ENERGY SOURCES—ALTERNATIVES 303
This ratio may still change for the worse, since:1) the combustion of oil will decrease, especially in
power plants;
2) the burning of coal will increase until nuclear
energy can substantially replace the coal in new
power plants—supposed nuclear power plants
will be accepted by population also in the larger
future.Therefore new coal burning power plants will be equipped
with desulfurization equipment that may eliminate in increas-
ing measure the emission of sulfur dioxide.
Emission factors are given for various uncontrolled
sources in Table 8 as a function of the per cent sulfur con-
tent of the fuel. The average sulfur content of coal is about
2.5% by weight while that of crude oil is closer to 0.3%, but
these levels can vary considerably depending on the source.
In petroleum refining, the sulfur generally remains in the
heavier fractions and reaches much higher concentrations in
residual fuel oil, for example. Sulfur dioxide emissions may
be controlled by fuel selection using either low sulfur fuels
or by removing the sulfur from the fuel before burning it. At
the beginning of the seventies other techniques have been
proposed and developed to capture sulfur compounds during
combustion or to remove them from the flue gas before it isdischarged to the atmosphere. Flue gas scrubbing processes
developed, under test and further improvement include the
use of MgO, alkalyzed alumina, or limestone to neutralize
oxides of sulfur, forming precipitates of sulfites and sulfates
that can be collected as solids. A catalytic oxidation pro-
cess, based on the technology of sulfuric acid production,
that captures sulfur emissions as SO 3 in a weak acid solution
is also quite promising.Oxides of Nitrogen,^17 Hydrocarbons,^18 and
Photochemical oxidants^19
NO x and reactive organic substances such as hydrocarbons
(HC), when exposed in the atmosphere to solar ultraviolet
radiation, form photochemical smog consisting of aldehydes
and a group of oxidants including ozone and peroxyacyl
nitrates.
Oxides of nitrogen are produced in all combustion pro-
cesses that use air as the oxidant at a rate that is dependent
on both the flame temperature and air supply. Nitric oxide
(NO) is formed at high combustion temperatures and nor-
mally would convert to the more stable nitrogen dioxide
(NO 2 ) as the gas temperature is reduced. The kinetics of that
reaction are slow, however, and the gases cool to near ambi-
ent conditions before very much NO 2 can be formed. In the
combustion of methane, the peak equilibrium NO x emission
concentration is developed with about 15% excess air while
the peak flame temperature occurs at a slightly deficient air
fuel ratio.
With different fuels the NO x emissions will vary, depend-
ing on the theoretical flame temperatures and the combustion
process used. Table 9 gives average uncontrolled emission
factors for NO x. These emissions may be reduced, but not
eliminated, by the adjustment of the air supply to a fuel-rich
condition and operation at as low a combustion temperature
as possible by means of two stage combustion, exhaust gas
recirculation, or water injection. Some of the flue gas scrub-
bing techniques proposed to control SO 2 emissions in power
plants may also be effective for the removal of NO x from the
gas stream.
The main sources of hydrocarbon emissions are evapo-
ration of fuel from vehicles and in fuel handling operations,TABLE 7
Particulate emission factors^11Average emissions per
unit of fuel burned
General 16 Aa lb/ton
Dry bottom 17 A lb/ton
Wet bottom 13 A lb/ton
Cyclone 2 A lb/ton
Spreader stoker
Greater than 10 106 btu/hr 13 A lb/ton
Less than 10 106 btu/hr 2 A lb/ton
Hand-fired equipment 20 lb/ton
Utility 8 lb/1000 gal
Industrial and commercial 23 lb/1000 gal
Industrial and commercial 15 lb/1000 gal
Household 10 lb/1000 gal
Utility 15 lb/million ft^3
Industrial 18 lb/million ft^3
Household and commercial 19 lb/million ft^3
Gasoline-powered vehicle, urban 1970 8 lb/1000 gal
Diesel-powered bus and truck 13 lb/1000 gal
Aircraft, long range turbofan 8 lb/engine-flightb
Jumbo jet 10 lb/engine-flightb
a Where letter A is shown, multiply number given by the per cent ash in
coal.
b Emission during portion of flight at altitudes less than 3500 ft, including
takeoff and landing.TABLE 8
Sulfur dioxide emission factors^11Average emissions per unit
of fuel burned
Coal 38 Sa lb/ton
Fuel oil 157 Sb lb/1000 gal
Natural gas
Gasoline-powered vehicle, urban 1970 5 lb/1000 gal
Diesel-powered bus and truck 27 lb/1000 gal
Aircraft, long range turbofan and jumbo jet 2 lb/engine-flightc
a S indicates % sulfur in fuel.
b This factor is for residual oil. For distillate oil use factor of 142 S.
c Emission during portion of flight at altitudes less than 3500 ft, including
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