In its proposed rules to address hazardous air pollutants (HAPs), the U.S. Environmental Protection Agency identified several technologies wet and dry scrubbers, selective catalytic reduction systems, dry sorbent injection, activated carbon injection and fabric filters - as proven and readily available.

None of these technologies applied individually are likely to enable a generating unit to meet the proposed HAPs requirements, which address a number of pollutants. In most cases, combinations of these technologies will be required.

Flue gas desulfurization (FGD or scrubber) systems use chemical and mechanical processes to remove sulfur dioxide (SO2) from the flue gas. There are several FGD system technologies; but fundamentally, they introduce an alkali reagent (often lime or limestone) to react with the acidic gas. The resulting product can be put to beneficial use or properly disposed and managed in a landfill.

When paired with other control technologies, FGD systems also help reduce other emissions, such as sulfur trioxide (SO3) and mercury.

AEP has extensive scrubber construction and operation experience, with systems installed on approximately 10,000 megawatts of generation. The company currently is constructing its first dry FGD system at the John W. Turk Jr. Plant in Arkansas.

Selective catalytic reduction (SCR) systems convert nitrogen oxide (NOx) to harmless nitrogen and water. While NOx is not addressed by the HAPs rule, the SCR operation can have the co-benefit of oxidizing elemental mercury so that it can be removed in the FGD process.

Activated carbon injection (ACI) systems are used to capture mercury in the flue gas. Mercury attaches to the carbon particles so they can be removed with other particulate matter by an electrostatic precipitator or in a fabric filter. While AEP’s ACI experience is limited, the technology has helped reduce mercury emissions where installed.

Dry sorbent injection (DSI) introduces an alkali (typically trona or lime) into the flue gas to neutralize acid gases. AEP developed, and has extensive experience, injecting trona to control SO3 emissions. This technology may have application for neutralizing other acid gases.

Fabric filters (baghouses) physically trap and filter out particulate matter (flyash and injected sorbents). The particulate matter builds up on the surface of the bags and forms a layer (or filter cake). When sorbents are present, the filter cake itself captures acid gas and mercury as flue gas flows through it. When cake layer thickness becomes excessive, the particulate matter is removed from the bags, collected and subsequently disposed.

Electrostatic precipitators (ESPs) historically have been used by AEP and the utility industry to remove flyash. As flue gas passes across a series of electrically charged plates and wires, particulate matter (ash and injected sorbents) is statically charged. These charged particles collect on the electrically grounded plates. The plates are periodically “rapped” to dislodge particulate matter, which is then collected and disposed.