Page 193 - New Trends In Coal Conversion

156 New Trends in Coal Conversion

distribution and the residence time are the most relevant parameters in spray dryers,
and they are crucial to enhancing SO 2 -sorbent contact and thus high SO 2 removal
efficiency.
The flue gas enters into the absorber bottom at 120e150 C and evaporates the wa-

ter introduced with the slurry. SO 2 is absorbed into the droplet surfaces and reacts to
produce a mixture of semihydrated calcium sulfite (Ca 2 SO 3 $1/2H 2 O) and gypsum

(Ca 2 SO 4 $2H 2 O). The flue gas is cooled up to 70 C through the spray absorber and
no further reheating is required before its emission into the atmosphere.
The by-products are partially collected at the bottom of the absorber, but most of
them are recovered in a particulate matter control device downstream, either an elec-
trostatic precipitator (ESP) or a fabric filter (FF). FF provides a higher time contact be-
tween the remaining SO 2 and the unreacted lime retained in the filter cake, providing
further SO 2 removal of up 20% higher compared with ESP devices (Poullikkas, 2015).
The by-product still contains unreacted lime, and it is partially recycled to the slurry
feed tank to enhance the efficiency of the sorbent used. Spray dryers require less capital
investment although the operation costs are elevated due to the cost of sorbents in com-
parison with wet FGD processes.
Sorbent injection. Sorbents for SO 2 abatement can be injected in several locations
throughout the power plant process. Four types of sorbent injections are established,
namely furnace sorbent injection, economizer sorbent injection, duct sorbent injection,
and hybrid sorbent injection, depending on the location of the sorbent injection point
(Poullikkas, 2015). Lime, hydrated lime, trona, activated carbon, and sodium bisulfite
are the most common sorbents used for sorbent injection applications.
Furnace injection adds solid limestone or hydrated lime into the boiler where the

temperature ranges from 750 to 1250 C. Economizer sorbent injection operates at
temperatures in the range of 300e650 C. In this case, hydrated lime is used to pro-

duce calcium sulfite as a main by-product. Duct injections operate at the lowest tem-

perature close to 150 C. Once SO 2 is absorbed from the flue gas, the dust is then
collected by the main particulate control device of the process. Although the operating
costs are low, a high amount of by-product recycling is necessary to achieve an
adequate sorbent use in sorbent injection applications (C ordoba, 2015).
Dry scrubbers. In this case, sodium bisulfite, hydrated lime, limestone, and dolo-
mite are used as sorbents. The SO 2 removal efficiency is quite low, around 50%,
and most of the by-product is recycled to further yield unreacted sorbent. Water injec-
tions are also required to improve the SO 2 -sorbent reaction. ESP is preferably used as a
particulate device control for this technology, and the flue gas must be additionally
reheated before its discharge (Roy and Sardar, 2015).

6.2.2.4 Technological challenges
Research and innovation activities have been focused on new insights that might lead
to further improvements in the overall performance of the most extended DeSO x tech-
nologies. Regarding wet FGD scrubbers, novel configurations of the process allowed
for building more compact and higher performance absorbers, whereas new reactor de-
signs such as jet bubble reactors have been proposed since the 1990s. In terms of