Aslan Core Melts Down on First Firing (picture heavy)

Hey all, I've been farting around here for a few months talking about stuff of which I know not (like most newbies do) but now I have been deflowered.

I built and tested my first CFB core yesterday and it it went straight from awesome to scary.


Here it is, 2300 degree ceramic fiber board, based on Matt Walker's 6" core but scaled up, and using a 2" thick 4 foot CF riser sleeve as a heat riser. I placed a 1300C (2372F) thermocouple six inches up from the riser/core interface and sticking about 4" into the gas stream.





Yesterday, I cut it all out, put it together and fired it up. No hiccups, everything worked perfectly. I also placed a copper wire about 56" down the riser to check temperature through melting.
[img src="https://drive.google.com/uc?id=165NjNsSey4FRjaP2PCci4YEQBHv5rccW" alt="" style="max-width:100%;"]


Everything was going great, burning out the binder in the board, and the temps continued to rise. Then it approached and passed 2300 degrees and that's when I started getting concerned because my materials are all rated at right around that. This was burning dry but not particularly well cured small split wood and some dry 2x4s and 2x6s.  At the end of the burn it was hitting 2300 degrees burning ONLY TWO 2X6s!

Here is the peak I measured. This is a screen cap from the video. 2446.7 was the last number the thermocouple read before going all -----. On the video, the temperature is still rising at a rate of 2-3 degrees every half second. So, though I don't have numbers, I would say it approached 2500 degrees before backing off.


[img style="max-width:100%;" src="https://drive.google.com/uc?id=158RUGvaIusQa2YGoaxKRVWMVaddpSQIY" alt=""]


Later during cool down, the riser made loud cracking noises. Disassembly showed cracks in the lower riser section. Cracks appear to propagate about half the thickness of the riser.

[img style="max-width:100%;" src="https://drive.google.com/uc?id=1AnhP2tBW73oCKEYHAxnK31Ez1EKjshRh" alt=""]


Copper melted up to just below the edge of the top of the burn tunnel.  Copper melts at 1984F. A small nodule of copper dripped down and made a pile (though not a puddle).

[img alt="" src="https://drive.google.com/uc?id=1iGBwjaPNnbR8mFukSIgSunchn91As1hp" style="max-width:100%;"]


Now the major problem, every piece of the internal layer of CFB warped and cupped. You can see in the picture down the riser above that the back of the burn tunnel piece curved into the flow path. This does not seem to have affected the draw or output of the core at all. The feed tube is still likely the most constricted point in the system.

[img style="max-width:100%;" alt="" src="https://drive.google.com/uc?id=1Zh5OITl3F8ayv6cYdpT-4tzsCQyJ_Rml"]


The outer layer of the CFB seems to have had little issue. After burning for several hours, there was maybe half a cup total of ash. Firebricks were unaffected, and were still at 200 degrees four hours later when I took them out and hauled everything inside to avoid rain.  Even though the thermocouple went well outside specifications, it seems to have survived unscathed.

[img src="https://drive.google.com/uc?id=1REVdfYsYlHSIVHrEkrilMoarDa78KYeD" alt="" style="max-width:100%;"]


I named this core the Aslan Core.  Points to the first person who figures out why.

[img style="max-width:100%;" alt="" src="https://drive.google.com/uc?id=1lJQLKPJFh8OMXmTbJcMkJUPwJe3xsLVN"]

It looks like it really got hot. Hot is good, too hot is not so good. But very good that it ran and ran well. I'm a little surprised the thermocouple survived and also that the riser tube cracked like that.

I'm going to guess "Aslan" after the lion in the Narnia stories because of the enormous roar of this little lion.

That got warm!!!

The stone table cracked when certain laws were tested and broken, in this case the laws of thermal ratings!

And James Alun is the winner!  I call it Aslan because it cracked the stone table.

I think I'm just going to replace the inner lining for the time being when I install it in a full RMH build. Need to find a way to burn out the binders ahead of time.

Hopefully being installed in a full system will tame it a little, and also I have some ways planned of dampering it just a little, to try to keep it from exceeding the speed limit.  Don't realistically need more than 2000.

If that's not good enough, I'll re-liner it some higher temp CFB.  I'll keep using the riser until it falls apart. I think I'll also do all firebrick floor and just eliminate that piece of CFB.

Thank you for such a detailed account with quality pictures!  Where did you purchase the CFB and the CF riser materials?  I'm trying to find alternate sources for my own custom build.

Solomon;
Turn that warped cfb around and fire it again.  It will go straight.

R Parian wrote:Thank you for such a detailed account with quality pictures!  Where did you purchase the CFB and the CF riser materials?  I'm trying to find alternate sources for my own custom build.

Zartech in Tualatin OR. The CFB price was okay compared to options that ship. The CF riser price was ridiculous, $225 for each of those two pieces.  So you can bet I'm going to use them until they crumble into dust.

thomas rubino wrote:Solomon;
Turn that warped cfb around and fire it again.  It will go straight.

Thanks. I'll try it. I'm definitely going to be building this RMH with rebuilding in mind. I never planned on this being the final design.

I wonder if you were able to overfuel it by scaling it up. Also, I wonder if having a cold outside air temperature on either end of an open system like that would "supercharge" the system, and increase the temperature to an unsafe degree? I bet that the same core installed into a full system, in a house that is above outside temperature, with a large thermal mass on the back end would behave differently. Pretty cool post, nice work! I'm surprised you didn't burn your eyebrows off when taking the photo down the riser..

That riser looks super cool, did you see any steam rising as that can cause hairline fractures.

Ryan Adobe wrote:I wonder if you were able to overfuel it by scaling it up.

Yeah, the dragon heater core claims to need two barrels to handle the radiant heat. People have reported double the output from a normal 8" compared to a normal 6". Fortunately, when I build this in, I have already collected two barrels to house it.

Ryan Adobe wrote:Also, I wonder if having a cold outside air temperature on either end of an open system like that would "supercharge" the system, and increase the temperature to an unsafe degree? I bet that the same core installed into a full system, in a house that is above outside temperature, with a large thermal mass on the back end would behave differently. Pretty cool post, nice work!

This is my idea also. Stick a barrel on top of a riser and your draw reduces drastically. Add a chimney and the chimney probably has more control than anything.

Ryan Adobe wrote:I'm surprised you didn't burn your eyebrows off when taking the photo down the riser..

I was standing on top of an 8 foot ladder and there was a slight breeze. When the breeze stopped, I couldn't do it. There was an incredibly hot plume of gases coming out the top. I don't know if I've ever seen heat shimmers like that. So hot.

I've never seen higher numbers posted by anyone, so I'm claiming the record.

Scots John wrote:That riser looks super cool, did you see any steam rising as that can cause hairline fractures.

No. This was stored inside since the point I took possession. It was pre cured and was perfectly dry. And if it wasn't it certainly was after two hours at 2000+ degrees. The exterior clocked at 250 degrees, well above boiling.

I believe the cracking was due to thermal shock due to the difference between the interior and exterior surface temperatures. I don't think this will happen inside a bell or barrel as the inside and outside will be more similar and cooling will happen much more slowly.

I am definitely not an expert in rocket mass heaters but with your entire set up exposed to air I believe you did not have an adequate heat sink around your material.  Perhaps, if it was completely surrounded by the cob media the heat would have been absorbed into the cob and your material might have faired better, or survived unscathed.  

Michael Fundaro wrote:I am definitely not an expert in rocket mass heaters but with your entire set up exposed to air I believe you did not have an adequate heat sink around your material.  Perhaps, if it was completely surrounded by the cob media the heat would have been absorbed into the cob and your material might have faired better, or survived unscathed.  

Yes, this is quite possible, and I have thought about that. Having a full feed tube would reduce the draft a little also, and the barrel over the top of the riser would definitely reduce the draft, and also, I don't need four feet of riser either. That would reduce the draft.  Definitely some areas that can be experimented with. And I have enough board to replace the burnt parts so I can certainly try again. And if the risers completely fail, I can just do CFB riser as well.

I contacted Zartech but haven't heard back yet.  How much was the CFB and was it 36"x48"?  Do they have CFB that has a higher temp rating than the 2300 F board?

Used to be responsible for the mechanical integrity of some big industrial furnaces...


Not saying it's what happened here, but drying refractory materials is more than just getting above the boiling point of water. As the materials used to form the refractory heat up, they actually release moisture at temperatures well above the boiling point of water. Gibbsite to boehmite at about 300 F, boehmite to alumina at about 500 F (don't quote me on those compounds or temperatures, it's been a while since I cared enough to look them up, and I'm sure there are other dehydration reactions in other refractory materials). As a consequence, the first time you fire up a furnace with fresh refractory in it, you heat it relatively slowly (depending on the thickness, I've had dryouts as slow as 25 F/hr for a dual-density transfer line in a steam methane reformer), and hold for extended periods of time (like 12 hours in some cases) at those dehydration temperatures so you don't generate a bunch of steam and spall everything off all at once. Once all the water has been generated and driven off, you bring it up to full operating temperature. It's not nearly as big a deal after the first firing, or if the refractory was fired prior to installation, but for that first burn, it can be a big deal.  

R Parian wrote:I contacted Zartech but haven't heard back yet.  How much was the CFB and was it 36"x48"?  Do they have CFB that has a higher temp rating than the 2300 F board?

Yes, standard pallet size, 36x48. It was about $94 per sheet for 1" thick, uncured. $154 for 2".

There exists 2600 and 3000 CFB, but I didn't ask at the time. According to their website, they do supply it.

Cera Materials has the 2600 for about $105 a sheet. They have the 3000 for $310 a sheet. But that's 4 sheets, FOB PA and you have to ship freight. They only ship the 2300 boards UPS, and they're smaller, 2x3.

Nick Williams wrote:Used to be responsible for the mechanical integrity of some big industrial furnaces...


Not saying it's what happened here, but drying refractory materials is more than just getting above the boiling point of water. As the materials used to form the refractory heat up, they actually release moisture at temperatures well above the boiling point of water. Gibbsite to boehmite at about 300 F, boehmite to alumina at about 500 F (don't quote me on those compounds or temperatures, it's been a while since I cared enough to look them up, and I'm sure there are other dehydration reactions in other refractory materials). As a consequence, the first time you fire up a furnace with fresh refractory in it, you heat it relatively slowly (depending on the thickness, I've had dryouts as slow as 25 F/hr for a dual-density transfer line in a steam methane reformer), and hold for extended periods of time (like 12 hours in some cases) at those dehydration temperatures so you don't generate a bunch of steam and spall everything off all at once. Once all the water has been generated and driven off, you bring it up to full operating temperature. It's not nearly as big a deal after the first firing, or if the refractory was fired prior to installation, but for that first burn, it can be a big deal.  

As I mentioned, the risers were pre-baked. The boards were not. It was the risers that had cracking.