Plasma based CO₂ and CH₄ conversion: A modeling perspective

Abstract

This paper gives an overview of our plasma chemistry modeling for CO 2 and CH 4 conversion in a dielectric barrier discharge (DBD) and microwave (MW) plasma. We focus on pure CO 2 splitting and pure CH 4 reforming, as well as mixtures of CO 2 /CH 4 , CH 4 /O 2 , and CO 2 /H 2 O. We show calculation results for the conversion, energy efficiency, and product formation, in comparison with experiments where possible. We also present the underlying chemical reaction pathways, to explain the observed trends. For pure CO 2 , a comparison is made between a DBD and MW plasma, illustrating that the higher energy efficiency of the latter is attributed to the more important role of the vibrational levels. K E Y W O R D S0D chemical kinetics model, CO 2 and CH 4 conversion, dielectric barrier discharges, microwave plasmas, plasma chemistry modeling | INTRODUCTIONIn recent years, there is increasing interest in plasma used for CO 2 and CH 4 conversion. Several types of plasma reactors are being investigated for this purpose, including (packed bed) dielectric barrier discharges (DBDs), [1][2][3][4][5][6][7][8][9][10][11][12][13][14] microwave (MW) plasmas, [15][16][17][18][19][20] ns-pulsed, [21] spark, [22][23][24] and gliding arc (GA) [25][26][27][28][29][30][31][32] discharges. Research focuses on pure CO 2 splitting into CO and O 2 , on CH 4 (and other hydrocarbons) reforming, and on mixtures of CO 2 with a hydrogen-source, that is, mainly CH 4 , but sometimes also H 2 O or H 2 , to produce value-added chemicals like syngas, hydrocarbons, and oxygenated products. Key performance indicators are the conversion and the energy efficiency of the process, as well as the possibility to produce specific value-added chemicals with good yields and selectivity. To realize the latter, the plasma should be combined with a catalyst (e.g., [3][4][5][6][7][8][9]33,34] ), as the plasma itself is a too reactive environment, and thus produces a wealth of reactive species, which easily recombine to form new molecules, without any selectivity.To improve the conversion, product yields and energy efficiency of this process, a good insight in the underlying plasma chemistry is crucial. This can be obtained by experiments, but measuring the reactive species densities inside the plasma is far from evident. Therefore, modeling of the plasma chemistry can be a valuable alternative, as it provides information on the most important chemical reaction pathways, and how to tune them to improve the conversion, energy efficiency, and product formation.In the 80s and 90s, some papers have been published on CO 2 plasma chemistry modeling, with applications to CO 2 lasers. [35][36][37] These models, however, did not consider the vibrational kinetics, which are important for energy efficient CO 2 conversion. [38] Some other papers have described the vibrational kinetics for gas flow applications, [39,40] but without focusing on the plasma chemistry. In 1981 Rusanov

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

“…The latter will be illustrated here, based on the modeling work performed within our group PLASMANT, for pure CO 2 splitting, as well as CO 2 /CH 4 ,CH 4 /O 2 , CO 2 /H 2 , and CO 2 /H 2 O mixtures. For more details about the modeling results in these mixtures, and more specifically the calculated conversions, product yields and energy efficiencies, and comparison with experiments, we refer to the original research papers mentioned below, as well as two recent review papers [84,116].…”

“…In the last decade, the research on plasma-based CO 2 conversion experienced a clear revival, and quite some plasma chemistry models have been developed in literature, for either pure CO 2 splitting [7, or CH 4 (of interest for hydrocarbon reforming) [49][50][51][52], as well as in various mixtures, that is, CO 2 /CH 4 [53][54][55][56][57][58][59][60][61][62][63][64][65][66], CH 4 /O 2 [66][67][68][69][70][71][72], CO 2 /H 2 O [73], and CO 2 /H 2 [74,75], of interest for producing value-added chemicals, or in mixtures of CO 2 /N 2 [76,77] or CH 4 /N 2 [78][79][80][81][82][83], more closely mimicking reality, as N 2 is a major component in effluent gases. Recently, we gave an overview of such 0D models for plasma-based CO 2 and CH 4 conversion [84], and we also presented a very comprehensive plasma chemistry model for CO 2 and CH 4 conversion in mixtures with N 2 ,O 2 , and H 2 O [85]. These plasma chemistry models can provide detailed information on the underlying chemical reaction pathways for the conversion or product formation.…”

“…The thickness of the arrow lines corresponds to the importance of the reaction paths. Reproduced from [84] with permission. Figure 5.…”

Steam reforming of methane in a temperature-controlled dielectric barrier discharge reactor: the role of electron-induced chemistry versus thermochemistry

“…This is probably one reason that most modeling studies in literature have been performed at room temperature [25]. Nevertheless, looking at the effects of the abovementioned parameters, such as E/N and Pdis for a given mixture composition, on the electron-impact dissociation and ionization, we find valuable connections with the experimental data.…”

Special issue: Plasma Conversion

“…CH 4 and CO 2 reforming is one of the hot topics in plasma catalysis and research has been conducted mostly by experimental approaches. Bogaerts et al discuss CH 4 reforming and CO 2 splitting and various mixtures from a modeling perspective, paying special attention to plasma chemistry and the role of plasma-generated vibrationally excited molecules [2]. Reactions involving vibrationally excited species need to be explored further and a numerical approach is truly beneficial providing insightful information towards the reaction mechanisms.…”