Page 192 - New Trends In Coal Conversion

Technologies for control of sulfur and nitrogen compounds and particulates 155

between 5 and 6.5 to ensure the SO 2 absorption capacity of the slurry (C ordoba, 2015).
Gypsum is then produced following Reaction 4:

CaCO 3 þ O 2 þ 2H 2 O/CaSO 4 $2H 2 O (R4)

The gypsum aqueous solution, typically ranging from 15% to 18% wt gypsum in
water, is dehydrated to produce a marketable by-product. Hydrocyclones and vacuum
filters are employed to produce a gypsum with a moisture content ranging between 7%
and 10% wt (Poullikkas, 2015; Wang et al., 2005). SO 2 removal efficiencies up to 98%
can be achieved by this approach (European Commission, 2006).
Seawater scrubbing. Seawater scrubbing is considered the most feasible option for
coastal industries due to its low operating and solvent costs compared with other FGD
technologies. The presence of bicarbonates in seawater enhances its alkaline proper-
ties. Seawater is capable of absorbing SO 2 from the flue gas in sulfite and sulfate forms,
which are natural components of the seawater (Poullikkas, 2015). The acidified
seawater leaving the absorber must be mixed with fresh seawater to adjust the pH
for the oxidation process. Lime and limestone can also be added. Air is then passed
through the seawater to force the oxidation sulfite and the dissolved CO 2 is also elim-
inated. The O 2 content and the pH are restored before discharge back into the sea. This
FGD process avoids the disposal of any material into landfills, but it causes ecological
damage in aquatic environments close to the used seawater discharge. The SO 2
removal efficiency ranges from 85% to 98% (C ordoba, 2015).
Dual-alkali scrubbing. This process employs a sodium alkali slurry containing
sodium-based sorbents, mainly sodium hydroxide (NaOH) and sodium carbonate
(Na 2 CO 3 ), to produce sodium sulfite (Na 2 SO 3 ) and sodium bisulfite (NaHSO 3 ) once
SO 2 is absorbed from the flue gas. To regenerate the sorbent, lime or limestone is
added into the alkali slurry. Calcium sulfite and sulfate is then formed and precipitates
from the slurry. The initial sodium-based alkali solution is recovered and reintroduced
into the absorber for further SO 2 absorption. Ninety percent SO 2 removal efficiency
can be achieved using this method (C ordoba, 2015).
Others. Although the most relevant wet FGD technologies are already described,
there are several sorbents that are feasible to be used for SO 2 scrubbing. Sodium hy-
droxide (NaOH), ammonia (NH 3 ), and hydrogen peroxide (H 2 O 2 ) are considered
available FGD technologies which can achieve elevated SO 2 removal efficiency,
and they are included as best available technology for the European Commission
(European Commission, 2006). Magnesium-based sorbents such as magnesium hy-
droxide (MgOH) and magnesium oxide have been used not only as promoters in lime-
stone scrubbing but also in aqueous solutions for adequate SO 2 abatement (European
Commission, 2006; Roy and Sardar, 2015).
6.2.2.3.3.2.2 Dry/semidry processes Spray dryer scrubbers. Spray dryers are
commonly used in FGD technology, accounting for 11% of the total installed FGD
units around the world. In particular, they provide high performance for low- to
medium-sized industrial plants using medium sulfur content fuel (C ordoba, 2015).
Lime and hydrated lime are typically used as sorbents. They are mixed with water
to produce a slurry, which is atomized in the spray absorber. The droplet size