THE COMTHERM ON-LINE TECHNICAL MANUAL

FLAME TEMPERATURE

When fuel and air are combined in a combustion reaction, the heat released serves several functions including; heating the reaction, radiation to the surroundings, and conduction to the surroundings by contact.

It is very difficult to measure flame temperature or even to calculate it with any degree of accuracy. Consequently it is customary to speak of 'theoretical flame temperature’ which is the temperature which would be obtained if combustion took place instantaneously, completely, and without loss of any heat to the surroundings; that is, if all the heat of combustion were used to heat the gaseous products of combustion.

Comtherm table CT-4a provides a summary of quoted flame temperatures of fuel gases, taken from various technical reference sources.

In the case of natural gas data showing the affect of excess air and excess fuel is supplied. A small amount of excess air or excess fuel can greatly reduce the flame temperature.

Comtherm fig. CT-5b shows the dramatic affect on the temperature of the flame (products of combustion) of introducing large quantities of excess air, this information is important when considering direct fired process air heating applications.

The method used for the calculation of flame temperature is shown by the relationship :-

Theoretical Flame Temperature = (H_C + H_F + H_A) ÷ (Q_P x S_M)

H_C = heat of combustion
H_F = sensible heat in fuel
H_A = sensible heat in air
Q_P = quantity of products of combustion
S_M = mean specific heats of exhaust gases

This expression gives a high result because all the heat of combustion is not available for heating the products of combustion; losses to the surroundings are not considered, therefore, actual flame temperatures will always be lower than the theoretical.

Theoretical Flame Temperature may be further defined as that at which the total sensible heat in the products of combustion just equals the sum of the net heat value, plus the sum of the sensible heats of the gas and air, minus the heat absorbed by the dissociation of the CO₂ and H₂O.

The dissociation is caused because the combustion reactions are reversible so that the products break down into combustibles, oxygen, free radicals and atoms. This process absorbs some of the heat until equilibrium is reached.

The flame temperature obtained in the combustion of gases is very important; it can have a large affect on the speed and efficiency of a heating process. In order to maximise flame temperature when high flame and furnace temperature are required, accurate fuel-air ratio control is critical.

The thermal efficiency of a process can be measured by the ratio (T_F – T_E) : T_F

T_F is the flame temperature and T_E is the temperature of the exhaust combustion gases. (T_F – T_E) represents the amount of heat used up in the process and T_F the heat input.

The higher the flame temperature (T_F) the greater becomes the possible efficiency, because T_E is usually fixed by the nature of the flame application. Additionally the rate of heating of an object is proportional to the temperature difference (T_F – T_E) between flame and the object.

This assumes that the heated object is heated to the temperature of the exhaust gases during the process; in reality in most applications the exhaust gas temperature will be higher than the object temperature.

For example, a gas having a flame temperature of 1,800°C will heat an object to 1,400°C much faster than a gas with flame temperature of 1,500°C.
Elevated flame temperatures and correspondingly greater heat transfer rates can be obtained when the combustion air is preheated.

If instead of air, oxygen is used for combustion, then even higher flame temperatures and even higher process heat transfer rates can be achieved. Oxygen has been used in such applications as flame hardening, steel cutting and various glass working operations.

The use of Oxy-Fuel burners as a source of high temperature and high heat transfer rates is often an attractive proposition.

It is interesting to compare the performance of an air-towns gas flame with the performance of a oxy-towns gas flame. The flame temperature and the velocity of the combustion products both affect the rate of heat transfer from a flame to a relatively cool surface. The degree of dissociation of the combustion products into atoms and radicals also has a considerable affect on the flame temperature and heat transfer process.

The temperature of a typical oxy-towns gas flame is 2700°C, compared with 2000°C for towns gas/air. The oxygen flame therefore has an advantage in temperature of 700°C. However the presence of a large proportion of nitrogen in the combustion products of air-gas flames, results in a higher mass and flow rate for a given quantity of gas than in an oxy-gas flame and therefore gives the air-gas flame a mass flow advantage.

However the oxy-gas flame usually has an advantage because generally operates at a much higher supply pressure.

The decisive difference between the heat transfer rates of air-gas and oxy-gas flames is due mainly to the different degrees of dissociation of the combustion products. At temperatures above 2000°C the molecules of a gas begin to split up into free radicals and atoms and in doing so absorb appreciable quantities of heat.

The degree of dissociation increases with temperature and the fraction of the heat content of the flame in the form of latent heat becomes considerable. The air-gas flame contains only small parts of dissociated products, whereas about 25% of the products of an oxy-gas flame are dissociated.

It is mainly the heat release of the recombination of the dissociated species on a cool surface which gives rise to high rates of heat transfer characteristics of oxy-gas flames.

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