Improving the pollutant removal efficiency of packed-bed plasma reactors incorporating ferroelectric components
Graphical abstract
Introduction
One of the most common applications of dielectric barrier discharge (DBD) plasmas is the decomposition removal of gaseous pollutants [1], [2], [3], [4]. A great number of works have been published about the use of these plasma discharges for the removal of volatile organic compounds (VOCs) present in the air as a result of different industrial and in-door activities [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. However, although a large variety of reactor set-ups and working conditions have been proposed to maximize the efficiency of these processes [19], [20], [21], [22], [23], most studies have addressed the kinetics of the removal process and paid little attention to the analysis of the plasma, the electrical operation parameters or the effect of the reactor configuration. Common working conditions in these works are a low concentration of pollutants in air, of the order of hundreds or, maximum, thousands ppms, and the pursuit of the full oxidation of VOCs to CO2 and H2O (and other fully oxidized sub-products if the pollutant contains heteroatoms) with no formation of intermediate hazardous molecules.
Advanced arrangements of DBD reactors for gas reaction processes such as the reforming of hydrocarbons, CO2 conversion or the synthesis of ammonia incorporate a ferroelectric moderator between the metallic electrodes [24], [25], [26], [27], [28]. This design is advantageous with respect to classical configurations incorporating dielectric materials because the operating voltages can be smaller [29], [30] and high plasma currents can be maintained for relatively large separation gaps between electrodes [31], [32]. Among other factors, these improvements could be linked with the substantial amount of electrons that, emitted by polarized ferroelectric materials [33], [34], may contribute to increase the plasma current and to decrease the breakdown voltage. In this work we have used pellets of lead zirconate titanate (PZT), a rather seldom ferroelectric material when dealing with plasma DBD reactors. The study presents a fundamental study about the design of DBD reactors and the effect of ferroelectric materials on their performance for the abatement of VOCs and other hazardous gases like methane in air. For this purpose, a series of ferroelectric packed bed reactors with a parallel electrode configuration (most geometrical configurations in the literature use a cylindrical geometry and a two coaxial electrodes system [12], [15], [16]) have been used to test the influence of the electrode area (i.e., size of the reactor) and the effect of incorporating a ferroelectric plate together with the ferroelectric pellets filling the inter-electrode space. Although ferroelectric plates incorporated onto DBD electrodes have been demonstrated to be quite effective in reducing the ignition voltage in surface discharge plasmas [35], to our knowledge their incorporation together with pellets in a packed-bed reactor has not been intended up to now. This unprecedented trial has effectively demonstrated that these kinds of mixed barriers render more efficient the packed-bed reactors. Besides analyzing the influence of the ferroelectric material distribution within the reactor, we have also studied the effect of reactant flow and other working parameters on the process efficiency. Finally, a comprehensive study of the electrical response of the reactor, both theoretically by means of the COMSOL software [36] and experimentally through the determination of the average electron energy and system capacitance, has allowed us to correlate the efficiency of the decontamination processes with the architecture of the ferroelectric barrier and other operating parameters. Such an analysis has provided important clues to understand the system behavior and the possibilities for improvement and extrapolation to other reactor configurations. We think that the obtained results can be relevant to design more efficient packed-bed plasma reactors through the adjustment of the average energy of plasma electrons to values higher than the threshold required to effectively oxidize organic pollutant compounds in air.
Section snippets
Materials and methods
According to Fig. 1(a), the reactors utilized for the present investigation have a parallel plate configuration with a fixed gap space of 3 mm. The inter-electrode space was filled with quasi-spherical pellets of PZT (lead zirconate titanate) with 1.25 mm of average diameter that were synthesized in our laboratory according to a complex process described in detail in [26] and that involves the sintering and sieving of pellets. The relative dielectric constant of PZT is approximately 1900 (APC
Results and discussion
Safety of decomposition products, energy consumption and removal efficiency are three major concerns by the construction and operation of DBD reactors intended for decontamination purposes [1], [2], [3], [4], [18], [19], [23]. In this regard, incorporation of catalysts in the packed bed is a general strategy in order to favor the total oxidation of organic pollutants [5], [11], [24]. Since in the present study no significant traces of other byproducts apart from CO2 and H2O could be detected in
Conclusions
The previous results and discussion have shown that packed bed parallel plate DBD reactors are quite efficient for the removal of VOCs and other contaminants (results are shown for acetone, toluene, chloroform or methane) in air. They have also proved that a parallel plate design permits to assess the effect of increasing the active discharge area, and therefore the reactor size, without affecting other working parameters such as distance between electrodes or gas flow distribution. In this
Acknowledgments
This work has been carried out thank to the financial support of the Junta de Andalucía (project P12-FQM-2265) and EU social founds MINECO-CSIC (project RECUPERA 2020). One of the authors, A. Gómez-Ramírez acknowledges financial support from MINECO (Spain) through the “Formación Postdoctoral 2013”.
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These authors contributed equally.