TM 5-815-1/AFR 19-6
SULFUR OXIDE (SOx) CONTROL SYSTEMS
Formation of sulfur oxides (SOx)
(3) When choosing a higher quality fuel, as in
changing from residual to distillate fuel oil,
a. Definition of sulfur oxide. All fossil fuels contain
modest modifications, such as changing
sulfur compounds, usually less than 8 percent of the
burner tips, and oil feed pumps, are required.
fuel content by weight. During combustion, fuel-bound
c. Changes in fuel properties. Consideration of pos-
sulfur is converted to sulfur oxides in much the same
sible differences in fuel properties is important. Some
way as carbon is oxidized to CO2. Sulfur dioxide (SO2)
and sulfur trioxide (SO3) are the predominant sulfur
(1) Higher ash content increases particulate emis-
oxides formed. See equations 10-1 and 10-2.
(2) Lower coal sulfur content decreases ash
fusion temperature and enhances boiler tube
b. Stack-gas concentrations. In efficient fuel com-
(3) Lower coal sulfur content increases fly-ash
bustion processes, approximately 95 percent of the
fuel-bound sulfur is oxidized to sulfur dioxide with 1
to 2% being coverted to sulfur trioxide.
(4) Low sulfur coal types may have higher
c. Factors affecting the formation of SOx.
sodium content which enhances fouling of
(1) 503 formation increases as flame temperature
increases. Above 3,150 degrees Fahrenheit,
(5) The combination of physical coal cleaning
503 formation no longer increases.
and partial flue gas desulfurization enables
(2) SO3 formation increases as the excess air rate
many generating stations to meet SO2
standards at less expense than using flue gas
(3) SO3 formation decreases with coarser
d. Modification of fuel. Some possibilities are:
(1) Fuels of varying sulfur content may be mixed
Available methods for reducing SOX
to adjust the level of sulfur in the fuel to a low
enough level to reduce SO2 emissions to an
a. Fuel substitution. Burning low sulfur fuel is the
most direct means of preventing a SOx emissions prob-
(2) Fuels resulting from these processes will
lem. However, low sulfur fuel reserves are decreasing
become available in the not too distant future.
and are not available in many areas. Because of this,
Gasification of coal removes essentially all of
fuel cleaning technology has receive much attention.
the sulfur and liquification of coal results in a
There are presently more than 500 coal cleaning plants
reduction of more than 85% of the sulfur.
in this country. At present, more than 20% of the coal
e. Applicability of boiler conversion from one fuel
consumed yearly by the utility industry is cleaned.
type to another. Table 10-1 indicates that
Forty to ninety percent of the sulfur in coal can be
can be converted to other type of firing but that policies
removed by physical cleaning, depending upon the type
of the agencies must also be a consideration.
of sulfur deposits in the coal. As fuel cleaning tech-
nology progresses and the costs of cleaning decrease,
fuel cleaning will become a long term solution
available for reducing sulfur oxide emissions.
b. Considerations of fuel substitution. Fuel sub-
stitution may involve choosing a higher quality fuel
grade; or it may mean changing to an alternate fuel
type. Fuel substitution may require any of the following
(1) Alternations in fuel storage, handling, prepa-
(2) When changing fuel type, such as oil to coal,
a new system must be installed.