Background
It is well known that NO and NO2 (NOx) are emitted from internal combustion engines as a byproduct of combustion, but the NO2:NOx ratio produced can vary depending on engine specifications and operating conditions. For example, stoichiometric gasoline engines produce nearly all NO, while diesel engines may produce varying NO2:NOx ratios depending on the given operating condition.
Southwest Research Institute (SwRI) utilizes the burner-based Exhaust Composition Transient Operation Laboratory™ (ECTO-Lab™) to replicate exhaust profiles produced by light and heavy-duty engines for aftertreatment aging and evaluations. This system can generate and dose NOx over transient cycles from a range of 20 ppm to 1200 ppm where the NOx is generated by the in-situ decomposition and combustion of a nitrogen containing fuel. During the combustion and decomposition of this fuel, over 95% of the NOx generated is in the form of NO. To accurately evaluate aftertreatment systems and simulate real engine exhaust is necessary to account for the distribution of NO and NO2 within the exhaust stream. Since previous work has established that the decomposition of nitric acid can be utilized as a method to generate NO2, the objective of this project was to develop control of NO and NO2 within SwRI's ECTO-Lab through the decomposition of nitric acid (HNO3). In addition to this, a system was built capable of generating NO2 and NO independently of the ECTO-Lab via nitric acid decomposition.
Approach
In this program, a baseline HNO3 decomposition system was developed, and steady-state experiments were performed to determine the kinetic parameters associated with nitric acid decomposition reactions. The kinetic experiments conducted included the determination of the activation energy and reaction order associated with HNO3 decomposition. It was expected that the reaction rate would be first order with the rate increasing with increasing HNO3 concentration, but it was found that at low concentrations the reaction rate did not increase with increasing HNO3 concentration at equivalent residence time. This indicated that the rate of evaporation of HNO3 was being measured, rather than the reaction rate of HNO3 decomposition. To assist in increasing the rate of reaction, a pre-heater was added to the system and experiments were performed to determine the optimum HNO33 flow rate, carrier N2 flow rate, and oven temperature required to minimize HNO3 slip and carrier N2 required. The optimum conditions were found, where an oven temperature of 475 °C, a N2 flow of 10 l/min, and 15 l/min HNO3 flow are required to maximize the NO2:NOx ratio at 0.90 and minimize the HNO3 slip to 2 ppm. Once the kinetic experimentation was complete, the baseline HNO3 decomposition system was modified for transient operation. The Rachford-Rice Method and Antoine's Equation were used to predict the vapor/liquid equilibrium compositions of the NO2/H2O mixture that results from HNO3 decomposition. It was found that the separation was inefficient, and when 90% of the vapor stream is NO2, only 5.1% of the total feed was in the vapor stream and 86.1% of the NO2 was lost in the liquid waste stream. Because of this, it was determined that the products of the HNO3 decomposition would be stored in the vapor phase to maximize NO2 utilization. This lead to the development of a steady state NO2:NOx control system capable of decomposing nitric acid to either NO or NO2 which is then injected into the ECTO-Lab exhaust stream.
Accomplishments
It was determined that NO2:NOx control can be achieved by varying the nitrogen containing fuel flow rate and nitric acid flow rate. It was determined that total NOx control was approximately ± 20 ppm with a target NOx setpoint of 1000 ppm, and the gas phase HNO3 concentration was maintained below 3 ppm for all operating points. The maximum NO2:NOx ratio achieved was 90 %, and the minimum was 2%. This proves that when utilizing the HNO3 decomposition system, the ECTO-Lab has the capability of controlling the NO2:NOx distribution at steady-state. The HNOx decomposition system also underwent experimentation to demonstrate NO generation, without the use of the nitrogen containing fuel. It was found that the HNO3 decomposition system could maintain a NO concentration of approximately 240 ppm when the total NOx was 250 ppm.