Session GT4: Energy and the Environment

Live

Plasma-based CO$_{\mathrm{2}}$ conversion is gaining increasing interest [1]. To improve this application in terms of conversion, energy efficiency and product formation, a good insight in the underlying mechanisms is desirable. We try to obtain this by computer modelling, supported by experiments. We will first provide a brief overview of the state of the art in plasma-based CO$_{\mathrm{2}}$ (and CH$_{\mathrm{4}})$ conversion, with different types of plasma reactors. Subsequently, we will present some recent results obtained in Antwerp in this domain, including experiments and modeling for a better understanding of the underlying mechanisms. This includes modeling the plasma chemistry as well as the reactor design, in different types of plasma reactors commonly used for gas conversion, i.e., dielectric barrier discharges (DBDs), gliding arc (GA) discharges, microwave (MW) plasmas and atmospheric pressure glow discharges (APGDs). For the plasma reactor design, we use 2D or 3D computational fluid dynamics modelling [2]. For the plasma chemistry, we make use of zero-dimensional chemical kinetics modeling, which solves continuity equations for the various plasma species, based on production and loss terms, as defined by the chemical reactions [3]. We will show the role of vibrationally excited CO$_{\mathrm{2}}$ levels for energy-efficient CO$_{\mathrm{2}}$ conversion, as well as the role of thermal conversion in warm plasmas (such as GA and MW plasmas) and quenching after the plasma. We will also show how the performance in CO$_{\mathrm{2}}$ conversion and energy efficiency can be improved in novel reactor designs, developed based on CFD modeling [4,5]. [1] R. Snoeckx and A. Bogaerts, \textit{Chem. Soc. Rev.} 46, 5805-5863 (2017). [2] A. Bogaerts, A. Berthelot, S. Heijkers, St. Kolev, R. Snoeckx, S. Sun, G. Trenchev, K. Van Laer and W. Wang,\textit{ Plasma Sources Sci. Technol.} 26, 063001 (2017). [3] A. Bogaerts, C. De Bie, R. Snoeckx and T. Koz\'{a}k, \textit{Plasma Process. Polym.} 14, e1600070 (2017). [4] G. Trenchev, A. Nikiforov, W. Wang, St. Kolev and A. Bogaerts, \textit{Chem. Eng. J.,} \textbf{362}, 830-841 (2019). [5] G. Trenchev and A. Bogaerts, \textit{J. CO}$_{2}$\textit{ Utiliz.,} \textbf{39}, 101152 (2020).   

Live

In consideration of the increasing consciousness for environmental protection, energy efficient processes for purification of polluted gas streams, e.g. in industrial plants or living space, are growing in demand. These gas streams can be contaminated with pollutants such as numerous hydrocarbons and other chemicals, which are known to be detrimental to the environment and human health. A novel twin surface dielectric barrier discharge for the conversion of volatile organic compounds from gas streams is studied regarding its electrical discharge parameters, power efficiency, gas phase chemistry, and conversion of frequently used hydrocarbons and other chemical pollutants. To this end, techniques such as flame ionization detectors, as well as online gas chromatography-mass spectrometry are used amongst others, to gain insight into the occurring gas-phase chemistry, possible reaction pathways, and advantages of the presented discharge over comparable techniques.   

Live

Volatile organic compounds (VOCs) are detrimental for the environment. Therefore, an efficient removal of VOCs from exhaust gases is necessary. The removal by plasma-assisted catalysis may be a promising replacement for energy-demanding techniques. Dielectric barrier discharges (DBDs) have already shown promising results for VOC conversion in atmospheric pressure plasmas. A combination of plasma with well-known oxidation catalysts can lead to synergistic effects. The applied twin surface DBD geometry has the advantage of a thin catalyst layer which can be deposited in a well-controlled distance to the plasma-ignited area. The thermal oxidation of n-butane has been performed in synthetic air (20.5\% O$_2$, 79.5\% N$_2$) over MnO$_2$ as catalyst up to 450 $^\circ$C. MnO$_2$ achieves conversion of 5\% at 180 $^\circ$C and 95\% at 319 $^\circ$C with closed carbon balance. For the plasma operation n-butane was oxidized in synthetic air. Degrees of conversion of up to 36\% without catalyst and 46\% with mask-coated catalyst were reached. The presence of the catalyst increased both CO$_2$ selectivity and carbon balance. By heating the reactor to 140 $^\circ$C in the presence of the catalyst, conversion increased further to 58\% demonstrating synergistic plasma-catalyst interactions.   

Live

Nanosecond Repetitively Pulsed (NRP) discharges can be used to stabilize lean flames with a low power budget. In this work, a lean premixed methane-air flame (power: 13.6 kW, equivalence ratio: 0.8) is stabilized by NRP discharges at 20 kHz. In this application, the influence of the discharge on the combustion chemistry is mainly driven by (i) the temperature and (ii) N$_{\mathrm{2}}$ excited states (e.g. two-step ultrafast dissociation of oxygen). The plasma temperature can be determined from the rotational temperature of excited species, but this measurement relies on the quenching rate which are rarely known at high temperatures. To assess the validity of the temperature measurements, N$_{\mathrm{2}}^{\mathrm{+}}$(B), N$_{\mathrm{2}}$(B), and N$_{\mathrm{2}}$(C) rotational temperatures were determined. Within uncertainties, they are found to be equal. Then, we performed absolute optical emission spectroscopy of N$_{\mathrm{2}}$(B-A), N$_{\mathrm{2}}$(C-B), and N$_{\mathrm{2}}^{\mathrm{+}}$(B-X) to quantify the number densities of N$_{\mathrm{2}}$(B), N$_{\mathrm{2}}$(C), and N$_{\mathrm{2}}^{\mathrm{+}}$(B). They correspond to previous results with the same discharge in preheated air, despite the different gas composition. Also, using Stark broadening of the H$_{\mathrm{\alpha \thinspace }}$line, the electron number density is found to increase up to approximately 10$^{\mathrm{16}}$ cm$^{\mathrm{-3}}$ ($\approx $ 1{\%} of ionization).   

Live

Low-temperature plasmas have recently emerged as possible pathway for decentralized nitrogen fixation, a promising alternative to the Haber Bosch process. While several reports confirm that it is possible to produce ammonia using a plasma, these works also highlight a weak dependence on the catalyst material of choice. This last aspect is not consistent with established models. In this contribution, we present the results of a combined experimental/computational investigation on the formation of ammonia in a low-pressure hydrogen-nitrogen discharge. In good agreement with previous literature, we observe that the degree of nitrogen fixation has a weak dependence on the catalyst of choice. Ab-initio Born Oppenheimer molecular dynamics simulations are deployed to elucidate the origin of this behavior. It is found that ammonia production follows an Eley-Rideal mechanism. The weak difference in the ammonia production yield displayed by the materials stems from their different ability to store atomic nitrogen at their surface. The atomic hydrogen flux to the catalyst surface, characterized via optical emission spectroscopy, largely exceeds that of atomic nitrogen, making the ammonia production yield mass transport limited by the flux of atomic nitrogen.   

Live

PFAS is a persistent contaminant now ubiquitous in the environment and particularly in freshwater bodies. The contaminant is derived from industrial processing, fire fighting foaming agents, and consumer products such as nonstick coatings. This contaminant bio-accumulates in humans and can be a source of a variety of diseases including cancer. PFAS compounds feature the fluorine carbon bond, which is among the strongest in organic chemistry making it difficult to decompose by conventional means such as advanced oxidation. The bond can be destabilized via reduction processes driven by solvated electrons. Reduction driven decomposition of PFAS has been demonstrated by the Clarkson group using a multidischarge apparatus with argon gas. The multidischarge improves the plasma liquid contact surface area. Recently a plasma reactor with dielectric layers has been developed at Michigan that optimizes the plasma liquid contact surface area by disposing water into a multiplicity to narrow water jets with high surface area to volume. Plasma can be excited between the water jets akin to a packed bed dielectric barrier discharge but in this case with water as the dielectric barriers. The plasma is in contact with significant surface area and can be considered de facto in volume water treatment. This high contact area geometry affords plasma electrons to solvate through much of the water as it passes through the reactor. We demonstrate the ability of this device to efficiently decompose PFAS using air as the cover gas. The time evolution of PFAS contaminated well water as a function of time is presented for water derived from two highly contaminated wells with and without pretreatment (removal of suspended solids associated with the well water). Resulting plasma induce water chemistry is also commented upon.   

Live

The thermal and chemical effects of nanosecond discharges have been thoroughly investigated in previous work at atmospheric pressure in preheated air at 1000 K. N$_{\mathrm{2}}$ is excited by electron impact reactions, radicals and heat are produced via dissociative quenching of excited electronic states of N$_{\mathrm{2}}$ by O$_{\mathrm{2}}$. In this work, we extend these investigations to NRP discharges in partially burned, premixed methane-air flames. A burst of 500 discharges is applied to stabilize a 10-kW lean flame at atmospheric pressure. The shape of the flame is strongly modified by the heat and radicals produced by the discharge. We study the transient production of OH radicals and heat to identify the effects that promote flame stabilization. First, we record time-resolved OH* chemiluminescence images during the transition from a flame without plasma to the steady-state plasma-stabilized flame. Steady state is reached at the end of the burst. Then using calibrated optical emission spectroscopy, we measure the rotational temperature and the N$_{\mathrm{2}}$(C) number density evolution during the transient stabilization. Laser-Rayleigh scattering is also applied to confirm the gas temperature evolution obtained by emission spectroscopy.   

On Demand

Plasma-assisted catalyst reduction of NO$_{\mathrm{x}}$ is an effective exhaust gas treatment method at low temperatures. Compared to traditional catalyst, the energy can be more efficiently used when the plasma is combined. To date, most of plasma-catalytic studies on NO$_{\mathrm{x}}$ reduction have been investigated in DBD or packed-bed reactors with a narrow discharge gap. Environmental catalysts should be able to treat a large flow rate, but there are several bottlenecks for practical applications of the packed-bed or DBD-based systems in terms of pressure drop and scale-up, though the plasma-catalyst combination has been demonstrated to be successful in small-scale systems. Such problems can be overcome by using honeycomb catalysts. A honeycomb catalyst can avoid the pressure drop and allow a large flow rate. In this study, corona plasma discharge in honeycomb catalyst has been investigated for improving the catalytic activity at low temperatures. The corona discharge is created with two perforated disks serving as high-voltage and ground electrodes. The optimal condition for corona discharge inside the honeycomb channels is determined, and then the effect of various parameters on the NO$_{\mathrm{x}}$ reduction is examined   

Ion Energy and Angular Distributions onto Surfaces of Catalysts in Atmospheric Pressure Plasmas

Plasma catalysis is gaining increasing interest as a means of efficient and selective chemical conversion. The manner in which plasmas activate catalytic processes is poorly known. In a packed bed reactor (PBR) the discharge propagating along the surface of a catalyst pellet can produce electric field enhancement in addition to that naturally generated by the polarization and shape of the pellet. This electric field enhancement can then accelerate ions into the surface of pellet which, even at atmospheric pressure, can produce bursts of ion energies in excess of tens of eV. These high energy ions in turn can affect the surface chemistry. In this paper, we report on results from a computational investigation of the ion energy and angular distributions (IEADs) to surfaces for atmospheric pressure plasmas propagating in a PBR in the presence of catalyst particles. The two-dimensional \textit{nonPDPSIM} modeling platform was used in which IEADs are computed using Monte Carlo techniques. We show that ions with high energies can be delivered to surfaces while changing the surface conditions and thus influence the behavior of surface streamers. We discuss the details of the IEADs which depend on the size of the catalyst, its shape, dielectric constant and conductivity.   

On Demand

Shockwaves produced by underwater plasmas have the ability to induce cracks in rocks. Underwater plasma spark (nanosecond pulsed) initiates a cavitation bubble similar to a snapping shrimp's burrowing activity. The cavitation event is accompanied with intense shockwaves capable of cracking rock surface. This reduces the integrity of the rock which makes it easier to drill and potentially increases drilling rate of penetration. The effect of low energy (\textasciitilde 80J) nanosecond plasma pulses on crack extension and cutting energy reduction in granite rock was studied. A 56\% reduction in cutting energy was observed with cracks extending up to 0.3 inches in length. Nanosecond plasma pulsing was also carried out at downhole pressure conditions (5000 psi) and its effect on rock cracking was studied. This work also presents the incorporation of SPARC plasma technology in traditional drill bits. SPARC-drill bits were tested in an in-house fabricated ambient pressure laboratory drill rig that uses a rotating air spark switch to produce nanosecond plasma pulses. The observed reduction in cutting energy due to SPARC show that it may be useful in aiding the traditional drilling method, thereby reducing drilling operational time.