Page 51 - Impiantistica Italiana 5/2016
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Fig. 3 - Overview of Secondary Measures for SOx reduction (BREF 7.2006)
Technical Solutions for and sulphur and are classified with respect to flue
Retrofitting Existing Large gas cooling approach:
Combustion Plants • the wet type FGD is based on wet scrubber
De-SOx technology; the flue gas temperature is cooled
Sulphur oxides (SOx) are formed during the com- down below the water dew point and the sor-
• tbheentsuemseidwinetthtyepperoFcGeDsswisitlhimsepsrtaoynedr(yCaaCbsOo3r)-;
bustion of coal and lignite through oxidation of the ber technology or Scrubber technology based
on CFB; the flue gas temperature is cooled
sulphur content in the two feedstocks. SOx are down above the water dew point and the sor-
bent used in the process is lime (CaO);
mainly SthOe2 and SSOO3xbeumt iitssisioSnO. 2 that contributes to • the dry type FGD is based on dry sorbent
almost total injection technology; the sorbent used in the
process is hydrated lime (Ca(OH)2).
The basic primary measure to reduce the SOx
formation is the use of a low sulphur fuel or, for
Circulating Fluidized Bed (CFB) boilers, fuel with
alkaline ash compounds that promote internal de- Wet limestone scrubbers are the most widely used
sulphurization. among all the FGD systems because of their high
Another primary measure, mainly used in coal fired SarOe2threemmoovsatl efficiency and reliability and therefore
appropriate technology for retrofitting
LCPs based on fluidized bed technology, is the use
of adsorbents like limestone which reacts with SO2 existing LCPs.
producing calcium sulfate at 850 °C.
The wet scrubber FGD technology is described in
When primary measures cannot be applied, the the simplified scheme reported in figure 4.
secondary measures to reduce SOx consist of Flue The flue gas leaving the dust control system enters
Gas Desulphurization (FGD) technologies, as sum- rthecetFcGoDntaacbtsworitbhelri,mineswtohnicehsSluOrr2yisspreramyeodvecdoubnytedri--
marized in figure 3.
In the regenerative processes, the spent sorbent current into the absorber. The flue gas flows up-
is re-used after thermal or chemical treatment to ward to the mist eliminator located at the top of the
pcorondvuecrteedcotonceelnetmraetnetdalSsOu2lp, hwuhr.icThheissethpenrocuessusaellsy absorber and is released to the atmosphere from
a stack.
are not generally attractive because are not com- Several key factors shall be taken into consider-
mercially referenced, due to their high investment ation for the retrofit of the wet FGD:
cost, and produce sulphur which is often difficult to • constructability factors, such as: site conge-
commercialize. stion, space, soil conditions, access roads
The FGD technologies generally adopted for the and seismic;
SOx reduction are non-regenerative process types, • existing equipment design factors, such as:
which are all based on a reaction between calcium fan capacity, allowable pressure drop, site au-
Impiantistica Italiana - Settembre-Ottobre 2016 49
