Page 52 - Impiantistica Italiana 5/2016
P. 52
Fig. 4 - Schematic flow diagram of a limestone wet scrubber FGD process for SOx reduction (BREF 7.2006)
xiliary and support facilities; technology (SCR) or Selective Non-Catalytic
• existing plant operation factors, such as: tie-in Reduction technology (SNCR).
outage timing and duration; An overview of the primary measures is provided
• economic factors, such as investment cost. in figure 5.
The introduction of primary measures to retrofit ex-
De-NOx isting LCPs shall be carefully evaluated as implies
Nitrogen oxides (NOx) formed during the combu- combustion modification and may have adverse
impact on PC boiler performance or contribute
stion of coal and lignite are Nsu2Ob,seNqOueanntdmNeOa2s..uTrehse, to other pollutant formation. However it has to be
NOx control is achieved by said that the LPCs based on PC boilers technol-
ogy designed in the last twenty years are already
that can be implemented independently or in com- equipped with primary measures and therefore the
retrofit for the control of NOx emissions primarily
bination: focuses on secondary measures.
Secondary measures are Selective Catalytic Re-
• Primary Measures (PM) control the NOx for- duction technology (SCR) and Selective Non-Cata-
lytic Reduction technology (SNCR).
mation by acting on operational or design The SCR technology is based on the selective re-
CidnuOgc-atNiogHnen2o)tfinisNtOihnejxepcwtreeitdhseainnmctoemtoohfneaiaflcu(aNetHagl3ya)ssot.ur Tpuhsreetraerea(NdmuHco2--f
parameters of combustion installation so that the catalyst and NOx conversion takes place on
the catalyst surface.
the formation of NOx is reduced inside the PC SCR systems are operated with a careful manage-
ssmklleiiepppn,titsrienomsfutathlhtineientgaSciaCnfrteoRadmlytasottthamaesniNsnduiHmare3ucemtlxohcseteeosNcsmOotexnhetarrttoetimlhsoeohf vrateahsqle.tuoNNirbeHHde33
boiler prior to their release; NOx (3 ppm).
The catalyst selection and the position of the SCR
• Secondary Measures are end-of-pipe techni- reactor into the flue gas path is strictly depen-
dant on the feedstock characteristics and on the
ques to reduce the NOx in the flue gas and boiler technology adopted, which affect the flue
gas composition: in case of retrofitting of existing
are based on Selective Catalytic Reduction LCPs burning coal and lignite, the SCR is generally
placed between the economizer and the air pre-
Fig. 5 - Overview of PM for NOx reduction (BREF 7.2006) heater, as shown in figure 6, in order to operate
within the required temperature range.
Several key factors shall be taken into consider-
ation for the retrofit of the SCR:
50 Impiantistica Italiana - Settembre-Ottobre 2016
