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