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are directed toward testing various forced oxidation designs to find the best oxida-
tion system using the least energy.
2. The chemical reactions of the wet scrubbing FGD process are discussed and presented
next. As stated previously, in a lime slurry system, the sulfur dioxide and sulfur trioxide
react with lime (CaO) to form calcium sulfite and water, based on the following reaction:
SO + CaO + H O → CaSO + H O
x 2 3 2
When limestone (CaCO ) is used instead of lime, it results in a similar chemical reac-
3
tion, but also yields carbon dioxide:
SO + CaCO + H O → CaSO + H O + CO
x 3 2 3 2 2
In the forced oxidation reaction, the oxygen in air oxidizes the calcium sulfite CaSO
3
to calcium sulfate CaSO as present in the following reaction:
4
CaSO + H O + 0.5 O → CaSO + H O
3 2 2 4 2
where SO = SO + SO , CaO = lime, calcium oxide, CaSO = calcium sulfite, CaCO
x 2 3 3 3
= limestone and, CaSO = calcium sulfate.
4
Example 27
Dry scrubbing is a feasible process FGD (32,66,67). Discuss the following:

1. The process description, performance, and future of the dry scrubbing FGD process.
2. The chemical reactions of the dry scrubbing FGD process.

Solution
1. The following presents the process description, performance, and future of the dry
scrubbing desulfurization process. Dry scrubbing is a modification of the wet scrub-
bing FGD technology. As in other FGD systems, the exhaust gases combine with a
fine slurry mist of lime or sodium carbonate. This system, however, takes advantage
of the heat of the exhaust gases to dry the reacted slurry into particles of calcium sul-
fite and sodium sulfite.

The particles generated by this dry scrubbing process are then collected along with
other particles from coal combustion in a baghouse collector. This collector uses fab-
ric bags that function similarly to those in a vacuum cleaner, which collect particles
while permitting cleaned gases to escape.
Dry scrubbing typically removes 70% of the sulfur dioxide in a waste gas stream. It
is 15–30% less expensive to install and operate than a conventional wet scrubbing
system. However, because dry scrubbing is less efficient than wet scrubbing, the tech-
nology has been limited to use with low-sulfur coal.
Plans for future research include evaluating the performance and reliability of a full-
scale utility boiler equipped with a spraydryer SO control system. Improvements
2
could make these dry scrubbing systems acceptable for general use by the early 2000s.
2. The chemical reactions of the dry scrubbing desulfurization process are as follows:
SO + CaO → CaSO
2 3
SO + Na CO → Na SO + CO
2 2 3 2 3 2
where CaO = lime, calcium oxide, CaSO = calcium sulfite, Na CO = sodium car-
3 2 3
bonate, and Na SO = sodium sulfite.
2 3