Applications of Dithering Control for NSCR Catalysts
Control of exhaust pollutants from stationary, reciprocating engines by the use of three-way catalysts, i.e., Non-Selective Catalytic Reduction (NSCR) systems, remains an area of continued study. Research by the manufacturers of catalytic converters into catalyst chemistry as well as development work by manufacturers of air/fuel ratio controls has resulted in practical systems capable of achieving very low pollutant levels for long periods without requiring operator adjustment. One air/fuel ratio control method that affects the design of both the catalyst and the controls system requirements is dithering. Dithering, a rapid and intentional periodic fluctuation of the pre-catalyst air/fuel ratio set point, has been utilized in stationary natural gas engine controls for many different purposes. This sometimes misunderstood control method has pronounced and important influences on catalyst behavior, including the stabilization of post-catalyst chemistry during steady-state and transient events. This paper presents a basic description of the functioning of an NSCR catalyst, including an overview of catalyst chemistry and the mechanism of oxygen storage and release. In addition, it presents some rules for catalyst sizing and reviews how catalyst composition and capacity and details such as space velocity and cell density all interact and react to a dithering air/fuel control algorithm. This is followed by a presentation of dithering methods, control loops and required hardware using some real-world examples from the natural gas industry. Finally, laboratory and field test data are presented illustrating (1) applications of dithering and the resulting improvements in control fidelity, (2) continuous compliance with pollutant limits, and (3) improvements to the emissions control system robustness.