POWER February 2012 - 76

MERCURY REMOVAL TECHNOLOGIES
5. Going down. Tests on units of different
sizes confirmed that increasing the application
rate of pre-combustion oxidizer and/
or PAC reduces stack Hg emissions. Source:
Alstom Power
106 ppm Br
130 ppm Br
mercury levels. Mercury compliance levels
can be met by varying PAC injection rate
and/or pre-combustion oxidizer application
rate. This approach is useful to optimize performance
with variations in raw coal Hg content
occurring on a daily basis.
Unit size also has an effect on Hg capture,
0.90
Unit #1 (310 MWe-net)
Unit #3 (740 MWe-net)
so a field test prior to contract execution is
recommended for identifying unit-specific
requirements for system design and PAC
and/or pre-oxidizer consumption rates in order
to ensure compliance with environmental
regulations.
Results show that, regardless of unit size, in0
50 100 150 200 250
PAC injection rate (lb/hr)
300 350 400 450 500 550 600
creasing the application rate of pre-combustion
oxidizer and/or PAC reduces stack Hg emissions
and offers plant operators additional " tuning
knobs " to improve the cost-effectiveness of
the Hg reduction process or to account for variation
in raw coal mercury content. All the tested
conditions resulted in Hg emissions well below
the 0.9 lb/TBtu level (Figure 5).
Testing on Unit 1 demonstrated how daily
6. Variables. A significant drop in raw coal
mercury content allowed for substantially reduced
usage of the additive materials. Source:
Alstom Power
83 ppm Br (6.0 gph oxidizer)
106 ppm Br (7.2 gph oxidizer)
71 ppm Br (4.8 gph oxidizer)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
changes in coal quality can dramatically affect
the amount of pre-combustion oxidizer
and/or PAC required to meet stack mercury
compliance levels (Figure 6). With all boiler
operation and coal sources remaining constant,
a considerable drop in raw coal Hg
content allowed for substantially reduced
usage of the additive materials. Variations in
raw coal Hg content frequently occur. PAC
injection rate is adjusted automatically by
the control system to maintain the stack Hg
emissions within compliance.
A brief parametric test was also conCoal
quality change
0 10 20 30 40 50
PAC injection rate (lb/hr)
60 70 80 90
7. Rate gain. Without the Mer-Cure processor,
achieving the same level of mercury
reduction requires much more PAC injection.
Source: Alstom Power
Avg. Hg total
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
20 40
Processed
PAC rate
(lb/hr)
60
80 100
120
PAC rate required
with processor
bypassed
76
ducted on Unit 1 to assess the value of using
the Mer-Cure processor (Figure 7). In
this test, the Mer-Cure system was operated
normally with the processor " in service. "
This resulted in a stack total mercury
emission level of 0.60 lb/TBtu at the PAC
usage rate of 30 lb/hr. The processor was
then taken out of service (bypassed), and a
substantial increase in stack Hg level was
noted. The PAC rate was then increased to
obtain a comparable 0.60 lb/TBtu level.
The difference in PAC rate required to
achieve the same stack Hg level would indicate
that use of the Mer-Cure processor
allows the plant to use 50% less PAC than
would be required if a conventional PAC
system were used.
The entire Colstrip site consumed approximately
1.2 million pounds of PAC
in the first quarter of 2010 with the commercial
Mer-Cure system (Figure 7). Estimates
indicate that PAC usage would
approximately double if a non-Mer-Cure
system were employed. For example, with
a PAC delivered cost of $1.00/lb, the plant
using Mer-Cure technology potentially reduces
operation and maintenance costs of
www.powermag.com
mercury compliance by $1.2 million per
quarter, or roughly $4.8 million annually.
Long-Term Operating Experience
During the first year of operation, the
daily variations in total stack gaseous mercury
levels were monitored using the stack
CMM system on all four units at Colstrip.
These variations, largely attributable to
variations in raw coal mercury content, required
the plant to frequently adjust PAC
and pre-combustion oxidizer feed rate(s)
in order to meet the required compliance
level of 0.9 lb/TBtu on a rolling 12-month
average basis. The reported data after two
full years of operation have confirmed that
the plant maintains compliance with the
Montana Mercury Rule adopted in 2006
and promulgated as Administrative Rule
of Montana 17.8.771.
Here are some observations from longterm
operation:
■ Erosion was observed on a limited number
of components initially. Upgraded materials
were installed and are reliable to date.
■ PAC and flow rate variability were observed;
tuning of the gravimetric feed system
is critical to stable operation.
■ Stack mercury CMM operation requires
considerable maintenance. The plant has
several staff dedicated to ensuring functionality
of the four stack CMMs.
■ PAC was initially observed to be contaminated
by foreign objects (likely during
shipping), which can cause equipment
damage. Rocks were found in some of the
delivered PAC but have not been observed
since system initial startup.
■ Opacity average increases of approximately
0% to 2% were noted.
In combination with a pre-combustion
oxidizer, the Mer-Cure system has been
in service since January 2010 and has enabled
Colstrip to comply with the state of
Montana's mercury limits. The field results
prove that Mer-Cure is an effective tool in
the global drive to cut mercury emissions
from coal-fired plants.
Mer-Cure technology is currently protected
by several U.S. patents. The most recent,
U.S. Patent Number 8,069,797 issued by the
U.S. Patent and Trademark Office on Dec. 6,
2011, covers technology to improve the performance
of activated carbon injection for
control of mercury emission through injection
upstream of the air heater. ■
-John J. Iovino (john.iovina@power
.alstom.com) is product manager and
Richard LaFlesh (richard.laflesh@
power.alstom.com) is principal consulting
engineer for Alstom Power.
POWER | February 2012
Stacking total (lb/TBtu)
Stacking total (lb/TBtu)
Stacking total (lb/TBtu)
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POWER February 2012

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