Effects of a Combined Diesel Particle Filter-DeNOx System (DPN) on Reactive Nitrogen Compounds Emissions: A Parameter Study

Abstract

The impact of a combined diesel particle filter-deNO(x) 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 deNO(x) process. The DPN consisted of a platinum-coated cordierite filter and a vanadia-based deNO(x) 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 deNO(x) 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 <0.045 to 0.12, 1.82, and 12.8 g/h with maxima at highest temperatures and shortest residence times. Most HNCO is released at intermediate residence times (0.2-0.3 s) and temperatures (300-400 °C). Total RNC efficiencies are highest at α = 1.0, when comparable amounts of reduced and oxidized compounds are released. The DPN represents the most advanced system studied so far under the VERT protocol achieving high conversion efficiencies for particles, NO, NO₂, CO, and hydrocarbons. However, we observed a trade-off between deNO(x) efficiency and secondary emissions. Therefore, it is important to adopt such DPN technology to specific application conditions to take advantage of reduced NO(x) and particle emissions while avoiding NH₃ and HNCO slip.

Investigating diesel engines as an atmospheric source of isocyanic acid in urban areas

“…This allowed the SCRequipped diesel engine to meet and exceed the most recent EPA Tier 4 emission standard of ∼ 1.6 g of NO x kg fuel −1 (or 0.4 g of NO x kW h −1 ). Similar to earlier work (Wentzell et al, 2013;Heeb et al, 2012Heeb et al, , 2011Suarez-Bertoa and Astorga, 2016), we observed HNCO when no NH 3 was injected, implying that the HNCO was produced either in the engine cylinder or in the aftertreatment devices upstream of the SCR (DOC + DPF). For six of the seven experiments performed with diesel fuel, HNCO emissions were reduced by 5-40 % with increasing NH 3 injections.…”

“…Fourth, the emission factors for HNCO from this study (31-56 mg kg fuel −1 ) compared reasonably well with those from an SCR-equipped diesel engine tested by Heeb and coworkers (Heeb et al, 2011(Heeb et al, , 2012) (29-32 mg kg fuel −1 ) but were slightly higher than the two SCR-equipped diesel vehicles tested by Suarez-Bertoa and Astorga (2016) (1.3-9.7 mg kg fuel −1 ). However, in sharp contrast to our findings from Fig.…”

“…Since we are modeling an episode prior to when diesel engines were required to have a DOC, we have only used non-DOC data to calculate low and high estimates for HNCO : CO ratios. Based on this work, and that of Link et al (2016), Heeb and coworkers (Heeb et al, 2011(Heeb et al, , 2012, and Wentzell et al (2013), we loosely calculated a lower bound HNCO : CO ratio of ∼ 0.001 that reflected diesel engine operation at lower engine loads and a higher HNCO : CO ratio of ∼ 0.01 that reflected diesel engine operation at higher engine loads; the HNCO : CO data from all sources are tabu- Brady et al (2014). b This work, Link et al (2016), and Heeb et al (2011.…”

“…In Fig. 3, we compare HNCO emission factors for diesel fuel with all earlier work involving diesel engines: Link et al (2016), Wentzell et al (2013), Heeb et al (2011Heeb et al ( , 2012, and Suarez-Bertoa and Astorga (2016). We also compare our results to the average HNCO emissions from eight light-duty gasoline vehicles measured by Brady et al (2014).…”

“…Heeb et al (Heeb et al, 2011 performed experiments on an on-road diesel engine and found an order-of-magnitude increase in HNCO emissions with the SCR system engaged, implying that SCR systems could potentially be a source for HNCO. However, Heeb et al (Heeb et al, 2011(Heeb et al, , 2012 used an offline technique to measure HNCO, which lacks the time resolution and sensitivity found in online mass spectrometry instrumentation. To date, there has only been a single study that has used a large-scale model to simulate ambient concentrations of HNCO from biomass burning and biofuel combustion.…”