air pollution_plume rise

8/12/2019 Air Pollution_Plume rise

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PLUME RISE


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FATE OF THE PLUME AS IT LEAVES THE SOURCE

1. Plume rise

2. Downwind transport and dilution


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Plumes are mixtures of both gases and particulate matter.

Large particles have appreciable settling velocity and settle near thesource.

Smaller particles tend to remain suspended in the atmosphere for longerperiods of time and their dispersion behaviour may be similar to thegases.

The gaseous nature of the plume would, given sufficient time, allow forits dispersion by simple diffusion (Diffusion is random movement of

gaseous molecules from an area of high concentration to an area of lowconcentration.)

Although diffusion is an important parameter in the dispersion of aplume, it cannot by itself account for the dispersion observed.

FATE OF THE PLUME AS IT LEAVES THE SOURCE


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PLUME RISE

h = Stack height/Physical stack height

h = Plume rise

H = Effective stack height= h + h

The additional height above the physical stack height up to which aplume rises is called plume rise

h

H

h

u


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PLUME RISE

Reasons of plume rise

Momentum of the exhaust gasBuoyancy of the exhaust gas

Exhaust gas hotter than surrounding air Mass and velocity of the gas

When molecular weight of the exhaust gas is lower than that of air


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PLUME RISE

Initial plume rise is important, because

The height of the plume substantially influences the downwind groundlevel pollutant concentration

The larger is the plume rise and the more is the effective stack height.

Increased effective stack height results in

Greater downwind distance the pollutant will travel along with the

plume before they reach the ground

Greater dispersion of the pollutants

Reduced ground level concentration


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Temperature of the emission

Height of the plume rise can be increased by emitting pollutant athigher temperature.

Temperature difference between the ambient air and stack gasdetermines the thermal buoyancy the plume will achieve as itexits the stack.

FACTORS o n w h i c h PLUME RISE DEPENDS


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Horizontal wind speed

More horizontal wind speed at plume exit point

reduction of height of plume rise

quick bending of the plume and hence net downwinddistance the plume travels decreases

However, dispersion may not be affected adversely as high windusually enhances dispersion process.



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Vertical temperature gradient

Temperature gradient determines the atmospheric stabilityand therefore influences the height of the plume rise.

Emission velocity

With increase of emission velocity, the plume rise increases.



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Height of the plume rise can be increased by increasing stack height.

Stack heights of 250 to 300 m are common for fossil fuel

plants.

Some modern power plants have stack height up to 400 m.

PLUME RISE


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PLUME RISE ESTIMATION

Mostly based on review of empirical data performed by Briggs. Itrequires extensive calculations.

More simplified approach is the use of Hollands formula.

h = Plume rise, m vs = Stack gas exit velocity, m s -1

d = Stack inner diameter, m

u = Wind speed at actual/physical stack height, m s-1

P = Atmospheric pressure, millibar (If pressure is given in KPa, then in the formula,10 -3 should be replaced with 10 -2, because 1 mb = 10 -3 bar and 1 bar = 100 KPa; Thus 1 mb= 100 x 10 -3 =10 -1 KPa)Ts= Stack gas temperature, KTa = Atmospheric/Ambient temperature at stack height, K

= 1.5 + 2.68 10 3


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PLUME RISE ESTIMATION

The Hollands formula is applicable for a neutral plume (Atmosphericstability class D)

Correction factors for varying atmospheric stability:

Atmosphericstability class Atmospheric stability Correctionfactor

A - B Extremely - Moderately unstable 1.15

C Slightly unstable 1.10

D Neutral 1.00

E - F Slightly Moderately stable 0.85


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PASQUILL STABILITY CLASSES


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