ECO-SO2 System Overview

Introduction
The Electro-Catalytic Oxidation (ECO) system provides high removal of sulfur dioxide (SO2), nitrogen oxides (NOx), mercury, and fine particulate matter (PM2.5) emissions. In applications where additional NOx removal is not required, the ECO technology can be installed without the NOx control component. The resultant absorber installation (ECO-SO2) achieves major reductions in emissions of SO2, SO3, PM2.5, oxidized mercury and Hazardous Air Pollutants, such as hydrochloric acid (HCl) and hydrofluoric acid (HF).

As shown in the process flow diagram below, in the ECO-SO2 configuration, the ECO technology is supplied as two-stage emission removal process consisting of an ammonia absorber column and a tubular, vertically oriented wet electrostatic precipitator (wet ESP). The ammonia absorber column captures SO2 and oxidized mercury from the flue gas. After exiting the absorber, the flue gas enters the wet ESP, which collects acid aerosols, air toxics, and fine particulate matter. Next the ash is filtered, and the captured mercury is removed from the scrubbing liquor by a treated activated carbon adsorption bed, which isolates the mercury for separate disposal. The fertilizer co-product processing facility then crystallizes the liquid fertilizer into a high value, crystalline product.

The ECO-SO2 system is installed downstream of a power plant's existing electrostatic precipitator or fabric filter, which removes the majority of the ash particles. The flue gas passes through a booster fan to the absorber vessel and into the wet ESP.  The cleaned flue gas exits the wet ESP to a wet stack.

Absorber Vessel —Removes SO2 and oxidized mercury

The absorber tower consists of a saturation section, an absorption section, and a wet ESP. The flue gas enters the absorber and turns upward into the lower sprays, where it is contacted by process liquor.  Here the flue gas is cooled and saturated by evaporating water from the recycling liquid, which also serves to concentrate the ammonium sulfate (AS) liquor.  The liquid falls into the recycle tank located at the bottom of the tower.  The liquid is pumped by the recycle pump back to the spray headers, with a bleed stream to the co-product system.  The water lost to evaporation and to the bleed stream is made up by water addition.

The saturated flue gas passes through the tower packed section where it contacts the recycle liquor.  The liquor is distributed on top of the packing section by a liquid distributor for uniform distribution and efficient mass transfer.  The SO2 and SO3/H2SO4 in the flue gas transfers into the liquid, forms sulfite, and reacts with the ammonium hydroxide and dissolved oxygen in the recycle liquid to ultimately form ammonium sulfate. To maintain the desired sulfate concentration as SO2 is absorbed, liquid is bled from the recycle tank and water is added.

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

After exiting the absorber vessel, the flue gas enters a wet ESP, which has a minimum of two fields.  The wet ESP is intermittently washed with makeup water to prevent fouling.  The wash liquid drains to the recycle tank.  The modular design of the wet ESP allows for online cleaning of each section without degradation of the overall removal efficiency.  The wet ESP operates with a low gas pressure drop (~1”wc).

In addition to aerosol capture, the wet ESP also captures fine particulate matter not captured by the plant’s particulate collection device.  Oxidized mercury capture is also enhanced by the wet ESP.

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

The co-product of the ECO-SO2 scrubbing process is liquid ammonium sulfate.  As is done with a full ECO system, the clear liquid ammonium sulfate is drawn off the scrubber, filtered for ash, passed through an activated carbon adsorption bed to remove mercury, and then crystallized.  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.