SNCR is a post-combustion NOx control process developed to reduce NOx emissions from fossil-fuel combustion systems. SNCR processes involve the injection of a chemical containing nitrogen into the combustion products, where the temperature is in the range of 1600°F – 2200°F (870°C – 1205°C). In this temperature range, the chemical reacts selectively with NOx in the presence of oxygen, forming primarily nitrogen and water. Although a number of chemicals have been investigated and implemented for SNCR NOx reduction, urea and ammonia have been most widely used for full-scale applications.

In understanding the NOx reduction performance potential of SNCR, it is important to recognize that its performance is not only a function of process chemistry, but also of furnace parameters. When applying SNCR to a utility boiler, the furnace essentially becomes the chemical reactor for the process. This presents challenges not encountered in systems where one has more freedom to design the chemical reactor to meet the process requirements. Although the SNCR processes superficially appear simple, implementation of these processes entails a number of challenges. These challenges arise primarily due to the relatively narrow temperature "window" over which the chemicals selectively react with NOx. SNCR has the capability of NOx reductions in the range of 30-60%, depending on the specific retrofit application. Since catalysts are not involved, equipment costs are considered to be relatively low compared to other post-combustion NOx control technologies.

Although the SNCR process has many attractive features, it does have several disadvantages. One drawback is the relatively narrow temperature window (i.e., 1600°F to 2200°F; 870°C-1205°C) over which the process is effective. Another disadvantage is the possible emission, at least under some operating conditions, of undesirable by-products, such as NH₃, CO, or N₂O. Reactions between SO₃ (prevalent in boilers utilizing high sulfur coal) and NH₃ resulting in air preheater deposition can be a major balance-of-plant impact. To date, it is not always possible to assess all of these issues a priori, due to the complexity of the interaction of the SNCR process and several basic boiler design features (e.g., boiler flue gas path, temperature-time history, physical access, available residence times, and gas path velocities).

The overall goal of the project was to demonstrate the technical feasibility of applying the SNCR process to a large (600-MW) coal-fired utility boiler and to assess balance-of-plant impacts. The technical objective was to demonstrate an additional 30-percent NOx reduction (above that from the LNBs) across the load range with urea-based SNCR while maintaining acceptable levels of ammonia slip and balance-of-plant impacts. AEP Cardinal Unit 1 was retrofit with Fuel Tech’s urea injection system, the NOx OUT^® Process, in December 1998. The project was structured to assess the environmental and boiler performance impacts of the SNCR process. Key issues addressed included:

• NOx removal efficiency
• By-product emission characteristics (e.g., NH₃ slip, N₂O and CO)
• Balance-of-plant performance impacts

The test program was divided into two phases. Following construction and startup of the SNCR system, the first phase of testing – system optimization – determined the SNCR parameters to be used during automatic operation. This effort comprised 226 tests conducted over the period from March 15, 1999 through April 27, 1999. Once the optimization tests were completed, Fuel Tech, Inc. then programmed into the PLC the selected SNCR parameters to be used over the load range. Phase II consisted of automatic operation of the SNCR system while the boiler was under normal load dispatch. This long-term demonstration, performed for nominally six weeks, demonstrated the day-to-day NOx reductions of 30% were achievable across the load range tested, and documented balance-of-plant impacts, including air preheater deposition and NH₃ absorption on ash.