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TM 5-815-1/AFR 19-6

CHAPTER 10
SULFUR OXIDE (SOx) CONTROL SYSTEMS
10-1.

Formation of sulfur oxides (SOx)

a. Definition of sulfur oxide. All fossil fuels contain
sulfur compounds, usually less than 8 percent of the
fuel content by weight. During combustion, fuel-bound
sulfur is converted to sulfur oxides in much the same
way as carbon is oxidized to CO2. Sulfur dioxide (SO2)
andsulfur trioxide (SO3) are the predominant sulfur
oxides formed. See equations 10-1 and 10-2.

b. Stack-gas concentrations. In efficient fuel combustion processes, approximately 95 percent of the
fuel-bound sulfur is oxidized to sulfur dioxide with 1
to 2% being coverted to sulfur trioxide.
c. Factors affecting the formation of SOx.
(1) 503 formation increases as flame temperature
increases.Above 3,150 degrees Fahrenheit,
503 formation no longer increases.
(2) SO3 formation increases as the excess air rate
is increased.
(3) SO3 formation decreases with coarser
atomization.

10-2.

Available methods for reducing SOX
emissions

a. Fuel substitution. Burning low sulfur fuel is the
most direct means of preventing a SOx emissions problem. However, low sulfur fuel reserves aredecreasing
and are not available in many areas. Because of this,
fuel cleaning technology has receive much attention.
There are presently more than 500 coal cleaning plants
in this country. At present, more than 20% of the coal
consumed yearly by the utility industry is cleaned.
Forty to ninety percent of the sulfur in coal can be
removed by physical cleaning, depending upon the type
ofsulfur deposits in the coal. As fuel cleaning technology 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 substitution may involve choosing a higher quality fuel
grade; or it may mean changing to an alternate fuel
type. Fuel substitution may require any ofthe following
considerations:
(1) Alternations in fuel storage, handling, preparation, and combustion equipment.
(2) When changing fuel type, such as oil to coal,
a new system must be installed.

(3) When choosing a higher quality fuel, as in
changing from residual to distillate fuel oil,
modest modifications, such as changing
burner tips, and oil feed pumps, are required.
c. Changes infuel properties. Consideration of possible differences in fuel properties is important. Some
examples are:
(1) Higher ash content increases particulate emissions.
(2) Lower coal sulfur content decreases ash
fusion temperature and enhances boiler tube
slagging.
(3) Lower coal sulfur content increases fly-ash
resistivity and adversely affects electrostatic
precipitator performance.
(4) Lowsulfur coal types may have higher
sodium content which enhances fouling of
boiler convection tube surfaces.
(5) The combination of physical coal cleaning
and partial flue gas desulfurization enables
many generating stations to meet SO2
standards at less expense than using flue gas
desulfurization alone.
d. Modification of fuel. Some possibilities are:
(1) Fuels of varying sulfur content maybe mixed
to adjust the level of sulfur in the fuel to a low
enough level to reduce SO2 emissions to an
acceptable level.
(2) Fuels resulting from these processes will
become available in the not too distant future.
Gasification of coal removes essentially all of
the sulfur and liquification of coal results in a
reduction of more than 85% of the sulfur.
e. Applicability of boilerconversion from one fuel
type to another. Table 10-1 indicates that most boilers
can be converted to other type of firing but that policies
of the agencies must also be a consideration.

10-1

TM 5-815-1/AFR 19-6
f. Approach to fuel substitution. An approach to fuel
substitution should proceed in the following manner:
(1) Determine the availability of low sulfur fuels.
(2) For each,...