Urea-based Selective Catalytic Reduction Systems Research Articles

Experimental study on pollutant emission characteristics of diesel urea-based selective catalytic reduction system based on corrugated substrate

In this study, a corrugated substrate of diesel engines for urea-based selective catalytic reduction (urea-SCR) with V2O5-WO3/TiO2 (V-SCR) and Cu-based (Cu-SCR) catalysts is proposed and the effects of the ammonia-to-nitrogen ratio α on the NOx emission, NH3 slip, and N2O emission were experimentally investigated on a diesel engine test bench with the World Harmonized Transient Cycle (WHTC). The volume ratio of the catalyst to the engine capacity is 1.6. With the increase of α, the NOx reduction reaction of V-SCR performed better NOx emission characteristics than that of Cu-SCR. When α increased from 1.0 to 1.2, the NOx emission of V-SCR and Cu-SCR decreased from 0.44 to 0.26 g/kW·h and from 0.63 to 0.36 g/kW·h, respectively. However, the NH3 slip of Cu-SCR was less because of its absorption capacity of NH3. The NOx conversion efficiency β and N2 selectivity δ of V-SCR outperformed those of Cu-SCR. At α = 1.1, β and δ of V-SCR were 1.07% and 3.25% higher than those of Cu-SCR, respectively. The dynamic response of V-SCR to NH3 showed high competitiveness compared to Cu-SCR. It was concluded that, based on the corrugated substrate, the V-SCR showed better pollutant emission characteristics than did the Cu-SCR.

Mathematical Modeling of Urea Reaction with Sulfuric Acid and Phosphoric Acid to Produce Ammonium Sulfate and Ammonium Dihydrogen Phosphate Respectively

Urea is the final product of protein metabolism in mammals and can be found in different biological fluids. Use of mammalian urine in agricultural production as organic fertilizer requires safe handling to avoid the formation of ammonia that will decrease the fertilizer value due to the loss of nitrogen. Safe handling is also required to minimize the decomposition of urea into condensed products such as biuret and cyanuric acid, which will also have a negative impact on the potential sustainable production of crops and sanitation technologies. The study of thermodynamics and reaction kinetics of urea stabilization plays a key role in understanding the conditions under which undesirable compounds and impurities in urea-based fertilizers and urea-based selective catalytic reduction systems are formed. For this reason, we studied the reaction of urea in acid media to achieve urea stabilization by modeling the reaction of urea with sulfuric acid and phosphoric acid, and estimating the reaction enthalpy and adiabatic heat difference for control of the heat released from the neutralization step using Ca(OH)2 or MgO for the safety of the process. Numerical and simulation analyses were performed by studying the effect of the surrounding temperature, the ratio of acid reagent to urea concentration, the rate of addition, and the reaction rate to estimate the required time to achieve an optimum value of urea conversion into ammonium dihydrogen phosphate or ammonium sulfate as potential technological opportunities for by-product valorization. Full conversion of urea was achieved in about 10 h for reaction rates in the order of 1 × 10−5s−1 when the ratio of H2SO4 to CH4N2O was 1.5. When increasing the ratio to 10, the time required for full conversion was considerably reduced to 3 h.

Impact of Choice of Spray Inputs on Simulations of Urea-Based Selective Catalytic Reduction Systems

Impact of Choice of Spray Inputs on Simulations of Urea-Based Selective Catalytic Reduction Systems

Insights on the morphology of air-assisted breakup of urea-water-solution sprays for varying surface tension

The efficacy of NOx reduction in diesel engines is mainly dependent on how uniformly urea-water solutions (UWS) are dispersed onto the catalyst surface of the Selective Catalytic Reduction (SCR) systems. The urea-based SCR systems also suffer drawbacks due to the formation of urea deposits onto the walls of after-treatment devices due to poor atomization characteristics of UWS. In this work, the impact of lowering the surface tension of UWS on the morphology of UWS sprays was explored using high-speed shadowgraph imaging techniques. The surface tension of UWS was lowered by adding surfactants; two surfactants viz., Sodium Dodecyl Sulfate (SDS) and Dodecyl-Dimethyl-Amine-oxide (DDA) were considered in this investigation. The surface tension of UWS was reduced to a maximum from 73.7 to 30.2 mN/m and 39.8 mN/m with the addition of DDA and SDS respectively at 75% of its respective Critical Micelle Concentration (CMC) in UWS. Even at a very low-pressure difference of 500 mbar of co-flowing air, the surfactant-added UWS tends to break-up relatively closer to the nozzle tip due to flapping-induced bag breakup, which improved its drop-size distribution. Under a relatively higher pressure difference of 2000 mbar of co-flow atomizing air, the liquid breakup was mostly due to surface stripping in surfactant-added UWS sprays that generated a large number of fine droplets. The image analyses of sprays were performed at far downstream locations from the nozzle to quantify the variations of their droplet-sizes caused by varying the surface tension of UWS. The surfactants added UWS sprays revealed a considerably narrower drop-size distribution by up to 43% compared to UWS sprays under high-pressure conditions, and this was due to a combination of flapping-induced bag breakup, surface stripping and secondary atomization of big droplets caused by reducing the surface tension of UWS. Reducing the surface tension of UWS has the potential to improve NOx reduction in SCR systems due to the reduction in droplet sizes of UWS sprays and also to reduce the formation of urea deposits.

Estimation of Ammonia Storage Nonuniformity for Urea-Based Selective Catalytic Reduction Systems

Ammonia storage nonuniformity has a significant impact on the emission reduction performance of urea-based selective catalytic reduction (SCR) systems. In this paper, a unique SCR platform with two catalysts in a parallel configuration was created for investigating the impact of ammonia storage nonuniformity on the emission reduction performance in a simulation environment. The established two-cell SCR platform allows users to independently control the ammonia-to-NOx ratio (ANR) for each catalyst using two independent urea solution injectors. Simulation results over US06 cycle demonstrate that, compared to the case without ammonia storage nonuniformity, the tailpipe NOx and ammonia emissions can be increased by 6.73% and 22.0%, respectively, due to the nonuniform ammonia storage in the case of an ANR nonuniformity index (NUI) at 0.2. Furthermore, an innovative model-based method was proposed for estimating the ammonia coverage ratio nonuniformity (i.e., ammonia storage nonuniformity if storage capacity is known) by utilizing a control-oriented SCR model and the tailpipe NOx and ammonia measurements at the confluence point. Simulation results proved the effectiveness of the proposed method in estimating the ammonia coverage ratio nonuniformity.

4-Nitrophenol, 1-nitropyrene, and 9-nitroanthracene emissions in exhaust particles from diesel vehicles with different exhaust gas treatments

The dependence of nitro-organic compound emissions in automotive exhaust particles on the type of aftertreatment used was investigated. Three diesel vehicles with different aftertreatment systems (an oxidation catalyst, vehicle-DOC; a particulate matter and NOx reduction system, vehicle-DPNR; and a urea-based selective catalytic reduction system, vehicle-SCR) and a gasoline car with a three-way catalyst were tested. Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) and nitrophenols in the particles emitted were analyzed by thermal desorption gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry. The secondary production of nitro-organic compounds on the filters used to collect particles and the adsorption of gaseous nitro-organic compounds by the filters were evaluated. Emissions of 1-nitropyrene, 9-nitroanthracene, and 4-nitrophenol in the diesel exhaust particles were then quantified. The NOx reduction process in vehicle-DPNR appeared to remove nitro-hydrocarbons efficiently but not to remove nitro-oxygenated hydrocarbons efficiently. The nitro-PAH emission factors were lower for vehicle-DOC when it was not fitted with a catalyst than when it was fitted with a catalyst. The 4-nitrophenol emission factors were also lower for vehicle-DOC with a catalyst than vehicle-DOC without a catalyst, suggesting that the oxidation catalyst was a source of both nitro-PAHs and 4-nitrophenol. The time-resolved aerosol mass spectrometry data suggested that nitro-organic compounds are mainly produced when an engine is working under load. The presence of 4-nitrophenol in the particles was not confirmed statistically because of interference from gaseous 4-nitrophenol. Systematic errors in the estimated amounts of gaseous 1-nitropyrene and 9-nitroanthracene adsorbed onto the filters and the estimated amounts of volatile nitro-organic compounds that evaporated during sampling and during post-sampling conditioning could not be excluded. An analytical method in which all gaseous compounds are absorbed before particles are collected, and in which the volatile compounds are derivatized, would improve the precision and the accuracy of the data.

NOx Sensor Reading Correction in Diesel Engine Selective Catalytic Reduction System Applications

In this paper, we investigate NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor reading correction in diesel engine selective catalytic reduction (SCR) system applications. It is well known that the urea-based SCR system is promising to remove engine-out NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> emissions and achieve strict legislative emission regulations. However, if the urea is overdosed, the system also leads to ammonia slip in the tailpipe. In order to control the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> emissions and constrain the ammonia slip, a feedback loop with NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and ammonia sensors may be needed. An interesting phenomenon in the closed loop is that a well calibrated NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor does not work well in the urea-based SCR systems. For this typical application, there may be several sources for the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor reading mismatching issues. Due to the existence of the gaseous ammonia, the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor reading is approximately a combination of the actual NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> concentration and the ammonia concentration. Another phenomenon is that the influence of the ammonia concentration to the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor reading is significantly different with respect to different temperatures. Based on such observations and the importance of the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> concentration reading, we devote to develop an algorithm to correct the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor reading. With considerable experimental data which cover most of the typical SCR operating temperature range, we obtain the relationship between the cross-sensitivity factor and the temperature by employing the adaptive-network-based fuzzy inference system (ANFIS). The developed fuzzy inference system is then applied to experimental validation tests. Compared with the existing extended Kalman filter (EKF), the developed fuzzy inference system has prominent advantages on the reading correction performance.

Sliding-mode observers for urea selective catalytic reduction system state estimations based on nitrogen oxide sensor measurements

The state estimations for urea-based selective catalytic reduction systems have received growing interest in the past few years because of their importance to selective catalytic reduction controls and onboard diagnostics, as well as the high cost and difficulties of direct sensor measurements. Most of the existing estimation techniques suffer because of the ammonia cross-sensitivity from the nitrogen oxide sensor and require an ammonia sensor to be available downstream of the selective catalytic reduction, which may increase the costs of production urea-based selective catalytic reduction systems. The purpose of this study is to design observers for estimating the key states (the nitrogen oxide concentration, the ammonia concentration, and the ammonia coverage ratio) of the urea-based selective catalytic reduction system at the selective catalytic reduction outlet in various situations, using only a nitrogen oxide sensor. The sliding-mode observers were designed to ensure high robustness against the ammonia cross-sensitivity from the nitrogen oxide sensor and uncertainties from the selective catalytic reduction inputs. Simulation verification results under the Federal Test Procedure 75 test cycle showed that the proposed observers were able to estimate the states accurately, in hot and cold exhaust conditions, for both single-cell selective catalytic reduction systems and two-cell selective catalytic reduction systems. The proposed observer-based estimation technique has the potential for reducing the cost of advanced selective catalytic reduction control systems and for applying it to onboard diagnostics.

A novel cost-effective robust approach for selective catalytic reduction state estimations using dual nitrogen oxide sensors

Urea-based selective catalytic reduction systems have become a promising technique in controlling nitrogen oxide emissions from diesel engines in the past decade. One of the continuing issues is the cross-sensitivity characteristics of the current nitrogen oxide sensors to ammonia in the exhaust, which makes nitrogen oxide sensor readings inaccurate in the presence of ammonia. Although prototype ammonia sensors for mobile selective catalytic reduction applications have been introduced for diesel emission control in the past few years, ammonia sensors are still too expensive to be available for mass-produced diesel engines and selective catalytic reduction systems. In order to address these issues, in this paper, a novel, cost-effective, and robust approach is proposed for estimating the key selective catalytic reduction states including the nitric oxide concentration, the nitrogen dioxide concentration, the ammonia concentration, and the ammonia surface coverage ratio, by utilizing two post-selective-catalytic-reduction nitrogen oxide sensors with different ammonia cross-sensitivity factors. Experimental verification results show that the proposed approach has the potential for precisely estimating the four selective catalytic reduction states. Such a novel approach can be instrumental in reducing the cost of selective catalytic reduction diagnosis, selective catalytic reduction estimation, and advanced selective catalytic reduction controls.

Reactive nitrogen compounds (RNCs) in exhaust of advanced PM–NOx abatement technologies for future diesel applications

Long-term exposure to increased levels of reactive nitrogen compounds (RNCs) and particulate matter (PM) affect human health. Many cities are currently not able to fulfill European air quality standards for these critical pollutants. Meanwhile, promising new abatement technologies such as diesel particle filters (DPFs) and selective catalytic reduction (SCR) catalysts are developed to reduce PM and RNC emissions. Herein, effects of a urea-based SCR system on RNC emissions are discussed and we quantified the highly reactive intermediates isocyanic acid (HNCO) and ammonia (NH3), both potential secondary pollutants of the urea-based SCR chemistry. A diesel engine (3.0 L, 100 kW), operated in the ISO 8178/4 C1, cycle was used as test platform. A V2O5-based SCR catalyst was either applied as such or down-stream of a high oxidation potential-DPF (hox-DPF). With active SCR, nitric oxide (NO) and nitrogen dioxide (NO2) conversion efficiencies of 0.86–0.94 and 0.86–0.99 were obtained. On the other hand, mean HNCO and NH3 emissions increased to 240–280 and 1800–1900 mg h−1. On a molar basis, HNCO accounted for 0.8–1.4% and NH3 for 14–25% of the emitted RNCs. On roads, SCR systems will partly be inactive when exhaust temperatures drop below 220 °C. The system was active only during 75% of the test cycle, and urea dosing was stopped and restarted several times. Consequently, NO conversion stopped but interestingly, NO2 was still converted. Such light-off and shutdown events are frequent in urban driving, compromising the overall deNOx efficiency. Another important effect of the SCR technology is illustrated by the NH3/NO2 ratio, which was >1 with active SCR, indicating that exhaust is basic rather than acidic after the SCR catalyst. Under these conditions, isocyanic acid is stable. The widespread use of various converter technologies already affected RNC release. Diesel oxidation catalysts (DOCs) and hox-DPFs increased NO2 emissions, three-way catalysts (TWCs) those of NH3. The investigated SCR technology substantially lowered NO and NO2 emissions, while NH3 levels were comparable to those of TWC vehicles (300–1500 mg h−1). If applied in the future, the combined DPF/SCR technology will change ambient RNC levels, PM compositions and atmospheric redox- and acid/base-chemistry in traffic-affected areas.

Non-thermal plasma application to the abatement of noxious emissions in automotive exhaust gases

Experiments and numerical model calculations on non-thermal plasma treatment of lean combustion exhaust gases were reviewed. It was found that because of the oxygen concentration of several per cent, oxidation of noxious compounds is the prevailing non-thermal plasma-induced process. Therefore nitric oxides cannot be reduced directly, but hybrid processes combining non-thermal plasma pre-treatment with catalytic reduction using either hydrocarbons or ammonia-based reducing agents have to be applied.Plasma-enhanced selective catalytic reduction (PE-SCR) of the nitric oxides emitted from a modern car's diesel engine for values of more than 60% was demonstrated in test bench experiments. For these experiments, a compact dielectric barrier discharge reactor with a flow cross section of 15 cm2 excited by a semiconductor switched pulse voltage source and a urea-based selective catalytic reduction system were applied. The average fuel penalty for this process under urban driving conditions was estimated to be around 2%.Thus PE-SCR has the potential to reduce the NOx emission of diesel cars to values well below future emission standards to be set in force in 2007. A number of investigations on the non-thermal plasma-induced oxidation of diesel soot showed very encouraging results.