Final Report: Mixed-Potential Based Miniature Sensors for Real-Time On-Vehicle NOx Monitoring From Mobile SourcesEPA Contract Number: 68D03061
Title: Mixed-Potential Based Miniature Sensors for Real-Time On-Vehicle NOx Monitoring From Mobile Sources
Investigators: Nair, Balakrishnan
Small Business: Ceramatec Inc.
EPA Contact: Manager, SBIR Program
Project Period: October 1, 2003 through December 31, 2004
Project Amount: $224,979
RFA: Small Business Innovation Research (SBIR) - Phase II (2003) Recipients Lists
Research Category: Air Quality and Air Toxics , SBIR - Air Pollution , Small Business Innovation Research (SBIR)
Oxides of nitrogen, primarily nitric oxide (NO) and nitrogen dioxide (NO2) (jointly represented as NOx), constitute one of the primary classes of environmental pollutants. According to a report published by the U.S. Environmental Protection Agency’s (EPA) Office of Air Quality Planning and Standards in 1999, mobile sources generate 56 percent of the overall domestic nitric oxide (NOx) emissions. Both on-road mobile sources such as diesel trucks (34% of domestic NOx) and off-road mobile sources such as tractors and lawn mowers (22% of domestic NOx) can be significant polluters. To ameliorate these problems, the 1990 Clean Air Act Amendments were proposed that required major sources of air emissions to limit NOx emissions. Further emissions reduction regulations are expected in the near future. A number of promising NOx control technologies for engines have been developed in recent years, such as NOx adsorbers, selective catalytic reduction (SCR) with injection of urea/ammonia into the inlet gas stream, exhaust gas recirculation (EGR) to decrease combustion temperatures, and oxygen-enriched air injection (OEAI) technology. For efficient application of these techniques for NOx reduction, monitoring of the NOx concentration generated by combustion is absolutely critical.
The information obtained by online NOx monitoring devices can be fed back into the process control system so that process variables such as inlet concentrations of ammonia/urea in SCR, the volume of exhaust gas recirculated in EGR, and the oxygen/air ratio in OEAI can be adjusted to account for time-dependent fluctuations, thereby minimizing overall NOx emissions. In addition to the obvious requirement of miniaturized size of the sensor so as to be easily incorporated into exhaust tubing, there are other requirements that need to be met before the use of NOx control systems can be practical. The sensor must have a fast enough response time to be useful as part of a control system (i.e., less than 1 second). The sensor must be sensitive to fluctuations in NOx concentration; engine manufacturers typically demand sensitivity down to 15 ppm of NOx. Further, the sensor signal should have no cross-interference with other expected components of the exhaust stream such as water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2), and sulfur dioxide (SO2). Current sensors cannot meet all of these demanding requirements.
This research project addressed the pending need for the development and commercialization of miniature sensors that have fast response and high-sensitivity to NOx, low cross-sensitivity to other species, and can perform for extended periods in engine exhaust from mobile sources. Such sensors will help EPA monitor emissions and enforce emissions regulations for both on-road and off-road mobile sources.
The goal of this research project was to develop robust sensors that can perform under the operating conditions expected in engine exhaust using ceramic NOx sensor technology currently being developed at Ceramatec, Inc. The approach involved developing a mixed-potential based ceramic electrochemical sensor to provide the necessary combination of robustness, sensitivity, and fast-response time required of a NOx sensor used for monitoring and process control in mobile sources. A mixed-potential sensor requires an oxide electrode material that does not promote the equilibration of the NOx gases at the electrode/electrolyte interface. It is the non-equilibrium condition at this interface that generates the mixed potentials. The specific focus was on development of sensors for monitoring NOx in diesel engine exhaust, but the technology developed also is relevant for gasoline engines and coal-based power generation systems. The work included materials development and optimization, system engineering, detailed sensor testing, and characterization. A leading diesel engine manufacturer participated in the project and provided testing support from an end-user perspective by testing the sensors under simulated operating conditions.
The development efforts were highly successful at accomplishing the technical objectives. Through this project, some of the key problems that have been associated with mixed-potential sensors for a long time have been overcome through innovative engineering solutions. One of the main requirements for a NOx sensor is that it be capable of operating in the temperature range typically found in an engine exhaust stream (i.e., 500-600°C). Results showed that the ceramic-based mixed-potential sensor is suitable for operation in the temperature range required for automotive and diesel engine applications. The exact temperature of operation needs to be optimized while considering other system parameters such as response time and cross-sensitivity effects.
Another major challenge associated with mixed-potential sensors for measuring NOx concentrations is the cross-sensitivity between NO and NO2. It is commonly observed that the voltage signal for these two gases is of opposite sign, thereby making it impossible to establish a meaningful relationship between output voltage signal and total NOx concentration. Through system engineering, Ceramatec demonstrated a sensor system in which regardless of the gas input NO/NO2 ratio, the sensor voltage signal is proportional to the total NOx concentration.
Conventional mixed-potential sensors also have problems of cross-sensitivity to other species encountered in engine emissions. A series of tests were performed to study the effect on the sensor voltage response when exposed to other gases typically found in an exhaust gas stream. CO2 and water vapor had no intrinsic response on the sensing electrode. It was determined, however, that the electrode had sensitivity to carbon monoxide (CO) and sulfur dioxide (SO2) in addition to NOx. Through engineering design of the sensor, a strategy was developed to mitigate this cross-sensitivity and demonstrate no cross-sensitivity to up to 1,500 ppm of CO and 15 ppm of SO2. Intermediate-term exposure of the sensor to high levels of sulfur (15 ppm) also was carried out, and it was determined that exposure to sulfur resulted in a change in baseline voltage of the sensor, but the sensor still performed effectively as a NOx sensor. Strategies were identified to mitigate the effect of sulfur on the sensor, and to regenerate the sensor in the event of over exposure. A strategy and method also were developed to mitigate ammonia cross-sensitivity in the sensor. Further, the sensor response is a function of oxygen concentration in addition to NOx concentration; this was addressed by integrating an oxygen sensor into the system. Thus, by accurately measuring the oxygen concentration and monitoring the response of the NOx sensor, an accurate NOx concentration can be measured.
Because the sensor is designed to be part of an emissions control system that would reduce the NOx emissions to a significantly lower level than current levels, several end-users suggested that it would be important to demonstrate sensitivity to very low NOx levels on the order of 1 ppm. Therefore, sensor testing was performed at very low NOx levels of 1-20 ppm. It was found that, as expected due to the logarithmic nature of the response, the sensitivity to low levels of NOx was very high and NOx levels as low as 1 ppm could be detected accurately.
As a final step in Phase II, Ceramatec integrated and miniaturized the sensor into a small device that could be threaded into an exhaust port and tested in simulated engine exhaust. This device was installed at an emissions test facility of a major diesel engine manufacturer and the sensor performance was evaluated. The sensor performed very well in this test, demonstrating no cross-sensitivity to CO and CO2, and working well in determining both oxygen and NOx concentrations.
The new sensor technology, which is a mixed-potential sensor, has a number of significant advantages over other NOx sensor systems, and therefore represents an exciting technology for diesel and gasoline engine manufacturers to meet the demanding requirements of emissions control sensors. Other competing high-temperature sensors are amperometric sensors that depend on the use of a very small orifice that acts as a gas-limiting element that controls the amount of gas that can enter the pumping chamber. To make these amperometric sensors reproducible from one to the next, the size of this orifice has to be very accurately controlled. In addition, the orifice can become plugged with time or simply change geometrical dimension through long-term annealing or partial clogging of particles on the edges. Other advantages of a potentiometric mixed-potential type sensor over an amperometric sensor include very high sensitivity to low concentrations of NOx due to the logarithmic response curve, much lower power requirement for operation, and substantially lower design complexity.
During this research project, Ceramatec stayed in close contact with potential end users of the technology such as diesel engine and automobile manufacturers, as well as system integrators. A major U.S. diesel engine manufacturer participated in the study, and Ceramatec currently is negotiating a significant Phase III program funded by this diesel engine manufacturer for prototype development, scale up, and commercialization of the sensor technology. It is anticipated that the sensor will be in commercial production well in time to address the 2009 EPA emissions requirements.
Because these negotiations were underway during the Phase II project, Ceramatec requested Foresight Science and Technology to perform a commercialization study focused on other applications of NOx sensors such as coal. Foresight identified a number of targets, including an Australian oxygen sensor manufacturer that is interested in entering the NOx sensor market. Ceramatec currently is negotiating a technology transfer agreement with this company for NOx sensors to be used in stationary power generation applications.