EXAM 3 CHEM

combustion:

chemical reaction between fuel and O2 that is initiated by and releases heat

Fire triangle OG

* fuel
* heat
* O2

O2

important for intensity of fire

Fuel

* plant biomass

fuel composition

* wood ( dominate compounds )
* Extractives
* waxes
* fat
* terpenes
* mineral parts

What are the three parts of heat?

* preignition
* combustion
* heat transfer

Preignition:

* endothermic so need energy
* outside source gives energy = pilot

Combustion

* exothermic so release energy
* released energy feeds back to preignition phase to continue loop

heat transfer

* energy transferred due to difference in temperate
* heat must be transferred from combustion to preignition somehow
* convection
* radiation
* conduction

Convection

* transfer of energy via fluid movement ( liquid and gasses )
* flame contact
* circulation of heated air to fuel

radiation

* transfer of energy by electromagnetic waves
* visible light
* infrared light

Conduction

* transfer of energy through matter ( solids )

Preignition in detail

* heat transfer (endothermic)
* radiation or convection of warm air
* steps when you raise fuel temp:
* dehydration ( evaporation of water and volitiles
* volatilization of extractives - terpenes, waxes, and fats
* pyrolysis ( themeral degradation of fuel )
* chemical change without combustion
* creation of char
* more volatiles
* more h2o lost
* creation of mineral ash

Ignition

* rapid transition between preignition and combustion
* required a temp. between of around 600º F
* rapid initiation of exothermic reaction

Combustion

* Flaming combustion: oxidation of gasses above the fuel
* gasses = volatiles and tar
* flame = combustion of gasses
* volatiles x O2
* Smoldering / glowing combustion: oxidation of char on the surface of the fuel
* example: smoldering glow cigar

smoking glowing cigar:

1. tightly packed = limit air flow
2. fuel = tobacco leaves
3. pilot source = lighter
4. glowing char once lit ( red glowy shit )


1. moving forward as burns = dries out and releases volatiles and smoke
2. CO2 is mainly released but so is H2O, Tar,
3. smoke = smoke particles of incomplete combustion
5. end of what was burned = mineral ash

ground fire

* most intense
* caused by extended droughts
* recovery could take years

surface fire

* cool fast moving
* fueled by surface litter
* does not affect underground shit
* opens cones, kills some seedlings

crown fire

* forest community
* caused by lightening, intense surface fire
* kills mature trees

prescribed natural fire

* intentional ignition of plants/forest fuels for specific purposes according to predetermined conditions

Fire environmental triangle:

* fuel
* weather
* topography
* fire
* all influence each other

live fuel

* receives moisture from soil

dead fuel

* more sensitive to changes in weather

fine fuel

* sensitive to weather
* flashy fuel:
* ignites easily
* moisture sensitive to weather

coarse fuel

* heavy fuels
* more heat and time to ignite
* increase energy needed to burn

topography

* slope influences intensity and rate of spread

low slope

* slow spread and low intensity
* convection is not in contact w the fuel

high slope

* fast spread and high intensity
* convection in contact w fuel
* shorter radiation path
* heat energy from combustion more efficiently arriving to unburn fuel for preignition

aspect:

* the direction of a slope is facing
* effects:
* angle and amount of solar radiation
* position relative to the wind
* higher wind speed on south facing sun, where more solar energy hits

elevation

* effects:
* temperature and humidity
* growing season = fuel availability and plant species, fuel types

weather

* state and changing nature of the atmosphere → troposphere

fire weather

* atmospheric elements that influence wildland fire

fire climate

* weather over many years

fire season

* atmospheric trends throughout the year

3 things that affect fuel moisture

* temperature
* relative humidity
* precipitation

wind

* dominate influences on fire behavior
* supplies O2
* dries fuel
* effects fires rate of spread
* fire creates wind = creates own weather

head fire

* flames move in direction of wind
* increases:
* radiation efficiency
* flame contact w unburned fuel
* convection heat transfer
* fire spread rate

backing fire:

* burning against the wind

wind is hard to predict bc variability in:

* speed
* direction
* topography vegetation
* local heating and cooling

wind measure at 20 ft height in US must be adjusted

\
* mph x adjustment = answer
* exposed fuel = 0.6 - 0.4
* partially sheltered fuels = 0.3
* fully sheltered ( open ) = 0.2
* fully sheltered ( dense ) = 0.1

point source fire

* fire from lightning strike
* cigarette
* there is a point that is hit w fire
* spread going in direction of wind = fastest
* spread going against direction of the wind = slowest
* spread going anywhere else = intermediate
* two sides going off from the fastest direction = flanks

point source fire assumption

* homogenous fuel bed

point source fire - biggest things influencing ROS

* wind direction
* fuel

prescribed burn (strip ignition)

* need perimeter around the area = mineral fire break, no fuel wihtin this space
* 1st lightning strike:
* fire burns against the wind and has a slow ROS
* 2nd lightning strike:
* the flanks of the zone, burns and goes up and out to burn edges
* 3rd lightning strike: head fire moves w wind quickly across the area, will reach edges that have already been burned = fire won’t go out of control
* fire spreads from perimeter // fire grows from burning embers transported from wind or convection column

flame residency time

* how long a unit undergoes flaming combustion

2kg log ( coarse fuel ) vs 2kg praire grass ( fine fuel )

* both release the same amount of heat bc they contain the same amount of carbon bonds
* different rates of spread
* grass burns more intensely = release all energy at once

hardwood litter:

* low rate of spread, low heat per area
* not a lot of fuel = not fast moving = low intensity

short grass

* high rate of spread, low heat per area
* not a lot of fuel but burns fast

slash

* waste left in logging forest after logs are removed
* low rate of spread, high heat per area bc coarse fuel = lots of heat released over time

chaparrel

* S. california, mediterranean climate
* cool wet winters, dry hot summers, dense shrublands, not so coarse, high fuel load

rate of spread R =  I(r)ξ( 1 + Øw + Øs ) / Pb x દ x Qig assumptions

* homogeneous fuel bed
* constant wind speed
* constant slope

rate of spread = intensity / heat sink

* = energy received by unburned fuel ( propagating flux ) / energy required to ignite the unburned fuel ( heat sink )
* intensity: how much heat and how quickly it reaches unburned fuel
* heat sink: energy you need to keep fire going
* incorporates fuel moisture and fuel size packing

I ( R) =

* reaction intensity - theoretical maximum
* intensity is based on
* fuel load, moisture content, mineral content
* surface area, volume ratio of fuel
* packing of fuel particle

wildland fuels triangle

* fuel


* weather
* topography
* fuel
* heat
* O2

Fuel in Wildland fuel triangle:

* affects:
* ease of ignition
* fire size
* fire intensity

plant biomass

quantity of living plant material mass per area/volume

phytobiomass

* plant material above mineral soil

total fuel =

* total phytobiomass

potential fuel:

* material that might burn in the most intense fire possible < total fuel

available fuel:

* fuel that is available to burn in a particular fire

total fuel at a location is governed by

* establish and growth of plants
* longevity / turnover time of plants and or plant parts
* rate and extent of decomposition

the growth of plants, turnover time, and decomposition rate are controled by

* plants available to occupy a site


* physical environment ( temp, pecip, light, soils )
* biological interactions ( competition, herbivory, decomposer community )
* time since establishment / disturbance

Fuel is not

static

fuel properties

* fuel state
* size and shape
* quantity
* compactness
* arrangement

fuel state:

* alive vs dead
* moisture content
* state / stage of decomposition

size and shape determines…

* ease of ignition ( smaller easier than large )
* rates of drying / wetting
* duration of flaming and smoldering

quantity

* fuel load

compactness

* spacing between fuel particles
* INC compactness DEC spread rate = loosely packed fuel reacts faster
* bulk density = dry fuel weight / volume

arrangement

* orientation and continuity of fuel through horizontal and vertical space
* crown fuels
* ladder fuels
* surface fuels
* ground fuels

Crown fuels

* aerial fuels
* overstory fuels
* large shrubs

ladder fuels:

* vines
* midsize trees
* needle draps
* provide vertical continuity from surface to crown

surface fuels:

* vegetation and litter on the ground

ground fuels:

* fuels in the ground
* duff
* leaf litter
* roots
* peat
* buried logs

fuel bed is characterized by

* structure
* continuity and composition of phytobiomass

different time scales for fuel changs

* abrupt
* diurnal: every 24 hours
* INC temp. = DEC RH
* seasonal
* annual

sucessional:

* changes in vegetation type and structure over long timescales

ecological sucession:

* process by which natural communities replace one another over time = predictable
* each community creates conditions that allow the next commnuity to establish and thrive

pioneer species:

* first ones
* bare rock
* lichen
* small annual plants

intermediate species

* grasses perennials / shade tolerant ( some )

climax community

* shade tolerant trees and understory

primary sucession

* starts on new space

climax community:

* hypothetical end point of succession when the community is in equilibrium with climate

secondary succession:

* established community is impacted by a discrete event = disturbance

disturbances

* can act on living and dead fuel to change the magnitude, trend, and direction of fuel accumulation in space and time

what is a major heat sink source?

* water
* where most of the energy goes in preignition

specific heat

* energy required to raise 1g of material 1º C
* wood: 1-2 - 2.5 J/gº C
* water: 4.0 J/gº C
* heat of vaporization of water = 2260 J/G
* energy to change 100ºC liquid water to 100º water vapor

fuel moisture %

100x amount of water / dry weight (is this right??)

dead fuel moisture

* 2 - 30 % range
* mix of past and present weather conditions
* all moisture exchange happens at the surface on the fuel

time lag:

* how quickly a fuels moisture content changes toward a new equilibrium

equilibrium moisture content (EMC):

* value that fuel moisture approaches if exposed to constant atmospheric humidity and temperature for an indefinite time

Live fuel moisture

* moisture of living vegetation
* typical range is 35 - 300% for the living fuel moisture

live fuel moisture equation

weight of H2O / dry weight fuel x 100 = fuel moisture %

live fuels contribute to

1. serve as a major heat sink ( DEC FIRE SPREAD / DEC INTENSITY )
2. The amount of energy released by fire

factors that affect live fuel moisture:

* state of vegetation
* weather and topography influence temp and Rh