ATS2545 Lecture Notes - Lecture 14: Pyrolysis, Sclerophyll, Combustibility And Flammability
25 May 2018
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Lecture 14 - Wildfire and Logging: hydrologic effects
Fire and Logging - background
• Hydrologic change arises from many causes - global and regional
environmental change, clearing and change in dominant type of vegetation,
urbanisation
• Fire and logging have superficial similarities in their effects - loss of
vegetation cover and increasingly bare surface
• Both are worth considering in more detail in relation to the reliability of water
yields from catchments supplying urban requirements
• Both can have marked local effects on water yield, that are additional to the
global-scale implications of tropical forest decline already considered
o Africa - 68.4%% of MODIS area (total area burned)
o Australia - 17.3% every year
o South America - 4.7%
▪ Due to long droughts and vegetation flattening
Fire (pyrolosis) - some basics (preheating, flaming combustion, glowing combustion)
• Flammability of eucalypt vegetation:
• Flammable foliage
o Easily ignited and continues to burn
• High litter loads
o Especially dry sclerophyll - owing to dryness and refractory nature of
nutrient poor leaves
o Eucalypt leaves fall after 1-3 years
• Loose aerated litter structure (twigs, leaves, bark)
• Spotting ahead of fire by bark firebrands
• Droughts
• Flammability of Australian vegetation
o Mutch hypothesis re-phrased
o Dry sclerophyllous vegetation burns intensely and sustains the
conditions resulting in flammability because of distant evolutionary
selection driven by aridity and nutrient scarcity
o Feed back loop
▪ Flammability results in ongoing loss of nutrients and organic
matter in post fire runoff
▪ Declining nutrient abundance retards rate of litter breakdown
and increases fuel loads, increasing fire severity
The Nature of Fire and Flames
• Warming and drying of fuel: preheating phase
• Ignition of fine dry fuels causes heating of other fuels by radiation, conduction
and convection
• At 200˚c, pyrolysis reaction beings (heat decomposition)
• Releases vapours of waxes, oils, resins and pyrolysis products
• Gases then burn - flaming combustion
o Results in char (burnable residue) and ash (not burnable)
• Wind is head of fire front
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Fire Influences
• Burning out of roots system (consequence)
• Great variety of fire intensities related to differing conditions of
o Amount of standing vegetation and its type (amount of litter/fuel)
o Amount of litter and its wetness or dryness
o Weather conditions (temperature, humidity, windiness)
o Topography
o The season (amount of growth, litter, wetness of the terrain and effect
on weather)
• The energy of typical forest fuels is about 20 MJ per kg
• Once sufficiently warm, the chemical reaction pyrolysis beings
• Pyrolosis is formal name for chemical reactions that occur and releases heat
during a fire → exothermic oxidation reaction
Some Properties
• Need to warm fuel to its kindling temperature for the reaction with oxygen
(pryolosis) to begin
• Takes more heat energy if the fuel is damp
• Recognisable phases of fire in vegetation thus include
o Preheating
o Flaming combustion
▪ Burning of hydrocarbon gases released from the fuel
o Glowing combustion
▪ Slow burning from the solid state later in the fire event (logs)
find more resources at oneclass.com
find more resources at oneclass.com
