Indirect Partial Oxidation of Methane using a Counter-Current Moving

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Kinetics, Catalysis, and Reaction Engineering

Indirect Partial Oxidation of Methane using a Counter-Current Moving Bed Chemical Looping Configuration for Enhanced Syngas Production Deven S. Baser, Sourabh Gangadhar Nadgouda, Anuj Sanjiv Joshi, and Liang-Shih Fan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02520 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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Industrial & Engineering Chemistry Research

110th Anniversary: Indirect Partial Oxidation of Methane using a Counter-Current Moving Bed Chemical Looping Configuration for Enhanced Syngas Production Deven S Baser±, Sourabh G Nadgouda±, Anuj S Joshi, L.-S. Fan* ±Co-first authors William G. Lowrie Department of Chemical and Biomolecular Engineering 151 West Woodruff Avenue, The Ohio State University, Columbus, Ohio 43210, USA *Corresponding Author. Telephone: +1 614-688-3262; Fax: +1 614-292-3769; Email: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Syngas is a valuable chemical intermediate for producing commodity chemicals like olefins, methanol, liquid fuels, etc. The chemical looping route for syngas production presents an attractive alternative to state-of-the-art technology like partial oxidation, auto-thermal reforming, and steam methane reforming. Out of the several chemical looping configurations, the co-current moving bed reactor with iron titanium composite metal oxide particles has demonstrated a high purity syngas production. In this study, an alternative reactor configuration (Indirect chemical looping system) is proposed to the co-current moving bed reactor system (Direct chemical looping system) to enhance the syngas yield. The indirect chemical looping system consists of a fuel reactor and a syngas generation reactor, both operated in countercurrent mode with respect to the gas-solid flow, as opposed to just one co-current fuel reactor in the direct chemical looping system. This unique gas-solid contact pattern in the indirect chemical looping system aids in greater utilization of CO2 and H2O and improves the thermodynamic performance for syngas production. Thermodynamic simulations in Aspen Plus software are performed for system analysis and comparison under isothermal and auto-thermal conditions. Isothermal analysis at several different temperatures and pressures, with and without co-injection of CO2/H2O, is conducted to explain the behavior of the proposed system. Auto-thermal operation of the system under different pressures is also evaluated to determine the maximum syngas yield within the constraints of a practical system for syngas production to further produce liquid fuels via Fischer-Tropsch synthesis. The results from these simulations are compared against the direct chemical looping system to highlight the difference in thermodynamic constraints between the two processes. The oxidation behavior of reduced Fe2O3MgAl2O4 with CO2 and H2O is experimentally tested at different pressures and temperatures to gain an understanding for the syngas generation reactor in the indirect chemical looping system.


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1. Introduction Syngas is an important intermediate chemical that facilitates the conversion of different fossil fuels to petrochemicals and synthetic fuels. Several technologies exist that convert various compositions of syngas into value-added products such as gasoline, methanol, hydrogen, etc. The abundance of shale gas along with the high H/C ratio of methane makes the prospect of syngas production from methane very attractive. As a result, a wide variety of catalytic and non-catalytic processes have been developed and commercialized for syngas production. Steam methane reforming (SMR) is a popular technology to produce hydrogen from methane. However, the H2/CO ratio produced from SMR is not compatible with Fischer-Tropsch (F-T) system to produce liquid fuels.1 Consequently, technologies such as two-step reforming and non-catalytic partial oxidation are used to produce syngas with an H2/CO ratio of 2.2,3 This syngas ratio is essential for liquid fuel production from a Co-based F-T process.4 For a Gas-to-liquids (GTL) plant, a syngas production facility accounts for a major portion of the capital and operational cost.5 The traditional GTL plant requires several auxiliary unit operations to achieve a high syngas purity and syngas yield. They must rely on economies of scale to be feasible since the air separation unit (ASU) is the most energy and cost-intensive unit operation. Thus, the chemical looping reforming (CLR) technology developed by The Ohio State University (OSU) is an attractive alternative that eliminates the ASU and maximizes the thermodynamic potential to produce syngas. CLR technology reduces the methane requirement by up to 20% as compared to the baseline case for syngas production.6,7 Iron oxide-based oxygen carriers are popular for application in chemical looping combustion and CLR process.8 The low cost, widespread availability and low toxicity of iron oxide make it economical and safe to use for commercial processes.9 The oxygen carrying capacity of iron oxide is relatively high compared to other metal oxides. Additionally, the ability of reduced iron oxide phases to react with steam renders it suitable for H2 production.3,10 Therefore, the iron oxide-based metal oxide is used as an oxygen carrier in this study. The CLR process is described in Figure 1(a), where methane is indirectly oxidized to syngas by air via Fe2O3 in 2 cyclically operated reactors. Specifically, methane reacts with Fe2O3 to form syngas and reduced iron oxide phases. These reduced phases are conveyed to the second reactor, where they are re-oxidized to Fe2O3 by air. Details on the role of the two reactors and iron oxide as the oxygen carrier are described in detail in section 2. The



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novel moving bed design with the co-current flow of oxygen carrier and methane aids in the high syngas productivity of the system. The thermodynamics of this system was explored and corroborated with results in a sub-pilot unit.11 Several other configurations of CLR have been developed using fluidized bed and fixed bed reactors. Ni-based oxygen carriers were investigated using several fluidized bed configurations for CLR.12-15 However, due to the back-mixing of oxygen carrier particles present in fluidized bed reactors, the selectivity towards syngas was poor. These Ni-based oxygen carriers also suffer from deactivation due to carbon deposition.16,17 These issues were rectified by using steam or CO2 co-injection with methane, which promotes gas phase shift reactions and increases the syngas purity.3 Several oxygen carriers were tested in a fixed bed reactor, which cycles between reduction and oxidation stages.18-20 The fixed bed mode of operation eliminates back-mixing and enhances syngas purity at the cost of poor heat transfer from/to the bed. Majority of the research focuses on developing efficient oxygen carrier materials and testing them at atmospheric conditions. However, downstream F-T process operates at pressures ranging from 10 to 100 atm, based on the end product produced.21-24 Therefore, it is important to investigate syngas production at elevated pressures. Thermodynamic and process simulation analysis was conducted on CLR systems with iron-titanium composite metal oxides to evaluate the effect of pressure on syngas production.25 Under no H2O/CO2 co-injection, methane conversion and syngas yield drop with an increase in reactor pressure. This effect is attributed to the reactants being thermodynamically favored as compared to the products when the reaction pressure is increased. This reaction is depicted in Equation (1), where the moles of gas increase towards the product side: CH4 + Fe2O3 ↔ CO + 2H2 + 2FeO

(1)

The loss in syngas productivity with pressure increase can be mitigated by co-injecting CO2 and/or H2O, which promote gas phase reactions and improve syngas production. A unique variable pressure CLR unit was developed by OSU to improve the system efficiency for syngas production by reducing the compression requirement on both fuel and air reactor gas streams. An economic analysis on the traditional equal pressure and the novel variable pressure systems was performed. The latter system indicated a reduction of ~29% in capital cost, due to the elimination of air compressors.25,26 To combat the negative effects of pressure on syngas production, an alternative moving bed reactor configuration is envisioned. This study investigates the new configuration that


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produces syngas with a different gas-solid contact pattern and achieves higher syngas yields through improved utilization of CO2 and H2O at high operating pressures.

2. Direct vs Indirect chemical looping system

Figure 1. Process flow diagram for syngas generation using (a) the 2-reactor system or direct chemical looping (DCL) and (b) the 3-reactor system or indirect chemical looping (IDCL). The two moving bed configurations capable of generating syngas with iron-based oxygen carrier are shown in Figure 1. Figure 1(a) represents the direct chemical looping (DCL) system, where Fe2O3 is reduced to Fe/FeO in the fuel reactor, which is then re-oxidized to Fe2O3 in the air reactor.8,9 Several studies have shown that the formation of Fe/FeO at the bottom of the fuel reactor is crucial for the generation of syngas.11 Thermodynamically, this iron oxide phase ensures a high H2/H2O or CO/CO2 ratio in the gas stream exiting the fuel reactor. However, with the increase in reaction pressure from 1 atm to 30 atm, the methane conversion and thus the syngas yield decreases.25,26 This occurs because reaction equilibrium for Equation (1) favors the side with lower stoichiometry at higher pressure, i.e. reactants are thermodynamically favored than the products. The distinctive thermodynamics of iron oxide along with the use of a moving bed system permits an alternate syngas generation configuration, Indirect chemical looping (IDCL) system, shown in



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Figure 1(b). In the IDCL system, methane is combusted to CO2 and H2O in the fuel reactor. These combustion products are sent to the syngas generation reactor (SGR) to produce syngas. Just as the DCL system, the air reactor oxidizes the oxygen carrier back to its original oxidation state. The reactions occurring in the two chemical looping systems can be broadly classified into gas-solid and gas phase reactions. A list of all the main reactions is given by Equations (S1)-(S22) in the Supplementary material. The gas-solid reactions comprise of different phases of iron oxide reacting with gases like CH4, CO2, CO, H2 and H2O in the fuel reactors and SGR. Some of these reactions are selective towards syngas production [Equations (S4)-(S6)] while others either result in complete combustion [Equations (S1)-(S3)] or carbon deposition [Equations (S13)-(S16)]. Based on fixed bed experimental results and thermodynamic analysis, Fe2O3 and Fe/FeO are observed to be least and most selective, respectively, towards syngas production.11,27 In the air reactor, the reduced iron oxide phases react with O2 in air and enough residence time is provided to ensure complete oxidation to Fe2O3 [Equations (S17)-(S19)]. Both the fuel reactor and the SGR, in the IDCL system, are operated as countercurrent moving bed reactors. The countercurrent gas-solid contact ensures that the syngas exits in equilibrium with solids that are in their lowest oxidation state. This lower oxidation state is depicted as Fe/FeO in Figure 1(b). Thermodynamically, high ratios of H2/H2O and CO/CO2 exist in the gas outlet from SGR that is in equilibrium with highly reduced solids.3 Also, the solids at the outlet of the SGR will be in equilibrium with product gas from the fuel reactor having high CO2/CO and H2O/H2 ratios. The favorable equilibrium at both the gas and solid outlets of the SGR results in selective and greater utilization of the lattice oxygen in Fe2O3 to produce syngas. Therefore, there is potential for Fe3O4 formation at the SGR outlet in the IDCL system as compared to Fe/FeO in the DCL system. Fe3O4 formation in the IDCL system indicates increased utilization of CO2 and H2O to enhance the syngas yield. Additionally, the equilibrium of CO2 [Equations (2) and (3)] and H2O [Equations (4) and (5)] oxidation reactions in the SGR are not affected by pressure because of equivalent moles of reactants and products. CO2 + 3FeO ↔ Fe3O4 + CO

(2)

CO2 + Fe ↔ FeO+ CO

(3)

H2O + 3FeO ↔ Fe3O4 + H2

(4)


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H2O + Fe ↔ FeO + H2

(5)

The potential advantages of the IDCL system over the DCL system are investigated in this work under both isothermal and auto-thermal conditions. The thermodynamic investigation is conducted in ASPEN plus simulation software with Fe2O3-MgAl2O4 as the oxygen carrier. An understanding of the IDCL system behavior is gained through isothermal analysis at different temperatures and pressures. The effect of H2O/CO2 co-injection on the system is also studied. Next, an auto-thermal operation is compared for both the systems under the constraints of producing syngas for liquid fuel production. Finally, experimental investigation of the rate of CO2 and H2O oxidation at several temperatures and high pressures is discussed to gain insights into the SGR reactor performance.

3. Material and methods 3.1 Aspen model The DCL and IDCL systems were simulated in ASPEN plus v10 software (ASPEN) under isothermal and adiabatic conditions. The parameters and unit operations used to set up the ASPEN simulation model is given in Tables S1 and S2 in Supplementary material. The fuel reactor and SGR for the IDCL system are counter-current moving bed reactors. A counter-current moving bed reactor is ideally represented by an infinite series of RGIBBS reactors where the gas and solids flow in opposite direction to each other.28 A RGIBBS reactor assumes well-mixed conditions and minimizes the Gibbs free energy of the system. Hence, it is the ideal unit operation in ASPEN to determine the thermodynamic performance of a reactor. It is observed that the performance of a series of RGIBBS reactor stages does not vary significantly after 5 stages. Therefore, only 5 stages are used to represent the counter-current moving bed reactor to simplify the simulation model and reduce simulation time. The fuel reactor in the DCL system is a co-current moving bed reactor and it is represented by a single-stage RGIBBS reactor in ASPEN. These representations of co-current and counter-current moving bed reactors in ASPEN were justified by a close match of thermodynamic simulations with the experimental results for bench and sub-pilot scale systems.11,28-30 The air reactor for both DCL and IDCL system is a fluidized bed reactor represented by a single-stage RGIBBS reactor. This selection of RGIBBS reactor for the air reactor is under the assumption of ideal mixing and attainment of reaction equilibrium because of fast air oxidation kinetics at high temperatures. It is to be noted that reaction kinetics are not considered in simulating



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the performance of any reactor since the current study is to compare only the thermodynamic limits for syngas production. Detailed reaction kinetics and gas-solid flow dynamics modelling are beyond the scope of this paper. For a comprehensive comparison between DCL and IDCL systems, isothermal and auto-thermal conditions were simulated in ASPEN. The isothermal simulations provide insights into the two systems when they are subjected to different operating conditions. These results highlight the key features of the systems and create a basis for further investigation. The range of temperatures and pressures analyzed for isothermal analysis are 700 to 1100 °C and 1 to 30 atm, respectively. For all the simulations, the CH4 flow rate is maintained at 1 kmol/hr as the basis for comparison between the simulations. Different conditions are simulated by varying the oxygen carrier flow rate or the co-injection stream flow rate. From a system point of view, an auto-thermal operation is desired since it would not require an external source of heat input. An auto-thermal operation for the chemical looping system is ensured by setting heat duty to be zero for the fuel reactors and SGR, while heat duty for the combustor reactor is ≤ 0. The auto-thermal operation analysis is done for 1 to 30 atm pressure range and 1100 °C as the solids inlet temperature to the fuel reactor. The air reactor was also maintained at 1100 °C for this analysis. Similar to the isothermal simulations, CH4 input of 1 kmol/hr at 600 °C is considered as the input in all auto-thermal thermodynamic analyses. The co-injection stream (CO2/H2O) and atmospheric air are assumed to be available at 600 °C and 25 °C, respectively. The thermodynamic phase diagrams of Fe-O-C and Fe-O-H systems are similar for temperatures greater than 700°C.9 Therefore, the syngas yield predicted by ASPEN is not affected by the co-injection stream composition. The primary effect of co-injection stream composition is on the H2/CO ratio in syngas, which is not considered as part of the isothermal analysis. Hence, an arbitrary co-injection stream composition of 70% H2O and 30% CO2 is considered for the isothermal analysis. An 80% H2O and 20% CO2 co-injection stream composition is used for auto-thermal analysis to meet the H2/CO constraint for liquid fuels production. Fe2O3 (15 wt%)- MgAl2O4 (85 wt%) is used as the oxygen carrier for both chemical looping systems, where MgAl2O4 is considered as inert support that acts as a heat transfer agent. Several parameters are evaluated to understand the thermodynamic performance of the system and they have been defined below. 𝐶𝐻4 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = (1 −

𝑛𝐶𝐻4 ,𝑟𝑒𝑎𝑐𝑡𝑜𝑟 𝑜𝑢𝑡𝑙𝑒𝑡 𝑛𝐶𝐻4 ,𝑓𝑒𝑒𝑑

) × 100,


(6)

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where “reactor” is the fuel reactor for the DCL system and SGR for the IDCL system. 𝑆𝑜𝑙𝑖𝑑 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = (1 −

3𝑛𝐹𝑒2 𝑂3 +4𝑛𝐹𝑒3 𝑂4 +𝑛𝐹𝑒𝑂 ⁄2𝑛 𝐹𝑒2 𝑂3 +3𝑛𝐹𝑒3 𝑂4 +𝑛𝐹𝑒𝑂 +𝑛𝐹𝑒 ) 3𝑛𝐹𝑒2 𝑂3 ⁄2𝑛 𝐹𝑒2 𝑂3

× 100,

(7)

100% and 0% solids conversion correspond to Fe and Fe2O3, respectively. 𝐹𝑒2 𝑂3 𝐶𝐻4

𝑛𝐹𝑒2 𝑂3

=𝑛

(8)

𝐶𝐻4 ,𝑓𝑒𝑒𝑑

𝑆𝑦𝑛𝑔𝑎𝑠 𝑦𝑖𝑒𝑙𝑑 =

𝑛𝐶𝑂,𝑠𝑦𝑛𝑔𝑎𝑠 𝑠𝑡𝑟𝑒𝑎𝑚 +𝑛𝐻2 ,𝑠𝑦𝑛𝑔𝑎𝑠 𝑠𝑡𝑟𝑒𝑎𝑚 𝑛𝐶𝐻4 ,𝑓𝑒𝑒𝑑

𝐶𝑜 − 𝑖𝑛𝑗𝑒𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 (𝐶𝐹) =

𝑛𝑐𝑜−𝑖𝑛𝑗𝑒𝑐𝑡𝑖𝑜𝑛 𝑠𝑡𝑟𝑒𝑎𝑚 𝑛𝐶𝐻4 ,𝑓𝑒𝑒𝑑

(9) (10)

In the above equations, 𝑛𝑋 represents the molar flow rate of X in the process stream. 3.2 Thermogravimetric analysis Thermogravimetric analysis (TGA) is used to gain a deeper understanding of the rate of oxidation in SGR at different reaction temperatures and pressures. A high-pressure TGA (DynTHERM, TA instruments) is used for this purpose, where the change in the weight of the oxygen carrier is correlated to the oxygen uptake from H2O or CO2. Oxygen carrier particles of 1.5 mm diameter with 50 wt% Fe2O3 and balance MgAl2O4 support are synthesized by sintering at 950°C under air. These oxygen carrier particles were activated by running several redox cycles before conducting oxidation kinetic studies. Both H2O and CO2 rate of oxidation are explored at reaction temperatures and pressures between 700-1000°C and 1 to 10 atm, respectively. The extent of oxidation [Equation (11)] with these reactions is investigated on the reduced oxygen carrier. The reduced oxygen carrier was synthesized in-situ reduction by reacting it with H2 to form metallic Fe. 𝐸𝑥𝑡𝑒𝑛𝑡 𝑜𝑓 𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 (%) = 100 − 𝑆𝑜𝑙𝑖𝑑 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛(%)

(11)

4. Results and Discussion 4.1 Isothermal Analysis The DCL and IDCL systems are investigated under isothermal conditions. Figure 2(a) depicts the syngas yield, calculated using Equation (9), as a function of moles of Fe2O3 per mole of methane added into the fuel reactor. As the Fe2O3/CH4 ratio (Equation (8)) increases, the syngas yield shows



a decreasing trend in both the systems, which has also been reported previously.3,8 This characteristic trend also depicts an initial region of carbon deposition. The carbon deposition decreases as the oxygen content or the Fe2O3/CH4 ratio increases. This increases the syngas yield up-to a maximum value after which it decreases with increase in Fe2O3/CH4 due to over-oxidation of the syngas product. It is important to note that between the two systems, the region of maximum syngas yield behaves differently at both 1 and 30 atm. For the DCL system, the maximum syngas yield is observed at a single Fe2O3/CH4 ratio. The maximum syngas yield is 2.95 and 2.58 at Fe2O3/CH4 of 0.324 and 0.396 for a pressure of 1 and 30 atm, respectively. These maxima points correspond to equilibrium between Fe and the gas phase products at the fuel reactor outlet. The IDCL system, on the other hand, has a maximum syngas yield region and not just a single point for both 1 and 30 atm. For 1 atm, this region has a similar maximum syngas yield as the DCL system over a Fe2O3/CH4 range of 0.36 to 0.468. Similarly, at 30 atm, the maximum syngas yield of 2.54 is observed over a Fe2O3/CH4 range of 0.468 to 0.612. The distinctive plateau region of the IDCL system is because the solids in contact with syngas remain as metallic Fe with some carbon deposition. Unlike the DCL system, carbon deposition on the reduced oxygen carrier from the fuel reactor does not reduce the syngas yield as it is oxidized to CO in the SGR reactor. The carbon deposition reduces to 0 at the end of the plateau region and the syngas yield trend beyond that region is similar to that of the DCL system. Apart from the maxima region, the syngas yield trend for both DCL and IDCL overlap. This overlap is because the same mixture of iron oxide phases is in contact with the syngas stream for both systems. For example, in the plateau region following the maximum syngas yield, the solids at the fuel reactor outlet are a mixture of Fe and FeO. Even though the amount of Fe is higher for the IDCL system than the DCL system, the syngas yield is not affected, as the relative amount of Fe and FeO does not affect the reaction equilibrium of Equations (3) and (5). The values for the amount of Fe and FeO are given for DCL and IDCL system in Tables S7 and S13, respectively, in Supplementary material.


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Figure 2. Syngas yield vs Fe2O3/CH4 for (a) the DCL and IDCL system at 1 and 30 atm and 1100°C and (b) different SGR temperatures in the IDCL system at 1 atm with the fuel reactor temperature held constant at 1100°C. (FR refers to fuel reactor) For the IDCL system, the fuel reactor and the SGR can be operated at different temperatures. Figure 2(b) demonstrates the syngas yield at two temperatures of the SGR, 700°C and 1100°C, while the fuel reactor temperature is 1100°C. The maximum syngas yield decreases with a decrease in SGR temperature as a result of low CH4 conversion (Equation (6)) at lower temperatures. Further, as the Fe2O3/CH4 ratio increases, the syngas yield drops as seen previously. This is accompanied by an increase in the CH4 conversion but a reduction in syngas selectivity, which is at par with the high temperature case. As the thermodynamic conversion of H2O/CO2 to H2/CO is favored at lower temperatures, the syngas yield after Fe2O3 /CH4 ratio of 1.152 is marginally higher for the 700°C than the 1100°C SGR by 6.1%. 4.2 Isothermal analysis with a co-injection stream In Section 4.1, it was observed that the maximum syngas yield did not coincide with an H2/CO ratio of 2 in the syngas product. A means to enhance the syngas yield along with an H2/CO ratio of 2 is to inject additional CO2 and H2O in the system.25 The added CO2/H2O participates in both gas-only and gas-solid phase reactions to form CO/H2. 2 different co-injection rates with the coinjection factor (CF), shown in Equation (10), of CF=1 (CF1) and CF=2 (CF2) are analyzed. The CF is defined with respect to the CH4 input with the co-injection stream consisting of CO2 and H2O. The composition of the co-injected stream was selected to be 70 vol% H2O and 30 vol% CO2 as explained in Section 3.1. The additional H2O/CO2 stream is injected with natural gas in the fuel



reactor and the SGR for DCL and IDCL system, respectively, as shown in Figure 1. The isothermal temperature for both the systems is 1100 °C at a pressure of 1 atm. A detailed comparison between the two systems at the above condition is given first, followed by comparison over a wide range of temperatures (700 °C to 1100 °C) and pressures (1 atm to 30 atm). The maximum syngas yield of both systems for CF1 and CF2 is 32% and 34.5% higher, respectively, compared to the case without co-injection in Section 4.1. The added CO2/H2O improves the syngas yield by minimizing carbon deposition at low Fe2O3/CH4. This co-injection stream reacts with reduced solids and favors steam methane and dry reforming reactions in the gas phase. The effect of co-injection for DCL system is evident in comparing Figure 2(a) and Figure 3(c) and (d). For CF1 case, there is no carbon deposition for low Fe2O3/CH4 and hence the syngas yield is higher. Moreover, the initial increase in the syngas yield observed for the no co-injection case is not observed. The plateau region in syngas yield, representing equilibrium composition of the gas phase with a FeO/Fe3O4 mixture, is similar for both the cases. However, the magnitude of yield is higher for the CF1 case, due to the additional CO and H2 generated from the co-injected gas stream. The solids conversion (Equation (7)) at the fuel reactor solid outlet is higher for the CF1 case, especially at lower Fe2O3/CH4, due to additional oxygen provided by the co-injected gas stream. This also results in the solids conversion reaching the equilibrium value of 33.33% at a lower Fe2O3/CH4. The syngas yield and solids conversion for CF2 follow a similar trend to CF1 (Figure 3). However, the magnitude of syngas yield in the plateau region is higher for CF2 and the solids conversion reach 33.33% conversion at a lower Fe2O3/CH4 compared to CF1. As observed in the DCL system, the carbon deposition in the IDCL system is reduced upon coinjection at low solid flow rates. Since the co-injected gas stream is input to the SGR, there is still carbon deposition on solids at the solid outlet of fuel reactor for low Fe2O3/CH4, which gets oxidized in the SGR reactor. For most of the Fe2O3/CH4 values, the IDCL system has a higher syngas yield as compared to the DCL system in both CF1 and CF2 cases. The main reason for the higher yield in the IDCL system is the higher oxidation state of the solids at SGR solid outlet compared to that at the fuel reactor solid outlet for the DCL system. This higher oxidation state of the solids is an indication of the oxygen carrier being oxidized by CO2 and H2O. Further, a greater amount of oxygen transferred from the CO2 and H2O stream results in enhanced CO and H2


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production, respectively. As explained in Section 2, this is possible in the IDCL system because of the gas-solid contact pattern where the solids outlet of SGR reactor is in equilibrium with the co-injected gas stream that is of oxidizing nature.

Figure 3. Solids conversion as a function of Fe2O3/CH4 for DCL and IDCL system with (a) CF1 (b) CF2. Syngas yield as a function of Fe2O3/CH4 for DCL and IDCL system with (c) CF1 (d) CF2. The effect of pressure and temperature on both the systems, under isothermal conditions, with the co-injection of a CO2/H2O gas stream is observed in Figure 4. The syngas yield was evaluated at two different solids flow rates, Fe2O3/CH4 equal to 0.108 and 0.504. These flow rates were selected based on Figure 3 (c) and (d), where at 0.108 the syngas yields for both the systems are similar and they differ significantly at 0.504. It is observed that at Fe2O3/CH4=0.108 for CF1, even at other pressures and temperatures, the syngas yields for both DCL and IDCL systems are almost the same. However, for CF2 and Fe2O3/CH4=0.108, the syngas yield for IDCL system is slightly higher than DCL especially at high temperatures and low pressures. The difference between the yields for the two systems diminishes as the temperature is lowered at a given pressure or the



pressure is increased at a given temperature, as can be seen from Figure 4 (c). The major reason behind the trends observed above is that the thermodynamics for CH4 conversion to syngas is least favorable at lower temperature and high pressure and it affects the two systems equally. At low CH4 conversion, the solids conversion is also low and the benefit of the IDCL system in improving the solids conversion is not significant. Once methane conversion is not limiting due to high pressure, low temperature, low solids flow rate or low co-injection rate, the advantage of the IDCL system becomes apparent. A similar trend is also observed for Fe2O3/CH4=0.504 (Figures 4 (b) and 4 (d)), although the percent increase in yield for IDCL system over DCL system is higher compared to Fe2O3/CH4=0.108 which is consistent with behavior seen in Figure 3 (c) and (d).

Figure 4. Syngas yield in DCL and IDCL system as a function of temperature and pressure at CF1 with (a) Fe2O3/CH4=0.108 and (b) Fe2O3/CH4=0.504 and at CF2 with (c) Fe2O3/CH4=0.108 and (d) Fe2O3/CH4=0.504.


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4.3 Auto-thermal analysis The knowledge about factors governing syngas yield in both systems and the effect of pressure and temperature on system performance was gained from the isothermal analysis. This understanding was applied to evaluate and compare the auto-thermal operation of the two systems with co-injection of CO2/H2O stream. Figure 5 depicts the syngas yield as a function of pressure for both the DCL and IDCL system at 2 different co-injection rates under auto-thermal conditions. These results are obtained under the constraints that H2/CO ratio is 2, methane conversion is greater than 95% and the H2O concentration is 80% in the co-injection stream. For CF1, it is seen from Figure 5 (a), the syngas yield reduces with increasing pressure. The predominant reason for this trend is the reduction in CH4 conversion with an increase in pressure. The syngas yield for the IDCL system is within 0.5% of the DCL system because the solids conversion at the SGR outlet (IDCL system) is similar to the fuel reactor outlet (DCL system) as seen in Figure 6. This similar solid conversion along with comparable solids flow rate in the two systems result in the methane conversion and syngas yield to be equivalent. For CF2, the syngas yield follows a similar trend with respect to pressure as observed for CF1. However, the syngas yield for the IDCL system is about 12% higher than the DCL system at all pressures. As shown in Figure 6, the solids conversion at SGR outlet of IDCL system is about 11% lower compared to the fuel reactor outlet of the DCL system. In other words, a higher percentage of the co-injected CO2 and H2O is utilized to oxidize the solids in the IDCL system resulting in an increase in syngas produced. Also, since the reduction of CO2 and H2O is efficient in a countercurrent system, the syngas yield for CF2 is 14-17% higher than CF1 whereas this increase is only 2-5% for the DCL system. It is interesting to note that the low degree of solids conversion exiting the SGR results in a higher solids circulation rate which is required to maintain autothermal operation. This is because the exothermic heat released in the air reactor reduces as the solids entering the air reactor are at a higher oxidation state. To compensate for the low exothermic heat release, the solids circulation rate is increased. Therefore, even though the syngas yields for the IDCL system is higher, it is achieved at a high solids circulation rate. Also, the reduction in syngas yield due to an increase in pressure is less for CF2 compared to CF1. A higher co-injection rate ensures close to 100% CH4 conversion irrespective of pressure since the co-injected CO2 and H2O dilute the effect of volume expansion reaction.



Figure 5. Syngas yield in the DCL and IDCL systems with respect to pressure at (a) CF1 and (b) CF2 under auto-thermal operation.

Figure 6. The difference in the conversion of solids at the fuel reactor outlet (DCL system) and SGR outlet (IDCL system) for CF1 and CF2. 4.4 Thermogravimetric analysis The kinetics of fuel reactors in the DCL and IDCL systems have been investigated in several studies.11,31 However, the SGR component of the IDCL system is not explored with respect to a reaction engineering aspect. Thus, the rate of oxidation of the reduced particles is investigated at different temperatures and pressures with H2O and CO2 as the oxidizing gases to simulate SGR. The gas flow rates are adjusted based on the operating pressure to account for the residence time and the gas dispersion. These results serve to highlight the difference in oxidation rates of CO2 and H2O and kinetic parameter estimation of these oxidation reactions is beyond the scope of this


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study. The extent of oxidation trend [Equation (11)] for H2O and CO2, as seen in Figure 7, is similar to the air oxidation trend previously reported for iron oxide particles.32 Thermodynamically, oxidation with H2O or CO2 of a reduced Fe oxygen carrier is limited to 11% solid conversion or Fe3O4 phase. This limit was observed for both the oxidizing gases, thus corroborating the thermodynamic result. With both the oxidizing gases, the initial rate of oxidation is fast, which gradually slows down before complete oxidation to Fe3O4. This characteristic oxidation trend is explained by the fast kinetics of initial surface oxidation followed by slow lattice oxygen diffusion as the rate-controlling step.33,34 Figure 7 also shows an improvement towards the rate of oxidation for CO2 and H2O with an increase in pressure. Additionally, at 1000 °C H2O rate of oxidation is 17% faster than the CO2 rate of oxidation at 1 atm. This difference increases in the favor of H2O at higher pressures. At 10 atm, the H2O rate of oxidation is 39% faster than CO2, indicating a high affinity for oxygen abstraction from H2O as compared to CO2.

Figure 7 Oxidation rate of the reduced particle at a temperature of 1000 °C and pressures of 1, 5, 10 atm with (a) 10% CO2 and (b) 10% H2O Figure 8 depicts the extent of oxidation for up to 2.5 minutes of oxidation at various pressures and temperatures. The 2.5-minute mark is chosen to compare the rate of oxidation in the initial surface oxidation region for both H2O and CO2. Across the entire operational spectrum tested for the oxygen carrier, oxidation with H2O is faster than CO2. It can be further deduced that the increase in the rate of H2O oxidation is larger compared to that of CO2, with an increase in pressure. Specifically, at 700°C and 2.5 minutes of oxidation, the extent of oxidation with CO2 increases from 13.9% to 23.4% with pressure increase from 1 atm to 10 atm. This signifies an increase of



68.1% in the rate of oxidation with a 10-fold increase in pressure. Similarly, for H2O, the extent of oxidation increases from 19.7% to 39.5% signifying an increase of 100.86%. At 1000°C, H2O shows an 89.1% increase and CO2 shows a 79.7% increase in the rate of oxidation with the change in pressure from 1 atm to 10 atm. Although the H2O rate of oxidation is faster compared to that of CO2 at all pressures, the difference between them decreases with increase in pressure. These oxidation trends would aid in the design of the SGR and help maximize its utility by operating near the thermodynamic limit.

Figure 8 Solid conversion after 2.5-minute oxidation of the reduced particle at temperatures of 700, 800, 900 and 1000 °C and pressures of 1, 5, 10 atm in (a) 10% CO2 and (b) 10% H2O

5. Conclusions Syngas production at high pressure is critical as several downstream applications of syngas are operated at pressures ranging from 10 to 100 atm. However, syngas yields from methane are lower at higher pressures due to it being a volume expansion reaction. To overcome the low syngas production at higher pressure, an alternate reactor configuration (IDCL system) from the conventional co-current moving bed reactor configuration (DCL system) was investigated. Isothermal and auto-thermal analysis at temperatures ranging from 700 to 1100 °C and pressures from 1 atm to 30 atm were conducted to understand the behavior of the IDCL system and compare against the DCL system. The auto-thermal simulations were conducted under the constraints of CH4 conversion>95%, H2/CO ratio equal to 2 and steam composition in co-injection stream equal to 80%. At isothermal conditions, the maximum syngas yield was similar for both the systems, but this yield was obtained at a wider range of Fe2O3/CH4 for the IDCL system. With co-injection, the maximum syngas yield increased by 32-35% for both the systems. Reduction of carbon deposition and reduction of the added CO2/H2O by the solids were the main reasons for the increase in syngas


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yield. The effect of pressure and temperature was also tested for both systems, with co-injection of CO2/H2O, at two different solid flow rates. It was observed that the syngas yield for the IDCL system was significantly higher than the DCL system at lower pressures and high temperatures. The solids conversion at the outlet of SGR in IDCL system was observed to be higher than the outlet of fuel reactor in the DCL system. This resulted in greater utilization of the co-injected CO2/H2O and enhancement in syngas yield. The same reason explained the higher syngas yield during auto-thermal operation of IDCL system compared to the DCL system. The difference in syngas yield was 0.5% and 12% for operation with co-injection factor = 1 and co-injection factor = 2, respectively. Further, TGA analysis of CO2 and H2O oxidation was conducted at 4 different temperatures and pressures of 1, 5 and 10 atm. The H2O rate of oxidation was 17% and 39% higher than CO2 at 1 atm and 10 atm, respectively, at 1000 °C. The TGA results also validated the thermodynamic limit of oxidation state that can be achieved in the SGR. In summary, the IDCL system proved to enhance the syngas yield due to the gas-solid contact pattern which led to favorable thermodynamics for syngas production. Declarations of Interest None

Funding Source This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors. Supporting Information Reaction Network, ASPEN simulation parameters and assumptions, Process flow diagram with stream numbers, detailed stream tables for different cases. This information is free of charge via the Internet at http://pubs.acs.org

Corresponding Author *E-mail: [email protected].

Author Contributions



± Deven Baser and Sourabh Nadgouda contributed equally to this work and are co-first authors.

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For Table of Contents Only


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Indirect Partial Oxidation of Methane using a Counter-Current Moving

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