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CONTENTS SECTION

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1 SCOPE ....................................................................................................................................................... 3 2 REFERENCES............................................................................................................................................ 4 3 BACKGROUND.......................................................................................................................................... 5 4 DEFINITIONS OF AIR POLLUTION AND ITS DISPERSION .................................................................... 5 4.1 NORMAL UNITS IN AIR POLLUTION .............................................................................................. 5 4.2 AIR POLLUTION SOURCES ............................................................................................................ 6 4.3 REGULATORY TERMS .................................................................................................................... 6 5 BASIC AIR POLLUTION METEOROLOGY ............................................................................................... 7 5.1 ATMOSPHERIC STABILITY ............................................................................................................. 8 5.2 ATMOSPHERIC INVERSION CONDITIONS.................................................................................... 9 5.3 PLUME RISE .................................................................................................................................... 9 5.4 STACK TIP AND BUILDING DOWNWASH .................................................................................... 10 6 MODELS, METHODS, AND COMPUTER PROGRAMS ......................................................................... 10 6.1 MAXIMIZED CONCENTRATIONS.................................................................................................. 11 6.2 SELECTION GUIDANCE ................................................................................................................ 12 6.3 SIMPLE, SCREENING COMPUTER PROGRAMS ........................................................................ 13 6.3.1 Flares........................................................................................................................................ 13 6.3.2 Fumigation ............................................................................................................................... 14 6.4 REFINED COMPUTER PROGRAMS ............................................................................................ 14 6.5 DENSE GAS COMPUTER PROGRAMS ........................................................................................ 15 6.6 REACTIVE, PHOTOCHEMICAL COMPUTER PROGRAMS.......................................................... 16 7 NOMENCLATURE.................................................................................................................................... 17 8 SAMPLE CALCULATIONS ...................................................................................................................... 19 8.1 SIMPLE POINT SOURCE DISPERSION PROBLEM ..................................................................... 19 8.2 FUMIGATION PROBLEM .............................................................................................................. 21

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TABLES

Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9

Representative Ambient Air Quality Standards ............................................................... 23 Mass-Volume Conversion Factors .................................................................................. 24 Wind Speed Exponent, P ................................................................................................ 24 Pasquill-Turner Stability Categories................................................................................ 25 Surface Roughness Lengths Values for Typical Ground Cover ...................................... 25 Combinations of Wind Speed and Stability for Screening Modeling ............................... 25 Briggs Plume Rise Equations.......................................................................................... 26 Guidance For Selecting Computer Programs ................................................................ 27 Downwind Distance (KM) to the Maximum GLC for Inversion Break-Up Fumigation as a Function of Stack Height (h) and Plume Height (H)................................................. 28

FIGURES

Figure 1 Figure 2 Figure 3 Figure 4

Typical Modes of Plume Behavior................................................................................... 29 Coordinate System Showing Gaussian Distribution and Elliptical Concentration “Footprint” of the Ground Level Impact ........................................................................... 30 Horizontal Dispersion Coefficient (sy) as a Function of Downwind Distance and Stability Class ................................................................................................................. 31 Vertical Dispersion Coefficient (sz) as a Function of Downwind Distance and Stability Class ................................................................................................................. 32

Revision Memo

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06/04

This revision updates the list of dispersion models currently used within EMRE, corrects typographical errors within various texts, and updates Table 1, Representative Ambient Air Quality Standards.

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SCOPE

This section is about calculating concentrations of non-reactive pollutants emitted from typical process industry sources for comparison to appropriate ambient and work-place criteria. It describes methods and computer programs that can be used. Some methods are simple; many are very complex and require use of specialized computer programs. Some emissions are lighter than air and others are heavier than air; they are modeled very differently. Emission sources include conventional flue gas stacks from fired heaters, boilers, and gas turbines; flares; process vents; tanks; fugitive emissions; accidental releases of gases; and spills of volatile liquids. Emissions may be continuous, short-duration, or intermittent. Most methods apply to flat terrain and are used to predict hourly to annual ambient ground-level-concentrations (glcs) for distances out to about 10 – 50 kilometers (~6 – 30 miles). For calculation purposes, pollutants are assumed to disperse as gases. Here, the overall approach is to describe the basic method and then to provide guidance to select computer programs for air dispersion calculations. First, several necessary definitions are provided for general terms used in air pollution. Then, a brief overview of atmospheric meteorology is given. Finally, the basic calculation methods are presented for estimating plume rise; building and stack tip downwash; ambient, ground level concentrations for conventional flue gas stacks and flares; and the conversion of maximum ground-level-concentrations from one averaging-time to another. For routine calculations, computer programs are normally used. Therefore, guidance is provided for selecting proprietary ExxonMobil Engineering and selective public domain computer programs that can be used for either screening-quality or more rigorous air dispersion analyses. The screening-quality programs essentially computerize the basic dispersion calculation method for discrete meteorological conditions of emissions from a single stack. These results are intended to be more conservative (higher concentrations of pollutants) than those from the more rigorous tools. The rigorous computer programs have many capabilities to more realistically simulate the actual emissions sources, meteorological conditions, and receptor characteristics of the particular facility being analyzed. However, significantly more input data and professional time are required to use these programs. The more rigorous or refined programs are used widely in air quality permitting. Beyond permitting, dispersion results from appropriate computer programs are used in chronic and acute community health and risk assessment studies for toxic and flammable substances. Typically, advanced knowledge and expertise is required to conduct these studies. This expertise may be obtained from ExxonMobil Engineering's Environmental Engineering Section, selected individuals in the Safety, Civil, and Marine Section, or a qualified outside air dispersion modeling consultant. ç

Predicted pollutant concentrations using air dispersion models can be considered an estimate of the pollutant concentration that would be measured at a specified location and time after release. The agreement between calculated pollutant concentrations and actual ambient levels depends upon many factors, including: · Model used (e.g., screening vs. refined) · Dynamic variability in meteorological conditions, including atmospheric stability (e.g., measured meteorological data or average

stability class) and wind speed

· Terrain variability and urban structures · Actual vs. calculated plume rise · Uncertainty in the emissions rate and source compositions · Type of pollutant and concentration thresholds (e.g., odorant vs. toxic) · Type of source (area, volume, or point) · Assumed averaging time

Because these and other factors contribute to differences between calculated and actual ambient concentrations, it is important that persons familiar with ambient air quality impact assessments review calculation methodologies and results. ç

Note that metric units are provided as the primary convention for this section consistent with universally accepted practice for Air Dispersion Calculations. Customary units are provided within brackets, i.e., opposite to convention adopted for the DPs.

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REFERENCES

1.

Beychok, M. R., Fundamentals of Stack Gas Dispersion, Third Edition, Irvine, CA, 1994.

2.

Hanna, S. R., Briggs, G. A. & Hosker, R. P., Handbook on Atmospheric Diffusion, Oak Ridge, TN, Technical Information Center, U. S. Department Of Energy, 1982. Turner, D. B., Workbook of Atmospheric Dispersion Estimates: An Introduction to Dispersion Modeling, Second Edition, Boca Raton, FL, CRC Press, Inc., 1994. Lyons, T. J. and Scott, W. D., Principles of Air Pollution Meteorology, Boca Raton, FL, CRC Press, Inc., 1990. Hanna, S. R. and Strimaitis, D., Workbook of Test Cases for Vapor Cloud Source Dispersion Models, New York, NY, Center for Chemical Process Safety of the American Institute of Chemical Engineers, 1989. Hanna, S. R. and Drivas, P. J., Guidelines for the Use of Vapor Cloud Dispersion Models, New York, NY, Center for Chemical Process Safety of the American Institute of Chemical Engineers, 1987. EPA-450/2-78-027R, Supplement A, B, and C, Guideline on Air Quality Models, Revised, U.S. Environmental Protection Agency. EPA 450/4-80-023R, Guideline for Determination Of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations (Revised), U.S. Environmental Protection Agency, June 1985. EPA-454/R-92-019, Screening Procedures for Estimating the Air Quality Impact of Stationary Sources, Revised, U.S. Environmental Protection Agency, October 1992 Cole, C. F., and Hoffnagle, G. F., Review of EPA's Proposed Flare Plume Rise Procedure, American Petroleum Institute, November 1988. Auer, A. H., Correlation of Land Use and Cover With Meteorological Anomalies, Journal Of Applied Meteorology, 1978. Esmaili, E., User's Manual for BRIGGS – A PC Based Gaussian Air Dispersion Modeling Program, Exxon Research & Engineering Company, CPEE.43, October 1990. Esmaili, E. and Shin, S. H., PHRASE 2.2: Predicting Hazardous Release Rates Into the Atmosphere and Subsequent Environmental Impact, Exxon Research & Engineering Company, EE.113E.94, February 1995. EPA-454/B-95-004, SCREEN3 Model User's Guide, U.S. Environmental Protection Agency, September 1995. EPA-454/B-95-003a, User's Guide for the Industrial Source Complex (ISC3) Dispersion Models, U.S. Environmental Protection Agency, September 1995. Post, L, HGSYSTEM 3.0 User's Guide and Technical Reference Manual, Shell Research Limited, Chester, U.K., June 1995. DiCristofaro, D. C., and Hanna, S. R., OCD: The Offshore and Coastal Dispersion Model, Sigma Research Corporation, November 1989. ADMS 3, User's Guide, CERC Limited, Cambridge, U.K., February 1999. Seigneur, C., Pai, P., Louis, J.F., Hopke, P., and Grosjean, D., Review of Air Quality Models for Particulate Matter, American Petroleum Institute, June 1997.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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BACKGROUND

Throughout the world, dispersion calculation methods are used to quantify the impact of pollutant emissions on the environment. For example, regulatory agencies rely on these methods for permitting new and modified industrial facilities. Dispersion calculation methods offer the only technique available to forecast the impact of these new operations on air quality. Calculated impacts are compared to ambient air quality standards that have been promulgated by worldwide regulatory agencies (Table 1). More recently, regulations have been promulgated that require dispersion calculations in risk analyses of chronic and acute hazardous substances emitting from industrial facilities. As the United States (US) Environmental Protection Agency (EPA) dispersion calculation method is widely accepted throughout the world, it is the basis for the method included here. The US has led in the development of dispersion calculation methods for permitting of industrial facilities that emit the so called criteria pollutants (i.e., sulfur dioxide, particulate matter, nitrogen dioxide, carbon monoxide, and ozone) with the 1977 Clean Air Act Amendments (CAAA) and ensuing regulations. These regulations forced a reliance on dispersion calculation methods to a much greater extent than previously, and at the same time, restrict the number and type of calculation procedures acceptable for regulatory purposes. In the years that followed, most countries in which ExxonMobil has operations have accepted the dispersion calculation methods recommended by the EPA. As the EPA led in the development of methods for the dispersion of criteria pollutants, the Europeans have lead the development of dispersion calculation methods for predicting the dispersion of accidental releases of heavier-than-air gases. Major releases of flammable and toxic gases in Europe motivated this development as well as the original Seveso Directive. With the Bhopal disaster, the US included accidental release provisions in the 1990 CAAA. Also, the European Union (EU) has replaced the Seveso Directive with a new directive known as the Control Of Major Accident Hazards (COMAH). Through this legislation in the US and EU, further reliance is expected on dispersion calculation methods for assessing accidental hazardous releases. Because of regulatory constraints and the technical complexity of actual problems, users of this section will frequently require assistance either from EMRE's Environmental Engineering Section or from a qualified outside consultant. As a general rule, a user should consider asking for help, even if they are somewhat knowledgeable about dispersion calculations. This is especially important if the calculated results and the allowed maximum concentrations are the same order of magnitude (i.e., it is not absolutely clear-cut that the facility either complies or does not comply with the ambient criterion). 4

DEFINITIONS OF AIR POLLUTION AND ITS DISPERSION

In this section, when an important term is first introduced, it is shown in boldface. 4.1

NORMAL UNITS IN AIR POLLUTION

Air pollution concentrations are usually expressed in units of mass of pollutant per volume of contaminated air, e.g., mg/m3 (nlbm/ft3). If the contaminant is a gas, the concentrations may also be expressed in units of volume of pollutant per volume of contaminated air, e.g., cm3/m3 or the equivalent volume parts per million (vppm). Volume/volume units have the advantage in that they are independent of temperature. Table 2 gives factors for converting from one set of units to the other. Two specific concentrations of interest are the ambient concentration and the ground level concentration (glc). Though both terms are commonly used interchangeably, they are not identical. An ambient concentration is the concentration at any point of interest in the atmosphere. Ambient concentrations at points on or close to the ground due to all sources are glcs.

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4.2 AIR POLLUTION SOURCES Any substance discharged into the atmosphere from an air pollution source is an emission. A point source is one from which emissions of waste gas are discharged into the atmosphere through a well-defined conduit, and for which the downwind distance of concern is much larger than the conduit diameter. Most point sources are conventional flue gas stacks. A typical or ordinary industrial stack is usually characterized as one for which:

·

The minimum height to avoid ambient turbulence effects near the ground is typically above about 15 m (50 ft) while the maximum height can be limited by regulation.

·

The emissions are constant and continuous over a time period that is long with respect to both the time over which the concentrations are measured and averaged, and the downwind distance of concern divided by the wind speed.

·

The stack exit velocity is high enough with respect to the mean horizontal wind speed (at least 1–1/2 times) so that the pollutant plume is not sucked into the low-pressure region on the lee side of the stack.

·

The emissions are discharged at temperatures less than about 550°C (1000°F) and at exit velocities less than about 40 m/s (130 ft/s).

·

The average molecular weight of the emitted stream is close to that of air (28.97).

Process vents, safety valves, and tank vents are other point sources in process facilities. Typically, these sources have shorter stacks and emit streams that are significantly different than air. Also, exit (exhaust) velocities can be either much higher or lower than the velocities from conventional flue gas stacks, and release durations are short rather than constant and continuous. An area source, unlike a point source, does not emit pollutants through a well-defined conduit nor does the downwind distance of concern have to be larger than the area of the source. Frequently, even if a source is known to be emitted from individual points, it is considered to be an area source if the points are too numerous to consider individually or it is otherwise convenient to do so for the sake of calculation. Here, emissions are assumed to be discharged from the same elevation. A volume source is generally a combination of numerous point sources of different height. It represents a convenient mathematical construct to calculate dispersion for complicated sources. Fugitive emissions, usually treated as either an area or volume source, result from leaks in pollution control devices and other mechanical equipment. Blow-off from stored piles of crushed or ground solids is another example of fugitive emissions. Such emissions are usually not amenable to dispersion calculations using simple techniques. Consult the Environmental Engineering Section of the EMRE Plant Engineering Division or a qualified outside consultant for guidance. 4.3

REGULATORY TERMS

Ambient Air Quality Standards (AAQS) are those ambient air pollution concentrations, usually glcs, that a regulatory agency has decreed may either never be exceeded or may be exceeded only a certain, small percentage of the time. AAQS are intended to protect the public health and welfare. The current National AAQS (NAAQS) for the US and several other countries is listed in Table 1. AAQS represent the maximum total allowable contributions of pollution from all sources. These include natural and anthropogenic sources (i.e., stationary and mobile). The aim of regulatory agencies is to keep or bring ambient concentrations below the AAQS. Countries use different regulatory approaches to achieve this goal. For example, some AAQS promulgated represent the maximum allowable ambient ground level concentration for a given new or modified facility. This approach is not used in the US. In the US, regulatory authorities require a facility to demonstrate that it will raise existing ambient ground level concentrations by only a small portion of the total NAAQS. Further, many other restrictions and limitations are contained in US regulations. Because regulations differ between countries, specific guidance should always be obtained from the appropriate regulatory authority prior to conducting any facility compliance demonstration. This will ensure consistency with country-approved regulations. Sometimes, before a new or modified air pollution source is allowed to begin operations, the owner must decrease pollutants from other nearby sources so that the net increase in emissions is zero or even negative. The decreased emissions, or emissions offsets, can occur at sources belonging to a different owner than the owner of the new or modified source. Usually, but not always, emission offsets are required only for pollutants whose existing ambient concentrations exceed applicable air quality standards.

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For fugitive and accidental releases of hazardous substances, regulations often require hazard assessments. It is part of a quantitative risk analysis that estimates the downwind ambient concentrations for releases, and can be compared to ambient chronic or acute criterion. For chronic hazard assessments, continuous emissions of substances that are known to cause long-term health effects are calculated using conventional dispersion methods. For acute hazard assessments, accidental releases resulting from equipment failures are analyzed using dispersion methods developed for releases of substances in which heavier-than-air gas clouds are produced. Typically, acute hazard assessments are concerned with acutely toxic and flammable substances. 5

BASIC AIR POLLUTION METEOROLOGY

Air dispersion, or atmospheric diffusion, is the dilution of an air pollutant due to natural atmospheric movement and to the interaction between the momentum and buoyancy (if any) of the pollutant and natural convective mixing of the atmosphere. The atmosphere is constantly moving in all directions and on all size scales. For convenience, the average of the larger-scale slowly changing component is termed the mean wind speed, u, while the rapidly changing smaller-scale components are called turbulence and is treated statistically. Both are actually on one continuous spectrum. For air dispersion calculations, the assumption is usually made that the mean wind speed is purely horizontal (the vertical component is zero), and that turbulence is a totally random phenomenon. Nearest the ground, turbulence is caused by air convection and by mechanical mixing as the wind passes over and around natural and man-made surface obstructions. Surface roughness length is a measure of the mechanical turbulence. Plume dilution increases with increasing turbulence. Note that the wind (speed and direction) changes with elevation. This is called wind shear. When calculating air dispersion, the change of wind direction is almost never considered. However, the change of wind speed is normally taken into account by assuming an exponential increase with elevation. The often-used EPA method is the logarithmic wind speed power law as follows:

uz = um ( z / zm )p where: uz um z zm p

Eq. (1)

= = = = =

Wind speed at elevation z, m/s (ft/s) Wind speed at reference elevation, zm, m/s (ft/s) Desired elevation at which wind speed is to be calculated, m (ft) Reference elevation, m (normally 10 m) (~33 ft) Empirically derived wind speed exponent (Table 3)

Constant turbulence means that air pollution concentrations are always changing, and the highest measured concentrations increase as the averaging time decreases. Averaging time is the time interval over which the concentration is measured, and can vary from a fraction of a second (the response time of very fast instruments - there are no instantaneous measurements) up to a year or more. It has been found that, for non-reactive pollutants and sampling points not located between two or more sources of interest, the highest concentrations generally vary with averaging time according to the formula (c1 / c2 ) = ( t1 / t 2 )-a where: c1 c2 a

= = =

Eq. (2) Highest glc over averaging time t1 Highest glc over averaging time t2 Conversion exponent = 0.2

Calculated maximum concentrations may be converted from one averaging time to another (up to 24-hour averages) using this equation. For approximate estimates of annual average concentrations, a reasonable rule of thumb for the annual concentration value is about 10% of 1-hour maximum concentrations.

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ATMOSPHERIC STABILITY

Throughout the day, the dispersive capacity of the atmosphere changes as solar radiation and wind speed change. Atmospheric stability describes the dispersive capacity of the atmosphere in terms of convective and mechanical turbulence. To calculate pollutant dispersion, it is necessary to characterize this dispersive capacity of the atmosphere. However, the dispersive capability is hard to measure. What is done in practice is to define atmospheric stability by empirical or semi-empirical means based on easily observed phenomena, and then to use the stability so defined as an indicator of dispersive capability. Unfortunately, several empirical definitions of stability exist, and they are not consistent with each other. Also, the continuous turbulence spectrum of the atmosphere is frequently divided into discrete stability categories, and the several schemes that exist for doing this are not consistent with each other. Most schemes consider 4 or 6 discrete stability categories (more rarely 7 or some other number) with the designation “A” (or “1”) going to the most unstable atmosphere and subsequent letters (or numbers) indicating progressively more stable atmospheres. Neutral atmospheres are usually indicated by “C” or “D” (“3” or “4”). Note that the term “neutral atmosphere" does not denote zero dispersion. All atmospheres, even those more stable than neutral, disperse pollutants, although an extremely stable atmospheric layer (e.g., a layer associated with an inversion) can be impenetrable to a dispersing layer. By this approach, the local dispersive capability of the atmosphere defines the local stability.

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One way of defining divisions of the turbulence spectrum into stability categories is by means of the vertical temperature gradient or lapse rate, G. This is the rate at which atmospheric temperature decreases with elevation from the ground. In this scheme, an adiabatic lapse rate is defined as neutral. This is the change in temperature with height of a small volume of air rising in the atmosphere and expanding adiabatically as the pressure decreases (note: an adiabatic process is defined as one where a material changes its physical state (i.e., pressure, volume, or temperature) without any heat being either added to it or withdrawn from it). It is numerically equal to –0.0098°K/m (-0.55°F/100 ft); the negative sign indicates decreasing temperature with increasing altitude). If the air temperature decreases more rapidly with height, the lapse rate is super-adiabatic and the atmosphere is unstable. If the air temperature decreases more slowly or even increases with height, the lapse rate is sub adiabatic and the atmosphere is stable. For convenience, meteorologists have defined a potential temperature gradient, dq/dz, whose value is zero when the atmosphere is neutral, i.e., the potential temperature gradient is the actual temperature gradient minus the adiabatic lapse rate (the actual temperature gradient plus 0.55°F/100 ft or 0.0098°K/m). Because the adiabatic lapse rate is subtracted out, temperature differences due to pressure gradient are eliminated. The potential temperature gradient reflects only differences due to heat imbalances, and is zero for neutral atmospheres, positive for stable atmospheres, and negative for unstable atmospheres. Other definitions of atmospheric stability related to temperature gradient, are the Richardson number, which is the ratio of the buoyant forces caused by the lapse rate to the acceleration forces caused by wind shear and from boundary layer similarity theory, the Monin Obukhov length (L), which is the ratio of the fluctuating component of the wind to the atmospheric heat flux. The absolute value of the Monin Obukhov length can be thought of as the depth of the mechanically mixed layer near the ground. L is positive for stable conditions (usually at night), negative for unstable (usually daytime under clear skies), and approaches infinity for neutral conditions (dawn and dusk transitions periods and cloudy windy conditions). A commonly used definition of atmospheric stability, unrelated to and frequently inconsistent with temperature gradient, etc., is the definition by means of solar insolation (i.e., whether it is day or night and whether it is sunny or cloudy) and wind speed. The six Pasquill stability categories (“A” through “F”) are defined in this way. Table 4 presents the Pasquill-Turner allowed combinations of wind speed and atmospheric stability for rural areas (i.e., areas consisting of mostly flat, vegetated terrain). The corresponding atmospheric stability categories in an urban area are generally one or two categories more unstable than would be predicted using the Pasquill-Turner approach. In the EPA computer programs, specific dispersion coefficients have been recommended for rural and urban areas. The selection of rural or urban coefficient is often decided by using the Auer scheme for many regulatory applications. In its simplest form, if more than 50% of the area within a 3 km (~2 miles) radius of the emission source is vegetated, the area is considered rural. Otherwise it is urban. In more recent computer programs, such as AERMOD and ADMS 3 from the US EPA and the UK Meteorology Office, respectively, as well as programs for accidental releases of heavier-than-air gases (e.g., PHAST), the surface roughness length must be specified to account for the influence on dispersion of surface obstructions in the surrounding area. Here, it is assumed that the plume is always large in comparison to height of the surface obstructions. Typical surface roughness lengths for various ground conditions are shown in Table 5. Surface roughness length, zo, is effectively the height above the surface of the ground at which the mean wind speed, u, is equal to zero.

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Finally, many regulatory agencies insist on a thorough consideration of atmospheric stability and wind speed conditions in performing air dispersion calculations. This may require the gathering and utilization of hourly sequential meteorological data or the use of representative combinations of wind speed and atmospheric stability. It is always preferable to use on-site meteorological data when available, as it produces more accurate estimates of glcs. However, often meteorological data from a nearby location such as an airport, etc., are substituted. Using representative combinations of wind speed and atmospheric stability is the least accurate and most conservative alternative. When hourly sequential meteorological data are not available, recommended combinations of wind speed and stability can be used (Table 6). 5.2 ATMOSPHERIC INVERSION CONDITIONS Inversion conditions occur in the atmosphere near the ground and aloft. An inversion is a layer of air in the atmosphere in which the temperature increases with height rather than decreasing with height. Near the ground, diurnal variations typically cause them as well as their break-up. Aloft, it can be an impenetrable layer to plume dispersion. An elevated inversion is often called an inversion lid or a capped inversion. The height of the inversion lid is called the mixing height. Elevated inversions often persist for days or even weeks at a time. Inversions can strongly restrict pollutant dispersion and during break-up can cause high shortterm glcs.

At the ground, a surface inversion layer is typically formed at night under conditions of light wind and clear skies, when the loss of heat from the ground cools the air adjacent to it faster than the air aloft. At dawn, solar heating begins warming the ground producing an unstable air mass near the ground that pushes the more stable layer aloft. As the stable surface layer is initially below the height of the plume, pollutants from elevated stacks are trapped above the inversion resulting in no glc. As the depth of the unstable air mass grows, it reaches the height of the plume. Then, the pollutants are rapidly mixed (fumigated) in the unstable air mass, perhaps all the way to the ground, causing short term, high glcs. Elevated inversions are more common. In an elevated inversion the air temperature decreases with height from the ground to the base of the elevated inversion layer. Then, the temperature increases with height through the depth of air aloft to the top of the layer. Above the inversion, the temperature returns to the normal decrease of temperature with height. This condition can be caused by the gradual descent of air aloft accompanied by adiabatic warming from an increase in atmospheric pressure, by a sea breeze introducing a layer of cool air beneath a warmer air mass, or by the passage of a weather front. A method for calculating glcs for inversion break-up is provided in Equation (15). In Figure 1, simplistic sketches are presented of elevated plume behaviors under various atmospheric stabilities and mixing height conditions. In the small diagram of altitude versus temperature, the atmospheric condition conducive to plume formation is shown in terms of the relation between the existing ambient temperature gradient (solid lines) and the dry adiabatic lapse rate (dashed lines). 5.3

PLUME RISE

Plume rise (Dh) is the rise of a waste gas plume in the atmosphere due to its buoyancy or momentum. Essentially, the higher the plume rise the lower the glcs. Most calculation methods for plume rise treat it as if it were independent of dispersion. Since it is not, plume rise is usually better regarded as a convenient mathematical construct rather than an accurate description of reality. When the plume rise is added to the stack height (h), the result is called the effective stack height (H). H = h + Dh

Eq. (3)

For most applications and for nearly all regulatory purposes, the plume rise formulas by Briggs are favored. These formulas are based on theoretical considerations and observations of plume rise from a wide variety of sources. Separate formulas are used for buoyancy and momentum induced rise for various atmospheric stability conditions. The first step in calculating plume rise is to calculate the buoyancy flux factor, F in units of m4/sec3 (ft4/sec3): F = [ g ( Vs ) (d2 ) ( DT ) ] / [ 4 Ts ] where: g DT Vs d

= = = =

Eq. (4)

Acceleration of gravity, 9.8 m/sec2 (32.2 ft/sec2) Ts – Ta, Ts is the stack gas temperature, °K (°R), and Ta is the air temperature, °K (°R) Stack gas exit velocity, m/sec (ft/sec) Inside stack diameter, m (ft), at the release point to the atmosphere

In order to determine whether the plume is dominated by buoyant or momentum rise, the critical temperature difference, DTc, is calculated based on the atmospheric stability.

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Unstable or neutral atmospheres: For F < 55 (6366 ft4/s)

DTc = [(0.0297) (Ts) (Vs1/3)] / [d2/3]

Eq. (5)

For F ³ 55 (6366 ft4/s)

DTc = [(0.00575) (Ts) (Vs2/3)] / [d1/3]

Eq. (6)

Stable atmospheres: DTc = [(0.01958) (Ts) (Vs) (S1/3)] where: S G dT/dz

= = =

Eq. (7)

g(dT/dz -G)/Ta Adiabatic lapse rate (-0.0098 °K/m, -0.0054°R/ft) Actual temperature gradient (0.0102 °K/m or 0.0056°R/ft for “E” and 0.0252 °K/m or 0.014°R/ft for “F” stability)

If the actual DT exceeds DTc, the plume is buoyant dominated; if not, it is momentum dominated. The appropriate equation for calculating the plume rise, Dh, is then selected from Table 7. 5.4

STACK TIP AND BUILDING DOWNWASH

Aerodynamic downwash (commonly known as stack-tip, building, or terrain downwash) is the sucking down of a plume into the low-pressure cavity on the lee side of a bluff body (building or hill) nearby the stack. Stack tip downwash can generally be avoided by making the stack exit velocity at least one and a half times the ambient horizontal wind speed. If the stack exit velocity is very low, downwash (or down draft) may result instead of plume rise. Building and terrain downwash can generally be avoided by constructing a taller stack than the nearby obstruction. The stack height that is normally sufficient to prevent building or terrain downwash is called good engineering stack height,

Hg = Bh + 1.5L

Eq. (8)

where:

Hg Bh L

= = =

Good engineering stack height, measured from the ground-level elevation at the base of the stack Height of nearby structure measured from the ground level elevation at the base of the stack Lesser dimension, height or projected width, of the nearby structure

In Eq. (8), both the height and width are determined from the frontal area of the structure, projected onto a plane perpendicular to the direction of the wind. For the purpose of determining good engineering stack height, nearby is limited to five structure heights or widths, whichever is less, downwind from the trailing edge of the structure. The area of influence becomes diminishingly smaller as the height to width ratio of a structure increases. Therefore, an isolated and thin process tower should not be considered in good engineering stack height determinations. In areas with multiple towers and process structures, computer models are necessary to calculate good engineering stack height. Also, as process structures and towers are generally aerodynamically smooth and porous, Eq. (8) should be used with caution. Here, rule of thumb guidance is to calculate Hg using the dimensions of the solid portion of the structure that is enclosed by latticework. However, rigorous techniques using an environmental wind tunnel may be necessary to justify taller stacks to regulatory authorities in some cases. In this latter situation, assistance can be obtained from EMRE's Environmental Engineering Section or a qualified outside consultant. ç

ç

6

MODELS, METHODS, AND COMPUTER PROGRAMS

A model is a description of physical phenomena; a method is a specific set of equations conforming to a particular model, including all the constants necessary to obtain numerical results; and a computer program is an algorithm for solving the equations on a computer. For example, there is the Gaussian model, which states that plume concentrations downwind of a stack are normally distributed both horizontally and vertically. There is the EPA method, which incorporates specific numerical values for the standard deviations of these normal distributions, and also a specific equation to calculate plume rise and effective stack height. Finally, there are computer programs such as the AMS/EPA Regulatory Model (AERMOD), the Industrial Source Complex (ISC3), and the SCREEN3 codes from the US EPA, which solve the equations. AERMOD and ISC3 are refined regulatory computer programs, while SCREEN3 is a simple screening computer program. Both ISC3 and SCREEN3 use the Gaussian model for dispersion. AERMOD uses the Gaussian model for stable wind conditions, but uses a non-Gaussian probability density function for vertical dispersion and unstable wind conditions. It also uses more modern approaches to account for the Planetary Boundary Layer (PBL). EXXONMOBIL RESEARCH AND ENGINEERING, FAIRFAX, VA.

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The PBL roughly corresponds to the lowest 1 km of the atmosphere. In the PBL, the flow over a stationary underlying surface gives rise to a frictional drag force that is comparable in magnitude to the other terms in the horizontal equations of motion. The Gaussian model assumes that the pollutant concentration downwind of a source follows Gaussian (normal probability) distribution in both the crosswind and vertical directions (Figure 2). The common equation for a continuous release from a point source is shown below. It is derived by integrating the differential equations of motion under the assumptions of conservative (nonreacting) pollutant and stationary homogeneous turbulence (i.e., turbulence is independent of time and distance). c(x,y,z) = Q / (2p sy sz u) [exp –1/2 (y / sy )2] {exp[–1/2 ((z – H) / sz )2] + exp [–1/2 ((z + H) / sz )2]} where:

c x,y,z Q p sy

= = = = =

sz

=

u H

= =

Eq. (9)

Ground level concentration, g/m3 (lbm/ft3) Downwind, crosswind, and vertical location in space of the receptor with concentration, c Pollutant mass emissions rate, g/s (lbm/s) Mathematical constant pi = 3.1416… Standard deviation of concentration distribution in the crosswind direction, m (ft), at downwind distance, x (Figure 3) Standard deviation of concentration distribution in the vertical direction, m (ft) at downwind distance, x (Figure 4) Wind speed at the point of release, m/s (ft/s) Effective height of the centerline of the pollutant plume, m (ft)

Five points may be noted about the above equation. The first is that plume rise (Dh, included in the effective stack height, H) and dispersion are treated as mathematically independent. This two-step feature (plume rise and dispersion) is characteristic of all dispersion methods using the Gaussian model. The second point is that complete reflection at the ground is assumed. This is the elastic bouncing back into the air of all pollutants hitting the ground. This is an idealization, since in reality there is always some deposition. Reflection is accounted for by the second exponential term involving z, namely: exp [–1/2 ((z + H) / sz )2 ]

Eq. (10)

The third point is that the equation is for a single stack only. Concentrations for emissions from multiple stacks must be obtained by superposition. This is the process of obtaining the total pollutant concentration at a location by adding the contributions from each stack to the concentration at that receptor location. The procedure assumes that there is no interaction among the plumes from the various stacks. The fourth point is that there is no provision in the Gaussian equation for the effects of chemical reactions. And fifth, this formula assumes persistent wind direction. Often it is used for distances out to 50 kilometers (~30 miles), but more realistic distances might extend to about 10 – 15 kilometers (~6 – 9 miles). By the definition of a calculation method given above, whenever a particular set of dispersion coefficients (sy and sz) or a particular formula for calculating plume rise and effective stack height (Dh and H) is used with the Gaussian equation, a separate calculation method results. However, of all the possible methods, only one will be discussed here, the one recommended by the US EPA. 6.1

MAXIMIZED CONCENTRATIONS

The resultant glcs from an elevated point source pass through maxima with respect to both wind speed and downwind distance. The physical reasons for this are as follows:

·

Wind speed - At high wind speeds, glcs approach zero because the plume is rapidly diluted. At low wind speeds, glcs again approach zero because there is no wind to impede plume rise and the plume never reaches the ground. Therefore, a maximum must occur at some intermediate wind speed.

·

Downwind distance - Directly below the stack, the contribution to the glc is zero, because the emitted plume has not yet touched the ground. Far downwind, the plume is dilute and the contribution to the glc again approaches zero. Therefore, a maximum must occur at some intermediate point.

For a group of stacks, the maxima occur with respect to wind direction as well as wind speed, and downwind distance. ç

The maxima are important because they are the highest glcs that can occur, and thus are compared with AAQS. For calculation methods conforming to the Gaussian model, analytical solutions for these maxima are possible. For a single stack, methods to determine the maximum glcs are described in Reference 9, Section 4. A nomogram method for determining the distance to ground-level maximum and maximum concentration can be found in Reference 2, Chapter 2.10. Computer programs are the only practical approach to do this calculation for multiple stacks; these programs are routinely used for single stack applications as well.

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SELECTION GUIDANCE

Due to the complexity of air dispersion calculations, computer programs are normally used. In fact, many have been developed by different organizations for various applications with different capabilities. To provide guidance for all of them is not practical. By limiting the choice of computer programs to EMRE's proprietary and other public domain codes that are accepted by regulatory authorities, selection guidance can be provided for most ExxonMobil applications. Naturally, these are by no means the only computer programs that could or should be used. Other public domain and proprietary vendor computer programs are available. However, these other programs should only be used after a competent user has determined their appropriateness. Further, the selection of a computer program for a specific project application must consider the requirements of the appropriate regulatory authority. Additional guidance can be obtained from EMRE's Environmental Engineering Section or from a qualified outside consultant. Assuming no regulatory preference exists, guidance for selecting an appropriate computer program to conduct dispersion calculations is summarized in Table 8. Here, computer programs are listed for both screening and refined air quality analyses of continuous and accidental releases from point and area/volume sources. All listed computer programs, except UAM and HGSYSTEM, treat pollutant releases as chemically inert. The UAM program, developed for the US EPA, is for predicting the air quality impacts of ozone for entire urban areas resulting from emissions from mobile and stationary sources. HGSYSTEM, originally developed for the Industry Cooperative Hydrogen Fluoride Mitigation/Assessment Program (ICHMAP), can simulate the chemical reactions of pressurized liquid hydrogen fluoride (HF) discharged into moist ambient air. For the process industry, HF is the only chemical in HGSYSTEM in which atmospheric chemistry is treated. The other programs are for dispersion of combustion flue gas and lighter-than-air (LTA) gases, as well as heavier-than-air (HTA) gases. For the purposes of dispersion modeling, HTA gaseous releases are those whose density is greater than that of ambient air and whose release Richardson number is greater than 10. The release Richardson number is often defined as follows: æ rp - ra R io = g w o çç è ra

where: Rio g wo rp ra D u*

u p

ö D æpö ÷ ÷ u * 2 u çè 4 ÷ø ø

Eq. (11)

= = = = = = =

Release Richardson number Acceleration due to gravity, 9.8 m/s2 (32 ft/s) Initial plume speed, m/s (ft/s) Initial plume density, kg/m3 (lb/ft3) Ambient density, kg/m3 (lb/ft3) Initial plume diameter, m (ft) Ambient friction velocity, m/s (ft/s)(roughly equal to 0.065u, 0.2u, and 0.02u for neutral,

= =

unstable, stable atmosphere conditions, respectively) Ambient wind speed, m/s (ft/s) Mathematical constant pi, 3.1416

Both criteria should be met for a release to be considered HTA. All listed computer programs can be acquired by ExxonMobil personnel. ExxonMobil proprietary programs can be obtained from the HSME User Tools menu. The other programs listed can be obtained directly from the US EPA website www.epa.gov/scram001; from the National Technical Information Service, NTIS; or from outside vendors. Of the computer programs listed, ISC3 is the most frequently used code that is applied to typical emissions from refinery and chemical plant facilities. It is designed to predict hourly through annual average glcs for continuous, steady-state emissions of flue gas. However, it has been used for simulating the dispersion and ambient impact of fugitive emissions from process facilities, too. ISC3 can simulate releases from point, area, and volume sources for which the elevation of the surrounding terrain is above or below the minimum plume rise of the sources being analyzed. If the source of emission is located offshore (e.g., a drilling or production platform), ISC3 is not recommended for use. For this application, the OCD computer program is preferred.

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Accidental release hazard assessments should be conducted with either PHRASE or HGSYSTEM computer programs. As PHRASE, EMRE's proprietary hazard assessment code, is easier and faster to use, it should be the program of choice for most hazard assessment calculations. It contains codes for liquid spills on land, gas releases from a vessel, and gas releases from a pipeline. Further, it calculates the time dependent rate of release of either a single or multi-component stream. The physical properties of the gas release stream are calculated by using EDL III. Dispersion is simulated in PHRASE by the SLAB computer program developed by Lawrence Livermore National Laboratories. The HGSYSTEM computer program is not user friendly, but contains many capabilities including hydrogen fluoride chemistry, a program for jet releases, and algorithms for short, finite duration releases. These capabilities can make it the program of choice for some hazard assessments. Further, it uses EMRE's liquid spill model (LSM) for calculating pool evaporation. The US EPA accepted both SLAB and HGSYSTEM as appropriate state-of-the-art programs for hazard assessment for regulatory purposes. Additional information regarding these computer programs and program selection in general can be obtained directly from EMRE's Environmental Engineering Section or from a qualified outside consultant. 6.3

SIMPLE, SCREENING COMPUTER PROGRAMS

In some cases, calculated results from a simple screening program are sufficient to demonstrate compliance with air quality standards. However, if exceedances are predicted, then refined computer programs should be used. Also, refined programs are used when multiple sources at a facility must be analyzed. Screening programs are applied to a single stack. Further, screening programs can only use representative combinations of wind speed and stability as input meteorological data (Table 6). As a result, more conservative (higher) glcs are predicted from this class of programs. In contrast, refined programs use a full year or more of hourly sequential meteorological data and automatically calculated source contributions at specified receptor locations. Because of their ease of use, it is generally preferable to first conduct dispersion calculations using a screening program rather than starting with refined methods, if at all possible. 6.3.1

Flares

The air pollutant impact from flare emissions can be calculated using a method in which equivalent stack parameters are determined, allowing conventional dispersion programs to then be used. Since a flare does not have a characteristic diameter for the combusted waste gas, a single exit velocity, or one plume temperature, as do conventional stack emissions, it is necessary to first determine equivalent parameters that can be used in standard dispersion programs to calculate dispersion of flare emissions. This is accomplished by conserving the buoyancy flux of the release. The method uses the total heat release from the combustion of the flared gas [HF, cal/s, (Btu/s)]; computes the sensible heat release [QH (or ~ 65 ft/s) cal/s, (Btu/s)] by subtracting the fraction of heat, (1 – Fr), lost to radiant transfer; assumes an arbitrary value for the exit velocity (20 m/s) and stack gas temperature (1000°C or 1832°F); and then computes an equivalent stack diameter so that the thermal buoyancy flux of the flare and the equivalent stack are identical. The fraction of heat lost by radiant transfer, and therefore not available for plume rise, can vary depending on whether the flare is smoking or non-smoking. For smoking flare calculations, F equal to 0.55 should be used. For non-smoking flares, F can be determined using the following equation: Fr = 0.2 [ (50 MW + 100) / 900]1/2 where: Fr = MW =

Eq. (12)

Fraction of heat radiated Molecular weight of gas being burned

Calculated equivalent and assumed flare parameters are then used as input to the selected computer program along with the appropriate pollutant emissions rate. US EPA recommends using F = 0.55 for all flare release simulations. Since the data supporting this value is for smoking flares, EMRE recommends using this value only for smoking flares. The total sensible heat release (QH) is given by: QH = HF (1 – Fr)

Eq. (13)

The effective stack diameter is calculated using the following formula: ds = 9.88 x 10-4 [QH ]1/2 where: ds

= Stack inside diameter, m (ft)

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Eq. (14)

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Fumigation

As was indicated in the section on meteorology, an inversion is not uncommon in the atmosphere. When it occurs near the ground, it can significantly influence pollutant ground level concentrations. This occurs when pollutants emitted into the stable layer aloft reach more unstable air and mix vertically to the ground. Fumigation such as this can cause high, short duration ground-level concentrations. Such glcs are expected to persist about 30 to 90 minutes. Only refined computer programs contain methods for calculating the influence of elevated inversion on glc of averaging periods from 1 hour and longer. To estimate glcs under inversion break-up conditions, the plume is assumed to be transported aloft in stable air. The plume height, H, is estimated during “F” stability conditions and a wind speed of 2.5 m/s (~5.5 mph) using Eq. (3) and Eq. (7) (and either Eq. C or D in Table 7) assuming Vs > 1.5u. The downwind distance at which the maximum fumigation is expected to occur can be estimated from Table 9. The maximum fumigation concentration (cf) is calculated from the following equation: cf = (Q) / [(2 p)1/ 2 u{ s¢y + (H / 8)} {H + 2 s¢z }] where:

cf Q p u s¢y

= = = = =

Eq. (15)

Maximum fumigation concentration, g/m3 (lb/ft3) Pollutant mass emission rate, g/s (lb/s) Mathematical constant pi, 3.1416 Mean horizontal wind speed, m/s (ft/s) Buoyancy-induced dispersion adjusted crosswind dispersion coefficient = [sy2 + (Dh / 3.5)2]1/2

s¢z =

Buoyancy-induced dispersion adjusted vertical dispersion coefficient [sz2 + (Dh / 3.5)2]1/2

Dh = H =

Plume rise, m (ft) Effective stack height, m (ft)

Values for sy and sz can be determined by using the “F” stability curve in Figures 3 and 4. If the estimated fumigation concentration, cf, is less than the maximum 1-hour concentration, c1, then the effects of fumigation may be ignored. If the estimated fumigation concentration exceeds the maximum 1-hour concentration then the glc should be adjusted by calculating the time weighted average. The 1-hour concentration, c1 using Eq. (9) or an appropriate computer program should be adjusted using a weighted average of c1 and cf, assuming cf persists for 90 minutes. The weighted average, c1¢, for three averaging periods is calculated as follows: AVERAGING TIME

ADJUSTMENT OF cf FOR FUMIGATION

3 hours

c¢1 = (c1 + cf ) / 2

Eq. (16)

8 hours

c¢1 = (13c1 + 3cf) / 16

Eq. (17)

24 hours

c¢1 = (15c1 + cf) / 16

Eq. (18)

The adjusted value c¢1 should then be revised to the appropriate averaging time of concern using Eq. (2). ç

6.4 REFINED COMPUTER PROGRAMS

The most accurate, least conservative estimates of glcs are obtained using refined computer programs. This class of computer programs contains subroutines in which real world diffusion and transport of pollutants can more accurately be simulated. Further, the performance of these programs has been rigorously evaluated using observed air quality data from field studies. From these studies, the best performance occurred when predicted and observed glc were compared unpaired in space and time. Performance degrades for paired comparisons (either in time and/or space). Additional information about program performance can be obtained by contacting EMRE's Environmental Engineering Section or a qualified outside consultant. As noted earlier, the ISC3 computer program is often used for refinery and chemical plant air quality analyses. It was designed for multiple point and area sources for which plume rise is affected by nearby building and process structures. As with all refined computer programs, concentrations are calculated at user specified receptor locations, normally using either Cartesian or polar

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coordinates. Further, it contains algorithms for the dispersion and deposition of particulate matter emitted from sources. ISC3 requires specification of particle size distribution as program input. There is a short-term and long-term version of ISC3. The short-term version is used to calculate from 1 hour to annual averaging periods using sequential hourly meteorological data. The long-term version uses an annual frequency distribution of wind speed, wind direction, and atmospheric stability to calculate seasonal or annual glcs. As noted before, either version can consider terrain effects. To more accurately simulate the effects that terrain has on various meteorological variables (i.e., wind speed and direction) affecting pollutant dispersion, advanced computer models are now available (e.g., ADMS 3 and CALPUFF). However, computations with these programs are input-data and computer-time intensive. Expert's assistance is recommended. If additional information is needed, ExxonMobil affiliates should contact EMRE's Environmental Engineering Section or a qualified outside consultant. Finally, the Offshore Coastal Dispersion (OCD) computer program was originally developed by the US Mineral Management Service for predicting onshore air quality impacts from offshore emission sources. The API evaluated and further improved the OCD computer program. Now, it is the preferred dispersion model for these applications. The most accurate performance with it is obtained using over-water meteorological data, but the code also can use representative meteorological data. Detailed information about the OCD computer program can be found in the user's guide. Assistance can be obtained from EMRE's Environmental Engineering Section or a qualified outside consultant. 6.5

DENSE GAS COMPUTER PROGRAMS

In modeling accidental releases from process facility operations, the discharged substance seldom is released from a stack or vent. Normally, it is accidentally released from severing a small diameter pipe, a blow-out of a flange gasket, a mechanical failure of a pump seal, etc., The released substance may be a pressurized liquid or gas, and it may be a pure compound or a mixture. Either because of temperature, molecular weight, or both, the substance at atmospheric pressure is normally HTA. As such, the initial cloud can be thought to push back the ambient air and resist normal turbulent diffusion. The cloud slumps to the ground. During this process, air is entrained as the cloud spreads under stable stratified flow. Here, air is only entrained into the cloud by small eddy turbulence at the cloud edges. Depending upon the ambient pick-up rate and the wind speed, the cloud is transported downwind. Air continues to be entrained into the core of the HTA cloud. When the core is fully eroded by ambient turbulence, the cloud disperses according to normal Gaussian assumptions. There are three public domain computer programs for HTA dispersion recognized by the US EPA: namely, HEGADAS (HEavy GAs Dispersion from an Area Source), DEGADIS (DEnse GAs DISpersion), and the SLAB computer program. HEGADAS is contained in HGSYSTEM as shown in Table 8. EMRE has incorporated SLAB for the "Predicting Hazardous Release rates into the Atmosphere and Subsequent Environmental impact" (PHRASE) computer program. PHRASE 2.2 has the capability to simulate three different types of multi-component releases: (i) a hole in a gas-filled pipeline (BPIPE); (ii) a hole in a gas-filled vessel (BVESS); and (iii) a liquid emission from a hole in a vessel (LSM) in which flashing, aerosol formation, and liquid pool evaporation can occur. SLAB can be used to predict downwind concentrations from these types of releases if they are heavier or lighter-than-air gases. PHRASE is more user-friendly than either HEGADAS or DEGADIS computer programs and is computationally much faster on typical personal computers. For accidental releases where the release can be considered instantaneous or continuous, all three computer programs produce about equivalent results. However for releases under two minutes in duration, the HEGADAS program is preferred choice (see discussion below). Also, if releases of hydrogen fluoride are being analyzed, the HEGADAS program is the preferred choice among these public domain computer programs. There is a steady state and transient release version of HEGADAS in HGSYSTEM along with other programs. These other programs can simulate spill rates, jet entrainment, pool evaporation, and passive gas dispersion. As already mentioned, HGSYSTEM was originally developed to simulate the thermodynamics and chemical reaction of a liquid hydrogen fluoride release into ambient air. More recently, it has been enhanced by support from the API and PERF (Petroleum Environmental Research Forum) for multi-component gas releases and for simulating short duration releases during worst-case, stable low wind speed conditions. The DEGADIS computer program is similar to HEGADAS, but with many fewer capabilities and output options. While a technically credible computer program, ExxonMobil normally does not use it unless specified by the local regulatory authority as the program of choice. DEGADIS is only recommended for HTA releases. It will abort if the release gas is lighter-than-air. ç

More recently (since 2000), EMRE 's Environmental Engineering Section has been using PHAST to model accidental releases for dense gases. This tool (Process Hazards Analysis Software Tool) provides dispersion calculations using the Unified Dispersion Model (UDM), and also contains functionality for calculating explosion and radiative heat hazard potentials. PHAST has a number of modeling options and reporting features that make it a preferred tool for studies involving accidental releases.

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ExxonMobil affiliates use a variety of other vendor-developed computer programs for accidental releases of HTA gases. Many are more user-friendly and capable of more complete hazard assessment evaluations than the public domain, regulatory-approved codes. Further, several were shown to perform as well as HEGADAS, DEGADIS, and SLAB. However as the codes are proprietary, no independent technical peer review of these codes has been conducted. Therefore, it is not known as to the amount of tuning that has been done to provide satisfactory performance. ExxonMobil affiliates are cautioned to use the vendor codes only after a qualified expert has made a thorough technical review. Further information about these codes may be obtained from EMRE's Environmental Engineering Section or a qualified outside consultant. 6.6

REACTIVE, PHOTOCHEMICAL COMPUTER PROGRAMS

In several cases, emissions from ExxonMobil facilities are reactive in the atmosphere. For example, only about 10% of the NOx emission from combustion boilers or furnaces is nitrogen dioxide (NO2) with the remainder NO. The NO reacts to NO2 in the atmosphere. Another example is ozone (O3). While stationary (i.e., a refinery or chemical plant) or mobile (i.e., automobiles) sources do not emit ozone, it is formed in the atmosphere in complex reactions involving emitted NO2 and hydrocarbons. Nonetheless, ambient standards exist for both pollutants and compliance may be an issue. For NO2 , the normal approach is to calculate the glcs assuming the emitted NOx is 100% converted to NO2 in the stack. If this approach produces glcs at or above the ambient standard or criterion of concern, NOx to NO2 conversion can be estimated based on the available ambient O3 concentrations using the O3 limiting method. If this approach still produces glcs at or above the ambient criterion, advanced computer program techniques may be needed. Additional information regarding these computer programs can be obtained by contacting EMRE's Environmental Engineering Section or a qualified outside consultant. For O3, ambient impact analyses for specific facilities have not been required. In the US, compliance with O3 ambient standards is treated as an entire airshed problem. Typically, an airshed is an entire city or if long-range transport is involved, an entire region of a country may need to be analyzed. The EPA computer programs of choice are normally the Regional Oxidant Model (ROM) and the Urban Airshed Model (UAM). ROM is for regional transport, while UAM is for city specific analyses. Both are numerical grid models and calculate hourly averaged concentrations for a grid square. Input data requirements are extensive including detailed emissions, ground and wind field data as a function of elevation, and boundary conditions. Typically, these computer programs are used for only three to four day episodes due to the computer computational time involved for each day of simulation. Additional information regarding these computer programs can be obtained by contacting EMRE's Environmental Engineering Section or a qualified outside consultant. Fine particulate matter (< 2.5 microns) is a growing regulatory concern. Atmospheric fine particulate (i.e., PM2.5) consists of primary and secondary particles. A relatively small amount of PM2.5 is primary particles or that PM2.5 directly emitted into the atmosphere from numerous sources. The more significant amount of PM2.5 is secondary particles added to the particle phase as a result of gas-to-particle conversion reactions in the atmosphere. Several computer programs exist that have some application history, but all need improvements according to a recent API study. Additional information regarding these computer programs can be obtained by contacting EMRE's Environmental Engineering Section or a qualified outside consultant.

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NOMENCLATURE

Bh

=

Height of nearby structure, m (ft)

D

=

Initial plume or cloud diameter, m (ft)

d

=

Inside stack diameter, m, at the release point to the atmosphere

ds

=

Stack inside diameter, m (ft) ds = 9.88 x 10-4 [QH ]1/2 [See Eq. (14)]

dT/dz

=

Actual temperature gradient, °K/m (°R/ft)

F

=

Buoyancy flux factor, m4/sec3, (ft4/sec3) F = [ g ( Vs ) (d2 ) ( DT ) ] / [ 4 Ts ]

Fr

=

Fraction of heat radiated, Fr = 0.2 [ (50 MW + 100)/900]1/2

g

=

Acceleration of gravity, 9.8 m/sec2 (32.2 ft/s)

H

=

Effective stack height, m (ft) H = h + Dh

h

=

Stack height, m (ft)

HF

=

Total heat release from the combustion of the flared gas, cal/s (Btu/s)

Hg

=

Good engineering stack height, m (ft)

L

=

Lesser dimension, height or projected width, of the nearby structure, m (ft)

MW

=

Molecular weight of gas being burned, g/gmole (lbm/lbmole)

N

=

nano (10-9)

p

=

Empirically derived wind speed exponent. See Table 3.

Q

=

Pollutant mass emissions rate, g/s (lbm/s)

QH

=

Sensible heat release, cal/s (Btu/s)

Rio

=

Release Richardson number

S

=

Stability parameter for calculating DTc for stable conditions, S = g(dT/dz -G)/Ta

Ta

=

Air temperature, °K (°R)

Ts

=

Stack gas temperature, °K, (°R)

u

=

Wind speed at the point of release, m/s (ft/s)

u*

=

Ambient friction velocity, m/s (ft/s)

um

=

Wind speed at reference elevation, zm, m/s (ft/s)

uz

=

Wind speed at elevation z, m/s (ft/s)

Vs

=

Stack gas exit velocity, m/sec (ft/s)

wo

=

Initial plume speed, m/s (ft/s)

x,y,z

=

Downwind, crosswind, and vertical location in space of the receptor with concentration, c

z

=

Desired elevation at which wind speed is to be calculated, m (ft)

zm

=

Reference elevation, m (normally 10 m, or ~33 ft)

a

=

Conversion exponent = 0.2

G

=

Lapse rate, °K/m (°R/ft)

Dh

=

Plume rise due to momentum or buoyancy, m (ft)

[See Eq. (4)]

[See Eq. (12)]

[See Eq. (3)]

Hg = Bh + 1.5L

[See Eq. (8)]

[See Eq. (11)] [See Eq. (7)]

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DT

=

Ts - Ta, Ts is the stack gas temperature, °K, (°R)

DTc

=

Critical temperature difference, °K (°R)

dq/dz

=

Potential temperature gradient, °K, (°R), dq/dz = actual temperature gradient - adiabatic lapse rate

ra

=

Ambient density, kg/m3 (lbm/ft )

rp

=

Initial plume density, kg/m3 (lbm/ft )

sy

=

Standard deviation of concentration distribution in the crosswind direction, m (ft), at downwind distance, x (Figure 3)

s¢y

=

Buoyancy-induced dispersion adjusted crosswind dispersion coefficient = [sy2 + (Dh/3.5)2]1/2

sz

=

Standard deviation of concentration distribution in the vertical direction, m (ft) at downwind distance, x (Figure 4)

s¢z

=

Buoyancy-induced dispersion adjusted vertical dispersion coefficient [sz2 + (Dh/3.5)2]1/2

c

=

Ground level concentration, g/m3 (nlbm/ft )

cf

=

Maximum fumigation concentration, mg/m3 (nlbm/ft ) [ See Eq. (15)]

c1

=

Highest glc over averaging time t1 mg/m (nlbm/ft )

c2

=

Highest glc over averaging time t2

3

3

3

3

3

3

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8 8.1

Section

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SAMPLE CALCULATIONS

SIMPLE POINT SOURCE DISPERSION PROBLEM

Given:

A 58.6 MW (200 MBTU/hr) furnace burns fuel oil with 30% excess air in a rural plant. The fuel has a sulfur content of 0.010 gramsulfur/gram-fuel oil (lb/lb). The temperature of the flue gas released to the atmosphere is 204°C (~400 °F) from a stack 20 m (65.6 ft.) with an inside diameter of 1.5 m (4.9 ft.). A barrel of fuel oil weighs 159 kg (350 lbm) and yields 1.6 kcal (6.35 BTU), or 9980 kcal/kg (~18,000 BTU/lb). Each 454 grams (lb) of fuel oil, when burned, yields 5 m3 (~177 ft3) of flue gas at 15.5°C (60 °F) and 1 atmosphere (stoichiometric air) or 6.5 m3 (~230 ft3) (30% excess air). The prevailing wind speed at 10-meter (33 ft.) elevation is 5.0 m/s (~11 mph) , and the atmospheric stability is neutral. The ambient temperature is 21°C (~70 °F). Find:

What is the maximum ground-level-concentration downwind from the stack for a one-hour period? What is the estimated maximum 24-hour glc? Solution (Metric units):

The primary approach is to determine the pollutant emission rate and exhaust velocity. With this information and the data specified in the problem it can be solved using several of the equations in this section. ç

Step 1. Fuel Oil Consumption Rate:

[58 MW] [106 joules/sec / 4.186 joules] [kg-fuel / 9980 kcal] [3600 sec/hr] = 5050 kg-fuel/hr Step 2. SO2 Emission Rate: Assume all sulfur (S) goes up the stack as SO2:

(5,050 kg-fuel oil/hr)(0.01 kg-S/kg-fuel oil)(64.06 kg-SO2 /32.06 kg-S) = 100.9 kg-SO2/hr or 28 grams-SO2/sec. Step 3. Flue Gas Emission Rate: Assume flue gas molecular weight = 28.9 and use volume-mass conversion factor from Table 2.

[5050 kg-fuel oil/hr] [6.5 m3 flue gas/0.454 kg-fuel oil] [kg °K/m3 /0.00283(15.5 + 273) °K] = 88,555 kg flue-gas/hr = 24.6 kg flue-gas/sec. Step 4. Flue gas exhaust velocity: Assume the stack flue gas pressure is slightly above atmospheric pressure 101.3 kPa and it molecular weight is 28.97 kg/kgmole.

[24.6 kg/s][(8317 joules/kgmole °K)/(28.97 kg/kgmole)](477 °K)]/101.3 kPa

= [4 (33 m3/s /(3.14)(1.5 m)2] = 18.7 m/s = Vs

ç

Step 5. Effective stack height: Can be determined by adding the plume rise Dh to the stack height. To determine plume rise, the buoyancy flux factor, Eq. (4), needs to be used.

F = [(9.8 m/s2 )(18.7 m/s)(1.5 m)2 (477 - 294) °K]/(4 (477 °K) = 39.55 m4/s3 DTc = 0.0297(477)(18.7)1/3/(1.5)2/3 = 28.7°K and DT = [Ts - Ta] = (477 - 294) = 183°K \DT > DTc such that the plume is buoyant and with F < 55 (6366), then, h is calculated by Equation A of Table 7 Dh = 21.425 (39.55)3/4]/5.6 = 60.9 m

H = 20 + 60.9 = 80.9 m ç

Step 6. Maximum, ground-level-concentrations: Solve Eq. (9) with y = z = 0 and calculate the wind speed at plume height using Eq. (1) for neutral atmospheric conditions.

u = 5 m/s[80.9/10]0.15 = 6.8 m/s c(x) = (28.1)/(p 6.8 sy sz) exp[-1/2 ((80.9)/sz )2]

and solving with 1 hour values of sy sz obtained from Figures 3 and 4, respectively.

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DISTANCE, m (ft)

sy, m (ft)

sz, m (ft)

1-Hour GLC, mg/m3 (nlb/ft3)

300 (984)

23 (75)

14 (46)

2.3 x 10-4 (1.5 x 10-5) 0.52 (0.033)

500 (1,640)

35 (115)

20 (67)

1000 (3,280)

75 (246)

32 (105)

22.2 (1.4)

1500 (4,920)

120 (394)

48 (157)

54.7 (3.4)

2000 (6,560)

140 (459)

53 (174)

54.8 (3.4)

2500 (8,200)

190 (623)

63 (207)

47.8 (3.0)

4000 (13,120)

270 (886)

85 (279)

36.1 (2.3)

From the above table, the maximum, 1 hour glc is 54.8 mg/m3 at 2000 meters downwind from the stack during 5 m/s (winds and neutral atmospheric conditions. Step 7. Maximum 24-hour glc: This can be calculated using Equation (2). c1 = 54.8 mg/m3 (24/1)-0.2 = 29 mg/m3 Solution (in customary units):

The primary approach is to determine the pollutant emission rate and exhaust velocity. With this information and the data specified in the problem it can be solved using several of the equations in this section. ç

Step 1. Fuel Oil Consumption Rate:

[200 x 106 BTU/hr] [lb fuel oil/18,000 BTU] = 11,110 lb-fuel/hr Step 2. SO2 Emission Rate:

[11,110 lb-fuel/hr] [0.01 lb-S/lb-fuel] [64.06 lb-SO2/32.06 lb-S] = 222 lb-SO2/hr or 0.062 lb-SO2/s Step 3. Flue Gas Emission Rate:

[11,110 lb-fuel/hr] [230 ft3 flue-gas/1 lb-fuel] [1 lb °R/ft3 /(0.0252)(60 + 460) °R]

= 195,000 lb/hr = 54.2 lb-flue-gas/sec

Step 4. Flue gas exhaust velocity:

[54.2 lb/s] [(1545 ft-lbf/lbmole-°R)/(28.97 lb/lbmole)] [(400 + 460)°R/((14.7 x 144 lb/ft2)] = 4 (1174 ft3/s)/(3.14)(4.9)2 = 62 ft/s = Vs ç

Step 5. Effective stack height:

F = [(32.2 ft/ s2) (62 ft/s) (4.9 ft)2 (860 - 530) °R]/(4 (860 °R) = 4598 ft4/s3 DTc = 0.0443(860)(62)1/3/(4.9)2/3 = 52 °R and DT = [Ts - Ta] = (860 - 530) = 330 °R \DT > DTc such that the plume is buoyant and with F < 55 (6366), then, h is calculated by Equation A of Table 7 Dh = 6.58(4598)3/4]/18.4 = 199.75 ft.

H = 65.6 + 199.75 = 265.4 ft. ç

Step 6. Maximum ground-level concentrations.

u = 16.4 ft/s[265.4/32.8]0.15 = 22.4 ft/s c(x) = (0.062)/(p 22.4 sy sz) exp[-1/2 ((265.4)/sz )2]

and solving with 1 hour values of sy sz obtained from Figures 3 and 4, respectively.

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From the above table, the maximum, 1 hour glc is 3.4 nlb/ft3 at 6,560 ft downwind from the stack during ~11 mph (winds and neutral atmospheric conditions. Step 7. Maximum 24-hour glc: c1 = 3.4 nlb/ft3 (24/1)-0.2 = 1.8 nlb/ft3

8.2

FUMIGATION PROBLEM

Given:

The same stack parameters and emission conditions are again being assumed. Find:

Calculate the 1 and 24 hour ground-Level-concentration due to inversion break-up. Solution (Metric units):

The overall approach is to determine the pollutant dispersion under stable conditions and use Eqs. (15) and (18). But first, it is necessary to determine if the plume rise is buoyancy or momentum dominated. Then, the distance to the maximum inversion break-up glc is determined, and the fumigation maximum glc is calculated after corrections for buoyancy-induced dispersion are made. Finally, the weighted 24-hour glc is calculated using Eq. (18). Step 1. Compute Plume Rise: Assume stable (“F” stability) conditions and a stack height wind speed of 2.5 m/s.

H = hs + Dh, but first, it needs to be determine if the plume is buoyancy or momentum dominated, so using Equation (7): DTc = (0.01958) (477) (18.7) (0.0012) = 0.2 °K which is less than DT

so it is buoyancy-dominated. Dh = 2.6 [39.55/((2.5)(0.0012))]1/3 = 61.4 m \ H = 20 + 61.4 = 81.4 m Step 2. Distance To Maximum GLC. Use the effective plume height, H; the stack height, hs; and Table 9:

Distance is 4.3 km or 4300 m ç

Step 3. Maximum Inversion Break-Up GLC. The 1 hour cf, in mg/m3, is calculated using Equation (15) after s¢y and s¢z are

determined using Figures 3 and 4 at 4.3 km and then corrected for buoyancy induced dispersion. cf = (2.8 x 106)/[(2 p)1/2 (2.5) {150.1 + (81.4/8)} {81.4 + 2(32.5)}] = 190.3 mg/m3 ç

Step 4. 24 Hour Weighted Average GLC. Assuming that the maximum downwind glc calculated for neutral stability and 5 m/s winds is the maximum glc for all stability and wind speed combinations, Eq. (18) is used to calculate the 24 hour average glc. c¢1 = [(15)(54.8) + (190.3)])/16 = 63.3 mg/m3

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Solution (in Customary units): Step 1. Compute Plume Rise: Assume stable (“F” stability) conditions and a stack height wind speed of 8.2 ft/s.

H = hs + Dh, but first, it needs to be determined if the plume is buoyancy or momentum dominated, so using Eq. (7): DTc = (0.0059) (860) (62) (0.0012) = 0.38 °R which less than DT

so it is buoyancy-dominated. Dh = 2.6 [4598/((8.2)(0.0012))]1/3 = 201.7 m \ H = 65.6 + 201.7 = 267.3 ft. Step 2. Distance To Maximum GLC:

Distance is 2.6 mi. or 13,776 ft. ç

Step 3. Maximum Inversion Break-Up GLC. cf = (0.0617)/[(2 p)1/2 (8.2) {492.3 + (267.3/8)} {267.3 + 2(106.6)}] = 11.9 nlb/ft3

ç

Step 4. 24 Hour Weighted Average GLC: c¢1 = [(15)(3.4) + (11.9)])/16 = 3.9 nlb/ft3

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ç

June, 2004

Table 1 Representative Ambient Air Quality Standards

POLLUTANTS

COUNTRY (MEAN CONCENTRATIONS in mg/m3)

TIME

CANADA*

CHINA#

EU

GERMANY (Jan. 1, 2005)

Sulfur

Annual

30/60/---

60

Dioxide

24 hour

150/300/800

150

125

125(a)

450/900/--

500

350

350(b)

40

40(o)

Nitrogen Dioxide Particulate Matter

UNITED STATES

107

365(g)

20

3 hours 1 hour

JAPAN

80(f)

500(c)

Annual

60/100/--

40

24 hour

--/200/300

80

1 hour

--/400/1000

1300(g) 267 100(f) 77

200

200(d,d)

PM10

120 TSP

PM10

PM10

PM10

PM2.5

Annual

60/70/--

100

200

20

40

50(f,h)

15(f,i)

24 hour

--/120/400

150

300

50

50(e)

150(g)

65(j)

100 200

1 hour Carbon Monoxide

Ozone

Annual 24 hour 8 hour

6e3/15e3/20e3

1 hour

15000/35000

Annual

--/30/--

24 hour

30/50/--

4000 10000

1 hour

10000(g) 40000(g)

65(m) 120

100/160/300

11400 22800

10000

8 hour Lead

10000

160

Annual

1.0

Quarterly

1.5

110 180/200/360(n)

0.5

0.5

157(k) 118

235(l) 1.5

Legend / Notes: a = Maximum of 3 exceedances tolerable per annum. b = Maximum of 24 exceedances tolerable per annum. c = Alert value. Alert value implies that the authority has to / will take action. 3 3 d = After December 31, 2009, maximum of 18 exceedances tolerable per annum. The alert value is 400 mg/m . Tolerance of 80 mg/m starting 3 3 in 2002, ramping down to zero tolerance (must meet 200 mg/m ) by 2010 in annual increments of 10 mg/m . e = Maximum of 35 exceedances tolerable per annum. f = arithmetic mean g = Not to be exceeded more than once per year. h = To attain this standard, the expected annual arithmetic mean PM10 concentration at each monitor within an area must not exceed 50 ug/m3. i = To attain this standard, the 3-year average of the annual arithmetic mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15 ug/m3. j = To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 65 ug/m3. k = To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm. l = (1) The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is