Article pubs.acs.org/est
Effects of a Combined Diesel Particle Filter-DeNOx System (DPN) on Reactive Nitrogen Compounds Emissions: A Parameter Study Norbert V. Heeb,*,† Regula Haag,† Cornelia Seiler,† Peter Schmid,† Markus Zennegg,† Adrian Wichser,† Andrea Ulrich,† Peter Honegger,‡ Kerstin Zeyer,‡ Lukas Emmenegger,‡ Yan Zimmerli,§ Jan Czerwinski,§ Markus Kasper,⊥ and Andreas Mayer∥ Empa, Swiss Federal Laboratories for Materials Testing and Research, †Laboratory for Analytical Chemistry, ‡Laboratory for Air Pollution/Environmental Technology, Ü berlandstrasse 129, CH-8600 Dübendorf, Switzerland § UASB, University of Applied Sciences Biel, Laboratory for Exhaust Emission Control, Gwerdtstrasse 5, CH-2560 Nidau, Switzerland ⊥ Matter Aerosol AG, Bremgarterstrasse 62, CH-5610 Wohlen, Switzerland ∥ TTM, Technik Thermischer Maschinen, Fohrhölzlistr. 14b, CH-5443 Niederrohrdorf, Switzerland S Supporting Information *
ABSTRACT: The impact of a combined diesel particle filter-deNOx system (DPN) on emissions of reactive nitrogen compounds (RNCs) was studied varying the urea feed factor (α), temperature, and residence time, which are key parameters of the deNOx process. The DPN consisted of a platinum-coated cordierite filter and a vanadia-based deNOx catalyst supporting selective catalytic reduction (SCR) chemistry. Ammonia (NH3) is produced in situ from thermolysis of urea and hydrolysis of isocyanic acid (HNCO). HNCO and NH3 are both toxic and highly reactive intermediates. The deNOx system was only part-time active in the ISO8178/4 C1cycle. Urea injection was stopped and restarted twice. Mean NO and NO2 conversion efficiencies were 80%, 95%, 97% and 43%, 87%, 99%, respectively, for α = 0.8, 1.0, and 1.2. HNCO emissions increased from 0.028 g/h engine-out to 0.18, 0.25, and 0.26 g/h at α = 0.8, 1.0, and 1.2, whereas NH3 emissions increased from 98% over a particle size range of 20−300 nm.33 Two important classes of DPFs are distinguishable.35 High oxidation potential-DPFs (hox-DPFs) convert NO to NO2 and oxidize CO and hydrocarbons. Low oxidation potential-DPFs (lox-DPFs) convert hydrocarbons but do not oxidize CO and NO. On the contrary, lox-DPFs remove NO2. DeNOx Technologies for Diesel Vehicles. NO x emissions of diesel passenger cars exceeded those of comparable gasoline vehicles in the past, and their NOx emission limits were higher until recently. The proportion of diesel vehicles increases in many fleets. In 2010, the Swiss diesel vehicle fleet released 74% of the traffic-related NOx emissions.37 Estimates for U.S.A. report that more than 50% of NOx is released by heavy-duty vehicles (HDVs), which account for 2% of the fleet and 4% of the traveling distance only.38−40 Mean NOx emission factors of 9.1, 7.7, and 5.6 g/km are reported for Swiss HDV fleets in 2000, 2005 and 2010, whereas those of diesel passenger cars were 0.48, 0.32, and 0.23 g/km.37 In other words, HDVs are the largest contributor to traffic-related NOx emissions. In 2010, HDVs contributed 39% of a total of 33 000 t NOx/y.37 Additionally, NO2 proportions increased with the implementation of DOCs and hox-DPFs. Higher proportions of the oxidant NO2 together with decreased emissions of reductants such as CO, NO, and hydrocarbons are, among others reasons, responsible for the persistently high ozone levels in urban environments.28 Substantial NO and NO2 reductions are expected from future deNOx technologies for diesel vehicles.40,41 The effectiveness of urea-based SCR systems strongly depend on the chosen urea-dosing strategy and the exhaust gas temperatures reached under real-world driving. Herein, we report effects of a combined DPF−deNOx system on RNC emissions varying urea stoichiometries, engine loads, exhaust temperatures, and residence times. The DPN, consisting of a wall-flow filter upstream of a deNOx catalyst, was feed at urea stoichiometries of α = 0.8, 1.0, and 1.2 to study effects of under- and overdosing on emissions of isocyanic acid and ammonia, both toxic and reactive intermediates formed during urea decomposition. Our findings support the benefit/ risk assessment process of these new technologies and indicate further optimization potential for such developments.
Figure 1. Torque/revolutions-per-minute (RPM) diagram (top) and weighting factors for the 8 modes (M1-M8) of the ISO 8178/4 C1 cycle. Engine-out (dashed) and tail-pipe (solid) exhaust temperatures are given in the lower diagram. Urea dosing was active above 200 °C (gray).
are given in Table S1 of the Supporting Information. The lubricant (Mobil 1 ESP, 5W40) complied with the specifications of the engine manufacturer. SCR Catalyst, Particle Filter, Urea Dosing. The DPN system consisted of a platinum-based hox-DPF (Pt/cordierite, Pt 25 g/ft3, 9″ × 12″, 12.7 L, DINEX, Denmark) upstream of a vanadium pentoxide-based SCR catalyst (V2O5/corrugated ceramic, 2 bricks, 5.7″ × 9″, 6.0 L, DINEX, Denmark). Two NOx-sensors (Siemens VDO, Germany) up- and downstream of the SCR were used to control urea dosing. Urea feed factors were adjusted to α = 0.8, 1.0, and 1.2. Commercial aqueous urea solution (AdBlue) was injected via an external pump. Exhaust Sampling, Workup, Chemical Analysis. The regulated pollutants CO, NOx and hydrocarbons together with CO2, the latter is a metric for fuel consumption, were measured from hot, undiluted exhaust. NO and NO2 were monitored with chemiluminescence detection with and without NO2 converter (CLD, Horiba, Japan). NH3 emissions were determined with two independent techniques, laser absorption spectroscopy (LDS 6, Siemens, Germany) and Fourier transformation-infrared spectroscopy (FTIR, SESAM, AVL, Germany). NO and NO2 were also monitored by FTIR. We noticed severe sampling artifacts when monitoring NH3 in diluted exhaust of a constant volume sampling (CVS) system.23 Adsorptive losses and long-lasting memory effects in the order of hours were observed in the CVS tunnel. It is recommended to collect RNCs from hot undiluted exhaust and condensation of water has to be avoided with heated transfer lines. NO, NO2, and NH3 are typically found in the parts per million range and can be monitored online. HNCO, which is present in lower concentrations, was transformed to a stable derivative and analyzed off-line. The ISO8178/4 C1 cycle with relatively few load changes and long periods of steady state conditions is well suited to obtain flow-proportional exhaust samples. The
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EXPERIMENTAL SECTION Engine, Test Cycle, Fuel, Lubricant. A 3 L diesel engine with turbo charger and direct fuel injection (IVECO, type F1C, EURO-3, 100 kW, Torino, Italy) was used. The engine was operated in the ISO 8178/4 C1 cycle (Figure 1, modes M1M8). Commercial low-sulfur diesel fuels (SN 181160−1:2009, class 0) with comparable properties were used. Further details 13318
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Impact of the Urea Feed Factor (α). Tables S2−S4 of the Supporting Information report RNC emissions at different modes for engine-out and tail-pipe emissions at urea feed factors α = 0.8, 1.0, and 1.2. Mean (n = 3) values and standard deviations are given for all pollutants except for HNCO (n = 2). Also included are weighted cycle averages, amounts of consumed fuel (kg/h), effective power (kW), and emissions of major combustion products. The given data can be converted to consumption- or power-based emission factors. Figure 3 displays mean RNC emissions and their standard deviation. Engine-out NO emissions varied from 11.7 to 275 g/h with a cycle average of 146 g/h. The DPN lowered NO emissions to 29.5, 7.97, and 4.88 g/h at α = 0.8, 1.0, and 1.2 respectively corresponding to mean NO conversion efficiencies of 80%, 95%, and 97% (Figure 4, Tables S2−S8 of the Supporting Information). Engine-out NO2 emissions varied from 3.52 to 15.9 g/h with an average of 12.0 g/h. After the DPN, which included a hox-DPF, highest NO2 emissions were observed at modes M3, M6, and M7 at α = 0.8. Respective NO2 conversion efficiencies were low or even negative (Figure 4). It seems that not all NO2 formed in the hox-DPF was converted in the SCR under these conditions. However, even at α = 0.8, mean NO2 emissions were lowered to 6.91 g/h and levels of 1.57 and 0.073 g/h were achieved at α = 1.0 and 1.2 corresponding to efficiencies of 43%, 87%, and 99%. Engine-out NH3 emissions were 90% indicating that NO2 still is adsorbed and possibly converted in the SCR catalyst. A closer look reveals a net formation of NO in M4, with negative efficiencies of −0.13 and −0.10 at α = 0.8 and 1.0 and virtually no NO conversion (ηNO = 0.00) at α = 1.2 (Figure 4). We hypothesize that some NO forms again over the still hot SCR catalyst, either from NO2 reduction, as indicated by the still high NO2 conversion, or by NH3 oxidation, or by a combination of both processes. Figure 5 plots RNC emissions versus temperature at different feed factors. Only data with active deNOx system are given. The temperature dependence was visualized with second-order polynomial fits (solid lines). Engine-out NO increased with temperature. After the DPN, considerably lower NO levels
chemisorption method, which converts HNCO to a stable urea derivative according to reaction R8 (Figure 2) and the liquid
Figure 2. Chemical reactions of RNCs during urea-based selective catalytic reduction on a V2O5-catalyst. R1 displays the NO oxidation as occurring in DOCs and hox-DPFs, R2-R4 report stoichiometries of ammonia-supported deNOx reactions. R5 and R6 describe urea decomposition and hydrolysis reactions. R7 indicates the disproportionation of NO2 in water. R8 describes the derivatization of HNCO. In addition, nitrogen oxidation states are indicated.
chromatography−mass spectrometry method (LC-MS) have been reported elsewhere.42,43 In brief, aliquots of undiluted exhaust (20−35 L) were collected at individual modes of the test cycle (Figure 1). The exhaust was adsorbed in gas washing bottles containing dibutylamine/toluene solutions. After workup, aliquots were spiked with an isotopically labeled standard and analyzed with LC-MS as described.42,43
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RESULTS AND DISCUSSION A catalytic diesel particle filter supports various transformations at different time scales. These reactions occur in an oxidative environment. Operating both, a DPF and a deNOx-system requires the in situ production of a reducing agent (reductant) to convert nitrogen oxides to reduced nitrogen species, ideally to N2. Figure 2 displays some of the expected SCR reactions and reports oxidation states of the involved RNCs. Thus a DPN supports numerous reactions, both in oxidation and reduction zones. The amount of reductants needed varies at a time scale of milliseconds. Hence, fast-responding NOx sensors are necessary for proper dosing of the reducing agent. Two NOx sensors up- and downstream of the deNOx system were used to adjust the urea dosing. The urea solution is injected downstream of the DPF to produce ammonia, the required reducing agent for SCR in the V2O5-catalyst. In this configuration, response times are fast enough to operate the DPN under more transient conditions too.44 High deNOx efficiencies were obtained in the European Transient Cycle, where exhaust temperatures never fall below 200 °C and the catalyst was active throughout the entire cycle.44 Herein, we report emissions for different modes of the ISO8178/4-C1 cycle. Figure 1 displays the eight load stages M1-M8, their weighting factors (0.1 and 0.15) and the exhaust temperatures before and after the DPN. At low engine loads (M4, M8), exhaust temperatures drop below 200 °C and urea dosing stops (Figure 1, gray bar). The ISO8178/4-C1 cycle, widely used for VERT filter approval,33,35,36 is also well suited to study emissions of urea-based SCR systems, which are inactive at low engine operation but light-off at higher loads. 13319
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Figure 3. RNC emissions of a 100 kW diesel engine at modes M1−M8 of the ISO8178/4 C1 cycle and weighted averages. Engine-out emissions are compared with those downstream of a combined DPF−deNOx system at urea feed factors α = 0.8, 1.0, and 1.2. NO (gray), NO2 (white), NH3 (pink), and HNCO (red) are distinguished. Molar RNC proportions are also given. Modes M4 and M8 with inactive SCR systems are highlighted.
were found, but emissions still increased with temperature (Figure 5). Different trends were observed for NO2. Engine-out emissions were comparably high for the examined temperature range. After the DPN, highest NO2 emissions were noticed at lowest temperatures with α = 0.8. A similar behavior is observed for hox-DPFs, with highest NO2 formation at intermediate temperatures.35 NH3 emissions increased with α but also with temperature following the higher demand of reductants due to higher NO emissions of the hot engine. Maximum HNCO emissions are found at intermediate temperatures (Figure 5, volcano plots). HNCO is an intermediate of the urea decomposition. Its
occurrence is controlled by at least two processes, a formation and a conversion reaction (Figure 2, R5, R6). Both reactions are temperature dependent. At lower temperatures, the HNCO formation is slow but still faster than its conversion resulting in increased emissions. At higher temperatures, both formation and degradation are comparably fast. Thus, higher HNCO levels are found mainly in a critical temperature window of 250−350 °C. A similar behavior was noticed for TWCs, which support a secondary benzene formation from aromatic precursors such as toluene and alkyl benzenes in a critical temperature range.45,46 13320
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Figure 4. Conversion efficiencies of NO, NO2, and ΣRNC at different urea feed factors and modes of the ISO8178/4 C1 cycle. Modes M4 and M8 with inactive SCR systems are highlighted. Mean conversion efficiencies are indicated.
NO2 formation in the DPF. At α = 0.8, this extra NO2 is not converted completely in the SCR catalyst. However, at α = 1 and 1.2, NO2 conversion is high even at longer residence times. Ammonia emissions depend on α but also on residence time (Figures S1 and S2 of the Supporting Information). Ammonia slip is highest at shortest residence times indicating that higher space velocities are critical for this DPN system. HNCO emissions are highest at intermediate residence times (0.4 s), which is interpreted as a result of two competing processes, one affecting the HNCO formation the other its degradation (Figure 1, R5, R6). In conclusion, urea-stoichiometry, temperature, and residence time all affect RNC emissions. These parameters must be carefully adjusted for different diesel engine applications to minimize RNC emissions. The control and optimization of such a complex system is a multidimensional process that should include parameters like conversion efficiencies, consumption of reductants, and emissions of nonregulated pollutants. Changes of RNC Patterns. Figure 2 also includes RNC patterns at different modes and urea stoichiometries. Engineout exhaust is dominated by NO (95%) and molar proportions of NO2 remain at 5% in most modes except for M4 (21%) and M8 (16%), where lower temperatures prevail. The hox-DPF increased NO2 proportions, most visible at α = 0.8 and modes M3, M6, and M7, with proportions of 57%, 21%, and 32%. Mean NO2 proportions of 5%, 13%, 8%, and
