As with sulphur dioxide, it is possible to remove NOx after they have been formed in the flue gas of a power plant. The process involves the injection of either ammonia gas or urea into the flue gas stream. The chemical reacts with the NOx present, converting them into nitrogen and water.
If the ammonia or urea is injected into the hot flue gas stream, where the temperature is between 870°C and 1200°C, the reaction will occur spontaneously. This is called selective non-catalytic reduction or SNCR. At lower temperatures, however, a special metal catalyst is necessary to stimulate the reaction process. Where a catalyst is necessary, the process is called selective catalytic reduction or SCR.
SNCR will remove between 35% and 60% of the NOx from the flue gas stream. The technology has been demonstrated in a number of power plants in the USA and Germany. Nevertheless some technical issues remain to be resolved. Ammonia contamination of ash and ammonia slip, the release of unreacted ammonia into the atmosphere, are both potential problems.
An SCR system operates at a lower temperature than an SNCR system. Typical flue gas temperatures are 340-380°C. At these temperatures the reaction between ammonia and NOx must be accelerated by use of a solid catalytic surface. This is normally made from a vanadium-titanium material or a zeolite. The system is generally capable of removing 70-90% of the NOx emissions from a flue gas stream.
There are two drawbacks to SCR. First, it can only be used with low sulphur coals (up to 1.5% of sulphur) and secondly it is expensive. The catalyst also requires changing every 3-5 years. Even with low sulphur coal, SCR can lead to the formation of sulphur trioxide which becomes highly corrosive on contact with water when it forms sulphuric acid. Strategies are being developed to capture sulphur trioxide.
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