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80 New Trends in Coal Conversion

trisodium (TMT), and Fenton additives have been evaluated to prevent this effect
(Ochoa-Gonz alez et al., 2013). TMT is more effective for Hg 2þ capture and has the
advantage that only a small proportion of additive is necessary to stabilize the mercury
in WFGD systems.
Spray dry scrubber systems can remove both oxidized and elemental mercury with
total removal efficiencies as high as 90%, when coupled with a baghouse. Data from
ICR suggest that spray dry scrubber systems remove between 0% and 99% of the mer-
cury with an average removal of 38% (CATM, 2001).

3.5.3 Effect of NO x control

SCR is the most widely used technology to reduce NO x emissions following combus-
tion in coal-fired plant. These systems have been found to influence Hg emissions. Ac-
cording to Zevenhoven and Kilpinen (2001),NO x control systems have little influence
on TE behavior or removal. However, catalyst poisoning is a major issue in SCR sys-
tems, especially when firing coal with low Ca/As ratio, which may cause rapid catalyst
deactivation (Pandey et al., 2011). Catalyst poisoning is also discussed in detail in
several reports from the International Energy Agency-Celan Coal Centre (see www.
iea-coal.org for reviews and further information on NO x abatement and control).
Data from Gutberlet et al. (1992) and Fahlke and Bursik (1994) have suggested that,
when an SCR system is present upstream of an FGD unit, the SCR system causes
increased oxidation of the mercury in the flue gas and thus increased capture of this
mercury in the FGD system. There have been reports of damage to SCR catalysts
by arsenic (Berger and Krabbe, 1998). Similar results have been reported by Waldner
et al. (1999) who found that As concentrations decreased across the SCR system. No
data were found on the effects of selective noncatalytic reduction on TE emissions.
Mercury can be captured by unburnt carbon in fly ash. Low NO x burners or low NO x
combustion systems can cause an increase in unburnt carbon (5e30 wt% as loss on
ignition), and mercury can concentrate on the carbon-rich fraction of the fly ash and
thus be captured more efficiently in particulate control systems (Kolker et al., 2006).
SCR for NO x removal does not reduce mercury emissions. However, an SCR can
enhance mercury oxidation and therefore, if placed upstream of an FGD device, can
enhance mercury removal in the FGD. The range of mercury oxidation in SCR systems
is 30%e98% with an average of 72% for bituminous coal. The rate for subbituminous
coal is reported to be notably lower (Kolker et al., 2006), but no data are available for
lignite (Srivastava et al., 2006). Straube et al. (2008) have carried out bench-scale
studies investigating the relationship between mercury oxidation and the HCl concen-
tration of the flue gases around the SCR. It is possible that the SCR oxidation involves
chlorine and that explains the higher oxidation rate for bituminous coals (Kolker et al.,
2006). Any cobenefit mercury removal would need to occur without a significant reduc-
tion in NO x removal. Cobenefits will disappear if SCR catalysts lose their mercury
oxidizing capacity faster than their NO x reduction capacity (Offen et al., 2005).