Study finds high exhaust temperatures within NTE operation for heavy-duty diesels with aftertreatment systems contribute to high nanoparticle concentrations
Results of a new study led by a team from West Virginia University suggest that urea-based selective catalytic reduction (SCR)-equipped heavy-duty diesel engines operating within the Not-To-Exceed (NTE) operating zone over a critical exhaust temperature (about 380 °C) and under favorable ambient dilution conditions could contribute to high nanoparticle concentrations to the environment. The particle number count under those conditions increased by more than an order of magnitude in comparison to DPF-out particle concentrations.
The researchers also found that some of the high temperature modes resulted in diesel particulate filter (DPF)-out accumulation mode (between 50 and 200 nm) particle concentrations an order of magnitude greater than typical background PM concentrations. They concluded that sustained NTE operation could trigger high-temperature passive regeneration which, in turn, would result in lower filtration efficiencies of the DPF that further contributes to the increased solid fraction of the PM number count. Their paper is published in the ACS journal Environmental Science & Technology.
Current 2010 USEPA emissions standards for NOx and PM are set at 0.20 g/bhp-hr and 0.01 g/bhp-hr, respectively. Manufacturers have responded with different technologies and engine control strategies for compliance. Although there is a great deal of effort focused on the engine control strategies for combustion-based reduction of both, actual deployment is limited due to the well-known trade-off between the two criteria pollutants (reducing one increases the other) as well as a fuel economy penalty incurred with some in-cylinder technologies, the researchers note.
Diesel particulate filters (DPF) and urea-based selective catalytic reduction (SCR) exhaust after-treatment systems thus have emerged as feasible strategies to meet the 2010 USEPA regulations.
Diesel engines for use in the US are certified on the Federal Test Procedure (FTP) cycle. In addition, the EPA defined a Not-To-Exceed (NTE) control zone bound by certain speed and load conditions which encompasses a broad range of real-world load conditions not covered in the FTP engine dynamometer cycle.
The study by Thiruvengadam et al. work presents the findings of PM nanoparticle emissions downstream of a DPF-SCR after-treatment system, while operating at low and high exhaust temperature load conditions. The team used a MY 2007 Volvo MD11 engine compliant with USEPA 2007 emissions standards. The engine was calibrated to achieve the USEPA 2010 NOx standard, by involving strategies such as high EGR rates and optimization of the urea dosage in the SCR after-treatment system. The engine was also equipped with a catalyzed￼DPF system, with the capability to actively regenerate; due to lack of control over the active regeneration fuel injector, active regeneration was disabled. The fuel used in the study was Ultra Low Sulfur Diesel (ULSD) with sulfur concentration not exceeding 10 ppm.
Among the findings were that below a threshold temperature of about 380 °C, the contribution of the SCR catalyst toward precursors for nanoparticle formation is minimal. However, higher exhaust temperatures resulted in total particle number concentration that were orders of magnitude higher. This suggests strong effects of the after-treatment system on production of precursors for nanoparticle formation, the team concluded.
The particle formation mechanism from advanced after-treatment equipped engines could be attributed to sulfur oxidation over the catalyst. The fact that no distinguished particle size distribution is observed during low load, low exhaust temperature conditions, we can rule out that hydrocarbon precursors as a contributor to any significant particle formation. However, the nucleation mode distribution observed could be sulfuric acid based particles formed as a result of sulfur oxidation over the catalyst surfaces at exhaust temperatures over 380 °C...It is to be noted that the results of the current study points to the fact that SO3 formation could be more dominant with the DPF and SCR after-treatment systems placed in series compared to the presence of only the DPF system.
Although regulatory agencies have mandated ultra low sulfur diesel, contribution of sulfur from lubrication oil has proven significant toward producing precursors for sulfuric acid nucleation in the presence of exhaust after-treatment systems.—Thiruvengadam et al.
The California South Coast Air Quality Management District provided the funding for this study.
Arvind Thiruvengadam, Marc C. Besch, Daniel K Carder, Adewale Oshinuga, and Mridul Gautam (2012) Influence of Real-World Engine Load Conditions on Nanoparticle Emissions from a DPF and SCR Equipped Heavy-Duty Diesel Engine. Environmental Science & Technology doi: 10.1021/es203079n