Smokeless (sootless) combustion of vegetable oil

[Majorana]

TL;DR Summary

Aim: to design a small burner for vegetable oils which does not produce any soot (or VERY little of it).

Hello

I need to design a little burner, fed with vegetable oils like olive, soybean, sunflower and corn (alone or as a mixture).

The fundamental requisite of the burner is that it must not generate any soot (smoke): since "zero" is something hardly attainable in technology, "very little" can be an acceptable substitute

Power output should be in the range 600-800 W approximately.

It should not contain any electrical fuel pump: it must be either gravity- or capillarity-fed. The wick(s), if any, must be non-consuming.

In theory, as I could understand (I am no chemical engineer), there are two ways to burn a fuel with little or no output soot: 1-by an intrinsically soot-free combustion (smokeless flame), or 2-removing in some way the soot generated by a "conventional" flame.

Recently I learned that the problem of attaining a smokeless flame is anything but trivial:

On board the International Space Station (ISS), astronaut Christina Koch is lighting candles to help scientists back on Earth resolve the long-standing question of why flames in microgravity produce less soot.

Looks like somebody found the question to be DEFINITELY interesting...
Okay, I don't want to invent Capt. Kirk's antigravity in order to burn olive oil soot-free on Earth
Jokes apart, the first thing that comes to my mind is an electrostatic precipitator of some design (there are quite a few) following a conventional flame on non-consuming wick(s). But the plates of any electrostatic precipitator need to be cleaned periodically. Big installations in industrial chimneys are cleaned automatically by means of high-pressure water, a solution that is ruled out here, for obvious reasons.

Any ideas?...

 

[tnich]

How about blue whirl? https://www.sciencenews.org/article/flaming-fuel-water-creates-blue-whirl-burns-clean?tgt=nr, https://www.aiche.org/conferences/a...ential-fuel-flexible-and-soot-free-combustion

 

[Majorana]

How about blue whirl? https://www.sciencenews.org/article/flaming-fuel-water-creates-blue-whirl-burns-clean?tgt=nr, https://www.aiche.org/conferences/a...ential-fuel-flexible-and-soot-free-combustion

Look very interesting... but the first article is for subscribers only, and I can't understand how to read the second article, subscription or not...

 

[anorlunda]

@Majorana is correct. I can't read either of those two links. But a google search found



https://www.pnas.org/content/pnas/113/34/9457.full.pdfEdit: It looks like a very significant finding. If engineers could reliably create blue whirls in commercial burners, or wood stoves, it would have a big benefit. It is the model for how things are supposed to work. Science research first, then engineering development.

 

[Majorana]

This is one of the most fascinating things EVER! I want to read everything I can find on the web, and then EXPERIMENT (...without setting the house on fire... )

 

[Majorana]

I have just found this VERY interesting document, over 100 pages!

https://drum.lib.umd.edu/bitstream/handle/1903/19552/Hariharan_umd_0117N_18077.pdf

 

[256bits]

NOx emissions? Maybe no different than a regular blue flame, but I would think that should be checked for as well.

 

[TeethWhitener]

Smoke is basically just uncombusted carbon. The more oxygen-rich your flame is, the less smoke is produced. Hotter flames also increase the rate of reaction which leads to less smoke.

The issue with using vegetable oil as a fuel is that it has a very high boiling point, higher than its smoke point, so keeping it volatile enough to burn it completely in vapor form is a pain. As you increase the temperature, the oil tends to form a complex mixture of carbonaceous species, some of which are volatile and easy to burn completely, and some of which are tars and are not volatile at all, having a tendency to smolder instead of burn cleanly.

Diesel and biodiesel have the same problem, so it might be useful to take a look at the types of fuel injection systems used in modern Diesel engines. If you can nebulize the oil at a temperature below its smoke point, you might be able to completely combust the microdroplets quickly without generating too much soot.

 

[TeethWhitener]

Some poking around:
https://www.dieselnet.com/tech/diesel_fi.php
suggests that diesel fuel injectors are typically run at very high pressures, which is not terribly surprising, given the viscosity of diesel (and also oil) and the need to nebulize it into very small droplets.

 

[Majorana]

NOx emissions?

In my application, only soot emissions are a factor: NOx emissions aren't.

fuel injectors are typically run at very high pressures

Years ago I learned that diesel injectors must work at pressures well above 100 Bars in order to attain proper pulverization of diesel fuel. Apart from one basic requirement of my application ("the burner should not contain any electrical fuel pump") which rules out a high-pressure system like that, because my burner must be QUIET while an electrical 120 Bar fuel pump certainly isn't, I have a feeling that it would be VERY difficult to practically realize such a system for a power output of only, say, 800 W...

 

[neanderthalphysics]

My guess for why you don't get soot in zero gravity is because convection moves reactants to cooler areas of the flame, quenches the reaction and then you get soot (incomplete combustion products).

If you wanted smokeless combustion of vegetable oil, try doing it in an oxygen enriched environment.

 

[tnich]

My guess for why you don't get soot in zero gravity is because convection moves reactants to cooler areas of the flame, quenches the reaction and then you get soot (incomplete combustion products).

If you wanted smokeless combustion of vegetable oil, try doing it in an oxygen enriched environment.

It seems possible that the blue whirl might actually suck in the pre-ignition vegetable oil smoke and oxidize some of the incompletely combusted carbon.

 

[Majorana]

One of those two papers states that blue whirls can be obtained on a wide variety of fuels, including crude oil (certainly much "harder" than any vegetable oil...). What may change from a fuel to another in order to get a stable blue whirl is probably - as far as I learned to date - one parameter of the experimental setup: the gap width between the two half tubes. Investigation is needed. I would definitely stick with the blue whirl rather than some technique of fuel injection/pulverization as TeethWithener suggested: apart from the zero-noise requirement, I have a feeling that it would be extremely difficult (if possible at all) to design a high-pressure injector working at such very LOW fuel flow rates, where the fuel rate to feed the blue whirl described in those papers is about 1.1 cc/minute (1.1 milliliter/minute). I think that the blue whirl may actually be the perfect answer: silent, efficient, sootless, and no need for electricity.

 

[neanderthalphysics]

One of those two papers states that blue whirls can be obtained on a wide variety of fuels, including crude oil (certainly much "harder" than any vegetable oil...). What may change from a fuel to another in order to get a stable blue whirl is probably - as far as I learned to date - one parameter of the experimental setup: the gap width between the two half tubes. Investigation is needed. I would definitely stick with the blue whirl rather than some technique of fuel injection/pulverization as TeethWithener suggested: apart from the zero-noise requirement, I have a feeling that it would be extremely difficult (if possible at all) to design a high-pressure injector working at such very LOW fuel flow rates, where the fuel rate to feed the blue whirl described in those papers is about 1.1 cc/minute (1.1 milliliter/minute). I think that the blue whirl may actually be the perfect answer: silent, efficient, sootless, and no need for electricity.

It may be that the blue whirl over crude oil is burning the more volatile components, and what remains are the less volatile high carbon molecules.

 

[Majorana]

It may be that the blue whirl over crude oil is burning the more volatile components, and what remains are the less volatile high carbon molecules.

Yes, it's possible. But olive oil, or soybean oil, is much much lighter and purer than any crude oil. I have a feeling that the blue whirl is one of those ingenious and elegant solutions that Mother Nature sometimes draws from the hat, where elegant means simple, efficient, power free, and silent. I think we really have something here.

 

[Baluncore]

It may be that the blue whirl over crude oil is burning the more volatile components, and what remains are the less volatile high carbon molecules.

Burning heavy molecules involves cracking them thermally.

Heat radiated downwards by the blue whirl will rapidly vaporise the low molecular weight fraction on the surface, while breaking larger molecules into medium sizes that might normally have become smoke.

The blue whirl forms over a flat surface as the downward radiated heat supplies the "reactant smoke" that fuels the whirl above.

It seems the blue whirl takes medium weight molecules into the base flow, sorts, delays and processes them by weight, until they have reacted completely and are released as CO₂ and H₂O in the exhaust.

 

[chemisttree]

It should not contain any electrical fuel pump: it must be either gravity- or capillarity-fed. The wick(s), if any, must be non-consuming...

...the first thing that comes to my mind is an electrostatic precipitator of some design (there are quite a few) following a conventional flame on non-consuming wick(s).

So, no electrical fuel pump but an electrical precipitator is OK? You know how much electricity goes into electrostatic precipitation in coal plants?

Best to try for an oil burner coupled to a soot burner. Perhaps a soot burner based on a catalyst like platinum.

 

[chemisttree]

Platinum is probably the wrong catalyst. Ceria is more likely candidate. Soot combustion is accomplished with diesel particulate filter regeneration. Review article here: https://www.sciencedirect.com/science/article/pii/S0926860X15301939

 

[Majorana]

You know how much electricity goes into electrostatic precipitation in coal plants?

I don't know that exactly... but we are talking of a burner smaller than the smallest burner on a kitchen stove, and electrostatic precipitation, by own nature, implies just maintaining an electrostatic field rather than passing a significant current (I know, the corona discharge IS a current flow, but in my application it would be so small...) Anyway, the factor would not be the electrical power consumption but the NOISE produced by a pump. This burner must be silent. For that reason I would go for a wick-type combustion. Or a blue whirl...

Heat radiated downwards by the blue whirl will rapidly vaporise the low molecular weight fraction on the surface, while breaking larger molecules into medium sizes that might normally have become smoke.
The blue whirl forms over a flat surface as the downward radiated heat supplies the "reactant smoke" that fuels the whirl above.
It seems the blue whirl takes medium weight molecules into the base flow, sorts, delays and processes them by weight, until they have reacted completely and are released as CO2 and H2O in the exhaust.

This is far better than any engineer's wildest dream. Again, hats off to Mother Nature...

 

[chemisttree]

Yes, it's possible. But olive oil, or soybean oil, is much much lighter and purer than any crude oil.

Yes but the n-heptane used in this research is >99% pure and still it produced a thick viscous residue that didn’t burn but remained on the surface of the flat pool. You have traded soot for gooey tar. It will be much worse for vegetable oil since it is much less volatile and nowhere near as pure as >99% as is the n-heptane in the blue vortex work. I doubt it could even form a vortex over water as is described in this work since its pyrolysis temperature is many hundreds of degrees greater than the boiling point water (unlike low boiling heptane). Don’t even think about olive oil’s boiling point. It will pyrolyze way before it boils.

 

[Majorana]

the n-heptane used in this research is >99% pure and still it produced a thick viscous residue that didn’t burn but remained on the surface of the flat pool.

I admit I haven't read - yet- any of those papers to the end (lack of time... ), so the fault is mine, but where exactly did you find the reference about tar residue? I can't remember any mention of it

 

[chemisttree]

Page 40

“When water in the pan was not replaced after many tests, dark viscous liquids, presumably impurities in the fuel were observed to collect over the water surface. These globules floated on the water surface, and reduced the swirl on the water ... “

 

[Majorana]

These globules floated on the water surface, and reduced the swirl on the water

Sgrunt...

 

[Tom.G]

The fundamental requisite of the burner is that it must not generate any soot (smoke): since "zero" is something hardly attainable in technology, "very little" can be an acceptable substitute

“When water in the pan was not replaced after many tests, dark viscous liquids, presumably impurities in the fuel were observed to collect over the water surface.

So it leaves behind some 'ash' (probably less than a wood fire), that still seems to meet your original requirements.
Skimming, or simply dumping the water occassionally is no worse than washing dishes after use.

Or are we missing somehing?

Cheers,
Tom

 

[Majorana]

Or are we missing something

No, you aren't. It's all perfectly acceptable. My "sgrunt" was just a remnant from old Donald Duck & Uncle Scrooge strips... could not resist, sorry

 

[chemisttree]

So, have you given up on the wick/precipitator idea? Is cleaning the precipitator from time to time a problem? For your burner, how do you intend to use it? Is that purpose likely for a device that uses the blue swirl? Blue swirl needs a long chimney. For a precipitator equipped device?

 

[Majorana]

Well... an electrostatic precipitator suitable for this application should be designed, of course. And it should be effective, in the sense that it must capture more than 99% of the input soot. Cleaning the elements would not be too big a problem as long as "from time to time" is not once in a day or the like... I guess that burning a potentially smokey substance as oil - even vegetable, not mineral - would release quite a good deal of soot. As I mentioned in a previous post, the automatic cleaning systems used in big industrial precipitators are not suitable for such small (almost minuscule) designs. I haven't done experiments with the blue whirls yet, of course. But from what I could read about it, I think that the blue whirl is an extremely efficient, clean, simple, and "elegant" combustion regime. It's very close to perfection by very own nature. No cleaning systems, no electricity, no moving parts. And there is also another factor I didn't mention before. When you burn vegetable oils in the usual ways, you don't get just soot: you get odour , too. And you can't get rid of odours with just an electrostatic precipitator: you need other devices, like carbon filters or scrubbers, all devices that are either too complex or need scheduled replacement of parts. On the other hand, blue whirl attain a virtually perfect combustion. In the words of @Baluncore:

the blue whirl will rapidly vaporise the low molecular weight fraction on the surface, while breaking larger molecules into medium sizes that might normally have become smoke. [...] It seems the blue whirl takes medium weight molecules into the base flow, sorts, delays and processes them by weight, until they have reacted completely and are released as CO2 and H2O in the exhaust.

It seems that the blue whirl has the ability to break any molecule down to smaller and smaller molecules, and those molecules are not allowed to escape (presumable courtesy of the bright ring) until everything has been reduced to CO₂ and H₂O. In other words, what cannot burn is left as floating, cold tar, while all can burn is burned down to CO₂ and H₂O : odourless. Without noise, electricity, filters or the like. I cannot imagine anything more perfect nor more elegant.

 

[Baluncore]

I think there is a limitation due to using water as the fluid bed to the whirl. That limits pyrolysis of the heavier products to the boiling point of water, 100°C. It sets an upper limit to the molecular weight of the available reactants.

It might be possible to add something to the water, such as methanol, or maybe an acid, which could reduce recombination of pyrolysis products, and so avoid forming a hydroxide sludge.

Maybe raise the boiling point by adding salt to make a brine bed. Salt selection could also change the colour of the flame.

 

[chemisttree]

Well... an electrostatic precipitator suitable for this application should be designed, of course. And it should be effective, in the sense that it must capture more than 99% of the input soot. Cleaning the elements would not be too big a problem as long as "from time to time" is not once in a day or the like... I guess that burning a potentially smokey substance as oil - even vegetable, not mineral - would release quite a good deal of soot. As I mentioned in a previous post, the automatic cleaning systems used in big industrial precipitators are not suitable for such small (almost minuscule) designs. I haven't done experiments with the blue whirls yet, of course. But from what I could read about it, I think that the blue whirl is an extremely efficient, clean, simple, and "elegant" combustion regime. It's very close to perfection by very own nature. No cleaning systems...

Nothing was mentioned about the need to clean or not to except the mention of changing water. Soot IS produced, btw. Have you read the source you posted? It specifically states that wicks are not likely to be able to reproduce the whirl. Tests were conducted over a flat stainless surface and the whirl was observed so there is no absolute requirement for the water pool. No mention of pyrolysis. Combustion is believed to be via vaporization only which argues against the formation of thick, viscous and dark liquids via pyrolysis. No evidence given about the mechanism of fuel vaporization other than vaporization. A variety of low boiling fuels (ethanol, acetone, iso-octane, n-heptane and cyclohexane) were studied. None with low volatility like olive oil.

It’s a cool report but you are barking up the wrong tree I’m afraid.

You still haven’t described the purpose of the burner you want to build.

Have you read the source you posted?

 

[Majorana]

Hello, sorry for the delay in writing, I've been quite busy with my job and various neverending chores
@chemisttree you quite stole my thoughts I have read the part where they say that blue whirls can be produced also over a flat metal surface. It makes sense since, at least in my limited mind of a non-engineer, the only significant difference between water and flat metal (or even glass) would probably be that water allows the fuel to move in and all around virtually frictionless, not the same with metal or glass. Vapour from the water bed does not seem to play any part in the dynamics of the whirl. I have a feeling (just a gut feeling, no equations here ) that the frictionless movements of fuel on water should yeld a more stable and reliable blue whirl in comparison to metal or glass (especially with denser fuels, like oils), but probably no big differences in fundamental mechanics here.

It specifically states that wicks are not likely to be able to reproduce the whirl

I never thought to get a blue whirl from a wick... I mentioned wicks in my opening post only because, at that time, I had never heard a thing about blue whirls. Just that.

Soot IS produced, btw

My understanding is that soot is produced just as an "imperfection", so to speak, if and when the blue whirl is unstable (transient yellow hue/streaks). As long as it is stable, both in colour and shape/motion, soot does not seem to be produced, or maybe just in molecular amounts.
I have collected a total of 6 papers on the blue whirl so far (even one in Spanish, ¡olé! ), I haven't read them completely yet, due to lack of time, but I think I haven't missed any of the basics.

I think there is a limitation due to using water as the fluid bed to the whirl. That limits pyrolysis of the heavier products to the boiling point of water, 100°C. It sets an upper limit to the molecular weight of the available reactants.

I don't know... as I mentioned before, water does not seem to play any significant role in the mechanics of the blue whirl, except maybe for allowing the smoothest possible movements of the fuel "puddle". The fuel feeding the whirl is drawn from the surface of the fuel puddle, i.e. from the fuel-air interface: there is more fuel, not water, directly below the fuel being sucked up/vaporized. But I may be perfectly wrong here.