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Mercury abatement technologies
The mercury control solutions discussed in this article cover an array of treatment options. Each option has aspects that are both desirable and undesirable, but all focus on trapping the mercury and pulling it from the process. The best solution depends on the process mercury levels and the emissions reduction required.
Mercury removal can be grouped into three categories, targeting pre-process (raw materials), in-process (solids extraction) or after-process (polishing filter). Generally, the level and cost of mercury removal increases as one moves from feed materials to stack. Process complexity and product impact are a major issue when dealing with in-process treatment.
Selective materials sourcing
If the mercury problem originates with an additive raw material then substitution is an obvious solution. It has the least impact on existing operations, but alternative raw materials (if available) are generally more expensive. It is likely that low mercury content materials will be in short supply so this option is generally a short-term solution. If one feedstock is the main source of mercury then replacing it is a viable option. Depending on the material particle size distribution, separate treatment to remove the mercury may be possible with some of the new technologies.
Again, one must consider the oxidation state in the process. Swapping one material for another may adversely impact the oxidation state so reducing one material in favour of another may not provide the results expected. Understanding the mercury cycle is still a first step in the abatement process.
Breaking the mercury cycle
Breaking the process cycle is a well known practice in the cement industry. Many plants have existing alkali or chlorine bypass circuits that interrupt internal process cycles to reduce target compounds. This dust is wasted to manage the cycle at a cost in process equipment and loss of feed in the form of cement kiln dust (CKD). In practice, the bypass removes just a fraction of the target compound.
Fortunately, the mercury cycle is dependent on the absorptive qualities of the raw feed and much of the mercury is captured with the raw feed. This dust can be pulled continuously as in a bypass or where possible removed in a batch purge process. This is especially effective when mercury can be concentrated in a given area. There is existing literature citing removal efficiencies of 30 – 70%.4 The choice of batch versus continuous is case dependant. In both systems the disposition of the residual dust must be addressed since the quantities of slipstream dust can be significant.
Dust shuttling is an accepted means of reducing mercury emissions. The process was studied in detail by the Florida Department of Environmental Protection. In addition, the Florida Department of Transportation studied the effect on concrete. Both agencies have accepted the practice. There is minimal equipment required to divert the collector discharge to pneumatic transport and then to the finish mills. The reduction in material (dust) value and annualised capital contribute to a total cost of US$13 200/kg mercury removed.5 For the cost of an additional transport line, one Florida plant included a line to return the feed to the preheater. This was intended for additional process testing once a mercury CEMS was installed.
More recently, the option of processing the dust at an increased rate, removing the mercury and returning the raw feed to the process has been considered. There are a number of roaster technologies available as well as patented processes for mercury removal. All rely on thermal desorption and isolation of the resulting mercury laden gas, with feed returned to the process. This technology could also be applied to raw material treatment should the material be of powder form, lending itself to heating and the release of inherent mercury.
Flue gas mercury capture
There are a number of promising mercury capture options, as well as some new offerings that may eliminate powdered carbon management.
Activated carbon injection (ACI) in the power industry is gaining momentum, although results vary. The process is straightforward and can be evaluated at a given facility through slipstream testing. The need for a polishing filter mandates a significant capital investment. Carbon supply and disposal, combined with the additional fan capacity, increase operating costs as well. Many environmental groups see this as the best option for the US cement sector but there is limited experience with ACI in the industry with only two installations in place. These include polishing filters with mercury removal efficiency near 80%.
To reduce capital costs one could inject powdered activated carbon ahead of the existing raw mill bag filter. This would be done in combination with some level of dust shuttling or require wasting dust. This could be an option should dust disposal be a viable alternative at a given facility. In general, one would not choose to create another process waste stream. Nor would one introduce carbon to the final product.
The introduction of amended silicates provides for future opportunities. Testing indicates this material provides similar results as activated carbon injection using existing ACI equipment. The silicates as a mercury-enriched waste product would permit use in the cement milling process. Amended silicates could possibly be used ahead of an existing baghouse to increase mercury recovery if shuttling dust when the raw mill is offline. The efficiency of amended silicates is similar to ACI but not as efficient. These systems perform best with high levels of oxidised mercury in the gas stream.
Another new technology is the fixed sorbent bed mercury scrubber. This is a polishing filter that provides an operator advantages in terms of simplicity and safety. The system consists of a gas cooling station after the existing dust collector, followed by a fixed array of sorbent-containing modules. The system is being evaluated in slipstream installations with repeatable results in mercury removal. The process requires no powder injection and as a fixed unit is simple to operate. The sorbent modules are expected to operate for years with removal efficiencies tailored to the demand of the facility. Efficiencies in excess of 90% are reported with the benefit of slipstream testing assuring compliance.6 Based on the size and complexity of the modules, one would expect the capital costs to be more than an activated carbon injection system. The system by design has more opportunity to capture elemental mercury and should provide for reliable mercury capture.
The last systems for consideration in mercury abatement are systems designed for flue gas desulfurisation. These are the traditional wet scrubber technology as well as the newer semi-dry scrubbers.
The semi-dry variant of scrubber consists of a reactor chamber where process gas and lime/water slurry are managed to sustain a fluidised bed of reagent. Relying on significant recycling of the absorbent, this system reportedly provides significant emissions reduction with measured removal of mercury in excess of 90%.7 A number of systems are operating in the power industry, with one as a polishing filter in cement. This technology bears some consideration for mercury abatement if one is considering ACI. These systems are relatively compact, simple to operate and adaptable to changing process conditions. Depending on the process, the unit could be installed between the raw mill and bag filter with significant savings by negating the additional polishing filter.
The standard for flue gas desulfurisation is a wet scrubber using lime or ground limestone as an absorbent. Although there is evidence of excellent performance regarding the reduction of oxidised mercury in the utility industry, performance is process dependent and needs installation specific assessment in cement plants.
The US EPA initiated a study to evaluate the effectiveness of wet scrubber systems on mercury removal that found performance to be system dependent.8 Over 20% of the coal-fired utility boiler capacity in the US uses a PM control device upstream of the wet FGD scrubber for SO2 control. The study of these facilities found mercury removal efficiencies of 29 – 98% depending on the system configuration and source of coal. As in most absorbent based systems the removal efficiencies dropped with an increase in elemental mercury in the gas stream. Generally, high mercury capture was attributed to the configuration of the fabric filter ahead of the scrubber, where increased oxidisation and capture of Hg in the FF increased the system performance. Wet scrubbers require a very high first investment, as well as having significant operating costs. Management of the process waste is significant unless the sulfur content in the gas stream provides synthetic gypsum. Other lower cost alternatives should be considered if mercury control is the sole objective.
Over half of the US cement industry will need to invest in improved emissions control. All plants need to understand the mercury cycle in their process and evaluate the best system designed with their individual needs in mind. They should not accept emissions control systems designed for the power utility industry. Activated carbon injection and wet scrubbers may be a solution, but the related costs and subsequent waste streams mean a better solution could be found.
If ever there was a place for innovation it is in the development of a mercury capture system that is suited to the cement industry. Between a low and high cost installation there exists the ideal solution for any cement plant. It could be one of the new technologies, or a combination of any two. Whatever system is employed it should enhance the ability to produce a quality product with minimal environmental impact. It will be interesting to see what new solutions the mandated mercury reductions, effective from September 2015, will bring.
Written by Daniel Crowley, Titan America, USA. This is an abridged version of the full article, which appeared in the January 2014 issue of World Cement. Subscribers can view the full article by logging in.
This article is based on the paper ‘Mercury Emissions and Abatement Measures’ presented at the 7th International VDZ Congress 2013. It has been edited to World Cement’s house style.
4. GROSSMAN, D., ‘Alternatives to ACL,’ International Cement Review (May 2011).
5. CROWLEY, D., ‘Cement Kiln Mercury Reduction Strategies: A Case Study in Materials Management,’ IEEE-IAS/PCA 52nd Industry Technical Conference Record (2010).
6. KNOTTS, J., KOLDE, J., OLIZZI, C., and STARK, S., ‘A Fixed Bed Gas Remediation Solution For Mercury Control,’ IEEE-IAS/PCA Industry Technical Conference Record (2012).
7. MILLER, S., JENSEN, F., and NIELSEN, K. E., ‘The Gas Suspension Absorber at Norcem’s Brevik Plant,’ IEEE-IAS/PCA Industry Technical Conference Record (2011).
8. ‘USEPA Report: Control of Mercury Emissions From Coal- Fired Electric Utility Boilers,’ www.epa.gov/ttnatw01/utility/hgwhitepaperfinal.pdf
Read the article online at: https://www.worldcement.com/the-americas/10012014/opportunities_for_mercury_abatement_part_2_550/