Fire Science Show

187 - Smouldering of preserved timber with Wenxuan Wu

• Wojciech Węgrzyński

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Can a tiny amount of bio-protective coating completely change the fire behaviour of mass timber logs? If you asked me that some time ago, I would say it would probably be neutral.

Can a 0.5 x 0.5 m free-standing log of timber smoulder through without any external exposure to fire? If you asked me that some time ago, I would say no, and base that on observations of dozens of logs like this.

Yet, in Australia they’ve burned. And the hypothesis was that it has something to do with the preservative treatment.

My guest today, Wenxuan Wu from The University of Queensland was tasked with researching this problem and showed us the workshop used for this. In this podcast episode, we dive deep into measurements carried out in fire laboratories on the material scale and how use of different methods gradually increases our understanding of the problem. Their pursuit also opened new questsions - for example, why if timber would be subject to a very strong heat flux, the smouldering would not work anymore? This was also answered in the discussion.

If you would like to learn more about Wenxuan’s research, please refer to the published papers:

Also, this talk summarises the best poster at ITB-FRISSBE Summer School. What an event that was. Once again, we would like to thank our sponsors for that!

https://www.itb.pl/2024-summer-school/

Thank you to the SFPE for recognizing me with the 2025 SFPE Fire Safety Engineering Award! Huge thanks to YOU for being a part of this, and big thanks to the OFR for supporting me over the years.

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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.

Wojciech:

Hello everybody, welcome to the Fire Science Show. Today I'm taking you on a journey on how fire science is made, or perhaps rather how fire science can be used to solve a quite practical problem that one faces. Wenxuan Wu has submitted his PhD and he's on the final stretch, and what you're going to hear about the episode it's his PhD journey. But as well, he was charged with quite an interesting problem In Australia after bushfires. They were using quite a lot of utility poles you know, electricity, telephone but utility poles that are necessary to carry on cables across vast distances and when you lose them it's quite annoying to replace them. Utilize the walls that are necessary to carry on cables across vast distances, and when you lose them it's quite annoying to replace them, and you would just expect to put a pole and let it live for 70 years until you go back to it. And then they are faced with the problem that they're losing tens of thousands of them, and the way, how they're losing them, is not very well understood. So the team of Queensland University is being charged with investigating and oh boy, they did investigate. In this podcast episode you'll find out how preservative treatment of timber may change the swoldering behavior of solid timber material, which is very interesting on itself, perhaps even scalable to some other problems in fire safety engineering. But, more importantly, you'll learn about the workshop. You'll see the kitchen of how fire science is made. You'll learn a lot about interesting methods of measurements, like TGA for example, and you learn how they can be applied, how the scientists reads what they see in those measurements, how they plan the next step of their endeavor in order to unravel the truth about the problem they're solving. I found this very compelling and interesting and I hope this will be the same for you.

Wojciech:

A word of comment Wenxuan ended ended up in this spot. So half a year ago we have hosted a summer school together with Frisbee. It was ITB Frisbee summer school and we had a lot of feel like 40 people from across the world when she included and people came to this summer school with some ideas, with some work in progress, and they were showing them as posters. Those posters were evaluated by all the lecturers in the summer school and when she won the won the vote of the lecturers and I told him like this poster is so good, I'm to invite you to the podcast. And here he is. There was another honor I mentioned, matt Bowler won the popularity award on the facade stuff and I'm very sure Matt will come back to the podcast as well, but it was such a joy to see those young scholars present grow and be so proud about their research and there's a lot to be proud about, so I'm also happy to share this podcast episode for this reason with you.

Wojciech:

Anyway, enough talking. There's a lot of interesting fire science behind the intro, so let's spin it up and let's go. Welcome to the fire science show. My name is Wojciech Wigrzyński and I will be your host. This podcast is brought to you in collaboration with OFR Consultants, a multi-award-winning independent consultancy dedicated to addressing fire safety challenges. Ofr is the UK's leading fire risk consultancy. Its globally established team has developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and planets. Ofr is constantly growing and involved in fire safety engineering of the most interesting developments in the UK and also worldwide. In 2025, ofr will grow its team again and is keen to hear from industry professionals who want to collaborate on fire safety features this year. Get in touch at ofrconsultantscom. Hello, I'm joined today by Wangshan Wu from Queensland University. Hello, wangshan, hello Wojciech. By Wenxian Wu from Queensland University. Hello Wenxian.

Wenxuan Wu:

Hello Wojciech, Thanks for having me today.

Wojciech:

As promised, you've been a participant to our summer school Last year. You have won the best poster award and I said I'm going to interview the best poster. So here you are in the Fireside Show. I would have invited you anyway, but congratulations on that award. I hope you have some good memories from warsaw and our summer school yeah, for sure that will be the best.

Wenxuan Wu:

I wouldn't say a social event, but it's more like a conference experience for me would you?

Wojciech:

would you recommend to anyone who would love to to try that?

Wenxuan Wu:

um, yeah, actually I've briefly talked about my experience in itb summer school last year to all my colleagues and they would love to.

Wojciech:

They would love to participate if there's another one in the next year or in the following year yeah, I'll pass the feedback to Grundy Jumas and he'll be also happy, and we're so happy that our students are very pleased with the event. We've really poured everything we we had into organizing this. But anyway, you got there because you are a researcher, you are a PhD student, soon to be a doctor, and in our summer school we wanted to mix people who do a lot of consulting work or practical fice engineering and a lot of science research people at exactly your stage of career. You had to submit a topic that you work on currently when we also evaluated that and yours was very interesting and highly relevant. So perhaps, if you can give me a little bit of background on what exactly are you studying in UQ for your PhD?

Wenxuan Wu:

Yeah for sure. I enrolled my PhD in January 2021. So the PhD project itself was sponsored by a durability center and so it's an Australian government department working on the durability and wood protection. So my project was about the smoldering issues in preservative-treated timber. So what does preservative mean? So you know timber is a very popular construction material. It's used everywhere. But timber is a naturally organic material, so it is combustible. That's why many, many of our colleagues and researchers have been studying on that for many years. And it has another issue it's susceptible to biological decay.

Wenxuan Wu:

So if you use timber outside in the field, it might subject to the damage, the biodecay, the damage from the insects, from the ant, from the weathering attack, from whatever in the soil. So you need to do something to enhance the durability of the timber products. Let's say utility pole, fence posts or railway sleepers. Many of them were made by timber in Australia and globally. So one of the most effective way is to treat the timber products with preservative treatment, especially CCA. So CCA is a water-borne preservative treatment, chromated copper arsenate. So it contains two types of metallic components to enhance the durability of timber. Because you don't want to replace the timber products very often you expect the timber products to have a very long service life, for example 70 years. So you only look back at the timber infrastructure after 70 years. But there's a problem, so there's a problem.

Wenxuan Wu:

So I think this kind of treatment was established at least 40 or 50 years ago and it was actually quite popular over the world. It was actually quite popular over the world. Not only Australia but Europe, but Canada, us and Asian countries, africa, so everywhere used CCA at the very beginning. But at some point till 2000, like early 2000,. Then people realized oh, it's actually quite out-friendly to environment and to human beings Because Wayne Burns, he released gas-based products which is highly carcinogenic, actually quite unfriendly to the environment and to human beings, because when it burns it releases gas-rich products which is highly carcinogenic. So imagine your kid is playing in the background and they pick some just timber pieces and they burn it for barbecue and whatever. Then it's actually quite harmful. So due to the arsenic, the presence of arsenic, because arsenic is a bad guy. So till early 2000, most of the government banned the use of CCA as residential building. After that CCA can be only used for the outdoor or external infrastructure. Like I said, utility poles, fence poles, like those kinds of things utility poles, fence poles, like those kind of things. So it's more about the issue to the environment, to the human being, because it's highly carcinogenic.

Wenxuan Wu:

So I haven't moved to the smoldering part, because this is why we continue to use CCA in most of the countries in the world, like, of course, they have alternatives. So, for example, in Europe, in most of the European countries and US or Canada, they now use a tonnative preservation which is copper azole. Copper azole is still like copper with kind of organic fungicide or ACQ, so it's another one. So they use a tonnative, but they are more expensive, they are more corrosive and they are less durable. So CCA I think as of November 2023, cca still remains at almost 40% of the usage in Australia.

Wenxuan Wu:

And this is an issue because smoldering issues in this preservative-treated timber has widely reported since 1966. But no one has stressed this in detail Till the long-lasting bushfire in Australia from 2019 to 2020, if you remember that the bushfire has lasted for more than a year then we had more than 10,000 power poles, 10,000 utility poles have been destroyed. So, although the report hasn't identified the major issue is a flaming combustion from the white fire itself or the subsequent smoldering, but there are many news coverages or news reports and the field observations can confirm that it's not about the flaming combustion itself. It's about the subsequent smothering, but I haven't seen it myself right. So at the start, at the beginning of 2021, we cut a pole section from the real field, so we cut a, we cut a pole and we we put it back to the Was it disconnected from the network, or it just went completely rogue and just stole one that was operating.

Wenxuan Wu:

No, no, waste permission. Waste permission Because the project, as I mentioned, was sponsored by the Australian government. So waste permission. We somehow collected a pole section and put it back to our laboratory and burned it at a certain heat flux, which is 50. I know 50 is is pretty high heat flux, but we only burnt it for three minutes. So after three minutes we removed the heat source and the flame was out, just like any other timber, because everybody claimed that the char layer is gonna act as insulation to prevent the flame propagation, which which is true. So we didn't see the flame propagation, which is true. So we didn't see the flame anymore.

Wenxuan Wu:

But there's some subsequent smothering, which is also quite common, but it will come to an end from our expectation at that time. So we went back home. We went back home and we came the other day. So after 21 hours the core was still glowing and the major structure of the pole was destroyed. So the pole is like one meters high, half meters uh, half meter, half meter diameter, which is very heavy, so I can't carry it myself, but major structure was destroyed. So more than 30 percent of the mass has been consumed by the subsequent smoldering in the following 21 hours from the start point of ignition.

Wojciech:

And it was just a pole. That's how the project started. There was just a pole standing in a free space, no wall fluxes, no heat feedback from another fire around it, just a single log of timber that has been ignited for three minutes and built up its protective char layer, which is stronger than steel, which every architect knows today. Sorry, that was a bad joke, but it's interesting because normally you would not observe this type. Well, normally I'm not a smoldering and timber expert, but I've not seen a individual timber logs to to persist smoldering that long. It usually was in connection with some sort of architectural detail that would shield them or, you know, create those pockets of irradiating surfaces where the heat would have difficulty to escape In those conditions.

Wojciech:

Yes, we've seen smoldering persist and the story of a scientist coming back next day to the lab finding their sample completely burned down because of smoldering is a common one, I guess, in the timber research world. But you don't see that on like. If I tested the timber column and I vented the experiment, I would be pretty happy with leaving it alone. It shouldn't smolder through that column, right?

Wenxuan Wu:

Yeah, so you're exactly right. So just for like, a sole timber pole standing in the field without some more feedback from the surrounding, it's really hard to maintain the self-sustained smothering because we have to. So for sure, we've done the reference test with untreated timber pole. So exactly the same. Not exactly more or less the same dimension, because you can't step into the same river twice, so you can't find exactly the same timber in this world.

Wojciech:

So, anyway.

Wenxuan Wu:

So we've done the reference test. So no untreated. Untreated means like normal timber. Normal timber pole can never smother like that After the flame was out. That's the end of the story. Like that is very common because flames are generally considered as the major combustion in our fire safety engineering. So once the fire was out, if there's no subsequent smoldering, that means the end of the event.

Wojciech:

That's what you would expect and of course in some circumstances it could be different. But it's a heat balance equation. You need some energy to maintain smoldering. After all, in case of Australia and bushfires which you've now described, looks like something that's prone to ignition from a bushfire itself, because I imagine bushfire being a fairly rapid event, so the fire must go through fairly quickly. But if all you need is just a few minutes of exposure and bum, you get smoldering, and especially in an event of some firebrands and and uh and winds, probably that's even even extended. Indeed, every time a bushfire goes through an area you would lose the utility electricity poles.

Wojciech:

I can see, uh, where the um utility pun intended of your project is coming. So, uh, how did you tackle that? Because you are here left with the general idea that you have some practical problem in front of you. You have a good hypothesis that it's related to the treatment of the timber, but from that to a PhD, that's a pretty long way. Like it's not just you can publish an abstract. Oh, we believe it's the treatment that's doing some magical stuff to our timber. So so tell me the the rest of the story yeah, sure.

Wenxuan Wu:

So that was the beginning of the story because, like you said, it has some practical, practical implications in this case, because you don't expect the timber whatever in timber infrastructure can remain standing after a very severe bushfire, but if it's just firebrand or just a grass fire, like very, very mild fire source, can ignite the timber pole and and eventually it collapse. It's not ideal, so we have to address this. But because, like I said, no one has addressed this in detail. So we have to go back to the very fundamental question which, again, like you said, it's an energy balance. So if we believe the treatment does something to maintain the self-sustained smoldering, so in physics, it must satisfy the energy conservation which means the heat generated from the smoldering must be enough to enhance or to maintain the pyrolysis for the inner char layer.

Wenxuan Wu:

So because the smoldering is not like flaming combustion the flaming happens on the gas phase but the smoldering happens on the solid phase, or say heterogeneous solid gas, heterogeneous phase. So you have to have char first, you have to have the char formation first, then progress the smoldering. So, like I said, the energy generated from the oxidation must be enough to maintain the next step of pyrolysis, to form sufficient char to progress the self-sustained smoldering. So it's all about the energy balance and how the CCA preservation, the CCA preservative, can do that. It's just a very little amount Because, for your information, the normal CCA treatment is only 0.4% by weight.

Wojciech:

And you said it's water-based.

Wenxuan Wu:

Into the timber and it's water-based. So how is that small amount of chemical can do such a huge job to twist the whole energy conservation? So we need to look at the physics, we need to look at the chemistry at the very beginning. So after the demonstration from the pole testing, we actually started from the TGA thermogravimetric analysis, because the thermogravimetric analysis is a very commonly used technique in thermal study.

Wenxuan Wu:

Like many other materials, as long as it is combustible or organic, you can always see something from the thermogravimetric analysis. So the way it works is you put a very little amount of sample into a crucible and put a crucible into a burning furnace, so similar to you burn something in the big furnace for structural testing, but it's a small furnace and very little amount of sample, which is approximately like 10 milligrams. So let's say 10 milligrams of timber sawdust into a small furnace with a controlled environmentrolled environment refers to the gas environment, so you could just enter air like ambient air condition, or you could do a non-oxidative environment, say nitrogen or helium, if you only want to look at the pyrolysis behavior.

Wojciech:

So you don't burn the stuff that's produced from the pyrolysis?

Wenxuan Wu:

Yes, but the major difference from a micro-scale TGA test to a bench scale or even larger scale is it doesn't really flame inside the furnace because it's a controlled environment and you have purge gas all the time. All it can do within the furnace is to heat up the sample from the ambient temperature to a desired temperature. So let's say, if we heat up the timber up to 100 degrees or around 100 degrees, the moisture evaporation would occur.

Wojciech:

And how will you see that?

Wenxuan Wu:

You will see there is a mass loss. You will see a mass loss. So sorry, I forgot to mention that the very, very important quantification for the TGA testing is the mass, because it connects the sample you put in the crucible, connects to a scale. A very precise scale can measure the mass or mass loss over temperature so you can actually monitor the mass loss over a temperature change. So when it comes to around 100 degrees, you will see the mass loss which refers to the moisture evaporation. And when it comes to around 250 degrees and up to 300 degrees, you will see another huge mass loss which corresponds to pyrolysis.

Wojciech:

And if you have oxygen inside, will you have another one that corresponds to burning?

Wenxuan Wu:

Yes, if we have another one which corresponds to oxidative, let's say, because there are many ways to call it, but normally we just say oxidation, because it's not really burning inside the furnace. So, as I mentioned before, so flaming combustion occurs on the gas phase, but in the furnace environment it always has the purge gas to purge out whatever pyrolysis gas generated. And considering that small amount of mass of your sample, it's not likely to have a flaming combustion, but instead it becomes a very good representative of oxidation.

Wojciech:

I don't think we've talked to GA in the fire science show yet, so I'll perhaps rewrap that for the listeners. So you take a tiny sample of your material. You put it in a furnace in which you control the air that's in it. It can be nitrogen, so it's no oxygen in it. You can put normal air in it, you can put argon, helium, whatever you like. Then you start warming up the sample and that lasts for like 30 minutes hour.

Wenxuan Wu:

It's quite long right it really depends on the heating rate. So tga is like the techniques you can, you can just summarize in a few words, but it has many, many details.

Wojciech:

Yeah, of course, of course but I want I wanted to have listeners to have a really good general idea about the method. So you start increasing the temperature, it takes some minutes and and you see, okay, in 100 degrees I have this loss of mass. Then I increase from 100 to 150. Nothing happens, the mass remains the same. Then at 175 something starts to happen because I start to see mass loss. And then at 230 I have a maximum amount of mass lost ever. And then I increase the temperature to 300. Nothing happens again. And then at some temperature something happens again. Basically, like for every different material would give you a different curve of that mass loss to temperature. But that that's basically. Uh, how a scientist can tell what happens with the material at different temperatures, though it doesn't exactly. It doesn't tell you anything about the heat equation, right, because it's like infinite amount of heat for the material from the furnace, exactly.

Wenxuan Wu:

So the key information from here is you can see the peak temperature or the onset temperature of each reaction, because every reaction has an onset temperature. So whenever it starts to lose mass, that's an indicator of the beginning of the reaction. So for normal timber, just for reference for normal timber, so the drying is more or less the same, starting from 70 degrees. For the pyrolysis it starts from 250.

Wenxuan Wu:

For the cellulose and hemicellulose and for the oxidation it's normally around 450 degrees. So whatever heating rate you're using, it's like more or less about this range. But the problem is, once you put the cca you treated cca or eleazer preservative containing with copper into the timber, you don't see. You don't see any difference from the drying peak. You don't see any difference from the drying peak. You don't see any difference from the pyrolysis peak, but you see a huge shift from the oxidation which means the treatment into the timber changes, the onset temperature of the oxidation reaction.

Wenxuan Wu:

So in other words, your oxidation can occur at an early stage at a lower temperature, so you don't need to provide that much energy, but your oxidation could start.

Wojciech:

So also in the lay world, people love to measure fires with temperatures, which I always find ridiculous, because it's a heat transfer problem, not the temperature problem. Temperature is always at almost the same temperature. Anyway, when a reaction starts at a lower temperature, it would simply mean that you just need less heat to onset that reaction and, assuming that we're talking about oxidation, which is exothermic reaction, it produces its own heat. Once you have an onset of that reaction, you're pretty much done, because it eventually can become self-sustaining right At that point.

Wenxuan Wu:

At this point we couldn't really tell if the reaction can be self-sustained or not.

Wojciech:

You just see that it happens yeah.

Wenxuan Wu:

Yeah, tj is just simply a tool to tell you, like, the approximate temperature range for starting the reaction, but by doing different heating rates in the TGA. So that is another approach. So there's one thing we can calculate, which is activation energy. So by doing a series of TGA experiments you can actually calculate the apparent activation energy. Because one hypothesis we had is the CCA acts as a catalyst to promote the oxidation. So, like we have learned from the high school chemistry, the catalyst doesn't really involve into the reaction but as a promoter or accelerator to lower the activation energy for a certain reaction, or accelerator to lower the activation energy for a certain reaction or change the chemical pathway.

Wojciech:

Yeah, but at this point you don't know if it's copper, chromium or arsenic. That's right, but anyway, how did you figure out? It does accelerate the reaction from different temperatures.

Wenxuan Wu:

So again, to compare it with untreated, untreated like reference timber, untreated timber the activation energy for the oxidation reaction has markedly decreased. So for for the cca treated timber, it has a much, much lower activation energy. So now we can okay, so that's the very first step. Now we can confirm it's the catalytic effect from the cca. But you raised a very brilliant point. It is about copper or chromium or the arsenic.

Wenxuan Wu:

So we did, we did a lot, we did very intensive literature research and we realized for early organic, for many organic um polymers or organic material, the copper can act as catalyst for the oxidative thermal reaction, but the chromium has a stronger effect. The chromium has a much stronger effect and arsenic doesn't do much, but arsenic is very toxic. So now we need to think about something else. So, as I mentioned before, although some countries US, european countries, canada or some other Asian countries they replace CCA to alternative ACQ Canada or some other Asian countries they replace CCA to alternative ACQ CA, those preservation only containing copper instead of arsenic. So they are arsenic-free option, but they still contains copper. So it doesn't really resolve the smoldering issue, but we need to confirm that. So we have established a testing approach in bench scale apparatus to really quantify the smoldering rate of alien content material.

Wenxuan Wu:

It's beyond timber. It's beyond the CCA treated timber Because, as far as I know, there's no international recognized testing standard for smoldering, for the, let's say, propensity of smoldering or severity for smoldering, for the let's say propensity of smoldering or severity of smoldering. There is some european code for the building construction material to give you a very determined testic method to say yes or no to the material so if the material is going to smolder or not there is a method that you can use to assess how fast it propagates in the dust.

Wojciech:

So you have a sample of dust or solid material and you just observe how the front is moving through at different irradiance. I believe I remember someone tortured me with a method like this in my university times.

Wenxuan Wu:

That was such a long test. Yeah, I've seen that it's very similar to a facade test. They put a vertical sample in front of a radiant panel or a gas burner. They ignite the sample and remove the gas burner to see if the smoldering could propagate, identified by the temperature measurement of the thermocouple at different distances.

Wojciech:

Yeah, something like that yeah.

Wenxuan Wu:

Yeah. So we've developed something, I would say, in a very similar philosophy to simulate a passing bushfire scenario. Because this is just the very, very beginning. This is just the start of my PhD and we want to identify the question, we want to figure out why, but eventually the next step is to establish countermeasures.

Wenxuan Wu:

So no matter what you can do to the existing CCA or to replace the CCA to something else, you need a testing method to prove okay, now everything works.

Wenxuan Wu:

Or you want to see which type of preservation has the least effect on self-sustaining smoldering. So if we have established a testing approach I think back to 2022 that was my first paper for the ifss in japan, I think that that's where we met in person so we've established a modified con colorimetry test. So we burn the sample at a certain heat flux under the conic heater to maintain a certain period of flaming, depending on the fire intensity desired, then remove the sample to a separate load cell. So, after a certain degree of damage by the flaming combustion and remove it, transfer to a separate load scale at ambient temperature to measure the mass loss. So this is a very simple philosophy, just to see. So if there's nothing happened, then the sample is going to extinguish in both flaming and smothering. If the self-sustained smothering initiated after the flaming stage, you would see the mass loss and you can record the mass loss and simply do a first derivative to get the mass loss rate. Then you can compare which product has a faster smoldering rate.

Wojciech:

That's very smart and that's very practical actually, yeah this is very practical.

Wojciech:

You could use that for many construction products actually as an approach to test their how would you call it propensity to smoldering I don't know sensitivity to smoldering or ability to smolder at their own, pretty much. And perhaps if you had a little radiators over your scale you could test that at different heat fluxes. And I'm talking about the low values like 1 kilowattatt, two kilowatt square meter, three kilowatt square meter, to see like I'm I'm. You know I'm immediately turning my my mind into mass timber and fires. You know work of harry mitchell, imperial code reds and stuff where they had those pockets of smoldering in in the big clt compartment.

Wojciech:

We also, when we, when we were doing with Danny Hopkins and Mike Spearpoint the Work Package 5 experiments in Poland, work Package 6 for CLT, st SAG collaboration, we also had that persistent smoldering in some locations. It was like an endless discussion why does it persist in this location? Why does it not persist in other locations? It's a very intriguing concept because if you can identify the reason then you can act perhaps to some preemptive measures. We know it's about orientation and how the surfaces align with each other, about presence of cups.

Wojciech:

We've never considered the material properties actually For us timber was timber, and you are giving us an easy access to this interesting other world of timber. Perhaps, perhaps I'll steal your idea and do this.

Wenxuan Wu:

We'll see yeah, it's a very, it's a very.

Wenxuan Wu:

I have a scale, I have a cone, I mean it's a very simple idea and based on a standardized apparatus. It's like you said it's it's a little bit outside the scope of practical implication because the scenario is more complicated, with wind velocity, with thermal feedback from the surrounding, like you said, harry's cold railroad, right Cold railroad. They also have beams, they also have the corner, they have the corner effect, they have accumulation of heat, but that's another story. So this is more like a fast screening of the material itself. So because think, like under a steel condition, because for the for the modified cone smoldering test, if your material can smolder under steel environment, like there's no wind at all, the material is is like highly possible to get smolder in a real case scenario, because everything is more complicated and everything is more conservative. Um, so this is more like a fast screening methodology to determine and you call it mcc modified conchalometry I, I don't.

Wenxuan Wu:

I don't really give it the official name because it's based. It's based on the concolometry and we split the test into two parts the flaming combustion, then you transfer it to some somewhere else to do a smothering mass loss measurement.

Wenxuan Wu:

So there is actually an advanced option of this which is fire propagation apparatus. But but I believe it's not like every fire laboratory holding a fpa. So we also did a series of tests in fpa using identical timber samples. So identical mean very, very similar density with very, very similar cca or whatever chemical treatment concentration, to test out under different heat flux and different total exposure to see the thermal condition influence on the smothering. And so that's another work of mine, Because think about that, if you put the sample in the FPA you don't need to even transfer the sample, you just turn off the lamps After a period of time. Then you can have very you can have consecutive measurement of temperature by thermocouple, the mass of course separately, like either temperature or either mass and co co2 emission during the smoldering stage, which is very important because co co2 generation very important indicators of the combustion, either complete combustion or incomplete combustion in this context which is smoldering. So so those are two bench scale testing approaches we have established to exam the smoldering behavior of a certain material and then we can compare and if you run enough amount of tests then you can do a statistical analysis to see if the density plays a more important role in smoldering or the CCA concentration or something else. So yeah, that's the bench scale testing we've done so far.

Wenxuan Wu:

And I think at one time Jose visited Brisbane for a conference. Then we had a long discussion about this. He had the same idea, because he was confused about why the energy balance can be altered by a catalyst. So he doesn't believe it's about more heat generated from the oxidation to enhance the further pyrolysis and again the oxidation. So he suggested me to do the DSC DSC, which is differential scanning calorimetry to quantify the enthalpy of a certain reaction. So what was DSC?

Wojciech:

and how does it work?

Wenxuan Wu:

So DSC shares a very similar philosophy of the TGA. So again you use a very small amount of sample 10 milligrams or even less 5 milligrams and you put into a chamber In this case it's furnace again, because you actually manipulate the temperature increase depending on the heating rate, so you can heat faster, you can heat slower, depending on the application. So you heat up the sample and the sample will again have certain reaction, either exothermic or endothermic reaction. Then the apparatus can detect the heat flux or say the temperature, and convert to the measurement data and display in the software. Then you can have two ideas First, if the reaction is exothermic or endothermic.

Wenxuan Wu:

So you can basically see if there's energy released or there's energy absorbed by the reaction. So that's the first idea, and by calculating the area underneath the curve you can get the enthalpy, which means how much energy required or how much energy released from a certain exothermic or endothermic reaction.

Wojciech:

But are you modifying? Sorry, but is the flux modified in time or is it like a fixed temperature per second growth? You're keeping it at a constant temperature, or what?

Wenxuan Wu:

uh, normally, normally for not very normal configuration, you are heating up le sample at a constant heating rate okay, so like five degrees per minute like it's a linear heating yeah, like tga.

Wenxuan Wu:

Yes, exactly like tga even slower yeah, yeah. And so through through all the studies from bench scale, small scale and some, like I would say, large scale, so the one at the beginning I mentioned about the pole testing, there are some interesting side findings. So the major finding is the density definitely plays a role. So the smoldering severity or smoldering mass loss rate increased by decreased density and increased concentration of CCA. Okay, as well as prolonged heating.

Wojciech:

So the less than steam bird, the more copper in it, the worse it was.

Wenxuan Wu:

Yes, as expected. So those results are kind of like as expected, but you just need to confirm it and prove it by statistical analysis, like if you have run enough amount of repetitions. So as well as prolonged heating but not really a higher heat flux can produce an enhanced smoldering.

Wenxuan Wu:

So, there's a very interesting preliminary study. So we've done in the bench scale. So for 20, 25 or 15 kilowatts per square meters, heat flux for the initial flaming period, the subsequent smoldering, mass loss rate is pretty much the same. So like it's pretty much the same, but for 50, for 50 kilowatts per square meters. So one might expect the higher heat flux might might produce like more severe smoldering, but it's not smoldering at all for 50 kilos.

Wenxuan Wu:

You do not smolder at all yeah, so for the so for consuming the same amount of mass, so for different heat flux, but consuming the same 40 percent of the mass 40 is a lot for, like a timber block, 40 is a lot of mass. So after consuming 40 of the mass, no one was flaming at 10 kilowatts per square meters, did not smother at all. I was talking about the CCA-treated timber, just like the normal timber.

Wojciech:

Wow.

Wenxuan Wu:

So this is a very interesting side finding. So because everybody would expect oh, just like flaming combustion, you have a higher heat flux, then you might have more severe smothering. But this is not the case for the CCA-treated timber Because, in comparison to the normal timber, the normal timber is really hard to smolder like. Trust me, the normal timber is not like you burn it and you, you put it somewhere, like by itself and it could smolder for days or years. It's, it's not the case.

Wojciech:

That's why I was so surprised with your spool, like standing free in in like free space and smoldering. What the hell is this? It shouldn't be like that, sorry, continue.

Wenxuan Wu:

You have seen the image from my poster.

Wojciech:

It's in the paper as well.

Wenxuan Wu:

That was quite horrible. So yeah, for smoldering, this is a very interesting side finding because there are more evidence to prove the same thing, because not only in the preliminary study at 50, but we have found evidence with down the larger scale, with down the micro scale. So for the micro scale, so if you remember I said there's a temperature, the peak temperature shift for the oxidation to a lower temperature. But if you heat up so if you produce the char, if you produce the char in the TGA at a very high temperature, let's say 700 degrees, 700 degrees to produce the char in nitrogen environment then you cool down the char and heat up again in oxidative environment, which is air. You don't really see the shift in oxidation peak, which means in this case CCA is no longer a catalyst anymore or you couldn't see the effect from the CCA. So that's a very interesting side finding and from then I dive more into the chemistry instead of the fire dynamics.

Wenxuan Wu:

And that's the second journal paper we published. It's called Deactivation of CCA as Catalyst in Timber Smothering. So long story short, at very high temperature or equivalent high heat flux, the CCA will deactivate and the CCA treated timber will just behave like normal timber and it will not smolder as the other CCA treated timber does. It will just like normal timber. If the normal timber cannot sustain smoldering, then the high temperature produced CCA char cannot smolder.

Wojciech:

That's like so counterintuitive. We need bigger fires in bushes so your poles can survive in the bushfire. But yeah, very interesting findings there. And did you confirm the chemical composition of the ashes or of the timber after it burned down? Did you look into what remained after the fire?

Wenxuan Wu:

Yeah, I think that's a very good question, because one of the experimental techniques we used is to examine the remaining copper, chromium and arsenic by the ICP. So ICP is called induced coupled plasma. So it's it's a technique to confirm the metal, especially the metal contents, the metal contents by weight, like remaining in the char or remaining in whatever, like whatever material you have, because you first need to digest the material into a certain solution, then examine by the machine. Then the machine will tell you okay, you have this amount of copper, this amount of chromium, this amount of arsenic, so okay.

Wenxuan Wu:

So the conclusion is by increasing the fire intensity you start to lose more and more arsenic and chromium, but not really the copper, because copper is more stable. So even at very high temperature fire, you still remain at least 90%, 80% of the copper, but you lose half of the arsenic because the arsenic is easily to be gasified, then released with the gas. So my point is at the end of the high temperature fire, the ash or the char still maintain a sufficient amount of copper and chromium. So theoretically you should still sustain this molding right. But the major problem is not about the amount of copper or the amount of chromium, it's about the state of the copper and chromium, because we have confirmed that at very high temperature those metallic components will have a thing called metal agglomeration. So basically all the metals will be compact together. So normally the treatment process of the preservation is called full cell. So all the chemical composition, the copper, chromium, the arsenic complex, will be bound to the lignin, like everywhere.

Wojciech:

It's part of the material, okay, yeah.

Wenxuan Wu:

Yes, so you have a very high surface-to-volume ratio, which means every particle can be exposed to oxygen to have the catalytic effect for the whole thing, which promotes oxidation, which promotes the smoldering. But at the end of the high temperature condition, all the metals they just compact together. Then you lose a lot of active sites, you lose the active surface area, so the catalytic effect somehow disappears. And that's actually a very common issue with combustion engines. That's actually a very common issue with combustion engine. So many people vehicle engineers or chemical engineers have studied. They want to prevent this kind of phenomenon because they want to maintain a very high efficiency of the combustion, so they don't want all the metals to agglomerate together. So yeah, that's more into chemistry side of the of the study. Um, like, apart from the fire safety engineering but this is beautiful.

Wojciech:

So this is fire science at its finest. This is why I you've won the the poster award. The reason was I I can always speak to myself. It was an interesting discussion at the table, uh, because all all the lecturers have voted. But what you've shown was very high-quality fire science, very detailed investigation to really get into the bottom of a very practical fire engineering problem. That was the magical part. If you want to know why you have particularly won the poster, it's just an application, know, an application, a beautiful execution of a well-planned study for a very good hypothesis, but carrying on to confirm or disprove the hypothesis with a scientific method and at every step, learning a little bit more. This is what fire science is all about. So beautiful. I hope your viva will go well. Given this pre-viva, I would admit you to the full viva. You seem prepared.

Wenxuan Wu:

I don't think I organized very well the flow of the study in this interview because I haven't prepared much. I will never do that in my Viva. I will prepare a very complete story from the beginning to the end, other than those primary findings from the research. Just something to all the key findings on my research, first thing. First is some timber species can also smother inherently, so that is very surprising. So for normal pine it is fine, but for some certain timber species, like there's one called Queensland whitewood, Okay.

Wenxuan Wu:

It has inherent smoldering propensity. So, while the other timber species cannot smolder under the same fire condition, but this guy can have self-sustained smoldering and it can even transit to flaming at some point during the self-sustained smoldering stage. So yeah, that's a first, that's a first key finding from my phd. Some timber species can inherently smolder and we have to exclude that for the further study. Because I want to like, I want to figure out, I want to make a difference is this smoldering is from the, is from the chemical or is from the timber itself? So my thesis, chapter three uh, is only doing the native timber species, to figure out all the smoldering propensity from 12 timber species and choose the best candidate.

Wenxuan Wu:

The best candidate in this case means with no inherent or very less inherent smoldering propensity. Then we realize, okay, the smoldering severity is going to increase, with decreased density, as expected, and more concentrational CCA, of course, and prolonged heating, when the other parameters keep constant, but not the higher heat flux or high temperature, as I just explained. Then we realized, although the European countries and the US, they use alternative, they'll use alternative, preservative, treated timber, ca, acq, but they can still smother. So, long story short, the CCA is the worst, the CA is the best, but the CA CA corporate azole. I believe Poland uses corporate azole for the treated timber products, but the corporate azole is the most expensive one.

Wojciech:

We're a very rich country. Yes, yes, for sure, and then you might. This last thing was not published yet. I haven't seen a paper on that yet. Is there a paper in the making?

Wenxuan Wu:

That's an interesting story. So that paper was partially funded by the SFP student grant. So I wrote a proposal I think two years ago that I got the grant. Then I used them only to purchase the material, to purchase the preservative for sure. That's why I know CA is the most expensive one. Then I treated all the things with my bare hands but with gloves for sure. That's why I know CA is the most expensive one. Then I treated all the things with my bare hands but with gloves for sure. I treated all the samples. Then I did a lot of testing to see if their flaming, like ignition behavior is going to be altered by the preservative and the smoldering behavior like which preservative can enhance the smoldering the most. So that's actually a work I think I'll be presenting in the next sfp conference in vancouver. Okay, but we are also considering to publish it somewhere if the sfp conference doesn't require a full paper.

Wenxuan Wu:

Uh, yes, and so from from all the things we have done, we realized one thing that's the biggest issue, because we need something more practical. You can't just tell people yeah, those things have issues, we need to solve it. So we have two ways to solve it. First, to have another PhD student do the continuation of the work, to work on a modified formula to avoid the smoldering issues of the CCA. Because even now we still have a lot of existing CCA infrastructure and there's a lot of CCA in production. So we can't just give up CCA. So now we have a new PhD student working on it as a continuation of my work.

Wenxuan Wu:

Then the next thing is every smoldering test lasts more than two hours and I've done more than 150 of them so I've spent a lot of time in waiting. So that is not practical. So you can of course test some materials you want to see if they have smothering propensity. You can do a fast screening. But eventually, for any design purpose or engineering benefit, you need a computation or numerical tool to simulate it. So in my last chapter of the thesis we actually adopt a generalized pyrolysis model established by Lundenberger.

Wojciech:

They call it G-Pyro, G-Pyro okay.

Wenxuan Wu:

G-Pyro. Yes, to simulate this kind of self-sustained smothering, which was quite successful in comparison to the bench scale study we did in FPA. Because, you can't just run numerous, countless repetitions to prove one thing is going to smother.

Wojciech:

Of course you can. You just need a lot of students.

Wenxuan Wu:

Yeah, you just need a lot of students. It's just very people-consuming. Yeah, so that's the end. So that's the end of the, that's end of my phd and we hope the new student can continue and develop it a bit further fantastic.

Wojciech:

Good job there, mate and uh. Are you staying in academia? Are you going to industry? What's happening with you after your phd?

Wenxuan Wu:

um, it's not official yet, but I have started my postdoc research in the same group I've been staying for many years yes, yeah. So I'll be working on a totally different field, which is facade cladding material. Flame spread this kind of thing and and I'm very glad I have a lot of expert in flame spread like surround me fantastic good for you.

Wojciech:

I'm happy that you see your future in academia. We don't have enough good scientists in here, so always a shame to lose one to those pesky multi-billion dollar industries. I'm just kidding. Anyway, Wang Shen, thank you so much for coming to the Fire Science Show. Congratulations once again on your best poster award at our ITV Frisbee Summer School and all the best with your Viva and your future academic research.

Wenxuan Wu:

Thank you very much, Volchek. It's very nice. It's always nice to talk with you. I don't know why.

Wojciech:

Thank you Anyway, cheers and that's it. Thank you for listening. I promise you a journey, and a journey it was from very simple observations in full scale, from having a good research hypothesis through early material testing, identifying what's happening with the material, identifying the reactions, identifying the onset temperature of those reactions, already seeing some differences between the control sample and the sample with the coating, then going into more complicated stuff, assessing enthalpy of reactions, assessing the heat transfer and ending up with investigating the chemical composition, the structure. In his paper there's even pictures of those tiny bits of copper inside the char from a scanning electric microscope. And I say that because I know a lot of researchers use this type of pictures to enhance the value of their papers and usually they mean nothing.

Wojciech:

But in case of Wenxin it's a beautiful investigation and he really shows up this agglomeration of copper that deactivates the smoldering enhancement by this material. Really beautiful study, really beautifully executed study. Perhaps not the most useful if you're not in Australia and you're not dealing with electricity poles, but this type of materials, as Wenxian said, are used worldwide, so perhaps of some relevance. And the same type of metallurgy, the same approach could be used to study behavior of any additive on timber really to see how it changes the timber behavior. I said that we studied a lot on timber configurations, how the configuration of the materials changes the behavior of mass timber. Wenchun gives us a tool set to study the material part of that equation.

Wojciech:

So I find that beautiful and it's my mission to bring fire science to you, fire science that you may not even know that existed, fire science that I find intriguing and compelling, and bring that to the world of engineers so you can do with it whatever you like. I hope it's at least interesting and entertaining and hopefully practical and useful. Anyway, that's it for today's episode and next week. My mission statement does not change I'm going to bring you some good fire science on your next Wednesday. Thanks, see you there. Bye, thank you.