Yellow vs blue flames and fire

ChrisJ

Found some interesting info on Wikipedia and on several other websites regarding the colors of flame. I never knew this.


From :

http://archive.boston.com/news/science/articles/2007/11/19/why_are_some_flames_blue_and_others_yellow/


Flames are complicated objects that form when a fuel reacts with oxygen and releases heat and light. Clean fuels like propane or alcohol or natural gas burn mainly into carbon dioxide and water vapor, both of which are invisible. Blue flames, such as the ones from a propane torch or gas stove, are of this kind, and while quite a bit of heat is produced, you don't see much light. The exact colors depend on all sorts of details, but the important thing is that there isn't much light emitted.

Yellow flames such as those from a campfire or candle, come from the burning of relatively "dirty" fuels, in the sense that the fuel is not completely converted into carbon dioxide and water, but leaves little bits of unburned carbon. Those bits of carbon get hot and glow, making the yellow light that you see. Once they cool a bit, those bits of carbon - and other unburnt stuff that isn't carbon dioxide or water vapor - also go into making smoke, which is why the clean, blue flames are smokeless, while the dirty, yellow flames come with smoke.


From
https://en.wikipedia.org/wiki/Fire

A flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, the frequency spectrum of which depends on the chemical composition of the burning material and intermediate reaction products. In many cases, such as the burning of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of "fire".

Jamie Hall

Then when you add small amounts of other elements such as sodium (orange) or potassium (red) or copper (green and reddish) etc. you can get all sorts of interesting colours! When I was younger you could buy cans of salts of various kinds to throw on your fire for pretty flames...

ChrisJ

Jamie Hall said:

Then when you add small amounts of other elements such as sodium (orange) or potassium (red) or copper (green and reddish) etc. you can get all sorts of interesting colours! When I was younger you could buy cans of salts of various kinds to throw on your fire for pretty flames...

So does this mean an oil burner must have "soot" and other unburnt fuel in it's flame in order to be white?

dopey27177

Typically when you have a yellow flame in a burner chamber of a boiler the fuel is being starved of oxygen.

Jake

ChrisJ

dopey27177 said:

Typically when you have a yellow flame in a burner chamber of a boiler the fuel is being starved of oxygen.

Jake

So your oil burners run with a blue almost invisible flame?

SlamDunk

You can tune for a pretty blue but when you turn on your analyzer, the numbers will look ugly. I find that when the numbers are pretty, the flame is ugly.

Jamie Hall

Flame dynamics are actually incredibly complex. That said... an oil burner will tend to be bright and brilliant yellow, whereas a gas flame running right will be blue. Why? Because what you are seeing in the oil burner flame is actually only part of the combustion. If the burner is tuned right -- enough air but not too much -- the first stage of combustion involves breaking the rather long oil molecules apart, and there will be enough tiny particulate material formed at that stage to glow -- thermally -- that brilliant yellow. In a later stage that particulate material (pretty pure carbon, actually) will also be burned and as your combustion analyser and smoke tests will show there will be very little of that left -- just carbon dioxide, a trace of carbon monoxide, water vapour, and a trace of fine particulates. Natural gas, on the other hand, has no long molecules, and so very little opportunity to form even tiny particulates as part of the process. LP molecules are a little longer, but not much.

Now if you quench part of an oil flame too quickly, that second stage of combustion doesn't happen properly and you get a very rapid soot buildup -- too long a flame for the firebox, for example.

Wood combustion is much harder -- the molecules are much more complex. Getting a really clean burn off a wood fire is hard, and needs a lot of secondary air. Coal is similar that way, but not as variable.

ChrisJ

Jamie Hall said:

Flame dynamics are actually incredibly complex. That said... an oil burner will tend to be bright and brilliant yellow, whereas a gas flame running right will be blue. Why? Because what you are seeing in the oil burner flame is actually only part of the combustion. If the burner is tuned right -- enough air but not too much -- the first stage of combustion involves breaking the rather long oil molecules apart, and there will be enough tiny particulate material formed at that stage to glow -- thermally -- that brilliant yellow. In a later stage that particulate material (pretty pure carbon, actually) will also be burned and as your combustion analyser and smoke tests will show there will be very little of that left -- just carbon dioxide, a trace of carbon monoxide, water vapour, and a trace of fine particulates. Natural gas, on the other hand, has no long molecules, and so very little opportunity to form even tiny particulates as part of the process. LP molecules are a little longer, but not much.

Now if you quench part of an oil flame too quickly, that second stage of combustion doesn't happen properly and you get a very rapid soot buildup -- too long a flame for the firebox, for example.

Wood combustion is much harder -- the molecules are much more complex. Getting a really clean burn off a wood fire is hard, and needs a lot of secondary air. Coal is similar that way, but not as variable.

But according to those websites, in order for the flame to be any color other than colorless or blue, there must be incandescent material (soot etc) present.

So, how can it be that an oil burner flame is that clean if it's so visible to the human eye?

I've seen coal and charcoal burn with a blue flame and I'm pretty sure even wood can if in the right equipment, no?

mattmia2

Th oil particulates burn as the distance from the center of the flame increases and they have more exposure to air. There probably is an outer blue flame that you can't see because of the intensity of the yellow flame.

ChrisJ

Now what about Oxy Acetylene flames.
Those are far from invisible. What's producing the blinding visible light there?

captainco

Those articles might be great if you are a scientist or chemist but have no value if you are an HVAC tech. Flame color has been the most leading false information when it comes to determining if a burner is operating clean, I have measured blue flames that produces over 1000 ppm of CO and yellow flames that produced 0 ppm of CO. If we used 100% oxygen instead of 21% that might be different. On large commercial boilers with power burners, blue flame had higher CO levels most of the time. I get so frustrated when I still hear people teach look for a pretty blue flame!! Definitely not on oil and rarely on propane,

One additional comment is that you can make 10's of thousands of ppm of CO and have no soot!

ChrisJ

captainco said:

Those articles might be great if you are a scientist or chemist but have no value if you are an HVAC tech. Flame color has been the most leading false information when it comes to determining if a burner is operating clean, I have measured blue flames that produces over 1000 ppm of CO and yellow flames that produced 0 ppm of CO. If we used 100% oxygen instead of 21% that might be different. On large commercial boilers with power burners, blue flame had higher CO levels most of the time. I get so frustrated when I still hear people teach look for a pretty blue flame!! Definitely not on oil and rarely on propane,

One additional comment is that you can make 10's of thousands of ppm of CO and have no soot!

Does that suggest you can make zero CO and tons of soot?

captainco

Not on gas but I have seen oil burners operating at over #6 smoke and no CO

Lance

Only proper instruments can determine proper oxidizer fuel adjustment for a flame. Sure we can eyeball it, but that can't determine anything other than oxidizing of fuel is taking place. And an oxidizer does not have to be Oxygen, it could be: For example, hydrogen or acetylene can ignite spontaneously when in contact with chlorine, even without any oxygen present. Chlorine acts as an oxidizer.

ChrisJ

Lance said:

Only proper instruments can determine proper oxidizer fuel adjustment for a flame. Sure we can eyeball it, but that can't determine anything other than oxidizing of fuel is taking place. And an oxidizer does not have to be Oxygen, it could be: For example, hydrogen or acetylene can ignite spontaneously when in contact with chlorine, even without any oxygen present. Chlorine acts as an oxidizer.

This thread really wasn't about adjusting anything.

But that said, what's the typical difference in fuel consumption between a burner adjusted by eye and one done with an analyzer?

I'm not asking for the most someone can expect I'm saying what is the likely difference.

Someone good tunes a oil burner by eye and someone else comes in and uses an analyzer. What's the expected reduction in fuel?

Jamie Hall

My guess, @ChrisJ , is that it's going to depend on the eye... but the only good analogy I can think of is tuning a car (an old fashioned car. One of those things with little adjustment screws here and there...). A person with a really good ear can come astonishingly close to a good tune just by ear. But there may be a slight performance deficit, and a slight mileage loss. A few percent, perhaps? But if you take it to get the emissions tested, you may find that CO is quite high...

So, I would guess, with burners.

mattmia2

Wasn't oil burner combustion set with instruments long before electronic instruments were invented?

captainco

Gas and oil were set with analog instruments (fyrite/orsat bottles) before digital.

I have seen many burners tuned with analyzers that were off over 25%. Going to Pennsylvania next week to look at some commercial boilers that were off more than 30% after the commercial Boiler Burner company set them with their analyzer. I am showing the in-house HVAC maintenance crew how to set them correctly. The contractor refused to do so because he was trained with bad theory and opinion.

Eyeballing was not much closer. It is not the tool that makes the difference, it is the knowledge and experience of the person doing it. If you are following guidelines by persons that have done very little work on real jobs, then you are following bad information.

retiredguy

If you really want to see an oil flame in an industrial or commercial oil burner firing any grade of oil, "up close and personal", look at the flame through a cobalt blue lens.

Every tech I met in my 40+ years of servicing large boilers had his own way of setting up the combustion of the fuel they were burning, some good and some no so good. Any time that I had a little extra time to get to know how a burner fired a particular fuel, I would first try to set up the combustion by "sight and sound" before I got out the test equipment. Every burner has it's own flame pattern and characteristics. Some of the best looking flames produced the worst combustion readings. You could never rely on the color of the flame.

All I can say about coal is that soft coal is dirty and "YUK" and hard coal burns with a nice blue flame. I worked on coal units but my preference was nat gas and fuel oil.

ChrisJ

Ah, the nice blue glow.

captainco

If you remember, there was an oil burner called the Blue Flame, that burned with a blue flame. When the flame was blue, the CO was in the thousands, Recalled because of too many CO poisonings. When tuned to minimize CO the flame was more yellow.

ChrisJ

Just to avoid confusion, the picture I posted was of cherenkov radiation not of a flame.

Steamhead

captainco said:

If you remember, there was an oil burner called the Blue Flame, that burned with a blue flame. When the flame was blue, the CO was in the thousands, Recalled because of too many CO poisonings. When tuned to minimize CO the flame was more yellow.

You mean the Blueray?

captainco

Blue Ray is correct! Thanks.