Model for the Mass Transfer and Decay of Hydrogen Chloride in a Fire Scenario

It is now well established that hydrogen chloride is unusual among common fire gases in that it decays from the atmosphere. No model of hydrogen chloride transport and decay exists as yet, which has been formulated in such a way that it is generic enough to be used for scenarios different from the one in which the experiments were carried out, and can be incorporated into more comprehensive fire hazard models. The present paper introduces such a model. It deals with the influence of various surfaces [poly(methyl methacrylate) (PMMA), painted gypsum board, ceiling tile, cement block and Marinite], of surface to volume ratio and of humidity, on atmospheric hydrogen chloride concentration, both inside and outside the room of origin of the fire.

The present model incorporates generation of hydrogen chloride from poly(vinyl chloride), mass transfer to various wall locations, partition between the atmosphere and the surface, and a combination of diffusion and reaction inside the surface. The parameters in the model were fitted by using a non-linear (Marquardt) optimization procedure.

The model was corroborated using various experiments which involved the combustion of poly(vinyl chloride) in large- and small-scale scenarios.

It was found that, for a non-sorptive surface such as PMMA, the rate of mass transfer to the surface is much larger than the rates of the various reactions at the surface, in all cases. Such a surface, thus, allows much higher peak hydrogen chloride concentrations and much lower rates of decay than any of the sorptive surfaces investigated.

For sorptive surfaces and static systems the rate limiting process is the mass transfer to the surface. The activity of the various surfaces investigated was found to follow the order: ceiling tile > cement > Marinite ≥ painted gypsum board ≫ PMMA

The significance of this model is that it can predict hydrogen chloride decay in a real fire scenario. It is relevant to point out that normal construction surfaces are sorptive, and that hydrogen chloride decay will generally be quite fast in a fire, whereas it will be much slower in a small-scale toxicity test exposure chamber.