Conference report: SAE 2016 Heavy-Duty Emissions Control Symposium

30 September 2016

The 2016 SAE Heavy-Duty Emissions Control Symposium was held September 20-21 in Gothenburg, Sweden. There were 20 presentations in a single track and the conference was attended by about 125 participants. Presentations covered regulatory developments, potential technical solution for upcoming regulatory requirements and fuels.

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California. The meeting started with an overview by Alberto Ayala of developments in California affecting heavy-duty engines [Ayala, CARB]. California’s push to lower the NOx limit to as low as 0.02 g/bhp-hr is driven by the 2008 NAAQS for ozone of 75 ppb (8 hr). San Joaquin Valley and South Coast Air Basin are the worst cases for ground level ozone in the US; they are considered “extreme nonattainment regions” and have until 2032 to meet the 2008 standard. Planning for 2015 NAAQS 70 ppb ozone standard has not started yet.

In addition to the lower NOx limit, California may take additional measures to address some of its concerns related to HD vehicles. These include:

• A new low-load certification cycle
• The current NTE requirements do not adequately represent vehicle operation. Even in applications such as line-haul trucks, the time spent in the NTE zone, out of temperature exclusion zone and for longer than the minimum 30 s event duration can be as low as 5% (see figure below). Possible strategies to address this include: expansion of the NTE Control Area, a shortening of the minimum event duration, reduction of cold temperature exclusions and further exploration of work based window concept (such as used in Euro VI In-Service Conformity testing).
• Warranty period (100,000 miles), does not match truck lifetime (800,000 miles)
• High warrenty claims for some emissions related components (EGR coolers, fuel injectors, other EGR components, turbochargers and SCR).
• High emitters.

Rulemaking for the new heavy-duty emission regulations is expected to take place in 2017, adoption in 2019 and implementation in 2023 to 2027 [Johnson, Corning]. California is also pressuring the EPA to adopt a national 0.02 g/bhp-hr NOx limit for heavy-duty vehicles because a significant portion of the heavy-duty trucks that operate in California are registered out of state and attainment in the extreme nonattainment regions would be more challenging without a similar federal rule. Also, a California Phase 2 HD GHG proposal is expected in 2017 that will build on the current Phase 1 rule. Expect new GHG standards for medium and heavy-duty vehicles to take into account new technologies.

Southwest Research Institute is demonstrating that a 0.02 g/bhp-hr NOx limit could be met [Sharp, SwRI]. After evaluating a number of options, they selected a system with a passive NOx adsorber (PNA), mini-burner (MB), Cu-zeolite SCR on filter (SCRF), Cu-zeolite SCR and ammonia slip catalyst (ASC). The system is attached to a 2014 Volvo MD13TC Euro VI with turbocompounding. The PNA also serves as a DOC. A burner had lower GHG penalty than an electrically heated catalyst.

The focus of the technical solution is to reduce cold start NOx emissions. While the PNA is very effective at adsorbing NOx immediately after a cold start, the engine calibration was also modified to increase exhaust temperature more rapidly and reduce engine out NOx during the cold start. The calibration changes were released once aftertreatment system light-off occurred. Composite FTP NOx emissions were 0.012 g/bhp-hr with a GHG penalty of 2%. The GHG penalty break down was: 0.5% due to engine calibration, 0.5% due to increased SCRF regeneration and 1% due to the miniburner. Active regeneration of the SRCF was more frequent due to competition for NO₂ by SCR function. The additional active regenerations had the added benefit avoiding the need to desulfurize either the SCRF or SCR. Engine-out NOx emissions were 2.9 g/hp-hr and deNOx conversion were 99% for cold start and 99.7% for hot start. N₂O emissions were 0.07 to 0.08 g/hp-hr. Aging of the system is currently underway and is focused on thermal and chemical acceleration. The 1000 h test is meant to test exposure to full useful life active regeneration events and 25% of full useful life oil consumption. Program completion is expected at the end of October. Further information is available from California ARB.

Johnson Matthey also discussed a system similar to that used in the SwRI study for NOx levels similar to those expected in California [Walker, Johnson Matthey]. Instead of a burner, an electric heater was used. Presumably the heater after the PNA would increase exhaust temperature sooner to enable urea dosing earlier in the cycle and bring the SCR temperature up to light-off sooner. Potentially, it could also help with the relatively low release temperature characteristics of Johnson Matthey’s PNA technology. Another feature of the Johnson Matthey system is a combination of vanadium SCRF and a downstream Cu-zeolite SCR; the vanadium SCRF ensures low N₂O production from the system.

The EPA has seemed to open the door for more applications of vanadium SCR catalysts with the publishing of guidance to determine vanadium sublimation temperature. The EPA would expect to approve applications that include thermal management strategies which prevent exhaust gas temperatures from exceeding the specified sublimation temperature threshold (i.e., the temperature below which vanadium emissions are less than the method detection limit) [Johnson, Corning].

Aftertreatment Systems. Tim Johnson from Corning provided an overview of recent developments related to heavy-duty engines. Some interesting developments mentioned include [Johnson, Corning]:

• In NOx aftertreatment, sublimation of vanadium from SCR catalysts can occur from 650-700°C depending on the substrate.
• Sulfur removal from Cu-zeolite SCR catalysts can be aided by the addition of hydrocarbons or ammonia.
• Urea dosing can increase particle count by 460% to 610% over the WHTC compared to a non-dosing cases. It is speculated that HNCO polymerization, urea pyrolysis, and urea micro-explosions during evaporation are the leading causes. Under normal urea dosing conditions over 80% of the total particle count can be urea-based.
• In SCRF systems, initial soot penetration into the wall adversely impacts SCR efficiency while cake filtration has no impact.
• A pre-turbo DPF has advantages such as higher temperature, lower Δp, easier VGT optimization and a 3-5% BSFC advantage. However, a supercharger or hybrid electric drive is required to offset the increased turbocharger lag.

NGK reported further progress on their high porosity SCR substrates. Since gas reactor studies reported earlier this year, both high porosity SCRF and SCR substrates have been subjected to engine testing. In the SCRF configuration for a nonroad Stage V application, PN emissions were confirmed to be below the Stage V nonroad limit with comparable pressure drop and active regeneration efficiency to a catalyzed DPF. In another set of experiments to test the applicability to Euro VI applications where the high porosity substrate was used to maximize vanadia washcoat loading, the steady-state low temperature (200°C) NOx conversion of a degreened vanadium SCR catalyst was found to be comparable to Cu-zeolite. The high porosity substrate also showed the potential to reduce the SCR catalyst volume by 50% compared to a standard vanadium SCR catalyst. However, further improvement is needed to achieve the same performance level with coated Cu-zeolite SCR for Euro VI [Mori, NGK].

Improvements continue in Cu-zeolite catalyst formulations. Johnson Matthey has been able to enhance NOx conversion (to 85%) at low (200°C) and high (600°C) temperature after aging at 900°C for 2h. Extruded Cu-zeolite offers lower N₂O production than washcoated Cu-zeolite. Passive Cu-zeolite SRCF designs are available that provide similar NOx conversion to actively regenerated systems on the FTP and NRTC. The Ceramex ash cleaning process was found to remove almost all ash from an SCRF with no detectable impact on performance. New ammonia slip catalyst technologies are available that significantly improve N₂ selectivity to yield lower N₂O and NO and can have a significant benefit for enabling low NOx and N₂O emissions such as those expected in California.

The next SAE Heavy-Duty Emissions Control Symposium is expected in 2018.

Conference website: www.sae.org/events/hddec