EFFECTIVE CARBON DIOXIDE REDUCTION INTO CARBON MONOXIDE USING MILLICHANNEL EMBEDDED IN-LINE BARRIER DISCHARGE REACTOR Kimin Jun and Joseph M. Jacobson* Massachusetts Institute of Technology, U.S.A.

ABSTRACT

Plasma process is a possible method to decompose carbon dioxide(CO2) into carbon monoxide(CO) which is a valua- ble carbon source. To achieve this goal, we need to improve the conversion efficiency in process. In this research, we present a miniaturized discharge system. Electrode for dielectric barrier discharge(DBD) is aligned along millimeter scale channel. It is believed that plasma alignment along the flow direction and large plasma volume ratio to free space increase the reaction probability. We measured CO concentration of our system as well as large scale corona discharger. The results show significant improvement in conversion efficiency.

KEYWORDS: Carbon dioxide, carbon monoxide, plasma, dielectric barrier discharge, channel

INTRODUCTION

Carbon dioxide(CO2) reduction has importance not only in environmental benefit but also in possibly renewable ener- gy source. Hydrocarbon is still unmatched transportation fuel in terms of energy density and easiness of refueling. There- fore, synthetic hydrocarbon from renewable source is an attractive solution for the energy crisis. Currently, Fischer- Tropsch method is the most successful hydrocarbon synthesis method. This uses hydrogen and carbon monoxide(CO) as the primary reactants. The reduction of CO2 into CO then completes truly renewable carbon cycle[1]. However, CO2 re- duction has large thermodynamic barrier to necessitate huge energy supply. Therefore, plasma reaction, which is based on non-equilibrium state[2], possibly provides less energy-intensive reduction path. The key in plasma chemistry is how to increase the chances to be reacted. In this standpoint, we devised in-line DBD reactor in millimeter scale channel. By confining the geometry, we expect more chances of gas facing plasma region. The effective CO production rate was measured and compared with large scale corona discharger.

THEORY Dielectric barrier discharge(DBD) is a kind of plasma reactions in which at least one of the electrodes is separated by dielectric layer. Since this dielectric prevents unwanted arcing through the gas and helps distributing charges, the plasma is stable and uniform over the electrodes[3]. In its configuration, DBD is largely divided into volume discharge(VD) and surface discharge(SD)[4]. Volume discharge may produce larger plasma volume, but at the cost of higher voltage re- quirement. Considering possible minimum overhead for producing plasma and easiness of fabrication, we chose modified in-line SD configuration.

Figure 1: strength simulation. (a) corona discharger of 20mm diameter cylinder, and (b) DBD reac- tor of 1mm square channel. (unit V/mm)

978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS 321 14th International Conference on Miniaturized Systems for Chemistry and Life Sciences 3 - 7 October 2010, Groningen, The Netherlands SD mode can be readily localized since it facilitates asymmetric electric field. The plasma is primarily generated near the surface electrode due to the strong electric field. This phenomenon is analogous to corona discharge which also has asymmetric electrode geometry. Therefore, based on our physical setup(this will be described in more detail in EXPE- RIMENTAL section), model geometries and 2D cross sectional electric field analysis(by COMSOL Multiphysics) are provided in figure 1 for cylindrical corona discharger and square channel DBD reactor, respectively. The field strength(figure 1(a) and (b)) did not consider plasma potential. Due to Debye screening, the real potential line would be more compact than presented here[5]. Even in these sparse profiles, strong field is concentrated near the electrodes. Therefore, it is reasonable to shrink the channel dimension to increase the plasma volume ratio with respect to the free space. However, in case of corona discharge, limits the applicable voltage in small dimension since the mode is quickly changed to arcing. In contrast, dielectric barrier allows DBD reactor to be configured in small chan- nel.

EXPERIMENTAL The experiment was conducted both on in-line DBD reactor and commercial corona discharger. The cross section of in-line DBD reactor is schematically shown in figure 1(b). In 1mm by 1mm square channel(millichannel), surface elec- trode made of copper foil extrudes half way of the channel from one side. The buried electrode lies below the separation dielectric in opposite side. Since the electrode structure was symmetric, we built two identical channel on top and bottom of the dielectric. And, the millichannel spans along the electrodes interface line to form in-line configuration with total electrode length 65mm. We used 1mm thick slide glass for the dielectric as well as channel wall. Properly diced slide glass and copper foil were stacked as shown in figure 2(a). The side of this stack was sealed by epoxy and the end cap was molded by poly(dimethylsiloxane). The final assembly is shown in figure 2(b) and ignited plasma(in air) is shown in figure 2(c). Corona discharger(figure 2(d)) was extracted from commercial generator(Aqua-6, A2Z Ozone). The cylinder is 20mm diameter and 70mm long stainless steel tube. Pure CO2 was supplied to the discharging unit. The electrode of discharger was connected to AC power supply with fixed frequency(1.1kHz) and variable amplitude. The exit of the discharger was connected to the inlet of CO analyz- er(model 707, TPI). The experiment was conducted by varying CO2 flow rate and applied voltage. CO analyzer draws fixed volume(425sccm) of gas and measures the concentration in part per million(ppm). When CO2 flow rate was lower than the intake volume, fresh air compensates the remainder volume.

Figure 2: Experimental setup. (a) DBD reactor design, (b) fabricated DBD reactor, (c) ignited discharge, and (d) corona discharger.

RESULTS AND DISCUSSION

Figure 3 summarizes the results. Figure 3(a) shows measured CO concentration versus CO2 flow rate. Low voltage which produces nearly zero CO in corona discharger can reduce significant amount of CO2 in millichannel. This can be attributed to the low onset voltage of DBD. From this figure, it is observed that millichannel yields higher concentration of CO from given CO2 flow rate and voltage. The sharp dropping profile of large cylinder is understandable because the lateral mixing in large cylinder may not be so effective that many gas molecules simply bypass the plasma region as flow rate increases. That is, decreased residence time(here, it is defined as the average time a volume of gas passes the dis- charge reactor) lowers CO2 decomposition ratio. In contrast, DBD shows gradually increasing CO concentration with CO2 flow rate. However, this information might be misleading since the low CO2 flow rate is significantly smaller than sampling vo- lume. Therefore, we converted this raw value assuming entire sample consists of processed gas by multiplying ‘sampling volume/flow rate’. For the data points whose CO2 flow rate is already larger than sampling volume, the original value was used for further analysis. Also, considering the geometry, the flow rate was converted to residence time. Figure 3(b) is the converted plot. It describes millichannel requires orders of shorter residence time than its larger counterpart.

322 Current result, maximum conversion efficiency of about 0.85%, is based on 100% CO2 concentration. According to previous research[6], the conversion efficiency monotonically decreases as CO2 concentration increases. However, since lower concentration of CO2 consequently reduces CO yield, corresponding energy efficiency would be aggravated. Therefore, we believe that there should be optimal condition in terms of energy input, and it would be further research topic.

Figure 3: Result plots. (a) measured CO concentration with respect to CO2 flow rate and applied voltage, and (b) converted CO concentration regarding gas residence time and applied voltage.

CONCLUSION

This demonstrates that small scale channel DBD device effectively decomposes CO2. As many other chemical and physical processes, plasma process can benefit from scale-down system by increasing volumetric plasma ratio in free space. To maximize the conversion efficiency, it is required to conduct analytical studies on fluid mechanics and potential field in plasma region. The optimization strategy may focus on minimum energy input for recyclable energy application. At any case, small channel device has advantages on easily scale-up to parallel system as well as flexible configuration, which implies possible practical application.

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CONTACT *Joseph M. Jacobson, tel: +1-617-2537209; [email protected]

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