GCI TECH NOTES ©

Introduction

There is a growing level of concern about mercury emissions from cement kilns and interest in the industry in 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 third 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 the issue of controlling mercury emissions from modern cement kilns.

It should be kept in mind that mercury control technologies are under active development around the world as pressure mounts to reduce the total anthropogenic source of mercury to the mercury cycle. Most of this development has been focused on controlling mercury emission from coal fired electrical power utility boilers. Not all of the observations made regarding mercury emissions and controls for those systems can be directly applied to cement kilns. Further, the unique operating environment inside a cement kiln may present innovative and cost effective control methods for cement kilns that are impossible or impractical to apply to coal fired boilers.

Review of Control Options

During combustion, the mercury (Hg) in coal and other fuels is volatilized and converted to elemental mercury (Hg⁰) vapor in the high temperature regions of the cement kiln. As the flue gas is cooled, a series of complex reactions begin to convert Hg⁰ to ionic mercury (Hg²⁺) compounds and/or Hg compounds (Hg_p) that are in a solid-phase at flue gas cleaning temperatures, such as HgO and HgS, or Hg that is adsorbed onto the surface of other particles. The presence of chlorine gas-phase equilibrium can favor the formation of mercuric chloride (HgCl₂) at system temperatures depending on the competition of alkali-chloride reactions. The presence of significant levels of sulfide in the form of iron sulfide (pyrite) in some cement kiln raw feeds may favor the formation of HgS. However, Hg⁰ oxidation reactions are kinetically limited and, as a result, Hg can enter the air pollution control device(s) as a mixture of Hg⁰, Hg²⁺, and Hg_p with varying ratios depending on the conditions in the kiln system. This partitioning of Hg into Hg⁰, Hg²⁺, and Hg_p is known as mercury speciation, which can have considerable influence on selection of mercury control approaches. For this reason it is critical that assessing the range of operating conditions and the resulting speciation of Hg in any given cement kiln be the first step in determining the most effective control technology. A control technology that works on one kiln system cannot be assumed to be effective on another.

It is also critical to understand the role of CKD recycling in this process. Recycling CKD back into the kiln system can revolatilize mercury that has been captured, converting it from the particulate form to one of the more volatiles forms such as the chloride. 

Some control of mercury emissions from cement kilns is currently achieved via existing controls used to remove particulate matter (PM), sulfur dioxide (SO₂), and nitrogen oxides (NO_x). This includes capture of Hg _p in PM control equipment and soluble Hg²⁺ compounds in wet scrubber systems. Available data on electric utility boilers also suggest that use of selective catalytic reduction (SCR) NO_x control enhances oxidation of Hg⁰ in stack gasses and results in increased mercury removal in wet scrubbers. It is unlikely that the same would be true of SNCR NO_x control systems on cement kilns given the operating temperature of these systems. That said, the presence of excess ammonia in the stack gasses could impact available chlorides for reacting with and forming mercuric chloride.

There are three broad approaches to mercury control: (1) activated carbon injection (ACI), (2) multi-pollutant particulate control, in which Hg capture is enhanced in existing/new PM control devices, and (3) multi-pollutant wet scrubber control, in which Hg capture is enhanced in existing/new SO₂, and NO_x control devices. Relative to these three approaches, this paper describes currently available data, limitations, estimated potential, and research and development needs.

Activated Carbon Injection Control of Mercury Emissions

ACI has the potential to achieve moderate levels of mercury control. The performance of activated carbon is related to its physical and chemical characteristics. Generally, the physical properties of interest are surface area, pore size distribution, and particle size distribution. The capacity for mercury capture generally increases with increasing surface area and pore volume. The ability of mercury and other sorbates to penetrate into the interior of a particle is related to pore size distribution. The pores of the carbon sorbent must be large enough to provide free access to internal surface area by Hg⁰ and Hg²⁺ while avoiding excessive blockage by previously adsorbed reactants. As particle sizes decrease, access to the internal surface area of particle increases along with potential adsorption rates.

Carbon sorbent capacity is dependent on temperature, the concentration of mercury in the flue gas, the flue gas composition, and other factors. In general, the capacity for adsorbing Hg²⁺ will be different than that for Hg⁰. The selection of a carbon for a given application would take into consideration the total concentration of mercury, the relative amounts of Hg⁰ and Hg²⁺, the flue gas composition, and the method of capture (electrostatic precipitator (ESP), or baghouse). An important factor for some cement kilns will be the levels of hydrocarbons and the need to account for their sorption on to the carbon reducing the capacity of the carbon to adsorb mercury. In addition, bench-scale research shows that high SO₂ concentrations diminished the adsorption capacity of activated carbons. Both of these issues could prevent ACI from being an effective control on some cement kilns.

There has been only limited testing of ACI on low concentration mercury gas streams as are typical of cement kilns. Most of this work has been done on power plant boilers achieving control efficiencies of 25-95% depending on the type of coal being burned and a wide number of other factors. In many cases these plants already had some mercury control via the particulate control systems in place and enhanced control via ACI was as low as a 10% improvement.

ACI has the further disadvantage of requiring the disposal of the mercury contaminated spent carbon. Whether the carbon is cleaned and reactivated for reuse or disposed of, the ultimate fate of the mercury needs to be assessed to insure that the mercury will not be reintroduced into the global mercury cycle through some other means.

Mercury Emission Control via Control of Particulates in ESPs or Baghouses

ESPs and/or baghouses (fabric filters) can be an effective control for mercury from cement kilns if two critical conditions are met. First, the mercury must be in the particulate form. This may occur naturally in the system or may require reagents added to the right point in the process to oxidize or catalyze the oxidation of the Hg to HgS and/or HgO. One US wet process plant has demonstrated 95% control of mercury emission through their existing ESP system. High levels of pyrite in their raw materials may be a factor in producing this relatively high level of control. (Theoretically, efforts to reduct SO_x emissions by controlling pyrite in the raw feed could increase mercury emissions.) One experimental system (not in a cement kiln) uses UV light to shift the oxidation state of mercury.

The second critical step in the process is to remove a portion of the mercury-containing dust from the air pollution control system in such a way as to maximize the mercury removal and not place that portion of the dust back into the kiln system. In older wet process plants this has been done routinely by removing the dust from the final one or two stages of the ESP systems. Precalciner plants with in-line raw mills have a more complex scenario to consider. For plants that currently recycle all of their cement kiln dust, the mercury is simply returned to the system and recirculates until the concentration gets high enough that a portion is emitted out the stack in some form. It is critical to break this cycle. Without breaking this cycle the speciation of the emissions may have limited or no meaning relative in determining the most effective control technology.

A program to speciate mercury in dust samples taken during raw mill on and off conditions should be a first step in characterizing an operation to see if particulate control of mercury emissions is a viable option. It needs to be kept in mind that under current normal operations where all CKD is recycled back into the system that the mercury concentration in stack gasses and in the dust will be highly dynamic. The system probably never reaches a steady-state operation relative to mercury input and output. (This implies that there is no way to accurately determine mercury emissions with a stack test.)

Modeling of cement kilns with in-line raw mills suggests that removing a small portion of the captured dust when the raw mill is operating (and possibly when it is not on line as well) can break the recycle loop and may control mercury emissions with efficiencies in excess of 90% depending on speciation and a number of other factors including baghouse blowback cycles, baghouse (or ESP) operating temperatures, types of bags, etc. With the raw mill operating there is likely a very high level of sorption of mercury onto particles in the raw mill and in the baghouse (or ESP) – one set of tests on a precalciner showed sorption efficiencies of 98.5%. It has been typical to operate baghouses at higher temperatures when the raw mill is down and in the just mentioned case efficiencies dropped to 90%. This drop in efficiency is likely to have been due to the increase in the baghouse operating temperature from 100 °C to 175-200 °C.

Spray Tower/Wet Scrubbing of Mercury Emissions

Wet spray tower/slurry systems remove gaseous SO₂ from emissions by absorption. For SO₂ absorption, gaseous SO₂ is contacted with a caustic slurry, typically water and limestone or water and lime. Gaseous compounds of Hg²⁺ are generally water-soluble and can absorb in the aqueous slurry of a wet scrubber system. However, gaseous Hg⁰ is insoluble in water and therefore does not absorb in such slurries. When gaseous compounds of Hg²⁺ are absorbed in the liquid slurry of a wet system, the dissolved species are believed to react with dissolved sulfides from the flue gas, such as H₂S, to form mercuric sulfide (HgS); the HgS precipitates from the liquid solution as sludge.

The capture of mercury in wet scrubbers is likely dependent on the relative amount of Hg²⁺ in the inlet flue gas and on the PM control technology used. Electric utility boiler data reflected that average mercury captures ranged from 29 percent for one unit burning subbituminous coal with an ESP plus wet scrubber to 98 percent in a unit burning bituminous coal with a fabric filter baghouse plus a wet scrubber. The high mercury capture in the fabric filter baghouse plus wet scrubber unit was attributed to increased oxidization and capture of mercury in the baghouse followed by capture of any remaining Hg²⁺ in the wet scrubber. For cement plants with SO _x scrubbers this has particular potential. A system of bleeding the APCD particulate control system followed by a scrubber system for SO_x that coincidentally captures HgCl₂ may provide very high levels of mercury emission control on cement plants with the right chemistry.

Conclusion

While ACI and wet scrubbing may provide control of mercury emissions from cement kilns, the lowest cost option appears to be the use of the existing particulate control system in conjunction with a small bleed of dust from the primary air pollution control system. Plants that have an ESP, may find that the dust in the final stages of the ESP is even more enriched in mercury and that this simplifies the process of creating a “break” in the mercury recycle loop. For example, if it is found that this dust represents 1% of the total feed to the kiln system and is enriched to a factor of 100 times the average level of mercury in the system relative to raw feed; removal of that dust would effectively remove all the mercury from the system. This dust could then be sent to the finished cement blending silos with no appreciable impact on product quality. Investigation of the speciation and enrichment of mercury in the dust being captured in various stages of the ESP or bagouse with the raw mill both on and off is recommended as the first step in developing a dynamic model and from that a mercury control strategy for any cement plant wishing to reduce mercury emissions.

References

CRC Handbook of Chemistry and Physics 70th Edition (1989). Ed. Weast, Robert C., Ph.D., Florida: CRC Press, Inc.

Merck Index Twelfth Edition, The (1996). Ed. Budavari. New Jersey: Merck Research Laboratories, Division of Merck & Co., Inc.

Control of Mercury Emissions from Coal-Fired Electric Utility Boilers: An Update; EPA Air Pollution Prevention and Control Division, National Risk Management Research Laboratory, ORD: Research Triangle Park, NC, Feb 18, 2005; http://www.epa.gov/ttn/atw/utility/ord_whtpaper_hgcontroltech_oar-2002-0056-6141.pdf.