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Behavior of Mercury in Air Pollution Control Devices on Coal-Fired Utility Boilers

1
Behavior of Mercury in Air Pollution Control Devices
on Coal-Fired Utility Boilers
1


Constance L. Senior
Reaction Engineering International
Salt Lake City, Utah 84101

ABSTRACT

Coal-fired power plants are major point sources of mercury discharges into the
atmosphere. After considerable study of mercury emissions and their impact on the
environment, US EPA, in December, 2000, made a determination to regulate mercury
emissions from coal-fired electric utility boilers. EPA is to propose air pollution emission
regulations by December 15, 2003, and promulgate them by December 15, 2004.
Regulation of mercury emissions may necessitate additional air pollution control
devices being installed at utility power plants. Before regulations are imposed, it is important
to understand the behavior of mercury in existing devices. Extensive measurement of
mercury emissions at power plants have demonstrated that high levels of removal can occur
in existing devices. However, the complexity of mercury chemistry, the variability of coal
feedstocks and of boiler designs make it imperative that a clear understanding of the behavior
of mercury in air pollution control equipment be developed. In this paper, the database for
mercury speciation and stack emissions in coal-fired power plants is reviewed; this largely
consists of the Mercury Information Collection Request (ICR) initiated by the US EPA in
1999, designed to provide new information to help in making future regulatory
determinations on controlling mercury emissions from coal-fired power plants. Phase III of
this effort involved a plant testing program for mercury emissions including mercury
speciation from coal-fired power plants. Over 80 plants were statistically selected for this
testing based on several factors, which included boiler type, configuration of air pollution
control equipment, and fuel type. For each plant, measurements of mercury in the coal (along
with other coal composition data) and the flue gas were made. Flue gas measurements were
made at the stack and at the inlet to the last air pollution control device (APCD) using the
Ontario Hydro method, which provides mercury speciation data (elemental, oxidized, and
particulate-bound).
In this paper, ICR data on mercury speciation in flue gas, coal composition, boiler
design and operation, are examined to look for trends in the behavior of mercury in coal-fired
power plants. The speciation of mercury at the inlet to particulate control devices was found
to depend on the chlorine content of the coal and on the temperature at the inlet to the device.
Wet FGDs, dry scrubbers, and fabric filters can all remove a significant amount (50-90%) of
the mercury in the flue gas under certain conditions. Critical information is missing from the
ICR data, particularly the composition of the fly ash, and the lack of this information reduces
the quality of the model predictions.


1
Prepared for Power Production in the 21
st
Century: Impacts of Fuel Quality and Operations, Engineering
Foundation Conference, Snowbird, UT, October 28-November 2, 2001
2

INTRODUCTION

The United Stated Environmental Protection Agency (EPA) has estimated that during
the period 1994-1995 annual emissions of mercury from human activities in the United States
were 159 tons (Keating et al, 1997). Approximately 87% of these emissions were from
combustion sources. Coal-fired utilities in the U.S. were estimated to emit 51 tons of
mercury per year into the air during this period.
The form of mercury emitted from point sources is a critical variable in modeling the
patterns and amount of mercury deposition from the atmosphere (Pai et al, 1997). Both
elemental and oxidized mercury are emitted to the air from combustion point sources.
Elemental mercury has a lifetime in the atmosphere of up to a year, while oxidized forms of
mercury have lifetimes of a few days or less as a result of the higher solubility of Hg
+2
in
atmospheric moisture. Elemental mercury can thus be transported over long distances,
whereas oxidized and particulate mercury deposit near the point of emission. Once mercury
has deposited on land or water, it can transform into methylmercury, an organic form, and
thereby enter the food chain. Humans are most likely to be exposed to methylmercury
through consumption of fish.
In December of 2000, the US EPA made a decision to regulate the emission of
mercury from coal-fired power plants. A proposed regulation will be due no later than
December 2003 and promulgated no later than December 2004. Utility industry compliance
would have to be in place by December 2007. Compliance with the proposed regulation may
in some cases necessitate additional controls for mercury. Since some mercury is removed
by existing air pollution control devices (APCDs), it is vital to understand the behavior in
existing equipment in order to cost-effectively control emissions. The speciation of mercury
in the flue gas of a coal-fired power plant affects the amount of mercury retained in the air
pollution control devices (and not emitted out the stack) because the chemistry of elemental
mercury in flue gas is different from that of oxidized mercury.
In order to understand the technical and economic feasibility of mercury controls on
coal-fired power plants, it is therefore necessary to understand the chemistry of mercury in
flue gas and the potential physical and chemical interactions at various points in the system.
In this paper, data from full-scale power power plants are reviewed with the intent of testing
specific hypotheses about the behavior of mercury in coal-fired boilers.


BEHAVIOR OF MERCURY IN COMBUSTION SYSTEMS

Mercury is present in coal in low concentrations, on the order of 0.1 ppmw. In the
combustion zone of a coal-fired power plant, all the mercury in coal is vaporized as elemental
mercury, yielding vapor concentrations of mercury in the range of 1 to 20 µg/m
3
(1 to 20
ppbw). At furnace exit temperatures (1700 K), all of the mercury is expected to remain as the
thermodynamically favored elemental form in the gas. As the gas cools after combustion,
oxidation reactions can occur, significantly reducing the concentration of elemental mercury
by the time the post-combustion gases reach the stack. Equilibrium thermochemical
calculations predict that HgCl
2
will be formed at low temperatures in coal combustion flue
gas (Senior et al, 2000). However, the complete oxidation of elemental mercury that is
3
predicted from equilibrium is rarely observed in practice. This has led to the conclusion that
there are kinetic limitations to the oxidation of mercury in flue gas from coal-fired power
plants (Senior et al, 2000).
The major kinetic pathway to formation of HgCl
2
in flue gas is believed to be through
the reaction of atomic chlorine Cl with elemental mercury (Helble, et al., 2000; Sliger, et al.,
2000, Widmer, et al., 2000, Niksa and Helble, 2001)

. Although the oxidation of elemental
mercury in the convective pass is assumed to proceed primarily via gas-phase reaction,
experimental evidence suggests that some fly ash can catalyze oxidation of elemental
mercury. Iron oxide has been shown to promote this oxidation (Ghorishi, 1998). Other
constituents in the fly ash (carbon, calcium compounds) may also contribute. The presence
of acid gases (HCl, SO
2
, NO, NO
2
) in the flue gas has also been shown to cause oxidation in
the presence of fly ash (Carey, et al., 1998; Miller, et al., 1998). Furthermore, selective
catalytic reduction (SCR) technology for NO
x
control has been observed to oxidize a portion
of elemental mercury (Gutberlet et al, 1992, Fahlke and Bursik, 1995, Laudal et al, 2001).
Thus, the coal composition (in terms of chlorine content and ash composition), the
operation of the combustion system (in terms of unburned carbon in the ash), and temperature
and residence time in the particulate control device will all affect mercury speciation in the
gas and the amount of mercury adsorbed on the particulate matter. Other components of the
air pollution control system such as FGD and selective catalytic reduction (SCR) systems
may also affect both the speciation of mercury in the stack and the amount of mercury
removed in the air pollution control equipment as a whole.

FULL-SCALE MEASUREMENTS OF MERCURY SPECIATION

The Information Collection Request (ICR) initiated by the United States EPA in 1999
was designed to provide more information which could be useful for making a regulatory
determination about mercury emissions from coal-fired power plants. Data from Part 3 of the
ICR comprise a set of measurements of mercury speciation from coal-fired power plants.
Plants were selected based on the configurations of air pollution control equipment and fuel
type. For each plant, the input value of mercury in the coal was measured (along with other
coal composition data). Mercury measurements were made at the stack and at the inlet to the
last air pollution control device