A Scavenging Model Analysis Around a Large Coal-fired Power Plant in New Delhi With a Particular...

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A SCAVENGING YODEL ANALYSIS AROUND A LARGE COAL-FIRED POWER PLANT IN NEW DELHI WITHA PARTICULAR REFERENCE TO THE SCAVENGING ACTION OF THE MONSOONAL RAINS

S. Ghosh and M.P. Singh

Centre for Atmospheric and Fluids Scie?cesIndian Institute of Technology, New Delhi-119016.

Abstract. There have been reports in the recent past ahout the problem of SO2pollution over India. Some of them have even stated that corrosion of theAcropolis in Athelsis now matched by the corrosion of the Taj Mahal in Agra.'lathematical models and experimental analyses have been undertaken to addressthe problem of SO2 pollution over selected \letropolitan cities in India.

An important feature revealed from these model studies is that most urban airquality models in India grossly over predict ambient SO2 levels during themonsoon period. This is because washout calculations are not featured in thesemodels. In this paper we have tried to demonstrate the efficacy of the south-west monsoon rains to a scavenge a soluble pollutant like SO from an urbanenvironment. India in general does not face the acid rain t reat yet. However,results of rainwater analyses show that low pH of precipitation does occur atisolated pockets downwind of major industries and power plants. In this paperwe have determined the extent of acidic deposition in the near field of alarge coal-fired power plant in Delhi. SO2 concentration profiles, with andwithout washout,calculations have been shown. Prohable periods of the yearand the areas within the meteroplition regions of Delhi which could be worstaffected by .acid impaction have been identified on the basis of the modelsimulations with mean climatological data. Model computations show that maxi-mum pollution is brought in to the city from this power pl nt during themonth of October. The hourly GLC often exceeds 1000 ng m-J which is quitea close to the federal standard of 1199 ug m-3,;likewise the acid depositionflux is greatest during the month of August and is of the order of 59 ug m-2 s-lat a downwind distance of 1 km,

The nature of the washout coefficient during the monsoon pre and post monsoonperiods in relation to the relative importance of the atmospheric variablesconcerned has been investigated. The role of individual monsoonal showersto scavenge SO2 has been discussed.

This is perhaps the first work of its kind from India wherein representativewashout coefficients have calculated and subsequently featured as washout

effects in an urban air pollution model. Results show that this parametercould be as high as 16.93 x 10-5 s-1 during August and as low as 1.1 x lo-6s-1in April.

Keywords. Scavenging, washout coefficients, Ground level concentrations,Acid deposition flux.

INTRODUCTION ments indicate that rainwater aciditv in

In recent years we have witnessed an increa-sed awareness of a potential rise in the aci-dity of precipitation. This phenomenon, hasprompted considerable concern forecologists

in the regions of suspected maximum impactsuch as the Northeastern United States, South

eastern Canada, and Scandanavia .generally recognised that "acid rain"

It isis pri-

marily the result of long range transportand transformation of combustion productsfrom industrial and transportation sources.

Systematic work on measuring rainwater acidi-ty in India began with Mukherjee (1957)who re-ported pH measurements of Calcutta rainwaterand subsequently of Bombay rainwater. In one ofthe measurements near Calcutta, llukherjee(lg57)found that rainwater showed some acidity whenthe trajectory of air over the samnle collec-tion site was from the highly industrial area

Routine measurements on rainwater acidity inIndia have been carried out from 1982 onwards.These studies are being done by the BhabaAtomic Research Centre (BARC), Bomhay andalso by the network of background pollutionmonitoring.(BAPMON) stations established inIndia by the World Meteorological Organisa-tion (W?40). The results of these measure-

India has not so far reached alarminglevels . The high dust loads in theIndian atmospheres may be providing enoughmaterial to neautralise the acids evenwhen they are present. Sequira(l982),Daset. 'al(1981)and Handa(1969)measured the pHof rainwater at several places in India

and came to a similar conclusion.

Apart from the measurement of Mukherjee(1957) more recently high acid content ofrainwater has also been observed in smallpockets downwind of some factories andthermal power plants. The earlier work bythe BARC group in Bombay had shown thatareas like Tromhay and Chembur has acidicrains (with pH of 4.45 and 4.85 respecti-vely) during 1974.

More recently experts attending a NationalConference on pollution management havewarned against the possibility of acidrain over Agra and said that its effect on

the Taj Uahal will be disastrous .Acidrains would hit India in 10 years fromnow, if industrial units continue to burncoal at the present rate.

The above discussions highlight the factthat even though rainwater acidity in Indiahas not yet reached alarming levels at most

419



420 5th ICHN

places, in the immediate vicinity of fac-tories there still is an acid rain threat.Our interest Ginwater acidity,is therefore confined to its immediate effects onmaterials of construction and the preserva-tion of stone monuments in the close Gicini-ty of polluting sources. One of the firstrequirements in this direction, is the asse-ssment of the current situation, so that ifconditions are found to be deteriorating

measures can be taken to limit the emiss-ions. One of the ways by which this can bedone is by undertaking accurate quantitativemeasurements with a large network of monito-ring stations. However, this being a veryelaborate and expensive affair, involving alot of skilled manpower, the only other vi-able alternative left is by taking recourseto regional scale mathematical models.Thesemodels are quite capable of providing 'quicklook' estimates of the air-pollution statusaround a specific region and are also fairlyinexpensive to implement.

Having elaborated upon the justification forthe use of mathematical models for assessing

rainwater quality in a given region, thequestion of the choice of a suitable modelstill remains. Both Lagrangian and Eulerianmethods are used to assess the transformat-ion and transport of SO2 in the atmosphere.

For example Charmichael and Peters(l984)haveproposed an Eulerian combined transport/Chemistry/removal model that describes theregional distribution of SO2 and sulphate.

The mathematical basis for this Eulerianmodel is the coupled 3D-advection-diffu-sion equation. The set of equations is nume-rically solved using a Crank-Nicholson Galer-kin Vethod. This model is extremely elaborateand entails a large expenditure on expensive

computer time and so is not very suitablefor Indian conditions. Likewise, the 'Jniv.of Yichigan Atmospheric contributions toInter-regional Deposition (ACID) model isalso fairly elaborate. Both these models re-quire detailed meteorological inputs (inclu-ding variation of Topography) and chemicalkinetic data. Such detailed information areoften not available. The choice of a modeldepends on the question or hypothesis beingtested and the accuracy of the answer neededIn the N.E. IJnited tates, Canada and someparts of Europe where 'acid rain' is a poten-tial threat to everyone concerned,?t isjustified to use elaborate models like theACID model. For Indian conditions, mainlyfor a preliminary assessment purpose it see116most appropriate to use simple analyticalmodels, with an accurate input of releventmeteorological parameters.

With the above perspectives, we propose toundertake a scavenging model analysis aroundthe Indraprastha Power Plant situated inNew Delhi. The chief objectives of under-taking this study are:

a. To demonstrate the efficacy of the Southwest monsoonal rains to scavenge asoluble pollutant like SO2 from an urbanenvironment.

b. To recommend the use of washout calcula-

tions in all Indian urban air qualitymodels. Without such calculations,thesemodels will grossly over predict ambientSO2 le,vels uring the months of July to

September.

C. To determine the extent of acidic de-position within the power plants nearfield (within 10 km radius).

d. To evaluate the relative importance ofthe atmospheric variables concerned.

MATHEYATICAL FORVDLATION

Level terrain is assumed in our analysis.

!Ye onsider the steady state form of theatmospheric advection-diffusion equationwith washout effects:

32c+I),DYy2

ti--uac_f+lJ’ az2

ax

Here, x,y,z are the horizontal downwind,crosswind and vertical coordinates respec-tively; u is the constant average windspeed, and 6 is the washout coefficientfor SO2, c is the SO2 concentration at

(x,y,z), and Dy and Ds are the constant

eddy diffusivities in the crosswind andvertical directions, respectively.

For a continuous point source of strengthQ located at (x = 0, y = 0, z = h), theboundary conditions are given by:

weis

c(O,y,z) = ” A(Y) 6(z-h) (2a)

c(x,+ m ,z) = 0 (?h)

D %=9atz=OandHz a2 (3c)

assume that the pollutant concentrationzero far away in the lateral direction__

and that there is no pollutant flux at theground at a = 0 and at the top of themixed layer at a = H.

For constant Dy and Ds, the exact analytic

solution of the diffusion equation wasfirst given by Yonin(l959) and later bySmith(l982), triven and Fisher(l975), ae(1975)and rmak(1977). hese solutions duevarious authors, though basically similar,differ somewhat due to different sourceconditions, pollutant species and otherassumptions used in their studies.

In this paper the main emphasis will be onwet scavenging and on the extent of wetremoval with monthly variations of thewashout coefficient. We have not consider-ed dry deposition effects because it is

well known that washout when it occurs isa more effective mechanism for removingSO2 than dry deposition effects. It isalso worth pointing out that rainfall ratesare highly variable from January throughDecember over India. During the monsoonperiods the intensity of rainfall for anindividual shower could be of the order of25 mm/hr. This is a common occurrence overIndia unlike the situation in Britain wheretypical rainfall rates correspond to about1 mm/hr for a heavy shower . No workis reported in literature from India where

representative washout coefficients havebeen calculated based on Indian climatolo-gical data; neither have they been featu-

red as washout effects in any uhran airquality model. As a result these modelshave grossly over predicted ambient SO,levels during the monsoon months

(Gupta and Padmanahhamurty (1984)).



SCAVEAGIAG MODEL ANALYSIS 421

ANALYTICAL SOLUTIONS and along the plume-centre-line, where

Equation (1) is solved by applying Laplacetransforms w.r.t. 'x', Fourier transformw.r.t. !y', and Finite Fourier cosine 'trans-

form w.r.t., '2'. Using the boundary condi-tions (2a-2c), we obtain the following solu-tion

Q e-Y2/2$ e-6x/u m

c(x,y,s) = 11+2 zuH JZi? up n=l

This is the amount of SO2 deposited perunit time per unit surface area along theplume centre-line and hourly, monthly oryearly estimates at a given downwind dis-

tance from the source in a given wind di-rection can be easily made.

nnhx cos y cos H IMPLE'IENTATION OF THE IIODEL

D n2r2_z x

e ii3

HZ(3)

where ai = 2D xYG

(4)

In order to facilitate the practical appli-cation of the analytical solution the eddydiffusivity Dy is expressed in terms of a

Y'the standard deviation of the crosswindGaussian concentration distribution.

(1) Plume Trapping:

It has been assumed that neutral condi-tions prevail while it is rainingand it is reasonable to estimate o as:

yaY

= 6.36 x6.66 (5)

(Smith and Singer(l966))Following the current practise in EPA modelsmainly for a neutral atmosphere, the mixingdepth should be included in the algorithm.This is usually done through calculation ofmultiple eddy reflections both from the groground and the stable layer aloft, when theplume is trapped between these two surfaces.Our solution (3) can be easily recast into aform where the above effect is immediately

:pp_arant*By introducing the parameter

z- 2Ds x/u and the substitutions

"Z$(z&2 ; T = & (z?h) in the rela-

tion:

1+2 ; e-n2v

m

cos 2nt =J $ Ce-(t-nn)2/v

n=l n=_co

the solution (3) can be expressed in theform:

Q e-y2/20G e-9x/u

c(x,y,s) =JZi uaycrz

+m

xc.ze-(z+h-2nH)2/20~

n=_m

+ e-(z-h-2nH)'/2a~ 7 (6)

Multiple reflections are now immediatelyapparant in the above expression.

(ii) Surface Deposition Flux:

From (3), the precipitation induced

flux of effluents to the ground at any down-wind distance, x , is easily obtained as:

9 ge-13x/u e-Y2/20;

F(x,y) =Jzii a u

(7)

Y

(8)

A computer program has been developed forthe ICL 2960 system available at the IITDelhi. The data used in this model aregiven in Table 1.

The SO2 scavenging coefficients are com-puted after the method outlined by Samsonand Small(l934) and is given by

6 = 5 x lo4 P/H (9)

where P is the precipitation rate (mm perhour) and H is the mixing height.

This method was adopted in preference tothe more common method of adopting a con-stant 9 value, because in the present app-roach the scavenging coefficient is a fun-ction of both the precipitation rate andthe mixing height - both of which are high-ly variable under Indian conditions. Itwill therefore just not suffice to have*one representative scavenging coefficient"!Ve have considered the seasonal variationof the mean mixing depths over Delhi.The vertical eddy diffusion coefficient

was taken as 10 m2 s .1

The power plant in question is the Indra-prastha Power Plant located in the east ofDelhi city. The average amount of coalburnt per day is 3000 metric tons. Thesulphur content of Indian coal is .5% andwith these informations the total estimat-ed source strength of SO due to coal burn-ing at the Indraprastba 3ower Plant is

1.19 x 10-1

6 g/hr or 331 gs .

The effective stack height (plume rise plusphysical stack height) under neutral con-ditions was found to be ahout 150 m.

SESULTS AYD DISCTJSSIONS

(i) Ground level concentrations: (GLC)

The ground level (z = 9,) plume centre-line (y = 0) concentrations are shown inFigure 1 as a function of the downwind dis-tance x for the months of January (winter),May (summer), August (monsoon) and November(autumn). Concentrations are also shownwithout washout calculations for the sakeof comparisons.

During the winter months of December,January and February the maximum hourlyGLCs are less than 650 ng rnv3 and the winddirection is north-westerly. Washout eff-ects are also not significantly pronounced

during these months and the pollutants areblown away from the city's main lands.During October and November the maximumhourly SO2 concentrations are of the order

of 1000 ug m-3

upto 1 km downwind. (Riostfederal standars specify that the hourlyconcentration of SOP should not exceed



422 5th ICI4'I

TABLE 1: YODEL INPUT PARAMETERS

Mixing Depth H Wind Speed U Wind Rainfall Washoutm -1 _-

ms Direction rate-1 Coefficient 6

mmhs-l

JAN 760 3.22

FEB 1215 4.36

84AR 1356 4.47

APR 1500 5.07

MAY 1462 4.95

JUN 1545 6.40

JUL 1050 3.40

AUG 952 2.89

SEP 997 2.66

OCT 1354 2.03

NOV 1457 2.22

DEC 561 2.85

NW 0.8

mW 0.33

NNW 0.55

SW 0.12

W 0.64

NNE 2.07

SE 6.2

ESE 10.9

W 6.2

NE 0.97

NNW 0.13

NW 0.64

1.47 x 10 5

0.38 x 10 5

0.56 x 1O-5

0.11 x 1O-50.61 x 10 5

1.88 X 10 5

8.27 x 10 5

16.03 X 10 5

8.71 x 10 5

1.00 x 10 5

0.125 ~l'l-~ .

1.597 x1O-5

(The data presented above are mean hourly averages based on five years data)

1100 ugmm3). During the month of October,the prevailing wind direction being NE aggra-vates the situation further, because SO2 isblown into the city directly. Important stru-ctures like the Supreme Court buildings, theNational Stadium and the India Gate are thusdirectly affected.

The summer months of March. April,"fay ndJune are characterised by relatively lowerGLCs. The boundary layer is mostly in a stateof vigorous mixing. During these months themixing depths are quite large.

The months of July, August,and September arecharacterised by the monsoonal rains. Scaven-ging effects are significantly pronouncedduring the month of August when ttie ean rainfall rate is about 11 mmh-'. This is clear-ly evident from the Figure 1.

Fig. 1: Ground Level concentrations with andwithout washout effects at differentdownwind distances.

In Table 2 we have tabulated the GLCs, withand without washout effects for the month ofAugust. We find that unless washout calcula-

tions are featured in the model, the extentof over prediction is quite significant.This effect is more pronounced as onemoves further downwind from the power plant.This is because as the plume progressesdownwind the extent of scavenging keeps onincreasing (we have assumed that the sca-venging action is continuous within theone hour period) and by the time it hasadvanced 10 kms a significant fraction ofSO2 is washed down by the falling rain.

TABLE 2: GLCs WITH AND WITHOUT WASHOUTEFFECT

K, GLC in GLC in 96 f over-Km August

Mrnm3August

Mrnm3prediction

with withoutwashout washout'

1 593.6 627.5 5.72 528.8 590.4 11.63 354.4 418.6 18.14 259.5 324.1 24.95 196.6 259.5 32.06 153.5 214.2 39.57 122.2 180.3 47.5

8 99.2 154.7 55.99 81.8 134.8 65.510 68.2 118.9 74.3

During the months of July and August thewind is South easterly. A lot of SO2 isscavenged by the falling rain because ofthe fairly intense precipitation rates.Structures most vulnerable so far as pol-luted rain is concerned are Vikas "linar(Office of the Delhi Development Authority),Lady Hardinge College, 'laulana zad UedicalCollege, Lok Kalyan Bhawan etc. Rainwatermeasurements should be done at atleast oneof these sites to study the effects of an-thropogenic emissions on the precipitationquality. Unfortunately, todate not evenone sampling site is chosen around any oneof these identified localities. Shouldacid rains ever hit the city due to incrb-ased SO2 emissions from the l.P power plant,then these are the probable places wherethe effects would be the most pronounced.



423

(ii) Effect of individual monsoon storms:

Individual storms can wash down mostof the pollutants. Storms during the mon-soons are characterised by very high rain-fall rates. The storm discussed here had arainfall rate of the order of 50 mmh" ((August 1960). The scavenging effects of amonsoon storm on the GLC is depicted in theFigure 2. It is clear from this figure that

the fall in the GLC due to washout in astorm is more pronounced than in the case ofmean hourly washout.

(a) (b)

Fig. 2: Portrayal of scavenging effects ofindividual storms on the GLC duringthe monsoons.

(iii) Wet removal flux of SO2 during themonsoons:

Expression (8) was directly used toestimate the precipitation induced flux ofSO

zto the ground. As expected this is high-

es for August because of the large value of! 1 and a comparatively low value of the rni,+ing height, H, and ranges from about 50 ngm-2

s-l at a downwind distance of 1 km to about-2 -1

11 ugm s at 10 kms downwind, along theplume centre line. From the expression (8)it is clear that the variation of the wetremoval flux with downwind distance is expo-

nentially decaying, hence these profiles arenot explicitly shown.

(iv) Vertical profiles of the ambient con-centration during the monsoons:

The most important observation in thisconnection is that even during a fairly se-vere Strom (rainfall rate % 50 mmh-l) theshape of the vertical profiles essentiallyremain unaltered. One observes the familiarGaussian profile (with a peak concentrationlevel centered around the effective stackheight) in the near field (1 km downwind).Effects of ground reflection are apparant asone proceeds further downwind. These pro-files are shown in Figure 3.

Pig. 3: Vertical profiles of the ambientconcentration during the monsoons

(August).

LI'IITATIONS ND ADVAWTAGES

A constant diffusivity in the vertical istaken throughout. This is a major limita-tion, and the effects of a variable diffu-sivity profile will be featured in a sub-sequent work. Another major limitation ofthis model, as is the case in common withmost of the simple dispersion models, isits inability to allow for the change inwind velocity with height above the sur-face. Chemical transformation processesand dry deposition effects have also been

disregarded.

Inspite of all these limitations this .model has been put to practical use as aresult of simulation with mean climatolo-gical data. The main points are summarisedbelow.

1. During the months'of Julv and Aueust alot 07 SO released from-the 1.P:Power Plait is scavenged by the mon-soonal rains. The wind direction isSouth easterly and the pollutants arebrought in direc.tly o important areaswithin the Metropolitan regions ofDelhi city. The chances of precipita-

tion contamination within the powerplant's near field are the greatest inthese areas. Any proposed field studiesto monitor rainwater quality in Delhimust consider sampling sites at theseregions.

Scavenging effects are significantlypronounced during the months of July,August and September. Any urban airquality model must therefore featurewashout effects during concentrationsimulations at least during thesemonths; otherwise the model willgrossly over-predict concentrationlevels during these months. To date,no urban air quality model in India,

features such calculations.Being an analytical model the computa-tional expenses incurred are muchlower than a comparable numericalmodel.



424 5th Im

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