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290 10 Post-combustion Air Emission Control

Table 10.2 The most important FGD systems with non-regenerable sorbents
Process Sorbent By-product
Wet scrubbers Lime/Limestone Gypsum, calcium sulfate/sulfite
Lime/Fly ash Calcium sulfate/sulfite/fly ash
Spray-dry scrubbers Lime Calcium sulfate/sulfite
Dual-alkali Primary: sodium hydroxide Calcium sulfate/sulfite
Secondary: lime
Seawater Primary: seawater Waste seawater
Secondary: lime
Walther Ammonia Ammonia sulfate
Source http://www.iea-coal.org.uk/

Aqueous ammonia can also be an effective SO 2 absorbent. The corresponding
overall reactions can be described using Eqs. (10.12) and (10.13) that follow.

ð
SO 2 þ 2NH 3 þ H 2 O ! NH 3 Þ SO 3 ð10:12Þ
2
And the product of (NH 3 ) 2 SO 3 can further react with SO 2 to produce ammonium
hydrogen sulfite

ð
SO 2 þ NH 3 Þ þ H 2 O ! 2NH 4 HSO 3 ð10:13Þ
2
The resultant products are usually used as fertilizer feedstock.
The most important FGD processes as listed by IEA Coal Research are given in
Table 10.2.
The Walther process is based on scrubbing with ammonia water where the
product is mainly (NH 4 ) 2 SO 4 . However, the sulfur content in the fuel has to be
below 2 %-wt in order to minimize the risk of formation of ammonium sulfate
aerosols.

10.3.1.2 Renewable Sorbents
Over 80 % of the power utilities with sulfur emission control use a non-regenerable
sorbent based on calcium to remove the sulfur from the flue gas. However, in most
FGD processes, the sorbent is not completely used due to the nature of shell
formation surrounding the core lime [35]. It is a waste of resources to throw away
the “used” sorbents. In addition, this throwaway sorbent creates also extra cost for
landfill. Therefore, regeneration of “used” sorbent has been developed accordingly.
Important commercial FGD processes with regenerable sorbents are listed by IEA
Coal Research and summarized in Table 10.3.