The Role of Moisture Content in Heating
Vegetation to its Flash Point
entre
ire esearch
Imran Khan, John Dold
Fire Research Centre, University of Manchester, UK
Nomenclature
Summary
pyrolysate flux m
In a bushfire, vegetation is heated to a point at which its
constituents are broken down or pyrolysed to produce a
flammable vapour.
heat flux J
G
F
W
V
C
A
φ
T
ω
ρ
β
J
τ
R
M
P
MC
a
If the concentration of this vapour becomes high enough to
exceed its lean flammability limit then any nearby flame is able to
initiate a piloted ignition of a flame around the vegetation; in
essence, the vegetation reaches a form of flash point.
surface
temperature T
Samples of vegetation are considered to be subjected to a uniform
constant heat-flux at their surface. The model also includes the
vapourisation of moisture within the vegetation. Emerging fuel
and water vapour are considered to mix by diffusion with the
surrounding air.
pyrolysate vapour generated by
heating a sample of vegetation
The dimension, radius and moisture content all affect the
progress towards the flash point, with moisture content having a
marked effect.
Model
dry vegetation
fuel vapour
absorbed water
water vapour
char
air
porosity
temperature
reaction rate
overall density
thermal sensitivity of pyrolysis
heat flux
time scale
universal gas constant
mean molecular mass
pressure
moisture content
Numerical Profiles for G, F, W, V, C and A
Geometry takes the form of either a solid slab of thickness 2a (e.g. leaf) or a cylinder
of radius a (e.g. stalk of grass).
slab
cylinder
1
The sample is subjected to a uniform heat flux J and radiant heat loss.
1
t = 18.6 s
MC = 0%
0
0
1
1
Charring (slow pyrolysis) and fast pyrolysis are modelled as one-step reactions.
MC = 10%
0
t = 36.7 s
1 mm
1
Governing equations
0
30 cm
t = 47.3 s
1 mm
Conservation equations for mass, energy and species.
1
The ideal gas law is used for gas components with pressure held constant.
0
MC = 0%
10 cm
5 mm
MC = 10%
Charring:
Vapourisation:
G → C + V : ωC =
W→V
:
−1
ωV = τW ρW
βF (1 − Tc/T )
βC (1 − Tc/T )
t = 31.9 s
0
5 mm
MC = 10%
t = 132.0 s
0
0
5 mm
55.5 cm
10 cm
t = 54.3 s
50 cm
5 mm
100 cm
1
MC = 20%
t = 206.3 s
0
ρG τc−1 exp
:
20 cm
1
1
MC = 20%
ωF =
G→F
1 mm
t = 43.1 s
0
5 mm
Fast pyrolysi:
0
1
1
t = 19.1 s
MC = 20%
50 cm
16.4 cm
MC = 0%
ρG τc−1 exp
10 cm
1 mm
1
0
Pyrolysis and water vaporisation rates
t = 15.4 s
MC = 10%
5.3 cm
MC = 20%
Saturated vapour pressure PVb for water in vegetation determines the vapourisation
rate ωV .
5 cm
1 mm
5 cm
1 mm
Model takes moisture into account and its vapourisation to water vapour.
t = 11.1 s
MC = 0%
vegetation
water
100 cm
char
200 cm
fuel vapour
t = 78.4 s
50 cm
5 mm
water vapour
air
Results are shown for a=1mm and 5mm slabs and cylinders
heated at 20kW/m2 for different moisture contents.
RT ρV 1−
MV φPV b
◦
where τc ≈ 30 min., Tc ≈ 290 C, βF ≈ 52, βC ≈ 32 and τW ≈
1
10
s.
Conclusions
The fast pyrolysis reaction is dominant over the charring reaction at high temperatures
(typically above 300◦C). Water vapourises mainly around its boiling point.
For the slab, moisture delays the flash point time and pushes its
location away from the surface of the slab.
Resulting Mass Fluxes and Time/Position at Flash Point
For the cylinder the location of the flash point remains at the surface
of the cylinder for low moisture contents.
0%
30%
60%
Increasing the radius of the sample delays the time to reach the flash
point.
: fuel
: water
20
mass flux
A 2mm slab
heated at
20kW/m2.
In nature vegetation would rarely be represented by isolated cylinders
or slabs so that gas and vapour mixing from multiple sources would
act together.
Mixing would also be more complex, involving air-flow and
turbulence.
0
60
time (s)
120
Water vapour is produced even for zero initial moisture content
through the charring reaction.
Fuel vapourisation is delayed for increased moisture content.
150
Time and position when a
flammable mixture is produced
(2mm slab at 20kW/m2).
position (cm)
time (s)
0
moisture content (%)
60
A minimum amount of moisture content is required to drive the location of the flash
point away from the sample.
The Role of Moisture Content in Heating
Vegetation to its Flash Point
References
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(3) Varhegyi, G., Jakab, E., Antal, M.J. (1994), Is the Broido-Shafizadeh model for cellulose
pyrolysis true?, Energy and Fuels 8. 1345–1352.
(4) Antal, M.J., Varhegyi, G. (1995), Cellulose Pyrolysis Kinetics: The current State of Knowledge?,
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(5) Dold, J.W. (2007), Premixed flames modelled with thermally sensitive intermidiate branching
kinetics., Combustion Theory and Modelling 11. 909–948.
(6) Law, C.K. (2006), Combustion Physics., Cambridge University Press.
(7) Broido, A., Nelson, M.A. (1975), Char yield on pyrolysis of cellulose., Combustion and Flame
24. 263–268.
Imran Khan, John Dold