Hydrogen production from methane in a dielectric barrier discharge using oxide zinc and chromium as catalyst

https://doi.org/10.1016/j.jcice.2007.10.001Get rights and content

Abstract

The hydrogen fuel cell is a promising option as a future energy resource; however, the nature of the gas is such that the conversion process of other fuels to hydrogen on board is necessary. Among the raw fuel resources, methane could be the best candidate as it is plentiful. In this experiment, the possibility of producing hydrogen with less carbon formation from methane by a dielectric barrier discharge (DBD) was investigated. Without the addition of a catalyst, the formation of hydrogen reached between 30% and 35% at methane residence time of 0.22 min and supplied powers in the range of 60–130 W. The hydrogen selectivity increased at higher supplied power, but the process efficiency, defined as a ratio of the produced hydrogen to the supplied power, decreased slightly. In order to boost the hydrogen production with less carbon formation, a mixed oxide catalyst of zinc and chromium was added to the reactor. It was shown that the production of hydrogen was ca. 40% higher than the non-catalytic plasma process.

Introduction

The development of fuel-efficient engines that produce fewer pollutants has been a major objective for many years. Regulatory requirements have been and will be of major importance in defining the standards (Cooper, 1994). Increasingly stringent legislation directs attention at factors that reduce polluting emissions, such as a cold-start process and lowering sulfur levels in fuel (Matsumoto et al., 2000). Among many selective compound candidates, hydrogen and methanol have been considered as green fuel starting materials. Compared to a hydrogen-based process, the kinetics of methanol conversion was relatively slower and catalyst deactivation occurred simultaneously. As a result, hydrogen is preferred (Cameron, 1999, Ralph and Hards, 1998, Hoogers and Thompsett, 1999), but presents obvious problems with generation, storage, and distribution of the gas. Pressurized vessels or metal hydrides of hydrogen in a vehicle are relatively dangerous because hydrogen is categorized as a highly flammable material.

Currently, attention has been focused on the design and operation of compact and efficient devices designed to generate hydrogen on board a vehicle. Jamal and Wyszynski (1994) reviewed many possible onboard hydrogen generators and the alternative fuels for spark ignition engines. Hydrogen production from methane (CH4) and methanol has attracted the consideration of many experts. Although methanol has more advantages than methane, e.g. easier storage and safety, along with the by-product of carbon monoxide being annoying for both environmental and human health (Liu et al., 2004, Velu et al., 1999). On the other hand, nowadays, methane for fuel is gaining popularity and widespread use, especially in the transportation sector.

In order to produce hydrogen from methane, a methane cracking process is necessary. However, the conventional thermal method requires a high temperature condition to achieve high methane conversion (Indarto et al., 2006a) and can be very costly for the onboard process. In particular, decomposition of methane using plasma could be a candidate for this purpose as the existence of high-energy electrons were able to decompose methane conversion at lower temperature. Yao et al. (2001) investigated the methane conversion using non-thermal plasma and showed that the methane conversion could be dramatically improved through this method.

As high hydrogen production is the goal of the research, process modification, e.g. by the addition of a catalyst, is necessary to change the nature of the product distribution. In thermal-based processes, the catalytic conversion of methane to hydrogen is usually carried out by the catalyst of Ni or Pt (Peña et al., 1996, Twigg, 1989, Rostrup-Nielsen, 1984). Our previous research using Pt/γ-Al2O3 catalyst to convert CH4 in a DBD showed a negative effect on the production of hydrogen, as higher hydrocarbon transformation was the more dominant phenomena (Kim et al., 2004). Better results employing Ru and Rh catalysts were obtained in a catalytic thermal process (Rostrup-Nielsen and Hansen, 1993). That result was supported by our investigation that Ru and Rh were also active metal catalysts for converting methane to hydrogen in a non-thermal plasma (Indarto et al., 2007, Indarto et al., 2008), but, as the price in the market is relatively expensive, this material has been rarely used in the industry.

Bridger et al. (1970) developed a relatively cheaper mixture catalyst based on Zn and Cr oxide metal for hydrogen production. Using a mixture of CH4 and CO2 as the reactants, the production of hydrogen yielded 35% at a relatively low temperature (ca. 100 °C). Davies and Hall (1976) thoroughly investigated the activity of Zn catalyst for H2 production. Recent experiments by Liu et al. (2004) showed that the existence of ZnO-based catalyst increased the production of hydrogen from methanol. The activity of ZnO–Cr2O3 catalyst at low temperature condition has been investigated by Ohta et al. (2004) for the dehydrogenation of isobutane. However, no publication was found for the use of mixed oxide of Zn–Cr catalyst for methane conversion to produce hydrogen both by thermal and plasma methods.

In this research, the direct methane conversion to hydrogen was investigated using a dielectric barrier discharge (DBD). DBD is a widely used plasma technique for many applications because it is an easily installed and low-cost of operation instrument. In accordance with the above discussion, a catalyst based on Zn and Cr oxide metal was made and employed in order to boost the production of hydrogen. In order to increase the lifetime of the catalyst, the research would like to suppress the production of carbon. The deposition of carbon by covering the surface of catalyst is a major reason of the catalyst deactivation (Venugopal et al., 2007).

Section snippets

Experimental setup

The schematic diagram of the experimental setup is shown in Fig. 1. The reactor was a quartz tube with an inside diameter of 6 mm and length of 20 cm. A thin silver film, serving as the outer electrode, coated the outer wall of the reactor. The inner electrode was two stainless wires (∅ = 0.2 mm) located in the center of the reactor. The plasma was generated by a high-voltage alternating current (AC) generator (Auto electric, model A1831) that has maximum voltage of 10.0 kV and maximum frequency of 20

Non-catalytic plasma reaction

The conversion of methane using non-thermal plasma devices, especially dielectric barrier discharge, has been commonly used. Numerous reports and papers have been published with differing results and conclusions as many aspects and variables exist in a DBD (Indarto et al., 2005, Indarto et al., 2006a, Indarto et al., 2006b, Kim et al., 2004, Yang, 2003, Yao et al., 2001). Although plasma reaction is known as one of the most difficult reactions to determine the kinetics, the tendency to follow a

Conclusions

The production of hydrogen from methane by a dielectric barrier discharge with mixed oxide catalyst of zinc and chromium was investigated. Non-catalytic plasma reactions resulted in hydrogen with selectivity of ca. 30–35%. The selectivity of hydrogen increased with increasing the power supplied to the reactor. The addition of calcined mixed Zn–Cr oxide catalyst increased the hydrogen production 40% higher than the non-catalytic reaction. The existence of a catalyst also increased the methane

Acknowledgements

The author thanks to the Korea Institute of Science and Technology (KIST) and the Korea University (KU) for the financial of the study. The author also would like to express his appreciation to the Università degli studi di Torino for the support during study period in Turin, Italy.

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