RMH Riser material preference

I have amassed most of the materials for my RMH and asked a number of questions here (many thanks to you all for your input and answers - even your questions, as they make me think harder).  But as with most projects, there ALWAYS seems to be more questions!  

Previously, I asked about the sizes of White ovens in use, but have had little response; I also asked the same question at a few of the designer sites.  One response asked why I was using insulated fire brick for my riser instead of the "5-minute" version.  The short answer is - I already have the K-28 bricks.  

But since he is in the business of masonry builds, I thought I should see if there are any down sides to the use of IFB instead of ceramic board and wrap. Suggestions, anyone?

Thanks!
Randy

Randy,

IFBs are not designed to work in the fire path. That's it. They are porous and soft and will get eroded, If you don't care about any reasonable lifespan of the riser then you may use them. I understand that you want to use what you have, but it does not mean it's the correct material. I would suggest to use them AROUND the riser build form heavy duty firebricks.

Hi Randy;
I attempted to use modern harder 1750F IFB for a batch box and riser.
They failed at the port and the riser within a week. Temps were just too high
Older insulated firebricks were very soft and easily eroded.
Modern IFBs are much more durable.
K-28s are good for 2800F,  Your batch riser should be around 2000F..
My prediction is you will have a top-notch riser that lasts indefinitely.
The beauty is you already have them! If they do not hold up then switch over to a 5-minute riser.

Many insulating firebrick are used in gas- and wood-fired pottery kilns which routinely reach 2300 F, where they can last for decades. Obviously low-grade IFB would not be as durable, so it helps if you know what you have.

Thank you Glenn.

So how do I know what grade?  I thought the K-26 and K-28 designations were enough.

The ones I have are dull orange in color and if I rub two together, they do sluff off.  There are no other markings and no boxes (previously owned, unused).

It is interesting. Erosion of bricks is in the first place caused by chemical attack. Wood burning, even as efficient as in rocket stove is still not as clean as gas fire and the fumes are acidic.
Insulating bricks have porosity of at least 45% and up and the quality dense firebricks 15-20%. Better and more expensive firebricks have lower porosity and higher specific gravity. Higher porosity causes faster chemical erosion.
Insulating firebricks are used as lining of ceramic kilns, which are heated by electricity or gas. I would like to see their performance in direct wood fire contact, so I'm really curious of the outcome.

It might be a good idea to coat the insulating fire brick with Zircon or the equivalent American trade name coating.

I have not visited donkey32 website for over two years. Yesterday I found a TERRIFIC thread started in 2022 about refractory material selection for wood burning devices.
It is written by one of the forum users and he seems to have good knowledge of refractory material science.
I have read before on some website created by refractory material specialist, that if home stove builders applied modern discoveries in refractory industry they could gain a lot.
The linked thread tone may seem at first a little bit anal when you factor that people of Europe were building wood fired masonry heaters for few hundred years, and nobody was too worried about some high tech material selection, but their designs were dealing with lower temperatures.
It turns out that burning wood in high temperatures is more demanding that some metallurgical processes. Heat resistance itself means not much if taken ouf the contents of chemicals.
I recommend reading all six pages. Pure essence.

https://donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux

As a conclusion I have to say that you HAVE to use the zirconia 100HT coating to protect your insulating bricks from chemical erosion (ash fluxing) that leads to spalling.

The second argument for the coating is this:

"In laymen's terms, that means: porous refractories don't work as efficiently at holding heat in your combustion zone when it crests 300ºC, and so a method of blocking [reflecting] infrared radiation is needed if you want to boost performance... as in: if you want to have efficient combustion and clean exhaust, especially A) early in the burn cycle, when the system is coming up to optimal operating temp, and B) at the end of the burn cycle, when the fire is dying down and temperatures are tapering off, but lingering carbon still needs to be cracked into CO2, rather than CO, which requires a *minimum* sustained temp of 600ºC / 1100ºF, IIRC.)"

So it's a double edged sword - use of insulating bricks may seem like a best solution, because starting the clean burn will be fast, but then above mentioned 300 C the heat of the riser will radiate out through the heated insulating bricks instead of staying inside.

I think the best solution offered in this thread is to use the dense firebricks and use zirconia coating that will radiate the heat back to the riser. At the same time I would be a little bit cautious about this approach, because cumulative heat reflected back to the firebox and the riser may create higher than normal gas temperatures, which may be good for the clean and efficient combustion but may affect the design of the stove ceiling - materials, distances, etc.

I understand that for some people this information may be irrelevant, because they like to rebuild their stoves every few years, but I like solutions that last. It's especially important if someone wants to popularize rocket stoves ideas among ordinary consumers.

I have a couple of video on my youtube channel about applying Zircon, it might of been around for years but only been marketed recently so there  is not much long term info out there.
From my limited experience, it is working extremely well so far, I can imaging that it would  be at its best coating insulating brick.