Full Length ArticleInfluence of hydrogen addition on methane coupling in a moderate pressure microwave plasma
Graphical abstract
Introduction
Methane is believed to play a key role as a hydrocarbons feedstock in the near future [1], [2], [3]. One of the main reasons is that, as opposed to depleting crude oil reserves that are commonly used in the petrochemical industry, the number of methane sources is constantly increasing (new deposits of natural gas and alternative sources of methane) [1], [2]. As a result, much effort is put into developing a process that allows for efficient methane conversion, especially to valuable C2 products (C2H2 and C2H4) that are commonly used as feedstock in the chemical industry [4], [5], [6]. Despite the scientific attention given, no significant industrial-scale breakthrough has been achieved in that matter yet. Conversion of methane to valuable chemicals can be done by indirect routes involving multiple steps, i.e. via production of synthetic gas or methanol, that further can be processed into olefins [7]. The main drawback of such a process is the complexity, leading to high costs and the usual need for additional H2. Moreover, with the hydrogen being produced coming mainly from fossil fuels by steam reforming, these methods would contribute greatly to CO2 emission.
Another possible process of C2 compounds production is oxidative methane coupling (OMC). While this process has been intensively investigated over the last 20–30 years the main problematic issue - providing a perfect catalyst - remains unsolved [7], [8]. This issue is connected with oxidation of methane, resulting in formation of CO2, which is competitive to C2 production [9], [10]. Moreover, the secondary problem is the stability of the catalyst and its sintering in the high temperature of the process [8], [11] as well as the need for an expensive oxygen/nitrogen separation [8], [10].
With the European Green Deal, focusing on CO2 reduction, being implemented and the unavoidable shortage of oil reserves, the direct non-oxidative methane coupling into C2 compounds might become a highly desired process in the near future. However, conventional processes of thermal methane coupling are limited by the high energy required to activate the stable methane molecule [5], [7]. Additionally, these processes suffer from a few other drawbacks, like catalyst deactivation in the presence of soot, long start-up/shut-down periods, low energy efficiency, and indirect CO2/NOx production [5].
In that context, plasma non-oxidative methane coupling is considered as a promising method of C2 compounds production. Plasma provides a high concentration of chemically active species that can enhance methane activation. Moreover, plasma reactors have a low inertia and can be started /shut-down quickly all the while being considered CO2 neutral if powered with renewable energy. Plasma driven methane activation can be done in both thermal and non-thermal plasma; the thermal plasma, for example the Hüels process, is characterized by low energy efficiency and the need of intensive quenching [10], [12], [13]. A suitable alternative could be found in non-thermal plasma. Many non-equilibrium plasmas are described extensively in literature in the context of methane coupling [5], [14], [15]: dielectric-barrier discharge (DBD) [16], [17], [18], [19], nanosecond pulsed discharge (PD) plasma [1], [6], [20], [21], glow discharge [22], [23], corona plasma [24], [25], gliding-arc (GA) plasma [4], [26], [27] and microwave (MW) plasma [5], [10], [28]. Of these plasma sources, MW plasma is often claimed to show the highest rate and energy efficiency of methane conversion. The MW plasma parameters can vary in a wide range from strictly non-equilibrium to close to thermal equilibrium depending on the operating pressure and power [5]. Regardless of the condition, the temperature within the methane plasma is usually above 1000–2000 K [10], [29] providing a suitable condition for methane decomposition through thermal chemistry, thereby reaching conversions above 80% and maintaining high selectivity towards C2 compounds [14]. The MW plasma can be considered a warm plasma [26], [30], meaning, that despite the high temperature it can still show a vibrational non-equilibrium of its particles. In fact, some authors have stated that this non-equilibrium, as a result of vibrational excitation of methane, can enhance its decomposition [4], [10], [16], [31], [32]. However, experimental validation of vibrational non-equilibrium and its role in dissociation of methane was often lacking [14], [32], [33]. Moreover, recent research in this field suggests, that in fact this phenomenon is rather negligible in the commonly used experimental plasma reactors (including MW plasma), and the process of methane conversion is thermally driven [14], [33]. Nevertheless, applying MW plasma can also be beneficial from a technical point of view. Magnetrons used in microwave plasma generation are the same as those used in other industrial applications of microwaves, which are considered to be mature technologies, e.g. drying, food processing, and heating in general [34]. Microwave technology elements are relatively cheap and have a simple and compact construction [35], they are produced by many companies and their power can vary from few to hundreds of kW, creating the potential to scale up the technology.
Regardless of the method applied for methane coupling, one of the main, unavoidable products is hydrogen. Interestingly, the addition of hydrogen into the methane gas stream can significantly affect the process, i.e. methane conversion rate and distribution of the products. This effect can be twofold. In high temperature conditions, as in warm microwave plasma, the addition of hydrogen can result in the presence of additional highly energetic H radicals that can enhance methane conversion [36]. On the other hand, typical for conventional thermal pyrolysis of methane or strictly non-thermal plasmas [5], [37], the addition of hydrogen can result in both a lower conversion, due to methyl radical recombination into methane, as well as suppression of benzene and soot production by inhibition of acetylene decomposition. However, these two effects are not always contradicting. In works where atmospheric MW plasma was studied, a high hydrogen dilution (e.g. CH4:H2 ratio above 1:1) resulted in both increasing methane conversion and C2 compounds yield as well as inhibiting soot production [2], [3].
Despite the promising potential of MW plasma application in methane coupling and the possible benefits of hydrogen addition, the amount of works focusing on this issue is limited. The two prominent works in that field are the work of Heintze et al. [38] and the work of Shen et al. [3]. The first work used a moderate pressure (30 mbar) MW plasma and therein focused on the mechanism behind C2 compound formation and the effect of H-atoms on it. The H2 addition was limited to only 17% v/v. The latter work used an atmospheric MW plasma reactor and studied a wide range of CH4:H2 ratios (from 5:1 to 1:5) in order to find the effect of H2 addition on methane conversion and product distributions. This work lacked any information considering plasma parameters, and the mechanisms behind the hydrogen addition were only briefly discussed.
Taking into account the limited literature data and the variety of possible process conditions (i.e. MW plasma pressure and temperatures, hydrogen dilution), this work aims to investigate the impact of hydrogen addition (CH4:H2 ratios of 3:1 and 1:1) on methane coupling in a moderate pressure (55 mbar and 110 mbar) MW plasma reactor. The work involves the determination of the methane conversion products along with the investigation of the plasma temperatures.
Section snippets
Experimental setup
The experimental layout is schematically shown in Fig. 1. A magnetron power source (IBF PGEN2450/1-2KW5CSW) applies microwaves of up to 1000 W peak power via a WR340 waveguide to a 27 mm inner diameter quartz tube, where the plasma is generated. An EH tuner and an adjustable short are used to tune the electrical field for minimal reflected power. Gas is injected tangentially, which is a common design in MW plasma reactors [29], [39]. The tangential injection creates a vortex in the quartz tube
Reactions of methane conversion
The process of methane decomposition, requiring a high temperature (>1273 K [5]) due to methane’s stability [10], can be described with a simplified stepwise reaction (R1):
Since it is a gradual process, the distribution of the products depends on the temperature and reaction time. Many researchers agree that the primary product is C2H6, however it is promptly dehydrogenated into further products [5], [50]. This phenomenon is mostly affected by the fact that
Plasma diagnosis
Fig. 2 presents the spectrum of the CH4-H2 plasma measured with the HRC29301 spectrometer. It can be seen the dominant emission in the spectrum is from the CH (A-X) system. The other components that are observed in the spectrum are the C2 Swan system and the Hβ line.
The measured systems were used to determine the rotational and vibrational temperatures. In the case of the CH (A-X) system, the Lifbase [48] software was used (Fig. 3a). For the C2 Swan (Fig. 3b) the determination of temperatures
Energy costs and comparison with other plasma techniques
To determine the energy costs of methane coupling three parameters are used, i.e. specific energy input (SEI), specific energy requirement (SER), and energy requirement (ER), calculated as in eqs. (9), (10), (11), respectively. The SEI is constant due to the fixed input volumetric gas flow rate (6 SLM) and magnetron power consumption (1000 W) and its value is 224 kJ/mol. The SER and ER are presented in Fig. 10, Fig. 11, respectively. As it can be seen, the SER is gradually increasing with the CH
Conclusions
Methane coupling to valuable C2 compounds by means of plasma was studied. The focus of this work was the impact of hydrogen addition on the non-oxidative methane coupling in a warm, moderate pressure microwave reactor in two different regimes of radial confinement. Optical emission measurements were coupled with downstream measurements of conversion and product distributions.
The dilution with hydrogen has a twofold beneficial effect. Firstly, it increases the conversion rate of methane and
CRediT authorship contribution statement
M. Wnukowski: Conceptualization, Methodology, Investigation, Validation, Writing - original draft. A.W. van de Steeg: Methodology, Investigation, Validation, Writing - original draft. B. Hrycak: Formal analysis, Writing - original draft. M. Jasiński: Formal analysis, Writing - original draft. G.J. van Rooij: Methodology, Resources, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Polish Ministry of Science and Higher Education (subvention number 049M/0014/19) and received support from the Netherlands Organization for Scientific Research (NWO) in the framework of the CO2-to-Products program with kind support from Shell, and the ENW PPP Fund for the top sectors.
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