Emissions Control Technology

Oxidation Catalysts

 

Overview

Some SCR installations for both reciprocating engines and gas turbines incorporate CO catalytic oxidation modules along with the NOx reduction catalyst for simultaneous CO/NOx control.

Oxidation catalysts have been applied, in limited cases, to oxidize CO in engine exhaust, typically from lean-burn engines that do not require NOx reductions. Lean-burn technologies may cause increased CO emissions and the application of catalytic oxidation has been shown to be effective in reducing CO emissions from lean-burn engines. In a catalytic oxidation system, CO passes over a catalyst, usually a noble metal, which oxidizes the CO to CO 2 at efficiencies of approximately 70 percent for two stroke lean burn engines and 90 percent for four stroke lean burn engines.

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 hazardous air pollutant (HAP) 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 and water 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-plus percent control of CO and about 85 to 90 percent control of formaldehyde. Similar emission reductions are expected on other HAP pollutants. This could become an important control mechanism as the new Maximum Achievable Control Technology (MACT) standard for formaldehyde for new gas turbines is 91 ppb (parts per billion) at 15% O 2 .