Effects of Nonthermal Plasma on Microstructure and Oxidation Characteristics of Particulate Matter

Abstract

To investigate the effect of nonthermal plasma (NTP) on the microstructure and oxidation characteristics of particulate matter (PM) from diesel engines at different oxidation stages, a self-designed NTP reactor was used to conduct a timevarying oxidation test on PM samples. The oxidized PM samples were analyzed via thermogravimetric analysis and Raman spectroscopy. The results indicated that the effect of NTP could allow the elemental carbon (EC) to more easily start ignition. The oxidation activity of the EC decreased when the action time of the NTP was less than 5 min. Conversely, when the NTP action time was more than 5 min, the EC oxidation activity gradually increased. When the NTP was active for more than 10 min, it rapidly reacted with the EC, and the oxidation priority of the volatile fraction was higher than that of the EC. During the oxidation process, there are many forms of carbon structures in the particles and they have a mutual transmission relationship. The variation trend of the graphitization degree was consistent with that of the thermogravimetric results, indicating that the degree of graphitization directly affected the PM oxidation activity.

Impact of High-Voltage Discharge After-Treatment Technology on Diesel Engine Particulate Matter Composition and Gaseous Emissions

“…This was explained by the partial diesel soot oxidation on the DPM's surface, DPM mainly consisting of VOC, which has a higher tendency to be oxidized by free radicals in comparison to carbon in the soot, and resulted in a decreased particle diameter. 41,43 Ji et al 39 reported the alteration of the DPM microstructure from a large aggregate and longer chain structure into a smaller aggregate and shorter chain, which is consistent with the results shown in Figure 3.…”

“…However, the mass oxidation profiles (Figure 5A) indicated a shifting toward a higher temperature, which suggested less DPM reactivity. With the limited available quantity of highly reactive free radicals generated and the gas resident time within the reactor (governed by GHSV), this time-varying soot oxidation phenomenon was experimentally confirmed by 39 using indirect soot oxidation with NTP-induced O 3 , and it was reported that the VOC/EC ratio was higher for diesel soot under treatment at a relatively short duration (<5 min) and VOC was the first to be oxidized by HVD-generated radical species (mainly O 3 ).…”

“…The ignition temperature onset of EC (at the EC mass loss of 5%) was also evidently shifted toward a higher temperature, from 416.7 to 426.3 °C, which further confirmed that partial oxidation by HVD-generated free radicals reduced the DPM reactivity when exposed to O 3 for a short duration. 39 The MMLRT of the HVD-treated DPM samples demonstrated a noticeable shift toward higher temperature, from 510.9 to 520.6 °C. The significantly lower mass loss rate (dm/dT) suggests that free radicals generated by HVD and direct contact of the HVD with the EC altered the EC nanostructure to a more orderly organization or a higher degree of graphitization to form more graphene layers, 38,39,43 which resulted in lower soot reactivity.…”

“…39 The MMLRT of the HVD-treated DPM samples demonstrated a noticeable shift toward higher temperature, from 510.9 to 520.6 °C. The significantly lower mass loss rate (dm/dT) suggests that free radicals generated by HVD and direct contact of the HVD with the EC altered the EC nanostructure to a more orderly organization or a higher degree of graphitization to form more graphene layers, 38,39,43 which resulted in lower soot reactivity. Furthermore, activation energy (E a ) analyses were carried out to evaluate the oxidation kinetics of the DPM samples, where the lower E a results in less energy being required to overcome the energy threshold to begin the oxidation process of the elemental carbon.…”

The effect of nonthermal plasma on the oxidation and removal of particulate matter under different diesel engine loads

“…The effects of NTP on PM microstructure under different loads were investigated by Gao et al [20,21] The results showed that the PM oxidation by NTP first occurred on the particle surface, and the lattice arrangement of the particles was more compact after the NTP treatment. Ji et al [22] investigated the effect of NTP on PM oxidation using Raman spectroscopy and thermogravimetric analysis (TGA). The results showed that the oxidation activity of EC was directly affected by the NTP treatment.…”