Influence of hydrogen addition on methane coupling in a moderate pressure microwave plasma

Highlights

• Moderate pressure microwave plasma coupling of methane was investigated.

• The main products were acetylene, ethylene, hydrogen, and soot-like material.

• H₂ addition increased methane conversion and C₂ compounds yield and selectivity.

• The coupling process was mainly thermally driven with the key role of H radicals.

Abstract

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 CH₄:H₂ 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 CH₄.

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/NO_x 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.