THE CHEMISTRY OF FOREST FIRES

The Chemistry of Forest Fires

By Kurtis Neeson

Background information

COMBUSTION:

• Complete Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O - Methane's transformation during forest fires. [Image 1]

• Incomplete Combustion: C₆H₁₀O₅ + insufficient O₂ → CO₂ + CO + C(s) + H₂O - Cellulose's incomplete oxidation.

• Flaming vs. Smoldering Combustion: The presence of visible flames versus glowing embers.

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PYROLYSIS:

• Pyrolysis Reaction: Organic Material + Heat → Pyrolyzate(g) + Char(s) + Ash(s)"

• Significance in Fire Development: Dictates the types of gasses and particulate matter released.

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FIRE CHEMISTRY AND THE FIRE TRIANGLE:

• Fire Triangle: Heat + Oxygen + Fuel = Fire
• Oxygen's Role: Accounting for approximately 21% of Earth's atmosphere, it is the primary oxidizer.
• Oxidation Half Reaction: C + 2O2−→ CO2 ​+ 4e−Carbon atoms lose four electrons to oxygen, causing the release of energy as heat and light

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Main Research

Pyrolysis And Combustion In The Life Cycle Of A Forest Fire

INCIPIENT STAGE:Combustion:

• Initial Combustion: The rise to 400°C, fuel dehydration, and endothermic reactions.
• Pre-heating of Fuels: Driven by convection and radiation, leading to pyrolysis.

• Combustion Emergence: Flaming complete combustion from gas oxidation and smoldering incomplete combustion from char oxidation.

Pyrolysis:

• Pyrolysis Stages in Wood: Below 200°C - slow decomposition; at 300°C - volatile gas buildup; around 325°C - rapid pyrolysis.

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GROWTH STAGE:Combustion:

• Temperature Surge: 400-1000°C, rapid spread with visible flames.
• Chemical Breakdown: Cellulose, hemicellulose, and lignin degradation in plants.
• Complete Combustion of Cellulose: 4C6H9O4 + 25O2 → 24CO2 + 18H2O + ~5.2MkJ energy.
• Combustion Transition: From smoldering to flaming combustion.

• Flashover Point: Simultaneous ignition of combustible surfaces, leading to full engulfment.

Pyrolysis:

• Gasification Stage of Pyrolysis: Char burning at 500-600°C, releasing volatile gasses.
• Emissions Diversity: Over 130 compounds identified, including flammable gasses.

Decay Stage:

• Reduction Factors: Limited airflow, moisture increase, and fuel depletion.
• Smoldering Combustion: Low-temperature oxidation of char and residual organics.
• Chemical Shift: Predominance of carbon dioxide, carbon monoxide, water vapor, and ash emissions from incomplete combustion.
• Extended Duration: Persistent low-temperature pyrolysis and smoldering.
• Heat Trapping: Insulation by the forest floor and incomplete combustion beneath soil.

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Post-Fire Analysis:

• Temperature Normalization: Setting the stage for ecosystem assessment.
• Soil and Ecosystem Impact: Evaluating the changes in soil quality due to heat and pyrolysis gasses.
• Analytical Techniques: Utilizing microscopy, gas chromatography, and mass spectrometry.
• Collaborative Efforts: Combining research findings with firefighting strategies.

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Conclusion:

• Summary of Findings: The crucial role of pyrolysis, incomplete and complete combustion.
• Implications: Enhanced firefighting, environmental management, and policy implementation.
• Future Directions: Further research on the environmental impact of forest fire emissions and recovery.

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Research Information References

• USDA Forest Service. 'Fire Behavior.' (Accessed 09/19/23). https://www.fs.usda.gov/research/fire/firebehavior [Cached version accessed 11/09/23]
• Santoso, M. A., et al. 'Review of the Transition From Smouldering to Flaming Combustion in Wildfires.' Frontiers in Mechanical Engineering, vol. 5, 2019. DOI: 10.3389/fmech.2019.00049. (Accessed 09/20/23)
• Friedli, H. R., et al. 'Volatile organic trace gases emitted from North American wildfires.' Global Biogeochem. Cycles, 15(2), 435–452, 2001. DOI: 10.1029/2000GB001328. (Accessed 09/20/23)
• Datta, R. 'To extinguish or not to extinguish: The role of forest fire in nature and soil resilience.' Journal of King Saud University - Science, Volume 33, Issue 6, 2021. DOI: 10.1016/j.jksus.2021.101539. (Accessed 10/16/23)

• Falk, D. 'Lec. 1.5: Where there’s fire, there’s smoke', University of Arizona Department of Agriculture, Life and Environmental Sciences, https://cales.arizona.edu/classes/rnr355/lectures/Lec_1_5_12.pdf (Accessed 10/27/23)
• Hubbard, W. G. 'Wood bioenergy.' In Bioenergy (Second Edition), Academic Press, 2020, Pages 69-87. DOI: 10.1016/B978-0-12-815497-7.00004-X. (Accessed 10/27/23)
• Chatelon, F.-J., et al. 'Generalized Blaze Flash, a “Flashover” Behavior for Forest Fires—Analysis from the Firefighter’s Point of View.' Open Journal of Forestry, 2014, 5, 590-610. DOI: 10.4236/ojf.2014.45059. (Accessed 11/3/23)

Image References

• Image 5: https://www.vnews.com/Killington-Vt-forest-fire-burns-underground-for-three-days-and-counting-36375963
• Image 6: https://www.insurancebusinessmag.com/us/news/catastrophe/new-solution-to-extinguish-wildfire-damage-95673.aspx
• Image 7: https://extension.oregonstate.edu/catalog/pub/em-9342-smoke-management
• Image 8: https://onetreeplanted.org/blogs/stories/fire-ecology-after-forest-fires
• Image 9: https://www.usfa.fema.gov/blog/new-research-on-inhalation-exposures-for-wildland-firefighters/

• Image 1: https://mytutorsource.com/questions-answers/chemistry/297/what-is-the-difference-between-complete-and-incomplete-combustion-of-a-hydrocarbon/
• Image 2: https://www.researchgate.net/figure/Schematic-diagram-of-pyrolysis-and-oxidation-for-a-forest-fuel-element-Pyrolysis-and_fig2_228370869
• Image 3: https://www.firerescue1.com/fire-products/apparatus-accessories/articles/what-is-a-fire-triangle-4HSY7X5xagWZR5KQ/
• Image 4: University of Arizona Department of Agriculture, Life and Environmental Sciences, Dr Don Falk (School of Natural Resources), https://cales.arizona.edu/classes/rnr355/lectures/Lec_2_1_12.pdf

THANK YOU!