Fire and Fuels8/31/2010

OXYGEN HEAT

FUEL

THE FIRE TRIANGLE

FIRE

Weather Topography

Fuels

The Wildland Fire The Wildland Fire EnvironmentEnvironment

((influences fire behavior)influences fire behavior)

FIRE

What are Fuels?

Any organic material that is living or dead that can ignite and burn.

Fuels can be found in the ground (ground fuels), on the ground (surface fuels), or in the air (aerial fuels).

Ground fuels

•below surface (“duff”)

•usually compact --> smoldering, creeping

•deep roots, duff, buried logs

•important in terms of line construction and mop-up

Surface fuels

•on surface

•litter, grasses, shrubs (to 6 ft.)

•important in terms of line construction

and mop-up

• Aerial fuels

•crown or canopy

•open vs. closed canopy

•important in terms of fire spread and fire

behavior due to torching, crowning, and spotting

Principle Fuel Characteristics

1. Fuel Loading

2. Size and Shape

3. Compactness

4. Horizontal Continuity

5. Vertical Arrangement

6. Chemical Content

7. Moisture Content

1. Fuel Loading

The oven dry weight of fuels in a given area, usually expressed in tons/acre or lbs/acre

2. Size and ShapeSurface-area-to-volume ratio (SA / V)

– ratio of fuel’s surface area to total volumeExample: Grass vs. Logs – which has a higher SA/V ratio?

Fuels with high surface area

to volume ratios (fine fuels)

lose moisture more quickly

than larger fuels, therefore

they ignite more quickly

Grass = high SA/V; Log = low SA/V * The higher the ratio, the finer the fuel

How does SA/V affect fuel moisture?

3. Compactness= The spacing between fuel particles. Closely Compacted

– less surface exposed (remember SA/V ratio?)

– restricts oxygen and inhibits convection and radiation

Loosely Compacted

– larger amount of surface exposed

– allows for oxygen circulation and heat movement

How does compactness affect ignition andcombustion?

5. Horizontal ContinuityThe horizontal distribution of fuels at various levels or planes.

How does horizontal continuity affect fire behavior?

Influences direction of fire spread, rate of spread,

and level of fire travel (surface vs. aerial)

Ladder Fuels

Vertical Arrangement

The relative heights of fuels above the ground as well as their vertical continuity.

6. Chemical Content

Certain types of fuels contain volatile chemicals which increase their flammability. Chemicals include, pitch, tar, resins, oils, wax,etc.

Examples include- chaparral, palmetto, fountaingrass, pine, juniper

7. Fuel Moisture Content

The amount of water in a fuel expressed as a percentage of the oven-dry weight of that fuel.

= (amount moisture/dry weight) * 100

Definition of Equilibrium Moisture Content:

No net gain or loss of moisture between fuels and the surrounding air.

Does equilibrium moisture content occur

for all fuels?

* Occurs in small, fine fuels.* Never occurs in larger fuels.

Dead Fuel Time-Lag

Timelag- the rate at which a fuel gains/loses moisture

relates to relative humidity, precipitation, size of fuels…

* Will small pieces of fuel gain/lose water faster or slower than large pieces of fuel?

Time-Lag Categories

1-hour - 0 to 1/4” in. diameter

10-hour - 1/4” to 1” in. diameter

100-hour - 1” to 3” in. diameter

1000-hour - 3” to 8” in. diameter

Which category gains/loses moisture fastest? Why?

Definition of Moisture of Extinction:

Moisture of Extinction is the fuel moisture content at which a fire will not spread, or spreads only sporadically and in a non-predictable manner.

- usually around 25% - lower for grasses; higher for logs - also depends on tissue chemical content

Moisture content of live vs. dead fuels:

Are they different?Why or why not?

Living fuels: hold between 30 % to over 300 % water*fluctuate slowly

Dead fuels: hold between 2 % to 30 % water *fluctuate rapidly

Live-to-Dead Ratio

Amount of live fuel to dead fuel.

-- How does live-to-dead ratio affect fire behavior?

Over mature fuel complexes, fire damaged, insect damaged, diseased

Examples of low live-to-dead ratios:

Higher amounts of dead to live fuels = higher flammability (less moisture to drive out).

**Factors Influencing Fuel Moisture Content

- Weather (ppt, dew, ground moisture,

wind, RH-evaporation)

- Biological/physiological processes in

plants

- Disturbance

Stage Moisture content %

1. Fresh young foliage

2. Maturing foliage.

3. Mature foliage

4. Entering dormancy, coloration starting, some leaves dropping.

5. Completely cured (= dead fuel).

Stages of Vegetative Development

300

200

100

50

< 30

Disturbance Factors Contributing to Live Fuel

Moisture Loss – may cause Abnormal Fire Seasons:

– Long drought periods.– Natural disease and insects.– Early frost.– Harvesting of timber.– Blowdown and ice storms.

Total fuel

NOT ALL FUEL WILL BURN!

Potential fuel

Available fuel

– all the plant material

– the material on a site that might burn in an intense fire (less than the total fuel)

– the fuel that is available for combustion in a given fire.* varies by site and environmental conditions

Available FuelFuels that will ignite and support combustion at the flaming front

under specific burning conditions.

What factors influence fuel availability?

Total fuel, fuel type, weather, chemical content, fuel moisture, season, live:dead, …

What will ignite more easily: Solid wood, decayed wood, or litter?

And why?

• Thermal conductivity

• Heat capacity

The quantity of heat transferred per unit of area per unit time per degree of temperature gradient.- Increases as the density and size of the fuel increases.

The amount of heat required to raise the temperature.- Increases with density.- High density-fuels require more heat for ignition than

low-density fuels.

Homework - Select a recently or currently burning fire…

• Name & location

• How did it start?

• Fuel type?

• Topography?

• Fire behavior?

• Weather conditions (RH/TEMP – see outcome sheet)