Elsevier

Applied Energy

Volume 211, 1 February 2018, Pages 174-186
Applied Energy

Syngas production on a Ni-enhanced Fe2O3/Al2O3 oxygen carrier via chemical looping partial oxidation with dry reforming of methane

https://doi.org/10.1016/j.apenergy.2017.11.018Get rights and content

Highlights

  • CLPD was derived by merging dry reforming into chemical looping partial oxidation.

  • Results of CLPD were calculated using the ASPEN Plus simulator.

  • Syngas with a H2/CO ratio of 2 was produced through the CLPD process.

  • Ni-enhanced Fe2O3/Al2O3 showed the enhanced CLPD activity without rare earth metals.

  • Stabilized Ni in Al2O3 promoted dry reforming with suppressed carbon deposition.

Abstract

A novel chemical looping process was introduced by combining partial oxidation and dry reforming of methane on a cost-effective iron-based oxygen carrier to produce high-purity syngas with a H2/CO ratio of 2. The rationale for the proposed chemical looping process was substantiated with the thermodynamic data, which showed increased syngas purity and an H2/CO ratio close to 2 by introducing the CH4-CO2 mixture feed. Compared with the general chemical looping process, the calculated carbon deposition with the CO2 emission of the proposed process was dramatically decreased by using CO2 as a co-feed with CH4. Due to the exothermic heat from the oxidation reaction of the oxygen carrier, the net heat duty of the novel chemical looping process was much lower than that of the dry reforming process. To validate the thermodynamic results, a Ni entrapped Fe2O3/Al2O3 oxygen carrier was synthesized by increasing the metal-support interaction through a sol-gel route. It is striking that the formation of Ni aluminate phase in the Ni-reinforced oxygen carrier facilitated dry reforming with partial oxidation while suppressing methane decomposition. By supplying a nonstoichiometric CH4-CO2 mixture feed (CO2/CH4 ratio = 0.38) to the 1 wt% Ni-entrapped Fe2O3/Al2O3 oxygen carrier at 900 °C, an H2/CO ratio of 2.09 and high CO selectivity of 96.76% were achieved with minimized carbon deposition. These results were close to the calculated equilibrium value while a Ni-impregnated Fe2O3/Al2O3 oxygen carrier showed an increased H2/CO ratio of 2.36 with severe carbon deposition by the promoted methane decomposition. In addition, the Ni-reinforced oxygen carrier also showed stable redox activity during successive reduction and oxidation cycles.

Introduction

The economical accessibility of shale gas reservoirs has been increasing due to advances in horizontal drilling and hydraulic fracturing technologies, which will almost double shale gas production by 2040 [1]. This increased production has attracted public attention as not only a promising fuel for power production but also as a chemical raw material [2]. However, when methane (CH4), a primary resource of shale gas, is directly used as a combusted fuel, anthropogenic carbon dioxide (CO2) is emitted, contributing to global warming [3]. The direct conversion of CH4 to chemicals is also difficult due to localized Csingle bondH bonds with a high bond energy of 413 kJ mol−1 and the absence of empty orbitals with a low bond energy [4].

The conversion of CH4 to syngas, a mixture of CO and H2, should be done first to synthesize valuable liquid hydrocarbons, because syngas is as an essential intermediate [5]. Steam reforming of CH4 [SRM, Eq. (1)] is mainly adopted to produce syngas from CH4 in the current mature industry. However, its endothermic reaction requires an energy intensive process, and the stoichiometric syngas ratio (H2/CO) of 3 is unsuitable for the syngas-to-liquid hydrocarbon process, where a H2/CO ratio of 2 is preferred [6], [7].CH4+H2OCO+3H2ΔH298K0=206kJmol-1CH4+(1/2)O2CO+2H2ΔH298K0=-36kJmol-1

Partial oxidation of CH4 [POM, Eq. (2)] is a promising alternative to SRM, which achieves both auto-thermal operation by its exothermic reaction and a H2/CO ratio of 2. Despite these advantages of POM over SRM, the safety issue related to mixed oxygen (O2) and flammable gases makes it difficult to scale up POM. There is also an economic burden because an air separation unit (ASU) is required to produce the concentrated syngas by supplying high-purity O2 [8].

Chemical looping partial oxidation of methane (CLP) is a deployable solution that can achieve sustainable syngas production with minimized safety and air separation cost issues, because the concept of chemical looping divides the oxidizing/reducing reactions into two separated reactors [9], [10], [11], [12], [13], [14], [15], [16], [17]. In CLP, the reducible oxygen resource is transported from air to CH4 via a metal oxide without being diluted by N2. The oxidized MeOx is reduced by CH4 in the fuel reactor with the production of syngas, and the reduced MeOy is re-oxidized by air with the generation of heat in the air reactor, where MeOx and MeOy represent the oxidized and reduced transition metal oxides, referred to here as the oxygen carrier (Fig. 1a). Therefore, direct contact of oxygen with flammable gases is avoided [18], [19], [20]. Also, an ASU is not required, because the produced gaseous streams are not diluted by N2.

However, a suitable oxygen carrier and its supporting matrix to achieve a H2/CO ratio of 2 are very limited in CLP. To the best of our knowledge, ceria (Ce)-containing materials [21], [22], [23], [24], [25], [26] or perovskite-type oxides [27], [28], [29], [30] with rare earth metals have been almost exclusively studied for CLP due to their high CO selectivity from the abundant oxygen vacancies. Although CeO2 preferentially yields syngas as the product from the reaction with CH4, not H2O and CO2 [31], the scarcity and high cost of CeO2 make Fe-Ce mixed oxides an attractive alternative to pure CeO2 [22], [23], [24], [25], [26]. In addition to the low cost of Fe based materials, the smaller size and lower valence state of Fe3+ relative to that of Ce4+ create structure defects, which enhance the redox activity of the Fe-Ce mixed oxide [32], [33], [34]. However, the dissolution of Fe3+ into bulk Ce was restricted to only 15% of the dopant content by a hydrothermal route [35], thus suggesting a high cost for the Fe-Ce mixed oxide oxygen carrier.

Perovskite-type oxides have emerged as an attractive candidate oxygen carrier for CLP due to the high oxygen mobility originating from the capability of the concentrated oxygen vacancies [36]. Various kinds of rare earth and transition metals have been used for their synthesis depending on the applications [27], [28], [29], [30], [37], [38], [39], [40], [41]. Nevertheless, the economic concern of using rare earth metals has not yet been solved and mechanical stabilities of perovskite particles in CLP should be evaluated further so that they can be used in a scaled-up process [12].

In addition to the type of oxygen carrier, the feed composition of CLP also plays an important role in determining the product composition. CO2 has been utilized as an oxidizing reagent to remove the deposited carbon and oxidize the reduced oxygen carrier [42], [43], [44], [45], [46], [47]. CO2-utilized chemical looping combustion was initially proposed, which produces a H2O-CO2 mixture from CH4 in the fuel reactor and high-purity CO from CO2 in the oxidation reactor [42]. Various oxygen carriers have been investigated for this CO2-utilized chemical looping combustion such as Fe/barium hexaaluminate [43], CeO2-modified Fe2O3 [44], and iron nickel oxide [45]. CO2-utilized chemical looping reforming, which produces syngas from CH4 in the fuel reactor and CO from CO2 in the oxidation reactor, was subsequently proposed [46], [47]. However, the produced H2/CO ratio was not closely monitored [47] or fluctuated during the reduction reaction [46], because CH4 and CO2 were separately supplied to the fuel and oxidation reactor, respectively. CO2 have only recently begun to be considered as the co-feed with CH4 [48], [49], [50]. In a moving bed reactor system, the CH4-CO2 mixture was reacted with iron-titanium composite metal oxide to produce a high-purity syngas while reducing the CH4 feed usage [49]. The chemical looping process with the CH4-CO2 mixture feed was also modularized, potentially providing economic and environmental benefits [50].

The aim of this study is to achieve a H2/CO ratio of 2 produced through a chemical looping process in a fixed bed reactor system without using expensive oxygen carriers such as CeO2 and perovskites. To this end, Fe2O3 on a supporting matrix of Al2O3 is used as the oxygen carrier due to its abundance and low cost, contributing to the economic feasibility of the chemical looping process. To compensate for the high H2/CO ratio caused by the catalytic methane decomposition [CMD, Eq. (3)] of Fe2O3/Al2O3 oxygen carrier, an external oxygen resource is supplied by CO2 with co-feeding of CH4 to the fuel reactor (Fig. 1b). Because the stoichiometric H2/CO ratio is 1 by the reaction of CH4 with CO2, which is dry reforming of CH4 [DRM, Eq. (4)], we conjecture that the addition of CO2 can adjust the produced H2/CO ratio to 2 even on a Fe2O3/Al2O3 oxygen carrier by promoting DRM with the production of additional CO.CH4C(s)+2H2,ΔH298K0=75kJmol-1CH4+CO22CO+2H2ΔH298K0=247kJmol-1

In this way, this study introduces the concept of CLP combined with DRM, hereafter called chemical looping partial oxidation with dry reforming of methane (CLPD), by controlling the molar ratio of CO2 to CH4 in the feed stream (CO2/CH4). To increase the catalytic activity toward DRM, a small amount of Ni, known as an effective catalytic metal [51], will be added to the oxygen carrier. However, Ni is highly active for not only DRM but also for CMD [52], [53], which leads to increases in both the H2/CO ratio and carbon deposition. Thus, to promote DRM while suppressing CMD, this study will show how the Ni and support interaction is enhanced by mixing the Ni precursor and the Al precursor before the hydrolysis of Al precursor through a sol-gel synthesis route. It will be demonstrated that this increased interaction between the Ni particles and the supporting matrix can minimize the carbon deposition by preventing the separation of the Ni particles from the supporting matrix. The Ni-enhanced Fe2O3/Al2O3 oxygen carrier can achieve the production of high-purity syngas with a H2/CO ratio of 2 through repeated reduction/oxidation cycles, implying the feasibility of the proposed CLPD process.

Section snippets

Thermodynamic analysis

Because the RGIBBS module in ASPEN Plus minimizes the Gibbs free energy, chemical equilibrium compositions of the gaseous products of the chemical looping process were simulated by using this module with various [O]/CH4 and CO2/CH4 ratios. The oxygen capacity ([O]) was defined as the moles of the reducible oxygen in the oxygen carrier, and the molar ratio of [O] in the oxygen carrier to CH4 in the feed stream is referred to as the [O]/CH4 ratio [Eq. (5)] [54]. The molar ratio of CO2 to CH4 in

Comparison between CLP and CLPD

The equilibrium compositions of the product streams were calculated with various [O]/CH4 ratios in chemical looping processes. Because the compositions are significantly affected by the reduction properties of the oxygen carriers, the inorganic and solids databanks from ASPEN Plus were used to accurately estimate the physical and chemical properties of the oxygen carriers [54]. Different from CeO2, which prefers partial oxidation of CH4 over almost the whole range of the [O]/CH4 ratios [54], Fe2

Chemical looping process with various [O]/CH4 and CO2/CH4 ratios

To translate the thermodynamic analysis into chemical looping experiments in a fixed bed reactor, CLPD with various [O]/CH4 and CO2/CH4 ratios was performed on Fe2O3/Al2O3 and compared with the calculated equilibrium state. Because the [O]/CH4 ratio varied with the reaction time as previously explained, the average [O]/CH4 ratio was taken to represent operating conditions [Eq. (8)]. Three different average [O]/CH4 ratios of 0.24, 0.48, and 0.72 were selected as the experimental conditions, and

Conclusions

A methane-to-syngas chemical looping process was investigated from both aspects of thermodynamic and experimental data. The equilibrium compositions of the product stream in the chemical looping process were simulated on a Fe2O3 oxygen carrier by introducing CO2 as a co-feed with CH4. When compared with a general chemical looping process (CLP) whose H2/CO ratio was up to 70, the H2/CO ratio of the novel chemical looping process (CLPD) dropped sharply to around 2 as the CH4-CO2 mixture feed was

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