Permit Regulations
4.3 CLASSIFYING SOURCES BY METHOD OF EMISSION
plume is applied. Under these conditions dispersion should most readily be described by Gaussian models.
4.2 THE TALL STACK
The Tennessee Valley Authority (TVA) has pioneered the use of tall stacks in the U.S. and has carried out extensive experiments, collected data, and determined the design variables and mathematical models to predict minimum ground-level con-centrations. The 170 ft stacks provided at the first large steam plant constructed by TVA at Johnsonville, TN, in 1952 were soon found to be inadequate. These stacks were then extended to 270 ft in 1955, and TVA stack height has crept upwards ever since. As evidence, the large coal-fired power plant at Cumberland City, TN, has two 1000-ft stacks, and the Kingston and Widows Creek Plants which each have a 1000-ft stack, topping the former tallest stacks at the Bull Run and Paradise plants by 200 ft.
Ever since structural steel became plentiful and strong enough to carry extreme loads, longer and taller structures have been built. Competition in this area is keen, and one wonders whether stack structures grow out of a rational need to reduce ground-level concentrations, or out of man’s need to excel. Whatever the reason, it is amusing to compile and contemplate the statistics on tall structures, as listed in Table 4.1. Table 4.2 has the details of the TVA stacks at their major steam plants.
4.3 CLASSIFYING SOURCES BY METHOD OF EMISSION
Table 4.3 summarizes useful methods by which air pollution sources can be classi-fied. Dispersion models exist which fit into this scheme. For stationary sources three cases are defined: area sources, process stacks, and tall stacks.
4.3.1 A DEFINITION OF TALL STACKS
Adopting the TVA viewpoint to define a tall stack requires reference to the amount of furnace heat input which should be greater than 293 MW (109 BTU/h). A furnace TABLE 4.1
The Size of Tall Things
Sears Tower 1450 ft INCO Stack (Sudbury, ON) 1250 ft
World Trade Center 1350 ft Kennecott Company (Magna, UT) 1215 ft Empire State Building
(1475 ft to top of mast)
1250 ft Penn Electric Company New York State Electric Co.
(Homer City, PA)
1210 ft
Chrysler Tower 1040 ft American Electric Power Mitchell Plant (Cresap, WV) 1206 ft
Eiffel Tower 984 ft Keystone Group (Conamaugh, PA) 1000 ft
Gateway Arch 630 ft TVA-Cumberland Plant 1000 ft
Washington Monument 555 ft TVA-Widows Creek Plant 1000 ft
TVA-Kingston Plant 1000 ft
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with this heat input would require 9072 kg/d (10,000 tons/d) of coal with a total heating value of 27.89 × 1010 J/kg (12,000 BTU/lb). A 100 MW plant would use about 771 kg/d (850 tons/d) and could qualify as having a tall stack. Most tall stack sources will be associated with fossil fuel burning steam electric power generating facilities. Another method to identify a tall stack is through the heat emission rate.
This quantity should be greater than 20 MW (68.24 × 106 BTU/h) to define a tall
Cumberland 1972 1–2 1300 2600 2 1000
Bull Run 1967 1 950 950 1 800
Paradise 1963 1–2 704 2558 2 600
3 1150 1 800
Allen 1959 1–3 330 990 3 400
Gallatin 1956 1–2 300 1255 1 500
3–4 327.5 1 500
Colbert 1955 1–2 200 1397 2 300
3–4 223 2 300
5 550.5 1 500
John Sevier 1955 1–2 223 846 1 350
3–4 200 1 350
Kingston 1954 1–4 175 1700 2 1000a
5–9 200
Shawnee 1953 1 150 1750 2 800b
2–7 175
8 150
9 175
10 150
Widows Creek 1952 1–2 140.6 1978 1 1000c
3 150
4,5,6 140.6
7 575 1 500
8 550 1 500
Johnsonville 1951 1–4 125 1485 4 270d
5–6 147 2 270d
7–10 173 2 400
Watts Bar 1942 1–4 60 240 4 177
a Original heights units 1–4, 250 ft, units 5–9, 300 ft.
bReplaces ten 250 ft stacks.
c Original six stacks 170 ft high, raised to 270 ft, then replaced.
dOriginal height 170 ft.
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Heat input is not the only requirement for establishing a tall stack. Such stacks produce plumes with great buoyancy, and these plumes have a high plume rise after leaving the stack. Furthermore, the exit velocity is high enough to avoid any building downwash. Rules of thumb to estimate the required exit velocity and stack height are
TABLE 4.3
Classifying Air Pollution Sources by Method of Emission
Moving Sources
Result of space heating and trash burning Homes, apartments, commercial buildings
Improper firing of furnaces, poor quality coal, uncontrolled emission Models: Require extensive source-emission information
Process Stacks
Chemical and Petroleum Processing Space heating and process steam
May be result of leak or venting waste inorganic or organic gases Heights up to about 250 ft
Low buoyancy, high velocity — could be a pure jet emission — plume rise not great
Model: Gaussian, but must evaluate the effects of stack and building downwash and surrounding topographical features
Tall Stacks
Fossil Fuel Burning for Electrical Power Production Heights up to 1250 ft
High buoyancy and velocity Plume rise significant
Furnace heat input: 109 BTU/h (100 MW plant and larger) Heat emission rate: 19,000 BTU/s
Stack height: 2.5 times height of tallest structure near stack Stack velocity: 1.5 times maximum average wind speed expected
Model: Gaussian, maximum concentration encountered depends upon regional meteorological conditions and topographical features
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where
hs= height of stack vs= stack exit velocity
hb= height of tallest structure near stack
u = maximum average wind speed that will be encountered–
When a stack satisfies all these conditions, it may be considered a tall stack, and calculations are simplified.
4.3.2 PROCESS STACKS
All other point sources differ in several ways. Most process stacks are not connected to sources with a high furnace heat input. Thus buoyancy is limited, and plume rise may be smaller. Quite often these stack plumes will have a high velocity, but little density difference, compared to ambient conditions. Thus the plumes might be considered as jets into the atmosphere. Furthermore, since these stacks are usually shorter than 400 ft, the plumes may be severely affected by the buildings and the terrain that surround them. If stack efflux velocity is low, stack downwash may become prominent. In general, this is the kind of stack that is found in a chemical or a petroleum processing plant. Emissions from such a stack range from the usual mixture of particulates, sulfur oxides, nitrogen oxides, and excess air to pure organic and inorganic gases. To further complicate matters, these emissions usually occur within a complex of multiple point emissions; the result being that single-point source calculations are not valid. A technique for combining these process complex sources must then be devised.