EnE 301: ENVIRONMENTAL ENGINEERING

1. EnE 301: ENVIRONMENTAL ENGINEERING • 3.1 Physical and Chemical Fundamentals • 3.2 Major Air Pollutants and their Effects • 3.3 Origin and Fate of Air Pollutants • 3.4 Micro and Macro Air Pollution and Meteorology • 3.5 Atmospheric Dispersion and Indoor Air Quality Model • 3.6 Air Pollution Control of Stationary and Mobile • Sources • 3.7 Clean Air Act of 1999- RA8749 and Its Implementing • Rules and Regulations 3.0 Air Pollution and Control

2. Factors Affecting Dispersion of Air Pollutants • The factors that affect the transport, dilution, and dispersion of air pollutants can generally be categorized in terms of the emission point characteristics, the nature of the pollutant material, meteorological conditions, and effects of terrain and anthropogenic structures. ATMOSPHERIC DISPERSION (1) Source Characteristics • Most industrial effluents are discharged vertically into the open air through a stack or duct. • As the contaminated gas stream leaves the discharge point, the plume tends to expand and mix with ambient air. • Horizontal air movement will tend to bend the discharge plume toward the downwind direction. • While the effluent plume is rising, bending, and beginning to move in a horizontal direction, the gaseous effluents are being diluted by the ambient air surrounding the plume.

3. As the contaminated gases are diluted by larger volumes of ambient air, they are eventually dispersed toward the ground. • The plume’s buoyancy is related to the exit gas mass relative to the surrounding air mass. • Increasing the exit velocity or the exit gas temperature will generally increase the plume rise. • The plume rise, together with the physical stack height is called the effective stack height. (2) Downwind Distance • The greater the distance between the point of discharge and a ground level receptor downwind, the greater the volume of air available for diluting the contaminant discharge before it reaches the receptor. (3) Wind Speed and Direction • The wind direction determines the direction in which, the contaminated gas stream will move across local terrain. • An increase in wind speed will decrease the plume rise by bending the plume over more rapidly.

4. The decrease in plume rise tends to increase the pollutant’s ground level concentration. • On the other hand, an increase in wind velocity will increase the rate of dilution of the effluent plume, tending to lower the downwind concentrations. (4) Stability • The turbulence of the atmosphere follows no other factor in power of dilution. • The more unstable the atmosphere, the greater the diluting power. • Inversions that are not ground based, but begin at some height above the stack exit, act as a lid to restrict vertical dilution. Dispersion Modeling • A dispersion model is a mathematical description of the meteorological transport and dispersion process that is quantified in terms of source and meteorological parameters during a particular time.

5. The resultant numerical calculations yield estimates of concentrations of the particular pollutant for specific locations and times. • The meteorological parameters required for use of the models include wind direction, wind speed, and atmospheric stability. • In some models, provisions may be made for including lapse rate and vertical mixing height. • Most models will require data about the physical stack height, the diameter of the stack at the emission point, the exit gas temperature and velocity, and the mass rate of emission of pollutants. Point Source Gaussian Dispersion Model Assumptions: 1. Atmospheric stability is uniform throughout the layer into which the contaminated gas stream is discharged. 2. Turbulent diffusion is a random activity and hence the dilution of the contaminated gas stream in both the horizontal and vertical direction can be described by the Gaussian or normal equation.

6. 3. The contaminated gas stream is released into the atmosphere at a distance above the ground level that is equal to the physical stack height plus the plume rise. 4. The degree of dilution of the effluent plume is inversely proportional to the wind speed. 5. Pollutant material that reaches ground level is totally reflected back into the atmosphere like a beam of light striking a mirror at an angle. Stability Criteria: Stability A = very unstable atmosphere Stability B = unstable atmosphere Stability C = slightly unstable atmosphere Stability D = neutral atmosphere Stability E = stable atmosphere Stability F = very stable atmosphere

7. Dispersion Model/ Equation: • The dispersion model gives the ground level concentration of pollutant at a point downwind from a stack with an effective stack height H. c(x,y,0,H)= ground level concentration, g/m3 E = emission rate of pollutant, g/s sy, sz= plume standard deviations, m (empirical formulas) vw = wind speed, m/s x, y, z = coordinate distances, m exp = exponential e, Naperial base e H = effective stack height, m

8. The value for the effective stack height is the sum of the physical stack height, h and the plume rise DH. vs= stack velocity, m/s d = stack diameter, m vw= wind speed, m/s P = pressure, kPa Ts = stack temperature, K Ta = ambient temperature, K where, • The values of sy and sz depend upon the turbulent structure or stability of the atmosphere where constants a, c, d, and f are defined in Table 6.7 and the equations yield standard deviations in meters for downwind distance x in kilometers.

9. Figure: Plume Dispersion Coordinate System

11. Example: It has been estimated that the emission of SO2 from a coal-fired power plant is 1,656.2 g/s. Determine the centerline concentration (y = 0) of SO2 at downwind distance of 3 km if the wind speed is 4.50 m/s in a neutral atmosphere. Stack Parameters: Height = 120.0 m Diameter = 1.5 m Exit Velocity = 10.0 m/s Temperature = 315 oC Atmospheric Condition: Pressure = 95.0 kPa Temperature = 25 oC

12. Inversion Aloft: • When an inversion is present, the basic diffusion equation must be modified to take into account the fact that the plume cannot disperse vertically once it reaches the inversion layer. • The plume will begin to mix downward when it reaches the base of the inversion layer.

13. The downward mixing will begin at a distance xL downwind from the stack. • The distance xL is a function of the stability in the layer below the inversion. • It has been determined empirically that the vertical standard deviation of the plume at the distance xL is: where, L= height to bottom of inversion layer, m H = effective stack height, m • When the plume reaches twice the distance to initial contact with the inversion base, the plume is said to be completely mixed throughout the layer below the inversion. • Beyond a distance equal to 2xL the centerline concentration of the pollutants may be estimated as: Inversion Form of Dispersion Equation

14. Example: Determine the distance downwind from a stack at which we must switch to the inversion form of the dispersion model given the following meteorologic situation: Effective Stack Height = 50.0 m Inversion Base = 350 m Stability Condition = Stable Atmosphere Indoor Air Quality Model Example: An unvented kerosene heater is operated for one hour in an apartment having a volume of 200 m3. The heater emits SO2 at a rate of 50 mg/s. The ambient air concentration and the initial indoor air concentration of SO2 are 100 mg/m3. If the rate of ventilation is 50 L/s, and the apartment is assumed to be well mixed, determine the indoor air concentration of SO2 at the end of one hour. The reaction rate coeff. for SO2 is 6.39 x 10-5 s-1.

15. Absorption • Control devices based on the principle of absorption attempt to transfer the pollutant from a gas to a liquid phase. • This is a mass transfer process in which the gas dissolves in the liquid. The dissolution may or may not be accompanied by a reaction with an ingredient of the liquid. • Mass transfer is a diffusion process wherein the pollutant gas moves from points of higher concentration to lower concentration. AIR POLLUTION CONTROL OF STATIONARY SOURCES Adsorption • This is a mass-transfer process in which the gas is bonded to a solid. It is a surface phenomenon. • The gas (adsorbate) penetrate into the pores of the solid (adsorbent). • The bond may be physical or chemical. Electrostatic forces hold the pollutant gas when physical bonding is significant. Chemical bonding is by reaction with the surface.

16. Combustion • When the contaminant in the gas stream is oxidizable to an inert gas, combustion is a possible alternative method of control. • Typically, CO and hydrocarbons fall into this category. Both direct flame combustion by afterburners and catalytic combustion may be used. Flue Gas Desulfurization (FGD) • FGD is a technology used to remove sulfur dioxide from the exhaust flue gases of fossil fuel power plants. Fossil fuel power plants burn coal or oil to produce steam for steam turbines which in turn drive electricity generators. • Flue gas desulfurization systems fall into two categories; non-regenerative and regenerative. • Non-regenerative means the reagent used to remove the sulfur oxides from the gas stream is used and discarded. • Regenerative means that the reagent is recovered and reused.

17. Gasoline Engine • One kg of gasoline can burn completely when mixed with about 15 kg of air. However, for maximum power, the proportion of air to fuel must be less. • Combustion is incomplete, and substantial amounts of material other than carbon dioxide and water are discharged. • One result of having an inadequate supply of air is the emission of carbon monoxide instead of carbon dioxide. Other by-products are unburned gasoline and hydrocarbons. AIR POLLUTION MOBILE SOURCES Diesel Engine • A diesel normally operates at a higher air-to-fuel ratio than does a gasoline engine. • the fuel is injected directly to the combustion chamber, so no carburetor is required. The power output is changed by the rate of fuel injection.

18. There is no spark ignition system. The air is heated by compression. That is, the air in the engine cylinder is squeezed until it exerts a pressure high enough to raise the air temperature to about 540 oC, which is enough to ignite the fuel oil as it is injected into the cylinder. • A well-designed, well-maintained, and properly adjusted diesel engine will emit less CO and hydrocarbons than the four-stroke engine because of the diesel’s high air-to-fuel ratio. • However, the higher operating temperatures lead to substantially higher NOx emissions.