Wood Boiler Secondary Burn

[ramboiavzl]

Today I tried to create secondary burn in my wood boiler, but I didn't see it ocorring.
I installed 1 tube of 1,3m x 28mm diameter where the exhaust gases passes to pre-heater secondary air. Then with a U curve I pass this pre-heated air to another tube with exactly the same measurements with 5 mm holes every 1 cm.
What I have done wrong?

 

[maple1]

It is very hard if not near impossible to create a secondary burn inside a water jacketed combustion chamber. You're dealing with a liquid cooled firebox.

 

[ramboiavzl]

Why?

 

[maple1]

Liquid cooled firebox. It doesn't get hot enough in there. Not saying it isn't possible but I don't think I've read any success stories.

 

[ramboiavzl]

I understand that.But when is burning the temperature is always hot,but probably isn't hot enough. Even if I super heat the secondary air,passing one tube through hot ashes doesn't work?
If not, what modification can I do to make secondary combustion happening?

 

[maple1]

None that I know of. Like mentioned, I don't think I've read any success stories. Also there has been an immense amount of r&d done by boiler manufacturers over the years to get secondary combustion and I think every successful outcome has involved an insulated secondary burn chamber.

 

[ramboiavzl]

It's not possible to build another structure to divert flue gases to there and in where trying to create the secondary burn?

 

[maple1]

Another structure? Not sure what you mean.

 

[ramboiavzl]

Something like a "wood stove" that receives the flue gas from the water boiler and makes the secondary burn happening there?

 

[maple1]

That's starting to sound like something that isn't a boiler anymore. How would you recover the heat?

 

[TCaldwell]

Not sure what kind of boiler you are starting out with, natural draft or induced draft. Secondary burn requires 3 things to be sustainable, flue gas of 1200degf , turbulence to mix the preheated secondary air with the flu gas, and resonance time for the reaction ( this happens in a insulated refractory tunnel). Also the ability to provide equal amounts of primary and secondary air. Most primary burn chambers are not designed to just add secondary air and have it work. The idea of a wood stove as a reaction chamber I doubt would work, as the boilers fluegas would be too cold by then, as well as the secondary air. If by chance you had some luck the wood stove wouldn’t stand the secondary burn temps . I give you credit for your imagination, but your ideas could burn the house down.

 

[ramboiavzl]

Natural Draft. I have already these water boiler at amost 20 years. The only thing I want to do it's trying to increase is eficiency. But with all of yours recommendations it seems almost impossible to do in this boiler. Probably for doing that I have to rebuild all the boiler to make secondary burn happening.

 

[SuperSpy]

I'm not sure I've ever seen an example of a boiler with a secondary chamber on the top, The problem is all the heat is in the glowing embers at the bottom of the fire box. The exchanger tubes, by definition, will be the coolest spot in the unit.

 

[maple1]

Yes, that's another aspect of the gasifying boiler design that has evolved - gases go through the coal bed immediately before they do the second stage burn. That is very important as anyone who has a gasifier and has had bridging happen in the firebox knows.

 

[SuperSpy]

Yeah I've had my CB unit do that a few times, super frustrating getting a 'Fire Out' text message and finding a nearly full (but cold) fire box with a basketball-sized hole hollowed out in the center.

 

[TCaldwell]

Yes it’s doubtful you will get a secondary burn in your boiler, possibly you could look at the different gasifier designs on line and build something.

 

[BoiledOver]

I see the potential to achieve a minimal level of gasification. Kinda tough to say for sure as your photos are clear but of narrow view. Could make a better assessment with a wider view of your unit. It appears that you have 3 access doors; ash clean out, primary firebox and exhaust tube access. Is that correct? I cannot see where your combustion air is entering the firebox. Is the middle door a double? One closing over the other from each side?

Firstly, to maximize the boilers intended performance, the exchange tubes should be kept clean as possible. Weekly or more often cleanings. Secondly, firewood moisture content will have a big influence. I would guess you will want your firewood to be below 20% moisture content, ideally at 17% or below.

It appears that you have the intake of secondary air coming from behind the boiler, in the shiney pipe. Is that correct? And then it runs through an exchange tube into the firebox. Is that correct? Are you depending on a natural draft to draw the air through the pipe? It appears to be a horizontal run, not much help there. I am thinking you would like to introduce the secondary combustion air where the unburned gasses enter the exchange tubes, not into the firebox directly. A pipe beginning low and ending high can get some natural draft. This would be your chance at creating secondary combustion going through the exchange tubes. As it looks in the photos, if any air draws through the pipe, it is just adding more air to the firebox. Same thing happens when opening the damper wider so no big gain there.

Does the back of the firebox have water behind it? If not, maybe entering your air feed pipe down low into the firebox from the back and take it up in the back end of the firebox, and then forward along the ceiling to the exchange tube entrance area. I did that same thing years ago in a similar attempt as you are trying. The preheated oxygen (air) mixing with the unburned gasses heading for the exit will get some secondary combustion. Or, maybe there is a way to run it from the front at the ash clean out to the back and then up the back and across the top. If successful secondary occurs, the exchange tube access door might get too hot, possible warping may occur.

I did get results in secondary gasification but faced other challenges that wouldn't bend to my way of thinking. One problem in my case was the flue temps skyrocketed beyond safe temperatures. Was scary to see the temp gauge bury itself, thank goodness for double wall flue pipe. My unit was not built for high temps and over time suffered severe warpage to some steel components.

Hoping you can get to a safe satisfying burn procedure. Firstly clean boiler, secondly truly seasoned firewood. Then think about other adjustments.

 

[ramboiavzl]

I can only take more photos in Thrusday because I'm not in house until Thusday.My boiler have 3 doors in front: In the top is the part where the flue gas transfer heat to water, the second door in the middle I have my combustion chamber and in bottom I have primary air control and ash clean out. For testing I'm just using natural draft but I have here 1'' air blower. I try with booth but without any sucess secondary burning. All the back off boiler is covered with water .The only way to pass secondary air from back is how I have try.The other way is trying but front but I have to modify my doors. I hope I answer everything, but if I put the pipes from behind you think I can get secondary burn?

 

[BoiledOver]

Or, maybe there is a way to run it from the front at the ash clean out

Going through the water jacket would be the last choice. In from the front down low and going up to exchange tube area. This will preheat the oxygen and also assist in a draft through the newly routed secondary combustion air intake pipe.

 

[BoiledOver]

You may be able to get it done without any door modifications. You have primary air control at the bottom. Maybe you can start your pipe inside the door and share from the primary air entry. In my EKO boiler, one fan pushes air through the primary channel and the secondary channel at the same time. Baffles on each are adjusted to get a correct balance for optimum burns.

 

[ramboiavzl]

Correct me if I'm wrong,maybe the best ideia yes to pass the tube in the all the way in the bottom of firebox to heat oxygen and after in the firebox background comes back?

 

[BoiledOver]

Correct me if I'm wrong,maybe the best ideia yes to pass the tube in the all the way in the bottom of firebox to heat oxygen and after in the firebox background comes back?

That sounds like a good plan.

 

[ramboiavzl]

I have to do few modifications.I will try and after I put here the result.I hope that works :D

 

[cumminstinkerer]

ramboiavzl,
I have done something similar to what you are trying to do in my box in box boiler and have a fair amount of success, still not perfect but way better than just a box in box. PM and I will pass along my phone number its easier for me because I only get on the web once in a while at work. you might search my posts too for a little background info on my project.
Jason

 

[BoiledOver]

Ramboiavzl may have a Portugal tele number.