Page 53 - Impiantistica Italiana 5/2016

Fig. 6 - Example of SCR retrofit

• constructability factors, such as: site conges- quent NOx formation, or the conversion rate is too
low and additional ammonia can be formed.
tion, space, soil conditions, access roads, The SNCR technology has a lower efficiency and
a higher ammonia slip with respect to the SCR
seismic and wind requirements; technology and therefore is more suitable with
feedstocks that lead to a lower NOx formation or
• existing equipment design factors, such as: where higher NOx emission limits are allowed, such
as the case of existing LCPs burning lignite.
fan capacity, allowable pressure drop, flow
De-dusting
distribution, temperature and ammonia mixing
During the combustion of coal and lignite, the mi-
requirements; neral matter converts to ash and partly leaves the
PC boiler as fly ash along with the flue gas. The
• eexnivsitrionngmpelanntat lofpaecrtaotriso,nsfuacchtoarss,tshuecNhHa3ss: ltiipe;-in particles suspended in the flue gas as fly ash con-
• stitute the primary particle matter entering the de-
dusting systems.
outage timing and duration; The characteristics and the amount of the fly ash
depend on the feedstock characteristics and on
• economic factors, such as investment cost. the boiler technology adopted: figure 7 provides
an overview of the currently used de-dusting sy-
Another secondary measure that can be taken into stems.
consideration for the retrofitting of LCPs is the Se-
lective Non-Catalytic Reduction (SNCR) that acts
on the same principle of the SCR but is operated
without a catalyst.
The reducing agent is injected at several levels into
the upper part of the PC boiler furnace where the
flue gas is between 850 and 1100 °C. The flue gas
temperature range is of considerable importance
as either ammonia oxidation occurs, with conse-

Fig. 7 - Overview of de-dusting systems for dust reduction (BREF 7.2006)
Impiantistica Italiana - Settembre-Ottobre 2016 51