Ammonia slip is actually the controlling factor in the operation of an SCR unit. In reality almost any installation can achieved any desired rate of NOx reduction if ammonia slip level is ignored. In practice, though, this is an unacceptable option. Ammonia slip must be limited due to downstream impacts associated with corrosion, fouling, or ammonia levels on ash, or in the case of gas-fired applications, ammonia slip is limited by regulatory limits. Several factors affect the ammonia slip including the original system design, amount of catalyst present, rate of catalyst deactivation, NOx and ammonia distributions at the inlet and within the reactor, and the operating conditions of the unit (including flow rate, temperate, and deNOx rate). For these reasons, the tracking of ammonia slip is an important tool in assessing the long-term performance of your SCR system.

For coal-fired facilities, ammonia slip can be tracked using measurements of the amount of ammonia that is adsorbed on the fly ash. This method is frequently used in Europe as a tool in assessing both performance of the SCR system as a whole (and is especially useful in flagging sudden adverse changes in operation), as well as tracking long-term catalyst deactivation. However, particular attention must be made as to the sampling process itself, and the frequency of sampling. Ash ammonia concentrations are know to vary greatly with ash sampling location and are particularly prone to show wide variations over time, even for very similar operating conditions. Thus, a large number of measurements must be made, over a relatively long period of time to fully establish the ammonia slip trend for a particular SCR installation. For instance, measurements may be required on a once per shift basis for several weeks or even months at similar operating conditions to fully establish an ammonia slip trend. Only with this long term tracking can the gradual effects of catalyst deactivation be seen. However, sudden sustained high levels of ammonia on ash may indicate an operational problem with the SCR or with the control system.

W. S. Hinton & Associates has a great deal of experience with the general phenomenon of ammonia adsorption on fly ash, as well as the sampling of ash and tracking of ammonia slip. We can assist in developing a comprehensive SCR tracking program which will insure that you achieve consistently optimal rates of NOx removal, while minimizing adverse impacts associated with ammonia slip.

See Also
Ammonia on Fly Ash and Related Issues
Catalyst Development
Selective Catalytic Reduction Process

 

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