In the industrial process heating business, the term "flame temperature" gets kicked around a lot. It doesn't take too long before you realize it means different things to different people. To some, it's the temperature of the combustion chamber the flame occupies. To others, it means the temperature of that glowing jet of gases coming out of a burner. To still others, it's a number quoted with authority from a handbook, while others will tell you that number is never seen in real life. In most practical situations, it really doesn't matter, as long as the heat processing equipment gets the job done. Just the same, life would be easier if everyone had a common understanding of the concept.
Well, here goes. But first, I'll define a few terms I'll be using along the way.
- Adiabatic is a term used to describe a combustion reaction in which all heat generated is retained in the products of combustion -- none is lost to the flame's surroundings.
- Dissociation is a reaction involving the breakdown of chemical compounds. In the case of combustion, these are water vapor and carbon dioxide.
- Stoichiometric is not an imported vodka. It describes correct mixture of ingredients in a chemical reaction. After the reaction is over, no surplus ingredients will be left. In combustion, the stoichiometric ratio also is called correct, ideal or perfect ratio.
The flame temperatures published in handbook tables usually are adiabatic flame temperatures for combustion at stoichiometric ratio. Unless specified otherwise, they're for fuel burned in air, with the incoming ingredients at room temperature. Change the temperature of the ingredients or the oxygen content of the air, and you'll change the adiabatic flame temperature. If you look up the flame temperature of natural gas in air, you'll probably find a value between 3,400 and 3,600°F (1,871 and 1,982°C).
That's not very precise. If all the handbooks agree on the air and gas temperatures, the oxygen content of the air and the ratio, how come they can't pin the temperature down closer than that? Easy -- because the chemical composition of natural gas varies from place to place. Some ingredients in the gas burn hotter than others. If the gas contains more of those constituents, it will have a higher flame temperature. Conversely, many natural gases contain small amounts of inert ingredients like nitrogen and carbon dioxide. They contribute nothing to combustion, and they drag flame temperatures down. To keep things simple from here on, I'll assume a natural gas with an adiabatic flame temperature of 3,600°F.
OK, is that the temperature you get if you operate our burner on stoichiometric ratio?
No. It will be lower. For one thing, dissociation will knock a few degrees off the top. Dissociation can be looked at as sort of reverse combustion. You go to all that trouble to mix fuel and air and burn them to produce CO2 and water vapor, only to find that at really high flame temperatures, some of those combustion products break back down into carbon monoxide, hydrogen and oxygen, reabsorbing the combustion energy they gave off when they were formed. Below 2,800°F (1,538°C) flame temperature, dissociation isn't significant, but from there up, even small temperature increases cause big jumps in the rate of dissociation. It's a classic Catch-22 situation -- the closer you operate to stoichiometric, the hotter the flame gets. The hotter the flame, the greater the amount of dissociation, forming increasing amounts of unburned combustion products and a greater drag on flame temperature. For our natural gas, the flame temperature will be about 3,450°F (1,899°C) after dissociation does its dirty work. Dissociation is one of the reasons so-called "on-ratio" combustion applications are usually operated with a small amount of excess air -- it keeps large amounts of carbon monoxide from forming.
All right, 3,450°F isn't too bad. That's what we have to work with, right?
Sorry, but if you look back at the definition of adiabatic combustion, you'll see it assumes no heat is lost to the flame's surroundings, and that doesn't happen in the real world. No sooner do the air and fuel begin to react and create heat, and some of that heat escapes to the surrounding combustion chamber or heating enclosure and all the product and fixtures it contains. It's like a water bucket with a big hole in the bottom. You can't fill it up because it's losing water almost as quickly as you pour it in.
So what's the bottom line on temperature in industrial heating equipment?
It depends on several factors. Burners that mix and combust the fuel and air quickly tend to develop higher flame temperatures because they get a little quicker jump on the heat loss to their surroundings. Flame temperatures tend to be higher in high temperature processes because the process doesn't suck the heat out of the flame as quickly. The mass of the combustion chamber and work load exposed directly to the flame also plays a big part. The greater that mass, the quicker it will pull heat out of the flame. When all is said and done, it's rare to find a flame temperature much above 3,250 to 3,300°F (1,788 to 1,816°C) in a practical combustion application. In low temperature industrial heating applications of interest to most of the readers of Process Heating, 3,000°F (1,649°C) may be as good as it gets.