By Sharon Sjostrom, Michael Durham and Connie Senior, ADA Environmental Solution
Mercury control for coal-fired power plants is not a “one size fits all” technology.
Some plants can achieve compliance-level controls by relying on the co-benefits of technologies designed to control other emissions. Others must install specific technologies to achieve sufficient controls. This article provides some general guidelines when assessing control options to meet the proposed Mercury and Air Toxic Standards (aka Utility MACT).
Mercury is present at trace levels in all coals. When coal is burned, gaseous mercury is released into the flue gas. The form, or speciation, of the mercury in flue gas is important to estimate potential removal in existing air pollution control equipment. In the flame, mercury is present in the gaseous elemental form. As the gases cool, some mercury reacts with halogens to form gaseous oxidized mercury. Units firing higher-halogen coals, which include most bituminous coals, often have high fractions of halogenated, oxidized mercury, such as HgCl2, in the flue gas downstream of the air preheater.
Oxidized mercury compounds (e.g., HgCl2) are water soluble and readily removed in SO2 scrubbers. According to reports from the US Department of Energy, 62 percent of the 316 GW of U.S. coal-fired capacity has invested in scrubbers1. The fraction of oxidized mercury can be increased by adding halogens to the coal. SCR units installed to control NOx emissions are also fairly effective at increasing the fraction of oxidized mercury when sufficient halogens are present in the flue gas.
Many plants affected by the Cross States Air Pollution Rule (CSAPR) have already invested in both SCRs and scrubbers to meet compliance requirements. For plants firing higher-halogen coals and configured with SCRs and scrubbers, compliance-level mercury removal can often be achieved without adding mercury-specific controls. Although this is good news for these plants, they still may want to consider installing an activated carbon injection (ACI) system as a low-capital-cost backup in order to insure compliance over a range of coal halogen content and operating conditions.
Good news doesn’t necessarily translate to zero risk, especially when multiple pollutants are involved in compliance decisions. The catalysts used in SCRs will convert a fraction of the incoming SO2 to SO3. For units firing higher sulfur coals or with high conversion catalysts, SO3 can cause balance-of-plant impacts related to air preheater pluggage and downstream corrosion from acid deposition, as well as opacity issues when the sulfuric acid aerosol forms a “blue plume” upon exiting the stack. In addition, SO3 impacts the effectiveness of both unburned carbon and activated carbon to capture mercury. Depending on coal characteristics and boiler operating conditions, SO3 interference can be a problem even for plants that are blending low-sulfur coal (e.g., PRB coal) with as little as 15 percent bituminous coal. Because of this interference, it may be necessary for some plants to add SO3 emission mitigation technology, such as dry sorbent injection (DSI), to the pollution control strategy in order to achieve mercury limits included in Utility MACT by the use of activated carbon injection.
Mercury re-emission is a concern for units with wet scrubbers. Some of the oxidized mercury absorbed in the scrubbing solution can be reduced to elemental mercury in solution. Elemental mercury has very low solubility, causing it to come out of solution and be re-emitted into the flue gas. The chemistry of the wet scrubber is important as several factors can contribute to re-emission, and not all are well understood. Important factors include the type of scrubber, pH of the scrubbing solution, and the concentration of halogens and trace metals in the scrubber solution. Several vendors are evaluating chemicals and technologies to reduce re-emissions.
To consistently meet the Air Toxics proposed mercury limits for the U.S. fleet, strategies to manage mercury in scrubbed units must be fully evaluated. Additional controls that can be employed upstream of wet scrubbers, or in conjunction with dry scrubbers, should be incorporated to minimize the risk of non-compliance and to allow flexibility in fuel choices. For example, for new power plants that have to meet strict permit limits for mercury emissions, the emission control equipment should include both DSI and ACI systems to supplement mercury capture in the SCR and wet scrubber.1
Mercury Removal with Carbon
Many plants cannot rely on co-benefits from SCRs and scrubbers alone to achieve mercury compliance and other capture mechanisms must be used. Many plants, especially those with low NOx burners and those firing bituminous coal, have 2 to 20 percent unburned carbon present in the fly ash. Unburned carbon can adsorb mercury; removal of fly ash in the particulate control device therefore assists with overall mercury control. However, the native carbon is often not sufficient to meet a strict mercury emission limit and it is necessary to supplement with activated carbon to achieve compliance-level mercury control.
ACI is a low capital cost technology that will be a key industry tool to reliably meet mercury emissions control limits. In many cases, ACI can be used as a stand-alone technology for mercury control. In other cases, it must be applied in conjunction with other technologies. Consistently achieving the proposed compliance level requires an understanding of factors that can impact the final emissions. ACI can be very effective for mercury control under the appropriate conditions, and provides fairly low cost “insurance” to allow some fuel flexibility in the future.
The effectiveness of carbon for mercury removal is dependent on several factors, including the properties of the unburned or activated carbon, the concentration of halogens and SO3 in the gas, the fraction of oxidized mercury in the gas, and the temperature of the gas. Higher temperatures and higher SO3 concentrations tend to impede mercury capture. Plants with these conditions will require a thorough review of options to determine their best approach to compliance.
ACI is often very effective for mercury control on sites firing western fuels, especially at plants where SO3 concentrations in the flue gas are low. The combination of low SO3 and sufficient halogens in the flue gas can result in optimal conditions for mercury control with activated carbon. Recent results suggest that halogen-based coal additives can be used to reduce the amount of carbon required to reach control targets. Performance appears to vary from plant to plant so that full-scale, plant-specific demonstrations must be conducted to determine precise levels of performance of this approach. Long-term testing will define whether there are any balance-of-plant issues such as corrosion in downstream equipment.
Another issue relates to units that inject SO3 to improve ESP performance, which is used on approximately 25 GW of power generation capacity. The injected SO3 can detrimentally impact the effectiveness of ACI to achieve compliance mercury control (see Figure 1). This plot shows that mercury capture is reduced as the concentration of SO3 increases. At injected SO3 concentrations of 5 to 10 ppm, there is a significant impact on ACI performance for mercury control. Therefore, it is likely that plants burning low-sulfur coal that have small ESPs will have to find alternatives for meeting particulate control requirements without injecting SO3. One option is a non-SO3-based flue gas conditioning chemical which has proven to be effective for achieving high mercury capture with AC while maintaining ESP performance. Another option is adding a fabric filter downstream of the existing ESP, which could be done through the Electric Power Research Institute’s Toxecon technology. Toxecon offers additional benefits including separating injected carbon from the bulk fly ash, providing additional particulate trim control, and providing an option to accommodate additional particulate loading associated with DSI for HCl control.
In assessing the viability of ACI for mercury control, there are many interactive parameters that must be considered including coal characteristics, boiler design, air pollution control configuration, residence time upstream of the particulate control equipment and gas temperature. There is a significant database available to provide general guidance; however, there are enough unique characteristics at each plant, that it is often still necessary to perform field tests to quantify performance.
The emissions control industry is gearing up to provide the consulting and testing services to assist plant owners and operators in making the right mercury control choices for their plant. It is expected that over 600 ACI systems will be needed to comply with the new mercury regulations. An increased demand for coal additive systems for plants firing low-halogen fuels and DSI systems for plants with moderate to high levels of SO3 is also expected. We recommend that plant operators take advantage of the large and diverse knowledge base in the industry in order to meet these challenges in a timely and cost-effective manner.
1. Mercury and Air Toxics Standards (MATS) for Power Plants: http://www.epa.gov/airquality/powerplanttoxics/actions.html
2. Annual Energy Outlook, DOE/EIA-0383(2011), April 2011.
3. Campbell et al. “Mercury Control with Activated Carbon: Results from Plants with High SO3” Paper #08-A-174. Presented at the Power Plant Air Pollutant Control “Mega” Symposium, Baltimore, MD, Aug. 25-28, 2008.
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