Powdered Activated Carbon Injection System to Control Mercury Emission
Mercury is still used today in a wide range of products, including batteries, paints, switches, electrical and electronic devices, thermometers, blood-pressure gauges, fluorescent and energy-saving lamps, pesticides, fungicides, medicines, and cosmetics.
As reported in the Global Mercury Assessment 2013 by the United Nations, many spent products and the mercury they contain enter waste streams. While mercury in landfills may slowly become re-mobilized to the atmosphere, waste that is incinerated – or melted, in the case of scrap – can be a major source of atmospheric mercury.
There are three main forms of mercury in flue gases: elemental Hg (Hg0), particulate Hg (HgP) and reactive (divalent) gaseous mercury (RGM, Hg(II) or Hg(g)2+) such as HgCl2.[2,3]
Emissions from steelmaking have historically been believed to be comprised of approximately 80% Hg0, 5% HgP and 15% Hg(II).
Steel plants and, in particular, electric-arc furnaces (EAFs) are recognized as a significant source of mercury emissions right after coal-fired power plants, which represent the biggest source of mercury emissions in the atmosphere. Different approaches and different technical solutions have been adopted to prevent or to control the mercury emissions from stacks, both in Europe and in the U.S., and the differences between steel plants and coal-fired power plants are significant.
Despite the efforts of the U.S., Europe and Russia, which have significantly reduced mercury emissions from 1990 to 2005, such emissions have increased in the same period of time in areas like China, India and some regions of Africa and South America.
EAF Mercury Rule
Mercury-emissions limits at the stack are currently governing steel plants in Europe but not in the U.S. In Europe, those limits are expected to become more restrictive as general awareness increases and best-available techniques (BATs) become available to the industry.
In some European steel plants, emission control devices are installed to ensure compliance with the imposed mercury concentration at the stack. When the limits are more severe, the Activated Carbon Injection (ACI) system has been demonstrated to be one of the most environmental and cost-effective solutions. A possible alternative technology for mercury removal from flue gases is the fixed-carbon bed technology. Differences between the two technologies are explained later.
As a general clarification, it has to be noted that the U.S. Environmental Protection Agency (EPA) rule in force today is based on pollution prevention.[6,7] Such rules require eliminating mercury at the source. Scrap from motor vehicles has to be decontaminated from mercury switches before it can be melted in EAF facilities. U.S. steelmakers are effectively complying with the rule, contributing to the reduction of mercury emissions. EAF production accounts for about 8% of total U.S. man-made mercury emissions. Coal-fired power plants are the leading source of mercury emissions at 48%.
Based on pollution prevention, the EPA’s Maximum Achievable Control Technology (MACT) standards for mercury require an EAF to obtain motor vehicle scrap only from scrap providers participating in an EPA-approved program for the removal of mercury switches. The National Vehicle Mercury Switch Recovery Program (NVMSRP) is an approved program under the final standard issued in 2007. The goal of the program is to collect 80-90% of available mercury switches by 2017.
There are a few cases for which local regulations impose emission limits and stack testing. ACI is currently considered by the EPA only as a potential “beyond-the-floor” option (i.e. option that pushes control technology to perform at levels below the MACT floor) for mercury control in the EAF.
The EPA is constantly reviewing its standards for mercury emissions. In fact, it has recently updated emission limits for new power plants under the Mercury and Air Toxics Standards (MATS). The rules for EAF steelmaking facilities haven’t changed recently, but future revisions could lead to mercury stack emission limits and continuous emission monitoring requirements. Although not anticipated, ACI implementation for EAF plants is then possible.
According to IPPC (International Pollution Prevention and Control) directive, the BAT for EAF processes in Europe is to prevent mercury emissions by avoiding, as much as possible, raw materials and auxiliaries that contain mercury. The BAT-associated emission limit is expressed in terms of mercury concentration at the stack, equal to 0.05 mg/Nm³. In some cases, more stringent local rules pushed the operator to find additional control solutions.
Difference Between ACI and Fixed-Carbon Bed
In this article, several cases are presented in which AC is used as sorbent material for capturing mercury and other pollutants present in the EAF flue gases. The cases include:
• Two EAFs based in Italy, in which the ACI system was installed with the scope of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) emission control, with consequent and significant reduction on mercury emission
• A single EAF facility in France, provided with the ACI system because of a very stringent mercury emission limit imposed by local authorities
• A facility based in Norway, where a fixed-carbon bed system is installed for mercury removal
The main difference between the two technologies mentioned previously is that ACI can be operated in order to achieve the specific limit of mercury emission, injecting enough carbon in the flue gases upstream of the filtering unit to comply with the value required by authorities. ACI is a dynamic system, and the carbon injected is modulated according to the target to achieve. On the other end, the fixed-carbon bed is installed downstream of the filtering unit and, due to its fixed nature, works as a “cartridge” to accumulate mercury. When the accumulation capacity has reached its maximum level, the “cartridge” has to be replaced with a new clean one, maintaining the emission at the stack always under the value required by authorities.
It is then not appropriate to compare the two systems in terms of efficiency in mercury removal. As a matter of fact, the two systems have been proven to be effective in their primary objective of removing mercury from flue gases. The systems are applied in steel plants on a case-by-case basis, depending on factors such as opex, capex, dust classification for a particular area, etc.
Mercury Emissions to Air in EAF Steelmaking
In EAF steelmaking, different grades of scrap metal are recycled. This material is contaminated by the presence of chemical sub-stances. Therefore, various root sources of emissions are plausible. The most common source of mercury comes from scrap obtained by old motor vehicles containing mercury switches. Nonetheless, other sources have to be considered too. Mercury coming from non-automobile scrap can be a significant portion of total mercury present in the charge, and it is not removed by the switch-removal program.
Scrap quality is the most influential factor for mercury emissions from EAF facilities, but other raw materials should be considered as mercury sources. According to a study by the Swedish Environmental Research Institute, fluorspar (CaF2) contains 1.1 ppm of mercury. Other raw materials contain up to 0.2 ppm of mercury, as per a more recent investigation by the Norwegian steel plant Mo i Rana in 2004. Based on an addition of 150 kg of fluorspar per heat, the concentration of 0.2 ppm of mercury from the raw material becomes 0.3 grams of mercury per heat.
Figure 3 shows the mercury content of various raw materials involved in the EAF steel melting process. In Nordic steel mills, continuous measurement of mercury emissions was implemented in the years following these studies, but a consistent correlation between raw-materials input and emission peaks has not yet been demonstrated.
Table 1 provides an overview of the specific emission factors that can be used as reference for the emission floor assessment in the U.S. and Europe.
Powdered Activated Carbon Injection System
The typical low temperature at the EAF bag filter inlet (in the 50-125˚C range) and the off-gas characteristics lead to having elemental mercury as the predominant mercury gaseous form in this section of the plant. It is difficult to capture because of its higher volatility and chemical inertness, and it cannot be captured on the bag’s surface by filtration only. For this reason, injecting a sorbent material upstream of the PM device (ideally a bag filter), which is able to capture gaseous mercury in the Hg0 and RGM forms, can be a very effective way to achieve the required mercury abatement.
The basic principle under which the elemental mercury control by ACI is performed is the adsorption. This term means the attach-ment of molecules (in this case, mercury, PCDDs/Fs, PAH and other pollutants) to the surface of a solid. To obtain an efficient adsorption, the following requirements need to be satisfied:
• Sorbent must have large surface areas
• Sorbent must have micropores
• Sorbent chemical characteristics must be considered especially for the case of mercury control
• Good contact time for an efficient separation (a good distribution in the flow is mandatory)
• The sorbent dosage must be regulated according to the fumes’ variable conditions
To reach the required results, the following factors should be considered in the design. The system operates using PAC as sorbent material. Each PAC particle has an extremely large surface area with micropores, which helps it to easily adsorb gaseous pollutants. Different types of PAC are available on the market, so the choice must be done in consideration of the process characteristics. Chemically embedded activated carbon (specifically sulphur, chlorine, bromine) enhances the uptake of mercury. This solution is sometimes adopted in coal combustion facilities, depending on the chemical composition of the flue gas.
The contact between PAC and gaseous mercury is very strong when the fumes pass through the dust cake accumulated on the fabric bags, so it is very important to ensure a good PAC distribution on the whole filtering surface. A CFD model is usually developed to improve the distribution of activated carbon, taking into account the plant peculiarities.
Installations and Results
Table 2 shows ACI systems installed in European facilities to comply with more stringent emission limits.
In the case of plants A and B in Italy, the system was primarily installed for the PCDD-F control, with the scope to obtain concentra-tion at the stack lower than 0,1 ngI-TEQ/Nm³ (ngI-TEQ: International Toxic Equivalent nanograms, according to the toxic equivalent factors defined by NATO 1988). Although these systems are not designed for mercury control, a significant reduction of the Hg emis-sion is reached in full compliance with the imposed limits.
Plants B and C are performing Hg emission levels in compliance with the imposed limits with a very low PAC consumption. The abatement performance can be further improved, if required by future regulations, by increasing and adjusting the injection rate.
In the case of plant C in France, the more stringent local mercury emissions limit pushed the company to install an ACI system for Hg control. Optimization of the system was achieved by adjusting the PAC granulometry and the concentration of carbon in the off-gas.
In plant D, Norway, a different solution was adopted, with fume passing through activated-carbon fixed beds before entering the stack. This solution is effectively working but, because of the sizeable dimensions of the system and considering additional pressure loss, is not suitable for high-fume flow-rates (i.e. in the case that EAF primary and secondary emissions are both conveyed to the same dedusting unit. Its installation in existing facilities requires operation stop and modification of the dedusting system of the plant.
Emission control is successfully performed in all the mentioned cases. Activated carbon technology is able to comply with the mercury emission limit that cannot be met by means of pollution-prevention practices.
The activated carbon beds technology is achieving very good results, and pollutants are recovered in a fixed quantity of activated carbon that can be properly treated or disposed after the use. Its installation tends to be more complicated for existing plants com-pared to an ACI system because it can be physically large to install and potentially have more complicated flue-gas logistics.
Significant ACI system features are the adaptability to all the dedusting system configurations. Also, installation is possible without the need for dedusting system modifications. Pollutants are captured by the PAC and collected together with the EAF dust, and PAC content usually results in lower than 3% of the total dust.
As the EAF dust itself contains mercury, adding PAC to it would just increase its mercury content, more or less significantly depending on the plant. In any case, with or without PAC injection, the produced dust will be properly treated in the reusing or disposal process. IH
For more information: Contact Francesco Memoli, VP Steelmaking, Tenova Core, 100 Corporate Center Drive, Coraopolis, PA; tel: 412-262-2240; e-mail: firstname.lastname@example.org; web: www.tenovacore.com and www.tenovagroup.com or Aldo Giachero, TTF Srl – Tecnologie per l’ambiente, Via Degli Artigiani, 58G, 16162 - Genova, Bolzaneto – Italy; tel: +39 010 2518836; e-mail: email@example.com; web: www.ttf.ge.it
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