Fire Fundamentals: Ignition, Pyrolysis and Combustion

• These notes cover the fundamentals of fire, its behavior, and its impact on the ecology and geography of landscapes.

Fire Fundamentals

• Defined as a rapid oxidation reaction, it emits heat and light while producing various chemical products.
• It's a type of chemical reaction between oxygen and fuel and differs from the original starting material.
• Fuel requires heating to its ignition temperature for combustion.
• Continued presence of heat, fuel, and oxygen will sustain fire.

Ignition Sources

• Lightning is a natural ignition source.
• Activation energy is required to overcome an energy barrier.
• Energy released during combustion needs to surpass the initial activation energy for the fire to continue (positive feedback).
• In combustion the products have less energy than the reactants.
• Anthropogenic sources include recreation, electric utilities, transportation, and arson.

Charring/Pyrolysis

• Involves a two-stage process in wood burning.
• Pyrolysis occurs when there is an absence of oxygen, leading to charcoaled materials.
• Combustion happens when oxidative components combine with oxygen in the air.
• Undesirable materials and gases are produced, some that remain (ash, charcoal) and others, like white smoke, are transported away.
• Pyrolysis creates flammable gases and liquids, but not H2O and CO2.
• Fires are a significant source of greenhouse gases (CO2, CO, CH4, NOx).

Fire Types

• Surface fires consume litter and ground fuels, like foliage, brush, and grass.
• Crown fires involve shrubs and aerial fuels, with branches and trees burning intensely.
• Ground fires occur in thick humic soil layers, burning contained soil and organic matter.
• Peat fires signify extreme ground and subsurface fires, rare but hazardous, causing damage and contributing to global warming.

Fire Effects on Soils

• Fire intensity is the measure representing energy released.
• Fire severity: The amount of damage done to vegetation.
• Burn severity: The amount of loss to organic material.
• Hydrophobicity refers to water repellency, seen more in dry sandy soils. Waxy residues on soil particles repel water, causing this.

Post-Fire Erosion & Deposition

• Major factors include fuel build-up, fire intensity/severity, and soil composition.
• Hydrological events involve the breakdown of organic material and root systems, stabilizing soil, and increasing soil hydrophobicity, which can increase runoff.
• Sediment transport and storage occur from heavy rainfall over burned areas, leading to debris flows, mudflows, slope failures, and alluvial fan deposition.
• Vegetation regrowth helps stabilization because it prevents sediment movement and removal.

Fire and Vegetation

• Vegetation types are fuels and influenced by and may adapt to fires.
• Fuel loads, flammability, surface roughness, and litter production vary across land covers.
• Slash refers to downed vegetation, while vegetation snags are standing dead or dying plants.
• Some ecosystems have plants that developed fire-resistant traits because of frequent surface fires. Some plants even need fire for reproduction and even promote fires.

Fire and Climate/Weather

• Climate is the ultimate control over wildfires because it provides ignition and spread conditions.
• Air humidity and wind are critical fire weather.
• Environmental conditions decide if fire spreads and is put out.

Fire Triangles

• Heat, oxygen, and fuel are the three components of a fire.
• Sufficient biomass, its dryness, fire spread, and ignitions are data collected on a regime model.

Fire Return Interval

• Definition: the time between two fires in a set area.
• Long intervals allow stems to grow to fire-resistant sizes, canopy close, leading to a decrease in grasses.
• Plant invasion: Grasses burn easier than other vegetation types. Flammable nonnative grasses may transform the woodlands and forests.
• Loss of woody biomass encourages drier microclimates allowing grasses to take up more nutrients and spread.

Combustion & Heat Transfer

• Fire behavior includes ignition, development, spread, and extinguishment; Rate and direction of spread, Size and shape (flame dimensions), Heat generation and sound produced (intensity).
• Combustion and heat transfer processes involve fuel consumed during combustion and smoldering phases that are too difficult to separate and quantify. Both forms of fuel are consumed; live and dead.
• Rate of spread (ROS) speed perimeter expands depends on windspeed, slope, fuel amount and water content.
• Flame front (head) speed estimates fire spread.

Wildland Fire Environment

• Conditions, influence and modifying forces determine inception, growth and behavior.
• Fuel, Topography, and Air (weather) are considered in a triangle.
• Air condition including humidity, wind, and precipitation influence ignition, spread and suppression.
• Fuel types: herbs, sub-shrubs, shrubs, and trees are located in the ground, surface ladder and aerial strata.
• A fuelbed represents a fire environment, with inherent aspects i.e. mineral, heat and chemical content as well extrinsic i.e Quantity, size, shape, arrangement.
• Dead fuel moisture (DFM) is water in dead plant debris that co-varies with atmospheric humidity.
• Live fuel moisture consists of water in living plants and can exceed 100%.
• The details of the natural and artificial features of an area are topography.
• Fuel, topography, and weather interact.
• DFM can measured destructively, or electronically.

Wildland Fire Behavior

• Rate and direction of spread, plus size and shape are measures of behavior.
• Smoke color can show general behavior.
• The leading edge of a fire spreading into unburned fuel is the flaming front.
• Shape includes being circular (calm) or elliptical (windy).
• The speed perimeter expands is the rate of speed.
• Flame front dimensions and characteristics: height, depth, length, angle, tilt
• If the fire spread rapidly, or exhibited contious crowning from the shrubs, the branches and are on fire the fire would be very difficult to contain and consider extreme.
• Caused by tornado that is caused by fire-generation with breezes.
• Pyro-cumulus, and all lighting show a convection column.

Pyrogeography

• The interplay of fire and life is related to the global pattern and how it changes through time.
• Climate influences vegetation zones, while ecozones include landforms, soil, and ecology.
• Spatial and climate affects the relationship between Net Primary Production (NPP) and areas affected by fire as seen by remote sensing.
• Four switch regime model includes sufficient biomass, adequate dryness, fire spread, ignitions which reflect the fire productivity.
• Human impacts on pyrogeography may also include more hotter droughts, flammable plants and urbanization.
• Fires can influence and control biome boundaries.
• Fire regime components: regularity, season, type, intensity, loss and location.
• Manifestations in biomes: soil and stability affected.
• Ecology: Complex interactions

Explore the fundamentals of fire including rapid oxidation, ignition sources like lightning, and anthropogenic factors. Understand the two-stage process of charring or pyrolysis in wood burning, and sustained fire.