Boiler Plants - Energy Conversion and Management - page 88

Except for sulfur compound particles, most of the sulfur contained in the fuel is
converted into SO 2 , but a small percentage of the total oxides is converted into
sulfur trioxide SO 3 . Sulfur oxides combine with the moisture present in stack flue
gas to form sulfuric acid, some of which can condense and corrode metallic parts of
the boiler. Similarly, in the atmosphere a portion of SO 2 is converted into SO 3 and
then into sulfuric acid and sulfur compounds.
The quantity of sulfur oxides formed during combustion is closely related to the
sulfur content of the fuel and does not depend on boiler operating conditions. Thus,
reduction of sulfur oxides can be obtained by using low-sulfur fuel, natural gas in
particular, or by introducing abatement devices such as stack gas scrubbers, which
remove SO 2 from stack flue gas.
The maximum SO 2 concentration in stack gas must be kept lower than
1,500-2,000 mg/Sm 3 or 525-700 ppm (see Table 6.8 ).
Emulsifying fuel oil with water or other compounds and atomization at the
burner are other techniques widely applied to solve problems due to the use of
heavy oil.
6.11.2 Nitrogen Oxides
Nitrogen oxides NO x are formed during combustion by the combination of oxygen
and nitrogen at high temperature. Both components are naturally present in com-
bustion air and nitrogen in the fuel itself.
NO x emissions are mainly NO (95 % in mass) and NO 2 (5 % in mass). In the
atmosphere NO combines with oxygen to form NO 2 .
NO x formed during the combustion process can be classified as Thermal, Fuel, or
Prompt:
• Thermal NO x (via Zeldovich mechanism) is formed due to the oxidation of
atmospheric nitrogen at temperatures higher than 1,273 K (1,000 C, 1,832 F).
It becomes important when the temperature reaches the range 1,600-1,800 K
(1,327-1,527 C, 2,421-2,781 F);
•FuelNO x is formed due to the oxidation of the nitrogen in the fuels (its value
ranges between 0.05 and 1.5 % in mass depending on the kind of fuel; lower
values for natural gas and light oil, higher values for heavy oil and coal). The
oxidation is accelerated if excess air value is high;
• Prompt NO x derives from the reaction of atmospheric air with hydrocarburic
radicals.
The concentration of NO x in stack gases is related mainly to the quantity
of nitrogen in the air; typical values are 50-200 mg/Sm 3 or 102-410 ppm
(see Table 6.8 ).