Why FireBricks / Refractories Fail: Silica+Flux, Not Heat
Mar 23, 2022 12:01:48 GMT -8
Dan (Upstate NY, USA), hallinen, and 5 more like this
Post by Forsythe on Mar 23, 2022 12:01:48 GMT -8
An alt title for this thread could be:
I've seen a handful of posts recently (both here and on other forums) about firebrick or castable refractory that crumbled apart at what should have been otherwise-tolerable temperatures, with a recurring assumption stated along the lines of "the temperature must have gotten too high." ...which... isn't really an accurate assessment of the problem-at-hand. From what I've seen, proceding from that incorrect assumption will likely lead to further disappointment; repeated refractory failures, requiring reconstruction of entire stoves, prematurely — that is, if the person hasn't given up on the idea of using a masonry heater or rocket-mass stove due to the disappointingly early failure of their first build's materials.
I hope to shed some light on what's happening, and offer some ideas for what can be done to mitigate this thermochemical decomposition caused by the slagging, fluxing effects of ash (and sometimes iron) on aluminosilicate-based refractories used in our stoves.
As such, it is meant to help with firebrick / refractory selection and, to some small degree, system design to help make your stove last on the order of decades or generations, rather than months or years, in a structurally sound and reliably operable condition.
A disclaimer: At present, my knowledge of the ceramic material science is greater than my knowledge of the finer points of natural-draft combustion design, so I hope the suggestions at the end will be seen as a jumping-off point to make your own builds more successful and more durable. I defer to the expertise and working knowledge of the more experienced stovebuilders here like Peter (Peterberg) and Trev (Vortex) —among many other senior forum members— in stove design from a clean, efficient, wood combustion standpoint.
We'll start with the [simplified] ceramic thermochemistry of the problem-at-hand, some recommendations for potential solutions, and then I'll follow-up with citations to the ceramic / refractory science on what's happening when these firebricks and/or refractory components erode, clinker, crack and crumble, melt, or disintegrate to dust. I also intend to start a running list of known/available refractory coatings later-on.
This topic is closely related to what was discussed in the Disintegration of Superwool Ceramic Fibre Board thread (linked here,) but the carcinogenic risk of high-aspect-ratio fiber particles (and cristobalite conversion of ceramic fibers in particular) are much more of a human health hazard, [discussed in detail in that thread] — whereas this topic is intended to explain the mechanisms by which even the non-fibrous brick and castable types of refractory materials can be subject to deterioration and structural failure. It is apart from the toxicological risk context of inhaling disintegrated fibers, which doesn't really apply here to the larger and heavier, less-airborne, low-aspect-ratio particles of eroded dense refractories.
Let's dive in:
The vast majority of refractory materials we will encounter for use in our stoves will be made of aluminosilicate, which at its most fundamental is a mixture of Alumina (AKA "aluminum oxide," "Al2O3," or "α-alumina") + Silica (AKA "silicon oxide," "SiO2," or "quartz".) [There are other types of boutique refractories like dead-burned Magnesia, Chrome-Magnesia, Carbide-composites, Zirconia, and pure Silica firebrick, (among others) which are tailored to application-specific industrial processes, but those types are not exactly consumer-accessible, nor would most of them be very useful even if they were affordable for our application — of intermittent woodburning as we're doing. So, this will focus on aluminosilicate refractories only.]
• Alumina by itself is a remarkably inert "amphoteric" oxide, and thus is fairly resistant to the fluxing effects of most metallic slags. Silica is not. Silica is an acidic mineral, and is very reactive with alkaline metallic oxides, even under only low-to-moderate heat.
• The higher the Alumina content, the more resistant to fluxing slags the remaining Silica in the aluminosilicate will be. Plus, the higher the Alumina content of a refractory shape, the better it will be at keeping any slagging, fluxing effects to its surface as superficial glazing or clinkering, rather than wicking those fluxes deeper into the refractory where there's more free, reactive Silica... ultimately causing internal damage of the refractory unit and potential structural failure of the stove.
...Why not just use pure Alumina with no Silica?
• Alumina and Silica are the oxides of Aluminum and Silicon, respectively, both of which are metallic elements, themselves. Pure Alumina alone and pure Silica alone each have melting points far higher than would be useful for forming into structural shapes. Added together, aluminosilicates have softening points lower than the melting points of either Alumina (2072°C) or Silica (1710°C). In other words, Silica fluxes Alumina into softening at lower temperatures, which is useful for ceramic-, brick-, and refractory-making. In short, we [usually] need at least a tiny amount of Silica to make Alumina stick together in moldable, vitrifiable ceramics. The silica "binder," as it were, can also help the alumina avoid thermal shock cracking by buffering between grain boundaries [but only if-and-when the silica is used in low enough proportions].
• Silica is one of the few metallic oxides that can readily flux otherwise pure Alumina, but its presence makes the whole aluminosilicate mix more susceptible to further fluxing by the introduction of other oxide "slags"... So the inverse of the 2nd bullet point is also true: The higher the amount of Silica in relation to Alumina, the more susceptible the whole aluminosilicate mixture is to the fluxing slag damage of woodash.
• Fluxing Slags are the oxides of yet other metals, which decrease the thermo-mechanical strength of Aluminosilicates by lowering the softening point even further than Silica does alone. These include metal oxides such as Potassium, Sodium, Magnesium, Calcium, Iron, Titanium, Zinc, Chromium, Boron, Lithium, etc.
• Woodash is comprised almost entirely of alkaline metallic-oxides (slags) in finely-divided powdered form. Finely-divided powders are exceptionally reactive because they have a massive surface-area-to-volume ratio, with every particle surface acting as a potential reaction point of contact. They can also rapidly diffuse into the pores of refractories — and into crevices between brickwork— and they can absorb and retain further oxidative moisture (condensation) more readily than solid chunks of "clinker" slag can.
• The largest component of most woodash (~15-30%) is usually Calcium oxides, followed by Potassium (~10-25%) and Sodium oxides (% varies widely). [There is a lot of variation in woodash composition, though, which can make its fluxing effects unpredictable, depending on what species of wood you burn, and the soil composition where each individual tree happened to grow.]
• Potassium is normally the worst culprit in destroying aluminosilicate refractories used in biomass burning devices. Even though Potassium fully liquifies at a higher temperature than Sodium, Potassium begins to soften and "wet" the aluminosilicate at a lower temperature (around 600ºC) making it adhere with a problematic "stickiness" to the brick- or refractory-face earlier and begin deteriorating it sooner.
• Next to Potassium, the ash's Sodium content is the second biggest culprit in degrading aluminosilicate refractories used in the burning of woody biomass.
• Calcium is usually not highly reactive with aluminosilicate... unless there is also Iron present. When combined with finely-powdered Iron oxides, (like could potentially result from the burned-away steel from things such as fire grates, or preheating secondary air tubes,) the Iron fluxes the Calcium, which can *then* flux the Silica at drastically lower temperatures— and can absorb very deeply into the refractory quite easily. [This coupled-flux effect happening between Iron+Calcium to Silica is analogous to what happens between Potassium+Silica to Alumina, and between Sodium+Silica to Alumina.]
• Once the refractory face has been wetted by —and has absorbed— the aforementioned fluxing slags, the softening-point of its surface is reduced (IE: it is no longer capable of reaching the "temperature rating" on the label without softening-expansion upon heating.) The softened, expanded surface thus causes more and more fluxing slag to adhere and absorb deeper with each subsequent exposure to ash and heat [as in: every additional firing of the stove.] ...Then...The more and deeper the slag penetrates into and fluxes the refractory, the more the softening-expansion occurs upon heating, which ultimately causes uneven shrink-cracking upon cooling.[(‡)] This is why the brickwork or refractory surface can appear unharmed for a period of time in the early stages, but then shows signs of wear increasingly rapidly as the fluxing effects accelerate exponentially ...or can even appear to suddenly crumble with little warning, and at temperatures well below the original rated threshold. That's why the Alumina content of your refractory matters more than the temperature rating on the label. High silica/low alumina content, (combined with the fluxing slags of ash created during woodburning,) actively lower the thermal tolerance of the refractory — which nullifies the original temperature rating. Another way to think of it: The temperature rating is valid only when the refractory is heated indirectly, while dry, and without ever being heated in direct exposure to ash.
[(‡) In the pottery, tile, and other non-refractory ceramics world —where mineral forms of potassium, sodium, calcium, magnesium, etc. are intensionally added to aluminosilicate clays and glazes as the method for lowering their softening and thus vitrification temperatures— this heating-softening-expansion and cooling-shrink-cracking of fluxed silicates are referred to as "quartz-cristoballite inversion." ...And the only way for potters to avoid cracking or shattering their aluminosilicate wares is by **very slowly** heating-up and cooling-back-down through the temperature range spanning 210°C - 600°C where quartz-cristobalite inversions occur.
...However, in the stove world, this slow-heating and slow-cooling would make for spectacularly dirty, incomplete woodburning cycles. It would be not only antithetical to efficient combustion, it would be virtually impossible to achieve, given the way rocketstoves and mass heaters inherently function by design. That is: to fire intermittently, heat-up rapidly, combust completely, and burn down rapidly — precluding smoldering, smoky, low-temperature fires.
Thus, our stoves' refractory ceramics need to be able to resist flux penetration, because it is only when they avoid the fluxing action of slags in woodash and iron that they can maintain their ongoing ability to withstand the thermal shock of rapid-cycling through that 210-600ºC temperature range and beyond.]
Goals:
A-> Decrease the amount of free Silica available with which the woodash (and potentially iron) can react in your firebox/core and riser/secondary-burn-chambers' refractory surfaces
B-> Reduce surface porosity in the firebox and riser/afterburner/secondary-burn-chamber to prevent ash from accumulating in and on them
C-> Prevent eroded Iron particles from co-mingling with the ash in- and on- the refractory surfaces
Potential Methods Toward Those Ends:
1) The best way to reduce the ratio of exposed, free Silica —with which the ash [and potentially iron] can react, flux, and erode— is by starting with an increased alumina content. The more Al2O3 in the mix, the more resistant to fluxing slags the refractory will be. Therefore: use the highest alumina-content refractory material you can afford in the firebox and flame path (AKA the "rocketove core"). 60% Al2O3 is probably the *minimum* needed for the hottest zones exposed to the most ash, like the port, firebox walls, and transitional area to the secondary burn chamber. [That is: when we're dealing with rocketstove temperatures, which are higher than bread ovens and other stoves / fireplaces.] 70-80% Alumina is what I would recommend if you can find and afford it. 90-95% Alumina is also available, which typically contains fused "tabular" alumina (as corundum aggregate instead of grog) and is probably even better for the core's longevity, but as the alumina content rises, so does the price.
2) Use a high-density, [low-porosity] high-alumina castable-refractory at least for the "hot-face" lining of the firebox. Porous bricks and refractories (while they do insulate and help combustion efficiency) will accumulate more ash more quickly than dense and/or smooth-surfaced ones. Goal B is to prevent the ash from accumulating in porous refractories or on rough-surfaces — and thus prevent that ash from sticking to-, fusing with-, melting and absorbing into- firebox materials...because the places where flame meets ash in contact with silica will begin to liquefy on a molecular level (or "wet") at rocketstove temps, and that's when the trouble starts. So, denying the ash these entry points and places to coalesce in the hottest areas will go a long way in preserving the refractory's integrity.
...or...
3.1) If using porous, insulating firebrick or ceramic fiberboard for the structural body of the firebox / core, [including the secondary burn chamber / afterburner / riser] : use a slag-resistant high-alumina-content refractory coating to seal and smooth the surface, which can be laid on in a relatively thin layer. (Many such coatings have high infrared reflectance values, which will actually *improve*, rather than detract-from the insulative performance of those materials in those areas. This coating could be considered analogous to the "parge" coating on the brickwork in a traditional fireplace, and the protective refractory hard-coat used in small home forges and foundries, which will conveniently also serve in protecting from abrasive mechanical wear. In a rocketstove, such mechanical wear is the result of things like wood loading, ash removal tools scraping against the inside of the firebox, gas and flame impingement erosion, etc.) [Sidenote: later on in this thread, I'll start a list of refractory coatings that we can hopefully all add to and keep updated with product specs. There are a whole lot of options available, and I'm certainly not aware of all of them. Additionally, this refractory coating option is probably the least-invasive way of improving stove longevity without disrupting the functionality of existing builds and designs]
(Manufactured in USA, available on Amazon, and carried by many pottery- and metalworking- supply stores [places you'd go to buy blacksmithing or forging/smelting/welding gear.]) www.itccoatings.com/products-1
"Zircon" from Vitcas
(Manufactured in the EU, available online & in EU pottery- and metalwortking- supply stores. shop.vitcas.com/vitcas-zircon-paint-coating.html
"HeatGaurd Refractory Coating" from Simond Refractories
(Manufactured in India, available on Amazon & online store simondstore.com/heat-guard-refractory-coating-3270f-5-lbs.html)
"Furnascote" from Consolidated Refractories
(Manufactured in Australia, available in AU and NZ) consolidatedrefractories.com.au/products/coatings/
"RTZ Washcoat from Matthews Refractories
(Also manufactured in Australia) mathews.com.au/company-profile/products/rtz-washcoat/
"Zircoat" from Jyoti Refractories
(Manufactured in India, appears to be available worldwide via their webstore www.jyoticeramic.com/zircoat.php
"Zircar Zirconia Coating Type ZC-2" From Zircar Refractories
(available online, apparently globally) www.zircarceramics.com/product/zc-2/
"Hi-Purity Zirconia Coating EQ-634-ZO-LD" from MTI Corporation
(available online globally with various distributers carrying it in stock) https:www.mtixtl.com/1800c3270fhi-purityzirconiacoatingquart.aspx
3.2) Highly-porous insulative firebrick or ceramic fiber is fine to use *behind* a hot face where it will not be exposed directly to ash and flame impingement. (And indeed, the more insulated your firebox and secondary burn chambers / risers / etc., the better they will perform in combustion efficiency, especially early-on in the burn cycle, (When much of the heat being produced is absorbing into the refractory materials, rather than contributing to secondary combustion.) [The quandary over insulative refractories in burn chambers is one of the "Triad of Compromises" that any refractory material must make between |1|high mechanical strength & homogenously dense| -vs- |2|highly insulative & porous voids| -vs- |3|thermal shock resistance & heterogenous grains and grain-boundaries|] ...(Just be sure you seal any fiber products to avoid the respiratory carcinogen risks if using ceramic fiber.)
...or...
4) Use a rebuildable firebox lining (often done with dense firebricks set on edge [AKA "shiners"] and layed dry) as the "hot-face," which can be replaced if/when the internal surface wears out. This design concept [originally put forth by Albie Barden of Maine Woodheat & MHA] is becoming common practice for many modern commercial masonry heaters in North America, as it allows for refurbishing the high-wear areas without compromising the whole heater's structural integrity, which would require a costly and time-intensive total tear-down and reconstruction of the entire build.
5) If laying your firebox core and secondary-burn-chamber/riser brick in clay mortar: use a high-alumina fireclay or kaolin with either high-alumina grog —or— fused pure-alumina sandblasting grit (AKA "Dead-burned Al2O3" AKA "corundum") instead of sharp sand (which is silica) for the aggregate. [Alumina sandblasting grit has the advantages over grog of (a) angular "sharp" edges which hold the clay in place better than rounded grog particles, and (b) the dead-burned corundum phase of alumina is highly inert to fluxing slags even at extremely high temperatures, well above what your stove could produce.] This should help ensure that the clay mortar doesn't become a weak point for the flux to penetrate the firebox walls... thus avoiding thermal expansion cracking between and/or through bricks, and allowing the core to be dismantled and rebuilt without breaking the brick units themselves.
6) Lower-alumina-content (and cheaper) firebrick and castable may be used downstream in the flue path as thermal mass for heat harvest and storage, (places where it is not exposed to bottom-ash in the firebox, nor to the combination of A) fly-ash with B) direct flame impingement, and C) the highest heat.)
...regarding preheated secondary air (goal C)...
7) Consider using high-temp "superalloys" for preheated secondary air tubes which can withstand the high temperatures they'll be exposed to without oxidative, carbogenic or nitrogenic embrittlement and spalling, or other erosion. Alloys like 330 Stainless Steel, Inconel 600, or Inconel 625 are usually expensive, but they may be options you can find on the scrap market. I've found several useful pieces on eBay over the years. For example: one seller I know works as a refurbisher of nuclear power plant heat exchanger coils — and he sells the cut-off scrap ends of (unused) inconel 600 tube for a reasonable price.
8) If using lower-grade, lower service-temperature steels for preheated secondary air: try to shield them with refractory from direct flame impingement, and —if possible with your design— only use them outside the firebox where they can gather heat conducted or radiated through firebox walls or roof. If placing them in high-heat zones, it's probably best to ensure that they can always have air flowing through them while the stove is operating (IE: self-regulating secondary air.) Closing-off a steel secondary air tube's inlet while the stove is running will prevent the metal from self-cooling via internal air flow, causing it to superheat, and accelerating its deterioration.
ITC 213 is one type of coating available for coating metals, (as you need a special formulation to bond to the metal) discussed HERE, and HERE
9) Dense carbide- / nitride ceramic tubes with good heat conducting properties do exist, can be found in small quantities on eBay, and may be options for preheated air tubes which will withstand direct flame contact. While they are extremely tough and abrasion resistant, they —like most dense ceramics and Tungsten composites— are usually brittle as a consequence, and are thus susceptible to impact, even if/when they are resistant to thermal shock. I have not personally tested this option, but it's on my bucketlist. The prices for these things are all over the map, but the nitride ceramics in particular are becoming cheaper and cheaper as manufacturing methods improve.
10) It may be possible to introduce preheated secondary air via the shape of the design, and/or by using nothing more than the existing refractory materials with holes strategically molded or drilled within/through them. (Trev and Peter have both made advances in this area with things like airframe doors and holes drilled through afterburner walls, etc.)
11) If you do choose to go the route of using steel for preheated secondary air tubes as consumable components intended to be worn out and periodically replaced: then high-purity alumina refractory in your firebox and secondary burn chambers will be all-the-more important, and will be better-able to withstand the Calcium+Iron slag fluxing effect. That...or you could design your entire core lining to be rebuildable / replaceable every few years, using the same general approach to stove maintenance as the steel air-tubes. (That's not my personal preference for my own, but hey, it's your heater and maybe you enjoy building things more than once. Perhaps you view it as a chance to play around with design variations in each rebuild, and that's cool, too. If it weren't for people who kept tinkering, we wouldn't have the improved rocketstove designs we currently do.)
• Corrosive Effect of Wood Ash Produced by Biomass Combustion on Refractory Materials in a Binary Al–Si System
www.ncbi.nlm.nih.gov/pmc/articles/PMC9416287/#!po=48.8506
• Refractory Materials for Biofuel Boilers
Original Citation: www.intechopen.com/chapters/52553
Archived: drive.google.com/file/d/19DlwfqFfdrlJE6BpLKEEjsSRh454MSqt/view?usp=sharing
• Refractory Materials for Biomass Combustion
aip.scitation.org/doi/pdf/10.1063/1.5132743
• How to Avoid Full Refractory System Failure in Wood Combustion
www.powermag.com/how-to-avoid-full-refractory-system-failure-in-wood-combustion/
• Degradation of Mullite Based Materials by Alkali Containing Slags - Jesper Stjernberg Ph.D. Thesis
www.diva-portal.org/smash/get/diva2:999761/FULLTEXT01.pdf
• Refractory Corrosion in Biomass Gasification - Markus Carlborg Ph.D. Dissertation
www.diva-portal.org/smash/get/diva2:1256564/FULLTEXT02.pdf
• Interaction of High Al2O3 Refractories with Alkaline Salts Containing Potassium and Sodium in Biomass and Waste Combustion
pubs.acs.org/doi/10.1021/acs.energyfuels.8b03136
• Analysis and Prediction of Corrosion of Refractory Materials by Potassium during Biomass Combustion-Thermodynamic Study
www.ncbi.nlm.nih.gov/pmc/articles/PMC6315771/pdf/materials-11-02584.pdf
• Biomass Refractories: One Size Does Not Fit All
biomassmagazine.com/articles/16689/biomass-refractories-one-size-does-not-fit-all
• Materials Issues In Biomass Gasification
www.osti.gov/servlets/purl/1407726
• Surface Modifications to Mitigate Refractory Degradation in High-Temperature Gasifiers
smartech.gatech.edu/bitstream/handle/1853/10488/pallay_krista_j_200605_mast.pdf
• CFB Refractory Improvements for Biomass Co-Firing
www.power-eng.com/renewables/cfb-refractory-improvements-for-biomass-co-firing/#gref
• High-Temperature Corrosion of Refractory Materials in Biomass and Waste Combustion: Method Development and Tests with Alumina Refractory Exposed to a K2CO3-KCl Mixture
Original Citation: www.researchgate.net/publication/318220982
Archived: drive.google.com/file/d/1L4jGkLF8ExCXuCjUC6gY7Ttg7Sp4nRcC/view?usp=sharing
• Thermo-Physical Properties of Insulating Refractory Materials - Diana Vitiello Ph.D. Thesis
tel.archives-ouvertes.fr/tel-03289147/document
• Mullite–zirconia composite for the bonding phase of refractory bricks in hazardous waste incineration rotary kiln
pdf.sciencedirectassets.com/271630/...
• Densification of zirconia-based anti-corrosion coatings for application in waste-to-energy plants
edoc.ub.uni-muenchen.de/20584/1/Mueller_Dirk.pdf
"Why the Alumina (Al2O3) Content of your Refractories and Firebricks Matters More than their Temperature Rating"
TLDR:
It's because we're burning wood and creating woodash
(And many of us are burning steel "consumable" parts, adding iron to that woodash)
Combinations of which are fluxing slags that lower the refractory's strength in relation to its Silica content
I've seen a handful of posts recently (both here and on other forums) about firebrick or castable refractory that crumbled apart at what should have been otherwise-tolerable temperatures, with a recurring assumption stated along the lines of "the temperature must have gotten too high." ...which... isn't really an accurate assessment of the problem-at-hand. From what I've seen, proceding from that incorrect assumption will likely lead to further disappointment; repeated refractory failures, requiring reconstruction of entire stoves, prematurely — that is, if the person hasn't given up on the idea of using a masonry heater or rocket-mass stove due to the disappointingly early failure of their first build's materials.
I hope to shed some light on what's happening, and offer some ideas for what can be done to mitigate this thermochemical decomposition caused by the slagging, fluxing effects of ash (and sometimes iron) on aluminosilicate-based refractories used in our stoves.
As such, it is meant to help with firebrick / refractory selection and, to some small degree, system design to help make your stove last on the order of decades or generations, rather than months or years, in a structurally sound and reliably operable condition.
A disclaimer: At present, my knowledge of the ceramic material science is greater than my knowledge of the finer points of natural-draft combustion design, so I hope the suggestions at the end will be seen as a jumping-off point to make your own builds more successful and more durable. I defer to the expertise and working knowledge of the more experienced stovebuilders here like Peter (Peterberg) and Trev (Vortex) —among many other senior forum members— in stove design from a clean, efficient, wood combustion standpoint.
We'll start with the [simplified] ceramic thermochemistry of the problem-at-hand, some recommendations for potential solutions, and then I'll follow-up with citations to the ceramic / refractory science on what's happening when these firebricks and/or refractory components erode, clinker, crack and crumble, melt, or disintegrate to dust. I also intend to start a running list of known/available refractory coatings later-on.
This topic is closely related to what was discussed in the Disintegration of Superwool Ceramic Fibre Board thread (linked here,) but the carcinogenic risk of high-aspect-ratio fiber particles (and cristobalite conversion of ceramic fibers in particular) are much more of a human health hazard, [discussed in detail in that thread] — whereas this topic is intended to explain the mechanisms by which even the non-fibrous brick and castable types of refractory materials can be subject to deterioration and structural failure. It is apart from the toxicological risk context of inhaling disintegrated fibers, which doesn't really apply here to the larger and heavier, less-airborne, low-aspect-ratio particles of eroded dense refractories.
Let's dive in:
The vast majority of refractory materials we will encounter for use in our stoves will be made of aluminosilicate, which at its most fundamental is a mixture of Alumina (AKA "aluminum oxide," "Al2O3," or "α-alumina") + Silica (AKA "silicon oxide," "SiO2," or "quartz".) [There are other types of boutique refractories like dead-burned Magnesia, Chrome-Magnesia, Carbide-composites, Zirconia, and pure Silica firebrick, (among others) which are tailored to application-specific industrial processes, but those types are not exactly consumer-accessible, nor would most of them be very useful even if they were affordable for our application — of intermittent woodburning as we're doing. So, this will focus on aluminosilicate refractories only.]
The Problem-at-Hand, Thermochemical Mechanisms & Conclusions:
• Alumina by itself is a remarkably inert "amphoteric" oxide, and thus is fairly resistant to the fluxing effects of most metallic slags. Silica is not. Silica is an acidic mineral, and is very reactive with alkaline metallic oxides, even under only low-to-moderate heat.
• The higher the Alumina content, the more resistant to fluxing slags the remaining Silica in the aluminosilicate will be. Plus, the higher the Alumina content of a refractory shape, the better it will be at keeping any slagging, fluxing effects to its surface as superficial glazing or clinkering, rather than wicking those fluxes deeper into the refractory where there's more free, reactive Silica... ultimately causing internal damage of the refractory unit and potential structural failure of the stove.
...Why not just use pure Alumina with no Silica?
• Alumina and Silica are the oxides of Aluminum and Silicon, respectively, both of which are metallic elements, themselves. Pure Alumina alone and pure Silica alone each have melting points far higher than would be useful for forming into structural shapes. Added together, aluminosilicates have softening points lower than the melting points of either Alumina (2072°C) or Silica (1710°C). In other words, Silica fluxes Alumina into softening at lower temperatures, which is useful for ceramic-, brick-, and refractory-making. In short, we [usually] need at least a tiny amount of Silica to make Alumina stick together in moldable, vitrifiable ceramics. The silica "binder," as it were, can also help the alumina avoid thermal shock cracking by buffering between grain boundaries [but only if-and-when the silica is used in low enough proportions].
• Silica is one of the few metallic oxides that can readily flux otherwise pure Alumina, but its presence makes the whole aluminosilicate mix more susceptible to further fluxing by the introduction of other oxide "slags"... So the inverse of the 2nd bullet point is also true: The higher the amount of Silica in relation to Alumina, the more susceptible the whole aluminosilicate mixture is to the fluxing slag damage of woodash.
• Fluxing Slags are the oxides of yet other metals, which decrease the thermo-mechanical strength of Aluminosilicates by lowering the softening point even further than Silica does alone. These include metal oxides such as Potassium, Sodium, Magnesium, Calcium, Iron, Titanium, Zinc, Chromium, Boron, Lithium, etc.
• Woodash is comprised almost entirely of alkaline metallic-oxides (slags) in finely-divided powdered form. Finely-divided powders are exceptionally reactive because they have a massive surface-area-to-volume ratio, with every particle surface acting as a potential reaction point of contact. They can also rapidly diffuse into the pores of refractories — and into crevices between brickwork— and they can absorb and retain further oxidative moisture (condensation) more readily than solid chunks of "clinker" slag can.
• The largest component of most woodash (~15-30%) is usually Calcium oxides, followed by Potassium (~10-25%) and Sodium oxides (% varies widely). [There is a lot of variation in woodash composition, though, which can make its fluxing effects unpredictable, depending on what species of wood you burn, and the soil composition where each individual tree happened to grow.]
• Potassium is normally the worst culprit in destroying aluminosilicate refractories used in biomass burning devices. Even though Potassium fully liquifies at a higher temperature than Sodium, Potassium begins to soften and "wet" the aluminosilicate at a lower temperature (around 600ºC) making it adhere with a problematic "stickiness" to the brick- or refractory-face earlier and begin deteriorating it sooner.
• Next to Potassium, the ash's Sodium content is the second biggest culprit in degrading aluminosilicate refractories used in the burning of woody biomass.
• Calcium is usually not highly reactive with aluminosilicate... unless there is also Iron present. When combined with finely-powdered Iron oxides, (like could potentially result from the burned-away steel from things such as fire grates, or preheating secondary air tubes,) the Iron fluxes the Calcium, which can *then* flux the Silica at drastically lower temperatures— and can absorb very deeply into the refractory quite easily. [This coupled-flux effect happening between Iron+Calcium to Silica is analogous to what happens between Potassium+Silica to Alumina, and between Sodium+Silica to Alumina.]
• Once the refractory face has been wetted by —and has absorbed— the aforementioned fluxing slags, the softening-point of its surface is reduced (IE: it is no longer capable of reaching the "temperature rating" on the label without softening-expansion upon heating.) The softened, expanded surface thus causes more and more fluxing slag to adhere and absorb deeper with each subsequent exposure to ash and heat [as in: every additional firing of the stove.] ...Then...The more and deeper the slag penetrates into and fluxes the refractory, the more the softening-expansion occurs upon heating, which ultimately causes uneven shrink-cracking upon cooling.[(‡)] This is why the brickwork or refractory surface can appear unharmed for a period of time in the early stages, but then shows signs of wear increasingly rapidly as the fluxing effects accelerate exponentially ...or can even appear to suddenly crumble with little warning, and at temperatures well below the original rated threshold. That's why the Alumina content of your refractory matters more than the temperature rating on the label. High silica/low alumina content, (combined with the fluxing slags of ash created during woodburning,) actively lower the thermal tolerance of the refractory — which nullifies the original temperature rating. Another way to think of it: The temperature rating is valid only when the refractory is heated indirectly, while dry, and without ever being heated in direct exposure to ash.
[(‡) In the pottery, tile, and other non-refractory ceramics world —where mineral forms of potassium, sodium, calcium, magnesium, etc. are intensionally added to aluminosilicate clays and glazes as the method for lowering their softening and thus vitrification temperatures— this heating-softening-expansion and cooling-shrink-cracking of fluxed silicates are referred to as "quartz-cristoballite inversion." ...And the only way for potters to avoid cracking or shattering their aluminosilicate wares is by **very slowly** heating-up and cooling-back-down through the temperature range spanning 210°C - 600°C where quartz-cristobalite inversions occur.
...However, in the stove world, this slow-heating and slow-cooling would make for spectacularly dirty, incomplete woodburning cycles. It would be not only antithetical to efficient combustion, it would be virtually impossible to achieve, given the way rocketstoves and mass heaters inherently function by design. That is: to fire intermittently, heat-up rapidly, combust completely, and burn down rapidly — precluding smoldering, smoky, low-temperature fires.
Thus, our stoves' refractory ceramics need to be able to resist flux penetration, because it is only when they avoid the fluxing action of slags in woodash and iron that they can maintain their ongoing ability to withstand the thermal shock of rapid-cycling through that 210-600ºC temperature range and beyond.]
Goals:
A-> Decrease the amount of free Silica available with which the woodash (and potentially iron) can react in your firebox/core and riser/secondary-burn-chambers' refractory surfaces
B-> Reduce surface porosity in the firebox and riser/afterburner/secondary-burn-chamber to prevent ash from accumulating in and on them
C-> Prevent eroded Iron particles from co-mingling with the ash in- and on- the refractory surfaces
Potential Methods Toward Those Ends:
1) The best way to reduce the ratio of exposed, free Silica —with which the ash [and potentially iron] can react, flux, and erode— is by starting with an increased alumina content. The more Al2O3 in the mix, the more resistant to fluxing slags the refractory will be. Therefore: use the highest alumina-content refractory material you can afford in the firebox and flame path (AKA the "rocketove core"). 60% Al2O3 is probably the *minimum* needed for the hottest zones exposed to the most ash, like the port, firebox walls, and transitional area to the secondary burn chamber. [That is: when we're dealing with rocketstove temperatures, which are higher than bread ovens and other stoves / fireplaces.] 70-80% Alumina is what I would recommend if you can find and afford it. 90-95% Alumina is also available, which typically contains fused "tabular" alumina (as corundum aggregate instead of grog) and is probably even better for the core's longevity, but as the alumina content rises, so does the price.
2) Use a high-density, [low-porosity] high-alumina castable-refractory at least for the "hot-face" lining of the firebox. Porous bricks and refractories (while they do insulate and help combustion efficiency) will accumulate more ash more quickly than dense and/or smooth-surfaced ones. Goal B is to prevent the ash from accumulating in porous refractories or on rough-surfaces — and thus prevent that ash from sticking to-, fusing with-, melting and absorbing into- firebox materials...because the places where flame meets ash in contact with silica will begin to liquefy on a molecular level (or "wet") at rocketstove temps, and that's when the trouble starts. So, denying the ash these entry points and places to coalesce in the hottest areas will go a long way in preserving the refractory's integrity.
...or...
3.1) If using porous, insulating firebrick or ceramic fiberboard for the structural body of the firebox / core, [including the secondary burn chamber / afterburner / riser] : use a slag-resistant high-alumina-content refractory coating to seal and smooth the surface, which can be laid on in a relatively thin layer. (Many such coatings have high infrared reflectance values, which will actually *improve*, rather than detract-from the insulative performance of those materials in those areas. This coating could be considered analogous to the "parge" coating on the brickwork in a traditional fireplace, and the protective refractory hard-coat used in small home forges and foundries, which will conveniently also serve in protecting from abrasive mechanical wear. In a rocketstove, such mechanical wear is the result of things like wood loading, ash removal tools scraping against the inside of the firebox, gas and flame impingement erosion, etc.) [Sidenote: later on in this thread, I'll start a list of refractory coatings that we can hopefully all add to and keep updated with product specs. There are a whole lot of options available, and I'm certainly not aware of all of them. Additionally, this refractory coating option is probably the least-invasive way of improving stove longevity without disrupting the functionality of existing builds and designs]
Brand names that I'm aware of so far:
"ITC-100HT" from International Technical Ceramics (Manufactured in USA, available on Amazon, and carried by many pottery- and metalworking- supply stores [places you'd go to buy blacksmithing or forging/smelting/welding gear.]) www.itccoatings.com/products-1
"Zircon" from Vitcas
(Manufactured in the EU, available online & in EU pottery- and metalwortking- supply stores. shop.vitcas.com/vitcas-zircon-paint-coating.html
"HeatGaurd Refractory Coating" from Simond Refractories
(Manufactured in India, available on Amazon & online store simondstore.com/heat-guard-refractory-coating-3270f-5-lbs.html)
"Furnascote" from Consolidated Refractories
(Manufactured in Australia, available in AU and NZ) consolidatedrefractories.com.au/products/coatings/
"RTZ Washcoat from Matthews Refractories
(Also manufactured in Australia) mathews.com.au/company-profile/products/rtz-washcoat/
"Zircoat" from Jyoti Refractories
(Manufactured in India, appears to be available worldwide via their webstore www.jyoticeramic.com/zircoat.php
"Zircar Zirconia Coating Type ZC-2" From Zircar Refractories
(available online, apparently globally) www.zircarceramics.com/product/zc-2/
"Hi-Purity Zirconia Coating EQ-634-ZO-LD" from MTI Corporation
(available online globally with various distributers carrying it in stock) https:www.mtixtl.com/1800c3270fhi-purityzirconiacoatingquart.aspx
3.2) Highly-porous insulative firebrick or ceramic fiber is fine to use *behind* a hot face where it will not be exposed directly to ash and flame impingement. (And indeed, the more insulated your firebox and secondary burn chambers / risers / etc., the better they will perform in combustion efficiency, especially early-on in the burn cycle, (When much of the heat being produced is absorbing into the refractory materials, rather than contributing to secondary combustion.) [The quandary over insulative refractories in burn chambers is one of the "Triad of Compromises" that any refractory material must make between |1|high mechanical strength & homogenously dense| -vs- |2|highly insulative & porous voids| -vs- |3|thermal shock resistance & heterogenous grains and grain-boundaries|] ...(Just be sure you seal any fiber products to avoid the respiratory carcinogen risks if using ceramic fiber.)
...or...
4) Use a rebuildable firebox lining (often done with dense firebricks set on edge [AKA "shiners"] and layed dry) as the "hot-face," which can be replaced if/when the internal surface wears out. This design concept [originally put forth by Albie Barden of Maine Woodheat & MHA] is becoming common practice for many modern commercial masonry heaters in North America, as it allows for refurbishing the high-wear areas without compromising the whole heater's structural integrity, which would require a costly and time-intensive total tear-down and reconstruction of the entire build.
5) If laying your firebox core and secondary-burn-chamber/riser brick in clay mortar: use a high-alumina fireclay or kaolin with either high-alumina grog —or— fused pure-alumina sandblasting grit (AKA "Dead-burned Al2O3" AKA "corundum") instead of sharp sand (which is silica) for the aggregate. [Alumina sandblasting grit has the advantages over grog of (a) angular "sharp" edges which hold the clay in place better than rounded grog particles, and (b) the dead-burned corundum phase of alumina is highly inert to fluxing slags even at extremely high temperatures, well above what your stove could produce.] This should help ensure that the clay mortar doesn't become a weak point for the flux to penetrate the firebox walls... thus avoiding thermal expansion cracking between and/or through bricks, and allowing the core to be dismantled and rebuilt without breaking the brick units themselves.
6) Lower-alumina-content (and cheaper) firebrick and castable may be used downstream in the flue path as thermal mass for heat harvest and storage, (places where it is not exposed to bottom-ash in the firebox, nor to the combination of A) fly-ash with B) direct flame impingement, and C) the highest heat.)
...regarding preheated secondary air (goal C)...
7) Consider using high-temp "superalloys" for preheated secondary air tubes which can withstand the high temperatures they'll be exposed to without oxidative, carbogenic or nitrogenic embrittlement and spalling, or other erosion. Alloys like 330 Stainless Steel, Inconel 600, or Inconel 625 are usually expensive, but they may be options you can find on the scrap market. I've found several useful pieces on eBay over the years. For example: one seller I know works as a refurbisher of nuclear power plant heat exchanger coils — and he sells the cut-off scrap ends of (unused) inconel 600 tube for a reasonable price.
8) If using lower-grade, lower service-temperature steels for preheated secondary air: try to shield them with refractory from direct flame impingement, and —if possible with your design— only use them outside the firebox where they can gather heat conducted or radiated through firebox walls or roof. If placing them in high-heat zones, it's probably best to ensure that they can always have air flowing through them while the stove is operating (IE: self-regulating secondary air.) Closing-off a steel secondary air tube's inlet while the stove is running will prevent the metal from self-cooling via internal air flow, causing it to superheat, and accelerating its deterioration.
ITC 213 is one type of coating available for coating metals, (as you need a special formulation to bond to the metal) discussed HERE, and HERE
9) Dense carbide- / nitride ceramic tubes with good heat conducting properties do exist, can be found in small quantities on eBay, and may be options for preheated air tubes which will withstand direct flame contact. While they are extremely tough and abrasion resistant, they —like most dense ceramics and Tungsten composites— are usually brittle as a consequence, and are thus susceptible to impact, even if/when they are resistant to thermal shock. I have not personally tested this option, but it's on my bucketlist. The prices for these things are all over the map, but the nitride ceramics in particular are becoming cheaper and cheaper as manufacturing methods improve.
10) It may be possible to introduce preheated secondary air via the shape of the design, and/or by using nothing more than the existing refractory materials with holes strategically molded or drilled within/through them. (Trev and Peter have both made advances in this area with things like airframe doors and holes drilled through afterburner walls, etc.)
11) If you do choose to go the route of using steel for preheated secondary air tubes as consumable components intended to be worn out and periodically replaced: then high-purity alumina refractory in your firebox and secondary burn chambers will be all-the-more important, and will be better-able to withstand the Calcium+Iron slag fluxing effect. That...or you could design your entire core lining to be rebuildable / replaceable every few years, using the same general approach to stove maintenance as the steel air-tubes. (That's not my personal preference for my own, but hey, it's your heater and maybe you enjoy building things more than once. Perhaps you view it as a chance to play around with design variations in each rebuild, and that's cool, too. If it weren't for people who kept tinkering, we wouldn't have the improved rocketstove designs we currently do.)
Source Info and Relevant Published Research Regarding Refractory Corrosion
Under Biomass Burning Conditions and Exposure to the Slagging, Aluminosilicate-Fluxing, Alkali Residues Present in Wood Ash
Under Biomass Burning Conditions and Exposure to the Slagging, Aluminosilicate-Fluxing, Alkali Residues Present in Wood Ash
• Corrosive Effect of Wood Ash Produced by Biomass Combustion on Refractory Materials in a Binary Al–Si System
www.ncbi.nlm.nih.gov/pmc/articles/PMC9416287/#!po=48.8506
• Refractory Materials for Biofuel Boilers
Original Citation: www.intechopen.com/chapters/52553
Archived: drive.google.com/file/d/19DlwfqFfdrlJE6BpLKEEjsSRh454MSqt/view?usp=sharing
• Refractory Materials for Biomass Combustion
aip.scitation.org/doi/pdf/10.1063/1.5132743
• How to Avoid Full Refractory System Failure in Wood Combustion
www.powermag.com/how-to-avoid-full-refractory-system-failure-in-wood-combustion/
• Degradation of Mullite Based Materials by Alkali Containing Slags - Jesper Stjernberg Ph.D. Thesis
www.diva-portal.org/smash/get/diva2:999761/FULLTEXT01.pdf
• Refractory Corrosion in Biomass Gasification - Markus Carlborg Ph.D. Dissertation
www.diva-portal.org/smash/get/diva2:1256564/FULLTEXT02.pdf
• Interaction of High Al2O3 Refractories with Alkaline Salts Containing Potassium and Sodium in Biomass and Waste Combustion
pubs.acs.org/doi/10.1021/acs.energyfuels.8b03136
• Analysis and Prediction of Corrosion of Refractory Materials by Potassium during Biomass Combustion-Thermodynamic Study
www.ncbi.nlm.nih.gov/pmc/articles/PMC6315771/pdf/materials-11-02584.pdf
• Biomass Refractories: One Size Does Not Fit All
biomassmagazine.com/articles/16689/biomass-refractories-one-size-does-not-fit-all
• Materials Issues In Biomass Gasification
www.osti.gov/servlets/purl/1407726
• Surface Modifications to Mitigate Refractory Degradation in High-Temperature Gasifiers
smartech.gatech.edu/bitstream/handle/1853/10488/pallay_krista_j_200605_mast.pdf
• CFB Refractory Improvements for Biomass Co-Firing
www.power-eng.com/renewables/cfb-refractory-improvements-for-biomass-co-firing/#gref
• High-Temperature Corrosion of Refractory Materials in Biomass and Waste Combustion: Method Development and Tests with Alumina Refractory Exposed to a K2CO3-KCl Mixture
Original Citation: www.researchgate.net/publication/318220982
Archived: drive.google.com/file/d/1L4jGkLF8ExCXuCjUC6gY7Ttg7Sp4nRcC/view?usp=sharing
• Thermo-Physical Properties of Insulating Refractory Materials - Diana Vitiello Ph.D. Thesis
tel.archives-ouvertes.fr/tel-03289147/document
• Mullite–zirconia composite for the bonding phase of refractory bricks in hazardous waste incineration rotary kiln
pdf.sciencedirectassets.com/271630/...
• Densification of zirconia-based anti-corrosion coatings for application in waste-to-energy plants
edoc.ub.uni-muenchen.de/20584/1/Mueller_Dirk.pdf
