Integrated System to Reduce Emissions from Natural Gas-Fired Reciprocating Engines — Performance Assessment of Amperometric NOx/O ₂ Sensors

Emission reduction from Natural gas-fired reciprocating engines (NGFREs) is of high priority to regulatory agencies due to their significant contribution to overall pollutant emissions. NGFREs are well-known for their simplicity and are designed mainly to work at specific Air-to-Fuel ratios (AFRs). The AFR has been used as an effective parameter to control emissions in modern engines. However, such AFR control systems are not present in many NGFREs. To solve this challenge, a novel, cost-effective retrofit kit is proposed here that can be integrated with typical NGFREs to control AFR and improve their performance and emissions at a wide range of operating conditions. The retrofit kit comprises an air bypass mechanism combined with a control unit and amperometric sensors adapted from the automobile industry. The amperometric sensors operate in the limiting current region, which enables the measurement of a wide range of NOx/O₂ concentrations. This paper thoroughly describes these sensors and presents models that are developed to map their performance. The models incorporate electrode polarization, which includes activation, ohmic and concentration losses as well as diffusion mechanism through the diffusion barriers. The characteristic current-voltage (I-V) curves for different mole fractions of NOx/O₂ are generated successfully using Engineering Equation Solver (EES) software. The models are employed to show the sensor performance as a function of various parameters such as exhaust gas temperature, cell resistance, kinetic parameters, and diffusion coefficient. Such a model is an essential tool to estimate the electrochemical performance of NOx/O₂ sensors. It will be employed in the subsequent phases of this study to develop an appropriate control algorithm for the effective management of the retrofit kit mentioned above. The sensor is intended for calibration at very high excess O₂ in the following phases of our study, as described in this paper.