Upload
samrat-salikineedi
View
31
Download
1
Tags:
Embed Size (px)
Citation preview
Dispersion of Air Pollutants
Depends on meteorological conditions:
wind speed and atmospheric stability class (adiabatic lapse rate,
see diagrams at left)
2
Effect of stack parameters
PLUMERISE
X
PLUMECENTERLINE
z
Z RELEASEHEIGHT
he
Plume rise: fairly complex, depends on velocity and temperature of flue gas, as well as on ambient atmospheric conditions
3
Turbulence• Circular eddies of air movements over short timescales than
those that determine wind speed (unstable)• Mechanical Turbulence: – Caused by air moving over and around structures/vegetation– Increases with wind speed– Affected by surface roughness
• Thermal Turbulence: – Caused by heating/cooling of the earth’s surface– Flows are typically vertical– Convection cells of upwards of 1000 - 1500 meters
What is the effect of turbulence on pollution?Is turbulence desired?
04/07/2023 4
Atmospheric Stability
• Concept that describes (non-)movement of air near the surface
• Characterized by vertical temperature gradients (Lapse Rates)– Dry adiabatic lapse rate () = 0.976 oC/100 m ~ 1 oC/100 m– International standard lapse rate = 0.0066 oC/m
Does dry or moist air have a larger temperature change for the same change in elevation? Why?
Does lapse rate have anything to do with air quality?
04/07/2023 5
• First Law of Thermodynamics
• Barometric Equation
gdZdP
dPdTCdPdhdq p 1
= 0 for adiabatic expansion
p
p
Cg
dZdT
gdZdPdTC
1
How much is dT/dZ if Cp = 1.0034103 m2/s2-K? What if Cp = 1.856103 m2/s2-K? (for dry air and moist air)
Lapse Rate
04/07/2023 6
Stability Conditions
Adiabatic lapse rate
Environmental lapse rate
04/07/2023 7
Superadiabatic Lapse Rates (Unstable)
• Temperature decreases are greater than -10o C/km• Occur on sunny days• Characterized by intense vertical mixing• Excellent dispersion conditions
04/07/2023 8
Neutral Lapse Rates• Temperature decreases are similar to the adiabatic lapse rate• Results from:
– Cloudy conditions– Elevated wind speeds– Day/night transitions
• Describes good dispersion conditions
Isothermal Lapse Rates (Weakly Stable)• Characterized by no temperature change with height• Atmosphere is somewhat stable• Dispersion conditions are moderate
04/07/2023 9
Inverted Lapse Rates (Strongly Stable)• Characterized by increasing temperature with height
Does it occur during the day or at night?Is it associated with high or low pressure systems?Does it improve or deteriorate air quality?
www.ew.govt.nz/enviroinfo/air/weather.htm
www.co.mendocino.ca.us/aqmd/Inversions.htm
Inversion
04/07/2023 10
Inverted Lapse Rates (Strongly Stable)• Characterized by increasing temperature with height
Does it occur during the day or at night?Is it associated with high or low pressure systems?Does it improve or deteriorate air quality?
www.ew.govt.nz/enviroinfo/air/weather.htm
www.co.mendocino.ca.us/aqmd/Inversions.htm
Inversion
04/07/2023 11
Inversion• Definition: temperature increases with altitude
04/07/2023 12
Inversion
http://www.co.mendocino.ca.us/aqmd/pages/Inversion-Art-(web).jpg
04/07/2023 13
Inversion• Two major types of inversion:– Subsidence Inversion: descent of a layer of air within a high
pressure air mass– Radiational Inversion: radiation at night from the earth’s
surface into the local atmosphere
04/07/2023 14
Radiational Inversions
• Result from radiational cooling of the ground• Occur on cloudless nights – nocturnal• Typically surface based• Are intensified in river valleys• Cause pollutants to be “trapped”
What happens to inversion when sun rises?
www.co.mendocino.ca.us/aqmd/Inversions.htm
Fig 3.3
04/07/2023 15
Radiational Inversions
• Elevated inversions are formed over urban areas– Due to heat island effect– Due to dust dome
Fig 3.4
04/07/2023 16
Radiational Inversions• Breakup after sunrise• Breakup results in elevated ground level
concentrations• Breakup described as a fumigation
de.wikipedia.org/wiki/Smog
04/07/2023 17
Radiational Inversions
• Elevated inversions are formed over urban areas– Due to heat island effect– Due to dust dome
Fig 3.4
04/07/2023 18
Subsidence Inversion• Associated with high-pressure systems• Inversion layer is formed aloft• Covers hundreds of thousands of square kms• Persists for days
Fig 3.5apollo.lsc.vsc.edu/.../smog_var_geo.html
04/07/2023 19
Subsidence Inversion• Migrating high-pressure systems: contribute to the hazy
summer conditions in Midwest, SE and NE• Semi-permanent marine high-pressure systems
www.oceansatlas.org/.../datard.htm
– Results in a large number of sunny calm days
– Inversion layer closest to the ground on continental side
– Responsible for air stagnation over Southern California
Where else on earth would have similar phenomenon?
04/07/2023 20
Inversions
• Frontal - warm air overrides cooler air• Advective - warm air flows over a cold surface or
cold air
www.atmos.ucla.edu/.../inversions/Note03.html
Qualitative Descriptions• Plume rise h
H=hs + h• Driving forces– Buoyancy– Momentum
• Different phases– Initial phase– Thermal phase– Breakup phase– Diffusion phase
Qualitative Descriptions
• Influencing factors– When there is no downwash
• Exit velocity• Stack diameter• Stack gas temperature• Ambient temperature• Wind speed• Atmospheric stability• Wind shear
– Downwash
Holland Plume Rise Formula
• Simple• More suitable for power plant• For neutral conditions
The wind speed ū is adjusted to the stack height.
• For non-neutral conditions
ss
asss dTTT
Puvd
h 31068.25.1
hCFh
StCF
CF
)(
7.010
Briggs Plume Rise Formulas
• More complicated• Buoyancy flux parameter
• Momentum flux parameter
a
asssb T
TTdgvF
4
2
s
assm T
TdvF
4
22
Briggs Plume Rise Formulas• Determination of buoyancy dominated or
momentum dominated plumes– Calculate (T)c
• For unstable or neutral (A-D)– For Fb <55
– For Fb55
• For stable (E,F)
– If T (=Ts-Ta) (T)c , it’s buoyancy dominated– If T (=Ts-Ta) < (T)c , it’s momentum dominated
3
2
31
0297.0
s
ssc
d
VTT
3
1
32
00575.0
s
ssc
d
VTT
21
01958.0)( sVTT ssc
Briggs Plume Rise Formulas
• For buoyancy dominated plume under unstable or neutral conditions (A-D)– x* = distance at which atmospheric turbulence
begins to dominate entrainment• For Fb55 m4/sec3, x*=34 Fb
2/5
• For Fb<55 m4/sec3, x*=14 Fb5/8
– xf=distance to the final rise, m• xf=3.5x*
– Final plume rise:
uxF
h b3
2*31
)5.3(6.1
Briggs Plume Rise Formulas
• For buoyancy dominated plume under stable conditions (E and F)– Stability parameter, s
• Default values for
– 0.02 K/m for E stability– 0.035 K/m for F stability
TT
gsa
z
Briggs Plume Rise Formulas
– Final plume rise
– Distance to final rise
31
6.2
suF
h b
210715.2s
ux f
Briggs Plume Rise Formulas
• For momentum dominated plume under unstable or neutral conditions (A-D)
• For momentum dominated plume under stable conditions (E,F)
– Calculate both and use the lower one.
uvd
h ss3
31
5.1
suF
h m
Briggs Plume Rise Formulas
– Final plume rise
– Distance to final rise
31
6.2
suF
h b
210715.2s
ux f
Briggs Plume Rise Formulas
• For momentum dominated plume under unstable or neutral conditions (A-D)
• For momentum dominated plume under stable conditions (E,F)
– Calculate both and use the lower one.
uvd
h ss3
31
5.1
suF
h m
Briggs Plume Rise Formulas
• Gradual rise• Distance < distance to final rise (i.e., x<xf) and
Buoyancy dominated plume
uxF
h b3
23
1)(6.1
Briggs Plume Rise Formulas
• Distance < distance to final rise (i.e., x<xf) and momentum dominated plume– Jet entrainment coefficient
– Unstable conditions (A-D)3
1
22
3
u
xFh
j
m
sj v
u
31
Briggs Plume Rise Formulas• X=downwind distance with max value of:
Xmax=49Fb5/8 for 0<Fb<55 m4/sec3
xmax=119Fb2/5 for Fb> 55 m4/sec3
– Stable conditions (E,F)
• with
0)3(4 2
max
bs
ss FForuvuvd
x
31
2
/sin(3
suusx
Fhj
m
sux 5.0max
Briggs Plume Rise Summary
Unstable and neutral
Stable
Buoyancy
Momentum
uxF
h b3
2*31
)5.3(6.1
31
6.2
suF
h b
uvd
h ss3
31
5.1
suF
h m
Buoyancy Induced Dispersion• Air entrainment due to “boiling-like action” enlarges
the plume• Small impact on ground level concentration in most
cases• The impact can be reflected in – Initial plume size
– Effective dispersion coefficients5.300h
zy
5.020
2
5.020
2
)(
)(
zzze
yyye
• Calculate the final plume rise from a power plant for the following conditions:
• Atmospheric Stability D• Vs =19 m/s• ds =3 m • U 10 m =4 m/s • Ts =400 oK • Ta =283 oK • Stack Height= 67 m
• Deacon power law for calculating wind speed at stack height • u = u1 * (z/z1)p
• Where,• u = desired but unknown wind speed, (us)
• u1 = wind speed at known height, (u10)
• z = height where wind speed is unknown, hs
• z1 = height where wind speed is known, 10m• p = exponent from table 3-3 in the text = 0.15• Therefore, u = u1 * (z/z1)p = 4 * (67/10)0.15 = 5.3 m/sec
• 2) Check for downwash:• Vs / u >= 1.5 (downwash conditions need not
be considered) = 19.0/5.3 = 3.571 >1.5 (therefore downwash need not be considered)
• Where,• Vs = stack velocity in m/sec• u = wind speed at plume elevation
• 3) Calculate buoyancy flux parameter• Fb = g * vs * d2 * ΔT / (4 * Ts) • = 9.81 * 19* 32 * (400 - 283) / (4 *400) = 123 m4/s3 (Fb >
55m4/s3)• 4) Calculate temperature difference• ΔT = Ts - Ta = 400 - 283 = 1170K• 5) Calculate cross over temperature difference (ΔT)c
• for Fb > 55m4/s3
• (ΔT)c = 0.00575 * Ts * vs 2/3 / ds 1/3 = 0.00575 * 400 * 192/3 / 3 1/3
= 11.40K
• 7) Calculate final plume rise Δh• for Fb > 55m4/s3 Δh = 38.71 * (Fb
3/5 / u ) = 38.71 * (1233/5 / 5.3) = 130m
• 8) Calculate final effective plume height H• H = 130 + 67 = 197m • This is less than the typical 300m night time
inversion height; so plume rise may be reasonably accurate