关于“木材热释放速率中第二个/末端峰值的奇怪情况:锥形量热计研究”的评论,Sanned等人。

IF 2 4区 材料科学 Q3 MATERIALS SCIENCE, MULTIDISCIPLINARY
Vytenis Babrauskas
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引用次数: 0

摘要

几十年来,众所周知,燃烧木材样本显示出热释放率(HRR)曲线,在开始时有一个峰值,在结束时有第二个峰值,或多或少在在中间有一个平稳期。作者认为第二个峰的存在是“奇怪的”,但他们将其特征仅归因于样品背面的热效应。这种分析忽略了木材热解和燃烧的一个极其重要的特征:在热降解过程中存在(至少)两种不同的化学途径。从20世纪50年代中期到60年代中期,许多研究人员研究了木材降解的化学动力学,到1968年,Shafizadeh能够总结出对其机制的理解。最初的机制涉及脱气过程。这会释放出可燃挥发物,这些挥发物可以在样品表面上方燃烧,并产生可见的火焰。第二种机制是碳化,发生时间较晚。随着挥发物逐渐离开,碳质炭残留下来。这种炭不能通过挥发燃烧(因为它不挥发),但可以通过表面氧化燃烧,即在表面发光燃烧。整个过程必须从燃烧开始,因为一开始还不存在焦炭。但在炭化开始发生后,这两个过程可以同时进行。然而,当燃烧发生时,只有少量的发光燃烧可以发生。这是因为火焰中的反应消耗掉了氧气,火焰和试样表面之间的区域几乎没有留下氧气。在挥发物的大部分燃烧停止后,氧气可以很容易地到达表面,然后焦炭氧化发光燃烧可以不受阻碍地进行。任何观察到壁炉着火的人都能看到这个过程中的现象。起初,只能看到燃烧的原木,但当火焰熄灭后,原木会因表面氧化而变成鲜红色。最终,即使是木炭也会被消耗掉,只有原木中的矿物质含量才能使其成形,用扑克戳时会碎掉。如果将一块木头放入氧弹量热计中,则总燃烧热约为19.2–21.8 MJ kg 1,净燃烧热约17.8–20.4 MJ kg。因此,早期的火灾科学研究人员曾经假设,在以木材为燃料的实验火灾中,可以使用大约20 MJ kg 1的常数来从质量损失率中得出HRR。但早在1989年,Heskestad和Delichatsios就指出,这绝对是不正确的,应该使用12.5 MJ kg 1的值。原因是火灾试验通常在其活性燃烧状态下进行研究,通常在火焰熄灭之前熄灭,只有焦炭氧化继续。荒地火灾研究人员过去也有类似的假设。但在2006年,我报道了一项关于燃烧花旗松的研究,在那里测量了有效燃烧热。对于在森林中燃烧的整棵树,建议燃烧热的有效值为12.5 MJ kg 1。我在SFPE手册中发表了一张图,显示了锥形量热计中测试的木材样品的有效燃烧热的实时演变(图1)。这是在该仪器中测试木材样品的典型结果。大约在试验运行的前3/4,可以看到大约12.5MJ kg 1的值。之后,该值上升到30MJkg附近的峰值。作为参考,可以注意到,对于石墨形式的碳,碳的燃烧热为32.8MJ/kg 1,而对于木炭,其为34.3MJ/kg 1。人们可能会注意到,从锥形量热计测试中获得有效的燃烧热结果需要从具有一定时间响应特性的两个信号中形成一个数值比,因此可以从这个来源中预期适度的异常。尽管如此,结果表明,12.5 MJ kg 1是测试第一部分的合理代表,而接收日期:2023年2月1日接受日期:2022年4月4日
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Comments on “The curious case of the second/end peak in the heat release rate of wood: A cone calorimeter investigation,” by Sanned et al.
It has been well known for some decades that burning wood specimens show a heat release rate (HRR) curve with a peak at the start, a second peak near the end, and more or less a plateau in the middle. The authors consider the presence of the second peak to be “curious,” but they attribute its characteristics solely to thermal effects at the back face of the specimen. This analysis overlooks one exceedingly important feature of the pyrolysis and combustion of wood: the presence of (at least) two different chemical pathways in the thermal degradation process. Numerous researchers from the mid-1950s to the mid-1960s studied the chemical kinetics of wood degradation, and by 1968 Shafizadeh was able to summarize an understanding of the mechanisms. The initial mechanism involves a degasification process. This releases combustible volatiles which can burn above the surface of the sample with a visible flame. The second mechanism is carbonization, which occurs later. As the volatiles progressively depart, a carbonaceous char remains. This char cannot burn by volatilization (since it is not volatile), but can burn by a surface oxidation, that is, glowing combustion at the surface. The whole process necessarily starts with flaming combustion, since char does not yet exist at the beginning. But after charring starts to occur, both processes can go on simultaneously. However, while flaming is taking place, only a small amount of glowing combustion can take place. This is because the reactions in the flame use up oxygen, leaving little for the zone between the flame and the surface of the specimen. After most of the combustion of the volatiles has ceased, oxygen can readily reach the surface, and char-oxidation glowing combustion can then proceed unhindered. The phenomena of this process are visible to anyone who observed a fire in their fireplace. Initially, only flaming is seen, but when the flames have died down, the logs turn bright red due to surface oxidation. Eventually, even the char gets consumed and only mineral content of the log remains to give it shape, which can crumble when poked with a poker. If one puts in a piece of wood into an oxygen-bomb calorimeter, values of the gross heat of combustion of around 19.2–21.8 MJ kg 1 are found, with the net heat of combustion being around 17.8– 20.4 MJ kg . Thus, early fire science researchers used to presume that in experimental fires fueled by wood, a constant of around 20 MJ kg 1 could be used to derive the HRR from the mass loss rate. But already in 1989, Heskestad and Delichatsios pointed out that this is definitely incorrect, and a value of 12.5 MJ kg 1 should be used, instead. The reason is that fire tests are normally studied in their active flaming state and are usually extinguished before flaming dies out and only char oxidation continues. Wildland fire researchers used to assume something very similar. But in 2006, I reported on a study of burning Douglas-fir trees, where the effective heat of combustion was measured. For whole trees burning in the forest, an effective value of heat of combustion of 12.5 MJ kg 1 was recommended. I published in the SFPE Handbook a figure which shows the realtime evolution of the effective heat of combustion for a wood sample tested in the Cone Calorimeter (Figure 1). This is a typical result for wood specimens tested in that apparatus. A value around 12.5 MJ kg 1 is seen for about the first 3/4 of the test run. After that, the value rises to a peak which is in the vicinity of 30 MJ kg . For reference, it may be noted that the heat of combustion of carbon is 32.8 MJ kg 1 for carbon in the form of graphite, while it is 34.3 MJ kg 1 for charcoal. One may note that obtaining effective heat of combustion results from Cone Calorimeter testing requires forming a numerical ratio from two signals which have certain time– response characteristics, thus moderate anomalies can be expected from this source. Nonetheless, the results suggest that 12.5 MJ kg 1 is a reasonable representation for the first part of the test, while a Received: 1 February 2023 Accepted: 4 April 2023
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来源期刊
Fire and Materials
Fire and Materials 工程技术-材料科学:综合
CiteScore
4.60
自引率
5.30%
发文量
72
审稿时长
3 months
期刊介绍: Fire and Materials is an international journal for scientific and technological communications directed at the fire properties of materials and the products into which they are made. This covers all aspects of the polymer field and the end uses where polymers find application; the important developments in the fields of natural products - wood and cellulosics; non-polymeric materials - metals and ceramics; as well as the chemistry and industrial applications of fire retardant chemicals. Contributions will be particularly welcomed on heat release; properties of combustion products - smoke opacity, toxicity and corrosivity; modelling and testing.
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