Ammonia Scrubber for Dilution Probes
Ammonia is becoming a very common topic when discussing CEMS. Most new plants built today utilize SCR’s for NOx control and many existing plants are being retrofitted or will be adding SCR’s in the near future. Ammonia (NH3) is a very sticky and reactive gas and brings a unique set of problems to any device or piece of instrumentation exposed to it.
One of the problems present when monitoring for gaseous emissions after an SCR, or on the inlet of an SCR, is the NH3 present in the gas stream will react with the SO2 present to form a white particulate. This material coats the inside of a probe, sample lines, and even the inside of the analyzers over a period of time. This build-up can cause false readings and adds to the maintenance required for the system.
The best approach for handling this problem is to remove the NH3 from the sample stream as early as possible in the sample handling system (i.e. probe, umbilical, scrubbers, dryers, etc.). This can be accomplished with a special mantle for an in-stack dilution probe, which removes the NH3 before it enters into the system. Scrubbing the NH3 in-situ as opposed to after the dilution takes place offers many benefits:
1. By removing the NH3 before the critical orifice, build-up occurring inside the probe and analyzers can be prevented.
2. NH3 scrubbing media works best at an elevated temperature. By placing the scrubbing media in the gas stream, the need for added heater blankets to heat the media’s canisters is eliminated.
3. The NH3 is scrubbed at the highest concentration. This helps to minimize any breakthrough from the media.
4. Even when the feed rate of NH3 is closely controlled, there are periods of time when the level present can be high enough to cause problems. It is much easier to replace the scrubber media when doing probe maintenance than to have the probe sent out for cleaning.
The scrubbing of NH3 before the critical orifice also helps to prevent build-up from occurring on the quartz wool and then reacting with the SO2. This can lead to false readings and calibration drift problems.
Predictive or Direct Ammonia measurement?…
One question that comes up frequently when discussing ammonia measurement with SCR’s is whether to measure the ammonia directly (utilizing an NH3 converter, dedicated umbilical, and special analyzer), or predict ammonia slip utilizing two NOx monitors – one up and one downstream of the SCR.
While the later method is sometimes utilized, it’s not used as often because of naturally inherent problems. The term used for NH3 is " NH3 Slip ". This term “slip” refers to the excess NH3 present in the gas stream after the SCR. A large portion of the NH3 comes from bypassing or "slipping" around the catalyst.
The ammonia that passes around the catalyst cannot be predicted by measuring NOx. All that one can do is assume some efficiency value for the catalyst and that all the NH3 contacts the catalyst. This is the only way of predicting the NH3 levels using two NOx analyzers and does not provide very accurate or reliable results. At the present time, converting the NH3 to NO, measuring it, and then subtracting the NOx readings is the best way to measure NH3.
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