COMPARISON OF SHORT TERM CONCENTRATION OF SULPHUR DIOXIDE (SO2) , AND Suspended particulate matter (SPM) PM POLLUTION FROM A POINT SOURCE USING DIFFERENT PLUME RISE FORMULAE

Neetu Saxena

PGD Environmental Management, Bareilly College, Bareilly, U.P., India

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Abstract

Two plume rise formulae viz. Briggs equation and Holland equation has been computed to estimate the short term concentration of SO2 and SPM from a bolier stack. It has been found that the formulae shows a wide variation in the short term concentration. Whereas, Holland equation under estimates the plume rise and hence gives high values of short term concentration.

Keywords

Dispersion concentration, Plume rise.

Introduction

The calculation of plume rise is often a vital consideration in predicting dispersion of harmful effluents into the atmosphere. The engineers or meterologist must choose from more than 30 different plume rise formulas and a casual search through the literature for more help in choosing is likely to be confusing. In spite of the importance of plume rise in predicting dispersion, there is much controversy about how it should be calculated. Several investigations have proposed formulae for the estimation of plume rise under given conditions. Plume rise can be calculated as a function of source parameters, such as byoyancy and meterological conditions. In the present study, the short term ground level meterological conditions. In the present study, the short term ground level concentration of air pollutants downwind of an elevated point source ( a boiler) has been computed, incorporating Briggs and Holland plume rise formulae and Pasquill Turbulence type dispersion parameters in the Gaussian Dispersion model.

Methodology

The ground level short term concentration (μg/m3) along the plume center line has been computed according to floowing equation (Smith, 1968).

Plant characteristics

A single boiler stack of 86 m height and 5 m diameter near Gurgaon has been considered for the computation of ground level concentration (glc) with the follwoing emission rate -

SO2 emission rate = 5.93 x 107 μg/s

SPM emission rate = 1.13 x 107 μg/s

Effective stack height

The effective stack height, He is calculated as

Two different formulae viz. Holland equation (Holland, 1953) and Briggs equation (Briggs, 1969, Padmanabhamurty and Gupta, 198) are used to calculate the value of plume rise.

Holland equation

Briggs equation

(ii) Stable conditions (stability class E & F)

The values for pwoer profile exponent are used according to Irwin (USEPA 1987).

Dispersion coefficients

The dispersion characteristics σy and σz depends upon the turbulent structure of the atmosphere and the downwind distance. The procedure for determining the stability class for the calculation of dispersion coefficient is according the Pasquill (1961), modified by Grifford (1976), and a dopted by the US Public Health Service (Turnner, 1970).

The values of y and z are computed using the follwoing equations (Martin, 1976) :

Where a, c, d and f are constants and whose values depends on the stability class.

Results and Discussion

The Gaussian dispersion equation is computed for the estimation of short term concentration of pollutants under different stability classes using Briggs and Holland plume rise formulae. Ground level concentration (glc) is estimated at different distances from the point source, i.e. 1 km, 2 km, 5 km, 10 km, 15 km, 20 km, 25 km and 30 km.

Under extremely unstable atmospheric conditions (stability class ‘A’), according to Briggs formula, maximum glc of 862 and SPM is at 2 km while Holland’formula.gives maximum glc at 1 km .The value of ground level concentration is very high than that of given by Briggs formula upto 10 km distance, after which both the formulae had given almost same results (fig. 1 & 2). Under moderately unstable conditions (stability class ‘B’ and ‘C’), max. glc of SOa is found at 10 km by Briggs equation, whereas, it is maximum at 1 km by Holland equation (fig. 3). The ground level concentration of SPM is negligible on computing Briggs equation while Holland equation gives high SPM concentration with maximum concentration at 1 km (fig. 4). According to Briggs equation, glc of 862 & SPM is almost nil under neutral atmospheric conditions (stability class’D’) while Holland equation give high values (fig. 5 & 6). Under stable conditions (stability class ‘E’ & ‘F’), Holland equation shows high values of SC>2 & SPM concentration at all distances (Fig. 7 & 8).

Figure 1: Short term concentration of SO₂(μg/m³) under extremely unstable atmospheric conditions (stability class ‘A’)

Figure 2: Short term concentration of SPM (μg/m³) under extremely unstable atmospheric conditions (stability class ‘A’)

Figure 3: Short term concentration of SO₂ (μg/m³) under moderate atmospheric conditions (stability class ‘B’ and ‘C’)

Figure 4: Short term concentration of SPM (μg/m³) under emoderate unstable atmospheric conditions (stability class ‘B’ and ‘C’)

Figure 5: Short term concentration of SO₂ (μg/m³) under neutral atmospheric conditions (stability class ‘D’)

Figure 6: Short term concentration of SPM (μg/m³) under neutral atmospheric conditions (stability class ‘D’)

Figure 7: Short term concentration of SO₂ (μg/m³) under stable atmospheric conditions (stability class ‘B’ and ‘F’)

Figure 8: Short term concentration of SO₂ (μg/m³) under stable atmospheric conditions (stability class ‘B’ and ‘F’)

From the results, it has been found that the value of glc of any pollutant is highly dependent on the value of plume rise and hence effective stack height. Holland equation underestimates plume rise, which results in high values of ground level concentration. Briggs equation, on the other hand over estimates plume rise and therefore, gives low values of glc. Moses & Storm(1961) have also made a comparative study of some formulae and found that “ there is no one formula which is outstanding in all respects”. Guldberg (1975), after an extensive comparison of plume rise formulae, concluded that Briggs model (1969, 1971, 1972) predicts the best results of the observed plume rise during periods of low wind speed, and at higher wind speeds the Taennesse Valley Authority (TVA) model performs the best. High emission velocity and higher temperature of the effluents than the ambient air at the stack top enhances the effective stack height above the physical height of the stack. Hence, in any determination of concentration downwind from a source, plume rise estimation plays a vital role and it is essential to select a suitable formula forestimation of effective stack height.

References

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