Full Length ArticlePlasma reforming of tar model compound in a rotating gliding arc reactor: Understanding the effects of CO2 and H2O addition
Graphical abstract
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.
Section snippets
Equipment
Fig. 1 shows the schematic diagram of the experimental set-up. The RGA reactor comprises a plasma reaction zone generated between a conical inner electrode (anode) and a cylindrical grounded outer electrode (cathode), while both electrodes are fixed coaxially and insulated by a Teflon base. A ring magnet is placed outside the cylindrical cathode, generating an upward magnetic field to stabilize and accelerate the arc. The gas mixture is injected into the RGA reactor through three tangential
OES diagnostics of the RGA plasma
Optical emission spectroscopy diagnostics have been used to understand the formation of reactive species generated by the RGA plasma using different carrier gas compositions and to get new insights into the possible reaction mechanisms and pathways in the plasma reforming of toluene. Typical emission spectra of the RGA plasma with different carrier gas compositions are shown in Fig. 2.
Conclusion
The influence of CO2 and steam, two major oxidants in the producer gas, on the destruction of toluene as a tar model compound in a novel RGA plasma reactor has been investigated in terms of the conversion of toluene, gas production and the energy efficiency of the plasma process. The presence of CO2 in the plasma reforming reduces the destruction of toluene, while the addition of appropriate steam enhances the conversion of toluene with a maximum toluene destruction of 85.2% achieved at the
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.
Acknowledgements
The support of this work by the National Natural Science Foundation of China (No. 51576174, No. 51706204 and No. 51621005), EPSRC SUPERGEN Bioenergy Challenge (Ref. EP/M013162/1), EPSRC Impact Acceleration Account (IAA), Newton Advanced Fellowship (Ref. NAF/R1/180230) and the Foundation of State Key Laboratory of Coal Combustion (No. FSKLCCA1805) is gratefully acknowledged.
References (72)
- et al.
Waste gasification vs. conventional waste-to-energy: a comparative evaluation of two commercial technologies
Waste Manage
(2012) - et al.
Biomass gasification technology: the state of the art overview
J Energy Chem
(2016) - et al.
New concepts in biomass gasification
Prog Energy Combust
(2015) - et al.
Recent advances in the gasification of waste plastics. A critical overview
Renew Sustain Energy Rev
(2018) - et al.
Tar property, analysis, reforming mechanism and model for biomass gasification—an overview
Renew Sustain Energy Rev
(2009) - et al.
A review of the primary measures for tar elimination in biomass gasification processes
Biomass Bioenergy
(2003) - et al.
The reduction and control technology of tar during biomass gasification/pyrolysis: an overview
Renew Sustain Energy Rev
(2008) - et al.
Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: a review
Renew Sustain Energy Rev
(2011) - et al.
Gliding arc plasma oxidative steam reforming of a simulated syngas containing naphthalene and toluene
Int J Hydrogen Energy
(2014) - et al.
Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis—a review
Renew Sustain Energy Rev
(2013)
Catalytic reforming of toluene and naphthalene (model tar) by char supported nickel catalyst
Fuel
Catalytic cracking of tar component from high-temperature fuel gas
Appl Therm Eng
In situ catalytic conversion of tar using rice husk char/ash supported nickel–iron catalysts for biomass pyrolytic gasification combined with the mixing-simulation in fluidized-bed gasifier
Appl Energy
Preparation, modification and development of Ni-based catalysts for catalytic reforming of tar produced from biomass gasification
Renew Sustain Energy Rev
Plasma dry reforming of methane in an atmospheric pressure AC gliding arc discharge: co-generation of syngas and carbon nanomaterials
Int J Hydrogen Energy
Steam reforming of toluene and naphthalene as tar surrogate in a gliding arc discharge reactor
J Hazard Mater
Plasma assisted dry reforming of methanol for clean syngas production and high-efficiency CO2 conversion
Chem Eng J
Reactive species distribution characteristics and toluene destruction in the three-electrode DBD reactor energized by different pulsed modes
Chem Eng J
Tar removal from biomass-derived fuel gas by pulsed corona discharges
Fuel Process Technol
Steam reforming of toluene as biomass tar model compound in a gliding arc discharge reactor
Chem Eng J
Advanced VOCs decomposition method by gliding arc plasma
Chem Eng J
Gliding arc plasma for CO2 conversion: better insights by a combined experimental and modelling approach
Chem Eng J
Gliding arc gas discharge
Prog Energy Combust Sci
Non-oxidative decomposition of methanol into hydrogen in a rotating gliding arc plasma reactor
Int J Hydrogen Energy
Optimization scheme of a rotating gliding arc reactor for partial oxidation of methane
P Combust Inst
Plasma enhanced catalytic reforming of biomass tar model compound to syngas
Fuel
Steam reforming of toluene as model compound of biomass pyrolysis tar for hydrogen
Biomass Bioenergy
Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound
Appl Catal B-Environ
Plasma activation of methane for hydrogen production in a N2 rotating gliding arc warm plasma: a chemical kinetics study
Chem Eng J
Destruction of toluene by rotating gliding arc discharge
Fuel
Microwave plasma application in decomposition and steam reforming of model tar compounds
Fuel Process Technol
Plasma-catalytic reforming of CO2-rich biogas over Ni/γ-Al2O3 catalysts in a rotating gliding arc reactor
Fuel
Tar evolution in a two stage fluid bed–plasma gasification process for waste valorization
Fuel Process Technol
Decomposition of benzene as a tar analogue in CO2 and H2 carrier gases, using a non-thermal plasma
Chem Eng J
Review of catalytic conditioning of biomass-derived syngas
Energy Fuels
Upgrading of raw gas from biomass and waste gasification: challenges and opportunities
Top Catal
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The first two authors contributed equally to this work.