POWER October 2014 - 62

MERCURY CONTROL
Non-Carbon Reagent Injection for
MATS Compliance
With the compliance deadline for the EPA's Mercury and Air Toxics Standards
less than a year off, plant owners need to decide now on the best approach
to control mercury emissions. The use of non-carbon sorbents may offer
substantial benefits over standard powdered activated carbon injection.
Jim Butz
T
he upcoming implementation of the
Environmental Protection Agency's
(EPA's) Mercury and Air Toxics Standards
(MATS) is putting pressure on coal-fired
power plants to develop and implement compliance
strategies that are effective, economically
efficient, and environmentally advantageous.
MATS, scheduled to go into effect in
April 2015 for most power plants, requires
that vapor-phase mercury stack emissions be
less than 1.2 pounds per trillion Btu (lb/TBtu)
for bituminous- and subbituminous-fired
units and 4 lb/TBtu for lignite units.
Plant compliance with these emissions
standards must be reported on a 30-day rolling
average basis. The EPA has authorized a
one-year extension for utilities that apply to
state regulators; a second-year extension is
available only under extreme circumstances.
Compliance options include mercury capture
in a wet flue gas desulfurization (FGD)
scrubber or the use of powdered sorbents to
capture mercury upstream of the particulate
control device (PCD), with the best solutions
dependent on individual unit configuration
and operating conditions.
Wet Scrubbers
Wet FGD capture requires high (typically
>90%) levels of mercury oxidation, as oxidized
mercury has much greater solubility in the
scrubber solution. Oxidation occurs naturally
with high-chloride-content coals or can be facilitated
by the addition of oxidizing agents to
the boiler coal feed. An alternate method relies
on mercury oxidation as a co-benefit of installation
of a selective catalytic reduction (SCR)
module for NOx
control. Capital and operating
costs for these options depend on the technology
employed and site conditions.
Wet scrubbers are installed on many plants
that burn higher-sulfur eastern bituminous
coals. While these systems typically provide
high native capture of vapor-phase mercury,
most cannot consistently meet the MATS standard.
In addition to coal oxidation additives
noted above, wet FGD units sometimes employ
62
additives to the scrubber liquor to reduce or
eliminate a phenomenon called " re-emission. "
In the circulating FGD liquid, oxidized
mercury in solution may be reduced back
to its elemental state, with lower solubility.
This leads to saturation of the liquid with respect
to elemental mercury and release of the
elemental mercury back into the gas phase.
Strong evidence of this process is measurement
of vapor-phase elemental mercury at
higher concentration in the stack than at the
inlet to the wet FGD. Several scrubber additive
products are commercially available to
convert the soluble oxidized mercury to another
form, which eliminates the potential for
re-emission.
Powdered Sorbents
Dry sorbent injection captures mercury on
fine particulates added to the flue gas after
the economizer, typically either upstream or
downstream of the air heater. The mercurycontaining
particles are then removed in the
unit's PCD. Powdered activated carbon (PAC)
and non-carbon sorbents, such as Novinda's
Amended Silicates products, are options.
PAC is commercially available in standard
and brominated forms, with brominated
materials used where vapor-phase mercury
is mostly elemental. The bromine promotes
oxidation on the particle, facilitating capture
and removal from the flue gas.
The most efficient and economic option
for a specific coal-fired unit depends on
many factors and is best determined via a
trial where mercury capture is measured during
implementation of candidate technologies.
Trials require temporary installation
of equipment to deliver the mercury control
technologies of interest, as well as mercury
measurement instruments to document mercury
capture. Trials can be as short as a week
or continue for a month or more if the host
utility is interested in evaluating longer-term
performance and balance-of-plant impacts.
Utilities often wish to understand the effect
of mercury control on the salability of fly ash
as a cement replacement or on operation of
the host unit PCD.
Prior to the availability of the Amended
Silicates reagent technology, coal-fired power
plants that were looking for a dry sorbent
mercury (Hg) removal solution had little
choice but to use carbon-based products. The
non-carbon Amended Silicates platform was
under development for several years, with
the most recent version-Amended Silicates
HgX-ESP-introduced in June 2014.
1. Quick results. The cumulative average of stack Hg emissions during injection of Amended
Silicates AS-HgX for a Powder River Basin (PRB) coal-fired unit are shown here. Courtesy: Novinda
◆ Cumulative average stack Hg
6
5
4
3
2
1
AS injection started
MATS limit
11/1 11/4 11/7 11/10 11/13 11/16 11/19 11/22 11/25 11/28 12/1 12/4 12/7 12/10 12/13
www.powermag.com
POWER | October 2014
HgT (lb/TBtu)

http://www.powermag.com

POWER October 2014

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