Atmospheric Layers

Discussion of the layers in the Earth’s atmosphere is needed to understand where airborne pollutants disperse in the atmosphere. The layer closest to the Earth’s surface is known as the troposphere. It extends from sea‐level to a height of about 18 km and contains about 80% of the mass of the overall atmosphere. The stratosphere is the next layer and extends from 18 km to about 50 km. The third layer is the mesosphere which extends from 50 km to about 80 km. There are other layers above 80 km, but they are insignificant with respect to atmospheric dispersion modeling. On a mesoscale, Figure 2.2 shows the four regions of the atmosphere, as temperature varies with altitude, so does density.

The lowest part of the troposphere is called the atmospheric boundary layer (ABL) or the planetary boundary layer and extends from the Earth’s surface to about 1.5–2.0 km in height. The air temperature of the atmospheric boundary layer decreases with increasing altitude until it reaches what is called the inversion layer (where the temperature increases with increasing altitude) that caps the atmospheric boundary layer. The upper part of the troposphere (i.e. above the inversion layer) is called the free troposphere and it extends up to the 18 km height of the troposphere.

The ABL is of the most important with respect to the emission, transport, and dispersion of airborne pollutants. The part of the ABL between the Earth’s surface and the bottom of the inversion layer is known as the mixing layer. Almost all of the airborne pollutants emitted into the ambient atmosphere are transported and dispersed within the mixing layer. Some of the emissions penetrate the inversion layer and enter the free troposphere above the ABL.

In summary, the layers of the Earth’s atmosphere from the surface of the ground upwards are the ABL made up of the mixing layer capped by the inversion layer, the free troposphere, the stratosphere, the mesosphere, and others. Many atmospheric dispersion models are referred to as boundary layer models because they mainly model air pollutant dispersion within the ABL. To avoid confusion, models referred to as mesoscale models have dispersion modeling capabilities that extend horizontally up to a few hundred kilometers. It does not mean that they model dispersion in the mesosphere.

EXAMPLE 4.11

A 500‐MW coal‐fired power plant is burning coal with a heating value of 12 000 Btu/lb. The coal contains 2.0% sulfur and 150 ppb of mercury. The plant has a thermal efficiency of 38% and uses an air/fuel ratio of 14 : 1 (mass basis). Calculate (a) the coal firing rate (lb/h), (b) the flue‐gas flow rate (scfm), (c) the SO₂ removal efficiency needed to meet a standard of 1.4 lb/MWh gross electrical energy output, and (d) the overall mercury capture percentage that is required to meet a mercury standard of 0.015 lb/GWh of gross energy output.

Given: Heating value = 12 000 Btu/lb

Coal content: 2.0% S, 150 ppb Hg

η_thermal = 38%

A/F = 14 : 1 (mass basis)

SOLUTION

1. Coal‐firing rate
2. Flue‐gas flow rate:A/F = 14 : 1, so flue‐gas mass flow is 15 times coal feed rate
3. SO₂ removal efficiency
4. Overall mercury capture percentage

Gross energy output: 

Uncontrolled mercury emissions: 

Mercury capture percentage: