Page 311 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

288 10 Post-combustion Air Emission Control

Table 10.1 Particle size collection efficiencies of various wet scrubbers
Type of scrubber Pressure drop in Pa Minimum collectable particle diameter in μm
Gravity spray tower 125–375 10
Cyclonic spray tower 500–25,00 2–6
Packed bed scrubbers 500–4,000 1–5
Venturi scrubber 2,500–18,000 0.5–1
A minimum particle size collected with approximately 85 % efficiency
Source http://www.epa.gov/ttn/caaa/t1/reports/Sect.5-4.pdf

Table 10.1 shows the comparison of size collection efficiencies of various wet
scrubbers for particulate separation. Obviously a gravity spray tower has the lowest
pressure drop as well as the largest minimum collectable particle diameter of 10 μm.
Among them, a venturi scrubber seems to be the most effective and it also has the
highest pressure drop.

10.3 Flue Gas Desulfurization

FGD, one major post-combustion SO x control technology, has been developed for
three temperature ranges, low (<200 °C), medium (200–400 °C) and high
(>600 °C). The former two are usually in duct and the last one right after the
furnace. FGD can also be classified into wet, dry, and semi-dry processes,
depending on the presence of water in the sorbent and the flue gas.
Data on worldwide FGD applications show that wet limestone FGD have been
predominantly selected over other FGD technologies. Wet FGD is a well-established
technology with high sorbent conversion rate and high desulfurization efficiency.
The main challenges facing the wet FGD system are the high capital costs and
extensive post processing of liquid FGD byproducts. As a promising alternative to
wet FGD, dry or semi-dry FGD at medium temperatures have been developed for
potential cost and energy savings and relative ease of operation compared to wet
FGD. At a medium temperature, the reaction rate of Ca(OH) 2 with SO 2 should be
very fast, allowing higher conversions because the sinterization of the sorbents can
be avoided and a better contact between the solid and the flue gas can be reached.

10.3.1 Wet FGD

In a wet FGD process, a solution of lime (Ca(OH) 2 ) with a high content of calcium
and magnesium are sprayed into an absorption tower to capture the SO x in the flue
gas. CaO is produced by calcining the CaCO 3 at temperatures above 800 °C with
the release of CO 2 .