Despite its ubiquity in
human life, chemists have still barely unlocked what’s happening amid the
flames. Kit Chapman reports
By
Kip Chapman for Chemistry World [a
publication of the Royal Society of Chemistry]
July
20, 2020 -- On 14 June 2017, the UK woke to images of a black vortex of smoke
above a raging inferno. In west London, the 24-storey Grenfell Tower was on
fire, the external cladding causing a stack effect – similar to a chimney –
spreading its destruction throughout the building. It was the UK’s worst
residential fire since the second world war, causing 72 deaths as residents
became trapped on the upper floors. Fire scientist Claire Benson remembers
watching the news that morning in horror. ‘I woke up that morning and I was
furious,’ she recalls. ‘Because it shouldn’t have happened.’
Grenfell,
along with other major blazes such as the destruction of Notre Dame Cathedral
in France and the 2019/20 wildfires in the US and Australia, has shown how much
work still needs to be done to understand how to prevent and control fires.
Despite being one of the most familiar phenomena in most people’s lives, it’s
surprising how little we truly know about how and why things burn.
Well Worth the Candle
Although
it’s more than 170 years since Michael Faraday’s lectures on the chemical
history of a candle, we still struggle with a complete understanding of fire. A
candle remains, however, a perfect place to start. This is a diffusion flame,
where burning happens only at the interface between fuel and air – a region
called the reaction zone, which is about 200µg deep. The interior of the flame
itself is a completely oxygen-free environment.
But
even this can be overly simple, explains Ludovico Cademartiri, an associate
professor at the University of Parma, Italy. ‘If you spoke to someone in the
street and said “What do you need for fire?” They’d say you need air, you need
a fuel, you need combustion. That’s it. It’s not wrong, but it gives a false
impression of simplicity. Combustion is the prototypical complex system,’
Cademartiri says. ‘The simplest hydrocarbon combustion – methane and oxygen –
produces hundreds of different intermediates and byproducts through hundreds of
different chemical reactions occurring at different rates. Those
rates depend on temperature, which changes dramatically across a flame. Among
the byproducts is soot, which is a solid with a sizeable heat capacity, so will
get heated up and glow. For most people it’s hard to grasp how much complexity
can hide beneath the reaction of just two reagents.’
This
complexity is why fire remains so hard to grasp. While completing his
postdoctoral work at Harvard University in the US, Cademartiri worked with
fellow postdocs in George Whitesides’ lab and the US Defence Advanced Research
Projects Agency to develop flame suppressants. At one point he asked one of the
US Navy’s combustion experts when, on a chemical level, a fire is extinguished.
‘I asked if we can really determine the causal chain of events that lead to a
specific flame going out. He said no.’
Between
2008–2011 the group’s task was to find alternative means to control fires in
enclosed environments, such as the interior of an armoured car, where water
damage can be just as devastating. Whitesides’ team focused on tactics to
extinguish fires that wouldn’t require suppressants – and succeeded. ‘The
combustion of hydrocarbons produces ions and electrons, which become part of the
composition of the flame itself. So, effectively, part of a flame is a dilute
plasma,’ Cademartiri says. By creating an oscillating, highly concentrated
electric field, the team were able to deflect and extinguish flames about
a metre tall. The idea proved so popular it was even used by DC Comics, with
new Superwoman Lana Lang manipulating electricity to snuff out a fire and
defeat Lex Luthor. Unfortunately, in the real world the gradient of the
electrical field required meant it couldn’t be scaled up beyond flames with a
base of a few centimetres.
The
team were also able to use sound waves to suffocate the flame’s reaction zone
by hindering convention of air to the flame, Cademartiri says. ‘But we needed
very low frequencies, between 55Hz and 60Hz, at about 130 decibels. On the
other hand, it could scale much better than the electric field. We thought
about using it for something like a wildfire, just trying to contain it. But we
were never able to do the feasibility study.’
Where the Wildfires Are
Wildfires
are also diffusion flames and present an immense challenge worldwide. In 2019,
the US National Interagency Fire Center reported 50,477 wildfires in the US,
destroying more than 18,600km2 of land – a five-year low – with
annual suppression costs totalling around $3 billion (£2.4 billion). It’s a
level of devastation that atmospheric chemist Krystal Vasquez has witnessed
first-hand. A PhD candidate at the California Institute of Technology, US, in
2019 she found herself among a team of other scientists and their instruments
in the hull of a converted Douglas DC-8. They were flying out of Boise, Idaho
to sample the smoke plumes of conflagrations raging across the Pacific
Northwest. During an undergraduate internship, Vasquez took part in atmospheric
sampling missions across central California and fell in love with flying
fieldwork. When she found out there was a seat on a plane assessing wildfires
for her PhD, she leapt at the chance to come aboard.
‘There
was one fire that we went to that was toward the evening and you could see the
glow of the flames and all of the smoke.’ Vasquez recalls. ‘It was pretty wild.
The plane is doing zigzags across the plume, and you have to time everything so
you can get that one or two seconds when you’re actually in the plume itself.
Then you have the excitement of the cabin filling up with smoke, which is
frightening. And you have the convection for the heat of the fire, so the plane
is turbulent and bouncing. It’s like a fun rollercoaster… you learn to take a
lot of [motion-sickness medication] Dramamine.’
The
flight was part of the Firex-AQ mission, a joint venture between Nasa, the
National Oceanographic and Atmospheric Administration (Noaa) and more than 40
partners to assess the complex chemistry of smoke. Vasquez’s role, with another
colleague, was operating a time-of-flight ionisation mass spectrometer, which
uses a fluorinated reagent ion that clusters with her target compounds. ‘On the
flight the team measured aerosols, nitrogen oxides (NOx),
hydrocarbons,’ she says. ‘I measured oxygenated hydrocarbons – the oxidation
products that end up produced in smoke. Not to brag or anything, but they’re
really difficult to measure because they are so reactive. We had to position
our instrument at the window of the plane [which has a custom-built air inlet];
a lot of our compounds are super-sticky, such as nitric acid, so we needed to
the inlet to be extremely short or they’d get stuck on the walls of the
instrument.’
Much
of wildfire smoke remains a mystery. Although a previous Firex mission identified
the chemical mechanisms of furan-type compounds in the smoke, a large portion
of the reactions are still unknown. Isocyanic acid (HNCO) was only found to be
up to 30% of NOx in 2010, while there remains a large
quantity of unidentified semi-volatile organic compounds in fires that impact
modelling. And, even when they have been identified, how they work is not
understood. For example, the NOx and volatile organic
compounds emitted by the plumes undergo photooxidation but only sometimes
produce ozone. This seems to depend on the precursors in the fire, the speed at
which the plume cools and how efficiently the NOx is
converted into products such as peroxyacetyl nitrate.
‘Wildfires
can produce ozone and all these toxic gases,’ Vasquez explains, ‘but the
compounds they produce is very dependent on the characteristics of the fire and
the relative humidity. The main motivation [of Firex] is to understand the
chemistry and composition of smoke plumes, so we can create models that
forecast how fires impact air quality and maybe give first responders an idea
of how fire might move. We also look at public health and land management,
because a lot of farms end up using fire to just clear off the fields before
next season. Being able to know what weather conditions are the most
appropriate to do that in is important for air quality in the region.’
Dust Devils
While
blazes in the wilderness can produce a host of unpleasant gases, an entirely
different airborne cocktail can be created in city fires. In Paris, France,
restoration scientists from the Historical Monuments Research Laboratory are
working in the gutted ruins of Notre Dame de Paris, the historic cathedral
where a fire on 15 April 2019 shocked the world. The French government has
since pledged to reopen the cathedral within five years, and researchers have
pored over its exposed remains to understand historical processes. But from a
health perspective, an even greater tragedy has unfolded than the lost art – as
the strange yellow tinge to the fire’s smoke revealed.
During
the inferno, the hundreds of tonnes of lead that lined the cathedral’s roof
melted. It was on a scale never seen before – so much so that Parisian police
warned people to wipe their homes and premises to avoid lead poisoning. It
wasn’t until September that an investigation by the New York Times revealed
the dust deposited around the cathedral was up to 1300 times higher than safety
guidelines, with its spread covering much of central Paris.
Such
toxic exposure happens every day around the world, albeit on a much smaller
scale. Jennifer Keir is an environmental chemist and toxicologist at the
University of Ottawa, Canada, who studies exposure of firefighters to heavy
metals. The majority of studies have looked at firefighter exposure during
training scenarios, Keir explains – but a burning wood pallet in a training
house doesn’t reflect the variety of contaminants in a real fire. ‘We wanted to
capture what they were being exposed to while on the job during emergencies,’
Keir says. ‘So we collaborated with the Ottawa fire services and looked at
urine samples, wipes from their skin and air samples. We found they were
significantly exposed. In terms of urinary metabolites, we saw huge spikes
after firefighters attended a fire.’ Keir’s samples detected concentrations of
metals such as antimony and lead on skin under the firefighters’ protective
equipment, as well as increased levels of polycyclic aromatic hydrocarbons.
‘Some of these bioaccumulate,’ she says. ‘You’ll excrete a lot of them within
days, but even when you’re excreting them, they’re still doing damage.’ Not all
of the dangers are from the fire, Keir says. Often, protective equipment and
firefighting foams include perfluoroalkyl and polyfluoroalkyl substances
(PFAS), a family of fluorinated compounds used as flame retardants and
surfactants but known to be detrimental to health. ‘Somehow that seems to
be getting into firefighters as well,’ she notes.
Firefighters Are Significantly Exposed
to Contaminants
Understanding
exposure is only half of Keir’s work. As firefighters can’t prevent exposure –
air flow is needed to keep cool and prevent them overheating during a call-out
– she focuses on post-fire decontamination. ‘There’s this movement in the fire
service that you immediately wipe down after a fire, because we know about skin
exposure. There have been companies promoting wipes to clean skin before
firefighters get back to the truck, before they can get back to the station and
shower. But there’s been no real scientific studies to look at this, and
whether those decontamination methods actually remove a significant amount of
the hazards they are exposed to.’ Keir’s work is currently on hold due to the
Covid-19 pandemic, but once complete will help protect firefighters when fires
do break out.
Razing the Standard
Perhaps
the greatest role for fire scientists is preventing blazes to begin with.
Chemists have been involved in fire safety throughout history – from Faraday’s
mentor Humphry Davy’s role in safety lamps through to the creation of
americium-241, an isotope that does not exist naturally on Earth, for use in
smoke detectors. It’s this drive for safety that left Benson so aghast as she
saw the Grenfell disaster unfold. A senior lecturer at London South Bank
University, Benson’s primary interest is fire in low pressure environments, but
she has also worked with the London Fire Brigade and has an interest in the
chemistry of materials used for buildings. The problem is that safety standards
are just as complex as fire itself.
Perhaps
the greatest role for fire scientists is preventing blazes to begin with.
Chemists have been involved in fire safety throughout history – from Faraday’s
mentor Humphry Davy’s role in safety lamps through to the creation of
americium-241, an isotope that does not exist naturally on Earth, for use in
smoke detectors. It’s this drive for safety that left Benson so aghast as she
saw the Grenfell disaster unfold. A senior lecturer at London South Bank
University, Benson’s primary interest is fire in low pressure environments, but
she has also worked with the London Fire Brigade and has an interest in the
chemistry of materials used for buildings. The problem is that safety standards
are just as complex as fire itself.
‘I
have a lot of sympathy for people who have to put standards together,’ she
explains. ‘For sofa materials, we can say it has to resist a flame of a certain
wattage for a certain period of time. We do the same for building materials.
The problem is that there are lots of different characteristics you could
choose. It’s one of those things that, while you could say something doesn’t
burn rapidly, it could still give off smoke. And even just burning itself is a
problem – are you talking about resisting ignition, that it shouldn’t ignite
until a certain point? Different materials may have a really high auto-ignition
temperature, but ignite really quickly when exposed to a naked flame.’
This
is just the first hurdle, Benson says. Materials might form carbon char layer,
caused by incomplete combustion, so that even if they do catch fire they go out
quickly. This could be safer than a material that takes longer to ignite, but
then won’t stop burning. ‘Then you’ve got flame speed across the surface of the
material. And heat release rate, when a material releases a lot of energy
quickly, which could be more dangerous in terms of onward combustion.’
The
UK Fire Protection Association’s (FPA) critique of the tests carried out on
Grenfell Tower’s cladding raised several concerns. The insulation was made from
polyisocyanurate, and was tested by creating a three storey tower. ‘At the
bottom, they created a wooden crib, a lattice of wood,’ Benson says. ‘It was
used so there was a known heat release rate, so we can extrapolate back and
it’s a nice, clean comparable test. But the point the FPA made was that it
wasn’t representative of the environment because so much of it is plastic, which
has a much higher heat release rate [than wood] … another excellent point is
that the test was done inside a very large warehouse, where there’s no air
flow. And I don’t know if you’ve ever put your hand outside a tower block
window, but the wind 20 or 30 storeys up is very different.’
The
hope is that these standards can be updated to better reflect the real world –
and prevent another disaster such as Grenfell. It’s a mission that will require
chemists and materials scientists working in partnership with fire services and
architects. And even then, we’ll have barely begun to understand the mysteries
of fire – and tackle the immensely complex scientific challenges it poses.
Kit
Chapman is a science writer based in Southampton, UK
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