ECO^® (Electro-Catalytic Oxidation) System Overview

Introduction
The Electro-Catalytic Oxidation (ECO) system is an integrated air pollution control technology that achieves major reductions in the primary air pollutants of concern from coal-fired power plants, specifically 99% reduction of sulfur dioxide (SO2) emissions, 90% of nitrogen oxide (NOx) emissions, 80-90% of mercury (Hg) emissions, and 95% of fine particulate matter (PM2.5) emissions. The system also provides high removal of other metals and acid gases such as sulfuric acid (SO3/H2SO4), hydrochloric acid (HCl), and hydrofluoric acid (HF). The ECO system produces a valuable, ammonium sulfate fertilizer co-product, reducing operating costs and minimizing landfill disposal of waste.

Powerspan is operating a 50-MW commercial unit at FirstEnergy's R. E. Burger Plant that employs this technology.

ECO Process Flow
The ECO process treats power plant flue gas in three steps to achieve multi-pollutant removal:

1. ECO Reactor — oxidizes pollutants;
2. Absorber Vessel — removes SO2, NO2, and oxidized mercury; and
3. Wet Electrostatic Precipitator (ESP) — removes acid aerosols, air toxics, and fine particulate matter.

In commercial operation the ECO system is installed downstream of a power plant's existing electrostatic precipitator or fabric filter as is shown in the figure below.

1. ECO Reactor—Oxidation of pollutants

Prior to entering the ECO reactor, the coal combustion flue gas passes through the plant's existing dry electrostatic precipitator (ESP) or fabric filter, which removes the majority of the ash particles. Next the flue gas is routed to the ECO reactor. This dielectric barrier discharge reactor uses non-thermal plasma to generate high energy electrons that have an energy ideal for creating hydroxyl (OH) radicals and atomic oxygen (O). These radicals are formed through the collision of the high energy electrons with water and oxygen molecules naturally occurring in the flue gas.

Once formed, the OH radicals and atomic oxygen act to oxidize pollutants present in the gas stream—particularly NO, SO2, and Hg—leading to the formation of soluble compounds and aerosol mists, which are more easily removed downstream. For example:

• NO gas forms nitrogen dioxide (NO2) gas and nitric acid (HNO3);
• A small portion of SO2 gas is converted to sulfur trioxide (SO3), leading to the formation of sulfuric acid (H2SO4) aerosol mist; and
• Hg is oxidized to mercuric oxide, a capturable form of oxidized mercury.

The removal of NO from the flue gas stream by the ECO process is driven by the ability of the ECO reactor to convert NO to NO2 and HNO3. Once in these more soluble forms, the ECO scrubber chemistry and wet ESP will capture both.

The ECO reactor is similar to gas reactors used in large industrial ozonators for water purification and disinfection. The following video depicts the operation of a shop scale ECO reactor. The blue-violet glow is a result of the excitation of nitrogen in the gas stream.

ECO Reactor Operation AVI file 500K (Windows^®)

ECO Reactor Operation QuickTime MOV file 800K (Windows^® & Mac)

2. Absorber Vessel—Removal of SO2, NO2, and oxidized mercury
Once the ECO process chemistry has been initiated by the reactor, the flue gas enters the scrubber, or absorber vessel. This two-loop scrubber performs three main functions:

1. Saturates and cools the flue gas;
2. Scrubs the remaining SO2 and NO2 from the flue gas stream; and
3. Concentrates the co-product.

Flue gas saturation and co-product concentration are accomplished in the "lower loop" while NOx and SO2 scrubbing is achieved in the "upper loop" (refer to the ECO process flow diagram above).

Ammonia is added to the upper loop to maintain the pH of the solution at a level that will result in a high SO2 scrubbing rate. At this pH, a high fraction of scrubbed SO2 forms sulfite ions, which then scrub the NO2 formed by the ECO reactor.

Although the absorber vessel is similar to those used in wet SO2 scrubbers (i.e. flue gas desulfurization systems), the ECO scrubbing chemistry that takes place at high pH differentiates the ECO process from conventional wet scrubbing systems. A smaller tower (approximately two-thirds the size of a conventional system) and lower liquid flowrates are required, resulting in a lower cost scrubbing system.

3. Wet ESP—Removal of acid aerosols, air toxics, and fine particulate matter

After exiting the absorber vessel, the flue gas enters a wet ESP. Aerosols generated in the ECO reactor and ammonia scrubbing process steps, along with air toxics and fine particulate matter, are captured in the wet ESP and returned to the lower loop of the scrubber.

Wet ESPs differ from dry ESPs in that liquid flows down the collecting plate, removing collected material from its surface as opposed to mechanically rapping or employing sonic horns to remove the material from the plate as is done in dry ESPs. The liquid layer created on the collection plate of wet ESPs prevents particle re-entrainment, improving its collection characteristics over dry ESPs. The improved collection permits higher gas velocities, limiting the equipment size required.

Wet ESPs have been used successfully in industrial applications to collect acid aerosols for over 50 years, particularly in metallurgical plants and in sulfuric acid manufacturing. Wet ESPs have shown to be efficient collectors of PM2.5 and hazardous air pollutants such as mercury.

Co-Product Processing—Mercury and ash removal from co-product stream; Production of commercial-grade fertilizer.

Ammonium sulfate created in the ECO process can be treated and used as a commercial fertilizer. Ash and mercury are removed from the co-product stream. Solids in the scrubber bleed stream from the lower loop, consisting of ash and insoluble metal compounds, are removed by filtration. The filtered material can then be discharged to the power plant's ash handling system along with ash collected from the plant's dry ESP.

The stream is then pumped through an activated carbon adsorption bed, which strongly adsorbs mercury compounds to the bed. The mercury is disposed of separately, and the spent activated carbon is replaced in the ECO process. It is estimated that the variable cost of mercury removal with activated carbon in the ECO process is $800 per pound of mercury, including the sorbent media and its disposal.

Liquid substantially free of mercury and ash may be used as a fertilizer in the liquid form, or may be sent to a crystallizer. The crystallization process consists of heating the liquid under a slight vacuum, boiling off the excess water.  Crystallizer design and operating parameters are established so as to produce crystals of the required size for sale into the fertilizer market without additional processing.