POWER July 2010 - 31

MERCURY REMOVAL
4. Mercury emissions increased to over 5 lb/TBtu during an apparent
re-emission event. Time series of mercury emissions, fractional unit load, wet flue gas
desulfurization (wFGD) pH, and wFGD SO2 removal performance during a high-mercury emission
event at Plant A are illustrated. Source: Southern Company
10
5
1
0.95
0.9
6.5
6
5.5
5
100
95
90
06:00
09:00
12:00
15:00
18:00
Time of day
5. Mercury emissions increased during full and partial SCR bypass. Time
series of mercury emissions and process parameters during cold start of Plant B are shown.
The three stages of start-up were full SCR bypass, partial SCR bypass, and no SCR bypass.
Source: Southern Company
1
1
0.5
1000
500
100
95
90
4
2
18:00
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00:00
06:00
12:00
Time of day
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18:00
00:00
06:00
2
3
21:00
00:00
03:00
06:00
erating setpoint of 5.2. Mercury emissions
increased synchronously with pH while SO2
control was maintained above 95% (Figure
4). This was likely a " re-emission " event in
which some of the oxidized mercury that
was collected in the wFGD before the pH
exceedence was emitted as elemental mercury
during this event. Note that not all
periods of pH exceedence were associated
with increased mercury emissions, and during
this event the wFGD liquor was also in
transition from highly oxidizing to moderately
oxidizing conditions as measured by
oxidation reduction potential (ORP). The
time scale of this event is consistent with
the characteristic times associated with dissolved
mercury residence in the wFGD liquor;
this suggests that the excess emissions
resulted from perturbations in the dissolved
mercury chemistry of the wFGD liquor.
Over the entire 40-month data set, five
such re-emission events like that shown in
Figure 4 occurred; these lasted for one to six
days. During these events mercury emission
rates averaged 3.0 lb/TBtu and totaled 5.8
lb. During all these events SO2
removal was
greater than 95%. These results lead us to
believe that under most wFGD conditions,
mercury sequestration chemistry is effective
at removing captured oxidized mercury;
however, on rare occasions mercury chemistry
in the wFGD liquor appears to be upset
by external inputs, leading to re-emission
events. These mercury emissions did not
exceed the mercury input rate, which suggests
that only freshly captured mercury is
reduced and re-emitted.
Mercury sequestration is not well understood
and was not directly observed; the current
data suggest that mercury re-emission
requires moderate-to-low ORP values.
Mercury Capture During Start-Up
Start-up of utility boilers and the associated
AQCS equipment may take 12 or more
hours. During this time co-benefit mercury
control is limited. Plants A through E start
up using oil igniters. During this period,
the SCR is fully bypassed in order to prevent
catalyst poisoning and potential fire
hazards. Full bypass continues until coal
firing begins and stable boiler operation is
achieved. Operational tie-in of the SCR is
initiated by a gradual opening of the SCR
inlet dampers. This partial bypass period
is required to minimize thermal stress,
ammonium bisulfate formation, and acid
corrosion. Mercury emissions are elevated
until all of the flue gas passes through the
SCR (Figure 5). Therefore, these start-up
periods could represent a significant portion
of a plant's emissions during a given
averaging period.
31
Hg emissions (lb/TBtu) SO2 removal (%)
SCR temp. (F)
Fractional load
SO2 removal (%)
FGD liquor pH
Fractional load
Hg emissions (lb/TBtu)

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POWER July 2010

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