Performance evaluation of non-thermal plasma on particulate matter, ozone and CO2 correlation for diesel exhaust emission reduction
Introduction
The road sector accounts for about three quarters of transport emissions and passenger cars and light trucks contribute to a considerable share of these emissions [1]. On the other hand, diesel engine applications in various heavy-duty and medium-duty vehicles are increasing compared to gasoline engines. The emissions produced by diesel engines, however, are a ubiquitous air pollutant consisting of a complex mixture of gases, vapour and particles. The negative health effects of diesel emissions have been emphasised in the literature [2], [3] and lately, diesel exhaust has been classified as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer (IARC) [4].
Carbon monoxide (CO), hydrocarbon (HC), Nitrous Oxides (NOx) and diesel particulate matter (DPM) have been globally regulated by diesel emission standards. DPM is basically composed of elemental carbons, which results in agglomerating and also absorbing other particles to form structures of complex physical and also chemical properties [5]. Up until now, several after-treatment technologies, such as diesel oxidation catalyst (DOC) [6], diesel particulate filter (DPF) [7], selective catalyst reduction (SCR) [8] and fuel borne catalyst (FBC) [9] have been employed to reduce diesel exhaust emissions. However, there are some drawbacks in using conventional after-treatment systems. For example, SCR catalysts require high temperatures (around 300 °C) for activation and there is the possibility of ammonia leakage, catalyst poisoning and catalyst discharge under high temperature conditions [10]. DPFs also produce an additional pressure drop inside the exhaust gas, due to the PM deposition. This deposition can cause filter choking and filter regeneration is required at about 600 °C. These effects cause more fuel consumption, which is not appropriate for low emission production and fuel economy. Furthermore, DPFs are inefficient in trapping small nanoparticles under 100 nm [11].
Considering the increasing environmental concerns and stringent emission standards, there is an imperative to develop new strategies for emission reduction [12]. Non-thermal plasma (NTP) technology has shown notable potential for emission control in various applications [3], [13], [14]. Plasma is the fourth state of matter that can be considered as an ionised gas. In the plasma state, sufficient energy is provided to free electrons from atoms or molecules and to allow species, ions and electrons, to coexist. Based on the relative temperature of the gas, plasmas can be classified into thermal and non-thermal plasma (NTP) [15]. In non-thermal plasma, the kinetic energy (temperature) of charged particles and kinetic energy (temperature) of background gas are similar [16]. In the NTP application for exhaust emission reduction, the input electrical energy is transferred to the electrons which generates free radicals through collisions of electrons and promotes the desired chemical changes in the exhaust gas. While the applied electric energy in NTP reactors will be consumed for the purpose of breaking the bonds in the parent molecules, there is no sensible heating of the gas, and discharged energy is not lost either in heating up the gas or to the surroundings [17].
A variety of research studies, concerning different aspects of NTP application for NOx removal, have been documented in the literature [3], [18]. NTP NOx reduction generally can be divided in two groups: NOx removal reactions that result in NOx reduction to N2 and NO to NO2 conversion reactions which is more dominant [3]. In comparison with the large amount of research conducted on the influence of plasma on NOx removal, much less has been dedicated to the effect of plasma on PM removal. Okubo et al. employed indirect or remote NTP for DPF regeneration [19], [20]. In this method, plasma is not introduced into the exhaust gas directly. Instead, plasma was introduced into the air and the NTP-treated air was injected into the exhaust gas, which causes the NO oxidation to NO2. This induced NO2 with other produced activated oxygen species by plasma oxidise deposited carbon soot on the DPF surface effectively. Furthermore, the simultaneous PM, HC and NOx removal efficiency of about 80%, 70% and 65% was reported in literature from the exhaust gas [21].
Few studies on PM removal of diesel engines have been conducted in the literature and removal mechanisms have not been studied extensively [3], [22], [23]. Furthermore, most of the research in this area was considered simulated diesel exhaust instead of actual exhaust. Many electron-impact reactions such as momentum transfer, dissociation, ionisation reaction, etc. and also numerous secondary reactions induced by products of the electron-impact reactions are possible in the plasma state [24]. This would be more complicated when plasma treatment of the diesel exhaust which is a complex mixture of thousands of gases is targeted. Therefore, the results of experiments for actual diesel exhaust may be different from what is happening in simulated exhaust. Furthermore, recently ozone has been studied as a possible solution for PM removal even in industries [12]; however, the advantage of ozone production in NTP systems for PM removal is not well established.
The objective of this paper is to investigate the effect of NTP on reduction of PM emitted by a diesel engine and oxidise it to less harmful combustion products (essentially carbon dioxide) at different discharge voltages and repetition rates. Furthermore, the mechanism of PM removal has been discussed in detail and the effect of ozone as the key parameter for PM removal has been highlighted. To examine the system in real condition which is the first step towards the applicability of NTP as an after-treatment system, the actual exhaust emission has been examined in this research.
Section snippets
Experimental setup
The experiments have been conducted using a modern turbo-charged 6 cylinder Cummins diesel engine. The engine has a capacity of 5.9 L, a bore of 102 mm, a stroke length of 120 mm, compression ratio of 17:3:1, and maximum power of 162 kW at 2500 rpm. All experiments have been conducted at a speed of 1500 rpm and at the load of 160 Nm to achieve the most uniform exhaust gas performance. For all experiments, exhaust gas from the exhaust pipe was passed through the dilution tunnel before flowing into the
Results and discussion
When plasma is introduced inside the exhaust gas, oxidation processes will be started and NOx, unburned hydrocarbons, and particulate matter can be oxidised [22]. In addition, due to the effectiveness of NTP on the removal of a variety of species inside the exhaust, the concentration variation of a special component may affect the other species differently during the experiments. In this paper, different contour plots have been presented to evaluate the effect of NTP on discharge power, ozone
Conclusion
In this paper, the NTP technique has been employed for emission reduction of actual diesel exhaust. NTP has been introduced inside the diesel exhaust by using a DBD reactor. Measurements have been conducted before and after introducing plasma inside the reactor to illustrate the effect of NTP on diesel emissions. The effect of NTP on PM, CO2 and ozone has been considered experimentally and the interrelationship between them has been clarified. Discharge power has been calculated at each
Acknowledgments
The authors would like to thank the Biofuel Engine Research Facility (BERF) at QUT for providing experimental facilities. The authors also gratefully acknowledge Mr. Mostafizur Rahman for his assistance during the experiments.
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