302 ENERGY SOURCES—ALTERNATIVES
design of the combustion chamber to prevent regions of
insufficient air and by providing residence time and tem-
perature conditions that will allow conversion of all of the
CO to CO 2. In cases where combustion conditions cannot
be adequately controlled to permit complete conversion, the
exhaust gases may be reacted with air either thermally or
catalytically to complete the reaction to CO 2.Particulates 12, 13
This category of pollutants includes any matter, solid or
liquid, in which individual particles are larger than molecular
size but smaller than 500 microns. The burning of coal in
central station power plants accounts for about 64% of all
particulate emissions even with the widespread use of elec-
trostatic precipitators with efficiencies of up to 99.5%. This
particulate release is mostly in the form of fly ash, which is
predominantly carbon, silica, alumina, and iron oxide, and
when recovered from stack gases, poses a solid waste disposal
problem. Markets for fly ash involve construction uses and
are growing. Oil has a lower ash content than coal but is also
a source of particulate emissions. The particulate emissions
from mobile power plants include lead compounds, carbon,
and metallic oxides. Lead alkyl additives used in gasoline to
raise its octane number are present in concentrations of 2 to
4 g/gal and this lead content is ultimately discharged to the
environment. The automotive and petroleum industries are
moving toward reduction or elimination of these additives.
Emission factors for particulates from uncontrolled
sources are given in Table 7. The emissions from the com-
bustion of coal are given as a function of the per cent ash
content, which depends on the grade of coal but averages
about 10%. Wet bottom and cyclone-type burners capture
more ash in the furnace in the form of bottom ash, which is a
solid waste, and thereby reduce the fly ash particulate matter
released with the flue gas. Control of particulate emissions
from power plants is provided by cyclone separation, fil-
tering of the flue gases, and electrostatic precipitation, and
these techniques are used extensively.
Particulate matter in the atmosphere has adverse effects
on materials and health and will have long-term effects on the
thermal energy balance of the earth. It provides condensation
nuclei which can increase cloud cover and thereby reflect
more of the solar energy input to the earth back into space.It has been estimated^14 that a 1% increase in the earth’s cloud
cover can reduce the average temperature of the earth by 1.4°F
and an increase of 5% could lead to the return of an ice age.
Volcanic eruptions are believed to account for most of the
particulate contamination of the upper atmosphere and in the
past have been a source of meteorological changes. Jet aircraft
exhausts add water vapor and particulates to the upper atmo-
sphere and can also contribute to meteorological effects.Sulfur Dioxides 15,16
SO 2 is produced in the combustion of sulfur-bearing fuels
such as coal and residual oil and is released to the atmo-
sphere in the flue gases from power plants using these fuels.
Approximately 60% of the SO 2 released to the atmosphere in
1968 was produced in the burning of coal while the combus-
tion of oil accounted for another 14%.TABLE 6
Carbon monoxide emission factors^11Average emissions per
unit of fuel burned
Stationary Sources
Coal:
Household and commercial 10 lb/ton
Industry 2 lb/ton
Utility 1 lb/ton
Fuel oil:
Household 5 lb/1000 gal
Commercial and industrial 0.2 lb/1000 gal
Utility 0.04 lb/1000 gal
Natural gas:
Household and commercial 20 lb/million ft^3
Utility and industrial 0.4 lb/million ft^3
Mobile Sources
Gasoline-powered vehicle urban 1970 2620 lb/1000 gal
Diesel-powered bus and truck 225 lb/1000 gal
Aircraft, long range turbofan 26 lb/engine-flighta
Jumbo jet 28 lb/engine-flightaa Emissions during portion of flight at altitudes less than 3500 ft, including
takeoff and landing.TABLE 5
Estimated nationwide emissions, 1968^10 (in millions of tons per year)Source Carbon monoxide Particulates Sulfur oxides Hydrocarbons Nitrogen oxides Total
Transportation 63.8 1.2 0.8 16.6 8.1 90.5
Fuel combustion in
stationary sources1.9 8.9 24.4 0.7 10.0 45.9Industrial processes 9.7 7.5 7.3 4.6 0.2 29.3
Solid waste disposal 7.8 1.1 0.1 1.6 0.6 11.2
Miscellaneous† 16.9 9.6 0.6 8.5 1.7 37.3
Total 100.1 28.3 33.2 32.0 20.6 214.2† Primarily forest fires, agricultural burning, coal waste fires.C005_006_r03.indd 302C005_006_r03.indd 302 11/18/2005 12:05:46 PM11/18/2005 12:05:46 PM