Impact of reactor materials on methane decomposition for hydrogen production

Highlights

• Investigation on the effects of operating parameters on hydrogen production in a lab-scale reactor.

• Catalytic activity of reactor’s wall on methane decomposition shows no significant change with time.

• Methane decomposition over the reactor’s internal surface shows temperature and VHSV dependencies.

• Quartz lined metal reactor could eliminate the influence of the reactor’s metal up to 925 °C.

• The results of EDAX confirm detachment of Ni from the reactor’s internal surface.

Abstract

In this study, the catalytic effect of the internal surface of reactors constructed from stainless steel (SS310) for hydrogen production via methane decomposition has experimentally been proved and compared with a quartz-lined reactor. The effects of the methane decomposition temperature and volume-hourly-space velocity (VHSV) on hydrogen production, conversion of methane, and initial decomposition rates were investigated in an SS310 lab-scale reactor. Within the duration of the experiments, no change was recorded for hydrogen and methane concentration in all the experiments. The catalytic activity of the SS310 reactor's internal surface increased with increasing methane decomposition temperature (850−950 °C) and decreasing VHSV (37−52 h⁻¹). The as-produced carbon was characterized using surface texture analysis, X-ray, and SEM-EDX.

Introduction

The current worldwide economy weighs heavily on fossil fuels, and this dependency leads to increase environmental catastrophes because of huge greenhouse gas emissions. Greenhouse gas emission is a major ongoing concern to scientists because of its environmental impact. Hydrogen is proposed as a promising energy carrier because it produces only water instead of greenhouse gases on its burning. Different methods are developed to extract hydrogen from natural resources, such as steam methane reforming (SMR), partial oxidation of methane, coal gasification, water electrolysis, thermochemical processing, and photocatalytic water splitting.

The production of hydrogen from natural gas, the main component being methane, has great practical importance because the world's known reserves of natural gas are increasing. Recently, thermocatalytic decomposition (TCD) of methane has attracted scientists' attention as an environmentally attractive way of hydrogen production (Muradov et al., 2005; Muradov, 2001; Rodat et al., 2009; Hirsch and Steinfeld, 2004). Details of methane TCD, such as economic aspects, alternative heating sources, and different employed catalysts, are thoroughly analyzed by Abbas and Wan Daud (2010). Unlike SMR, where water gas-shift and CO₂ removal stages are required, methane TCD is considered an attractive alternative because it is a straightforward single-step reaction that produces hydrogen and solid carbon. However, it is worth mentioning that this method also has drawbacks. It is well documented in the literature that using metal or carbonaceous materials as a catalyst for methane TCD will ultimately be deactivated due to fast carbon deposition. In addition, methane decomposition is an endothermic reaction that requires extremely high temperatures to decompose stable C–H bonds (Abbas and Wan Daud, 2010).

CH_{4 (g)} → C_(S) + 2H_2(g) ΔH^o = 75.6 kJ/mol

Citation of the literature shows that metal catalysts (Suelves et al., 2005; Pinilla et al., 2008a; Chesnokov and Chichkan, 2009; Pinilla et al., 2007) or carbonaceous materials (Muradov et al., 2005; Muradov, 2001; Pinilla et al., 2008b; Suelves et al., 2007; Lee et al., 2004) were utilized to diminish the cracking temperature of the methane because a reasonable yield was achievable only at high temperatures above 1200 °C. Ni (Suelves et al., 2005) or Fe (Jang and Cha, 2007) or combined metals like Co/Mo/Al₂O₃, Ni/Cu/Al₂O₃ (Qian et al., 2004), and Ni/Al or Ni/Cu/Al (Li et al., 2000) are the most investigated metal catalysts. Among different carbonaceous materials, most studies have been carried out on carbon black and activated carbon (AC) (Muradov et al., 2005; Muradov, 2001; Pinilla et al., 2008b; Suelves et al., 2007; Lee et al., 2004). In addition to the reduction of reaction temperature, carbonaceous materials serve to a) absorb reactor wall temperature and increase core temperature, b) provide nucleation site, c) become the radiation source itself, and d) enhance heat transfer via conduction.

Due to high activity, long-term stability, and its ability to co-produce carbon nanotubes or carbon nanofibers, utilization of metal-based catalysts for methane TCD, has attracted the attention of researchers. Generally, most investigations have concentrated on nickel-based catalysts, thanks to their high activity and potential to produce carbon nanofibers at mild temperatures. To increase Ni catalysts' thermal stability, they doped with metals like Fe and Cu (Abbas and Wan Daud, 2010).

Lab-scale units usually use quartz as construction material to prevent any tentative reactor's wall catalytic activity (Muradov et al., 2005; Muradov, 2001; Suelves et al., 2005; Dunker et al., 2006). The authors recognized that the catalytic effect of stainless steel as a reactor construction material on methane TCD and characterization of the carbon produced could be important to those concerned with conducting research and developing methane TCD technology. In previous work (Abbas and Daud, 2010a), the research objective was focused on the catalytic AC activity for methane decomposition. The result of a blank experiment conducted to investigate the catalytic effect of the rector's material on methane decomposition was the motive for the current study. The non-porous structure of catalyst composed of nickel, iron, and chromium and the high methane conversion obtained using a relatively small surface area of catalyst (1103 cm²), especially at a high temperature, indicate that detailed investigation should be carried out.

Literature surveys have indicated that the reactor material effect on TCD of methane requires further investigation. In the current research, the catalytic behavior of reactor construction material (SS310S) on cracking of methane in the temperature limitation of 800–950 °C was investigated. Authors (Ashik & Wan Daud) previously proved that the reactor wall materials are inactive for methane cracking below 800 °C (Ashik and Wan Daud, 2015). For the comparison purpose, the catalytic behavior of activated carbon originated from palm shell (ACPS) on TCD of methane was performed in a fixed bed reactor. The most important variables chosen in these researches were decomposition temperature, and volume hourly space velocity (VHSV) based on changing the methane flow rate at a constant catalyst area. The effect of decomposition temperatures on the methane TCD in a quartz-lined stainless steel tubular reactor was also investigated. The characterization of ACPS and as-produced carbon from methane decomposition was conducted using surface texture analysis, XRD, and EDAX.