Article pubs.acs.org/EF
Analysis of a Microplasma Fuel Reformer with a Carbon Dioxide Decomposition Reaction Peter J. Lindner,*,† Sang Youp Hwang,‡ and R. S. Besser‡ †
Department of Chemical Engineering, Manhattan College, 4513 Manhattan College Parkway, Riverdale, New York 10471, United States ‡ Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, 1 Castle Point on Hudson, Hoboken, New Jersey 07030, United States ABSTRACT: Microplasmas have become increasingly attractive for activating chemical reactions because of their advantages over conventional plasma technology and catalytic processes. Advantages of reduced power requirements, atmospheric pressure operation, portability, and the circumventing of catalyst issues have made microplasmas a fascinating alternative for hydrocarbon reforming. Through the course of previous work, these microplasma reactors have displayed process and plasma variability that make characterizing the microplasma reactor challenging. In the current study, 24 experiments were run using 16 different microplasma reactors, with carbon dioxide as the only reactant. Carbon dioxide decomposition was chosen because it provides a simple experimental reaction environment that eliminates the need for a carrier gas and reduces the number of undesirable products. Carbon dioxide decomposition also has potential application for greenhouse gas mitigation. For example, an array of microplasma devices on the exhaust of a fossil fuel power station or an automobile would not only reduce the amount of carbon dioxide in the air but the product, carbon monoxide, could be used as a fuel, resulting in a reduction in net “carbon footprint”. Carbon monoxide could be used in specific fuel cell types, such as solid oxide fuel cells, or in direct combustion. Despite the simplified carbon dioxide reaction chemistry, this study has revealed device variability in reaction outcomes while observed over operating lifetimes varying from 10 s to 180 min. Input electrical energy was found to have a direct link to the amount of carbon dioxide that is reformed. Variables such as repeatability, flow rate, and device dimensions were also evaluated to gain insights for the development of improved microfluidic plasma reactors.
1. INTRODUCTION Microplasma devices have gained interest because of recent improvements in fabrication technology.1 For chemical processing, microplasmas have many advantages over current technology. A microreactor device developed as a microelectromechanical system (MEMS) holds the advantages of portability, scalability by numbering up, reduced waste, and improved mass- and energy-transfer characteristics.2 Plasma reactors have many advantages when compared to current catalyst systems, such as near-room-temperature operation, instantaneous startup times, and the circumvention of catalyst issues, such as coking and poisoning from sulfur or other components.3 Microplasmas serve to improve upon conventional plasma reactors by being able to operate at much lower power inputs and at atmospheric pressure.1 The microplasma environment is also more electron-dense when compared to conventional-scale plasma reactors.1,4 These improved characteristics make microplasma an attractive environment for chemical reaction. Despite these advantages, research in this field has been limited. Previously documented studies from the authors5−8 as well as other researchers9−14 have successfully demonstrated chemical reactions in microplasma reactors but have had issues with device lifetime, system efficiencies, and plasma consistencies. These reactors have sown inconsistencies in power requirements, plasma volume and size, and plasma stability. This work presents an analysis of the microplasma reactor to further understand issues with device lifetime and consistency. © 2013 American Chemical Society
To better understand the reaction capabilities of the microplasma reactor, we studied carbon dioxide decomposition as a model reaction. CO2 → CO + 0.5O2
(1)
Although this reaction is of interest in its own right, the carbon dioxide system was chosen for several reasons. First, the decomposition is desirable for a microplasma reactor because it is an endothermic reaction (ΔHrxn = 282.98 kJ/mol). Endothermic reactions are ideal for plasma chemistry because a large percentage of the plasma discharge energy is transferred into vibrational energy. The direct vibrational excitation of molecules has been found to be effective in stimulating endothermic chemical reactions.15 Second, carbon dioxide produces stable plasmas and does not require an alternate carrier gas to aid in igniting and sustaining the plasma. This minimizes the variables of type and concentration of carrier and concentration of the feed. Third, this reaction is simple and minimizes the potential for undesirable side products. In addition to gaining a greater understanding of the lifetime and process variation, it is desired to determine, for planar microchannel-based microhollow cathode discharge (MHCD) Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: December 31, 2012 Revised: May 2, 2013 Published: May 2, 2013 4432
dx.doi.org/10.1021/ef302199a | Energy Fuels 2013, 27, 4432−4440
Energy & Fuels
Article
Figure 1. (a) Schematic of the microhollow cathode discharge design. (b) Schematic of the microplasma device and holder. (c) Image of the microplasma reactor. (d) Image of the microplasma device when the device is turned on, with the thermistor to the right of the device. (e) The 250 μm width channel without an enhanced perimeter. (f) The 250 μm width channel with an enhanced perimeter. 2.2. Experimental Setup. The reactors are tested by placing them into a custom-made Lexan/acrylic holder, depicted in Figure 1b. Within the holder, at the exit of flow, a thermistor is placed to measure the temperature for facilitating the energy balance. The holder has an open volume above the channel of approximately 0.3 cm3, while the microplasma volume ranges from 2 × 10−8 to 5 × 10−4 cm3. This excess bulk volume has been discussed previously as an issue that limits the amount of reactant that enters the high electron density microplasma environment.5 Despite the presence of this excess volume, high conversions have been found, indicating a highly reactive microplasma environment coupled with rapid reactant mass transport. The holder is fed with the desired reactant, in this case carbon dioxide, from 0.0625 in. outer diameter (1.5875 mm) tubing. The carbon dioxide is fed to the holder using an Aalborg mass flow controller (MFC). The exit of the microplasma reactor holder leads to a Valco Instruments Co., Inc., two-loop, eight-port sample valve. The valve sends 50 μL samples to a Varian CP-4900 Micro gas chromatography (GC) unit approximately every 2 min. Between samples, the outlet is analyzed continuously by a Stanford Research Systems quadrupole mass spectroscopy (QMS) unit. The GC and QMS data are employed to determine the mass balance (Figure 2). The microplasma is ignited by applying a high voltage (
