CO₂ reforming of CH₄ in single and double dielectric barrier discharge reactors: Comparison of discharge characteristics and product distribution

Highlights

• CO₂ reforming of CH₄ was performed in DBDs with different dielectric layers.

• More intensified filamentary microdischarges were displayed in DBD-SD.

• DBD-SD exhibited higher reactant conversion and liquid product selectivity.

• Higher gas product selectivity and stable carbon balance were achieved in DBD-DD.

• Higher energy efficiency for plasma reforming process was obtained in DBD-SD.

Abstract

CO₂ reforming of CH₄ in a non-thermal plasma process (e.g., dielectric barrier discharge, DBD) possesses dual benefits for our environment and energy needs. However, this process is strongly influenced by the dielectric structure of the DBD. Here, plasma CO₂ reforming of CH₄ has been performed in both single-dielectric and double-dielectric DBD (DBD-SD and DBD-DD) reactors under atmospheric pressure. Electrical and optical characterization, along with temperature measurements are performed to understand the influence of the DBD-SD and DBD-DD designs. Reactor performance for reforming is compared under different discharge powers. The results show that CO₂/CH₄ discharges in both DBD-SD and DBD-DD display typical filamentary microdischarges. Compared with the DBD-DD, the DBD-SD reactor exhibits a larger number and higher intensity of current pulses, which leads to a higher electron density and formation of reactive species. The highest conversion of CO₂ (24.1 %) and CH₄ (49.2 %) are achieved in the DBD-SD at a high discharge power (75 W). Moreover, higher selectivities of gaseous products are obtained in the DBD-DD, while the DBD-SD reactor shows a higher selectivity for liquid products, mainly including methanol and acetic acid. The highest energy efficiencies for reactant conversion (0.34 mmol/kJ), gaseous and liquid production formation (0.26 mmol/kJ and 0.015 mmol/kJ) are achieved in the DBD-SD reactor at a low discharge power (22 W), resulting from the low energy loss to the environment. However, the carbon deposited on the inner electrode surface in the DBD-SD would have an adverse influence on the reactor’s performance. Further research on the optimization of the DBD reactor to establish an efficient plasma-catalysis system is required for industrial applications.

Introduction

The unprecedented development of human society has consumed enormous amount of carbon-based fossil fuels (e.g., coal, nature gas and petroleum) with the demand for energy sources continuously growing. However, the consumption of fossil fuels not only results in the depletion of these limited non-renewable sources, but also leads to the huge volumetric emissions of greenhouse gases into the atmosphere, such as CO₂, CH₄, N₂O and SF₆, which contributes to the global warming and climate change [1]. Carbon conversion and utilization (CCU) can transform CO₂ into useful chemicals, bringing dual benefits for the environment and energy supply [[2], [3], [4], [5], [6]]. Among the viable solutions for CCU, CO₂ reforming of CH₄ has attracted significant attention [[7], [8], [9]], as it can simultaneously reduce the emission of the two main greenhouse gases (CO₂ and CH₄) and transform them into syngas, which can be used as a raw material to produce an array of chemicals.

$$\text{C}{\text{H}}_4+\text{C}{\text{O}}_2\to 2{\text{H}}_2+\text{2CO}\Delta H_{298\text{K}}=247\text{kJ}/\text{mol}$$

However, due to the high stability of both CO₂ and CH₄, high temperatures (> 700 °C) are required for thermal catalytic routes to achieve a reasonable reaction rate for CO₂ reforming of CH₄, resulting in a highly energy-intensive process. Moreover, catalysts are found to readily deactivate due to carbon deposition and the sintering of active metals under the high-temperatures, which limits the commercial development of this process [10,11].

In this regard, non-thermal plasma (NTP) can be a promising alternative to the conventional thermal reforming process. NTP contains energetic electrons, which have an average energy of 1−10 eV. Gas molecules can be activated in NTP through ionization, dissociation and excitation processes to generate various chemically reactive species for the initiation and propagation of the chemical reactions, while the bulk gas temperature remains close to room temperature [9]. Therefore, the thermodynamically unfavorable reactions can occur at ambient conditions with the aid of NTP [[12], [13], [14], [15]]. In addition, NTP has the property of rapid start-up and shutdown, which makes it more efficient compared with the traditional thermal processes. It also has the flexibility to be combined with renewable energy sources (e.g., solar, wind or hydraulic power) for energy storage and transportation in the form of chemical energy, especially when the renewable energy is produced during peak periods, leading to a significant decrease in the overall energy cost [9,16].

Various investigations have been performed on CO₂ reforming of CH₄ using different NTPs, such as corona discharge [17], atmospheric pressure glow discharge (APGD) [18], microwave discharge (MW) [19], gilding arc (GlidArc) discharge [20,21] and dielectric barrier discharge (DBD) [7,12,22]. Among these NTPs, DBD has received the most attention due to its mild operating conditions, simplicity of design, suitability for scaling and its successful experience for gas cleaning and ozone generation at commercial scale [23,24]. Currently, the energy efficiency for CO₂ reforming of CH₄ using DBD is typically low [9]. But on the other hand, valuable liquid chemicals can be directly synthesized from the one-step process for CO₂ reforming of CH₄ in DBDs, especially when appropriate catalysts are employed, which is more challenging in other plasmas (e.g., MW and GAD) and thermal-catalytic processes [9,[25], [26], [27], [28], [29]]. The performance of CO₂ reforming of CH₄ (e.g., reactant conversion, product yield and selectivity, energy efficiency) is found to be dependent on the reactor design [[30], [31], [32]], operating parameters [7,33,34], external conditions [35,36], catalysts and plasma-catalytic configurations [27,[37], [38], [39]].

A DBD is made up of metal electrodes and at least one dielectric layer [40]. For cylindrical DBDs, the structure of the dielectric tube is also a factor influencing the physical properties and reaction performance of plasma processing. In the DBD reactor with single dielectric tube (DBD-SD), the inner metal electrode is exposed to the plasma and reactant environment, which may result in the electrode corrosion and/or erosion and influence the performance of the plasma reactor [41]. To deal with this issue, DBD with double dielectric tube (DBD-DD) have been proposed by inserting another dielectric tube to protect the metal electrode from the chemically reactive discharge region [41]. However, the energy consumption will be higher, which will lower the electrical efficiency due to the increased stray capacitance by introducing the extra dielectric layer [42,43]. Comparisons between DBD-SD and DBD-DD have been widely investigated in the plasma treatment of waste gas and water, with DBD-DDs showing better performance in most cases [44]. Limited attention has been paid to investigating CO₂ reforming of CH₄ using DBDs with different dielectric structures. It remains unclear how the dielectric structure affects the plasma reforming process in DBD-SD and DBD-DD. Therefore, it is important to understand the roles of the dielectric structure on the physical properties and reforming performance to gain new insights into reactor design and process optimization for plasma CO₂ reforming of CH₄.

In this work, we investigated the plasma CO₂ reforming of CH₄ in coaxial DBD-SD and DBD-DD reactors. The role of the dielectric structure was evaluated from different perspectives, including electrical characterization, optical and temperature measurements, quantified reactant conversions and product distributions, and energy efficiency for both reactant conversion and product formation. Moreover, the energy efficiencies are compared with the results in the literature.