TSCREEN

1
TSCREEN

• A MODEL FOR CALCULATING TOXIC AIR POLLUTANT
  CONCENTRATIONS

2

• Prepared by
• David Deng
• Brian Kail
• Telicia Kuchan
• Jennifer Larkin
• Sankalpa Nagaraja
• Eric Sciorilli
• Patrick Stark
• Asciatu Whiteside

3
Why do we need a model for air dispersion?

•     When toxics are released via aerial
  dispersion
•    To correctly analyze toxic emissions and
  their subsequent dispersion
•     Determine compliance to environmental
  standards
•    Determine downwind concentration for future
  risk assessment

4
What is the difference between a screening model
as compared to a full-blown model?

• A screening model is a simplified model that will
  calculate downwind concentrations sufficiently
  conservatively, to determine whether a source 
• 1) poses a potential threat, or    
• 2) is not an air quality threat.
• The estimates represent maximum short-term ground
  level concentrations.
• Screening models are used to determine whether
  additional modeling is necessary. If EPA finds
  screening estimates objectionable, further
  analysis is needed. If estimates are found not
  objectionable, further analysis is not needed.
•  A full blown model is much more complex and time
  consuming. It incorporates
•        Refined emission data
•         Refined receptors
•         Refined meteorological conditions

5
What meteorological conditions are the air
dispersion calculations based on?

•  
•         wind speed
•        wind direction
•        ambient temperature
•        atmospheric pressure
•        atmospheric stability
•        turbulence
•  

6
TSCREEN SCENARIOS
7
PARTICULATE MATTER
Models the release of particulate matter from a
point source, such as a vertical smoke stack or
pipe. Modeling system assumes that particulate
deposition is insignificant. In order for a
model to be generated, the following information
is required         The buildings maximum
and minimum horizontal dimensions         The
aboveground release height         The building
height         Emission rate        
Temperature and release point         The
ambient temperature         The volumetric flow
rate
Exit velocity must be measured however, if this
is not possible, it can be calculated using a
formula in the users manual. Once all
required data is inputted, TSCREEN returns
maximum concentration of particulates to be
expected, and any variances in concentration
based on their relative distance and height from
the release point.
8
GASEOUS RELEASES

• TSCREEN has numerous scenarios for gaseous
  releases.
• The following is applicable to gas releases from
  properly designed flares, assumed
• to be 98 efficient.
• Input information required
• The total heat release rate
• The physical stack height above ground
• Effective release height
• Molecular weight of the material
• Volume flow rate
• Volume fraction of pollutant in the gas
• Similar to the particulate scenario, emission
  rates need to be known, or can be calculated
  using formulas in the users manual.

9
LIQUID RELEASES

• TSCREEN can run a model for release of materials
  stored under pressure, which may form a liquid
  phase if depressurization occurs.
• Consider the following continuous release of a
  pressurized liquid stored under saturated
  conditions in a cracked or defected container.
  Input required
• The area and diameter of the hole or opening
• The liquids heat capacity at the respective
  temperature
• The molecular weight of the gas
• The ambient pressure
• The reservoir pressure
• Total amount of material released
• The ambient temperature
• The containment normal boiling point
• The reservoir temperature
• The heat of vaporization at the normal boiling
  point
• The containment density
• User is required to calculate release rate,
  discharge density, and release duration.

10
SUPERFUND

• TSCREEN uses the situation of air stripping and
  release of VOCs from contaminated waters.
• In order for model to be successful, TSCREEN
  requires that the use know the concentration of
  the contaminant, and the incoming flow rate.
• With this information, TSCREEN can estimate the
  various concentrations to expect.

11
TSCREEN Models 4 Types

• SCREEN
• Has Gaussian dispersion model
• Is applicable to continuous releases
• Released particles (emissions) are assumed to be
  non-reactive
• Released particles (emissions) are assumed to be
  less dense than air
• Emissions can be point source, from a flare,or a
  fugitive source.
• Examples
• Stacks
• Vents
• Flares
• Pesticide volatalization
• Landfills
• Superfund air strippers

12
RVD

• For modeling denser-than-air emissions and
  aerosol emissions in vertically directed jet
  releases.
• Examples
• Leakage from reservoirs, thanks, and pipes
• Liquids of high and low volatility from
  pressurized tanks

Britter-McQuaid

• Used for
• Denser than air emissions
• Continuous emissions
• Instantaneous emissions
• Not vertically directed jet release
• Examples
• Leakage from reservoir, tanks, and pipes
• Liquids of high and low volatility from
  pressurized tanks
• Gaseous or liquid release

13
PUFF

• Used for
• Instantaneous emissions
• Non-denser-than-air emissions
• Released particles are assumed to be non-reactive
  and less dense than air.
• Examples
• Ducting/connector failure
• Discharges from equipment openings
• High and low volatility releases from tanks

14
PARAMETERS INPUT FOR FIRST RUN
15
CHANGES IN PARAMETERS
16
GASEOUS DISCHARGE PARAMETERS
CHANGES IN PARAMETERS
17
Example of raw data input screens
----- Continuous Particulate Releases from
Stacks, Vents - Scenario 1.1 ------ Based on
user input, SCREEN3 model has been selected.
SCREEN3 MODEL INPUTS - Page 1 of 7 Enter a
unique title for this data's model run
Simple RELEASE PARAMETERS
Emission Rate (Qm) -gt 1 g/s
Exit Velocity (ExitV)-gt 17.8 m/s
Release Height above Ground (Hs)
-gt 16 m Diameter at
Release Point (D) -gt .1 m
Temperature of the Material Released (Ts) -gt
310 K AMBIENT PARAMETER
Ambient Temperature (Ta) -gt 293
K ---------------------------------
------------------------------------------
ltF1gt Help ltF3gt Calculator ltF9gt Previous
Screen ltEscgt Abort
18
----- Continuous Particulate Releases from
Stacks, Vents - Scenario 1.1 ------

SCREEN3 MODEL INPUTS - Page
2 of 7

BUILDING
PARAMETERS
Building Height
(enter 0 if no building) -gt 19 m
Minimum Horizontal Building
Dimension -gt 19 m
Maximum Horizontal Building Dimension -gt 19
m

URBAN/RURAL CLASSIFICATION

Enter U for Urban - R for Rural -gt R


FENCELINE DISTANCE
Enter the
distance from the base of the stack
to
the plant fenceline -gt 100 m











-----------------------------
----------------------------------------------
ltF2gt Edit ltF9gt Previous Screen ltF10gt
Next Screen ltEscgt Abort ----------------
--------------------------------------------------
------------
19
----- Continuous Particulate Releases from
Stacks, Vents - Scenario 1.1 ------ SCREEN3
MODEL INPUTS - Page 3 of 7


TERRAIN TYPE
Is this a
FLAT or SIMPLE TERRAIN evaluation (Y/N) -gt Y


SIMPLE TERRAIN
Are
receptors above stack-base (Y/N) -gt N

SIMPLE
FLAT TERRAIN
Do you have specific
locations where you would like
pollutant concentrations to be
calculated (Y/N) -gt N

Do you have
receptors above ground level
(i.e. Flag Pole
Receptors) (Y/N) -gt N



You have completed simple terrain
inputs. Do you
want to continue with complex terrain (Y/N) -gt
N




---------------------------------
------------------------------------------
ltF2gt Edit ltF9gt Previous Screen ltF10gt Run
Model ltEscgt Abort ---------------------
--------------------------------------------------
-------
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
LIMITATIONS TO TSCREEN

• Only intended to give simplistic estimates of
  calculable scenarios.
• Can only be used for small to medium normal
  releases not for accidental releases.
• Air releases are assumed to be non-reactive and
  non-depositing.
• Liquid releases are assumed to be an aerosol with
  little fall out near source.
• Denser than air scenarios only look at
  contaminate in comparison to ambient air
  post-release thermodynamic behavior not handled
• Meteorological conditions not accounted for, e.g.
  evaporation of compound in relation to wind
  speed.
• Not able to calculate time-dependent emissions.
• Obstructions not accounted for, and therefore do
  not affect predicted dispersion.
• All receptors are assumed to be at stack base
  level or higher.

25
DISCUSSION

• TSCREEN calculates the downwind concentrations of
  air emissions through meteorological conditions.
• We tested various scenarios that would
  differentiate the meterological conditions that
  may be may be encountered and designed.
• All release scenarios take into account the
  buoyancy factor of the pollutants.
• If the emissions temperature is greater than the
  ambient temperature, it is known to be more
  buoyant and will be expected to disperse more in
  the atmosphere, with decreasing ground
  concentrations.
• However, it was found to have a negligible effect
  on the concentration at ground level.
• Thus, buoyancy is more important for model
  selections with less impact on downwind
  concentrations.

26
DISCUSSION (contd)

• Stack height variance will change concentrations
  because of the mixing height
• Mixing height is the boundary between surface air
  current and atmospheric air current
• Mixing height will vary with different
  meteorological conditions
• If the plume is released below the mixing height,
  concentrations will be high
• If the plume is released above the mixing height,
  concentrations will be low
• This has been corroborated with examples from
  power plants.
• Low stack heights will generally lead to higher
  concentrations
• High stack heights will lead to large dispersion
  in the atmosphere
• This is why most stacks heights are high!!

27
(No Transcript)
28
DISCUSSION (contd)

• BUILDING DOWNWASH
• Low stack heights with large structures nearby
  will hinder the dispersion of the pollutant in
  the atmosphere
• Large buildings create a strong downwash factor
  that will pull the plume towards the ground
  resulting in high ground concentrations.
• As noted in the various runs, stack heights that
  are lower than the structure or less than twice
  the height of the structure will be severely
  affected by the downwash factor.