Session DT3: Chemical Kinetics/Combustion & Environmental

Experimental Investigation of Pulsed Nanosecond Streamer Discharges for CO$_{\mathrm{2\thinspace }}$Reforming

Rapid global industrialization has led to an increase in atmospheric greenhouse gases, specifically carbon dioxide levels. Plasmas present a great potential for efficient reforming of greenhouse gases. There are several plasma discharges which have been reported for reforming process: dielectric barrier discharges (DBD), microwave discharges, and glide-arcs. Microwave discharges have CO$_{\mathrm{2}}$ conversion energy efficiency of up to 40{\%} at atmospheric conditions, while glide-arcs have 43{\%} and DBD 2-10{\%}. In our study, we analyze a single nanosecond pulsed cathode directed streamer discharge in CO$_{\mathrm{2}}$ at atmospheric pressure and temperature. We have conducted time resolved imaging with spectral bandpass filters of a streamer discharge with an applied negative polarity pulse. The image sequences have been correlated to the applied voltage and current pulses. From the spectral filters we can determine where spatially and temporally excited species are formed. In this talk we report on spectroscopic studies of the discharge and estimate plasma properties such as temperature and density of excited species and electrons. Furthermore, we report on the effects of pulse polarity as well as anodic streamer discharges on the CO$_{\mathrm{2}}$ conversion efficiency. Finally, we will focus on the effects of vibrational excitation on carbon dioxide reforming efficiency for streamer discharges. Our experimental results will be compared with an accompanying plasma computational model studies.   

Plasma assisted ignition with nanosecond surface dielectric barrier discharge. Two modes of nanosecond surface discharge

Nanosecond surface dielectric barrier discharge (nSDBD) is an efficient tool for a multi-point plasma-assisted ignition of combustible mixtures at elevated pressures. In combustible mixtures, nSDBD initiates numerous combustion waves propagating from the electrode. This work presents a comparative experimental study of the surface dielectric barrier discharge initiated by high voltage pulses (U $=$\textpm (20-60) kV) of different polarities in air at elevated pressures (P$=$1$-$12 bar). Discharge morphology, deposited energy, and spectroscopy of the discharges are analyzed. Differences between the discharges of the different polarity, as well as the changes in the discharge morphology with changing of a gas mixture composition, are discussed. The initiation of combustion with nSDBD was studied experimentally at high initial pressures up to 6 bar on the example of lean H$_{\mathrm{2}}$/Air. The ignition is initiated with two different discharge modes: streamer and filamentary. The influence of the discharge structure and energy deposition on the ignition is demonstrated. Three regimes of multi-point ignition were observed: ignition with a few kernels, quasi-uniform ignition along the edge of high voltage electrodes and ignition along the plasma channels.   

Green technology for conversion of renewable hydrocarbon based on plasma-catalytic approach

The ability to convert renewable biomass into fuels and chemicals is one of the most important steps on our path to green technology and sustainable development. However, the complex composition of biomass poses a major problem for established conversion technologies. The high temperature of thermochemical biomass conversion often leads to the appearance of undesirable byproducts and waste. The catalytic conversion has reduced yield and feedstock range. Plasma-catalytic reforming technology opens a new path for biomass conversion by replacing feedstock-specific catalysts with free radicals generated in the plasma. We studied the plasma-catalytic conversion of several renewable hydrocarbons using the air plasma created by rotating gliding discharge. We found that plasma-catalytic hydrocarbon conversion can be conducted at significantly lower temperatures (500 K) than during the thermochemical ($\approx$ 1000 K) and catalytic (800 K) conversion. By using gas chromatography, we determined conversion products and found that conversion efficiency of plasma-catalytic conversion reaches over 85 $\%$. We used obtained data to determine the energy yield of hydrogen in case of plasma-catalytic reforming of ethanol and compared it with other plasma-based hydrogen-generating systems.   

Enhancement of NO$_{\mathrm{x}}$ and hydrocarbon conversion in plasma-activated catalysis

Atmospheric pressure, non-thermal plasma-activated-catalysis is showing real promise in a number of applications. Here we report on how electrical, visible and FTIR spectroscopy and mass spectroscopy measurements in a kHz atmospheric pressure He plasma jet coupled with a Ag/Al$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$catalyst allowed us produce and confirm a strong enhancement of both NO$_{\mathrm{x}}$ and hydrocarbon conversion at a measured gas temperature of $\le $250 \textdegree C [1]. How these and other measurements have provided an insight into the fundamental physical and chemical processes in the plasma environment that have helped us move to a more efficient system and other processes will be discussed. [1] Stere C E,~Adress W,~Burch R,~Chansai S,~Goguet A,~Graham W G,~De Rosa F,~Palma V and Hardacre, C. ACS Catalysis \textbf{4} (2014) 666-673   

Conversion of CO2 to CO using radio-frequency atmospheric pressure plasmas

Low temperature plasmas can be used for the in situ generation of CO, from relatively non-toxic CO$_2$. CO is very useful in many industrial chemical processes and so, via low temperature plasmas, CO$_2$, a waste product, can be converted into a valuable chemical. The key challenges in using this method, for CO production, are optimising the energy efficiency, maximising the conversion of CO$_2$ into CO and then separating the CO from the other species produced in the plasma. Very high yields of CO, greater than 90\%, have been achieved at atmospheric pressure using argon as a carrier gas with admixtures up to 1.5\% with energy efficiencies of up to 4\%. The plasma generated in continuous and spatially homogeneous and is driven at a frequency of 40.68 MHz. A zero dimensional global model has also been used to simulate the chemical kinetics of the plasma to determine the dominant dissociation processes and is in good agreement with the experimentally determined yields. The model is used to determine how important a role the vibrational states of CO$_2$ are, in a highly collisional plasma, to the production of CO and there can provide insight into how to improve the energy efficiency and suppress unwanted reactions.   

Study of nanosecond discharges in different H$_2$ air mixtures at atmospheric pressure for plasma-assisted applications

This paper presents 2D simulations of nanosecond pulsed discharges between two point electrodes in different H$_2$/air mixtures and in air at atmospheric pressure. A fluid model is coupled with detailed kinetic schemes for air and different H$_2$/air mixtures to simulate the discharge dynamics. First, as the positive and negative ionization waves propagate in the interelectrode gap, it has been observed that in H$_2$/air mixtures with equivalence ratios between 0.3 and 2, major positive ions produced by the nanosecond discharge are N$_2^+$, O$_2^+$ and HN$_2^+$. The discharge dynamics is shown to vary only slightly for equivalence ratios of the H$_2$/air mixture between 0.3 and 2. Then, as the discharge transits to a nanosecond spark discharge, we have studied the different chemical reactions that lead to fast gas heating and to the production of radicals, as O,H and OH. Both thermal and chemical effects of the nanosecond spark discharge are of interest for plasma assisted combustion applications.   

Non-equilibrium kinetics of plasma-assisted combustion: the role of electronically excited atoms and molecules

A review of experimental and theoretical investigations of the effect of electronically excited atoms and molecules on the induction delay time and on the shift of the ignition temperature threshold of combustible mixtures is presented. At relatively low initial gas temperature, the effect of excited O($^{\mathrm{1}}$D) atoms on the oxidation and reforming of combustible mixtures is quite significant due to the high rates of reactions of O($^{\mathrm{1}}$D) atoms with hydrogen and hydrocarbon molecules. The singlet oxygen molecules, O$_{\mathrm{2}}$(a$^{\mathrm{1}}\Delta _{\mathrm{g}})$, participate both in chain initiation and chain branching reactions, but the effect of O$_{\mathrm{2}}$(a$^{\mathrm{1}}\Delta _{\mathrm{g}})$ in the ignition processes is generally less important compared to the oxygen atoms. To reduce the ignition delay time and decrease the temperature threshold of fuel-air mixtures, the use of gas discharges with relatively high E/N values is recommended. In this case the reactions of electronically excited N$_{\mathrm{2}}$(A$^{\mathrm{3}}\Sigma _{u}^{+} $, B$^{\mathrm{3}}\pi_{\mathrm{g}}$, C$^{\mathrm{3}}\pi _{\mathrm{u}}$, a'$^{\mathrm{1}}\Sigma_{u}^{-} )$ molecules, and atomic particles in ground and electronically excited states are extremely important. The energy stored in electronic excitation of atoms and molecules is spent on the additional dissociation of oxygen and fuel molecules, on the fast gas heating, and finally to the triggering of chain branching reactions.