Thermodynamic evaluation during the reduction of MWO4 (M = Fe, Mn, Ni) with methane for the production of hydrogen-syngas

https://doi.org/10.1016/j.ijhydene.2018.08.014Get rights and content

Highlights

  • A simulation model of POX of methane for syngas generation is proposed.

  • The use of MWO4 (M = Fe, Mn, Ni) as oxygen carriers for POX of methane by CLPO.

  • Regeneration of MWO4 with steam produce a high purity hydrogen stream.

  • Sensitivity analyses determined the optimal operating conditions in the process.

Abstract

In order to overcome one of the most important disadvantages of the partial oxidation of methane (POX) reaction, which deals with the use of pure oxygen as a gas feed, a mixed metal oxide (MWO4, M = Fe, Mn, Ni) is proposed as an oxygen carrier. The aim of this study is to evaluate the feasibility of these tungstates through the use of thermodynamic analyses and process simulations in an arrangement of two reactors. First reactor: CH4 + MWO4 = H2 + CO + M + W; CH4 + MWO4 = H2 + CO2 + M + W and possibly CH4 = C + 2H2. While in the second reactor: M + W + H2O = MWO4 + H2; C + H2O = H2 + CO; C + 2H2O = 2H2 + CO2. Then, the MWO4 is recycled back to the first reactor to make a continuous process. Simulation results of this process with the different MWO4 oxides are presented using Aspen Plus©.

Introduction

Despite the current weakness of world energy markets and the sluggish Chinese growth, demand for energy will continue to grow over the next 20 years and even later as the world economy expands and more energy is needed to push for greater industrial activity. According to the 2016 BP World Energy Outlook [1], global energy demand is expected to increase by 34% between 2014 and 2035, at an average of 1.4% per year. This growth, in demand, generally includes significant changes in the composition of the energy mix, with faster increase of low-carbon fuels than those with higher emissions as the planet begins the transition to a future with lower carbon emissions.

Even though the fast growth in other energy sources, it is predicted that fossil fuels will continue to be the predominant form of energy by 2035, accounting for 60% of the expected increase in demand and almost 80% of the world's total energy supply by 2035. Natural gas will be the fastest growing fossil fuel, with an annual increase of 1.8%, and oil will rise at a constant rate of 0.9% per year, although its share in the energy mix will continue to decline. Coal demand is expected to decline dramatically, so that by 2035 its share of the energy mix will be the lowest ever, being replaced by gas as the second largest source of fuel. Furthermore, natural gas supply is growing strongly thanks to a large increase in the production of shale gas worldwide, which is expected to expand by 5.6% annually. The share of shale gas in total gas production will intensify from 10% in 2014 to almost 25% in 2035 [1].

Moreover, hydrogen has been employed as a common raw material for a wide variety of processes, for example, in ammonia synthesis, pharmaceuticals, hydrogen peroxide generation, electronics and petrochemical industries [2], [3]. Nowadays, hydrogen has been considered as a clean, renewable and efficient fuel since, when used in combination with fuel cells, hydrogen can produce heat and electricity with the only waste being pure water. The fact that hydrogen is not a primary energy source like most common fossil fuels (coal or natural gas) makes it an energy vector that can be produced from many traditional energy systems (most hydrogen today is produced from natural gas). However, in order for hydrogen to be considered as a clean fuel this must come from renewable energy sources.

Furthermore, of equal importance to hydrogen, the mixture of hydrogen and carbon monoxide (H2 + CO), commonly called synthesis gas or syngas [4], [5], [6] is an important raw material for several industrial applications. Technically, syngas can be produced from any hydrocarbon feedstock [7]. Though, in many applications, natural gas is the main raw material [4].

Steam reforming of natural gas (mainly methane, SMR, reaction 1) is today the dominant process worldwide for the production of syngas [7], [8], [9], [10]. However, partial oxidation of methane (POX, reaction 2) for the production of syngas has been reported to present higher efficiencies than SMR [11]. Additionally, POX exhibits other benefits such as: less investment required to produce a syngas at a molar ratio of H2/CO = 2 (which is a very convenient gas composition for the Fischer-Tropsch process), an inherent exothermic reaction of this process, thus inferring substantial energy savings, fast kinetics derive in the use of small reactors and high methane conversions (≈90%) with elevated selectivity towards hydrogen (94∼̴99%). However, this process also presents some disadvantages such as high operating temperatures (900∼̴1000 °C), and the need for an oxygen plant, which makes this a highly cost process [11].CH4(g)+H2O(g)=3H2(g)+CO(g)2CH4(g)+O2(g)=4H2(g)+2CO(g)

While trying to overcome these drawbacks, research has been performed aiming to reduce syngas operating costs by POX and to lower working temperatures. One particular strategy is the elimination of the oxygen plant, which represents about half of the total process investment [12]. An example of research in the area, is the use of metal oxides employed as oxygen carriers, which is based on a variation of the partial oxidation of methane to produce syngas that consists in two steps: first, the demanded oxygen for partial oxidation is supplied by a metal oxide (MeO) containing oxygen in its crystalline structure, which is released under a reducing atmosphere to produce syngas and the reduced metal (Me) (Reaction 3). While in the second step, the reduced metal is back oxidized with water vapor to produce hydrogen and the regenerated MeO (reaction 4).CH4(g)+MeO=2H2(g)+CO(g)+MeH2O(g)+Me=MeO+H2(g)

The MeO is then consumed again in reaction (3) to complete a full cycle for the oxygen carrier (MeO), this process has been called POX-MeO or chemical looping partial oxidation (CLPO) [5], [11]. It is worth to notice that the overall reaction of this process is fairly equal to the SMR [12]. The CLPO system consists of two reactors. In the first reactor (fuel reactor) reaction (3) is carried out, where a fuel such as methane is fed along with a fresh oxygen carrier (MeO). The gaseous products from the first reactor may comprise H2, CO, CO2 and H2O, while the metal solid product from this reactor (Me) is fed in combination with a gaseous oxidant (steam) to a second reactor where these react through reaction (4) to produce H2 and the original oxygen carrier (MeO), which is sent back to the first (fuel) reactor to complete one full cycle of the oxygen carrier. Chemical looping partial oxidation of methane under this reaction scheme was first envisioned by Ryden and Lyngfelt [13] and continued by Mattisson and Lyngfelt [14], [15]. Chemical looping partial oxidation (CLPO) make use of the oxygen contained within the crystalline structure of a metal oxide to partially oxidize the hydrocarbon fuel (methane) without the use of pure oxygen. In contrast, in a conventional partial oxidation only pure oxygen is used, while autothermal reforming uses both steam and oxygen as oxidant gaseous a feed. Reactions (3) and (4) employed in CLPO are comparable to those in conventional reforming and partial oxidation, excepting that the metal oxide serves as the oxygen source for fuel oxidation. According to reaction (4) only a reduced number of metals can be thermodynamically oxidized by steam, such as Fe, Co, Mn, Zn, Mo and W among others [16].

There are a number of authors that report the use of this CLPO concept such as the syngas chemical looping (SCL) process proposed by Tong et al. [17], the iron-based integrated gasification combined cycle (IGCC) process developed by Cormos [18], the one-step decarbonization process envisioned by Mizia et al. [19] and the three-reactor arrangement projected by Kang et al. [20]. Nakamura [21] reported the hydrogen production using steam to oxidize reduced metals employing solar energy under a chemical looping process towards the production of syngas (CLPO), the main metal oxides studied were Ce, Fe, and Ni.

Due to the many limitations (thermodynamics, kinetics and high temperature sintering) of single metal oxides, binary metal oxides have been proposed to reduce sintering effects and enhance the selectivity of oxygen carriers for syngas production. Mixed oxides with spinel structure are among several proposed oxygen carrier (OC) materials to overcome such limitations. Examples of these OC are perovskite-type mixed oxides composed of transition metals that serve not only as lattice oxygen carriers but also as catalysts for methane activation as established by Ryden et al. [22]. Mihai et al. [23] reported that ceramic materials such as perovskite-type complex oxides (with formula ABO3), as oxygen carriers for CLPO applications, exhibit a well-defined crystalline structure along with high thermal stability towards sintering. These investigators claim that the cation A is responsible for the high thermal resistance, while the higher valence cations at the B position are responsible for the catalytic activity. Therefore, both A and B cations within the perovskite structure are of paramount importance towards the reactivity, stability and selectivity for the oxidation of CH4 that make use of the lattice oxygen stored in these materials. Examples of these perovskite materials are LaFeO3, and La0.7Sr0.3FeO3 [23].

Additionally, other researchers have focused their efforts towards the synthesis of oxygen carrier materials for syngas production using the solid diffusional reactive barrier concept aimed to avoid sintering of the material at high temperatures. This concept is based in mixed-metal oxide with a well-defined crystalline structure that when exposed to a reducing gas such as methane, this reacts producing reduced metal species. Furthermore, when these reduced metallic species are reoxidized by steam, they return to the original mixed-metal oxide crystalline structure. The re-formation of the original crystalline structure is thought to be responsible for the high thermal stability of these materials, thus avoiding sintering effects, accountable for the reduction of activity when oxygen carriers are exposed to several high-temperature reduction-oxidation cycles. Examples of these oxygen carrier materials are iron titanate (Fe2TiO5) reported by Luo et al. [24] for the partial oxidation of methane. Thermodynamic simulations and experimental results have established that chemical looping partial oxidation process (CLPO) employing Fe2TiO5 as oxygen carrier is able to produce more than 90% purity syngas close to a 2:1 H2:CO molar ratio. Additionally, De los Rios et al. [2], [11], [25] reported the use of cobalt tungstate (CoWO4) along with a nickel catalyst as an oxygen carrier for CLPO to produce high purity syngas and established the stability of this material under cyclic redox tests subjected to partial oxidation of methane for syngas generation.

Comparing Fe2TiO5 and CoWO4 as oxygen carriers for CLPO applications, it can be concluded that cobalt tungstate is a very remarkable material, since the reduced species produced by exposing Fe2TiO5 with methane were a mixture of Fe and FeTiO3 and Fe2TiO4 [24]. Alternately, the reduction of CoWO4 with CH4 is reported to generate only Co and W metallic species that when reacted with steam CoWO4 is the only solid specie produced [25].

From this noteworthy result, a number of other possible tungstate species are feasible to be considered under these moieties. For example, tungstate species having the structure MWO4 are expected to produce syngas during the reduction step with CH4 and be reoxidized back again with steam according to the following reactions:4CH4(g)+MWO48H2(g)+4CO(g)+M+WCH4(g)+MWO42H2O(g)+CO2(g)+M+WCH4(g)C+2H2(g)2CO(g)C+CO2(g)4H2O(g)+M+WMWO4+4H2(g)C+H2O(g)H2(g)+CO(g)C+2H2O(g)2H2(g)+CO2(g)where M = Fe, Mn or Ni. The reaction (5) produces syngas (CO + H2) together with M and W reduced metal species, while reaction (6) describes the possible complete oxidation of methane to produce CO2 and H2O. Under these oxygen starving conditions, other reactions may arise such as coal formation reactions 7(a) and 7(b), which correspond to methane decomposition and Boudard reactions, respectively. The reoxidation of the metallic Co and W species with steam is described by reaction (8) to produce the MWO4 oxygen carrier and H2, while if some carbon may have formed according to previous reactions 7(a) and 7(b), coal gasification reactions (9a) and (9b) may occur during the reoxidation stage leading to further hydrogen production and carbon oxide species (CO and CO2).

Due to the reported significant reaction behavior of CoWO4 oxygen carrier towards the production of syngas from methane and the reoxidation of its reduced species (Co and W) with steam to further produce H2, makes this reaction concept very attractive to be evaluated using other metallic tungstates such as FeWO4, MnWO4 and NiWO4 under a CLPO reaction scheme. Therefore, the aim of the present study is to evaluate the feasibility of these tungstate metal oxides through the use of thermodynamic analyses performed by process simulations to be used under a chemical looping partial oxidation (CLPO) reaction scheme for the production of syngas and hydrogen.

Section snippets

Process simulation

Concentration at equilibrium calculations in the CLPO for the MWO4 oxygen carriers was performed using the RGIBBS reactor model from Aspen Plus© process simulation software. This reactor model has been successfully employed in the simulation of several chemical looping reaction systems [26], [27], [28], [29]. This reactor model is able to determine the equilibrium amount of all different possible product species as well as their phases (gas, solid and liquid). For the methane partial oxidation

Thermodynamic analysis

Fig. 2 presents results from a thermodynamic analysis consisting in the calculation of the equilibrium amounts within the POX-MEO reactor using the RGIBBS model using a CH4 molar feed of 4 kmol/h.

In Fig. 2 the carbon formation C (kmol/h) is plotted as a function of the FeWO4 feed (kmol/h) and temperature (°C). In this Figure it is evident that the null generation of deposited carbon in the reaction products from the POX-MEO reactors comprised in a region from 700 °C to 1.4–3 kmol/h of FeWO4 and

Conclusions

Different types of MWO4 (M = Fe, Mn, Ni) were evaluated through thermodynamic analysis and process simulation for the production of hydrogen-synthesis gas (syngas) using Aspen Plus©. According to the thermodynamic analysis results, it is concluded that NiWO4 is the best oxygen carrier among the studied tungstate materials for the production of H2 and syngas. Simulation results found at the exit of the reduction reactor a free carbon deposition window located between a feed range of 1.6–3 kmol/h

References (35)

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