terrain stack height

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Presented at the 86th Annual AWMA Conference

Denver, Colorado, June 14-18, 1993

EFFECT OF A NEARBY HILL ON

GOOD ENGINEERING PRACTICE STACK HEIGHT

Ronald L. Petersen

and

Douglas K. Parce

Cermak Peterka Petersen, Inc.

1415 Blue Spruce Drive

Fort Collins, Colorado 80524

Jeffrey L. West

Metropolitan Edison Company

2800 Edison Company

Reading, Pennsylvania 19640

Richard Londergan

ENSR

95 Glastonbury BoulevardGlastonbury, Connecticut 06033


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INTRODUCTION

This paper describes the study conducted to determine the effect of a nearby hill on the "good

engineering practice" (GEP) stack height for Metropolitan Edison Company's (Met-Ed) Titus Generating

Station (TGS). TGS is located along the Schuylkill River about 3 km south of Reading, Pennsylvania asshown in Figure 1. The station has three 61.0 m (200 ft) stacks with a base elevation of 59.1 m, MSL (194

ft, MSL). To the west of the facility the terrain rises 73 m (240 ft) above plant grade within 1 km. A cross-

section of the plant and nearby terrain is shown in Figure 2.

Recent ISCST modeling conducted by Met-Ed has shown that the three existing stacks may

contribute to modeled accedences of the NAAQS. The high predicted concentrations are due to the combined

effects of short stack heights, nearby elevated terrain and building downwash. If the existing stacks are raised

to some taller height, the modeled accedences may be eliminated.

Before a source can consider a taller stack as a control measure, the EPA stack height regulations

(40 CFR 51.118) requires that the source demonstrate that "excessive concentrations" occur due to the

existing stack in the immediate vicinity of the source as a result of downwash created by nearby structures

and/or terrain. Once this demonstration is made a source can construct and receive regulatory modeling credit

for a taller stack. The maximum stack height that is creditable for regulatory modeling purposes is the GEP

stack height. Credit for a formula GEP stack height can be obtained without further analysis. Credit for a

GEP stack height taller than the formula must be proven through the use of wind tunnel modeling.

The formula based "good engineering practice" (GEP) stack height for TGS is approximately 98.3 m

(322.5 ft) based on the height of the 39.3 m (129 ft) high fan room building. This height stack would be

above the hill as indicated in Figure 2, but past studies by Castro and Snyder1 and Petersen and Ratcliff2

suggest that terrain wakes could still increase ground level concentrations by as much as a factor of two.

Hence, the GEP stack height based on the nearby terrain feature (Highs Hill) may significantly taller than

the formula height.

So the that Met-Ed can evaluate the feasibility of stack height increases for reducing maximum

ground level concentrations (as opposed to emission reduction), a study will following objective was

undertaken.

1) demonstrate whether excessive ground-level SO2 concentrations occur due to the existing

stacks (if they do occur a taller stack can be considered); and

2) determine the GEP stack height based on the nearby terrain.

To meet the project objectives, physical modeling of the atmospheric boundary layer and plume

dispersion was used. A 1:600 scale model of the construction features of the generating station andappropriate terrain features was constructed and placed in CPP's Environmental Wind Tunnel. Ground-level

concentrations of sulfur dioxide were determined with and without the nearby upwind terrain and structures

present by sampling concentrations of a tracer gas released from the model stacks. Tests were conducted to

simulate various plant and meteorological conditions to demonstrate excessive concentration for the existing

stacks and the maximum creditable (GEP) stack height for new stacks at TGS.


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2

0

=

H

e

U

VM

a

s

=

Included in this paper are a brief description of the similarity requirements for wind tunnel modeling,

a discussion of various technical issues, an explanation of the test methodology, and the results the

evaluation.

TECHNICAL CONSIDERATIONSSimilarity Requirements

An accurate simulation of the boundary-layer winds and stack gas flow is an essential prerequisite

to any wind-tunnel study of diffusion for power plant stack exhaust. The similarity requirements can be

obtained from dimensional arguments derived from the equations governing fluid motion. A detailed

discussion on these requirements is given in the EPA fluid modeling guideline3. The scaling criteria that were

used for this evaluation are summarized below:

match (equal in model and full scale) momentum ratio,Mo

(1)

match density ratio,

(2)

ensure a fully turbulent stack gas flow stack Reynolds number greater)/( = es dVRethan 670 for buoyant plumes or 2000 for turbulent jets4, or in-stack trip;

ensure a fully turbulent wake flow terrain or building Reynolds

number greater than 11,000 and/or Reynolds)/or/( == bHbtHt HUReHURenumber independence verification tests.

identical geometric proportions;

equivalent stability Richardson number in model equal[ ])/()( 2Hb UTHgRi =to that in full scale, equal to zero for neutral stratification; and

equality of dimensionless boundary and approach flow conditions;

where

Ve = stack gas exit velocity (m/s),

UH = ambient velocity at building top (m/s),

d = stack diameter (m),

= ambient air density (kg/m3),a


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= potential temperature difference betweenHb and the ground (K),T = mean temperature (K),

= stack gas density (kg/m3),s

= viscosity (m2/s),Hb = typical building height (m), and

Ht = typical hill top height (m).

Using the above criteria and source characteristics supplied by Met-Ed, the model test conditions

were computed for the stacks under evaluation. Model and full-scale parameters for one of the stacks with

a 4 m/s wind speed (at 6.1 m above grade at a nearby anemometer site) is presented in Table 1.

Determination of GEP Stack Height

In the stack height regulation, GEP stack height is defined to be the greater of:

(1) 65 meters,

(2) Hg = 2.5H(for stacks in existence in January 12, 1979), or Hg =H+ 1.5L (for all other stacks),

whereHg is the good engineering practice stack height,His the height of nearby structure(s) andL

is the lesser of the projected height or width of the structure,

(3) The height demonstrated by a fluid model or a field study approved by the EPA, State or local

control agency, which ensures that the emissions from a stack do not result in excessive

concentrations of any air pollutant as a result of atmospheric downwash, wakes, or eddy effects

created by the source itself, nearby structures or nearby terrain features."

When a source wants to increase the existing stack height up to the GEP stack height (and receivecredit for the increased stack height), the EPA stack height regulation also requires one of the following:

"(1) demonstrate by fluid modeling or a field study that both excessive concentration

criteria (discussed below) are met using the existing stack height and emission rate and

adding in background air quality, or

(2) show, by site specific information, that the stack is causing a local nuisance."

Since no site specific nuisance information was available for TGS, physical modeling was used to

conduct this demonstration. Once this demonstration is made for the existing stack, a new stack up to the

formula GEP stack height, or a height determined through physical modeling in the wind tunnel, may be

constructed. This wind tunnel determined height could be greater than the height determined from theformulas discussed above.

To quantitatively determine the GEP height through physical modeling, the stack height regulation

goes on to define excessive concentration as:


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"a maximum ground-level concentration due to emissions from a stack due in whole or in part to

downwash, wakes, and eddy effects produced by nearby structures or nearby terrain features which

individually is at least 40 percent in excess of the maximum concentration experienced in the

absence of such downwash, wakes, or eddy effects and which contributes to a total concentration

due to emissions from all sources that is greater than an ambient air quality standard."

Based on the above definitions, maximum ground level concentrations measured in the wind tunnel

due to each source with and without the nearby structures and terrain present are first compared. If the ratio

of these concentrations is greater than 1.4, one of the criteria for demonstrating an excessive concentration

will have been met. Next, the maximum concentration due to the combined impact of the three units is

determined and compared with NAAQS. If an accedence of the NAAQS is shown, then both excessive

concentration criteria will have been met. For the later determination, the background concentration due to

other sources can also be considered, to show an accedence. The 3- and 24-hour background values for this

region are 229.6 and 111.8 g/m3, respectively5,6.

Emission Rates

For the purpose of comparing wind-tunnel modeling estimates with NAAQS, SO2 emission rates

must be specified. To demonstrate excessive concentrations for the existing stacks an SO2 emission rate of

3.6 lbs/MMBTU was used (i.e., existing emission rate).

The stack height regulation requires that New Source Performance Standards (NSPS) be used to

define the emission rate when conducting a wind tunnel study to demonstrate a GEP stack height taller than

the formula height unless approval is obtained to use a different rate. The NSPS rate of 1.2 lbs SO2/MMBTU

was used for the terrain effects GEP stack height evaluation for the purpose of calculating full-scale

concentrations.

Nearby Structures and Terrain

In this study, the effect of nearby terrain features was primarily used to determine the GEP stackheight for TGS. However, nearby structures were also considered in the evaluation in case the structures

added additional turbulence to the flow. To evaluate the combined effects of nearby terrain and structures,

tests are first conducted with all terrain and structures included in the model. Next tests are conducted with

the nearby terrain and structures removed. A structure is defined as nearby7 when the distance from the stack

to the structure is less than or equal to five times the lesser of the height or cross-wind width of the structure.

The structures which were treated as nearby are shown in Figure 3.

A terrain feature is defined as nearby (40 CFR 51.1(jj)) if the feature achieves a height greater than

40 % of the formula GEP stack height within 0.8 km from the stack. Once a feature is defined as nearby the

portion of the feature that can be removed from the model is 10 times the maximum height of the feature,

not to exceed 3.2 km. Based on this definition, the terrain feature identified as Highs Hill in Figure 4 is

nearby. Figure 5 shows the nearby terrain "Out" configuration.Wind Speed, Direction and Persistence

The EPA stack height guideline8 requires that the design wind speed less than the 2 percent wind

speed (speed that is exceeded less than 2 percent of the time) unless it can be demonstrated that higher speeds

cause accedences of NAAQS limits. The 2 percent wind speed was determined by analyzing meteorological

data collected at two different anemometers located with 20 km of TGS. The results of the analysis


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an

anem

anemz

zUU

=

demonstrated that the 2% wind speed for the site is approximately 7.5 m/s at the 6.1 m height. All tests to

justify GEP stack height were conducted at or below this wind speed.

Wind speeds in the tunnel were set at a reference height of 600 m above plant grade. The speed at

this height was determined by scaling the 6.1 m wind speed up to the free stream height, 600 m above groundlevel. At this height, is it assumed that wind speeds at the site and at the anemometer location are the same

(i.e., local topographic effects are not important). The following equation define the procedure:

(3)

where is wind speed at free stream height (m/s), is free stream height (600 m), Uanem is wind speedU z

at 6.1 m height,zanem is height above grade for Uanem and Usite (6.1 m), and na is wind power law exponent at

the anemometer (taken to be 0.16).

WIND TUNNEL MODELING METHODOLOGY

Scale Model

A 1:600 scale model of TGS and surrounding terrain was designed and constructed. The entire area

modeled is depicted in Figure 1. A plan view of the turntable which was at the center of the modeled area

is given in Figures 4 and 4 for the terrain-in and terrain-out configurations, respectively. The location of the

TGS buildings is shown in Figure 3. The model included all significant terrain within a 1040 m (3400 ft)

radius of the stacks under evaluation. Upwind and downwind of the turntable, additional model terrain and

roughness elements were installed.

Model stacks were constructed of brass tubes. Trips were installed within the stacks to ensure that

the flow was fully turbulent upon exiting the stacks. The stacks were supplied with a premixed certifiedair-helium-hydrocarbon mixture. A precision gas flow meter was used to monitor and regulate the discharge

velocity. The stack parameters for all tests are provided in Table 1.

Concentration sampling taps were installed at numerous downwind sampling locations and were

positioned relative to the turntable center. For each test, appropriate sampling locations were used to ensure

that the maximum concentration was measured.

Wind Tunnel

All testing was carried out in CPP's environmental wind tunnel. A general description of the tunnel

is contained in Figure 6. A total of 10.7 m (35 ft) of model terrain was installed in the tunnel. Sheets of

cubical roughness elements arranged in a staggered pattern were placed up and downwind of the modeled

terrain to assist in the development and maintenance of an appropriate boundary layer. Additionally, eight

semi-elliptical spires and a two-dimensional trip were placed near the entrance of the tunnel to aid in

boundary-layer development.


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frf

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mo

rf

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Q

L

L

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Evaluation of Simulated Boundary Layer and Dispersion Comparability

The EPA fluid modeling guideline for determining excessive concentrations requires that certain

information be collected to document the boundary-layer and dispersion characteristics. All information as

required in the EPA Guidelines was obtained. In general, the results showed that a representative atmospheric

boundary layer was established and that the dispersion were characteristic of that observed in the atmosphere.These test have been performed for many such studies and similar results have been found9.

Calculation of Full-Scale Concentrations

The concentrations measured in the wind tunnel were converted to full-scale concentrations using

the following equation3:

(4)

where

Cf = full-scale concentration for pollutant of concern (g/m3),

(Co)m = tracer gas initial concentration in model (ppm),

L = model (m) and full-scale (f) length scales,

Ur = model (m) and full-scale (f) reference wind speeds (m/s),

Qf = full-scale SO2 emission rate (g/s),

V = model (m) and full-scale (f) volume flow rates (m3/s), and

(C)m = tracer gas concentration less background in model (ppm).

The full scale concentrations as determined from Equation 1 above represent 15 minute to one-hour

average concentrations in the full scale. For this evaluation, it will be assumed that the 1-hour averaging time

is appropriate. For conversion of hourly concentrations to equivalent 3-hour average concentrations, the EPA

guideline

10,11

recommends a multiplication factor of 0.9 0.1. To convert hourly values to 24-hour valuesEPA11 suggests a factor of 0.4 0.2. The guideline also notes that it may be desirable to increase the

conversion factors if terrain or building downwash is of concern. The guideline suggests that the appropriate

conversion factors be determined based on representative meteorological data for the site. Accordingly, a

wind persistence analysis was conducted that demonstrated a 0.542 24-hour conversion factor for a 347.5

degree wind direction (the direction with the terrain upwind of the stack).

Reynolds Number Independence Tests

Prior to conducting the evaluation of terrain wakes effects various documentation test were

conducted, one of which was to evaluate the effect of Reynolds number on the concentration results. Even

though the Reynolds number based on the height of the terrain was sufficiently high (17,600) such that

Reynolds number tests may not have been required, the tests were conducted to confirm Reynolds number

independence. For these tests, a neutrally buoyant tracer gas was released from each of the stacks underevaluation. The exit velocities for each of the stacks were set to be 1.5 times the wind velocity at the top of

the stacks (200 ft).

Three different model wind speeds (2, 4 and 6 m/s) were set at the reference height (600 m full scale)

and ground-level concentrations were measured for the "in" and "out" configurations. The maximum values

found at each downwind distance did not vary significantly for the range of wind tunnel speeds tested (terrain


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Reynolds numbers of 8,810, 17,600 and 26,400). All testing to evaluate the GEP stack height was conducted

using a wind tunnel speed of 4 m/s, where Reynolds number effects were shown to be insignificant.

EVALUATION OF GEP STACK HEIGHT

Excessive Concentrations for the Existing StacksThe first step in evaluating the GEP stack height is to determine whether excessive concentrations

(as defined previously) occur for the exiting 61 m stacks. To make this determination, ground-level

concentrations were measured for various wind directions and wind speeds and the model concentrations

were converted to full scale values using equation 1.

A summary of the maximum hourly, 3-hourly and 24-hourly ground-level SO2 concentrations for the

existing stack configuration is given in Table 2. Three and 24-hour average concentrations were computed

from the hourly values using the conversions factors previously discussed. Table 2 shows that the maximum

hourly SO2 concentrations for existing Units 1, 2 and 3 stacks are 2823, 2960 and 3070 g/m3. The maximum

combined concentration due all three stacks is 8812 g/m3. The individual maxima do not all occur at the

same spacial location, however, the overall maximum is based on a single location. The table also shows that

the 3- and 24-hour concentrations are above the NAAQS for SO2 for each stack individually and for all stacks

combined. Hence, the first criteria for demonstrating an excessive concentration is met.

Table 2 also shows the ratios of maximum concentrations with and without the terrain and buildings

present. For the 247.5 degree wind direction with 6, 8 and 9.6 m/s wind speeds (at 6.1 m height), the ratios

are found to be greater than the critical value of 1.4 and the second excessive concentration criteria is met.

Hence, these results demonstrate that excessive concentration occur for the existing stacks serving

Units 1, 2 and 3 as a results of adverse aerodynamic downwash effects created by nearby terrain and

buildings. Based on this result, Met-Ed can construct and receive credit for a taller stack, provided the stack

height is less than or equal to the GEP stack height. The results from the next phase testing will be used to

specify the actual GEP stack height.

Wind Tunnel Determined GEP Stack Height

As discussed previously, wind tunnel modeling must also be used to determine the GEP stack height

based on the effect of nearby terrain if a stack height taller than the EPA formula height is desired. For this

determination the EPA stack height regulation defines GEP stack height as one that avoids excessive

concentrations (as defined previously).

Preliminary tests were conducted to determine the wind direction and wind speed that produce the

highest concentration and largest differences in concentrations with and without the nearby terrain. The

preliminary tests to evaluate wind direction indicated that the highest ground-level concentrations occur for

a wind coming from 247.5 degrees (see Figure 1). The wind speed tests showed that the highest ground level

concentrations occur for an anemometer wind speed of 4 m/s (6.1 m height). The effect of stack height wasthen evaluated for the critical wind direction and wind speed. Figures 7 and 8 summarize the results. Figure

7 shows the maximum 3 and 24-hour SO2 concentration versus stack height for the 347.5 wind direction tests

with all terrain and structures present. The figure shows that the 3-hr NAAQS is exceeded at a 115 m stack

height and the 24-hour NAAQS is exceeded at a 175 m stack height.


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Figure 8 shows the concentration ratio (terrain amplification factor) versus stack height (normalized

by hill height). The figure shows that the concentration ratio is approximately 1.4 at a normalized stack

height of 575 ft (175 m). The figure also shows a comparison of the TGS data with curves developed by

Castro and Snyder1for an idealized 3-dimensional hill with a width to height ratio of 6 and infinity (2-

dimensional ridge) and a stack located at 1.25 times the cavity length distance (xr). The figures shows thatthe TGS concentration ratios compare well with those presented by Castro and Snyder1.

Based these results, a 175 m stack was selected as the preliminary GEP stack height. Documentation

tests were then conducted to verify and validate that 175 m is the GEP stack height for TGS. The results of

these tests are summarized in Table 3. The table shows that the maximum hourly ground-level SO2concentration for a 175 m stack at the existing Unit 2 stack location is 479 g/m3. The hourly values were

converted to 3-hour 24-hour averages using the 0.9 and 0.542 conversion factors discussed previously. This

resulted in 3 and 24-hour maximum concentrations of 431.2 and 259.5 g/m3. Adding the ambient

background concentrations to these values gives concentrations of 661 and 371 g/m3. The 3-hour

concentration is below the NAAQS for SO2 (1300 g/m3), but the 24-hour concentration exceeds the NAAQS

for SO2 (365 g/m3). Table 3 also shows the ratios of the maximum concentration with and without the

nearby terrain present. All ratios measured for a 175 m stack are greater than the critical value of 1.4. Hence,

the documentation tests have proven that the GEP stack height is 175 m for TGS.

SUMMARY

This paper has described the methods whereby the GEP stack height for TGS was determined based

on the effect of nearby terrain. To make this determination, excessive concentrations were first demonstrated

for the exiting 61 m stack. Thereafter, testing was conducted to find the GEP stack height. This testing

showed that a 175 m stack is GEP based on the nearby Highs Hill. This stack height is 2.6 times the height

of the hill. The concentration ratios (terrain amplification factors) measured as part of this study were

compared with those obtained by Castro and Snyder1 for idealized 3-dimensional hills. Good agreement was

observed.

REFERENCES

1. Castro,I.P., and W.H. Synder, A Wind Tunnel Study of Dispersion From Sources Downwind of

Three-Dimensional Hills,Atmospheric Environment, Vol. 16, No. 8, pp. 1869-1887, 1982.

2. Petersen, R.L. and M. A. Ratcliff, "Terrain Wake Effect On Dispersion Coefficients and Ground

Level Concentrations" AM Eighth Symposium on Turbulence and Diffusion, San Diego, CA, April

26-29, 1988

3. Snyder, W.H., "Guideline for Fluid Modeling of Atmospheric Diffusion," US EPA, Environmental

Sciences Research Laboratory, Office of Research and Development, Research Triangle Park, NC,

27711, Report No. EPA-600/8-81-009, 1981.

4. Arya, S.P.S., and J.F. Lape, Jr., "A Comparative Study of the Different Criteria for the Physical

Modeling of Buoyant Plume Rise in a Neutral Atmosphere,"Atmospheric Environment, Vol. 24A,

No. 2, pp. 289-295, 1990.


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5. Pennsylvania Commonwealth, "1990 Air Quality Report," Commonwealth of Pennsylvania,

Department of Environmental Resources, Bureau of Air Quality Control, Division of Technical

Services and Monitoring, DER#407-5/91.

6. Simonson, B., Personal Communication, June 1, 1992.

7. EPA, "Guideline for Determination of Good Engineering Practice Stack Height (Technical Support

Document for the Stack Height Regulation)," US EPA Office of Air Quality, Planning and

Standards, Research Triangle Park, NC, EPA-45014-80-023R, 1985a.

8. EPA, "Guideline for Use of Fluid Modeling to Determine Good Engineering Practice Stack Height,"

US EPA Office of Air Quality, Planning and Standards, Research Triangle Park, NC, EPA-450/4-81-

003, July, 1981.

9. Petersen, R.L., "Dispersion Comparability of the Wind Tunnel and Atmosphere for Adiabatic

Boundary Layers with Uniform Roughness," AMS Seventh Symposium on Turbulence and

Diffusion, Boulder, CO, November 12-15, 1985.

10. EPA, "Guidelines for Air Quality Maintenance Planning and Analysis Volume 10 (Revised):

Procedures for Evaluating Air Quality Impact of New Stationary Sources," US EPA Office of Air

Quality Planning and Standards, Research Triangle Park, North Carolina, EPA-450/4-77-011, 1977.

11. EPA, "Procedures for Implementing the Excessive Concentration Criteria Using Data from a Wind

Tunnel Demonstration," Fluid Modeling Demonstration of Good-Engineering-Practice Stack Height

in Complex Terrain, EPA 600/3-85-022, 1985b.


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Figure 1. Map showing Location of Titus Generating Station and area modeled.


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Figure 2. WSW through NNE terrain/plant cross-section for the Titus Generating Station.

Figure 3. Close-up plan view of Titus Generating Station showing the nearby buildings removed

configuration.


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Figure 6.CPP Boundary Layer Wind Tunnel


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Figure 7.Maximum 3- and 24-hour SO2 concentrations versus stack height.

Figure 8. Concentration ratio (or amplification factor) versus stack height for TGS compared with

values observed by Castro and Snyder1.


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