Experimental Assessment of an Electrofilter and a Tandem Positive-Negative Corona Charger for the Measurement of Charged Nanoparticles formed in Selective Catalytic Reduction Systems

Abstract

Onboard measurement of non-volatile particle number (PN) emissions with portable emissions measurement systems (PEMS) was introduced for the type-approval of passenger cars in Europe since 2017 and is foreseen for heavy-duty (HD) vehicles in 2021. First studies on the performance of PN-PEMS with HD engine exhaust revealed larger differences between established PN-PEMS techniques than what was observed for passenger cars. Particles forming in selective catalytic reduction (SCR) systems for NOx of late technology HD engines have recently been identified as a potential reason for the observed differences. The formed particles have a size distribution peaking below the regulatory limit of 23 nm and most importantly acquire high (more than one) positive charges at the elevated exhaust temperatures. Precise measurement of such highly charged nanosized particles with PN-PEMS instrumentation utilizing diffusion charger (DC) based counters requires proper conditioning of these charges. Two approaches were investigated in this study: (a) an electrofilter (EF) to completely remove charged particles below the regulated size and (b) a tandem negative-positive corona (TC) charger to directly condition pre-charged particles. The two technical solutions were tested alongside the unmodified DC-based PN-PEMS, a PN-PEMS utilizing a condensation particle counter (CPC) and a reference stationary PN system using exhaust of two SCR-equipped HD engines. The results confirmed that the particles forming in such SCR systems are responsible for the observed inconsistencies and that both technical solutions efficiently address the interferences of these pre-charged nanoparticles.

Effect of selective catalytic reduction on exhaust nonvolatile particle emissions of Euro VI heavy-duty compression ignition vehicles

“…When sampling particles carrying more charges of the same polarity, the charge will remain largely unchanged leading to an overestimation. This effect is illustrated in more detail on an accompanying paper that also describes some of the possible techniques to overcome this (Schwelberger et al 2019). Accordingly, the regulation specifications should be amended to require that the instrumentation can properly condition the highly charged exhaust particles of heavyduty diesel vehicles equipped with SCR systems.…”

Evaluation of a 10 nm Particle Number Portable Emissions Measurement System (PEMS)

“…The reference instrument was a CPC model 3792 from TSI, which was used to determine the particle number concentration reduction factor (PCRF) at 15, 30, 50, and 100 nm by measuring upstream and For the PEMS10 calibration, a neutralizer was also employed downstream of the DMA to neutralize the particles. This was necessary as the tested unit was not equipped with a precharger to neutralize precharged particles at the outlet of the DMA, which would thus affect the response of the instrument [52]. For the PEMS10 calibration, the particle generator was the graphite spark-discharge particle generator DNP 3000 from PALAS (Karlsruhe, Germany) [53], and the reference instrument was the CPC model 3775 from TSI.…”

Impact of pre-charged particles on steady state and pulsed modes of unipolar diffusion chargers

“…Charging can happen through several anthropogenic or natural processes, like static electrification or thermal ionization. Of particular concern are the anthropogenic sources of exhaust aftertreatment systems used in combustion based vehicles (Schwelberger et al 2019).…”