Guillermo Rein

Rory Hadden

School of Engineering

University of Edinburgh

From organic matter

to pyrogenic char to ash:

the role of smouldering combustion

Claudio Zaccone

Agro-Environmental Sciences

University of Foggia

Context

� Smouldering combustion of natural organic soils

like peatlands leads to the largest fires on Earth

� Poses a positive feedback mechanism to climate

change

1. Fate of organic matter during smouldering

2. A missing mechanism in biochar

stability/degradation

National Geographic 2008/ AP Photo/MODIS

2008 - The Evans Road fire, NC - burned for 7 months

100 km

� During worst drought on record

� 16,500 ha burned (2x annual avg.)

� 1 m deep into the soil

� Stopped by flooding and excavation

This plume is no larger than those from flaming fires in California, Greece, Australia,

etc, but it remained active for ~30 times longer than any flaming fire

Smouldering wildlfires propagate slowly during several months consuming

organic matter and threatening to release ancient carbon stored deep in the soil

Burning of natural organic soils

©

Organic soils, Mali

Moscow fires the summer of 2010Organic soils, Italy

Rothiemurchus, Scotland

Feedback Mechanism in the Earth System

topics I work onPermafrost thaw are already resulting inlarge smouldering artic fires (e.g., Alaska 2010).

320

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Mega-fires

The oldest continuously burning

fire on Earth is a smouldering coal

seam in Australia ignited >6,000

years old

In-depth spread over thick

peat layers consumes biomass

in the order of 100 kg/m^2,

this is 50 to 100 times larger

than flaming fires

Recent figures at the global scale

estimate average greenhouse gas

emissions from smouldering peat

is equivalent to >15% of man-

made emissions

smouldering

fire

Smouldering Combustion

� Flameless

� Low peak temperature ~600°C

� Low heat of combustion ~5 kJ/g

� Creeping propagation ~1 mm/min

� Incomplete combustion

� Heterogeneous combustion on fuel

surface (pores)

� Fuels: peat, coal, duff, organic soils

� Two-step combustion reaction:

R Hadden, UoE

OHCOCOash heat OChar 222 ++++→+

( )∑+→ ,PAHCHvolatiles charPeatheat

4,

video speeded up 600 times

1 s video = 10 min experiment

Smouldering spread 30x30 cm tray setup with 5 cm layer of peat

Top view, Visual camera Top view, Infrared camera

igniter

time

igniter

lS

tSresidual layer

of char and ash

0hundisturbed peat

leading edge

trailing edge

fate of organic matter

dSin-depth

Hadden et al, Proceedings of the Combustion Institute, 2012

Char is simultaneously product and

reactant in pyrolysis and oxidation

reactions, which initially results

in net char production and later

become net char consumption.

Carbon Balance

Ash

Hadden et al, Proceedings of the Combustion Institute, 2012

•Carbon fraction in char is ~1.5 times higher than peat

•Carbon fraction of ash is ~20 times lower than peat

• Carbon gaseous emissions mostly as CO2 and CO, but also CH4 and PAH

Depth from front (mm)

Zaccone et al., EGU2012-4795, SSS10.2 Poster XY648

The chemical analysis of the solid residue shows it is a mixture of ash and char with a strong increases of pH, higher C/H and lower C/N ratios relative to the virgin peat

Chemical Analysis of smouldering residue

Location of last combustion front in peat

Combustion Dynamics:

As the intensity of the fire increases (proxy via increasing oxygen concentration), the fraction of residual char rapidly decreases to zero.

Biochar degradation via fire1. Wildfires

2. Self-heating = Accidental Release

Self-heating refers to the tendency of certain reactive solids in oxidative

atmospheres to spontaneous exothermic reactions at low or ambient

temperatures. Initially, small amounts of heat are released and accumulate

during longer times when heat losses are low. This results in a sustained

increase of temperature and leads to thermal run away, and smouldering fire.

Stockpile

Temperature

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at

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generation

heatingSelf

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321 &43421 −

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Conclusions

� Smouldering are the largest and longest fires on Earth –100 times higher fuel consumption than flaming fires

� Consume organic matter and threat to release ancient carbon stored deep in the soil

� Smouldering combustion involves the simultaneous production and consumption of pyrogenic char (at different rates)

� Topic of global interest linked to ecosystem perturbation, carbon sequestration and climate change

� Smouldering illuminates the role of wildfires in pyrogenic char

� Fire is the fastest biochar degradation mechanism, not considered in the literature (?)

El Mundo

ThanksThanks

Belcher et al, PNAS 2011

Rein et al, Catena 2008

Hadden, PhD Thesis 2011

Rein et al., Proc Combustion Institute 2009

Rein, Int Review Chemical Engineering 2009

Zaccone et al., EGU2012-4795, SSS10.2 Poster XY648

Very long-term sequestration of solid carbonIn order to minimize and avoid the risk of an accidental release

associated with biochar storage, a new type of large facilities

for stable and very-long-term storage should be built.

Facilitates can be designed to store on surface, marine or

mining sites. The methods for designing these facilities would

use technological concepts borrowed from infrastructure fire

protection.

These facilities should be self-sufficient and passive (no input

of energy required). Technologies for designing these facilities

would drawn concepts from:

Stockpile size: As the size of the pile is made smaller, heat losses increase and

the risk of self-heating is reduced. The maximum safe stockpile size is given by

the ambient temperature.

Ventilation: Design features enhancing natural ventilation and cooling

Sealing: Storage in sealed compartments with low O2 (or inert) atmospheres. It

is known that smouldering fires cannot propagate at [O2]<17%.

Wetting: Material with large water contents (>125% dry base) do not ignite.

Inertation: Reducing reactivity by mixing biochar with inert material

New Concept: Inertationto reduce reactivity by mixing biochar with inert material, has been proven

for the first time in a series of experiments on samples of char/sand

(ratios 1 to 3).

The reactivity of the mixtures was measured for different basket sizes and

oven temperatures. Kamenetski’s theory allows then to obtain the

maximum safe stockpile sizes for different temperatures.

Inertation allows for 200 to 600% larger stockpile sizes, even for hot

countries.

Maximum safe stockpile sizes for the range of ambient temperatures found in hot to mild climates

Geo-Engineering facilities for biochar storage

The Royal Society defined in 2009 Geo-Engineering as:

“deliberate large-scale intervention in the Earth’s climate system, in order to moderate global warming”

The author thinks that designing and building facilities for very long-term sequestration of solid carbon would represent a engineering task at the Earth-scale and thus a Geo-Engineering topic, but with a lower level of geo-intervention compared to other proposal put forward.