SCR (Selective Catalytic Reduction) is a technique invented by Englehard Corp in 1957 as a method to reduce NOx emissions in exhaust gasses by converting the NOx to H2O and N2. It has been adapted to many applications including modern diesel engines and is used both in large scale power plants burning coal or oil and in Marine vessels. Ammonia or urea is injected into the exhaust gas or flue gas with a catalyst. NOx molecules are chemically reduced into molecular nitrogen and water vapor.
Ammonia slip an industry term that refers to the excess ammonia that is present in the final exhaust or “left over” from the SCR process . This happens for three reasons
- Added ammonia is never entirely consumed or too much is added
- Catalyst temperatures are not in the optimum range (too low) or the catalyst has degraded
- Too much ammonia is used
Some additional considerations:
- Range: 1 ppm up to 200+ ppm, with 5-20 ppm a typical measurement
- Release of NH3 may not be regulated or even reported
- Unfortunately, the Analyzer Shop does not control the rate of ammonia injection
- Process operators are often unconcerned about ammonia slip
The Excess ammonia can cause many problems for the CEMS sample conditioning system, including damaging downstream components. We have found all of these problems at our customers as these byproducts form in downstream sample lines.
Unwanted Consequences of Ammonia Slip
#1. NH3 is a catalyst in the formation of SO3 from SO2 and O2.
- As vapors condense, you also get: SO3 + H2O → H2SO4 (Sulfuric acid).
#2. NH3 + H2SO4 → NH4HSO4 (ammonium bisulfate – ammonia salts)
- due to sulfur content of the fuel source
- white and powdery when pure, it forms “large rhombic prisms” as it condenses
- collects and fouls solid surfaces in flues, probes, filters, sample lines and inside analyzers
- forms in tight spaces, eg. O2 analyzer paramagnetic mirror
#3. Conversion of NH3 back to NO
- especially with high temp converters, but also can happen at lower temperatures
- will artificially inflate NOx readings
- when every PPM counts, can cause a failed RATA
- are you paying for NOx, but not NH3, emissions?
Perma Pure offers a range of Ammonia Scrubbers to protect components from these unwanted consequences, and are used with Cooler/Chiller based Sample Conditioning Systems and Nafion based CEMS Sample Conditioning Systems.