Ammonia Slip Monitoring for NOx Control

Burning coal and other fossil fuels is widely used for generating electricity or thermal heat, but because of the combustion process, particulate matter and nitrogen oxides (NOx) are released into the atmosphere.

NOx and dust are the leading cause of human respiratory issues and government agencies worldwide are releasing stronger regulatory measures designed to reduce air pollution. In countries like the United States, offer NOx trading credits for excess reduction, making it more valuable than even fuel savings to a plant owner. Even if NOx credits are not available, prolonging the lifetime of the NOx reduction equipment also makes economic sense as it greatly reduces operating costs.

The two main NOx control strategies are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR), both based upon the use of ammonia (NH3) or urea to participate in the suppression of NOx formation. SCR and SNCR both rely on accurate NH3 or urea dosing to reduce NOx formation.

NH3 overdosing (called NH3 slip) causes the formation of ammonium bisulfate (ABS) precipitation - a white powder that forms when sulfur trioxide (SO3) is present (especially when coal is burned). The ABS deposits on and eventually plugs the NOx reduction catalyst, but it can also cause fouling or corrosion of boilers or, for those power plants that sell their fly ash to the cement industry, it results in sub-standard product which is no longer fit for use.

Traditional NH3 slip process monitoring methods have in the past used extractive sampling techniques with an infrared (IR) based analyzer. A sample of the gas must be transported to the analyzer which results in a delay of up to 30 seconds in the reported NH3 value which makes this an unreliable process control trigger.

A carefully controlled NH3 slip of < 2ppm is needed to optimize the process and without a direct gas measurement it will be difficult as many parameters influence the process including the inlet NOx concentration, fuel composition and catalyst performance. Rapid and accurate monitoring of the NH3 slip saves millions by providing a feedback process loop that reduces NOx and ABS formation.