Prediction of toxic species in fire

Most fire deaths are associated with the remote transport of toxic products produced in hot post-flashover fires, and with carbon monoxide (CO) in particular [1, 2]. Currently, numerical tools are effective at describing the transport of these toxic products, but incapable of accurately predicting the quantities generated in a fire - thus the source is missing [2, 3]. In order to extend the scope of fire safety engineering (FSE) methods, and provide more effective tools for practitioners, there is an urgent need for robust and well-validated methodologies which address the problem in its entirety, thus completing the chain and provided a true predictive capability [3]. This would open the door to a host of new applications, including fire forensics to assist in determining causes of fatalities, supplementing expensive full-scale fires tests, and ultimately in building design, and could transform the application and exploitation of FSE methodologies.

It is essential that any such methodology can be effectively exploited by the fire community, so it must be undemanding computationally (so that it can be run on computers typically used by consultants) and must effectively accommodate the specific requirements of real-world fires, i.e. large-scale building scenarios involving a very broad range of lengthscales, and multiple and often complex fuel sources, where significant contributions to toxic products yields may arise both from complex formation processes in the gas phase and directly from the solid-phase, via pyrolysis of combustible boundary materials [2].

In this project, approaches for predicting CO production in compartment fires are explored, based on the solution of a dedicated balance equation for CO. Finite-rate chemistry was addressed via two approximations for the CO source term: (i) adopting appropriate quasi-laminar chemistry in lower turbulence regions, in conjunction with the steady laminar flamelet model (ii) constructing the CO rate flamelets and effecting closure through 'presumed probability density function (pdf)' transport method. In each cases two-step chemistry is assumed, with rate constants drawn from simplified reaction mechanisms for oxidation of hydrocarbon fuels in flames. The models are tested against relevant experimental data from underventilated compartment fire scenarios. Predictions of the rate flamelet model can differ markedly from the steady flamelet yields but accuracy is variable and strongly dependent on the generality of the kinetic scheme adopted. In determining the turbulence threshold for the hybrid model it was found that quasi-laminar chemistry overpredicts CO production when applied globally, rapidly reaching the limits of available fuel carbon, thus requiring the steady flamelet model in higher turbulence regions, though invoking the latter does reduce the flexibility.

A detailed review of alternative approaches has been provided within a new textbook on "Fire toxicity" (eds. Hull & Stec, Woodhead Publishing, 2010) and direct comparison made with the models under development in FDS5, a fire simulation tool in widespread use in industry, in the paper presented in the final dissemination at the one day conference, FIRESEAT: "Fire safety engineering in the UK, the state of the art", held at the University of Edinburgh, November 2010 (80+ attendees). The paper and presentation are available here: http://www.see.ed.ac.uk/FIRESEAT/2010.html

References

1. Babrauskas, V., Levin, B. C., Gann, R. G., Paabo, M., Harris, Jr, R. H., Peacock, R. D. & Yusa, S. (1991) "Toxic potency measurement for fire hazard analysis", Special Pub. 827, NIST, Dec 1991
2. Pitts, W.M. (1995) "The Global Equivalence Ratio concept and the formation mechanisms of carbon monoxide in enclosure fires", Prog. Energy Combust. Sci., vol. 21, pp. 197-237
3. Purser, D. & Purser, J. (2003) "The potential for including fire chemistry and toxicity in fire safety engineering", BRE Client report 202804, 26/3/03

Most fire deaths are related to the toxic combustion products generated by whatever is burning but they can often be remote from the seat of the fire, i.e. resulting from the transport of those gases through a building or structure in the smoky hot layers. A variety of toxic species are harmful to health, causing asphyxiation and irritancy, with carbon monoxide (CO) being a prime example. State-of-the-art computer simulation models have the capability of describing the movement of these toxic products, i.e. the fluid dynamics part of the problem, but remain incapable of accurately predicting the quantities generated - thus the source is missing. In order to extend the scope of fire safety engineering (FSE) methods, and provide more effective tools for practitioners, there is an urgent need for robust and well-validated methodologies which address the problem in its entirety, thus completing the chain and provided a true predictive capability. This will open the door to new applications, including fire forensics to assist in determining causes of fatalities, supporting and interpretation the results of full-scale fires tests (which are usually very expensive), and ultimately in building design, and could transform the application and exploitation of FSE methodologies.
It is essential that any such methodology can be effectively exploited by the fire community, so it must be undemanding computationally (so that it can be run on computers typically used by consultants) and must effectively accommodate the specific requirements of real-world fires, i.e. large-scale building scenarios involving a very broad range of lengthscales, and multiple and often complex fuel sources, where significant contributions to toxic products yields may arise both from complex formation processes in the gas phase and directly from the solid-phase, via pyrolysis of combustible boundary materials. In this project we developed and tested methods to achieve this, presenting the detailed outcomes at relevant conferences and publishing them in the relevant technical literature.

Novel approaches for predicting CO production in compartment fires were explored and compared. In order to be of practical benefit, the computational methods must be relatively undemanding (so that they can be run on computers typically used by practitioners) and must effectively accommodate the specific requirements of real-world fires, i.e. large-scale building scenarios involving a very broad range of geometrical scales, and multiple and often complex fuel sources, and where significant contributions to toxic products yields may arise both from complex formation processes in the gas phase and directly from the solid phase, via pyrolysis of combustible boundary materials. We therefore adopted an approach which sought to compute the carbon monoxide concentrations by solution of a single additional expression for the reaction, this being drawn from the literature on simplified reaction mechanisms for oxidation of hydrocarbon fuels in flames. In addition a more demanding method was tested derived from the "flamelet" technique, where the complex gas-phase chemistry is precomputed in advance of the fire simulation analysis. The models were tested against relevant experimental data from scenarios where fires were monitoring in oxygen-starved conditions in compartments of various sizes. Valuable insights were gained into the performance capabilities of the different approaches. The results are strongly affected by the generality of the kinetic scheme adopted. A detailed review of alternative approaches has been published within a textbook on "Fire toxicity" (eds. Hull & Stec, Woodhead Publishing, 2010).