Plasma reforming of tar model compound in a rotating gliding arc reactor: Understanding the effects of CO₂ and H₂O addition

Highlights

• A 3-D rotating gliding arc plasma is investigated for model tar compound reforming.

• The role of H₂O and CO₂ that exist intrinsically in the producer gas is well studied.

• A toluene conversion of up to 85.2% can be obtained in the presence of H₂O.

• A small amount of H₂O enhances the toluene conversion due to the production of OH.

• Electrons, excited N₂, and O, OH radicals can destruct toluene via H abstraction.

Abstract

In this study, a rotating gliding arc (RGA) plasma reactor co-driven by a magnetic field and tangential flow has been investigated for the reforming of toluene as a tar surrogate from the gasification of biomass or waste. The effect of steam and CO₂ addition on the reaction performance of the plasma tar reforming process has been evaluated in terms of the conversion of toluene, gas production and energy efficiency. The presence of CO₂ in the reaction suppresses the conversion of toluene. By contrast, adding an appropriate amount of steam to the reforming process significantly enhances the conversion of toluene, while further increasing steam concentration reduces the conversion of toluene. The maximum toluene conversion of 85.2% is achieved at an optimal steam concentration of 16%. Optical emission spectroscopic (OES) diagnostics have been used to understand the generation of reactive species contributed to the conversion of toluene and reaction intermediates in the plasma reforming process. The possible reaction pathways and mechanisms have been discussed based on the analysis of gases and condensed liquid by-products combined with the emission spectra of the plasma in the presence or absence of steam and CO₂.

Introduction

The increasing depletion of fossil fuels and growing awareness of global warming is pushing up the development of biomass or municipal solid waste (MSW) utilization technology [1], [2]. Gasification, which is performed by partial oxidation of the carbon contained in biomass or MSW with a controlled oxidant amount, is considered as one of the most sustainable and promising thermochemical processes for biomass and MSW utilization. Gasification enables a high-efficiency conversion of biomass or MSW into value-added syngas mainly consisting of H₂, CO and CH₄, providing high flexibility for the generation of heat and electricity, or the synthesis of value-added platform chemicals and synthetic fuels [3], [4]. However, the producer gas is inevitably contaminated by some undesired impurities, such as tars, particulates, alkali metals and acid gases, of which the contamination by tars is a key challenge in the gasification of biomass or waste. As the presence of tars in the producer gas can cause fouling, obstruction and corrosion in downstream equipment, limiting the use of producer gas for energy applications [5], [6], [7].

Tar is a generic complex mixture of condensable hydrocarbons, including aromatics as well as multiple ring polycyclic aromatic compounds (PAHs) that may contain hetero-atoms, such as sulphur and nitrogen, and may be partly oxidized. A widely accepted definition of tar refers to ‘all organic contaminants with a molecular weight larger than benzene’ [8]. Typically, the tar content of producer gas varies from 0.5 to 100 g/Nm³, depending on the type of gasifiers, with a co-current flow of fuel and oxidant minimizing tars and counter-current operation favouring fuel economy, yet producing dirtier gas, since it contains the volatile matter of the feedstock. Most applications of producer gas require a low tar content. For instance, the tolerance of internal combustion engines for tars is well below 100 mg/Nm³. As for gas turbines, this constraint becomes even stricter, with a limit of 5 mg/Nm³ [8], [9], [10]. Thus, effective removal and conversion of tars from raw producer gas are of great importance for the gasification process.

Removing tars from producer gas can be accomplished through mechanical separation, including filters, cyclones, scrubbers, and electrostatic precipitators [10]. Physical methods may remove part of tars together with captured particulates. However, the chemical energy contained in the tars is lost, reducing the efficiency of the gasification process. Additionally, wet gas cleaning generates large amounts of contaminated water, requiring downstream treatment or recycling [5], [11]. Thermal or catalytic cracking can convert tars into light gases. However, high temperatures (>1000 °C) are required for thermal cracking to achieve sufficient tar destruction in a realistic residence time [12], incurring high energy cost and production of agglomerated soot particles [13]. Catalytic cracking can operate at the temperature of the gasifier [10]. Various catalysts have been explored for tar reforming, including Ni- or other metal-based catalysts, basic and acid catalysts, and activated carbon [14], [15], [16], [17], [18], [19]. Unfortunately, most catalysts can be easily fouled by coke formation or poisoned by sulphur and chlorine compounds. Finding cost-effective and stable catalysts remains a major challenge for catalytic cracking of tars from gasification.

Non-thermal plasmas have been regarded as a promising technology for the effective destruction and conversion of tars from the gasification of biomass or MSW. Plasmas are normally subdivided into thermal plasma and non-thermal plasma. Thermal plasma has a gas temperature of higher than 10⁴ K but lacks chemical selectivity because of the equilibration state between the electrons and heavy gas molecules. In non-thermal plasmas, the gas temperature (normally lower than 1500 K) is much lower in comparison to the electron temperature (normally 1–5 eV, 1 eV ≈ 10⁴ K), which means the energy input can be used to promote specific chemical reactions without heating the system. In a non-thermal plasma process, reactive species such as energetic electrons, ions, free radicals, excited molecules or atoms, are present and initiate a variety of chemical reactions [20], [21], [22], [23]. In addition, the plasma reforming process can be switched on and off quickly due to its instant reaction initiation with a high reaction rate. Moreover, non-thermal plasma can operate under mild conditions (atmospheric pressure and low temperature) and shows the merits of low investment and easy operation etc. However, very limited efforts have been devoted to the investigation of tar reforming using non-thermal plasmas. A few non-thermal plasma processes have been proposed for the conversion of model tar compounds (e.g., toluene and naphthalene) using nitrogen as a carrier gas, including corona discharge [24], [25], microwave discharge [26], dielectric barrier discharge (DBD) [27] and gliding arc (GA) discharge [28], [29]. These studies demonstrated the promising potential of using non-thermal plasma processes for tar reforming, but still facing challenges such as carbon deposition, polymerization, low processing capacity and low energy efficiency. Moreover, the influence of oxidative components (e.g. steam and carbon dioxide), which exist intrinsically in the producer gas, on the plasma reforming of tar model compounds is still unclear. Significant fundamental researches are still required to further enhance the conversion of tars and energy efficiency of the process through the design and development of new reactor concepts with enhanced processing capacity and flexibility.

Among different non-thermal plasma systems, gliding arc discharge is one of the most attractive because it can simultaneously provide a relatively high energy density, high electron temperature, good chemical selectivity, and low energy consumption [30], [31], [32], providing high flexibility to work in a wide range of reactant flow rates (up to 20 L/min in laboratory scale) and plasma power levels (up to several kW) [33]. However, in a traditional flat gliding arc discharge that consists of two divergent knife-shaped electrodes, the feed flow rates have to be quite high (e.g., 10–20 L/min) to maintain the gliding arc, which thus leads to a short residence time of reactant [34], [35], [36]. More importantly, a quasi-two-dimensional plasma reaction area that is confined by the flat electrodes leads to a limited fraction of the gas flow that processed by the plasma (e.g., around 20% depending on the reactor geometry) [36], [37].

To solve these problems, three-dimensional rotating gliding arc (RGA) reactors have been proposed, which can be driven by either magnetic field [38] or tangential flow [39]. Compared with traditional flat gliding arc discharge, the RGA reactor could provide a stable and large three-dimensional plasma zone for chemical reactions by creating a rotating “plasma disc” area with certain axial thickness. In this way, the plasma area can be enhanced with an elongated residence time of reactants. In this study, an updated rotating gliding arc plasma reactor co-driven by a magnetic field and tangential flow has been developed for the conversion of toluene as a tar surrogate from gasification. Toluene is commonly studied as a model tar compound both in plasma process [21], [27], [28], [40] and in traditional reforming process [41], [42], as it is stable and a good representative of aromatic compound. In addition, it is less harmful and has a simple structure and low boiling point, thus offering convenience for a good performance of the experiments. The RGA reactor can generate a synergetic effect resulted from the combination of a swirling flow and a Lorentz force, providing a steadier rotating arc volume with enhanced rotation frequency (up to 100 rotations per second) over a wider flow rate range (e.g., 0.05–40 L/min) [43]. It has been evidenced that the arc stability and rotating frequency can be increased under the effect of Lorentz force or swirling flow [44], [45]. Our previous study showed that the RGA reactor could provide a maximum tar conversion of over 95%, in a toluene destruction process in N₂ flow [46]. H₂ and C₂H₂ are the major gas products with selectivity of up to 39.4% and 27.0%, respectively.

It is well known that steam and carbon dioxide exist intrinsically in the producer gas with high content, and thus can react with tar compounds. However, the effects of steam and carbon dioxide on the reaction performance of plasma tar destruction have been scarcely investigated. Therefore, this work aims to evaluate the performance of the attractive RGA plasma system for model tar compound reforming in the oxidative atmosphere, with a specific focus on the effect of steam and carbon dioxide. Optical emission spectroscopic (OES) diagnostics have been used to get new insights into the formation of a range of reactive species generated in the plasma reforming process in the presence and absence of steam and CO₂. The plausible reaction pathways and mechanisms in the plasma reforming of tar model compound have been proposed and discussed based on the analysis of gas and liquid products combined with OES diagnostics. These works are expected to advance the industrial-scale application of this promising process.