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Making progress in mercury management
Setting emission standards for this sector, developing
implementation plans and measuring their efficacy is impeded by
continuing uncertainties in key areas of mercury management:
emissions measurement, availability of technologies for removing
mercury and the variability in the presence and behaviour of
mercury.
The
electric power generation sector is engaged in a number of
activities to try to reduce these uncertainties.
Improving emission measurements
For
a number of years, companies have been working to improve emission
estimates. This requires an understanding of the mercury content
in coal and what happens to that mercury when the coal is burned. Activities to improve this understanding have been confounded by
the variability in the levels of mercury found in coal and its
complex chemistry.
The
coal mercury content can vary dramatically across coal types and
within a single coal seam. Likewise, the chemical form in which
mercury is emitted can be highly variable, depending on coal and
ash characteristics, especially the sulphur and chlorine content,
as well as operating conditions.
Mercury emissions can
occur in three forms – particulate mercury, oxidized or reactive
(ionic) mercury, and elemental mercury. The particulate mercury
is a very small fraction of the total mercury. The ratio of ionic
mercury to elemental mercury depends upon the factors noted
above. The form of the emissions is a key factor in determining
viable management options.
Mercury is difficult to measure in coal-fired electricity
generation stack gases because its concentrations are so low –
near the minimum level for detection. For much the same reason,
laboratory analyses for mercury in coal or ash samples frequently
produce inconsistent results.
The
Ontario Hydro Method for measuring mercury emissions in combustion
gases (“stacks”) has been recommended by regulators across North
America, but it is relatively expensive to be considered as a
long-term measurement tool. Because of the low mercury
concentrations variable results are produced during repeated
tests, and present only a single point value (“snapshot”) as
opposed to continuous measurements. \Coal-fired utilities in
Canada continue to invest substantial resources in measuring stack
gases.
Continuous Emissions Monitoring systems (CEMs) are available, but
their long-term reliability has not been demonstrated for the low
mercury concentrations produced by coal-fired plants.
Companies have been investing in research to address these
measurement issues, testing coal, ash and stack gases. Activities
in these areas will increase significantly with the launch of an
intensive three-year Canadian Electricity Association Mercury
Program for Coal-fired Electricity Generators.
Reducing mercury emissions
Electric utilities have been actively taking steps to reduce
mercury emissions as part of ongoing integrated air emission
management programs. Many of the pollution control devices
installed to capture other emissions (e.g. sulphur dioxide,
nitrogen oxides, particulate matter and carbon dioxide) do capture
some particulate and ionic mercury. It has been estimated that 40
percent of the mercury in coal is removed from coal-fired boilers
in Canada in this fashion(1). However, a recent U.S. modeling
study regarding the four most significant air pollutants from
electric power generation has shown that "a mercury reduction
strategy by itself would not significantly reduce emissions of the
other three pollutants"(2).
The
technology receiving the most attention for mercury removal is
Activated Carbon Injection (ACI). It is still under study in
various programs in the U.S. and elsewhere: the final test results
on commercial volume emissions are expected in late 2003.
Installing ACI technology or adjusting air pollution control
systems can be effective in capturing mercury in its ionic form.
Neither of these options, however, captures elemental mercury
effectively.
Regional differences in the mercury content of coals used in
Canada and the lack of technical solutions for reduction in
elemental mercury emissions mean there is currently no
commercially demonstrated technology that can achieve uniform
reductions in mercury emissions from all coal-fired boilers(3).
CEA
members are monitoring and investing in a variety of U.S. mercury
research programs including control technologies and their
application to Canadian plants. The U.S. government is spending
many millions of dollars on mercury research, including pilot
programs for a variety of mercury capture technology options.
Important commercial demonstration projects will be completed in
2003.
(1) Smith, Ian in
Record of the Meeting of the Multi-Stakeholder Advisory Group and
the Mercury CWS Development Committee,
June 4-5, 2001
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