Keywords: tank fire, pollutant dispersion, smoke plume, turbulence, computational fluid dynamics, field models.

The recent large industrial accident on 11th December 2005 at oil storage depots in Hertfordshire, England resulted in a massive fire which engulfed over 20 large fuel storage tanks. The continuous generation of smoke from these fires, presented a potential environmental and health problem that is difficult to assess. In order to try and manage it, it is important to be able to estimate the concentration of the fire-plume pollutants for a wide range of conditions.

The purpose of the present effort is to estimate the dispersion of combustion products (smoke, CO, SO₂) and to evaluate the consequences to the environment from large hydrocarbon tank fires, as well as the height of the toxic plume (plume rise). For the application of the numerical simulation of toxic contaminants and plume rise, an external floating-roof tank has been selected, with dimensions of 85 m diameter and 20 m height. The tank is surrounded by bunds of 4 m height and 0.5 m width. Numerical simulations were performed with the use of CFD techniques for a physical domain of 20000 m length, 2200 m width and 2500 m height.

The mathematical model used in the present work consists of the full differential equations that describe turbulent flow and heat and mass transport [1]. A hybrid differencing scheme was employed to descretize all equations and the solution was obtained using the iterative SIMPLEST algorithm. The modified RNG (k-epsilon) model was employed with the modifications of Markatos and Pericleous [2] for the modelling of turbulence. For all walls the non-slip condition is applied for velocities and "wall functions" [3] for the near-wall values of the dependent variables. For the vertical extrapolation of inlet velocity the logarithmic profile is assumed in the planetary boundary layer and it is then kept constant above its height.

Parametric analysis is performed for four scenarios, all for an adiabatic atmosphere: crude oil and diesel fuel with two values of wind velocity, 8 m/s and 11 m/s were examined. According to the results, the highest plume rise takes place for the scenario of diesel with the lowest wind velocity (8 m/s) and the lowest plume rise occurs in the scenario of crude oil with the highest wind velocity (11 m/s). The worst-case scenario, under these circumstances of an adiabatic atmosphere, for the ground level concentration of CO and SO₂ proved to be the scenario (2) and for smoke the scenario (4). Subsequently, hazard identification analysis is performed, according to the ground level concentration of the pollutants and their safety limits. Finally, it is worth mentioning that there are no "death zones" due to the concentrations of smoke, CO and SO₂. At the first zone the concentrations which are observed are high, especially for smoke, but do not exceed the safety limits of LC₁ or IDLH.

1

H.K. Versteeg, W. Malalasekera, "An introduction to Computational Fluid Dynamics: The Finite Volume Method", Longman Group Ltd, London, England, 1995.

2

N.C. Markatos, K.A. Pericleous, "Laminar and turbulent natural convection in an enclosed cavity", International Journal of Heat and Mass Transfer, 27, 755-772, 1984. doi:10.1016/0017-9310(84)90145-5

3

B.E. Launder, D.B. Spalding, "The numerical computation of turbulent flows", Computer Methods in Applied Mechanics and Engineering, 3, 269-289, 1974. doi:10.1016/0045-7825(74)90029-2

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