Why do coal fires burn for so long?
Why do coal fires take so long to put out? How often do they happen?
Why do coal fires take so long to put out? How often do they happen? What are the impacts?
The bushfires burning across Australia during this long hot summer pose a particular risk to areas where coal is being mined.
In Morwell in Victoria, bushfires have ignited the coal seam face in an open cut mine which supplies a nearby electricity plant. Such coal fires, says University of Queensland mining safety expert Professor David Cliff, are much harder to put out than burning bushland.
"Unlike timber, coal when it gets hot has massive thermal mass which is very hard to extinguish. This Victorian brown coal is a low rank coal, geologically young, and very prone to being ignited. This is a thick-seam coal mine, 30 metres thick and very close to the surface so anything that intrudes into the mine will set fire to the seam," says Cliff.
Coal is made up of carbon (between 60 and 90 per cent — higher grades contain more carbon than lower grades), hydrogen (6 - 7 per cent), oxygen as well as very small amounts of nitrogen and sulphur.
Coal fires are created by the interaction of carbon, oxygen and heat, which in turn produces carbon monoxide, carbon dioxide, and more heat to fuel the reaction. Meanwhile, hydrogen in the coal is converted to water vapour, nitrogen to nitrogen dioxide and sulphur to sulphur dioxide.
While coal fires can be triggered by external sources of heat such as bushfires and lightning strikes, they can also start spontaneously as the coal oxidises, says Cliff.
"Spontaneous combustion is a well understood phenomenon. Coal, and particularly brown coal, is very reactive to oxygen, and will generate CO2 and that creates heat.
As the coal gets hotter it will eventually get to flame temperature and that coal will burn."
Coal fires, he says, are quite common.
"In Australia for example on average one underground coal mine has a major incident with the self-heating of coal every year, and you would find there would be a number of open cut mines that would be battling self-heating coal now."
Coal miners work to minimise interaction between air and coal to try to remove as much heat as possible and reduce the fire risk.
"Surface mines spray water to keep the seam cool. If coal is stockpiled they will often place a coating over the top to keep the air out, and design stockpiles so that wind doesn't blow too much air into them and cause the coal to oxidise."
In Victoria, says Cliff, coal is historically not stockpiled but mined by massive machines and sent straight to the power stations to burn.
"They don't like keeping it. If they do it dries out and starts reacting."
It is possible for coal to burn for many years if there is enough of it, says Cliff.
"There are a number of places known as 'burning mountains' in Australia where there are old underground coal deposits and cracks to the surface with smoke issuing from them. It will burn and continue to burn and can be very difficult to put out because the access to it is very deep."
He says there are fires both in Australia and overseas which are still burning after hundreds, sometimes thousands, of years.
One of these, at Burning Mountain in New South Wales,is a natural coal fire that has been going for at least 6000 years, burning at the rate of about one metre a year.
Cliff says coal fires can present a range of hazards.
Along with carbon dioxide and carbon monoxide, coal fires emit water vapour, soot, dust and sulphur dioxide, all of which disperse quite quickly but can be a significant problem to a local population depending on how close they are to the fire and what the weather is like at the time.
The bigger picture effect is a significant contribution to greenhouse gas emissions equivalent to the output of a coal fired power station.
Spot fires are likely when a bushfire burns into a coal seam as the fire spreads back into the bush. Last year. for example, a bushfire at Catherine Hill Bay in New South Wales was sparked by an underground coal fire.
Subsidence can occur in an underground mine but with an open cut would just be a localised collapse of a narrow band of the coal face and unlikely to be very hazardous as firefighters are unable to get that close.
Challenge for firefighters
A burning seam of coal presents a huge challenge to firefighters.
"The way to put a fire out is to cool it down and exclude the air, but the carbon dioxide and carbon monoxide fumes from such a large fire will be toxic at close range and the heat will be unimaginable so you can't get the firemen too close," says Cliff.
"You have to start with massive amounts of water from high pressure water hoses from a safe distance and once you can start to bring the temperatures down you can do other things to try to restrict the air access.
"In the case of this Victorian fire it's a matter of an awful lot of water and maybe bulldoze dirt over it if you can get access to it."
Cliff says the current fires are caused by exceptional circumstances.
"They've mined brown coal in Victoria for over a hundred years and it is generally done very safely. It is only under these extreme conditions when huge bushfires are raging close to the coal that the usual safety controls won't work."
Bushfires also ignited coal seams in the same area in 2006.
Professor David Cliff is director of the Minerals Industry Safety and health Centre at the University of Queensland. He was interviewed by Annie Hastwell.
Published 14 February 2014