CFD modelling of HCN formation in fires

Of the toxic gases generated by fire carbon monoxide (CO) has been assumed to be the greatest killer of people in connection to fires.

Increasingly one has realised that other more toxic gases, which are also produced in fires, may be responsible for an unknown percentage of fire deaths due to smoke inhalation. One of the most toxic gases in this context is hydrogen cyanide (HCN), which is extremely dangerous for humans to inhale. An initial study has been completed where the relative importance of HCN to CO in fire gases using burning material that are common in domestic applications has been reported previously in SP report 2000:27.

Model fire behaviour

Fire test in large scale are very expensive and it is practically impossible to make detailed measurements of toxic gases, such as HCN, at many points in order to estimate the pattern and flow of these species. Therefore, in recent years it has become more common to use CFD methods to model fire behaviour in complicated situations as an alternative to time consuming and costly experiments.

Computer models

In recognition of the importance of CO, computer models have been previously developed to calculate the production of CO (reported in SP report 1993:03). The results of the initial study show clearly that under certain conditions the production of HCN is significant and this can represent an important compounding issue in fire fatalities. Thus, it is important to include the production of both HCN and CO (and possibly other species) in computer modelling of fire development and fire gas spread.

Formation of HCN

The model has now been expanded to include also the formation of HCN. The calculation of the production rate of HCN is modelled using detailed chemical kinetics. The CFD programs then calculate the spread of HCN with fire-induced flows in multi-room buildings. The link between chemical kinetics and CFD is so called laminar flamelet model. The model has been made in collaboration between Lund University of Technology and SP Fire Technology under the auspices of the Swedish Fire Research Board (see SP Report 2003:10).

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