Monotube Flash Boiler Design

[Toymaker]

If the temperature of the pressurized air was hot enough that you were concerned about your plastic bottle, what about that poor motorcycle fuel pump? Isn't it going to get cooked in there? The temperature's gotta be way above the design rating of the the pump. It would seem to me that pump might last for a little while, then give up the ghost from overheating.

Thanks for your concerns for my fuel pump temperature limit, but my reason to not use a plastic bottle is entirely based on my fear of plastics becoming soft with even low temperature increases. Although I haven't measured the oil temperature inside the bottle, the outer surface of the air compressor measured 50⁰C max.

The plastic bottle I was using is made of PET (Polyethylene Terephthalate) which has a glass transition temperature of about 67⁰C. I don't have any hard information concerning at what temperature a PET bottle holding 20 psi would fail, but certainly that would happen somewhere below 67⁰C, which in everyday terms is the upper limit of a typical hair dryer. So maybe the plastic bottle would remain in it's safe temperature range,...I don't know,...which is one reason why I decided to go the safe route and use a metal bottle. The other reason to use the spherical container is the spherical shape makes an ideal air-oil separator.

I don't have the temperature specs for the fuel pump motor I'm using, but the majority of DC motors have an ambient temperature limit between 85⁰C and 100⁰C,...which is well above what I expect max oil temperatures to reach.

 

[ddmckee54]

Most electrical equipment - circuit breakers, switches and wiring, is rated at 75°C. The better stuff, read more expensive, is rated at 90°C. Most electrical equipment manufacturers don't want to see their stuff running at more than 40°C above ambient. I'd be concerned about the plastic parts of your pump getting soft and deforming. If you've got an all metal pump and fittings then you've probably got no worries.

 

[Steamchick]

From car experience: In cars that go to the Middle East (Arab states) temperatures can get up around 55deg C Max - when weather is hot and you drive over a hot road at around 55~60C.... - heated by the sun before you get there...
So fuel pumps run AT MAX at the fuel temp of around 60C. - but they are rated for 80~85C (depending on manufacturer) so they don't fail at temperatures that are possible... So no real worries below 60C.
(P,S: Petrol boils at 40C...)
K2

 

[vk7krj]

Steamchick- many years ago when I worked in Whyalla in South Australia, I had a car with a small (273 cu in) v8 in it. I fitted an accessory water gauge and was always intrigued to see the water temperature come DOWN to running temp. after the car had spent the day out in the sun. It was standard procedure to open all the doors and wait 5 minutes before getting in- and woe betide you if you sat on the vinyl seats when wearing shorts- blisters were the order of the day- ouch- hot pants before they became a fashion statement!

 

[The Ignoble Troll]

Thanks for your concerns for my fuel pump temperature limit, but my reason to not use a plastic bottle is entirely based on my fear of plastics becoming soft with even low temperature increases. Although I haven't measured the oil temperature inside the bottle, the outer surface of the air compressor measured 50⁰C max.

The plastic bottle I was using is made of PET (Polyethylene Terephthalate) which has a glass transition temperature of about 67⁰C. I don't have any hard information concerning at what temperature a PET bottle holding 20 psi would fail, but certainly that would happen somewhere below 67⁰C, which in everyday terms is the upper limit of a typical hair dryer. So maybe the plastic bottle would remain in it's safe temperature range,...I don't know,...which is one reason why I decided to go the safe route and use a metal bottle. The other reason to use the spherical container is the spherical shape makes an ideal air-oil separator.

I don't have the temperature specs for the fuel pump motor I'm using, but the majority of DC motors have an ambient temperature limit between 85⁰C and 100⁰C,...which is well above what I expect max oil temperatures to reach.

Is it too late to retouch on flame chemistry and flame color?

 

[Toymaker]

Is it too late to retouch on flame chemistry and flame color?

I'm continuously making adjustments and improvements to the fuel-air mix, so I guess we can still discuss flame color; the video in post #405 shows my current high fuel burn rate, which as you can see is almost entirely blue flame.
Without knowing your question(s), you may want to first Google "flame temperature color".

 

[Toymaker]

Steamchick- many years ago when I worked in Whyalla in South Australia, I had a car with a small (273 cu in) v8 in it. I fitted an accessory water gauge and was always intrigued to see the water temperature come DOWN to running temp. after the car had spent the day out in the sun. It was standard procedure to open all the doors and wait 5 minutes before getting in- and woe betide you if you sat on the vinyl seats when wearing shorts- blisters were the order of the day- ouch- hot pants before they became a fashion statement!

Reminds me of the 3 years I spent working in Tucson, Arizona.

 

[The Ignoble Troll]

I'm continuously making adjustments and improvements to the fuel-air mix, so I guess we can still discuss flame color; the video in post #405 shows my current high fuel burn rate, which as you can see is almost entirely blue flame.
Without knowing your question(s), you may want to first Google "flame temperature color".

It's the blue flame I wanted to comment on.

Blue flames are an internet obsession that are not always correctly used. If you look at a candle flame the outer envelope is blue. This blue layer radiates heat into the overly rich inner flame and causes carbon radical species to spontaneously emit visible light. Acetylene flames do the same so successfully that the carbon radicals led to the use of acetylene as a light source, much as candles still are.

The difference between a yellow flame, blue flame and a violet white flame, assuming a neutral, which means a slightly rich flame, is burn rate.

In the past I had a burner set up that preheated it's fuel, a disgusting mixture of motor oil, vegetable oil and anything else with a high flash point that was miscible and that the neighbors wanted to be rid of. The burner heated the fuel to around 1100°C in a vaporizing tube and then mixed it with air in a tube within a tube design and vented the flame out a tangential nozzle. Originally the steel vaporizing tube had a copper injector tube, but more on that later...

It was possible to change the nozzle so that different flame fronts could be created. The exact same mixture would cheerfully burn bright blue or yellow and soft or violent and whiteish violet.

All of these flames had the exact same air/fuel mixture but what I could change was how fast the flame front propagated. All of the flames had the exact same ability to heat a forge and a smelter/melter to the same temperature but the blue flame noticeably caused more erosion even to the ceramic furnace.

This is relevant in heating things because of factors like flame erosion and hot spotting.

You mentioned the paper cup of water on a Bunsen burner, are you familiar with a Meker burner? It is an example of the same fuel air mixture as a Bunsen burners yellow flame burning blue due to the many small short flame fronts.

while your blue flame is admittedly a wonderful achievement, it is likely a much harsher flame then a softer flame such as you had earlier on.


Because you are building a thermal turbo expander and attempting to convert as much of your thermal energy to compressed air, a softer flame may serve you better in a number of ways. so long as you have the chemistry right a softer flame is less likely to cause erosion damage to your heat exchanger. A softer flame aka a longer flame front will give the unit more time for more of the heat exchanger to exchange heat and reduce the chance of hot spotting.
If you get hot spotting, you could conceivably have pockets of thermal decomposition resulting in halogen radical contamination of your working fluid. Think Leidenfrost effect and cavitation. Possible metal damage and an incredibly toxic conversion of your working fluid to a metal eating monster of a deadly substance.

I experienced this with a well cooled copper injector in the above burner which had a tiny hotspot. It melted a segment of the copper injector which was inside of a steel white hot pipe. The result? A huge shot gun blast as I shut the burner down and vented air through the steel preheat line to clean and residual carbon deposits. The melted copper had reacted with the ultra hot fuel and the most likely result was a build up of Copper acetylide at the cold end of the injector. The thing was designed with no restrictions once the clean out was open so no harm done but it did nicely demonstrate to me the risk of hotspots on vaporizing tubes. My ears eventually stopped ringing and everything in the injector was converted to steel.

**Your design is interested in the volume of heat, not the gradient**


my suggestion, for safety, is embrace the softer yellow flame. There is a reason that diesel # 3 burning appliances favor a yellow flame with a swirl flame holder to extract the heat VS a much more compact flame with the same total caloric value but a substantially higher gradient.


The last example I would give of this is that in every steam engine that I am aware of, I know of no steam engine that used tubes in a fire pot of solid fuel to generate steam. The heat is removed from the fire pot and then run through the boiler. A solid fuel fire pot can be easily rigged to burn hot enough to set steel on fire (sounds like bacon frying) but that gradient is not used. A blue flame is roughly the same vs a softer yellow flame.


Just my 2 cents.

Great project. Love what you are doing.

 

[Toymaker]

It's the blue flame I wanted to comment on.

Blue flames are an internet obsession that are not always correctly used. If you look at a candle flame the outer envelope is blue. This blue layer radiates heat into the overly rich inner flame and causes carbon radical species to spontaneously emit visible light. Acetylene flames do the same so successfully that the carbon radicals led to the use of acetylene as a light source, much as candles still are.

The difference between a yellow flame, blue flame and a violet white flame, assuming a neutral, which means a slightly rich flame, is burn rate.

In the past I had a burner set up that preheated it's fuel, a disgusting mixture of motor oil, vegetable oil and anything else with a high flash point that was miscible and that the neighbors wanted to be rid of. The burner heated the fuel to around 1100°C in a vaporizing tube and then mixed it with air in a tube within a tube design and vented the flame out a tangential nozzle. Originally the steel vaporizing tube had a copper injector tube, but more on that later...

It was possible to change the nozzle so that different flame fronts could be created. The exact same mixture would cheerfully burn bright blue or yellow and soft or violent and whiteish violet.

All of these flames had the exact same air/fuel mixture but what I could change was how fast the flame front propagated. All of the flames had the exact same ability to heat a forge and a smelter/melter to the same temperature but the blue flame noticeably caused more erosion even to the ceramic furnace.

This is relevant in heating things because of factors like flame erosion and hot spotting.

You mentioned the paper cup of water on a Bunsen burner, are you familiar with a Meker burner? It is an example of the same fuel air mixture as a Bunsen burners yellow flame burning blue due to the many small short flame fronts.

while your blue flame is admittedly a wonderful achievement, it is likely a much harsher flame then a softer flame such as you had earlier on.


Because you are building a thermal turbo expander and attempting to convert as much of your thermal energy to compressed air, a softer flame may serve you better in a number of ways. so long as you have the chemistry right a softer flame is less likely to cause erosion damage to your heat exchanger. A softer flame aka a longer flame front will give the unit more time for more of the heat exchanger to exchange heat and reduce the chance of hot spotting.
If you get hot spotting, you could conceivably have pockets of thermal decomposition resulting in halogen radical contamination of your working fluid. Think Leidenfrost effect and cavitation. Possible metal damage and an incredibly toxic conversion of your working fluid to a metal eating monster of a deadly substance.

I experienced this with a well cooled copper injector in the above burner which had a tiny hotspot. It melted a segment of the copper injector which was inside of a steel white hot pipe. The result? A huge shot gun blast as I shut the burner down and vented air through the steel preheat line to clean and residual carbon deposits. The melted copper had reacted with the ultra hot fuel and the most likely result was a build up of Copper acetylide at the cold end of the injector. The thing was designed with no restrictions once the clean out was open so no harm done but it did nicely demonstrate to me the risk of hotspots on vaporizing tubes. My ears eventually stopped ringing and everything in the injector was converted to steel.

**Your design is interested in the volume of heat, not the gradient**


my suggestion, for safety, is embrace the softer yellow flame. There is a reason that diesel # 3 burning appliances favor a yellow flame with a swirl flame holder to extract the heat VS a much more compact flame with the same total caloric value but a substantially higher gradient.


The last example I would give of this is that in every steam engine that I am aware of, I know of no steam engine that used tubes in a fire pot of solid fuel to generate steam. The heat is removed from the fire pot and then run through the boiler. A solid fuel fire pot can be easily rigged to burn hot enough to set steel on fire (sounds like bacon frying) but that gradient is not used. A blue flame is roughly the same vs a softer yellow flame.


Just my 2 cents.

Great project. Love what you are doing.

First, thanks for the detailed info on combustion processes, very enlightening info concerning the eroding effects of blue flames.

The primary reason I've focused on producing a blue flame is fuel efficiency. The yellow flames seen in my earlier designs quickly coated everything they touched with a thin layer of black soot, which is a clear sign of incomplete combustion; the blue flames leave no residue, no black soot, indicating complete combustion. Since my end goal is to place the final engine in a car, fuel efficiency is very important to me.

Seems I have a choice. I can use blue flames and expect a shorter lifespan of the boiler tubes. Or I can use yellow flames and live with lower fuel efficiency and soot build-up inside the boiler.

 

[Steamchick]

Very interesting - I am learning loads from all the experts here. - THANKS!.
Just 2 pennorth... that may show how little I know, so I'll be glad of corrections here.
I the "chase" for more compact combustion and boiler improved efficiency (for my stuff) I favour using the flames to heat other stuff so the ratio of radiant heat to conducted heat (gas contact with surfaces) generated by a "fixed" gas and air system. Ergo using the very hot gases with low emissivity on something tolerant of flames (chemistry) and temperature to increase heat flow by radiating heat onto boiler surfaces.
E.g. on ceramic burners, I have adjusted air intakes to get "hotter" ceramic surfaces than "industrial" burners, to the limit where the near white-hot ceramic lasted only a few minutes before it over-cooked and cracked with thermal stresses. I have found industrial guides that explain the ceramic doesn't last long over "orange" heat (~900~950C). Also the ceramics - if given a bit too much air - can simply over-heat (they are cooled on the inside surfaces by air and gas flow) such that flash-back occurs. Yet, in copper boilers I find that any shortage of air "stinks" of combustion products that poison the lungs and brain? - But get it right and there is no smell (and I know CO doesn't smell either!). Unfortunately, in some boilers a I cannot see how the flame is burning - Just an orange glow from the ceramic surface as I look (with a mirror) down the chimney... - as the flames are very small across the surface of the burner.
What I see when the burner is outside the boiler is never the same as inside the boiler - due to back-pressure changing mixture and flame combustion/front speed.
With metal (pipes with slots, etc.) burners I can usually see the flames before determining the "Hottest" position for a radiant screen while tuning a burner outside the boiler... but then inside the boiler the screen may be in the wrong position due to the mixture changes and flame front combustion speed changes of the boiler confines, back-pressure, etc.
I am also aware that some boiler designs have flues or chimneys that are simply too small - by all standards - for the burner so generate significant back-pressure. (My father made a boiler (sized to "plans") - which didn't perform as expected - until I told him he needed twice the cross sectional area of the chimney. (30mm dia. not 20mm dia.). That made it a good boiler that could take a larger burner than the plans!).

It sounds from Troll's explanation above that I may be overheating some parts of a boiler if I over-do the radiant heating, as this could cause hot spots at the points closest to the radiant elements? - If water tubes or boiler surfaces, Kozo Hiraoka suggests the water cooling can keep the surfaces to only a few Deg.C. to ~40Deg.C.-ish over the gas temperature (when discussing effects of internal lime-scale) with maybe 1000 deg.C. gas temperature. - But high radiant heat input could increase this... ? - But as his max temperature are postulated as ~210deg.C on a scally boiler, I can't imagine getting much higher (only a further 50deg.C Max?) as the water is removing heat as fast as it boils...
Troll mentions that there is no consideration of water tubes or whatever in the fire, yet loco fireboxes often have a large red-hot bed of coals, contacting boiler internal surfaces at the sides of the firebox? The red-heat from primary combustion of coals to CO must be around 900deg.C. adjacent to water-cooled metal - so is this really a major risk? Do firebox walls corrode/erode more rapidly here? - I don't know (but I think "probably"?).

So really, on model (Small, not industrial large) boilers, I reckon you need a window on the combustion chamber inside the firebox part of the boiler to really see what is going-on? Will flames actually impinge on cooled metal? - or be blanketed by cooler/exhaust gases? Studies in fresh air only give a part of the story.
There is a lot I don't understand...
K2

 

[The Ignoble Troll]

First, thanks for the detailed info on combustion processes, very enlightening info concerning the eroding effects of blue flames.

The primary reason I've focused on producing a blue flame is fuel efficiency. The yellow flames seen in my earlier designs quickly coated everything they touched with a thin layer of black soot, which is a clear sign of incomplete combustion; the blue flames leave no residue, no black soot, indicating complete combustion. Since my end goal is to place the final engine in a car, fuel efficiency is very important to me.

Seems I have a choice. I can use blue flames and expect a shorter lifespan of the boiler tubes. Or I can use yellow flames and live with lower fuel efficiency and soot build-up inside the boiler.

Perhaps add a windshield pump on a timer and inject a little methanol or ethanol in the 70% range that gives a squirt when soot forms, so that the flame is briefly oxidizing?

A de-sooting stage?

 

[Toymaker]

Perhaps add a windshield pump on a timer and inject a little methanol or ethanol in the 70% range that gives a squirt when soot forms, so that the flame is briefly oxidizing?

A de-sooting stage?

Soot build-up isn't the problem, but rather a symptom of the real problem which is poor fuel efficiency due to incomplete combustion.

 

[The Ignoble Troll]

While soot is an excellent insulator, little amounts shouldn't be too upsettinging to your efficiency given the tempurature gradient from your flame to the working fluid, I would think.

If you did add a liquid injection that used oxygen rich fluids (water etc) to briefly oxidize away any carbon, you would be collecting the heat of combustion still, as it self cleaned.

Potential water injection could also let you reduce the heat in the flame can while increasing the mass energy transfer to your working fluid. Sorta like a booster.

Just a thought.

Is there an in-between flame you can make? Think redishblueish steaks without yellow streaks.

Soot typically is the deposition of those carbon bodies I mentioned, so if you can soften your flame without getting soot depositing strands of yellow...

Ether way your work is fascinating.

Have you considered adding a gas diverted and cooling down a sample of the exit gas enough to attach a suit of arduino sensors to quantify your exhaust, just for the sake of knowledge?

 

[Steamchick]

How about a car O2 sensor with appropriate circuitry to read the resistance and judge the mixture?

 

[The Ignoble Troll]

Also, how much soot are you talking about, if you were to scrape out 10 hours worth of soot and find it weighed less then a tenth of a gram, then your looking at an energy loss that would be far lower then the energy wasted by hanging a pinwheel off the grill of the car, right? On the other hand, if the thing is clogging with soot you are definitely loosing efficiency.

Chemistry and efficiency aren't always = if you know what I mean.


Even in 1950's vertical tube boilers, hot spotting was a threat. The papers I've read from the USA on such burners/boilers made it a non-issue via burner design but across the pond, many years later, the Russians decided to turbocharge vertical tube boilers with steam turbines and stuck those engines into three aircraft carriers.

From what I've read on their engine problems, hot spotting is a huge issue and no one is going to accuse those engines of anywhere near complete combustion. They literally overcook tubes filled with water, again from what I've read. That's impressive.


Your current auto gives off CO which is a reality of engines not being able to run stoichiometrically, because they would be too hot and lean for today's modern materials. They sacrifice some fuel to keep combustion as efficient as possible for the material constraints.


Thank you for the kind words Steamchick, but in fairness it is experts like you who got me into following this site. You are truly a wealth of information and experience.

 

[Steamchick]

Thanks Troll, kind words much appreciated. But beware. With the tasty bits on Chicken that we eat comes a ton of waste ... (****) so filter what it profess and enjoy the nice bits, but be selective to avoid the wrong (smelly) stuff.
On modern cars. The positive loop feedback from O2 sensors aims at stoichiometric mixture based on low O2 in the exhaust. Partly to avoid leaner mix high temperatures that create NOx over ~900°C. Just enough free O2 to assist the catalyst without cooking the cat. Emissions test results (post cat = tailpipe emissions) give very low CO, and hydrocarbons, which can't happen if away from stoichiometric input air and fuel. But the O2 sensor is a one sided measure. Once past the "no O2" point it can't determine the lack of O2 to bring it back from rich. On a car, the airflow meter and software send the measured impulses to the piezo-electric injectors to give the "target" correct mixture... Complexity not in use in these boilers.
I suppose some sort of hot wire element in the flame could be used, with a low voltage, low current, to monitor circuit resistance and hence be calibrated against flame temperature to give an input to a controller that varies fuel versus air input?
Geet complicated.... beyond my ken!
K2

 

[Toymaker]

My observations and analysis:

I hope we can all agree that a blue flame is indicative of complete combustion which releases all the BTUs within the fuel. Yellow, orange, & red flame colors all indicate carbon or carbon compounds produced from the fuel that didn't fully oxidize and represent wasted, unrealized BTUs as well as atmospheric pollution. Therefore, adjusting the air-fuel mix to give the most blue flames will result in the best fuel efficiency.

The only negative aspect of blue flames, which Ignoble has pointed out, is their possible corrosive nature. I say "possible" because I remain skeptical that blue flames are actually all that detrimental to metals. Millions of homes around the world have been cooking on gas stoves, adjusted to produce blue flames, for over a century, and I've never heard, nor experienced for myself, of a metal pot or pan, of any material, copper, stainless, aluminum, cast iron, etc, suffering any corrosive, or other negative affects from blue flames. Now, if you allow the pot or pan to become empty, then the flames will quickly overheat, or even melt the metal.

Even if placing the copper tubes in direct contact with blue flames turns out to be a problem, the position of the burner assembly is adjustable as shown by the two black arrows in the pic below, which shows the burner's position to be just slightly beyond the reach of the blue flames.



O2 sensors on a burner such as mine are unnecessary because the burner is completely un-coupled from the rest of the engine; once the fuel-air mix is adjusted for best performance at all power levels, unlike IC engines, that mix wont change no matter what occurs in the boiler, or the rest of the engine.

 

[Steamchick]

Hi Toymaker, I was postulating, based on my experience of un-forced air burners, that when fitted into the confines of the boiler firebox, that the inherent back-pressure may cause the flame front to increase velocity. Akin to changing the compression ratio in an internal combustion engine where - because of increased temperature at ignition, the mixture may need adjusting in a carburetted engine.
Of course, while your forced air system is analogous to a supercharged and fuel injected ICE, the control of mixture is different and seemingly "unchanged" at the air intake, the same principle applies. I cannot speculate of how much it may be significant, but I am sure that changing the environmental pressure where the flame occurs, because of a change of pressure immediately beyond the flame, there will be some change in the combustion speed of the flame front.
Therefore it is for you to determine how much this becomes significant due to the sizes of your passages and cooling effect on the exhaust beyond the flame front.
So far I think I have only seen the burner operating in air, not inside the boiler, where it is subjected to back-pressure, however small.
As Troll was explaining how flames can produce excess temperature at flame fronts - which may or may not impinge on boiler surfaces and affect the localise temperature of boiler material - you do seem to have a means of moving the flames back and away from direct contact, so this should not become a problem. I was not aware that you could back-off the burner as explained now.
My suggestion was simply that if there was a risk with excessive O2 in the flame, leading to excessive temperature, then the O2 sensor may be a device to give you a feedback of excess O2 to your control system.
It is good and efficient boiler practice to not have excess air, as this is effectively extra mass of ejected hot material at the exhaust so in effect there is less heat left in the boiler. It is simply being air-cooled by the excess air.
So that is another reason why you need to control the air accurately, and you have the controller and variable speed blower in place already. Adding a feedback loop from an O2 sensor may give you better control from excess air?
I appreciate not all suggestions are appropriate, and you are best placed to decide anyway.

K2

 

[Toymaker]

Hi Toymaker, I was postulating, based on my experience of un-forced air burners, that when fitted into the confines of the boiler firebox, that the inherent back-pressure may cause the flame front to increase velocity. Akin to changing the compression ratio in an internal combustion engine where - because of increased temperature at ignition, the mixture may need adjusting in a carburetted engine.
Of course, while your forced air system is analogous to a supercharged and fuel injected ICE, the control of mixture is different and seemingly "unchanged" at the air intake, the same principle applies. I cannot speculate of how much it may be significant, but I am sure that changing the environmental pressure where the flame occurs, because of a change of pressure immediately beyond the flame, there will be some change in the combustion speed of the flame front.

Recall that the burner assembly has a stainless steel round disc or plate (left photo below), which restricts the flow of exhaust flames exiting into the boiler. The small annular ring area where the exhaust gases exit is by far the smallest area the gases will encounter. The flame shown in the right photo below may appear to be solid circle, but in fact, it is ring shaped. Yes, the exhaust gasses will greatly expand as they flow through the boiler tubes which will represent a restricted flow,... but I doubt whatever backpressure the boiler offers will be greater than the backpressure produced by the restrictor plate.

Therefore it is for you to determine how much this becomes significant due to the sizes of your passages and cooling effect on the exhaust beyond the flame front.
So far I think I have only seen the burner operating in air, not inside the boiler, where it is subjected to back-pressure, however small.

Check your post #243

As Troll was explaining how flames can produce excess temperature at flame fronts - which may or may not impinge on boiler surfaces and affect the localise temperature of boiler material - you do seem to have a means of moving the flames back and away from direct contact, so this should not become a problem. I was not aware that you could back-off the burner as explained now.

Hey,...I cant give away all my design secrets all at once

My suggestion was simply that if there was a risk with excessive O2 in the flame, leading to excessive temperature, then the O2 sensor may be a device to give you a feedback of excess O2 to your control system.
It is good and efficient boiler practice to not have excess air, as this is effectively extra mass of ejected hot material at the exhaust so in effect there is less heat left in the boiler. It is simply being air-cooled by the excess air.
So that is another reason why you need to control the air accurately, and you have the controller and variable speed blower in place already. Adding a feedback loop from an O2 sensor may give you better control from excess air?
I appreciate not all suggestions are appropriate, and you are best placed to decide anyway.

K2

My goal is to adjust air-fuel mix to achieve the most blue flame over the entire operating range of the burner, while the burner is outside of the boiler where the flames can be observed. Once these settings are programed into the software, (which has already been done) the burner assembly can be tucked away inside the boiler coils as the ECU will always set the air-fuel mix for best burn.

 

[The Ignoble Troll]

My observations and analysis:







I hope we can all agree that a blue flame is indicative of complete combustion which releases all the BTUs within the fuel. ** Yellow, orange, & red flame colors all indicate carbon or carbon compounds produced from the fuel that didn't fully oxidize and represent wasted, unrealized BTUs as well as atmospheric pollution. Therefore, adjusting the air-fuel mix to give the most blue flames will result in the best fuel efficiency.**

I would actually respectfully disagree with you, but I'll address that lower down.

On erosion, you hafta look at when you have. You have a forced air burner with very high flow rates.

Take two examples with the same color flame, one is a USA space shuttle main engine, the other is a methanol spirt lamp.

Both flames are similar in color and are blue with high IR signatures.

Both are so faint that they are invisible in daylight.



The shuttle hydrogen/oxygen engine flame is clearly going to erode anything you part in front of it.


The soft methanol flame is going to clearly not erode materials put above it, unless they are very fine like transformer wire but that's extending the metaphor beyond meaning.



You built a jet engine combustion can. You pretty much have an electric Thermojet.



You need to look at the flame as something harsh, turbulent and fast moving, not at all like a stove flame.


On the note of burner flame color; soot, smoke, flaming black snow flakes (that's sorta what droplets of unburnt fuel look like as they eject with the flame) and so on, are the indicators of incomplete combustion.



As I mentioned earlier, I have and it is possible to burn the same fuel air mixture with all those colors - which means they are not indicators of completion of combustion efficiency but flame front speed.



I betcha that given your fuel flow rate that you could make a less efficient burner and get a two foot long yellow soft flame sootless. You've taken that and made it a few cm.


That means that the energy intensity, both thermal, kinetic and chemical are highly focused.

Imagine that you are looking at two 100% efficiency fixed displament pumps. One moves 1714 gallons at 1 psi the other moves 1 gpm at 1714 psi, each take exactly 1 horse power.


Which will cut off your finger?


For you, "combustion efficiency" should mean both that you have complete combustion as measured at the exhaust (easy to determine with a Grove multi gas sensor) AND that you are spreading that flames heat out for the whole boiler to absorb, because a Hotspot could be catastrophic.

Going waaaaaaaay back to your surmise that poping a leak will mean a jet of nasty gas not an explosion; I would personally bet on damage to the refrigerating cause by a hot spot to form an autocataltic reaction that could cause a broad failure as tlarge sections of the boiler corrode internally vs a pin hole. Aka if the seals don't go first, you could have an actual explosion realesing stuff that is more toxic then cyanide.


The web is sadly a midden for "blue" flames.

I'm not trying to preach or put you down or anything. Your project is well beyond my machining and electronic abilities. You are very smart, and it's clear.


I am just giving my .02$ as someone who spent a lot of time working with flame chemistry and is worried you may get injured due to the web's poor info.

All the best.

-Troll