OVEN BASICS
Burn-off, burn-out, heat-cleaning, pyrolysis… These ovens are described different ways but let’s try to simplify the whole thing.
The modern heat-cleaning oven is the product of many years of evolutionary engineering. Top-down, bottom-up, integrated controllers, water-spray cooled are some of the variations, but the basic concepts and science behind them are the same.
The modern burn-off oven consists of two chambers. They are in reality, two separate ovens tied together, often one on top of the other. Each chamber has its own burners, control mechanism and purposes.
There is a process chamber where “stuff” is processed. And there is another chamber where all the exhaust from the first chamber goes. This second chamber processes all the fumes, volatile components, and soot prior to emission to the outside world. This other chamber is referred to by different names such as the secondary chamber, oxidizer, thermal oxidizer, or the afterburner.
What’s it for?
The “stuff” that burn-off ovens process refers to many things: electric motor stators, armatures, alternators, engine blocks, painted parts, coated parts, waste water, reclaimed metal scrap. Many different industries use burn-off ovens to process a wide variety of materials.
The two most unusual applications I saw these ovens used for are reclaiming the metal from oil filters by burning off the oil and burning cows that died of mad cow disease.
It’s for recycling not disposal
They are not incinerators. They do not dispose of waste. They reclaim material and use it again with little or no change in the material being burned off. It is a controlled process where the substrate (usually metallic) is spared the damage caused by the high heat.
FIRST CHAMBER
The primary or process chamber of the burn-off oven operates at temperatures that thermally degrade and volatilize the organic material on the substrate without damaging the substrate. The required temperatures vary depending on the organic material, the type of metal, and what is burned off. It also varies on the end use. The nature of the organic substances burned off also determines at what temperature the chamber operates.
Some “stuff” such as electric motor components lose magnetic properties if heated for extended periods over 700 degrees F. Steel loses many of its properties at high temperatures and the normal maximum recommended temperature for steel parts is around 900 degrees F.
Usually nothing degrades until the part reaches 250 to 450 degrees F. But the thermal degradation will be at a low rate at these temperatures.
The parts should never be on fire
The purpose of this chamber is not to set the parts on fire. The purpose is to degrade and volatilize the organic material in a controlled and oxygen starved environment. The key is to limit the amount of oxygen so the process is controlled. There is not enough oxygen to burn quickly. This chamber is not totally air tight, but has a limited amount of air coming in so there is little oxygen and a slow reaction.
In the best-case scenario, no actual burning occurs but the organic material is simply vaporized. It vaporizes at a temperature that leaves the substrate unaffected. There should be no actual flame on the parts.
The flame in a burn off oven is normally contained within a firebox and only heat reaches the parts. If the flame reaches the parts then they can be badly damaged. Flame not contained in a firebox can damage carts or baskets that contain parts.
Any holes in the oven or open doors or bad door seals allow more oxygen in to the oven than is designed for the proper operation of the oven. Usually the only source of air in the lower chamber is the air intake at the burner, and this mostly inputs air during the “off” cycle of the burner. When it cycles on and off, we are also turning the oxygen valve on or off.
The key to this chamber is to control of the amount of fuel and oxygen that goes in. Fuel does not only mean gas. It also means the organic material burned off. The slowly burning organic material adds to the fuel value.
SECOND CHAMBER
The oxygen starved air from the lower chamber contains volatilized organic components. These are not completely burned but consist of gases, smoke, and soot. These gases and soot travel into the next chamber, the secondary or afterburner chamber of the oven.
This chamber runs at a higher temperature. It has an opening somewhere that allows air from the outside into the chamber at a controlled rate. Enough extra air is let in to complete the job started in the first chamber. The higher heat and more oxygen cause the gases and soot to completely combust. Little or no organic material is left. Theoretically the only thing that comes out of the stack in a properly operating oven is carbon dioxide and water vapor.
Oxygen and air flow control
And what controls the rate at which the gases leave the process chamber and enter the afterburner? The magical thing that allows the process to proceed at a safe and efficient rate? Mostly, the size of the hole in the wall between the two chambers.
Other factors also affect the velocity of the gasses leaving the one chamber and traveling to the next. It depends on the temperature of each chamber, the amount of extra air which comes into the second chamber, the height of the stack, and many other factors. It may even make a difference if it is summer or winter or if the plants doors are open or closed. But a properly operated, modern burn-off oven can compensate or be adjusted to control the rate of air transfer between the two chambers.
The size of the second chamber matters
Each afterburner or thermal oxidizer is sized to process a certain amount of organic material per hour. This is dependent on the speed of the gases entering the chamber, the temperature of the chamber and the amount of extra air that is added. A certain amount of retention time in the afterburner is required to properly process the organic vapors. If an oven has too big of a load and processes too much too fast, it will overload and push the organics out of the stack without being properly burned. This either causes a flame at the end of the stack where it hits a richer oxygen atmosphere or emission of unprocessed pollutants.
For example, carbon monoxide is always produced in the oxygen starved environment of the primary chamber. In order for carbon monoxide to completely burn, the exhaust gases must stay in the oxidizer close to a second at 1400 degrees F. (This may seem short but some oxidizers are not very large.) If the primary chamber is pushed too hard, with too high of a temperature or too big of a load, then the gases are forced through the afterburner too quickly and will release carbon monoxide out of the stack.
The speed at which an oven can process a load actually depends more on the size of the afterburner than the size of the process chamber.
Temperature
The thermal oxidizer of a burn-off oven runs at 1400-1800 degrees F.
The temperature that a thermal oxidizer (AKA afterburner/ AKA secondary chamber) needs to operate is set on a state by state basis by the state EPA or air quality agency. In general, the temperature in excess of 1450 degrees is needed, but this can vary. The higher the temperature is the faster the exhaust speed (draw) will be. This means a shorter retention time spent in the afterburner but a higher temperature also oxidizes the emissions faster so they balance one another.
That’s the basics and that’s really about it. Two ovens doing two different things with a hole in the middle. The rest is safety, efficiency, and preference.
NOT ROCKET SCIENCE
Now pyrolysis science is not rocket science. There is a lot of leeway and overkill built into a modern burn off oven. I met many inexperienced operators who spent hours with their oven door open trying to adjust the flame of their ovens to get the flame as blue as possible. A blue flame indicates that the gas/air mixture is at peak efficiency. A lot of furnace and boiler repair services think this way. After adjusting the flame as blue as possible, they close the door, cut off oxygen to the flame, the air/gas mixture is different, and it is no longer blue. But they never see that because the door is closed now.
The burners on a burn-off oven do not need a super efficiently burning flame. If a burner is not working efficiently and puts out carbon monoxide it is processed and burned completely in the afterburner. That’s what the afterburner does for a living.
I was at a customer recently and looking at their burn-off oven. They mostly burned off powder coated paint hooks. I discussed the oven with the operator and he seemed to have a handle on his operation. There were air leaks all over the oven but the results appeared good and the loads ran very quickly. While I was there the cycle timer ran down and the burn-off oven stopped. After a little while I asked him if we could open it up and look at the parts. He said ok, but it was still burning. We opened the door and the parts were covered in flames. He said that after the oven turned off, he left it closed for a while so the parts finish burning. So, the timer was finished, the burner was off, all pollution control elements were off and the parts were still burning.
Burn-off ovens of yesterday
This is not a new process. People at one time made a piles of wood and burned organic substances off of metal in the field behind the plant. They did not worry about pollution in the old days. This probably still goes on under the theory of “We always dunnit that way”.
The modern burn off oven did not come about just for environmental reasons. It came about also because of safety concerns both for the operator and the substrate being cleaned.
Early burn off ovens were furnaces that had a low air input and vented gasses to the outside at a controlled rate. The oven was heated to 700 to 800 degrees and as it rose in temperature it emitted gasses. It was dangerous because of the difficulty controlling the rate that gas was released and the gas could not always be emitted at a rate which would not cause fires or explosions
In the 80’s the increased use of new coating technologies such as powder coating and electrostatic coating required a good ground on paint fixtures. It also saw more chemically resistant coatings. These factors required new methods of stripping. This increased the use of cleaner burning ovens.
Over time the temperature required in the secondary chamber has risen. The retention time required has increased. Lots of changes have happened in the safety equipment on the ovens. There have been changes in the timing of when each of the chambers starts. But the basic principles have remained the same.
Burn-off ovens today
Today many companies have safe and efficient burn-off ovens, among them Bayco Ovens, Steelman, Pollution Control Products, GFS, Jackson, and ACE ovens.
PROBLEM PAINTS AND OTHER STUFF
Certain types of organic materials cause problems in the burn-off process. Epoxies for example create their own oxygen and start fires even in an oxygen starved atmosphere. These paints must be processed slowly and in a limited load so the afterburners are not overloaded.
Polyvinyl chloride and fluorocarbon, Teflon-like coatings also have problems both from a regulatory and an equipment standpoint.
METALS
Steel is easily and often processed through burn-off ovens. It withstands temperatures up to 1200 degrees without major changes to its properties. Good practice says to process steel below 900 degrees because parts which are “nested” together in a basket can reach higher temperatures. Often paint hooks sent out to outside processers are burned off as quick and hot as possible. This causes the hooks to lose their temper and not be able to support weight anymore. I have seen operators on paint lines hang a part on a line with a hook and the hook simply bends and the part falls to the floor.
Of course, certain metals such as zinc cannot be cleaning in a heat cleaning/burn off oven. They melt at too low of a temperature. And there is a possibility of fire when processing magnesium.
Aluminum can be a problem. Most aluminum alloys withstand the temperature of a burn-off oven, but thin aluminum often warps because of uneven heating. Aluminum castings change hardness and strength usually over 600 degrees F but this will vary from alloy to alloy. It is dangerous, for example, to burn off paint from aluminum wheels. This causes them to lose their strength and you create a disaster if the wheels break once they are on the vehicle.
There are the basics.
Contact us and keep reading our articles to go beyond the basics.
Thanks for reading.
If you need help fixing a burn-off oven or require a new one please contact us at 312-550-7083 or cdchemicals@att.net.
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