GCI
TECH NOTES©
Volume
11, Number 12
A Gossman Consulting, Inc.
Publication
December 2006
Cement Kiln Mercury (Hg) Emission Issues
by
David Gossman
Introduction
There is a growing level of concern
about mercury emissions from cement kilns and interest in the industry
on developing cost effective options for controlling these emissions.
Cement plants have a wide range of mercury inputs and resulting
emissions because of the wide variety of raw materials and fuels used
in the process. Further the current level of mercury emission control
at cement plants varies from 0% to as high as 95% using existing
particulate control systems. This is the first in a new series
of GCI TechNotes that will examine this issue.
Mercury emissions are regulated based on concern for mercury entering the food chain and bioaccummulating to significant levels that could impact people eating fish. The following is a brief review of the factors that impact this issue including the basic chemistry of mercury and mercury compounds, emission modeling issues and emission control factors. References at the end provide details on the health effects of various forms of mercury.
Mercury Forms and Fate in the Environment
There are four forms of mercury that have the potential to form from the cement kiln system. These are elemental mercury, mercuric chloride, mercuric oxide and mercuric sulfide. Table 1 provides a summary of some data regarding these forms.
TABLE 1
Mercury Forms
|
Compound |
Mercury |
Mercuric
chloride |
Mercuric
oxide |
Mercuric
sulfide |
|
Formula |
Hg |
HgCl2 |
HgO |
HgS |
|
Melting Point °C |
-39 |
276 |
500
(decomposes) |
584
(sublimes) |
|
Boiling Point °C |
357 |
302 |
N/A |
N/A |
|
Water Solubility |
low |
high |
insoluble |
insoluble |
Figure
1 from the Toxicological Profile for Mercury prepared by the Agency for Toxic Substances and Disease
Registry, USDHHS, shows how various mercury compounds can be
transformed in the
environment.
Source:
Stein et al. 1996.
Figure
1.
Transformation of Mercury in Air, Water and Sediment
Modeling Issues
Clearly the form of mercury found in stack emissions can have a significant impact on the fate of mercury in the environment and therefore have a potential impact on human health and the environment. Many of the existing programs for modeling emissions make worst case assumptions regarding the form of the mercury as it is emitted as well as the transformation path that the mercury takes once it is released to the environment. It is therefore critical that any modeling take into account the actual molecular form and valence state of any mercury that is emitted and make realistic assumptions regarding transformation of mercury emitted to the environment.
“Evaluating the Consequences
of Mercury
Emissions from a Point Source” by Zemba, Gossman,
Woodford, and Chrispell
provides an excellent analysis of the faults on this sort of modeling
when
applied to a cement plant.
Control
Issues
In
much the same way that the form of the
mercury can impact emission modeling it can also impact emission
control
technologies and their efficacy. Traditionally used methods such as
activated
carbon capture (ACC) have primarily been used on municipal waste
combustors
where mercury concentrations
in
the gas stream are relatively high and where there is a significant
presence of
chlorine in the gas stream – both of which enhance carbon
adsorption of mercury. To the
extent that any control technology
is used it is important that the mercury not be transferred from one medium to another in a
way that does not
result in its real removal from the global mercury cycle.
Conclusion
Clearly
the “best” environmental fate for mercury
is to sequester it in the form of the
insoluble oxide or sulfide in an environment where it is unlikely to be
altered
by microbes or bacteria. This has the potential to remove the mercury
from the
global mercury cycle. The cement manufacturing technology has that
potential
but each kiln system is different because of different raw materials,
fuels and other process conditions. Control technology that
might work on one kiln will not necessarily work on another.
Strategies to convert mercury in the process to insoluble and low
volatile oxide and sulfide forms that allow the exiting particulate
control systems to capture the mercury are likely to be the most cost
effective.
References
CRC Handbook of Chemistry and Physics 70th Edition (1989). Ed. Weast, Robert C., Ph.D., Florida: CRC Press, Inc.
Please contact David Gossman at 847-683-4188 or by e-mail at dgossman@gcisolutions.com for additional information.