A rare self-ignited fire

In December 2016, a pyrophoric fire broke out in a house in the Danish town of Hobro – something you only see every few years. According to a researcher in timber, we know very little about pyrophoric fires, because it is an extremely difficult subject to research.    

It was a tragic and extremely rare event that befell the Olesen family in December 2016. Their house was burned to the ground as a result of a self-igniting fire, or pyrophoric fire, to use its scientific name.

– In the burnt-out house in Hobro, we investigated all other possible causes of the fire, but in the end we had to conclude that it was a pyrophoric fire, explains DBI fire investigator Søren B. Mortensen, who investigated the cause of the fire in question.

Normally, pyrophoric fires are something you only see every few years in Denmark.

Long-time effects
Researcher in Timber, Emil Engelund Thybring from the University of Copenhagen explains that pyrophoric fires start when timber is converted into coal after having been exposed to a heat source, for example, a wood-burning stove or hot-water pipes, over an extended period of time.

– Through the effect of heat over many years, the timber dries out and the polymers break down causing chemical changes in the timber which is then converted into a kind of coal. In the presence of oxygen, the chemical reactions generate heat, which can raise the temperature. This can result in a self-perpetuating rise in temperature, causing the timber to smoulder, says Emil Engelund Thybring.

He explains that the lowest documented temperature that is able to cause this chemical change is 77 degrees. By way of comparison, the temperature of hot water in a water pipe is typically around 80 degrees.

Rarely happens
However, according to Emil Engelund Thybring, there has not been much research carried out into pyrophoric fires. The term has been in existence for more than 100 years but, back in the 1990s, many people still believed that pyrophoric fires were a myth.

– It is a difficult subject to research because it can take many years for the timber to be affected so much that there is a risk of it catching fire. It has been documented that the chemical changes in the timber can occur as a result of the timber being affected by heat for anything between three months and 15 years, and there has never been any thorough research over such a long period of time. Furthermore, pyrophoric fires require the ideal combination of oxygen and low humidity, says Emil Engelund Thybring.

A pyrophoric fire, whereby timber self-ignites, occurs very seldom. Over the last 20 years, fire investigator Søren B. Mortensen has investigated between 2,000 and 3,000 fires, and only a few times has he concluded that the cause of the fire was a pyrophoric fire. Prior to the fire in Hobro in December, the last time DBI had come across a case in which timber had self-ignited in this way was five years ago.