Session FF2: Green Plasma Science & Technology IV

Interplay of Transport, Plasma Concentration, and Chemistry in Microwave Discharges

Microwave plasma is investigated as gas conversion technology for its compatibility with intermittent power sources and its promise in efficiency and selectivity, e.g. in dissociation of CO₂ or carbon coupling in CH₄. Understanding of transport and power density and their relation to plasma concentration are key for optimization of performance and scale up of the technology. In this work, we demonstrate the dominant phenomena at play in a comparison of CO₂ and CH₄ discharges in Forward Vortex (FV) and Reverse Vortex (RV) flow, involving maps of composition (CO₂, CO, O₂ and O) and temperature measured with Raman scattering.

Large differences in temperature and composition are observed. In CO₂, we measure temperatures below 4500 K in RV versus 6000 K in FV and consequently lower dissociation degrees in the plasma core for RV versus FV. In CH₄, the macroscopic production of soot is entirely suppressed. It will be shown how these observations correlate with measured global gas conversion performance.  

In effect, a decoupling of core temperature and composition with power density is achieved, enabling concentrated plasma conditions at different gas temperature. Furthermore, the work shows that plasma concentration results from a balance between ambipolar diffusion and recombination. Such aspects underline the importance of gas flow geometry for reactor parameters and it will be sketched how these can play an important role in future reactor engineering and design, in particular in relation with ongoing R&D for electrification of process industry.

    

High Efficiency CO2 Conversion in the Rotating Argon Flow using Microwave Plasma at Atmospheric Pressure.

The CO₂ conversion in Ar flow using the microwave plasma was investigated with relatively high power (max. 1.5 kW) 2.45 GHz microwave cavity system at atmospheric pressure. We used small diameter quarts tubing (8 mm, inner radius) as a reactor. At the upstream, the rotating Ar flow was introduced to protect the wall of the reactor tube from the contact of high temperature plasma plume. CO₂ was introduced into the center of the tube by a small nozzle, then mixed with the rotation flow.  The maximum conversion efficiency and the energy efficiency were 76.5 % at 95 SCCM of CO₂ and 16.8 % at 6 SLM of CO₂, respectively in 15 SLM Ar rotating flow. In the case of the 6 SLM of CO₂, also we attained 20 % for conversion efficiency at 1.4 kW discharge power. In this study, we obtained comparatively higher efficiencies than that of the latest reported atmospheric pressure plasma systems. The rotating flow may not only protect the quarts wall from the plasma but also confine the plasma plume at near of the center axis that will proceed the decomposition reaction. According to the parameter analysis, we concluded that the higher power to the system, we will get the better conversion efficiency even as in the energy efficiency.

    

Direct non-oxidative methane conversion in arc plasma reactor: Physical and chemical solutions to lower energy cost

Methane conversion to higher hydrocarbons such as ethylene and aromatics has been referred as a great challenge in the petrochemical industry, which is gaining the increasing interest because of increasing cost of other fuel feedstock compared with methane cost (the least expensive feedstock). This work presents key solutions (i.e., related to both physics of plasma reactor and chemistry of the process) to increase the energy efficiency of methane conversion. The physics of plasma reactor herein mentioned to increase the energy transfer from arc to the gas against energy loss by modifying the plasma reactor design and increasing the length of arc column. The chemistry of reaction process herein referred to the effect of additional gases diluted with methane. The study indicated that modifying reactor design significantly increased the energy efficiency by virtue of focusing arc. Moreover, elongation of arc length is the key parameter to further reduce the energy cost. In the chemistry point of view, all of additional gases diluted with methane has a negative effect of energy cost of acetylene production. However, in the point of acetylene selectivity; hydrogen has a strong effect on the reaction kinetics of methane pyrolysis process in order to prevent methane dissociation and inhibit the carbon solid formation to increase acetylene selectivity. Finally, we successfully created the arc discharge in an optimized arc plasma reactor (called nozzle-type rotating arc reactor) using pure methane, which results in a very low energy cost (9.8 kW/kg-C₂H₂), reported the first time. This achievement is an important leap of methane-to-acetylene conversion process because of its extremely reduced energy cost and using the simplest discharge gas (pure methane); other additional gases like H₂, N₂, Ar, or He are removed from the discharge gas mixture, as a results, the burdens for experiment preparation and product separation are decreased.   

Effect of pulse repetition rate on filamentary discharge assisted low-temperature ignition in methane-air flows

Nanosecond repetitively pulsed (NRP) discharges are an efficient and fast source of radicals and heat and can ignite fuel-air mixtures in extreme conditions. We present pulse-to-pulse coupling in NRP discharges at 1 to 30 kHz pulse repetition rate (PRR) and its influence on ignition of methane-air flows. Contrary to NRP discharges in pin-to-pin configurations, our DBD electrode configuration allows pulse-to-pulse coupling in the entire studied PRR range while maintaining plasma in a non-equilibrium regime. We use three diagnostics tools for plasma and combustion parameters characterization: 1) High-speed intensified imaging, 2) phase-locked OES, and 3) electrical characterization. The effect of the number of pulses per burst and the PRR on ignition have been evaluated. We found that 2 to 5 pulses per burst are required to ignite the mixture depending on energy coupling into plasma per pulse. Moreover, the PRR affects the ignition kernel growth rate, but the final kernel size only depends on the number of pulses per burst. We estimate pulse-to-pulse plasma gas temperature and effective E/N using OES. We observe an increase in plasma pulse-to-pulse gas heating with increasing PRR even though energy coupling per pulse is similar which we attributed to filament contraction.   

Plasma induced conversion of CO2 with water to useful compounds

Global energy consumption will increase 28% between 2015 and 2040, according to a report by the U.S. Department of Energy’s Energy Information Administration (EIA). The increased energy use will be matched by a 16% increase in energy-related carbon dioxide (CO₂) emissions over that same period, with annual emissions rising from 33.9 billion metric tons in 2015 to 39.3 billion metric tons in 2040, according to EIA’s report. The CO₂ conversion to produce sustainable carbonaceous fuels is the most promising option to tackle greenhouse gases in the short to medium term. CO₂ is a promising renewable, cheap, and abundant C1 feedstock for producing valuable chemicals, such as CO.

Nonthermal plasma is a better option, as it can convert CO₂ at near-ambient temperature and pressure [1-3]. The reduction of CO₂ with H₂O was carried out using the Arc plasma and possible production of organic compounds in the liquid phase and formation CO in the gas phase. The measured net conversion of CO₂ to CO was close to 4% under optimal conditions. Although we also performed the 2D simulation using COMSOL Multiphysics® software to determine how the potential reaction generated gas phase was diffuse to the liquid phase.

 

Plasma-enhanced Carbon Capture and Utilization in CO2 Methanation

To realize a carbon neutral society, methanation of CO₂ can be used to convert exhausted CO₂ into renewable methane using CO₂-free energy source such as renewable electricity. This study focuses on nonthermal plasma effect on CO₂ methanation over Ru based multi-metallic catalyst. To this end, CO₂ conversion behaviors in a packed-bed dielectric barrier discharge (PB-DBD) reactor at 30 kPa were investigated. The low-temperature CO₂ methanation can be improved by applying DBD to the reaction because nonthermal plasma would contribute to activating CO₂ into vibrational states. Besides, Ruthenium plays a significant role in plasma-enhanced reaction performance while no promotion by plasma was found out on the catalyst without Ru. In addition, increasing discharge frequency at constant power can further improve methanation performance because high-frequency operation enhances generation of vibrationally excited CO₂, further accelerating carbonate formation. To better understand reaction mechanism in DBD environment, in situ Fourier-transform infrared spectroscopy (FTIR) was employed.   

Chemical Feedback and Control of Chemical Processes Using Non-Equilibrium Plasmas

A small-scale energy production plant must deal with variations in feedstock composition and flow properties near fossil resources. We use non-equilibrium plasmas and observe that varying the applied electric field, discharge frequency and specific energy input (SEI) maintains constant product selectivity and reactant conversion in the partial oxidation of methane. When SEI is constant, we observe that selectivity toward methanol remains constant and conversion of methane decreases with increasing voltage. Whereas for a given feedstock composition, changing SEI exchanges selectivity for conversion. We develop scaling and kinetic laws to describe this thermodynamic constraint and use it to select methanol production and methane conversion for different compositions and applied electric fields. We show that with fluctuations in feedstock composition at the reactor inlet, changing SEI and voltage enable dynamic control over the selectivity and conversion. Our measurements demonstrate that non-equilibrium plasmas, in combination with a feedback loop, enable active control over product purity and selectivity for chemical processes over distributed scales.

    

Photo-plasma: A new approach for efficient and enhanced mineralization of organic molecules

Water contamination from complex organic molecules has become a significant cause of worry due to conventional treatment systems' poor response to their removal. Therefore, developing an improved and environmentally friendly treatment system is critical to achieving water quality objectives and protecting human health. The present work develops a novel and chemical-free treatment system by coupling surface dielectric barrier discharge (SDBD) with high energetic UV-C source. The improved ability of the hybrid reactor (photo-SDBD) to degrade and mineralize complex organic molecules is demonstrated by taking brilliant red 5B (C₁₂H₁₈N₃Na₃O₁₄S) as the model pollutant. The influence of pollutant feed concentration, solution pH, and background salts on brilliant red 5B degradation is studied. The photo-SDBD reactor showed unprecedented improvement in mineralizing brilliant red 5B compared to SDBD and UV-C.

Notably, Glauber's salts and solution pH did not influence the photo-SDBD performance. The data reveals that the new system could overcome the quenching of hydroxyl radicals due to background salts and recombination of hydroxyl radicals (•OH) and their conversion to hydrogen peroxide (H₂O₂). The analysis of reactive chemical species confirms a five-fold increase in the generation of hydroxyl radicals in photo-SDBD compared to SDBD. The system also showed a 2 to 10 times reduction in degradation time than SDBD or UV-C. This is attributed to breakdown of H₂O₂ in the presence of UV-C to •OH and possibly increased contact of reactive chemical species with the target contaminant. We believe that the present study would open pathways for developing a more efficient and green advanced treatment system to remove complex and recalcitrant organic molecules.