Combustion Limits in Smouldering Peat Wildfires

8/10/2019 Combustion Limits in Smouldering Peat Wildfires

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Xinyan Huang 35th International Symposium on Combustion San Francisco, 5 August 2014

Xinyan Huang and Guillermo Rein

Department of Mechanical Engineering

Imperial College London

Haixiang Chen

State Key Laboratory of Fire Science

University of Science and Technology of China


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May 2013, Singapore

June 2013, SingaporeHaze

Pollutant Standards Index

(PSI) hit 401 (highest in

Singapore's history).

Human health issues and

traffic delays

Diplomatic strains are raised

among Indonesia, Singapore,

and Malaysia.


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Smoldering combustion[2]

Charring materials

Flameless, Low temperature (~600)Heterogeneous reactionincomplete

(gas + condensed phases)

Low heat of combustion (~ 10 MJ/kg) Creeping spread: ~O(1) mm/min Easy to ignite

Difficult to suppress Supply of oxygen

Heat loss

[2] T. Ohlemiller, Prog. Energy & Combust. Sci. 1985.

[3, 4] Frandsen, Can. J. For. Res. 1987, 1997

Primary factors for peat fire [3,4]:

o Moisture content (MC)

o

Inorganic content (IC)Other secondary factors:

o Peat density, porosity, thermal

conductivity et al.


7/19Xinyan Huang 35th International Symposium on Combustion San Francisco, 5 August 2014

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Small Modified Soil Samples

Dry organic peat,

0% 0% 110 kg/m3

Dry clay soil, 0%

100%Water,

Small modified soil sample [3]

+ +< 100%

Insolation

Coil heater x 3 min

Ignition box I

4 cm

Ignition No ignition

No ignition

moldering threshold

MC 1.1 1.35IC

Spread

direction

( I-D)

[3] Frandsen, Can. J. For. Res. 1987.



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Small Natural Soil Samples

Natural samples from multiple

field sites (North American)

With Natural MC, Natural IC [4]

Spread direction( I-D)

Insolation

Coil heaterx 3 min

Ignition box II

10 cm

5 cm

Dry peat

(1 cm)

ignition

[4] Frandsen, Can. J. For. Res. 1997.



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At 0 (top surface)

30 kW/m2 for first 3 min (as experiment), 10 W/m2-K, , 10 kg/m2-s At (top surface), 3 W/m2-K, , 0 kg/m2-s

Modelling assumptions:

1-D spread

No natural convection (small sample) Volume = Volume of Organic peat soil

+ Volume of clay soil

Water stays in the pores

Sample surface regresses

( )

=

,

=

( )

( )

Equations are solved in academic code Gpyro [5]

[5] Lautenberger,Fire Safety J., 2009.



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For fire simulation, number of parameters increases with model complexity whilelarge uncertainty involves in parameter selection

Balance between model

complexity and accuracy [6].

Challenges

Large variation on both chemical and

physical properties on solid species.

Measurements from experiments usuallyare insufficient for model input.

Bulk density (expansion during mixing):

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Effective conductivity (incl. radiation):

,1 ~ 104~10m

Permeability

~ 2 102~109m2 Averaged properties in each cell are

calculated by weighting appropriate

mass or volume fractions.

[6] Bal & Rein, Fire Safety J., 2014.



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=

, gas ,

=

, gas

where , is the stoichiometric coefficient.(, , ) (,)/ ,

,

5-Step global reaction scheme

1) Drying (dr):

Peat vdr,wH2O Peat v,wH2O(g)2) Peat pyrolysis (pp):

Peat ,pp-Char,pp Gas3) Peat oxidation (po):

Peat + ,poO2 ,pp-Char,pp Gas4) -char oxidation (o):

-Char + ,oO2 ,oAsh ,o Gas5) -char oxidation (o):

-Char +

,2 ,Ash

,Gas

Thermogravimetric analysis (TGA) [7]

[7] Huang and Rein, Combust. Flame 2014

Inverse modelling



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Base case

o

MC = 30%, IC = 40%o Kinetic parameters of Scottish peat

(a highly organic peat: IC < 2 %)

o Ignition: 30 kW/m2 x 3 min

The temperature profile first

decreases after ignition, and thanincreases in char-oxidation stage.

Three distinct fronts: drying, peat

pyrolysis, and char oxidation, are

observed, agreeing with previous plug-

flow model [6].

Drying front increases with time.

Peat oxidation is negligible in the in-

depth spread while peat pyrolysis is

dominant.

1 s = 1 min



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The surface regression is successfullysimulated, as observed in experiment

The mass-loss rate drops after

ignition, and tends to constant in

char-oxidation stage

Combustion duration (~1 h), similar to

the experiment

Mass of char reaches the peak when

the peat is just consumed

Base case

o MC = 30%, IC = 40%

o Kinetic parameters of Scottish peat(a highly organic peat: IC < 2 %)

o Ignition: 30 kW/m2 x 3 min



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Chemical Influence

Smoldering threshold is defined by the critical moisture and inorganic contents

(MCc & ICc), when most of the organic matter can be consumed.

The smoldering threshold of base

case across the experimental

data scatters

Threshold curve is nonlinear inthe whole range

Kinetic parameters of another 3

peat types are simulated

The decomposition chemistry can

be important in smoldering

ignition



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Ignition Protocol

Dry-peat ignition(~ 55 kW/m2)

30 kW/m2

(base case)

20 kW/m2

15 kW/m2

Minimum heatflux: 9 kW/m2

For high organic soils (IC < 30% i.e. peat), increasing the ignition powerextends the critical moisture content, no partial burn MCc,i for ignition.

For low organic soils (IC > 30%), critical moisture becomes insensitive toignition power, partial burn occurs MCc,x for extinction.

High organic soil Low organic soil



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Physical Properties

Three key thermophysical

parameters, peat bulk density,thermal conductivity, and heat

of combustion are studied:

Varied ranges

p 90 -130 kg/m3 kp = 0.8 - 1.2 W/m-K C

o = 15 - 25 MJ/kg

The predicted smoldering

thresholds covers most of

experimental data

Smoldering threshold increases with heat of combustion, while

decreases with peat bulk density and thermal conductivity,

agreeing with other experiments in literature [8].

[8] R. Hartford, Proc. 10th Conf. Fire Forest Meteorol. 1989.



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A comprehensive 1-D model for smoldering combustion of peat

is established with complex kinetic schemes.

The model successfully simulates Frandsens experiments in

1987&1997, and the detailed smoldering process is predicted.

Smoldering-threshold curve is predicted to be nonlinear,

implying the inaccuracy of a linear extrapolation fromexperiment.

The smoldering threshold can be appreciably influenced by

both chemical and thermophysical properties.

For high organic peat soils, the critical MC is found to be thecritical MC of ignition. While for low organic soils, it becomes

the critical MC of extinction.



This work was supported by the Department of Mechanical

Engineering at Imperial College and Santander Overseas

Research Scholarship.

Professor Naian Liu (USTC) for hosting the visit to SKLFS.

Professor Forman Williams (UC San Diego) and Dr Jian Gao

(Toyohashi Univ. Tech.) for valuable comments.

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19/19Xi H 35th I i l S i C b i S F i 5 A 2014

Xinyan Huang and Guillermo Rein

Department of Mechanical Engineering

Imperial College London

Haixiang Chen

State Key Laboratory of Fire Science

University of Science and Technology of China

Documents

Combustion Limits in Smouldering Peat Wildfires