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The transport of hydrogen chloride (HCl) gas in air was studied in a simulated heating, ventilating and air conditioning (HVAC) system. The test system was a tube of square cross section, 0.305 m (1 ft) on each side, with a total length of ca. 120 m. The sides of the apparatus were built of painted and unpainted gypsum wallboard, and poly(methyl methacrylate) (PMMA), ar ranged in two different configurations. All four tests were run at normal room temperature. Hydrogen chloride gas was injected into the air at the entrance to yield initial HCl concentrations of ca. 3,000-4,000 ppm (vol). The experiments were typically conducted for 30 min. Gaseous HCl concentrations were mea sured at 9 locations along the conduit, and the results were compared with pre dictions from an existing model for HCl transport and decay from fire at mospheres. The model was used in a purely predictive mode, i.e., unchanged, since the flow dynamics are well defined in this scenario and since HCl decay parameters for each type of surface used had been developed for the model in previous studies.

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.

The decay of HCI was investigated in two small-scale scenarios: pure HCI injection into a 3 I chamber and combustion of plasticized PVC in a 2001 chamber. The effects investigated included (1) humidity, (2) temperature, (3) concentration of HCI and (4) wall material. Surface materials studied were PMMA, ceiling tile (front and back), Marinite, painted PMMA, unpainted gypsum board and cement. In view of the very rapid HCI decay in most of those surfaces, the effects were often examined with small 'chips' of materials in a PMMA chamber, with fresh walls for each experiment. Experiments were also carried out to investigate the effect of surface ageing, with painted gypsum board, painted PMMA and unpainted gypsum board walls. HCI decay is very fast in cement or unpainted gypsum board surfaces (almost impossible to saturate with HCI) and almost as fast on ceiling tile and Marinite. Saturation of HCI can be reached on painted gypsum board and painted PMMA surfaces, albeit at different rates. An earlier empirical model from mathematical fitting had been followed by a new HCI generation, transport and decay model, with a sound physical basis. This allowed calculations of parameters for all the surfaces used. Much work has already been done in devising and writing a zone model for use together with fire hazard models (particularly the NIST model, FAST) to calculate correct HCL concentrations in various fire scenarios. This work, which concludes the investigation of these two static fire scenarios for the surfaces analysed, represents one more step in the pursuit of that goal.

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