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Frequently Asked Questions About Mercury Abatement in Air Emissions: What are the EPA's MACT Regulations? What are the EPA's MACT Regulations? The Clean Air Act Amendments of 1990 ("CAAA") directed the EPA to focus on three areas of focus, namely acid rain, urban air pollution and emissions of airborne toxins. The CAAA listed 189 air pollutants considered toxic and required the EPA to develop industry specific or source specific standards for regulating emissions of these toxins based on the Maximum Achievable Control Technology ("MACT") available in a given industry or for a given source. In general, the EPA determines the levels of pollutants coming from facilities operating with the best-performing pollution control technology and set these pollution levels as the new standards for the rest of the industry. As a result, the EPA has been promulgating MACT based emissions standards for different industrial sectors. Most recently, the EPA's new Mercury and Air Toxic Standards ("MATS") use MACT standards to control mercury and hazardous air pollutants from Coal Fired Power Plants, from Portland Cement kilns, from Major and Area Source Industrial Boilers, and from Commercial and Industrial Solid Waste. MACT Standard for Coal Fired Power Plants: In 2000, the EPA determined that mercury emissions from coal fired power plants represented a significant enough public danger to warrant a MACT standard. The EPA's initial efforts focused on developing a cap and trade program called the Clean Air Mercury Rule. This proposed program was however vacated by a Washington DC Circuit court of Appeals in February 2008 and the EPA was ordered to issue an appropriate MACT standard for power plants emissions. Within the framework of the MATS, the MACT standard for power plants was announced in 2011 and finalized in December 2012 after several court challenges. This standard includes provisions for lowering emissions of all major HAPs covered under the Clean Air Act as well as mercury. This new MACT standard for Coal Fired Power Plants has an initial compliance date of April 2015 for most coal fired power plants, with state agencies allowed to extend the deadline to 2016 for certain units which will require significant upgrade. MACT Standard for Portland Cement Kilns: In September 2010, the EPA issued MACT emissions standards for Portland Cement Kilns covering mercury and roughly 20 additional HAPs falling under the Clean Air Act. The Cement Kilns standard grant existing plants constructed before 2009 until Sept 2013 to comply with the new emissions guideline whereas newer facilities must be compliant with the standard from the first day of operations. MACT Standard for Major and Area Source Industrial Boilers: In April 2010, the EPA released draft emissions standards for industrial boilers classified into 2 types:
The current Industrial Boilers standards are a revised version of regulations originally issued by the EPA in 2004 which were vacated by a Washington DC Circuit court of Appeals in 2007. The EPA issued a new set of standards in 2011 and the rules were finalized in February 2013. The initial compliance deadline for existing industrial boilers is Feb 2016 with allowance for possible extensions. The compliance date for existing major sources is March 21, 2014 under the Clean Air Act, and state permitting authorities are able to grant an additional year as needed for technology installation. The EPA expects the MACT standard for Industrial Boilers will affect nearly all 200,000 existing sources and more than 8,500 new sources. The Industrial Boilers MACT standards are expected to most heavily affect industries with large scale boilers operations such as pulp and paper production, food and beverage manufacturing and chemical processing. MACT Standard for Commercial and Industrial Solid Waste Incinerators: The EPA finalized amendments to the March 2011 Commercial and Industrial Solid Waste Incinerators ("CISWI") and promulgated final MACT standards for CISWI on Dec 20, 2012. The latest compliance date for existing CISWI is February 2017 but no later than three years after the EPA approves the plan of the jurisdiction in which the CISWI is located, whichever comes first. For new CISWI sources which commenced construction on or after June 2010, the compliance date is 6 months after publication of the final rule in the Federal register or upon start up whichever is later. The EPA expects that the MACT standard for CISWI will affect only 106 existing sources at 76 facilities and six new sources over the next five years. More information is available on the EPA's website http://epa.gov/mats/. A sorbent is a material used to absorb or adsorb liquids or gases. A mercury sorbent is a substance that adsorbs elemental and oxidized mercury in its gaseous form. ECI's mercury sorbent is a powdered activated carbon (PAC) that is used in activated carbon injection (ACI) processes to adsorb mercury from industrial flue gases to meet MACT regulations. There are other non-carbon based mercury sorbents but at the moment, PAC is the EPA designated Maximum Achievable Control Technology to control mercury in industrial flue gases. What is Activated Carbon Injection? Over the past decade the DOE, in an effort to assist industry in developing a method to remove mercury from industrial flue gases, funded the development and testing of a process commonly known as activated carbon injection (ACI). At facilities employing ACI, PAC is injected (sprayed) into flue gas before a particulate control device, such as a fabric filter (FF) or electrostatic precipitator (ESP). The PAC adsorbs the flue gas mercury, and this PAC, containing the captured mercury in its pores, is removed when captured in the FF or ESP. Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. In terms of mercury adsorption it is the adhesion of mercury to the sorbent used, in most cases PAC. This process captures the mercury as a film on the surface of the adsorbent. Due to its high degree of microporosity, just one gram of PAC has a surface area in excess of 500 M2. This is one reason why PAC is used as mercury sorbent, its extensive surface area by weight provides greater opportunities for mercury to adhere. For mercury control in industrial flue gases, the most relevant test with regards to an activated carbon "activation level" and therefore its mercury capture performance is the iodine number test. The iodine number is a measure of the micropore content of a given PAC determined by its adsorption of iodine from a solution. This test measures the adsorption potential of a given PAC (higher number indicates higher degree of potential adsorption) and is measured in mg/g. It is correlated to the surface area of a given activated carbon which is another means to evaluate the adsorption potential of a given PAC. It should however be kept in mind that this particular test does not take into account the impact of PAC impregnation described elsewhere, which can increase the adsorption of a given PAC without increasing the iodine number test results. When operators affected by the EPA MATS regulations, are starting to consider their choice of PACs, ECI recommends performance testing of multiple PACs in the unit itself. As explained in What Affects A Company's Need For A Mercury Sorbent?, Hg behavior is impacted dramatically in the various phases of the creation and abatement of the flue gases, and its adsorption by any individual PAC will vary significantly based not only on coal choices, but also by a plant's emission control technologies, flue gas conditioning and usage of boiler additives. What Affects A Company's Usage Of A Mercury Sorbent? To understand a coal fired power plant operator's best use of mercury sorbent technology, one needs to understand how mercury arises and is affected by various processes from boiler to flue. Mercury is found in several different compounds in coal: Hg0 (elemental mercury) as well as HgCl2, HgBr2, and HgS among others. The form of mercury content of coal can differ by region and even by seam within regions. Additionally, these mercury species can vary substantially with heat and other conditions and can also break down to gaseous elemental mercury quite readily. As a result, not only the coal type but also the different process' thermochemical equilibrium impacts the existence of these mercury compounds in a coal fired power plant. Typically, the mass factors of the compounds essentially decline as that of elemental mercury increases with temperature. As a result, the largest mercury speciation occurs in the boiler where, in addition to heat, various additional compounds created in the boiler (S03, CO, H20, NOx and O2) influence this mercury speciation and its related capture. As a result, Mercury is typically present in flue gas in three basic forms: particulate-bound mercury (Hg[pl]), elemental (Hg0) and oxidized mercury (Hg2+). As it is in a coal fired power plant boiler that most elemental gaseous mercury is created, one successful and cost effective approach to limiting the gaseous elemental mercury exiting the boiler is the use of boiler chemical additives such as bromide and chloride salts. The halogen compounds oxidize the mercury increasing the fraction of oxidized mercury that can be captured downstream by particulate control devices and/or wet/ dry SO2 scrubbers as discussed below. But the mercury speciation does not stop at the boiler. The transfer of heat from flue gas to the steam lowers the temperature providing conditions for the gaseous elemental mercury to speciate again (re-emission), as does a pass through the air pre-heater providing again an opportunity for speciation back to higher gaseous elemental mercury levels. Similarly, a pass through the economizer again changes the temperature resulting in further mercury speciation. Additionally, gaseous elemental mercury, when entering an SCR can be converted to mercury oxide as a side effect of the catalytic reactions which can beneficially lower the gaseous elemental mercury in the flue gas. Flue gas residency in an ESP can also beneficially remove mercury by capturing particulate matter with imbedded mercury but when longer ESP residency s cools the flue gas, mercury speciation into elemental mercury can again take place. The use of a bag house has perhaps the most beneficial mercury abatement effect as it provides an environment for higher contact between the elemental mercury and ash resulting in the capture of particulate bound mercury including smaller mercury embedded particulate matter which typically can escape an ESP. And finally, a scrubber by cooling the gaseous elemental mercury can provide a favorable environment for re-emission while at the same time, providing optimal conditions for capture of HgO (elemental mercury). It should be noted however, that such scrubber capture potentially transfers an air pollutant to a water, or solid waste, pollutant. In short, Hg is impacted dramatically in the various phases of the creation and abatement of the flue gases, and will vary significantly based on coal choices and treatments as well as a plant's emission control technologies. For a more in depth source of the impact of mercury speciation on mercury removal from industrial flue gases, we recommend the reading of an extensive report issued by URS. Available Here. While an activation level sufficient for mercury sorption may be attained solely by the high surface area of a particular PAC, sometimes further chemical treatment can be used to enhance the PAC's adsorption properties. This is accomplished through the process known as impregnation where a chemical (typically sulfur or bromine) is added to the PAC while either in a salt phase or gas-phase to enhance the adsorptive properties of the PAC. These added chemicals react with the elemental mercury to form organic mercury that is more readily caught by existing emissions control devices (as noted above) as well as captured and fixated by the PAC. ECI's carbon feedstock used to manufacture EcoPAC-S has a higher percentage of native sulfur than traditional coal/lignite based PACs. This sulfur is resident in ECI's pre-activation process, where the carbon is heated to a temperature around 6000Ci which promotes a more uniform sulfur distribution in the PAC pore structure and results in higher mercury adsorption capability than a monolayer surface deposition impregnation, in particular at higher temperatures in flue gases. Further, greater sulfur concentration is achieved during the ECI activation process. This proprietary process, enhances EcoPAC-S mercury adsorption by further increasing its total sulfur content and decreasing its total micropore surface area and volume. . In particular, the impregnated sulfur reacts aggressively with elemental mercury (the hardest mercury to capture) creating mercuric sulfide on the carbon's surface and increasing the amount of elemental mercury captured and fixated. When is Impregnation Important? It Depends on Coal Selection. When the coal used produces flue gases relatively low in chloride and high in elemental mercury (Western coals such as Powder River Basin subbituminous, North Dakota lignite and Texas lignite) impregnation may be necessary to capture the high levels of elemental mercury if the unit does not have a bag house. If the unit is equipped with a fabric filter as opposed to an ESP the use of pre-spray's on the western coal together with non-impregnated, less-expensive PAC may result in less expensive mercury removal costs. Eastern bituminous coal results in flue gas with high levels of chloride and oxidized mercury and lower elemental mercury, and therefore using halogenated ACI to increase flue gas oxidized mercury does little to improve performance over standard PAC. This is because halogenated PAC's main function is to improve capture of elemental mercury not caught by the unit's traditional emission control technologies the concentration of which is relatively low in flue gas from bituminous coal. Back to top. Portland concrete is expensive so cement manufacturers replace up to 20% of concrete with fly ash in the manufacture of cement. Fly ash is actually beneficial to cement manufacturing as its particles are smaller than concrete, thereby filling smaller voids and additionally forming better bonds. To avoid cracking during freezing weather concrete must contain an optimal amount of air. In order to ensure such optimal aeration in concrete, natural air is supplemented with entrained air which is added by the use of air entrainment additives (AEA). Activated carbon in fly ash has been shown to adsorb AEA surfactants, and therefore to cause the need for addition of more AEAs to entrain sufficient air. The research is not yet definitive, but the most extensive independent, comprehensive tests to date were completed by URS. EPRI, LaFarge and Headwaters, Inc. Available Here. The one clear result is that a TOXECONTM configuration is the best method for ensuring fly ash marketability. However, the added capital expense of a bag house to capture the PAC injected after the primary fly ash capture device (typically an ESP) makes this configuration a very expensive solution. The remaining results are not as clear. Passivated PACs do have a slightly better performance with fly ash but the related higher costs of production must be factored in, as well as their possible reduced effectiveness of the PAC's primary role of mercury removal which also depends on the unit's emissions reduction equipment. In some facilities, un-impregnated PAC used in conjunction with coal treatment resulted in fly ash that was deemed useable by LaFarge. It should be noted however, that added quantities of AEAs in cement dictates lower prices for the ash to recover the added cost of surfactants. Halogenated PAC when used produced results just outside acceptable levels although with substantially more AEAs (8X) halogenated PAC fly ash could be used in cement manufacturing (this is probably due to the halogens higher sorbency of the air). Finally, URS and EPRI have indicated that the primary issue determining the quality of fly ash for cement manufacturing usage is the variability of PAC injection rates necessary to ensure required mercury capture in industrial flue gases. High injection rates variability may result in irregular ash quality. This creates a problem for concrete manufacturers who need consistency to determine the correct AEA dosing to ensure acceptable results. As coal choice, emission equipment selection and PAC all affect the carbon content of fly ash and impact its suitability for cement manufacture, extensive testing is possibly the best way to determine a specific PAC's impact in any one situation. Back to top. High concentrations of SO3 in flue gas can negatively impact a mercury sorbent by permeating the PAC's pores thereby limiting the surface area available for mercury sorption. SO3 can be present in larger concentrations because of high sulfur coal being used or by SO3 additions to improve ESP performance and facilitate better SCR performance. There is work being done on sorbents that will be SO3 resistant but none have yet to show commercial viability. There are however many ways to reduce SO3 such as: alkali sorbent/trona co-injection, injecting PAC upstream of the SCR before SO3 is added, and lowering the need for added SO3z for ESP conditioning by using chemicals other than SO3 or by using ESP power supplies that modify the shape and frequency of the ESP voltage and current. What Impacts the Cost of Mercury Removal? Almost every operating variable of a unit impacts PAC performance in removing mercury from industrial flue gases. The most important ones are as follows: coal choice, coal pre-treatment, PAC choice, emissions equipment, disposal costs, and ash sales. For further evaluation, we would suggest an EPRI document, that while somewhat old (2006), remains a very useful document in understanding the impact of all the operating variables and also a framework for determining overall cost of mercury capture Available Here.
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