Question

Describe the Selective Catalytic Reduction to CO CONTROL

Answer 1

Selective catalytic reduction (SCR) systems selectively reduce NOx emissions by injecting ammonium (NH3) into the exhaust gas stream upstream of a catalyst. Nitrogen oxides, NH3, and O2 react on the surface of the catalyst to form N2 and H2O. The exhaust gas must contain a minimum amount of O2 and be within a particular temperature range (typically 450–850°F) in order for the SCR system to operate properly.

The temperature range is dictated by the catalyst material, which is typically made from noble metals, including base metal oxides such as vanadium and titanium, or zeolite-based material. The removal efficiency of an SCR system in good working order is typically from 65–90%. Exhaust gas temperatures greater than the upper limit (850°F) cause NOx and NH3 to pass through the catalyst unreacted. Ammonia emissions, called NH3 slip, may be a consideration when specifying an SCR system.

Ammonia, either in the form of liquid anhydrous ammonia or aqueous ammonia hydroxide, is stored on-site and injected into the exhaust stream upstream of the catalyst. Although an SCR system can operate alone, it is typically used in conjunction with water-steam injection systems or lean-premix system to reduce NOx emissions to their lowest levels (less than 10 ppm at 15% oxygen for SCR and wet injection systems). The SCR system for landfill or digester gas-fired turbines requires a substantial fuel gas pretreatment to remove trace contaminants that can poison the catalyst. Therefore, SCR and other catalytic treatments may be inappropriate control technologies for landfill or digester gas-fired turbines.

The catalyst and catalyst housing used in SCR systems tend to be very large and dense (in terms of surface area to volume ratio) because of the high exhaust flow rates and long residence times required for NOx, O2, and NH3, to react on the catalyst. Most catalysts are configured in a parallel-plate, “honeycomb” design to maximize the surface area-to-volume ratio of the catalyst. Some SCR installations incorporate CO catalytic oxidation modules along with the NOx reduction catalyst for simultaneous CO/NOx control.

Carbon monoxide oxidation catalysts are typically used on turbines to achieve control of CO emissions, especially turbines that use steam injection, which can increase the concentrations of CO and unburned hydrocarbons in the exhaust. CO catalysts are also being used to reduce VOC and organic HAPs emissions. The catalyst is usually made of a precious metal such as platinum, palladium, or rhodium. Other formulations, such as metal oxides for emission streams containing chlorinated compounds, are also used. The CO catalyst promotes the oxidation of CO and hydrocarbon compounds to carbon dioxide (CO2) and water (H2O) as the emission stream passes through the catalyst bed. The oxidation process takes place spontaneously, without the requirement for introducing reactants. The performance of these oxidation catalyst systems on combustion turbines results in 90+% control of CO and about 85–90% control of formaldehyde. Similar emission reductions are expected on other HAP pollutants.