Burning Briquettes in gassification boiler

[David Kuznitz]

We have a new Polar G3 gassification boiler and have a shop with dry hardwood chips compressed into 2 1/2 diameter briquettes that are approximately 3 inches long for fuel.

Our boiler burns them nicely but smokes and am looking for ideas to burn them with proper gassification. From what I understand this has to do with the secondary chamber not burning properly. If I open the door the smoke clears but no improvement in trying a variety of air settings.

I'm a bit stuck...any ideas would be greatly appreciated.

 

[E Yoder]

I'm guessing the briquettes gasify at a higher rate than normal size firewood. You most likely need to modify the air intake beyond factory limits to allow more air in, especially in the reburn area. I don't know how doable that is in your unit as I haven't seen a polar up close.

 

[David Kuznitz]

I'm guessing the briquettes gasify at a higher rate than normal size firewood. You most likely need to modify the air intake beyond factory limits to allow more air in, especially in the reburn area. I don't know how doable that is in your unit as I haven't seen a polar up close.

I think you are correct in that they do gasify at a higher rate. I have tried opening up the secondary air to its limit...not sure how to go beyond that. The intake air is not forced, but the exhaust gas a fan. I suppose I could try a fan to pressurize the back of the unit to promote a higher air flow. Modifying the boiler seems like it should be a last resort.

I also read in another post that there always needs to be coals at the venturi... and briquettes do not coal well. Could this be an issue? Any ideas relating specifically to briquettes? They burn great in my standard wood stove....I know its all related to the gasses flowing through my boiler rather than being burned.

Thanks for the feedback!

 

[maple1]

I also read in another post that there always needs to be coals at the venturi... and briquettes do not coal well. Could this be an issue? Any ideas relating specifically to briquettes? They burn great in my standard wood stove....I know its all related to the gasses flowing through my boiler rather than being burned.

That could have something to do with it. I have been playing with a grate setup with mine, ever since I got it. Of varying materials, depending what I happen to find laying around or on the cheap. Whatever it is, it won't last too awful long so is practically a consumable. But it doesn't take much to hold a few coals back from falling through the gassing hole and help maintain a coal bed. Which partly helps the secondary burn better from the gasses being drawn through a very hot coal bed, but also at the same time slows the flow some of gas getting into secondary from primary. At least with my natural draft.

Have you tried slowing down primary air? I have no idea how air is controlled on the Polar. Or even the nozzle design. But thinking maybe a combo of some kind of grate effect, together with slowing primary air & the off gassing it makes, might help? If possible. I have found though, in my case, that it can be a fine line between a good coal bed, and too much stuff in the nozzle that can end up blocking gas flow too much.

 

[David Kuznitz]

That could have something to do with it. I have been playing with a grate setup with mine, ever since I got it. Of varying materials, depending what I happen to find laying around or on the cheap. Whatever it is, it won't last too awful long so is practically a consumable. But it doesn't take much to hold a few coals back from falling through the gassing hole and help maintain a coal bed. Which partly helps the secondary burn better from the gasses being drawn through a very hot coal bed, but also at the same time slows the flow some of gas getting into secondary from primary. At least with my natural draft.

Have you tried slowing down primary air? I have no idea how air is controlled on the Polar. Or even the nozzle design. But thinking maybe a combo of some kind of grate effect, together with slowing primary air & the off gassing it makes, might help? If possible. I have found though, in my case, that it can be a fine line between a good coal bed, and too much stuff in the nozzle that can end up blocking gas flow too much.

Thanks for the feedback....I'll experiment some more.....but do you really think restricting the air flow from the upper chamber is the solution? Opening the upper loading door clears the smoke, so lots of air. Any thoughts on how to load the briquettes? How many could be loaded at a time?

I ll try a grate of some sort as well, although my first naive attempt at using a standard fire grate suspended over the venturi a couple inches was a total failure....nothing would burn without smoking...a lot.

Do you burn briquettes?

Thanks!

 

[maple1]

No experience at all with briquettes, so could be that every thing I said turns out to be worthless.

But slowing air into (not necessarily out of) the primary chamber should slow the off gassing and reduce the fuel the secondary sees and by association reduce the secondary air needed. If it is already seeing the max secondary air possible. But I also have no experience at all with your boiler so don't know your primary & secondary air flows & controls. If the flow from primary into secondary is restricted, would that divert some air from the primary supply into the secondary? Again, just talking out loud in generalities since I don't know your boiler specifics.

 

[David Kuznitz]

No experience at all with briquettes, so could be that every thing I said turns out to be worthless.

But slowing air into (not necessarily out of) the primary chamber should slow the off gassing and reduce the fuel the secondary sees and by association reduce the secondary air needed. If it is already seeing the max secondary air possible. But I also have no experience at all with your boiler so don't know your primary & secondary air flows & controls. If the flow from primary into secondary is restricted, would that divert some air from the primary supply into the secondary? Again, just talking out loud in generalities since I don't know your boiler specifics.

It's worth a try. In also going to try some cordwood on the venturi to see if the issue has anything to do with the coals developed by the briquettes.

Early on I also tried burning some kiln dried oak scrap. This was a similar failure, tons of smoke, but at that time all my test firings were on a fire grate over the venturi. which I know now even prevents cordwood from burning properly.

DK

 

[SuperSpy]

I'm not familiar with your boiler, so maybe this isn't how yours burns, but what does the secondary burn chamber look like? I'm guessing these break up into sawdust and get blown down into the secondary chamber before they get a chance to get hot enough to release all their precious calories into the exhaust stream. If that guess is right, I'd bet there's tons of not-quite-ash piled up in the secondary chamber.

Maybe some sort of screen could help hold them back, but then you will be fighting clogs, and replacing melted screens.

 

[Fred61]

I think your briquettes are disintegrating before they carbonize and release their gas causing unburned wood to enter the nozzle and secondary combustion area. I don't know what the nozzle looks like on a Vigas but I think you should lay a brick over the nozzle, closing off half the opening and adjust the primary air way down.

When burning regular logs the burning surface is fully charred as it falls away from the piece.

 

[David Kuznitz]

These are both good responses, really helpful. There has got to be a solution. I'm still a little unclear and not quite sure why the boiler smokes. Here is an image of my burning briquettes.

Not a lot of ash in the secondary chamber, just a ton of smoke through the flue pipe. My guess is that lots of surface area means they burn too fast, maybe too rich? The briquettes are dry as well, maybe six percent. They burn leaving behind virtually no ash, in my conventional wood stove where I've burned a lot of them. I can open the ash door in the boiler and watch the fire feed down the venturi.

I'm going to try greatly reducing the primary air. Should I open up as much as possible the secondary air?

One other hint....when I close up the boiler burning the briquettes it seems to want to put the fire out.

 

[Fred61]

Before opening the door, I assume you must have opened a bypass so the photo shows a straight burn as opposed to fan forced burn you would have with the door closed. Is this assumption correct?

 

[maple1]

Seems the primary is making more smoke/fuel than the secondary can handle. Could be a combo of wrong balance of air supplies for the fuel (too much primary, not enough secondary), and maybe also like Fred suggested some of the raw feed is falling to the secondary chamber before it gets 'properly primaried'.

(Highly technical terms are used around here at times. )

 

[David Kuznitz]

The polar furnace does not have a bypass. All the smoke always passes through the secondary chamber. The bright burn is after the door is left open a bit. It's odd that all those nice coals from the briquettes dont give the secondary chamber what it needs to gassify correctly. When I shut the door the fire seems to go into smolder mode.

I'll try greatly reducing the primary air and maybe opening the secondary wide open. I think this is a good direction....perhaps too much smoke in the secondary chamber chokes the lower chamber burn.

I'm going to try some test firings with standard cord wood as well, maybe developing a nice coal bed and then adding some briquettes.

Something has got to work...

Thanks for the input I'm definitely more than a bit lost...

 

[Fred61]

You could have smoking little smoking particles going straight through the secondary chamber and up the chimney. Look around the outlet of the stack for little black dots on your roof. If you see the evidence that could answer the mystery of why you are not finding much ash in the boiler.

??? "PRIMARIED" ???

 

[David Kuznitz]

Some success...in my test firing.

Settings on the boiler air flow set to 60 % primary, 40 % secondary.

Lit up a fire with paper and kindling, no smoke.

Added larger hardwood kindling, no smoke, nice little fire at this point.

Added some oak cordwood, no smoke, let it burn maybe 45 minutes.

At this point I thought I'd see at what point the boiler would start smoking, so I added a couple of bundles of hardwood sticks.....that did it boiler started smoking. I tried primary air at 10%,,,, still smoking,,,,secondary air at 100%....still smoking....both primary and secondary air at 100%,,,, smoke cleared. I added briquettes, still no smoke, added more briquettes, no smoke.... I have some hope that there might be a solution.

The problem with the briquettes seem to be similar to burning dry kindling. Really dry fuel with lots of surface area burning too rapidly.

Odd that turning down the primary air didn't solve the problem.

Any implications with running the boiler wide open?

What complicates things for me is that my boiler serves the heating needs of a small shop, in California. My boiler needs a final approval by my local air quality agency, and I have to burn with emissions under a 20% Opacity rate with the exception of three minutes an hour where I can produce more smoke.

Thanks for the help!