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Electrical Generating Equipment - Natural Gas Engines

Three generic emission control techniques have been developed for reciprocating engines:

  • Advanced controls (timing and operating at a leaner air-to-fuel ratio)
  • Combustion modifications such as advanced engine design for new sources or major modification to existing sources
  • Post-combustion catalytic controls installed on the engine exhaust system.

Post-combustion catalytic technologies include selective catalytic reduction (SCR) for lean-burn engines, nonselective catalytic reduction (NSCR) for rich-burn engines, and CO oxidation catalysts for lean-burn engines

Nonselective Catalytic Reduction (NSCR)
Nonselective Catalytic Reduction (NSCR) uses the residual hydrocarbons and CO in the rich-burn engine exhaust as a reducing agent for NOx. In NSCR, O2 and NOx oxidize hydrocarbons and CO. The excess hydrocarbons, CO, and NOx pass over a catalyst (usually a noble metal such as platinum, rhodium, or palladium) that oxidizes the excess hydrocarbons and CO to H2O and CO2, while reducing NOx to N2. NOx reduction efficiencies are usually greater than 90 percent, while CO reduction efficiencies are approximately 90 percent. The NSCR technique is effectively limited to engines with normal exhaust oxygen levels of 4 percent or less because the catalyst will not function properly at higher oxygen levels. This includes 4-stroke rich-burn naturally aspirated engines and some 4-stroke rich-burn turbocharged engines. Engines operating with NSCR require tight air-to-fuel control to maintain high reduction effectiveness without high hydrocarbon emissions. To achieve effective NOx reduction performance, the engine may need to run with a richer fuel adjustment than normal. The excess oxygen in the exhaust would probably be closer to 1 percent.

Selective Catalytic Reduction (SCR)
Selective catalytic reduction is a relatively expensive post-combustion technology that has been shown to be effective in reducing NOx in exhaust from lean-burn engines. An SCR system consists of an ammonia storage, feed, and injection system, and a catalyst and catalyst housing. Selective catalytic reduction systems selectively reduce NOx emissions by injecting ammonia (either in the form of liquid anhydrous ammonia or aqueous ammonium hydroxide) into the exhaust gas stream upstream of the catalyst. Nitrogen oxides, NH3, and O2 react on the surface of the catalyst to form N2 and H2O. For the SCR system to operate properly, the exhaust gas must be within a particular temperature range (typically between 450°F and 850°F). The catalyst determines the temperature range. Exhaust gas temperatures greater than the upper limit (850°F) will pass the NOx and ammonia unreacted through the catalyst. Ammonia emissions, called NH3 slip, are a key consideration when specifying a SCR system. SCR is most suitable for lean-burn engines operated at constant loads, and can achieve control efficiencies as high as 90 percent. For engines that typically operate at variable loads, such as engines on gas transmission pipelines, an SCR system may not function effectively, causing either periods of ammonia slip or insufficient ammonia to gain the reductions needed.

CO Oxidation Catalysts
Catalytic oxidation is a post-combustion technology that has been applied, in limited cases, to oxidize CO in engine exhaust, typically from lean-burn engines. Lean-burn technologies may cause increased CO emissions. 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 CO2 at efficiencies of approximately 70 percent for two-Stroke Lean Burn engines and 90 percent for four-Stroke Lean Burn engines.