Selective catalytic reduction
Selective catalytic reduction (SCR) uses a catalyst and a reductant, usually ammonia, to convert nitrogen oxides in exhaust into nitrogen and water.
Ammonia: A nitrogen-and-hydrogen compound used as a refrigerant, fertilizer feedstock, and industrial chemical. SCR uses ammonia as the reductant that supplies hydrogen and nitrogen for NOx conversion.
Diesel exhaust fluid: A standardized aqueous urea solution injected into diesel exhaust-treatment systems. It is the common onboard reductant source for mobile diesel SCR.
Exhaust gas recirculation: An engine strategy that routes some exhaust back into the intake to reduce combustion temperatures and nitrogen-oxide formation. EGR suppresses NOx formation inside the engine, whereas SCR treats it after combustion.
Nitrogen oxides: A group of reactive gases, especially nitric oxide and nitrogen dioxide, produced by combustion and other processes. SCR targets these pollutants, which contribute to smog, acidification, and health harms.
Urea: An organic compound often used in aqueous solution as a source of ammonia. Diesel systems commonly inject urea, which decomposes to form SCR's active reductant.
Diesel engine: An internal-combustion engine that ignites fuel by compressing air to high temperatures. SCR is widely used to control nitrogen oxides from diesel exhaust.
Lean NOx trap: An exhaust catalyst that stores nitrogen oxides during lean operation and periodically reduces them during rich operation. It controls NOx by storage and regeneration rather than continuous reductant dosing.
Acid rain: Precipitation made unusually acidic by atmospheric pollutants, especially sulfur and nitrogen compounds. Reducing NOx emissions helps limit nitrogen-derived acid deposition.
Vanadium(V) oxide: An inorganic oxide used as a catalyst in oxidation and reduction reactions. Vanadium-based catalysts promote NOx reduction in many stationary SCR installations.
Selective catalytic reduction in power plants: SCR systems installed in stationary combustion plants to reduce nitrogen oxides in flue gas. Large boilers use SCR to meet emissions limits while treating high-volume exhaust.