flame temperature 2

[luft]

Hi all,
can somebody tell me how to calculate flame temperatures in combustion processes?

Thanks.

 

[25362]

The adiabatic (aka maximum or ultimate) temperature on combustion is that obtained assuming that all the energy, including that originally contained in the fuel and the air, and atomizing steam if used, plus that liberated in the combustion process, is retained in the gaseous products of combustion.

When burning hydrocarbons, etc., with the theoretical stoichiometric amount of air to perfect combustion these temperatures in ^oC are, for example:

H₂: 2210
CO: 2452
CH₄:2065
C₂H₆:2105
C₂H₄:2343
C₂H₂:2632

All these are influenced by the differing heats of formation.

In practice, no industrial furnace ever reaches these temperatures because of:

a. Release of radiant energy in status nascendi, energy absorbed by cold surfaces and partly lost to the surroundings thru furnace walls;
b. Excess air that gets heated up without participating in the combustion process;
c. Endothermal dissociation of CO₂ and water at temperatures above 1500^oC, a fact that applies brakes to the increase in temperature;
d. Air for combustion being added in two ways: primary pre-mixed and secondary to complete the combustion process, a fact that changes the flame characteristics as well as the speed of combustion.

A rigorous method would involve a series of iterations assuming temperatures and estimating the individual enthalpies of the various combustion-resulting gases, including the decomposition of water and CO₂, comparing the enthalpies with that entering to, and developed in, the process. I have with me tables showing the degree of dissociation of these molecules as functionn of temperature and partial pressure.

Fortunately, two gentlemen Rosin and Fehling, based on a lot of experimental measurements and statistical reasoning found that the enthalpy of (complete and perfect) combustion products at a given temperature depends only on xs air.

Nature came to help by making values of enthalpy and temperature of combustion products about equal for all fuels whether gas, solid or liquid !

R&F constructed a graph enthalpy vs temperature with a series of curves for different xs air values applicable to all fuels, that can be found in the literature. Upon their findings, estimated adiabatic temperatures corrected for % excess air, including CO₂ and water decompositon, are as follows:

For a gas with (P_i = 11000< (P_i <16000 kcal/Nm³) or a fuel oil with 9500< P_i ,10500 kcal/kg (P_i is the low calorific value) the values are :

xs air, % prod. enthalpy kcal/Nm^{3 o}C

0 860 1990
20 725 1800
40 627 1650
60 550 1490
80 490 1370

With an error of less than 15^oC, i.e., within the limits of the errors in determining calorific values.

Now, to your specific question.

First you determine the adiabatic &quot;corrected&quot; enthalpy in kcal/m³ of the combustion gases, by using the table above.

You must now state how much % radiation energy is lost to cold surfaces, deduct this percentage from the enthalpies above, enter the R-F graph and find the temperature you are looking for.

As an example, when burning a fuel with 20 xs air, radiating 40% of the combustion enthalpy, the remaining enthalpy would be 0.6 x 725 = 435 kcal/Nm³. Excess air is determined by measuring %O₂ and %CO₂ in the flue gases.

The graph will tell you that with 20% xs air the gas products will have a temperature of 1160^oC instead of 1800^oC as tabulated above for the adiabatic conditions.

All you need is the Rosin-Fehling graph, available in the pertinent literature.

I sincerely hope the above satisfies your query.

 

[luft]

Dear 25362,
thanks a lot for your answer.

Luft.

 

[25362]

An interesting site could be:

http://imartinez.etsin.upm.es/bk3/c15/Combustion kinetics.htm#Toc46315238