combustion temperatures

[atoussaint]

Hi! I'm working with a hot gas generator that uses heavy fuel. We wanted to know what is going to be the temperature that the gas generated will have. We did the calculation using the stoichiometric composition and we found around 1950°C (with 0% excess air), however when we talked with our burner provider he told us that the temperatures where much more higher. We where considering to have at least a 20% excess air but since we are not sure if our calculation is correct we dont know if the temperatures will be correct.

Could someone tell me a good way to calculated this temperature and also if you know of a calculator that I could use (website, applet, etc.)

Thank you for your help!!!

 

[FOURe]

When you get a chance, please check out a thread dealing with 'Combustion' at the following URL:

http://www.eng-tips.com/viewthread.cfm?qid=244351&page=1

http://www.engineering-4e.com

 

[25362]

From old statistical data in my antiquated notes the flame temperatures are never adiabatic because heat is transmitted in "status nascendi" to the colder parts of the heater (tubes, etc.).

Nevertheless, the flame temperatures for fuel oil are approximately as follows (the 0% xs air column gives the adiabatic flame temperatures):

t,^oC
heat radiated as a fraction
xs air 0 0.2 0.35 0.50
% (adiab.)

0 2000 1720 1460 1160
20 1800 1520 1270 990
40 1640 1360 1140 880
60 1480 1230 1010 780


The heat released by the burners (including any air preheat), the degree and type of atomization, the geometry of the combustion chamber and of the heat-absorbing surfaces may affect the amount of radiated heat. I think there is a formula by Hottel to that effect.

The enthalpy of the combustion gases for a typical fuel oil in kcal/nm³ (n=normal conditions, 0^oC and 1 ata) -taken from an old graph- are approximately as follows:

xs air
% kcal/nm³

0 860
10 790
20 720
30 670
40 620
50 580

These figures are valid for fuel oils with low calorific values ranging from 9500 to 11000 kcal/kg. There exists also an old graph by Rosin-Fehling that gives the enthalpy of the combustion gases for all kinds of hydrocarbon-based fuels as function of their temperatures.

 

[atoussaint]

Thank you for your answers.

Could we consider that the flame temperature and the gas temperature are close one to the other? In fact I need this info so we can design the hot gas chamber that goes just after the burner. Since the properties of the refractory bricks can change drastically from one temperature to the other I need a good estimate of the temperature to make my choice.

 

[25362]

The same old notes refer to the wall temperatures for two different heat loads per unit volume of the combustion chamber:

Heat release temperatures ^oC
kcal/(m³.h) nearest to burners farthest


225,000 1400 1250
150,000 1250 850

IMHO, the design of process fired heaters should be done by experienced companies, such as Broach, Sigma, Enercon, to name just a few.

 

[21121956]

Hello everybody:

In addition to the previous contributions, I would suggest that you go to

http://www.air-dispersion.com,

the website of Milton Beychok an honorable member of Eng-Tips Forums and contact him in order to obtain an Excel spreadsheet regarding with calculations of adiabatic flame temperature.

You can read as well thread71-189819 and thread135-182063.

 

[atoussaint]

Thanks a lot for your help, I will look into it and will tell you how it went.

 

[atoussaint]

Ok, I checked the values and when we do the stoichiometric combustion we have more or less the same values, around 2000°C, however this is when we do it with 0% excess air, the moment we include the 20% excess air we go down to 1750°C. This is the gas temperatures and not the flame temperature. Does this sound logical to you?
We are using the q = m*cp*dt formula to do our heat balance (in and out) and it all adds up but we still aren't sure about the temperature
Can the gas temp be different than the flame temp for more than 200°C?
Thanks for your help!

 

[atoussaint]

25362
Could you tell me from where you got your notes for the temperatures?
t,oC

heat radiated as a fraction
xs air 0 0.2 0.35 0.50 % (adiab.)

0 2000 1720 1460 1160
20 1800 1520 1270 990
40 1640 1360 1140 880
60 1480 1230 1010 780

I get the good values according to this table, could you tell me where you got it so I can use it as a reference?

thank you for your help

 

[25362]

My notes refer to an old book by Ing. Rodolfo Orel, titled: El c[á]lculo de la combusti[ó]n in Spanish, published by Editorial Alsina, Buenos Aires, 1954. It might have been translated to english by the EXXON group under whose auspices it was written.

It is offered second-hand in

http://www.masoportunidades.com.ar/aviso/3885448-3684-el-calculo-de-la-combustion

 

[atoussaint]

Thanks!!