October 1, 2009

Top Plants: Nebraska City Station Unit 2, Nebraska City, Nebraska

Dr. Robert Peltier, PE

Clearing the Air

The new plant incorporates state-of-the-art emission controls, including an Alstom spray dryer absorber and pulse jet fabric filter baghouse; an IHI selective catalytic reduction system (SCR); and fly ash – and bottom ash – handling systems (Figure 2). In the order of treatment, these are the components:

• High-efficiency burners. Low-NO_x burners limit emissions by carefully mixing and combusting the pulverized coal supplied by the mills.

• SCR system. The SCR system injects an anhydrous ammonia solution into the flue gas stream as the gas comes into contact with a catalyst, causing a chemical reaction that removes NO_x.

• Spray dryer absorber. Although OPPD uses low-sulfur coal, the scrubber uses a pebble-lime slurry to further minimize sulfur dioxide from the flue gas.

• Powder-activated carbon-injection system. The mercury removal system removes the trace amounts of mercury found in flue gas by injecting a very finely ground powder-activated carbon into the exhaust gas prior to the spray dryer absorber.

• Particulate baghouse. The baghouse collects fly ash. This ash is transferred to and stored on site in a specially designed landfill.

• Monitoring station. A continuous emission-monitoring system measures the concentration of different stack gas constituents, including CO₂, in the flue gas that exits the exhaust stack. NC2’s emissions limits are summarized in Table 5.

2.    All the right steps.  NCS2 uses an SCR followed by a spray dry absorber and a baghouse to keep the plant’s emissions within its permit limits. Courtesy: OPPD