Thermodynamic evolution of magnetite oxygen carrier via chemical looping reforming of methane

Highlights

• Magnetite ore is studied as a low-cost oxygen carrier via chemical looping reforming of methane.

• The Phase and activity experimental results agree with the thermodynamic analysis.

• Suitable operating conditions and magnetite mineral composition are proposed.

Abstract

The behavior of chemical looping reforming of methane over the magnetite oxygen carrier (OC) has been investigated by thermodynamic analysis and experiments, which clarified the appropriate operating conditions and material composition. The possible revolution of the reaction composition and the corresponding isothermal redox reactions also have been carried out. Compared with the effect of temperature rise (800–1000 °C) on the formation of carbon deposition, it is more conducive to the reduction of iron oxide. In the oxidation stage, the temperature of 400–650 °C is favorable for inhibiting the formation of C and Fe₃C, which greatly influenced the quality of hydrogen production. The compounds such as FeTiO₃, FeTi₂O₅ and Fe₂TiO₅ can be regenerated by water vapor. However, FeAl₂O₄ and MgFe₂O₄ are irreversible, resulting in the loss of reducible iron species. Equilibrium composition study revealed that syngas with an ideal ratio of (H₂/CO = 2) can be obtained at a temperature higher than 900 °C, while the carbon deposition has been inhibited. Both the thermodynamic analysis and experiment showed that the chemical thermodynamics analysis method is a leading approach for the reaction characteristic studies of chemical looping reforming of methane.

Introduction

Hydrogen is capable of meeting the demand of almost all kinds of energy requests to develop a permanent solution system for the current energy problems (Ren et al., 2015, Sun et al., 2014). It is advantageous because of rich source, high energy density, environment friendly and wide range of utilization (Hafizi et al., 2016). At present, the industrial production of hydrogen was mainly contributed by methane reforming (Song et al., 2017), due to the high availability and low cost of methane. Chemical looping reforming of methane (CLRM) is an advanced technology for the co-production of syngas and pure hydrogen, and it can be applied in large-scale production of hydrogen because of its low cost and convenient processing (Hosseini et al., 2020; Lu et al., 2020).

Compared with the traditional methane steam reforming technology, based on the two-step operation of the CLRM process, the mixed of the CH₄ or CO, and pure H₂ is avoided, the produced pure H₂ can therefore be readily obtained after condensation of excess steam and removal of the liquid water (Garai et al., 2020). This eliminates the need for additional energy supply for H₂ separation. Besides, the produced syngas with an H₂/CO ratio of 2.0 in the first step is suitable for further methanol production or Fischer–Tropsch synthesis (Hosseini et al., 2019). The CLRM is a cyclic process of oxidation (fuel reactor) and reduction stages (steam reactor) with an oxygen carrier (OC) circulating between the two reactors (Fig. 1).

Oxygen carrier is an oxygen transfer material which can release oxygen in an inert atmosphere and restore oxygen in a weak oxidation atmosphere (Zhang et al., 2020; Buelens et al., 2029). In the CLRM system,CH₄ is mainly converted to syngas (CO and H₂) and OC is reduced to the lower valence oxide in the fuel reactor, which then reacts with the steam (H₂O) to produce pure hydrogen in the steam reactor (Baser et al., 2019). The focus of the CLRM system is the highly active and low-cost OC for the feasibility of its application in large-scale production of hydrogen (Lu et al., 2018; Yu et al., 2019).

Iron-based oxygen carriers have attracted the most attention due to the excellent redox properties and environment friendliness. Forutan et al. compared many metal oxides (Co, Cu, Mn and Fe-based) as an oxygen carrier and showed that Fe based OC is the most suitable for the CL-SMR system (Forutan et al., 2015). From the thermodynamic analysis, Svoboda et al. found that Fe–Fe₃O₄ has a high potential for hydrogen production as compared with Ni–NiO and MnO–Mn₃O₄ based metal oxides (Svoboda et al., 2008). The study of the reduction of magnetite (Fe₃O₄) with methane to produce the synthesized gases and iron revealed that the equilibrium component in the solid phase is mainly metallic iron, and is composed of a mixed gas of 66.7% H₂ and 33.3% CO in the gas phase at temperatures above 1300 K (Steinfeld et al., 1993).

Taking economic and environmental considerations, natural iron mineral oxygen carriers have received more attention (Lu et al., 2018; Li et al., 2019). Iron ores are mainly composed of many kinds of iron oxides with some impurities (such as Al₂O₃, SiO₂, TiO₂, MgO and CaO), which can serve as the active phases and inert components (Li et al., 2020). It is noted that the phases of natural iron ore after simple processing were similar to the synthetic effective iron-based OC (Kang et al., 2018; Matzen et al., 2017; Tang et al., 2015). Hematite and ilmenite are widely investigated as iron-based oxygen carriers for the chemical looping combustion process, and magnetite ore is studied for chemical looping reforming (Ridha et al., 2016; Sun et al., 2017; Lu et al., 2020). Magnetite is an oxide-type mineral iron with iron content, that can reach up to 72.4%, and the various magnetite from different regions of China are listed in Table 1.

Compared with the artificial oxygen carriers, the magnetite oxygen carrier obtains a similar composition to the conventional iron-based oxygen carrier, and the preparation process avoids complicated procedures and pollution (wastewater, exhaust gas) caused by the synthesis process, which makes its cost cheaper and more environmentally friendly. The magnetite owner the highest iron content among the other Fe-contained minerals (hematite, limonite and siderite). For the low-cost Fe-contained minerals oxygen carrier used to CLRM, methane is converted to oxides (CO₂, CO, H₂ and H₂O) with the Fe₂O₃/Fe₃O₄ reduced to FeO/Fe, and the reduced Fe species (FeO or Fe) can only be recovered to Fe₃O₄ instead of Fe₂O₃ by water due to the weak oxidizing ability (Lu et al., 2018). It is worth noting that the magnetite with high contents of Fe₃O₄ may be an ideal candidate, due to it can achieve complete phase circulation, which is different from other iron ores that mainly contain Fe₂O₃.

Using magnetite as an oxygen carrier via CLRM, the presence of the impurities (components besides Fe₃O₄) may have a negative effect on the reactivity, stability and sintering or agglomeration by forming a non-reducible stable compound, resulting in the reduced oxygen content and weakened its reactivity with methane. Besides, the impurities may react with each other, affecting the mechanical strength of the OC and thereby reducing the number of cycles of the OC. On the other hand, many iron-based inert carriers reported are TiO₂, SiO₂, A1₂O₃, and MgAl₂O₄ (Karimi et al., 2014; Zhu et al., 2018; Gu et al., 2015). The addition of the inert supports, Al₂O₃ and TiO₂ allows OC to form a porous structure and large surface area (Karimi et al., 2014). The presence of SiO₂ in the impurity components could facilitate the sintering and reduces the selectivity of syngas due to the formation of an inactive compound (Fe₂SiO₄) with a low melting point (Yu et al., 2019). It was found that the incorporation of MgAl₂O₄ performed excellent thermal stability and carbon resistance at redox cycles (Zhu et al., 2018). In this case, it is necessary to clarify the effect of different components on a CLRM system, which is conducive to the applications of magnetite ore via a CLRM process.

This work will mainly, by the combination of thermodynamic analysis and experiments, study and discuss the magnetite use as OC for the CLRM process of hydrogen generation. The goal is to unveil the appropriate operating conditions and material composition by investigating the thermodynamic analysis and reactivity properties of the reactions. The possible reactions and new species produced in the fuel and steam reactors are listed below. The composition of the gas phase and solid phase of the system with temperature have been described in detail, and the corresponding experiment carried out to compare with the thermodynamic results.