Conference report: 23^rd ETH Conference on Combustion Generated Nanoparticles

10 July 2019

The 23^rd ETH Conference on Combustion Generated Nanoparticles was held on June 17-20, 2019 in Zürich, Switzerland. The Conference program included about 45 presentations and keynote talks, as well as 88 posters on a number of topics, including particle fundamentals, engine emission control, particles from aircraft and other combustion sources, emission measurement, and health effects of air pollution. There were about 380 registered attendees. The conference included an exhibition with the participation of suppliers of emission control devices, emission measurement instruments, and related products and services.

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The conference was officially opened by Prof. Gian-Luca Bona, the CEO of EMPA, Switzerland. His Opening Address—which briefly outlined the challenges faced by the world’s mobility sector—was followed by a Key Lecture by Prof. Wolfgang Lienemann of the University of Berne, titled Environmental Ethics in the High Risk Society, that aptly reflected the crisis in corporate ethics revealed by the Volkswagen emission scandal. After two days of technical sessions, on the last day of the conference, a Focus Event Not just Diesel Soot → Detox all Combustion Engines covered insights into toxic effects of all emissions from combustion engines, the importance of in-use emission control, as well as directions for future emission standards for new engines and vehicles.

Emission Measurement

The European particle number (PN) measurement procedure is being re-evaluated under the EU-sponsored DownTo10 (DTT) program [4348]. The research aims to develop modified sampling and measurement methodology that would account for particles smaller than the current 23 nm cut-off point, for both laboratory (CVS) and RDE (PEMS) measurements. The political objective is to amend the EU regulatory limits for solid PN emissions to include all particles above 10 nm, down from the current 23 nm cut-off size. The DTT system is based on the existing PMP method, using either dilution + evaporation or a catalytic stripper + dilution to remove volatile particles from the sample. As before, a CPC is used for particle counting, and a 50% counting efficiency at 10 nm has been demonstrated. The program is now testing both lab-based instruments and portable exhaust particle sampling system (PEPS) prototypes. A number of measurements that have been performed show that most vehicles are compliant with the Euro 6 PN limit of 6×10¹¹ 1/km for both > 23 nm and > 10 nm ranges. However, there are some technologies that can meet the current limit value in the PN₂₃ range, but would fail in the PN₁₀ range—in Figure 1, these points are located above the 100% PN₁₀ line while below the 100% PN₂₃ line. These technologies include mostly currently unregulated niche vehicle types such as performance motorcycles and CNG vehicles.

Measurements were also conducted below 10 nm, with the instruments sensitive down to 4 nm. There were no excessive PN increases in the below-10 nm region in technologies with diesel or gasoline particulate filters (DPF or GPF), which shows that filters are effective in removing very small particles. On the other hand, high PN_sub10 emissions were observed in several applications without particle filters—for CNG, over 130 times higher than the PN₂₃ levels.

The updated methodology for the measurement of PN₁₀ particle numbers was evaluated in a study conducted as part of another European sub-23 nm particle project, SUREAL-23 [4349]. Experiments on a GDI engine showed that lowering the cut-off value from 23 to 10 nm was associated with an increase of PN emissions by about 10-20%. Interestingly, the PN increase was not observed downstream of a catalyzed GPF (cGPF), and the particle size distribution after the filter was shifted towards larger particles. It was hypothesized that the smallest particles, below 23 nm, were trapped and oxidized within the cGPF. The same study investigated the effect of hybrid drivetrains on PN emissions and found that PN emissions increased by a factor between 2.5 and 4. This increase was caused by the rapid high load transitions the thermal engine in hybrid operation.

Horiba discussed the development of two PN₁₀ instruments—one for laboratory use and one for on-vehicle (PEMS) measurements—a development supported by the EU PEMS4Nano project. The devices utilize a catalytic stripper and a CPC. The laboratory PN₁₀ system can be applied like the existing SPCS (PN₂₃) system, while two PEMS devices (10 nm and 23 nm) are still under evaluation; an ambitious target of ±20% for SPCS/PEMS correlation was achieved in some tests [4350]. The diffusion charging (DC) sensor (Partector) that is at heart of two other PN PEMS systems—the AVL M.O.V.E. and NanoMet3 by Testo—has been also updated to the PN₁₀ measurement range [4351].

The Southwest Research Institute (SwRI) has been evaluating a number of spark-plug sized exhaust sensors for particulate emission monitoring [4352]. The work, conducted within the SwRI Particle Sensor and Durability Consortium (PSPD), supports the development of robust and accurate sensing technology and data transmission that could be used in future emission compliance mechanisms, and reduce the burden associated with current laboratory and in-use emission certification procedures. Three types of PM sensors are being evaluated: collecting, cumulative PM sensors based on resistance measurement (Bosch, Denso, Stoneridge); particle charging and net escaping current (NGK/NTK); and measurement of natural charge of particles (CoorsTek, formerly Emisense). Results were presented including sensors’ steady-state and transient response, and sensitivity after multiple exposures. In future work, the SwRI plans to focus on NOx/NH₃ sensors, is a project with potential participation of the Truck and Engine Manufacturers Association (EMA), US EPA, and sensor manufacturers.

Two presentations were given on instruments for the measurement of ambient concentrations of carbonaceous aerosols. The FHNW University, Switzerland, has discussed the development of a new total carbon analyzer, dubbed FAst Thermal CArbon Totalizer (FATCAT), that collects particles on a filter in the form of a sintered metal tube. The collected sample is then heated, passed over an oxidation catalyst, and the carbon mass is determined via NDIR CO₂ detection [4353]. The Paul Scherrer Institute (PSI) reported on the next version of the Single Particle Soot Photometer - Extended Range (SP2-XR) for black carbon measurements. The updated instrument uses the same laser-induced incandescence (LII) principle as its predecessor, the SP2, but features a smaller size and weight, improved stability, extended detection range (50-800 nm), and real time automatic raw data processing [4354].

Health Effects

Prof. Nino Künzli of the Swiss Tropical and Public Health Institute (Swiss TPH) has called for the adoption of strong clean air policies and global air quality standards based on the WHO exposure recommendations [4360]. The EU clean air policy has been described as “semi-ambitious”—the EU promoted (dirty) diesel until recently, delayed the introduction of DPF-forcing emission standards, and showed insufficient control of the industry, as demonstrated by the Volkswagen emission scandal. A central failure of EU clean air policy are the European exposure limits for PM₁₀ and PM_2.5—values not informed by science, but by the vested interest of the auto and coal industries. These EU limit values are twice as high as the WHO guideline and the respective PM limits in many other countries: for PM₁₀, the EU limit is 40 µg/m³ (annual mean), compared to 20 µg/m³ WHO value; for PM_2.5, the respective figures are 25 µg/m³ and 10 µg/m³. Prof. Künzli was very critical of many policies adopted in the aftermath of the VW scandal. The low emission zones (“diesel bans”) adopted in some cities were strategically located to protect air monitoring stations from non-compliance, rather than to protect public health. Several of the “derailed” emergency policies require upgrading to newer cars, which benefits the auto industry, while violating the principles of social justice, as the poor have no means to pay for new vehicles. As a result, public unrest erupted in some German cities, with groups—supported by some “manipulated” German media—calling for relaxing air quality limits. On a global scale, German “bans” result in exports of older cars to Eastern Europe and Africa. Such outsourcing of pollution is one of the reasons behind the ongoing deterioration of air quality in many poorer nations, and should not be an acceptable policy for the West.

Commenting on the health effects of ultrafine particles (UFP, particles < 100 nm), Prof. Künzli emphasized that claims that UFPs are more harmful than other pollution components are counterproductive, because the ultimate objective is to ensure that the air is clean of all pollutants—PM₁₀, PM_2.5, UFPs or NO₂. This topic was further discussed in the following presentation, which reviewed the existing body of literature on the health effects of UFPs [4361]. There is “suggestive evidence” for independent short-term health effects of UFPs on inflammation, autonomic tone and blood pressure. On the other hand, there is insufficient evidence on short-term effects on mortality, and insufficient evidence to draw firm conclusions on long-term effects of UFP—beyond those from PM_2.5 exposures. A new study on UFPs conducted in Ontario, Canada [4362] found some health effects (mostly cardiovascular) other than those from exposure to PM_2.5 or NO₂, as well as new evidence suggesting a possible link between UFPs and brain cancer.

Engine Research

A new generation of gasoline DI engines is being developed by the European PaREGEn consortium, with a target to reduce CO₂ emissions by 15%. Two demonstration vehicles, one Mercedes E180 and one Jaguar XE, will comply with Euro 6 RDE limits with PN emissions measured to a 10 nm size threshold. As part of the project, a team from ETH Zürich / Vir2sense has been working on a virtual gasoline particle sensor (vGPS) for direct injection SI engines [4363]. The virtual sensor estimates PN/PM emissions based on input parameters from the ECU and actual sensors (such as intake p & T) utilizing models for spray formation, wall film evaporation (impingement), combustion, and soot formation. The vGPS has been validated on a single cylinder naturally aspirated engine. Future plans include calibration on a multi-cylinder turbocharged engine.

Other noteworthy engine research work included an investigation of diesel-natural gas RCCI combustion, where it was shown that increasing the port fuel energy portion (i.e., NG) reduced PN emissions [4364]; and a study of an IC engine operated with a hydrogen-rich gaseous fuel (direct or port injection) obtained via steam reforming of methanol using exhaust gas heat to increase efficiency [4365].

Aftertreatment

The European Commission Horizon Prizes program includes a Cleanest Engine Prize: Design for the Future (a safety net alternative to electrification if growth is slow—apply by 20 August 2019) and a Retrofit Prize to clean high emitting vehicles on the road [4366]. The latter prize has been awarded last year for a Euro 5 vehicle retrofit with Amminex NH₃ storage and delivery system and Johnson Matthey SCR catalyst. Potential future retrofit areas envisioned in the presentation include GPF retrofits of gasoline vehicles, which can produce particles that are smaller than diesel and contain more PAHs, and even natural gas engines, with large PN shares escaping counting due to small size (3-10 nm) yet total count similar to unfiltered diesel.

The Danish Environmental Protection Agency (EPA) has started a project to reduce emissions from ships in Danish waters. One ship—the inland ferry M/F Isefjord—has been already fitted with Exilator DPFs and switched to a 50 ppm sulfur MGO fuel [4367]. The DPFs on main engines burn soot in normal operation (passive), while DPFs on generators burn soot at periodically increased load (controlled automatically). The expected ash cleaning interval is once a year. Both types of DPFs were found to reduce PM and PM emissions by over 99%, and to effectively reduce exhaust noise. The next demonstration will involve the installation of Purefi SCR and DPF systems on World Mistral (with 1,000 ppm S fuel, which presents a challenge for the selection of DPF technology), followed by another DPF+SCR installation on M/S Pernille, with 50 ppm S MGO fuel.

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The next, 24^th ETH Conference on Combustion Generated Nanoparticles is scheduled for June 22-25, 2020 at ETH Zürich, Switzerland.

Conference website: nanoparticles.ch