The technologies available for controlling Hazardous Air Pollutants (HAPs) are numerous. Since mercury is to be controlled for the first time, it may require the most in-depth control of any hazardous air pollutant. Mercury-specific control technologies include Activated Carbon Injection (ACI), halogen addition, and co-benefit methods of control, such as PM controls, dry sorbent injection (DSI) and dry and wet scrubbers (see Table 2).
ACI and halogen addition are the preferred methods for controlling mercury, said Brandy Johnson, manager of FGD project development at Babcock & Wilcox. ACI is essentially the injection of powdered activated carbon. The injected carbon is then captured using a downstream PM capture device (an electrostatic precipitator (ESP) or a baghouse). Halogen addition can be used to enhance the mercury removal process.
Many technologies provide co-benefit methods of control for mercury. Baghouses (or fabric filters) typically provide a higher co-benefit mercury capture than ESPs. Wet scrubbers with SCR controls upstream have also proven to be effective at removing oxidized mercury. One downside to this option is that a wet scrubber will take mercury emissions out of the air, but can leave mercury emissions in water.
Plants that only have an ESP may find it challenging to meet Utility MACT compliance, Mahabaleshwarkar said, because “ESPs remove limited mercury and HCl even with carbon/sorbent injection.” A plant that has an ESP and a wet scrubber installed will be more likely to meet compliance, while a plant with a dry scrubber and fabric filter will likely meet full compliance. Currently, more than 50 to 60 percent of coal-fired power plants have only ESPs installed.
Acid Gas Control. Strong acids like HCl and HF are formed as a result of the inherent halogen content in coal that is released during combustion to form acids as the flue gas cools. The proposed Utilty MACT rule sets a numerical emissions limit for HCl. This limit also functions as a surrogate limit for other acid gases, which are not given individual emission limits under the proposed rule. In principle, wet and dry SO2 scrubbers can be used for controlling HCl and HF on plant boilers.
Dioxin/Furan Control. Dioxins and furans are generally not present in coal, but can be formed during the molecular rearrangement of compounds and reactions of fly ash involving carbon, oxygen, hydrogen, chlorine and a transition metal catalyst. Because dioxins are formed through organic precursors, installing combustion controls or oxidation catalysts can help prevent or lower their formation. SCRs have also been proven to lower emissions of dioxins and furans. DSI upstream of the air preheater of a coal boiler may also be a way to reduce dioxin and furan formation.
Babcock & Wilcox offers a number of mercury-specific control options to meet various power plant needs. Its MercPlus System enhances mercury capture through halogen addition to the fuel to promote oxidation and minimizes powdered activated carbon usage at power plants that use low chlorine coals, such as PRB. The Absorption Plus (Hg) system is designed to inhibit mercury re-emission and to increase the total mercury captured and retained in a wet FGD system. Essentially, Absorption Plus maintains the Hg in the scrubber solution and prevents the mercury from changing back into elemental mercury and emitting from the stack. Babcock & Wilcox also offers powdered ACI, sorbent injection, baghouses and more.
ADA Environmental Solutions ACI systems can be tailored to meet specific requirements. Operators can choose between options like PLC or DCS controls, gravimetric or volumetric feeders, ladders or stairs, and standard or deluxe instrument packages. ADA also offers Dry Sorbent Injection (DSI) systems that use alkaline materials (hydrated lime, trona, sodium bicarbonate) to reduce SO2, SO3 and HCl from stack gases.
Skyonic has also produced a multi-pollutant control technology to help plants comply with HAPs regulations. The SkyMine emissions removal technology has 99 percent efficiency removal of SOx, NO2, mercury and other heavy metals. The technology is also capable of capturing carbon from flue gas stacks.
Eco Power Solutions, Comply 2000, is a multi-pollutant removal system that was selected by EPA to be included on a list of technologies that “offer the potential of reduced compliance costs and improved overall environmental performance” of the New Source Performance Standards and MACT. The Comply 2000 reduces SO2 emissions through injecting a fogging spray mixed with a hydrogen peroxide solution that is condensed concurrently with other pollutants over coils to remove all combustion emissions from the exhaust gas stream. This process converts NOx and SO2 to nitric and sulfuric acid in the wastewater stream, resulting in 99 percent removal of SO2, mercury, halogens including fluoride, chlorine and bromide, heavy metals include arsenic and cadmium, 2.5 and 10-micron PM, as well as 20 percent removal of CO2. (Power Engineering, 10/1/2011)
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