Ni-based Oxygen Carrier Research Articles

Density functional theory study on Support-Promoted CH4 reforming in Ni-Based oxygen carriers for chemical-looping conversion

Metal oxides play a pivotal role in the conversion of carbonaceous fuels. Nickel-based oxides, due to their affordability and high C–H activation activity, are regarded as promising oxygen carriers (OCs) for CH4 chemical looping technologies. Additives like Al2O3, ZrO2, and MgAl2O4 are used to boost the reactivity of OCs and mitigate agglomeration of OC particles at elevated temperatures. Despite extensive experimental studies on support selection, the influence of support materials on CH4 conversion remains unclearly. This research analyzed and compared the kinetic pathways of CH4 conversion on the supported and unsupported nickel-based OCs, utilizing the Density Functional Theory (DFT) model, to determine the rate limiting step (RLS) in the chemical looping process. The most promising support, NiAl2O4, weakens the Ni-C bond due to pronounced interaction between the OCs and supports, thereby leading to a reduced activation energy barrier for CH4 dehydrogenation. By probing the relationships between reaction kinetic barriers and critical electronic properties of OCs, a screening strategy is proposed to accelerate the development and optimization of novel OC/support materials for CH4 chemical looping conversion.

Theoretical Study of the Reactive Mechanisms of Li-Doped Ni-Based Oxygen Carrier during Chemical Looping Combustion.

Ni-based oxygen carriers (OCs) are considered promising materials in the chemical looping combustion (CLC) process. However, the reactivity of Ni-based OCs still offers the potential for further enhancement. In this work, the Li doping method has been employed for the modification of Ni-based OCs. The reactivity and microreaction mechanisms of different concentrations of Li-doped Ni-based OCs with CO in CLC are clarified using density functional theory (DFT) simulation. The structures, energy, and density of states are obtained through computational investigation of the reaction path in elementary reactions. The results show that (1) the adsorption energies of CO molecules on NiO surfaces with 4, 8, and 12% Li doping concentrations are -0.53, -0.48, and -0.54 eV, respectively, demonstrating an enhanced reactivity compared to that of pure NiO (-0.41 eV); (2) the calculation of the transition state indicates that the most favorable pathway for CO oxidation takes place on the surface of NiO with an 8% Li doping concentration, exhibiting the lowest energy barrier of 0.51 eV; and (3) the oxygen vacancy formation energies on the surface of NiO are 3.05, 2.30, and 2.10 eV for 4, 8, and 12% doping concentrations, respectively. Additionally, the decrease in oxygen vacancy formation energies exhibits a gradual decline with an increasing Li doping concentration. By comprehensive analysis, 8% is considered to be the optimal doping concentration of NiO for chemical looping combustion.

Catalyst-enhanced autothermal chemical looping reforming: Intrinsic SMR kinetics and numerical simulation

In this work, we propose a catalyst-enhanced autothermal chemical looping reforming (CE-aCLR) to address the environmental concerns associated with toxic Ni-based oxygen carrier (OC) and the low fuel conversion related to the low reactivity of Fe-based OC. In this process, a non-toxic noble-metal-based steam methane reforming (SMR) catalyst is employed for methane conversion, while a Fe-based OC is employed for oxygen transfer between two reactors. Firstly, a commercial Rh-based SMR catalyst is studied in a micro-fixed bed reactor and its intrinsic reaction kinetics are determined. Then, a 1-D CE-aCLR model is developed based on a verified multi-scale aCLR model to facilitate analyzing commercial-scale feasibility. This model accounts for the essentials of the reactor hydrodynamics, mass and heat transfer, detailed reaction kinetics, and the influence of the solids residence time distribution in the fuel reactor. The main dimensions of a commercial CE-aCLR unit are determined and this unit is simulated using the developed model. The results demonstrate the feasibility of the novel, environmentally friendly process that enables high methane conversion using Fe-based OC within realistically sized reactors and an acceptable solid circulation rate. Moreover, sensitivity study shows that the CE-aCLR allows using OCs with low activity (potentially low cost) while maintaining high CH4 conversion.

Numerical analysis of a solar assisted chemical looping combustion combined power and sea water desalination plant

Chemical looping combustion (CLC) with solar integration for power generation is viewed as a technology that would have many economic and environmental benefits. This is because the use of solar systems to provide heating for the CLC process could lead to improved power plant performance and significant reduction of emissions. The performance of energy resources for CLC systems can be further improved via poly-generation. In this study, a novel solar assisted, methane fired CLC power plant with CO2 capture combined with waste heat utilization for sea water desalination is proposed. The plant receives a total thermal energy input of 20 MW. Ni based oxygen carrier is used for the oxygen transfer cycle. The proposed model is developed using Aspen plus software with the consideration of the conservation of mass, momentum and energy. In the study, the effect of using solar energy for the provision of the required heating for the endothermic reaction process in CLC is thoroughly examined by varying the solar share. The results show that an efficiency of 60.56% for the system can be obtained when the solar energy contributes 15–20% of total thermal energy. The system performance is relatively poor for solar share values below 15%. The system performance is further improved (approximately three percentage points) by integrating it with a desalination unit. A parametric study was conducted to ensure that the operating parameters are at their optimum levels. The study also examines the effect of certain key operational parameters for the desalination system such as sea water temperature, sea water salinity and the top brine temperature. The water production is increased with the decrease of top brine temperature and the increase of sea water temperature and salinity.

Reaction performance and mechanism of a NiO/Ca2Fe2O5 oxygen carrier in Chemical looping gasification of cellulose

Chemical looping gasification (CLG) is a novel technology created to realize the clean and efficient utilization of solid fuels, where the syngas product avoids dilution by N2 and the pollutants are inhibited by using metal oxide as oxygen carrier (OC) instead of molecular oxygen. Cellulose, as the main component of biomass-based solid waste, was evaluated in the CLG process with NiO/Ca2Fe2O5 prepared by the impregnation method. Reaction mechanism and morphology evolution of the OC as well as the influence of key reaction parameters were investigated by fixed-bed reactor and thermogravimetric analysis experiments coupled with various characterization techniques. The results indicated that the addition of Ni not only improved the oxygen release performance of Ca2Fe2O5 but also helped tar cracking and carbon conversion. On the other hand, the excellent activity in the solid–solid reaction of Ca2Fe2O5 promoted the performance of the Ni-based OC. The test results revealed that Ca2Fe2O5 was a better base than CaFe2O4, and the carbon conversion efficiency reached 98% with the OC sample at 850 °C. NiO markedly improved the reactivity of Ca2Fe2O5 after 10 cyclic reactions. The temperature notably affected activity below temperatures of 850 °C and slightly above. Due to the limited reduction of Ca2Fe2O5 below 800 °C and the redox behavior improvement being minor above 850 °C, the temperature of 850 °C was marked as the optimum for the studied process Addition of water inhibited OC inactivation in cyclic reactions, which was mainly attributed to the crystal separation and agglomeration of OC particles. However, water inhibited the deep reduction of metal on the surface, which reduced the activity on tar cracking and carbon conversion to a certain extent.

Influence of an Oxygen Carrier on the CH4 Reforming Reaction Linked to the Biomass Chemical Looping Gasification Process.

A major challenge in biomass chemical looping gasification (BCLG) is the conversion of CH4 and light hydrocarbons to syngas (CO + H2) when the goal is the use for bioliquid fuel production. In this work, tests were performed in a batch fluidized bed reactor to determine the catalytic effect on the CH4 reforming reaction of oxygen carriers used in the BCLG process. Three ores (ilmenite, MnGB, and Tierga), one waste (LD slag), and five synthetic materials (Fe10Al, Fe20Al, Fe25Al, Cu14Al, and Ni18Al) were analyzed. These results were compared to those obtained during ∼300 h of continuous biomass gasification operation in a 1.5 kWth BCLG unit. The low-cost materials (ores and waste) did not show any catalytic effect in the CH4 reforming reaction, and as a consequence, the CH4 concentration values measured in the syngas produced in the continuous prototype were high. The synthetic oxygen carriers showed a catalytic effect in the CH4 reforming reaction, increasing this effect with increasing temperature. With the exception of the Ni-based oxygen carrier (used as a reference), the Cu-based oxygen carrier, working at 940 °C, showed the best catalytic properties, in good agreement with the low CH4 concentration values measured in the syngas generated in the continuous unit. The tests performed in a batch fluidized bed reactor were demonstrated to be very useful in determining the catalytic capacity of oxygen carriers in the CH4 reforming reaction. This fact is highly relevant when a syngas with a low CH4 content is desired as a final product.

Synthesis gas and H2 production by chemical looping reforming using bio-oil from fast pyrolysis of wood as raw material

• CLR of pine wood bio-oil has been demonstrated in a 1 kW th continuous unit. • Complete conversion of bio-oil was found during 60 h of operation. • Carbon deposition can be minimized and controlled in the continuous CLR unit. • Maximum H 2 production and H 2 /CO ratio were found at 650 °C with very low C deposition. • Auto-thermal H 2 production at 650 °C was 13.6 g/100 g of bio-oil with H 2 O/bio-oil = 6. The interest of using bio-oil produced by biomass fast pyrolysis to generate H 2 and high value chemicals is growing up in order to reduce CO 2 emissions by the use of biomass. In the present work, bio-oil Chemical Looping Reforming was demonstrated in a 1 kW th continuous unit during 60 h of operation using a Ni-based oxygen carrier to produce syngas/H 2 . No agglomeration of the oxygen carrier was detected. Complete conversion of the bio-oil was obtained in all cases, obtaining as main products H 2 , CO, CO 2 and in much lower concentration CH 4 . Carbon deposition in the bed appeared in almost all the experiments carried out during the bio-oil reforming, and CO 2 was found in the air reactor outlet, although did not affect to the process performance when oxygen carriers were used. This fact only let to a reduction of the CO 2 capture efficiency. Fuel reactor temperature was a key factor for the H 2 production, the H 2 /CO ratio and the carbon accumulated in the CLR unit. The maximum H 2 production together with the maximum H 2 /CO ratio of 6 were found at the lowest temperature used, 650 °C. In addition, the carbon formed in the fuel reactor was reactive enough so that it could be burnt in the air reactor with minimum accumulation in the continuous unit. The autothermal H 2 production was 13.6 g/100 g of bio-oil without water at 650 °C with a H 2 O/bio-oil = 6, which corresponds to 68 mol H 2 /kg of bio-oil without water. Therefore, the CLR process of bio-oil allows obtaining high H 2 production with low carbon deposition at 650 °C using less amount of water than in conventional bio-oil reforming.

Multifunctional Ni-based oxygen carrier for H2 production by sorption enhanced chemical looping reforming of ethanol

The sorption enhanced chemical looping reforming is a prospective technology for high-quality hydrogen production. In this technology, the full benefits of catalysis, oxygen transfer, and CO2 adsorption are achieved by the utilization of oxygen carriers and sorbents. However, most sorbents that are mixed physically into the reforming bed are liable to encountering ash fouling and porosity reduction, preventing CO2 adsorption and sorbent regeneration. In this work, we develop a multifunctional composite oxygen carrier, which combines the NiO/NiAl2O4 catalyst with the CaO sorbent into a one-body composite pellet. Compared with the conventional physically mixed oxygen carrier, the composite oxygen carrier shows uniform element distribution, superior reducibility, and shortened dead time, resulting in enhanced CO2 adsorption capacity, high H2 production efficiency, and excellent ethanol conversion. Furthermore, during 20 cycle tests, the stable CO2 adsorption capacity and H2 concentration of composite oxygen carrier were maintained, while the H2 concentration of mixed oxygen carrier sharply decreased because of deteriorated adsorption performance. The results confirm that the multifunctional composite oxygen carrier is more suitable than the mixed oxygen carrier for CO2 adsorption and high purity hydrogen production. This work opens a new avenue toward the performance improvement of oxygen carriers.

The Potential of Gas Switching Partial Oxidation Using Advanced Oxygen Carriers for Efficient H2 Production with Inherent CO2 Capture

The hydrogen economy has received resurging interest in recent years, as more countries commit to net-zero CO2 emissions around the mid-century. “Blue” hydrogen from natural gas with CO2 capture and storage (CCS) is one promising sustainable hydrogen supply option. Although conventional CO2 capture imposes a large energy penalty, advanced process concepts using the chemical looping principle can produce blue hydrogen at efficiencies even exceeding the conventional steam methane reforming (SMR) process without CCS. One such configuration is gas switching reforming (GSR), which uses a Ni-based oxygen carrier material to catalyze the SMR reaction and efficiently supply the required process heat by combusting an off-gas fuel with integrated CO2 capture. The present study investigates the potential of advanced La-Fe-based oxygen carrier materials to further increase this advantage using a gas switching partial oxidation (GSPOX) process. These materials can overcome the equilibrium limitations facing conventional catalytic SMR and achieve direct hydrogen production using a water-splitting reaction. Results showed that the GSPOX process can achieve mild efficiency improvements relative to GSR in the range of 0.6–4.1%-points, with the upper bound only achievable by large power and H2 co-production plants employing a highly efficient power cycle. These performance gains and the avoidance of toxicity challenges posed by Ni-based oxygen carriers create a solid case for the further development of these advanced materials. If successful, results from this work indicate that GSPOX blue hydrogen plants can outperform an SMR benchmark with conventional CO2 capture by more than 10%-points, both in terms of efficiency and CO2 avoidance.

Sulfur release and migration characteristics in chemical looping combustion of high-sulfur coal

Chemical looping combustion of coal has attracted great attention due to its advantage in carbon capture. Sulfur in coal may have negative impacts on this process including deteriorating the reactivity of oxygen carrier and generating gaseous sulfur species. In this research, the release and migration characteristics of sulfur in the chemical looping combustion of Xinzhou coal with two synthetic oxygen carriers, CuO/SiO2 and NiO/Al2O3, were investigated by experiments and thermodynamic simulation. The thermodynamic analysis showed that sulfur in coal mainly migrated to metal sulfides at lower temperatures and peroxide coefficients, but the formation of SO2 was favored at higher temperatures and peroxide coefficients. Experimental results showed that the sulfur conversion efficiencies (XS,g) during the reduction stage reached 62.34 % and 61.25 % under Cu- and Ni-based oxygen carriers, respectively, and XS,g during the reoxidation stage were 6.25 % and 7.44 %, respectively. For both oxygen carriers, the share of H2S and SO2 in the sulfurous gases exceeded 96 % during the reduction stage, while SO2 was the only sulfurous gas when air was introduced. Even though the metal sulfides (Cu2S, Ni3S2 and NiS) were observed in the reduced samples, those two oxygen carriers showed satisfactory regeneration performance after one redox process.

Use of bio-glycerol for the production of synthesis gas by chemical looping reforming

Glycerol availabity, a waste generated in the production of biodiesel, has increased during last years. In this work, Chemical Looping Reforming (CLR) of glycerol has been demonstrated in a 1 kWth continuous unit during 35 h using a Ni-based oxygen carrier to obtain a high purity syngas without CO2 emissions. Complete conversion of the glycerol and syngas composition close to thermodynamic equilibrium was obtained in the fuel reactor. Moreover, pure N2 was obtained as product in the air reactor. The influence of the main operating variables on the composition and flow rates of synthesis gas was evaluated. The oxygen-to-glycerol molar ratio was the main parameter since the amount of lattice oxygen transferred by the oxygen carrier in the fuel reactor controlled the syngas composition. The increase of the water-to-glycerol ratio produced an increase in the production of H2 and CO2 with a decrease in the CO content, increasing both the syngas yield and the H2/CO molar ratio in the gas. Fuel reactor temperature affected mainly to the CH4 content, being lower as higher was the temperature. Close to autothermal conditions, it is possible to obtain a syngas composed by H2 ≈ 48–50 vol%; CO ≈ 30–35 vol%; CO2 ≈ 14–18 vol%; and CH4 ≈ 1.6–3 vol%. In addition, different H2/CO ratios can be obtained by modifying the H2O/glycerol ratio and temperature. A H2/CO ratio of 2 could be reached using either a H2O/glycerol ratio of 1.5 at 750 °C or a H2O/glycerol ratio of 2 at 800 °C.

Renewable hydrogen production from chemical looping steam reforming of biodiesel byproduct glycerol by mesoporous oxygen carriers

Abstract The selection of metal oxides and supports to improve catalytic activity and stability of oxygen carriers (OCs) is crucial for chemical looping steam reforming (CLSR) to hydrogen production. In this study, the Ni-based MCM-41 and SBA-15 OCs with and without CeO2 promoter were prepared by direct synthesis and impregnation methods. The BET, XRD, TEM, ICP-AES, H2-TPR and DSC-TGA were used to analyze the synthesized OCs. Hydrogen production by CLSR using biodiesel byproduct glycerol was studied in a fixed-bed reactor with fuel and air stages. The DSC-TGA experiments were carried out to determine the thermal decomposition of biodiesel byproduct glycerol and the results indicated the exothermic and/or endothermic reactions as a result of secondary reactions and, the main degradation with the mass loss of 85.5 wt% occurred in the stages of 150–500 °C. The mesoporous OCs presented high specific surface area, large pore volume, and uniform pore size. It was found that the NiO and CeO2 in the mesoporous OCs were first reduced by fuels and the reduced OCs was responsible of steam reforming and water gas shift (WGS) to hydrogen production, and hydrogen selectivity was up to 90% using CeNiO/SBA-15. The mesoporous OCs with Ce promoter greatly shortened ‘dead time’ (the reduce time of OCs), and effectively suppressed carbon deposited on OCs. High loading of NiO and CeO2 on SBA-15 improved oxygen transfer and achieved excellent cyclic stability. These findings provide the fundamental understanding and principle of design on the conventional Ni-based OCs with room for further developments in chemical looping process.

A multi-scale simulation of syngas combustion reactions by Ni-based oxygen carriers for chemical looping combustion

Density functional theory (DFT) calculations of syngas combustion with the Ni-based oxygen carriers were conducted to reveal new insights on the elementary reaction mechanisms. For the first time, the reaction mechanism of syngas combustion using NiO was revealed. The CO oxidation was proposed as a 1-step reaction, while the H2 oxidation was proposed as a 3-step reaction. Among all the reactions, H2 decomposition was proved to be the controlling step that dominated the overall syngas combustion process. A DFT-based mean-field (MF) model was established to investigate the impacts of the operating conditions on the oxygen carrier (OC) behaviors inside the channel of OC particles. The results show that a high ratio of H2 and a high temperature benefit OC conversion. Also, the increase in pressure resulted in the reduction of the OC conversion ratio. The predicted outcomes from the MF model are in agreement with experimental observations, which further validated the proposed reaction mechanism from DFT analysis. Furthermore, results from the MF model show that high temperatures and low pressures lead to high CO2/H2O product ratios. This computational study illustrates the key factors that affect the performance of this process at the atomistic scale and inside the channel of OC particles.

A two carriers reactor configuration for chemical-looping combustion in a packed-bed

•A numerical model of a Chemical Looping Combustion (CLC) fixed bed reactors network for CH4 combustion is developed and studied in the present paper. The network is based on a parallel arrangement of couples of reactors in series. For every couple, the first reactor is filled with a Cu-based oxygen carrier (OC) while the second one exploits a Ni-based OC. The developed model allows to evaluate the performances of the proposed system in terms of both generated thermal power and flow and composition of the gaseous output streams.•For the same bed length and cross-sectional area, the proposed two-stage CLC reactor has similar performances to a single carrier one in terms of outlet gas temperature, allowing however for shorter cycle times and requiring a lower amount of active phases contents with respect to this latter. Moreover, fewer units to be arranged in the network are required for the proposed two-stage CLC system than the single-stage one.

Safety and environmental reasons for the use of Ni-, Co-, Cu-, Mn- and Fe-based oxygen carriers in CLC/CLOU applications: An overview

Chemical looping combustion (CLC) is presently considered as a cutting-edge combustion technology that enables capturing CO2 without major energy penalty for its separation. The technology involves the use of solid metal oxides, so-called oxygen carriers (OCs), which are alternately subjected to the redox reactions. In the last years, most investigations on the CLC process have been focused on the development of synthetic oxygen carriers, which are much more efficient and have greater resistance compared to natural materials. Synthetic OCs can be based on Ni-, Co-, Cu-, Mn- and Fe- oxides, the use of which however may cause various kinds of threats. The paper provides an overview of Ni, Co, Cu, Mn, and Fe-based OCs taking into account safety operation of CLC reactors as well as health and environmental issues. Therefore, the purpose of this work is to study the impact of different solid oxygen carriers on human beings, animals and natural environment. It has been revealed among others that OCs based on Ni- and Co- oxides may cause various environmental and health problems, while Cu- oxide might be hazardous especially for aquatic wildlife. Moreover, it was shown that Ni-based OCs have tendency to create Ni-S compounds, such as Ni3S2 or NiSO4, in reactions with combustible sulphur contained in the fuel. Toxicity investigations prove, for instance, that rats exposed to 15 mg/6 h/day of NiSO4 die in 10 days after the test. Otherwise, Mn- and Fe- oxides are generally safe in use.

Hydrogen production via chemical looping steam reforming of ethanol by Ni-based oxygen carriers supported on CeO2 and La2O3 promoted Al2O3

This work studies the effects of Ce4+ and/or La3+ on NiO/Al2O3 oxygen carrier (OC) on chemical looping steam reforming of ethanol for hydrogen production - alternating between fuel feed step (FFS) and air feed step (AFS). Suitable amount of Ce- and La-doping increases OC carbon tolerance. The solubility limit is found at 50 mol% La in solid solution. At higher La-doping, La2O3 disperses on the surface and adsorbs CO2 forming La2O2CO3 during FFS. From the 1st cycle, 12.5 wt%Ni/7 wt%La2O3-3wt%CeO2–Al2O3 (N/7LCA) displays the highest averaged H2 yield (3.2 mol/mol ethanol) with 87% ethanol conversion. However, after the 5th cycle, 12.5 wt%Ni/3 wt%La2O3-7wt%CeO2–Al2O3 (N/3LCA) exhibits more stability and presents the highest ethanol conversion (88%) and H2 yield (2.7 mol/mol ethanol). Amorphous coke on the OCs decreases with increasing La3+ content and can be removed at 500 °C during AFS; nevertheless, fibrous coke and La2O2CO3 cannot be eliminated. Therefore, after multiple redox cycles, highly La-doped OCs exhibits rather low stability.

Numerical Investigation of Nickel–Copper Oxygen Carriers in Chemical-Looping Combustion Process with Zero Emission of CO and H2

The chemical-looping combustion (CLC) technique with an intrinsic feature for carbon capture is playing a critical role in the industrial process planning. As the key technology in CLC, the behaviors of oxygen carriers with different weight ratios of NiO/CuO are investigated using the two-fluid model in this work. The kinetic theory of granular flow is employed for the description of particle motion, while the EMMS model is used to clarify the effect of mesoscale structure on gas–solid interaction in the fuel reactor. With the coexistence of Ni and Cu in the oxygen carriers, a zero emission of H2 and CO is observed at the outlet of the fuel reactor, while the intermediate products of H2 and CO arise in the system with Ni-based oxygen carriers. The complete conversion of CH4 is observed in the fuel reactor with various components of Ni–Cu oxygen carriers. The reactivity of CuO and NiO with gas fuel is analyzed, and the equilibrium state in the CLC system with different weight ratios of NiO/CuO is computed ...

Design of micro interconnected fluidized bed for oxygen carrier evaluation

Oxygen carriers (OC), one of the key factors in chemical looping combustion (CLC), are regularly evaluated using thermogravimetric analysis (TGA) and batch fluidized beds with oxidizing and reducing gas streams. Nevertheless, the reaction status in these devices has a specific deviation from a real CLC process. It introduces difficulty when assessing attrition rate, thermal stability, and reactivity. In this work, a micro interconnected fluidized bed (MIFB) is designed to provide a real CLC environment to increase the accuracy of OC evaluation. The air reactor and fuel reactor are identical two-stage bubbling beds, which have 30 mm ID and 100 mm height. With stable and flexible fluidization, the amount of bed inventory in MIFB was minimized to reduce operational cost. 350 g bed inventory was required after size miniaturization, structure optimization and circulation path simplification. Synthetic NiO/Al2O3 and natural hematite are employed as OCs to give insight into MIFB performance. Reactivity of hematite was observed in MIFB with conversion efficiencies of 100%, 36.8%, and 16% for H2, CO and CH4 respectively at 900℃. Ni-based OC displayed excellent reactivity and thermodynamics, but also exhibited severe carbon deposition at low temperature. OC lifetimes of 457 h and 680 h were estimated for hematite and NiO/Al2O3, respectively. In addition, particle size distributions were employed to evaluate OC attrition behavior in MIFB. Based on this approach, the potential of MIFB as an OC evaluation device is demonstrated, accurately reflecting fluidization hydrodynamics, OC reactivity, mechanical behavior and lifetime, the influence of temperature and carbon deposition phenomenon.

Sustainable iron-based oxygen carriers for hydrogen production – Real-time operando investigation

In this work, a spray-dried Fe-based oxygen carrier with an in situ generated Mg1-xAl2-yFex+yO4-support was investigated during packed-bed chemical looping operation with methane at 900 °C. The evolution of the solid-state chemistry taking place in the oxygen carrier material was investigated in real-time with synchrotron X-ray diffraction while the spatial distribution of the phases was investigated using X-ray diffraction computed tomography (XRD-CT). These measurements revealed that some Fe-cations were systematically taken up and released from the spinel support. This take-up and release was shown to be strongly related with the oxidation state of the active phase. Although this take-up and release of Fe-cations decreased the amount of Fe-oxides active in the chemical looping process, the oxygen transfer capacity was still sufficiently high. The microstructure of the oxygen carriers along the length of the packed reactor bed was also investigated with scanning electron microscopy (SEM). The experiments indicate that the MgFeAlOx support with an extra Fe-based active phase is a promising material for oxygen carriers, as it forms a sustainable non-toxic, stable and green alternative to the typical Ni-based oxygen carriers, for hydrogen generation by chemical looping.

Hydrogen Production via Chemical Looping Steam Reforming of Ethanol: Effect of Ce and La on Ni-based Oxygen Carrier Performance

Chemical looping steam reforming of ethanol for hydrogen production has been studied. In this process, the high hydrogen content compound, ethanol, was reacted with steam and oxygen carriers (OCs) through the redox reaction cycle to produce the clean fuel H2. The effects of the different promoters, CeO2 and La2O3, in Al2O3-supported Ni-based OCs were investigated. The mixed promoters Ni-Al2O3 OCs were synthesized by wet impregnation method. The fresh OCs were characterized by X-ray powder diffraction (XRD), N2 adsorption-desorption, H2-temperature programmed reduction (H2-TPR) and Scanning electron microscope/Energy dispersive x-ray spectroscopy (SEM/EDX) technique. The redox activity test was performed in a fixed bed reactor by alternating between fuel and air feeds at isothermal condition of 500°C. The fuel feed was composed of ethanol and steam with steam to ethanol (S/E) mole ratio of 3. It was found that addition of CeO2 and La2O3 on Ni/Al2O3 improved the metal dispersion, reduced the crystallite size and enhanced reducibility of NiO. The La2O3 promoted (Ni/La2O3-Al2O3) and CeO2 and La2O3 co-promoted (Ni/La2O3-CeO2-Al2O3) OCs showed high performance and stability in chemical looping steam reforming of ethanol with the maximum ethanol conversion of 98.3 and 92.5 %, respectively, with maximum H2 yield of 2.9 and 3.2 mol/mol ethanol, respectively. In addition, forming of La2O3-CeO2 solid solution phase in Ni/La2O3-CeO2-Al2O3 oxygen carriers enhanced the oxygen storage capacity (OSC) and high oxygen mobility (OM) of OCs, resulting in a lower carbon deposition.