Fig 7 - uploaded by Georgios D. Stefanidis
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Ethylene selectivity as function of reaction temperature; black line: gas phase hydrogenation reaction; red line: hydrogenation reaction catalyzed by the HV electrode; green line: hydrogenation reaction catalyzed by the ground electrode. The experiments were performed at a residence time of 40 s and ambient pressure.
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We present a study on the reaction pathways involved in non-oxidative methane coupling in a nanosecond pulsed discharge reactor using isotope analysis. Specifically, plasma-assisted reactions with isotopes, serving as tracers, are performed in two ratios (CH4:D2=1:1 and CH4:D2=1:3) and elevated pressures (up to 5 bar). Acetylene hydrogenation react...
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... (Fig. 6). Albeit stainless steel does not boost ethylene for- mation, higher ethylene selectivity is obtained when the hydrogenation reaction occurs in the presence of copper within 200-400 °C. Re- markably, ethylene selectivity significantly increases between 200 °C and 350 °C (4 and 2 times, respectively) as compared to the gas phase reaction (Fig. 7). In conclusion, homogeneous acetylene hydrogenation occurs in the post plasma zone of the reactor, due to the relatively high bulk gas temperature and hydrogen excess, and can significantly be promoted by the fabrication material of the plasma reactor ...
Citations
... In general, non-thermal plasmas such as dielectric barrier discharges (DBD), have lower conversion and higher selectivities for C 2 H 6 and C 3 -C 5 hydrocarbons [6][7][8]. Other plasmas that may transition between thermal and non-thermal states, such as pulsed discharges, microwave (MW) plasmas, and gliding arc discharges, tend to have higher conversion and higher selectivities for C 2 hydrocarbons and H 2 [6,7,[9][10][11][12][13][14][15]. It should be noted that MW plasmas can achieve > 90% conversion below atmospheric pressure; however, throughput is low due to the low pressures involved. ...
The kinetics of methane decomposition in low frequency (60 Hz) AC arc plasmas was investigated using on-line mass spectrometry and optical emission spectroscopy (OES) in a batch reactor configuration at pressures up to 3 bar absolute. Plasma conversion of CH4 results largely from thermal dissociation and was seen to follow first-order kinetics up to high conversions (> 90%) without observing any rate impedance from reverse hydrocracking. H– and C-atom selectivities for H2, C2H2, and C2H4 were 78% (1.56 mol H2/mol CH4 reacted), 36% (0.18 mol C2H2/mol CH4), and 30% (0.15 mol C2H4/mol CH4), respectively, at 3 bar. In other experiments, H2 diluent concentration played an important role in CH4 dissociation and final product distributions; H abstraction reactions increased the rate of CH4 decomposition at low H2 (yH2 < 0.6) while high H2 (yH2 > 0.6) impeded CH4 decomposition due to hydrocracking of C2 products. The rate of CH4 dissociation was seen to increase with pressure, up to 0.11 mol/m³/s, and the specific energy requirement (SER) decreased with pressure to 365 kJ/mol CH4 at 3 bar. The latter suggests that even higher operating pressures may improve the efficiency of plasma conversion of CH4, and ultimately that plasma pyrolysis may be a viable and energy efficient route to clean (turquoise) H2 and further implementation of chemical process electrification.
... It can dimerize into C 2 H 6 , which is considered the initial product of methane pyrolysis [10], or undergo subsequent dehydrogenation into CH x radicals (i.e., CH 2 , CH) [36]. Subsequently, the C 2 compounds can be produced via ethylene dehydrogenation and CH x radical coupling [37]. ...
... A significant contribution to the pulsed plasma application in methane coupling was made by S. Yao et al. [69,[74][75][76] at the beginning of the 2000s. In their studies, they In recent years, the application of nanosecond pulsed plasma in methane coupling was comprehensively investigated and developed by M. Scapinello, E. Delikonastantis, and G. D. Stefanidis [37,71,72,77]. In their research, they managed to significantly increase C 2 H 4 selectivity by increasing the reactor's pressure (up to 5 bar) and co-feeding hydrogen [70], as presented in Figure 6. ...
... It should be noted that no external hydrogen or heat was needed for this process, as both were provided by plasma methane decomposition. The reason for the increases in C 2 H 4 selectivity due to overpressure was investigated with the use of simulation and isotopes [37,72]. It was revealed that one of the main channels of C 2 olefins' formation goes through C 2 H 3 radicals, which tend to dehydrogenate at atmospheric pressure, forming C 2 H 2 , and hydrogenate at a pressure from 3-5 bar to C 2 H 4 [72]. ...
With the increasing role of hydrogen in the global market, new ways of hydrogen production are being sought and investigated. One of the possible solutions might be the plasma pyrolysis of methane. This approach provides not only the desired hydrogen, but also valuable carbon-containing products, e.g., carbon black of C2 compounds. This review gathers information from the last 20 years on different reactors that were investigated in the context of methane pyrolysis, emphasizing the different products that can be obtained through this process.
... The reaction pathways that shifted the product selectivity from acetylene to ethylene were determined via an isotope analysis. It was found that higher bulk gas temperatures imposed by the overpressure (>3 bar) activate direct gasphase methane coupling to ethylene and suggested that some acetylene hydrogenation to ethylene takes place at the copper-based reactor electrode [18]. ...
... At this pressure, the C 2 H 2 and C 2 H 6 yields account for less than 5% of product distribution. This product selectivity shift can be attributed to the direct CH 2 radical coupling (with CH 3 ) to ethylene and C 2 H 3 hydrogenation with H radicalsboth reactions are enhanced by high bulk gas temperatures imposed by the overpressure (>3 bar) -as revealed by the isotopic analysis previously performed by Stefanidis and co-workers [18] and further explored in the reaction pathway analysis provided by the modelled results (section 3.5). Moreover, in pulsed plasmas, catalytic hydrogenation occurring at the surface of the copper-based HV electrode also has an effect on the improved C 2 H 4 selectivity at higher pressures [18], owing to the ability of copper to promote C 2 H 2 to C 2 H 4 hydrogenation reactions [31]. ...
... This product selectivity shift can be attributed to the direct CH 2 radical coupling (with CH 3 ) to ethylene and C 2 H 3 hydrogenation with H radicalsboth reactions are enhanced by high bulk gas temperatures imposed by the overpressure (>3 bar) -as revealed by the isotopic analysis previously performed by Stefanidis and co-workers [18] and further explored in the reaction pathway analysis provided by the modelled results (section 3.5). Moreover, in pulsed plasmas, catalytic hydrogenation occurring at the surface of the copper-based HV electrode also has an effect on the improved C 2 H 4 selectivity at higher pressures [18], owing to the ability of copper to promote C 2 H 2 to C 2 H 4 hydrogenation reactions [31]. In a future follow-up, we intend to expand this work to investigate this effect under these conditions both on experimental and computational fronts. ...
... [40] Some literature also supports that hydrogenation of C 2 H 5 radical (C 2 H 5 þ H ! C 2 H 6 ) also contributes a lot for C 2 H 6 generation. [41][42][43] Furthermore, Shao et al. reported the formation of C 2 H 6 in a nanosecond pulsed discharge through a new pathway, which is divided into two steps, i.e., CH 4 ! CH 2 þ H 2 , and CH 4 þ CH 2 ! ...
... [44] C 2 H 4 can be formed either by electron-impact dissociation of C 2 H 6 and C 3 H 8 , or by recombination of CH 2 (R6), [37] and C 2 H 6 dehydrogenation (R12) has been considered to be the main route, which has been supported by most literature. [39,41,43] However, Bogaerts et al. found that, in a sinusoidal methane DBD, the self-disproportionation of C 2 H 5 radical ( C 2 H 5 þ C 2 H 5 ! C 2 H 6 þ C 2 H 4 ) contributes more than C 2 H 6 dehydrogenation (R12) for C 2 H 4 generation. ...
Non‐oxidative coupling of methane (CH4) by plasma has received particular attention due to the achievement of high methane conversion and product selectivity at a low temperature. In this work, experimental results suggest that the packing materials directly affect the electric field, which in turn determines the plasma discharge, the conversion of CH4 and the products selectivity. High selectivity to ethylene was obtained by BaTiO3 packing, which can be ascribed to the moderate electric field generated with special physiochemical properties. Furthermore, in the cases of packing by ZSM‐5, γ‐Al2O3 or SiO2 granules, small size of granules contributed to generate a stronger electric field, which improves CH4 conversion. However, reverse trend has been found in the case of packing by BaTiO3. Finally, several schematics (packing particles as micro‐electrode to generate micro‐discharge to enhance or decrease the electric field) were presented to describe the specific processes in micro‐electric field. The reaction pathways of methane DBD for production of C2‐C4 hydrocarbons were discussed. image
... 223 Acetaldehyde converts into CH 3 radicals and CO in a two-step (hydrogen abstraction and thermal cracking) reaction. CH 3 radicals form C 2 species and CH 4 through C-C coupling, hydrogen abstraction, and hydrogenation, 227 whereas formaldehyde gives syngas. 228 Overall, cellulose pyrolysis in a hydrogen plasma mainly results in syngas and, to a lesser extent, to light olefins. ...
Lignocellulosic biomass conversion to renewable, carbon-neutral materials, fuels, and chemicals is the cornerstone of the transition to a sustainable future bioeconomy. Green energy in the form of electricity needs to be coupled with or substitute conventional thermally driven processes to realize small-scale, economically viable and environmentally friendly biorefineries. Gas discharge plasmas enable the conversion of renewable electric energy, supplied in the form of an electric field, to chemical energy through the formation of a highly reactive environment that can induce several transformations related to agricultural waste valorization processes. Herein, we review the application of plasma technology to lignocellulosic biomass upgrade, aiming to provide the scientific background and technical challenges in this rapidly emerging research field. To bridge the gap between plasma science and biomass valorization technologies, we initially present the technical aspects of plasma reactors related to biomass processing and further discuss the advances in plasma processing for each biomass conversion technology, providing insights into the related plasma chemistry and interaction mechanisms. We first focus on the low and medium-temperature biomass conversion processes, including biomass pretreatment and delignification to promote enzyme or acid-catalyzed hydrolysis to sugars and biomass liquefaction using plasma electrolysis. Then we discuss the high and very high-temperature conversion processes, such as plasma-assisted pyrolysis and gasification to syngas and plasma application to tar removal, combustion, and vitrification. Overall, this review provides knowledge at the interface of plasma science and biomass conversion technology to promote the interaction between the individual communities, which is crucial for the further advancement of the field.
... 15 The bulk temperature stays low (generally less than 200°C), while a much higher electron temperature of up to ∼20 000 K can be achieved in NTP, where accelerated electrons, ions, and radicals are employed directly and efficiently to activate the C−H bonds without heating up the reactor to an elevated temperature. 16 Until now, various plasma types have been employed on NCM, such as gliding arc discharge (GAD), 17 corona discharge (CD), 18 microwave (MW), 19 and dielectric barrier discharge (DBD) 12,20 plasma. For example, Kim et al. 12 conducted the NCM in a DBD plasma reactor without any additives at room temperature. ...
... Methane conversion was about 5.5% and 6.5% in T-20 and T-200 samples, respectively. Hydrogen and acetylene are two most important products from plasma methane reforming [28,33,34]. Hydrogen concentration was about the same, and accounts for 4.6% and 4.8%, respectively. ...
Heavy oil conversion to fuel products and petrochemicals are both energy and greenhouse gas (GHG) emission intensive. Chemical reactions must be activated by high temperature and pressure or both with specific catalysts. Catalysts require constant regeneration by burning off coke which emits tremendous GHGs. Scale of reaction system is usually massive to maintain high energy efficiency. Here, we show an electrical method that uses nanosecond pulsed plasma to convert liquid fuels at ambient conditions with minimized GHG emissions. Plasma was generated on the interface of natural gas bubbles and liquid fuels (hexadecane). Interaction with plasma is able to convert both into valuable products including hydrogen and intermediates. Pulsing energy was the key to liquid conversion and product selectivity. Using lower pulsing energy (20 mJ), this process converts 13% of liquid fuel and 5% of methane primarily to hydrogen and C2H2 and to intermediate products (7%) and branched alkane products (3%). Energy cost was less than 1% of the energy content of processed fuel. Higher pulsing energy (200 mJ) primarily converts liquid fuels to solid products. This method is electrical, flexible on scale and easily integrated with renewable electricity.
... Results are averaged over eight tests at the same condition (Supplementary Table 1). Methane was consumed and primarily converted to hydrogen, acetylene, ethane and minimal higher carbon number species above C 4 , which agrees with literature results [45,63,64]. High yields of hydrogen and acetylene are strong indicators of non-thermal plasma reforming of methane [64,65]. ...
... Methane was consumed and primarily converted to hydrogen, acetylene, ethane and minimal higher carbon number species above C 4 , which agrees with literature results [45,63,64]. High yields of hydrogen and acetylene are strong indicators of non-thermal plasma reforming of methane [64,65]. Larger gas phase hydrocarbons, including C 3 -C 6 , were produced from the liquid phase hexadecane ( Supplementary Fig. 8-9). ...
... One advantage of two-phase plasma processing compared to a single phase is its high yields of both hydrogen and intermediate products due to the plasma-gas and plasma-liquid interactions. Methane conversion induced by nanosecond pulsed discharge was extensively studied in the literature [45,64,82]. Hydrogen and acetylene are the dominant products. ...
Electrifying refinery for unconventional/heavy oils, significant emission reductions, and alternative/bio fuels, faces major technological challenges: conversion, energy efficiency, modularity and integration with renewable electricity. Here, we introduce a multi-phase non-thermal plasma reactor that uses electrical discharges to co-convert liquid fuel and natural gas. Normal hexadecane was treated with methane plasma to validate conversion chemistry, and quantify the pathways of vapor, condensate, liquid and residue mass conversion. A complete mass balance and characterization of all products were determined. Using 500 kJ/kg-hexadecane energy input (∼1% of hexadecane’s energy content) this plasma process co-converts 9.36% of the hexadecane and 20% of the methane by mass. Distribution of products are: 2.18% hydrogen, 45.9% C2-C4, 28.9% high octane gasoline (C5-C11), 16.4% diesel (C12-C18), 2.78% heavier hydrocarbons, and 1.7% coke. Lighter product yields (C5-C15) were ∼9 molecules/100 eV, and modeled by a molecule dissociation mechanism. Hydrogen yield was 34.8 kWh/kg-H2 with minimal GHG emissions. Plasma-chemical conversion efficiency was ∼30%. This conversion process has higher efficiency, and reduces GHG emission compared to traditional technologies.
... Plasma discharges can be operated under different conditions of temperature and pressure, at varying degree of deviation from thermal equilibrium. Some non-equilibrium plasmas, like gliding arcs, sparks, nanosecond pulsed discharges (Heijkers et al., 2020;Dors et al., 2014;Scapinello et al., 2019;Delikonstantis et al., 2020) or microwave discharges (Dors et al., 2014;Heijkers et al., 2020), operate at high gas temperature (>800 K), while others, like corona (Yang, 2003a) or dielectric barrier discharges (DBD) (Xu and Tu, 2013;Toth et al., 2018;Wang et al., 2013;Saleem et al., 2019;Nozaki and Okazaki, 2013), operate at much lower gas temperature (generally below 500 K). ...
... On the contrary, when sticking for all species was not considered, no carbon losses were obviously observed, however much lower conversions were predicted, in large disagreement with the experimental data. For the latter simulation scenario, the much more populous H radicals, not getting lost to the walls anymore, appeared to promote the re-hydrogenation of CH 3 leading to an overall drop in conversion, unlike thermal plasma discharges (Scapinello et al., 2019;Li et al., 2004), where H radical-mediated hydrogen abstraction has been observed to promote methane conversion. In the experimental studies used for comparison in this work (Saleem et al., 2019;Xu and Tu, 2013;Wang et al., 2013), carbon losses up to 30% of the total carbon fed were reported indicating that more sophisticated models are required to properly describe this characteristic of the system and accurately account for its impact on kinetics. ...
A detailed kinetic scheme for non-thermal methane plasma is developed that considers the reactivity and relaxation of electronically and vibrationally excited species. An atmospheric pressure dielectric barrier discharge reactor for methane non-oxidative coupling is modelled. Via 1D fluid modelling short periods of time are investigated, while for longer periods of time, on the order of the reactor residence time, a combined 1D-0D approach is followed. Modelling results are in good qualitative agreement with literature experiments. Around 86% of the energy input is found to channel into the creation of excited species. The vibrationally excited states of methane exhibit very transient responses due to their rapid formation during electron streamers and fast quenching by VV and VT processes. The, higher energy, electronically excited states are rapidly converted, many of which essentially instantly dissociate. Over 70% of methane’s conversion proceeds via electronical excitation, while the contribution of vibrationally excited states is limited.
... The CH 3 radical can further dimerize into C 2 H 6 or undergo subsequent dehydrogenation, either thermally or by the H abstraction mechanism [38,53] as in R4-5: ...
... Subsequently, the C 2 compounds can be produced through two main pathways: dehydrogenation of ethylene, which might involve the H abstraction mechanism, and CH x radicals coupling [38,[53][54][55]. Some exemplary reactions of these pathways are presented in Table 2. ...
... The higher conversion rate of methane and methane radicals (R3 and R4-5) opens the pathway for dimerization reactions (R10-13). Moreover, the presence of H radicals enhances the important route of ethylene and acetylene creation [38,53], as in R6-9, which are favored at higher temperatures [38]. This results in the increase in acetylene and ethylene yield (calculated as in eq. ...
In this paper, the effect of hydrogen addition on methane coupling in a microwave moderate pressure (55 mbar and 110 mbar) plasma reactor has been studied. The use of optical emission spectroscopy allowed the determination of the rotational temperature of heavy particles and showed it to be in the range of 3000-4000 K. Due to the high temperature in the discharge the dominant product was acetylene and it was concluded that the methane coupling process is mainly through thermal decomposition with a key role of H radicals. It was revealed that the addition of hydrogen can increase both methane conversion and acetylene and ethylene yield and selectivity. With the CH 4 :H 2 ratio of 1:1, the methane conversion increased from 31.0% to 42.1% (55 mbar) and from 34.0% to 48.6% (110 mbar), when compared to pure methane plasma. Respectively, the yield of acetylene increased from 14.4% to 25.3% (55 mbar) and from 20.1% to 34.0% (110 mbar). Moreover, the addition of hydrogen decreased the output of the problematic soot-like product. These results indicate that hydrogen addition can be a simple yet effective method of increasing selectivity to desirable products in plasma reforming of CH 4 .
