Researchers from Brigham Young University, Utah, studied the chemistry of burning shrubs and made a model that can predict how forest fires will burn in the next 20 minutes instead of the next weeks.
The university posted a snippet of the study on their YouTube account, lead by Thomas Fletcher, a Chemical Engineering Professor along with his students, Seth Tollefsen, and Mahsa Alizadeh.
Fletcher said the team was trying to understand and "take some of the unpredictability out of the fire by doing experiments in well-controlled environments."
He added that they started by working on a small scale in hopes of getting some insight into what could happen on a large scale.
Tollefsen also explained, "There's some fairly good models already for how dry grass burns but nobody knows how these shrubs burn and that's when the fire gets unpredictable.
"The thing that's hard to model with a shrub is the geometry, how it grows from a few inches into a 1.8 meter (6 feet) or a 2.4 meter (8 feet) flame. How does that happen? We don't know." a researcher said.
The experiment involved loading leaves of 14 shrub species (the likes of Dwarf Palmetto, Fetterbush, Inkberry, Sparkleberry, and Wax Myrtle) into the crucible of a thermogravimetric analyzer.
Beginning with low heat, the researchers slowly turned it up to 800 °C (1 472 °F) while observing the leaves as they burn.
They classified the speed at which the plant burned and the chemicals that were given off by the heat.
In the experiment, the researchers also compared the impact of convective heat source, being wind-driven fires, and radiative heat source, from burning plant particles.
"We look for patterns in the flames, whether they merge, whether they don't merge, how the flames are behaving, how they interact with each other," Tollefsen said.
Alizadeh noted that they examined the chemicals that are being released when the leaves are burning.
"When something burns, it first gives off gasses and they are the things that really burn when we see a flame like that. It's not really the solid. We conduct these experiments with ceramic felt pads. They take the place of the leaves so we put them in different configurations, set them on fire, and we watch the flames as they burn," the researchers stated.
They tried to see what happens when two flames intersect, whether it would double in volume or show changes in its width.
"After that, we run it through a computer code that calculates the height of the flame with the flame, the area of the flame, and also whether the flames on the two pads are merging," a researcher noted.
As a result, the study showed that the chemistry of shrubs make an impact on how quickly they break down before it combusts.
In conclusion, it's important to know the chemistry because the type of plant found in the burning area can help predict how the fire will further burn, and how other plant species can get affected.
The study, according to the researchers, is an intermediate scale model that runs "quick enough for it to be useful for a fire manager" that asks where the fire is going to be.
"Our model can’t prevent a fire, but it can help with decisions on how to do managed fires so that when a fire starts, it doesn’t blow up into a huge, uncontrollable fire," Fletcher remarked.
"Pyrolysis kinetics of live and dead wildland vegetation from the Southern United States. Journal of Analytical and Applied Pyrolysis, 2019; 142: 104613 DOI: 10.1016/j.jaap.2019.05.002
The fundamental combustion behavior of live wildland vegetation is not fully understood. Since the combustion process during wildland fire starts with pyrolysis, there is a need for better understanding of pyrolysis to develop improved wildland fire models. The kinetics of pyrolysis of live and dead wildland vegetation has not been explored in detail. In this study, the pyrolysis kinetics were determined for 14 different plant species (live and dead) which are all native to the forests in the southern United States. Pyrolysis experiments were carried out in a Thermogravimetric analyzer (TGA) under inert conditions at 5 different heating rates ranged from 10 to 30 °C min−1. The iso-conversional and model-free method of KAS was used to calculate the activation energies and pre-exponential factors. The rates of pyrolysis as a function of conversion were calculated using the KAS method for all plant species (live and dead), and the results showed that during the sample decomposition, the highest rates were observed at the lower conversions. The dependence of activation energy on conversion fraction, type of plant species, and plant condition (live and dead) was investigated. For some plant species, an increase in activation energy with conversion was observed in the major pyrolysis zone for live samples. The effect of aging on apparent activation energy was studied by comparing the pyrolysis rates of live and dead longleaf pine foliage with longleaf pine litter (pine straw). The results showed that length of time since living increased the activation energy in the major pyrolysis zone. The average activation energy based on the KAS method for pyrolysis of abscised pine foliage (pine straw) was 183.4 kJ mol−1, while the activation energies were lower for live and dead longleaf pine foliage (167.3 and 167.4 kJ mol−1, respectively). Finally, the activation energies obtained in this study were compared with literature and the results showed that the average activation energies obtained in this study for palmetto, grass, broadleaf, and needle types are very close to the average activation energies obtained for different kinds of straw reported in the literature.
Featured image credit: Brigham Young University
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