Abstract

The upgrading of the bitumen extracted from the Athabasca Oil Sands results in a substantial amount of sulfur by-product; part of which is emitted to the atmosphere as sulfur dioxide. Present regulations require that a plant be designed such that the maximum ground level concentration for SO2 be 0.17 ppm or less in a Pasquill D atmosphere. Ignored or neglected are inversion layers and other atmospheric stability categories. Yet both occur with some regularity in the Fort McMurray area. This study shows that these factors have a strong bearing on ground level concentrations in the Oil Sands area, and that the problem is compounded when plume overlap occurs.

Introduction

That portion of the bitumen which can be recovered from the Athabasca Oil Sands by strip mining techniques is presently in the early stage of industrial development. Significant amounts of sulfur are present in the raw bitumen and after refining some of the sulfur is finally emitted to the atmosphere as sulfur dioxide. Present government regulations for stack emission design are based on experience with the conventional oil and gas industry. Due to their low frequency it appears that due regard has not been given to the effect of inversion layers, proximity of different emission sources or different stability categories. The objective of this study is to show that these factors should be considered in the oil sands area and that they can significantly increase the expected ground level concentrations.

EFFECT OF INVERSION LAYERS

Inversion layers because of their temperature stratification can prevent a plume from rising to its normal level. Briggs (1969) suggests that under certain circumstances penetration can occur and the plume will breakthrough if in light winds

Equation (1) Available in Full Paper

or for moderate winds

Equation (2) Available in Full Paper

Expressions 1 and 2 should be regarded as indicators only because they are based on limited data.

Inversion data are not presently available for the Oil Sands area; so an estimation of inversion height had to be made. Based on data at Fort Smith an inversion height of 200 – 600 m is thought to be representative of the Tar Sands area during the months October to March, with 200 m occurring in January and 600 m in October and March. For the purposes of this study 300, 400 and 600 m inversion heights have been considered. Typical ventilation coefficients at Fort Smith are 800 – 3500m2/s, the lower value occurring in January. Average monthly wind velocities, based on the above are:

  • u(Jan.) 4 m/s

  • u(Oct.) ≈ u(Feb.) 6 m/s

  • u(Nov.) = u(Dec.) ≈ u(Mar.) =5 m/s

The average annual measured wind velocity at Fort McMurray is approximately 3 m/s. The wind data for Fort McMurray are listed in Table 1. Various stack parameters used in this study are:

  • T(STACK) = 562 K, T(AMBIENT) 295 K

  • D = 6.7 m, Vs = 18.3 m/s

Typical emission rates and other data are given in Table 1.

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