A Simplified Stack Height Determination
Authors: Wentini J.J. , Reichert J.F. 
Published: November 2010

A method which was developed to optimize the stack height for either flares orincinerator stacks is discussed. The optimization is based on the Pasquillmethod(1) as it is modelled by the Alberta Department of Health(2). The Department program is designed to calculate estimatesfor contaminant gas concentrations at ground level, downwind from thestack.

In the application discussed in this paper, the method is used to generate ahypothetical stack gas plume (or smoke cloud), which is defined by a grid ofmaximum concentrations. Each of these maxima represents the highest value ofthe concentrations calculated for varying weather conditions at each grid pointin the atmosphere. The resulting plume grid is then used to define the contourof the maximum allowable ground-level concentration. This contour should clearthe topography surrounding the plant, thereby dictating a minimum required stack height.

The merits of alternate locations can be evaluated by applying corresponding to pographic profiles to the same plume plot.

A plume plot of maximum concentrations for a flare stack can be generated witha card deck containing no more than four data cards; a plot for an incineratorstack will require only three cards.

Introduction

INDUSTRY needs an answer to the question: "How high should this stack he forthis location and these operating conditions'!" A good answer to this questionis difficult to obtain, because the state of the art of predicting thedispersion of pollutant gases is a long way from being perfect, and because thelarge number of variables involved does necessitate an unreasonable number ofcalculations. Available models usually calculate the ground-levelconcentrations at specified locations for a particular weather condition, rather than the maximum concentration, that can occur at that particular spotfor all possible types of weather. By means of incremental changes of onevariable at a time, such programs can be used to determine the minimum requiredstack height, but it is a tedious job. Experience can reduce the number ofcalculations required, but it is difficult to judge whether the evaluation wasadequate. We decided that an optimization routine should be developed whichwould make use of an acceptable calculation method and which would generate asolution for a minimum height required for a particular stack.

The selection of the method was strongly influenced by the desire expressed bythe Alberta Department of Health that the Pasquill Method be used for stackdesign in uneven terrain.

All current calculation methods, including Pasquill's, are based on the stilllimited understanding of atmospheric dispersion. It is therefore advisable toregard all models with skepticism. Of the numerous stack design methodspublished in the literature, Pasquill's became one of the best known, and it isgenerally regarded as one of the more practical methods to calculateground-level concentrations.

It was therefore decided to base the optimization on Pasquill's method, bymodifying the Department of Health program.

The Background

The Department of Health version of the Pasquill method appeared to be particularly suitable for optimization techniques.