 (for methane [itex]n_{O2}=2 * n_{CH4}[/itex], ethane [itex]n_{O2}=7/2 * n_{C2H6}[/itex] etc):

So then the molar stoichiometric AF ratio would be 10,43 (with [itex]\lambda[/itex]=1,1 it would be 11,47 and with [itex]\lambda[/itex]=1,2 12,52).Code (Text):

moles O2 required (moles)

CH4 0,8951 1,7902

C2H6 0,058 0,203

C3H8 0,0225 0,1125

C4H10 0,009 0,0585

C5H12 0,0021 0,0168

C6H14 0,0006 0,0057

CO2 0,0085 0

N2 0,0042 0

===============

1 2,1867

N2 in the air = 3,77 * 2,1867 = 8,243859 moles

=> tot. required air for stoichiometric combustion =

= (2,1867 + 8,243859) mole = 10,430559 mole(air)/mole(natural gas)

And then the stoich AF ratio using masses would be

[itex]\frac{ m_{air} }{ m_{NG} } = 10,43 * \frac{ M_{air} }{ M_{NG} }[/itex]

Now the question is: are these calculations correct or should I approach this in some completely different way? Am I simplifying things too much / am I not taking something essential into account (that might render these calculations useless)?

If I measure the volumetric flow of natural gas to a burner (and convert it to massflow), can I then use

[itex]\dot{m}_{air} = 11,47 * \dot{m}_{NG} * \frac{ M_{air} }{ M_{NG} }[/itex]

to set the desired inlet airflow (for [itex]\lambda[/itex]=1,2)? How well do the theoretical calculations work in practice? (and by the way, if someone has any tips regarding adjusting/checking the adjustments of gas burners, I'd love to discuss the subject more).

Thanks in advance! :)

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# Combustion calculations and the required amount of air

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