The vast majority of anhydrous ammonia in power plants is used to reduce harmful nitrogen oxide (NOx​) emissions, which contribute to smog and acid rain. This is achieved through a technology called Selective Catalytic Reduction (SCR).
Selective Catalytic Reduction (SCR)
In fossil-fuel power plants (coal, gas, oil), combustion creates NOx​. The SCR system injects anhydrous ammonia into the hot exhaust gas (flue gas) before it passes through a specialized catalyst.
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The Process:
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Anhydrous ammonia is stored, often as a compressed liquid, and then vaporized into a gas.
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The ammonia gas is injected into the stream of hot flue gas.
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The mixture passes over a catalyst (typically made of vanadium, titanium, or tungsten).
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The catalyst causes the ammonia to selectively reduce the NOx​ molecules.
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The Reaction: The NOx​ is converted into harmless, naturally occurring substances:
4NO+4NH3​+O2​ Catalyst→ ​4N2​+6H2​O
(Nitric Oxide + Ammonia + Oxygen → Nitrogen + Water)
6NO2​+8NH3​ Catalyst → ​7N2​+12H2​O
(Nitrogen Dioxide + Ammonia → Nitrogen + Water)
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Result: SCR systems using ammonia are highly effective, often removing over 90% of the NOx​ emissions.
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Alternative Reductants
While anhydrous ammonia is the most effective and often preferred reagent for large utility boilers, some plants use:
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Aqueous Ammonia (NH4​OH): Ammonia dissolved in water; safer to handle but less concentrated.
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Urea (CO(NH2​)2​): A less hazardous solid that must be thermally converted to ammonia before injection.
