GCI TECH NOTES ©
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 563-652-2822 or by e-mail at dgossman@gcisolutions.com for additional information.