Conference report: 20th ETH Conference on Combustion Generated Nanoparticles
24 June 2016
The 20th ETH Conference on Combustion Generated Nanoparticles was held on June 13-16, 2016 in Zürich. The Conference program included 55 presentations and 109 posters. There were about 420 registered attendees. The conference also included an exhibition with the participation of 22 suppliers of emission measurement instruments, emission control systems and related products and services.
The 20th anniversary of the ETH Conference was commemorated by a Key Lecture by Jan Czerwinski [AFHB], who summarized the achievements of the Conference, which evolved from a small particle workshop first held in 1997 to a major scientific exchange forum on particle pollution and its control. The technical sessions that followed covered particle fundamentals, instrumentation, ambient exposures, health effects, emission aftertreatment, as well as particle emissions from aircraft engines and from biomass combustion. A Focus Event Particle Filter Quality under Real World Conditions included insights into the DPF technology from the fleet operators point of view, as well as lessons learned from DPF inspection and maintenance programs.
Particle Characterization. An improved thermophoretic-thermocouple method was proposed to study particle formation in flame combustion [G. De Falco, Università di Napoli]. In the thermophoretic particle densitometry (TPD) method, a thermocouple is inserted in a particle containing flame. Due to thermophoretic deposition of particles on the thermocouple junction, a change of measured temperature is recorded, which is dependent on particle concentration and properties. The developed method provides simultaneous measurement of volume fraction and particle emissivity, for the study of particle evolution in flame. In experiments with soot formation in rich flame, the method allowed to identify nascent soot particles with diameters of 2-5 nm, as well as 10-100 nm nanoparticles.
Transmission electron microscopy (TEM) is commonly used to characterize combustion particles. However, TEM produces 2D projections of particles and the lack of depth perception means that measured morphological parameters may be underestimated. A team from the University of Nottingham [E. Haffner-Staton] proposed a method to quantify the 2D uncertainties, based on the Fourier set theorem, which says that one can recreate the 3D particle based on a full set of 2D projections. The developed method involved tilting the TEM specimen stage at incremental angles over a ±60° range. By characterizing particles at each angle, it was possible to quantify the orientation uncertainty. A comparison of 2D and 3D derived results for a real particle was shown.
Carbon Nanotubes. One of the central themes of the Conference were carbon nanotubes (CNT). A recent medical paper showed that nanotubes were present in the lungs of children in Paris [F. Moussa, Université Paris-Sud]. The source of the CNTs was not clear. Using TEM images, it was shown that both dust and vehicle exhaust included CNTs.
The question whether CNTs can be present in diesel exhaust deserves a timely answer, because long fibrous particles can have adverse health effects years after the exposure—asbestos is the best known example. Long fibers and nanotubes cannot be cleared from the lung alveoli and cause inflammation with possible tumor formation. Two presentations discussed the lung cell interactions with CNTs and the susceptibility to lung disease, based on in vitro experiments [B. Rothen-Rutishauser, University of Fribourg] and in vivo experiments with mice [J. Bonner, North Carolina State University]. It was noted in the discussion that asbestos fibers have lengths on the order of 20 µm, while engine nanotubes appear to be only tens of nanometers long, therefore being less harmful.
Nanotube-like structures have been found in air in brake repair shops, near highways, in emissions from natural gas stoves and powerplants, as well as from propane stoves [D. Kittelson, University of Minnesota]. CNTs are not normally produced in diesel exhaust, but are possible under certain conditions. Commercial production methods of carbon nanotubes involve reacting carbon and/or hydrocarbons in a hot furnace in the presence of a metal-based catalyst (e.g., iron) and sulfur—a set of conditions that could occur in the diesel combustion chamber. Since CNT growth is facilitated by metals and sulfur, marine engines burning heavy fuels could be a potential source. However, even if CNTs are emitted from engines, contributions from other sources—such as brake wear—are very likely.
Researchers from the Minnesota State University [J. Swanson] conducted experiments in a single cylinder diesel engine using fuel doped with iron (ferrocene) and different sulfur levels. With fuel containing 4500 ppm S and 36 ppm Fe, a number of cylindrical structures (tubes or rods) were identified in the exhaust using TEM. The CNTs were approximately 75 nm long and had an aspect ratio of 10:1.
Diesel Particulate Filters. The effects of ceria-zirconia catalyst nanostructure on soot oxidation kinetics were studied by a team lead by Athanasios Konstandopoulos [CPERI-CERTH, Greece]. Ceria-zirconia catalyst samples were subjected to different milling protocols, to shift the catalyst particle size distribution. The produced catalysts were evaluated with respect to their soot oxidation activity. A multi-population kinetics model was developed to describe the soot oxidation process and assess the effect of catalyst particle size. Three populations of soot were identified with different activation energies, likely due to different structures of the surface oxygen complexes.
Liebherr [Y. Hohl] discussed their EU Stage IIIB and Stage V DPF systems. While the use of DPF technology at Stage IIIB was generally limited to the Swiss construction machinery, particulate filters will be introduced on all Stage V engines that have a PN emission limit. The Stage V aftertreatment system utilizes an SCRF (SCR on filter) followed by an SCR catalyst. The Stage V muffler is 25% longer than the SCR (non-DPF) version. The changes include different piping from the turbo and the addition of a DOC upstream of the SCR muffler. The system utilizes a vanadia SCR catalyst, and includes an ammonia slip catalyst. In most applications, the DPF will rely on passive regeneration.
An interesting talk on sintered metal filters was given by Rafael Rienks [HJS], who discussed the production process of their sintered metal DPFs and the quality assurance measures during filter manufacturing.
Aleksandar Bugarski [NIOSH, USA] and Brian Davies [University of Wollongong, Australia] discussed the experience with disposable exhaust filters used in underground coal mining in the United States and Australia, respectively. The pleated media filters, resembling intake air cleaners, are installed downstream of an exhaust scrubber that cools down the exhaust gases (as required by regulations for vehicles operated in explosive atmospheres). The filters last 8-50 hours and cost $100-$250 (AU data).
Field Experience with Particulate Filters. The California ARB [Alberto Ayala] undertook extensive field investigation of retrofit diesel particulate filters in on-road applications. Over 1000 trucks were inspected and vehicle testing, certification and compliance reporting data was analyzed. The study concluded that the DPF technology was working properly. Engine durability and inadequate maintenance practices were the main reason for occasional failure. In many instances, component failure (EGR valves, turbochargers, etc.) was incorrectly identified as a DPF failure.
Swiss regulations require that most diesel powered construction equipment be (retro)fitted with particulate filters. In the Zürich canton, the results of field inspections since 2006 indicate that about 80% of construction machines are correctly equipped with DPFs and comply with the regulations [A. Frölich, UGZ]. The Swiss inspection and maintenance program for construction machinery is moving away from opacity testing to instrumental methods based on a particle number measurement [T. Lutz, ETHZ]. Two instruments, one from Testo and one from TSI, have been approved for field measurements of PN emissions.
The public transit agency in Berlin operates a fleet of buses with DPFs since 2001 [B. Eberwein, BVG]. The current BVG fleet of 1400 buses includes older buses retrofitted with CRT filters and newer vehicles that were purchased with a DPF. The DPFs have been operating without problems, however, filters must be removed from vehicles and cleaned every 50,000-70,000 km. A few (3-4) filters a year become fully damaged, mostly due to turbocharger failures. BVG also operates 154 EEV buses with SCR+DPF emission systems, which experience regular problems with urea deposits due to the low temperature stop-and-go operation. In 2015, the cost of added maintenance to clean the urea deposits amounted to 197,000 euro.
VERT Retrofit Programs. VERT Association has been supporting a number of DPF retrofit programs around the world. Following a pilot DPF study in Tehran, the Iranian government adopted regulations that would require retrofitting of existing diesel vehicles with DPFs [V. Hosseini, Sharif University, Tehran]. The challenges include high sulfur content in the fuel. A number of DPFs using fuel borne catalysts (FBC) have been approved for the retrofit. Another DPF demonstration was conducted in Bogotá in 2014 [J. Rueda, District Secretariat of Environment Bogotá]. However, the retrofit program is falling behind schedule, due to concerns about the cost of the DPF systems.
One of the earliest VERT supported diesel retrofits has been conducted in Santiago, Chile [A. Reinoso, GEASUR]. About 3500 Santiago buses have been fitted with DPFs—including 1000 retrofits and 2500 OEM systems. An in-use emission testing program was conducted using portable particle number instruments, and a PN limit of 2.2×105 #/cm3 at low idle speed has been proposed as a good indicator for detecting buses with DPF efficiency below 90%. Abnormal (above the limit) emissions were found in 9% of vehicles from the 2010-2013 implementation stage of the retrofit, and in 84% of the 2005-2009 retrofits. The failure rates for retrofits and OEM DPFs were similar.
The Future—Low NOx Diesel. In California, the PM emission issue has been solved [A. Ayala, CARB]. Emission standards have been driving the use of DPFs on new vehicles, while in-use engine regulations require emission reductions from the existing diesel fleet. Under the provisions of the in-use Truck and Bus Regulation, all heavy-duty vehicles in California will be 2010 or newer (i.e., with DPF+SCR technology) by 2023. The remaining diesel emission issue is NOx. The ARB plans to develop—together with the US EPA or alone—low NOx standards for HD engines, targeting a NOx limit of 0.02 g/bhp-hr. The agency also intends to re-examine the NTE window, with the intent of extending it toward lower temperatures to improve low temperature SCR performance. A number of improvements are also expected in emission certification, compliance and enforcement—the VW emission scandal created public pressures and a good political climate to improve and expand laboratory and on-the-road testing and to enhance requirements for warranties, durability, in-use testing, certification, and public disclosure.
The Conference was closed with a talk by Norbert Heeb [EMPA], who provided the Swiss and European perspective on diesel emission control. NOx emissions were identified as the outstanding problem that haunts the diesel engine. Considering that PM emission issues have been largely solved, it was suggested that the scope of the ETH Conference be extended to also cover diesel NOx emissions.
The 21st ETH Conference on Combustion Generated Nanoparticles will be held on June 19-22, 2017 at the ETH Zentrum in Zürich, Switzerland.
Conference website: nanoparticles.ch