Hybrid SNCR-SCR Systems

In 1996, Applied Utility Systems, Inc. (AUS) was issued a U.S. Patent (copy available from AUS upon request) on the integration of SCR and SNCR in a low NO_x system. Most recently, the same patent has been issued in Germany, Italy, Holland, and Sweden.

The teaching of the AUS patent differs significantly from other competing patented processes. In competing processes, excess reagent is injected in the SNCR stage causing excess NH₃ to break through. This NH₃ is then used to reduce NO_x in the SCR stage. The deficiency of such teachings includes the following:

• An excessive amount of reagent must be used in the SNCR stage to produce modest levels of NH₃ for the SCR stage. Only 25 percent of excess reagent produces NH₃ for the SCR stage. This causes waste of reagent and a substantial increase in operating cost.
  


• NH₃ breakthrough at the discharge of the SNCR stage is generally not uniformly distributed. As a matter of fact, existing data suggest that significant stratification of NH₃ concentration would exist. Such stratification greatly compromises the effectiveness of the SCR system.
  


• In the teachings of competing technology, the SNCR process is used as the primary NO_x control technology. This leads to poor reagent utilization, since SNCR utilization efficiency of reagent is, at best, 50 percent compared to values approaching 100 percent with the use of a SCR system. Injecting excess reagent in the SNCR stage further reduces utilization to less than 25 percent.
  


• Selecting the SNCR process as the primary NO_x control technology requires the technology to be load following. Since the SNCR is very sensitive to products of combustion temperature, the requirement to design a load-following SNCR system adds a great deal of cost and complexity to the operation of this system. It also compromises the SNCR system NO_x removal capability.

The AUS patented process addresses the above deficiencies by selecting the SCR system to be the primary NO_x control measure. At low boiler load, the SCR system becomes the sole NO_x removal system for several reasons. At low loads, the products of combustion volume are also low allowing the SCR system to provide high NO_x control. Not only can significant NO_x removal be obtained by the SCR system at a lower load, but also the reduction in NO_x emissions is provided with full utilization of reagent.

At high load, the SNCR process is triggered to augment the NO_x reduction provided by the SCR system. As such, the SNCR system is required to operate in a narrow load band near full boiler load. Restricting the operation of the SNCR process to a certain narrow load range eliminates the requirement for having the system be load following. This provides substantial capital cost savings and offers a reduction in operation and maintenance costs.

Lastly, because the SNCR is only used as trim for the total NO_x emissions achieved by the integrated system, low NO_x to NH₃ mole ratio can be used in the SNCR stage. In the AUS process, no excess reagent is required to be injected in the SNCR stage to produce NH₃ breakthrough for the SCR stage. Instead, an NH₃ injection grid designed to provide uniform distribution of NH₃ in products of combustion is installed to supply NH₃ to the SCR stage.

Therefore, an effort is made to minimize rather than promote NH₃ breakthrough from the SNCR stage. This protects against having the NH₃ breakthrough from the SNCR stage and distort NH₃ distribution at the inlet to the SCR stage. It also allows high utilization of NH₃ in the SNCR stage. With the above in mind, the NO_x removal of the SNCR in the AUS patent is enhanced rather than diminished. The SNCR would be designed for a specific temperature range, used only at high initial NO_x concentration with reagent to NO_x mole ratio selected to maximize utilization.