Chemical Looping Methane Partial Oxidation Research Articles

Mechanistic study of chemical looping partial oxidation of methane over Ce2(SO4)3/MgO with CoO modification

Chemical looping partial oxidation of methane (CLPOM) to syngas provides an exceptional opportunity for methane utilization. However, it is still urgent to obtain efficient oxygen carrier (OC) and clarify its microscopic mechanism. In this work, CoO and MgO modified Ce2(SO4)3 OC are optimized based on various preparation methods and different CoO and Ce2(SO4)3 doping amount. Considering the conversion rate (XCH4), selectivities of CO (SCO) and H2 (SH2), H2/CO molar ratio, when the doping amount of CoO, Ce2(SO4)3 and MgO is 20 wt%, 30 wt% and 50 wt% respectively, the OC prepared by mechanical ball milling method has a better thermodynamic and kinetic properties in chemical looping reaction. Compared with literature, this OC has a larger XCH4. In multiple cycles, XCH4 is still higher than 90%, SCO and SH2 remain above 95%. Density functional theory calculation reveal that the Co doping reduces the energy barrier (Eb) of rate-limiting step by 24.3% and lowers the migration Eb of lattice oxygen by 78.2%. The OC with oxygen vacancy can further reduce the Eb of rate-limiting step, but the Eb basically remains unchanged when the oxygen vacancy (Ov) concentration is greater than 1.96%. Because of the high Eb of CO2 formation and low Eb of syngas formation, the syngas demonstrates high selectivity and does not produce CO2 in experiment. Meanwhile, the doping of CoO and MgO reduces carbon deposition, refines the nano-particles of OC, increases the specific surface area (SSA), and enhances resistance to sintering and aggregation. The unique pore structure also provides a favorable microenvironment for the reaction. These findings demonstrate that CoO and MgO modified Ce2(SO4)3 OC have the potential for application in CLPOM, and these basic insights can also be applied to other chemical looping systems.

ABO3 perovskite catalyst screening for chemical looping methane partial oxidation from descriptor-based microkinetic analysis

DFT calculations and descriptor-based microkinetic analysis have been performed to screen ABO3 perovskite catalysts for methane partial oxidation. The scaling relations indicate the adsorption energies of H@O and CH3@B have the capability of representing the energetics of methane oxidation, and bulk oxygen vacancy formation energy is identified to be a better single descriptor of the kinetics. The resultant volcano-shaped plot shows the catalytic activity varies in the order LaMnO3 > LaFeO3 > LaCoO3 ≈ LaCrO3, which agrees with the experimental findings and thus validates our theoretical predications. By screening 667 combinations of A- and B-cations based on electron neutrality, structural tolerance factors, and experimental verification, 77 ABO3 have been identified to be stable. Of them, SrRuO3, SmRhO3, LaIrO3, NdRhO3, and BaOsO3 are located at the summit of the volcano, which also exhibit satisfactory CO selectivity and enhanced carbon resistance compared to LaFeO3, and may act as the oxygen carrier candidates.

Oxygen activity regulation over LaNiO3 perovskites by Ti substitution for chemical looping partial oxidation of methane

The oxygen species in oxygen carriers directly influence the reaction performance in chemical looping processes. This paper describes the relationship between oxygen species and performance in chemical looping partial oxidation of methane (CLPOM) over LaNixTi1-xO3-δ (x = 0, 0.25, 0.5, 0.75 and 1) perovskites. Due to the increase of Ti substitution, the predominate reaction during CLPOM gradually changes from complete oxidation to partial oxidation. Quasi in situ XPS characterization suggests that the relative concentration of nucleophilic lattice oxygen species for partial oxidation increases with Ti substitution, while the relative concentration of electrophilic adsorbed oxygen species for complete oxidation decreases. The moderate Ti substitution for LaNi0.5Ti0.5O3-δ provides the suitable lattice oxygen activity for the methane partial oxidation. Moreover, LaNi0.5Ti0.5O3-δ exhibits high cyclic stability in redox cycles. This work demonstrates that the regulation of oxygen species distribution and activity is feasible to tailor the reaction performance of oxygen carriers in chemical looping processes.

Dynamic oxygen migration and reaction over ceria-supported nickel oxides in chemical looping partial oxidation of methane

The oxygen species over oxygen carriers determine the reaction performance in chemical looping partial oxidation of methane (CLPOM). This paper describes the dynamic migration and reaction of oxygen species over ceria-supported NiO oxygen carrier for CLPOM. At initial reaction stage, Ni-O species could be consumed rapidly and cause the full oxidation of methane to CO2. Afterwards, the active Ni-O-Ce species dominates the methane partial oxidation to promote the syngas yield. It is found that CO formation rate is linearly related with the content of oxygen species over Ni-O-Ce. Furthermore, the lattice oxygen from CeO2 would migrate through the bulk to complement the consumed Ni-O-Ce oxygen species. Eventually, 5NiO/(40CeO2-Al2O3) exhibit nearly three times as high as methane reaction rate of CeO2/Al2O3 and two times as high as CO formation rate of 5NiO/Al2O3. This work provides the comprehensive understanding of transport and reaction mechanism of oxygen species for chemical looping processes.

Enhanced performance of LaFeO3 oxygen carriers by NiO for chemical looping partial oxidation of methane

Partially oxidizing methane into syngas via a two step chemical looping scheme is a promising option for methane transformation. However, providing the optimum lattice oxygen to selectively produce syngas represents the major challenge for the development of oxygen carrier materials in chemical looping processes. In this work, a NiO-modified LaFeO3 (x wt% Ni/LaFeO3, x = 0, 3, 5, 7, 9, 11) oxygen carrier is proposed to enhance the activity of lattice oxygen for chemical looping partial oxidation of methane. The results show that the methane conversion rate of 5 wt% Ni/LaFeO3 is increased by 25% compared to that of unmodified LaFeO3. It is worth noting that the methane conversion and product selectivity mainly depend on the Ni loading, and 5 wt% Ni/LaFeO3 oxygen carriers has higher CH4 conversion (95.2%) and CO selectivity (85.1%). The results demonstrate that it is feasible to active the lattice oxygen of LaFeO3 perovskite oxygen carriers by surface modification of NiO, which ultimately influences their reaction performance in chemical looping partial oxidation of methane (CL-POM) processes.

Influence of the encapsulation degree of Fe0 active sites on performance of garnets for chemical looping partial oxidation of CH4

It is of challenge to improve the coke resistance of Fe-based oxygen carriers (OC) due to the conflict of Fe0 sites for CH4 activation but causing coke formation. The present work reported that carbon-tolerant of Y3Fe2Al3O12-T garnets (YFAO-T, T indicates calcination temperature) could be remarkably promoted by tuning calcination temperature for chemical looping partial oxidation of methane (CLPOM). This was because the encapsulation degree of Fe0 (1-Fe0exposed/Fe0total) became larger with reduction from 19% to 79% due to decreased oxygen mobility leading to more surface oxygen-deficiency for YFAO-T calcined at higher temperature, which not only resulted in Fe0 sites for CH4 activation but also accessible lattice oxygen of garnet shell for carbon oxidation formed on Fe0 sites. Comparatively, the encapsulation of Fe0 was destroyed leading to progressively naked Fe0 with large particle size with reduction for YFAO-T calcined at lower temperature, which induced carbon deposition due to inaccessible lattice oxygen of garnets.

Syngas production via chemical looping reforming using methane-based feed and NiO/Al2O3 oxygen carrier

Syngas production based on chemical looping reforming was experimentally studied using a fixed-bead reactor. The 15 wt% NiO/Al2O3 was used as the oxygen carrier. The reaction temperature was fixed as 800 °C. For reactant containing CH4 only, it was termed as chemical looping partial oxidation of methane (CL-POM). With five CL-POM cycles, no decay in oxygen carrier activity was found. A high H2/CO ratio resulted due to dominant H2 production reactions in the reduction stage. Carbon deposition on the oxygen carrier surface can be identified from the CO and CO2 formations in the oxidation stage. With CO2 added in the reactant in addition to CH4, this was referred to as the chemical looping dry reforming of methane (CL-DRM). Near theoretical amounts of H2 and CO yields were obtained. With both H2O and CO2 added in the reactant in addition to CH4, this was referred to as the chemical looping tri-reforming of methane (CL-TRM). Due to coupled steam reforming of methane (SRM), POM, and DRM, higher CH4 conversion, H2 and CO yields resulted compared with the CL-POM and CL-DRM cases. The average H2/CO ratio of 1.2 was obtained with reactant with molar ratio of CH4/CO2/H2O = 1/1/1.

On the ensemble requirement of fully selective chemical looping methane partial oxidation over La-Fe-based perovskites

The dynamic nature of active sites on La-Fe-based perovskites in chemical looping methane oxidation has been studied. According to experimental observations, the reduction of the oxygen carrier is divided into three stages occurring at different active ensembles where the Fe cations are in different coordination environments. The Mars van Krevelen mechanism formulated by microkinetic analysis describes well the effect of oxygen vacancy on the catalytic performance of LaFeO3. CO2 is not produced by CO oxidation but rather is a primary product of methane oxidation, and the presence of surface oxygen vacancies would dramatically increase the overall energy barrier for total combustion, thus decreasing the selectivity toward CO2. Hence, the Fe coordination environment (and hence the oxygen vacancy concentration) is the key parameter governing the catalyst selectivity, in the sense that methane oxidation can vary from total combustion on the O-rich surface to fully selective partial oxidation on the O-deficient surface.

Reaction performance and lattice oxygen migration of MnFe2O4 oxygen carrier in methane-carbon dioxide reaction system

Chemical looping partial oxidation of methane (CLPOM) coupled with carbon dioxide splitting process was proposed in this work to achieve high H2/CO ratio syngas and CO production simultaneously. Manganese ferrite (MnFe2O4) oxygen carrier with a spinel structure was prepared by using a co-precipitation phase inversion method for this methane-carbon dioxide reaction system. The reaction performance of oxygen carrier was investigated in fixed bed reactor combined with the analytical methods of XRD, TPR, H2-TG and XPS. The results revel that spinel structure MnFe2O4was successfully prepared and Fe is the main active spices to release lattice oxygen. TG analysis confirms that 43.3% of lattice oxygen can release from MnFe2O4 in H2 atmosphere with highest release rate 0.4%/min at 721 °C. The maximum conversion of CH4 and CO2 are 63% and 99% achieved in CLPOM coupled with CO2 splitting process and temperature makes a positive effect on the reaction process. Moreover, the reaction stability of MnFe2O4 oxygen carrier are verified in 10 cycles reaction of CLPOM coupled with CO2 splitting with the reaction path MnFe2O4→(FeO)0.798-(MnO)0·202/FeO →MnFe2O4. Moreover, The MnFe2O4 oxygen carriers with different reduction stages were analyzed by XPS to reveal lattice oxygen migration mechanism, which indicates that the bulk lattice oxygen of oxygen carrier gradually dissociated and migrated to the outer surface in reaction process where reacts directly with methane or converts to oxygen adsorption OII. The valence of Mn has not changed in the whole reaction process, whereas, Mn species forms the synergistic effect with Fe to improve the conversion of CH4 and CO2, promoting the oxidization of Fe2+ to Fe3+ in CO2 atmosphere. MnFe2O4 oxygen carrier is a promising candidate for the CLPOM coupled with CO2 splitting process.

Promoted methane conversion to syngas over Fe-based garnets via chemical looping

Fe-based oxygen carries (OCs) have attracted wide attention due to the low cost and environmental compatibility for chemical looping methane partial oxidation. However, they usually suffer from low CH4 reactivity mainly due to insufficient synergy between catalytic CH4 activation and lattice oxygen mobility. In this work, Fe ions with different amount were incorporated into a novel garnet structure (Y3FexAl5-xO12), which exhibited drastically increased CH4 conversion with reduction time. This resulted from in-situ formed Fe0 sites at the early stage of reduction, which led to a more efficient reaction route involving methane catalytic decomposition on Fe0 and the resulted carbon oxidation easier by lattice oxygen, compared with methane directly oxidized by OCs. Moreover, the amount of Fe ions in garnet influenced surface exposed Fe0 sites and oxygen mobility, inducing Y3Fe2Al3O12 performed the best with almost 94 % methane conversion thanks to the combined advantages of larger surface exposed Fe0 and higher lattice oxygen mobility.

Combining Exsolution and Infiltration for Redox, Low Temperature CH4 Conversion to Syngas

Exsolution of surface and bulk nanoparticles in perovskites has been recently employed in chemical looping methane partial oxidation because of the emergent materials’ properties such as oxygen capacity, redox stability, durability, coke resistance and enhanced activity. Here we attempt to further lower the temperature of methane conversion by complementing exsolution with infiltration. We prepare an endo/exo-particle system using exsolution and infiltrate it with minimal amount of Rh (0.1 wt%) in order to functionalize the surface and induce low temperature activity. We achieve a temperature decrease by almost 220 °C and an increase of the activity up to 40%. We also show that the initial microstructure of the perovskite plays a key role in controlling nanoparticle anchorage and carbon deposition. Our results demonstrate that microstructure tuning and surface functionalization are important aspects to consider when designing materials for redox cycling applications.

Near 100% CO selectivity in nanoscaled iron-based oxygen carriers for chemical looping methane partial oxidation

Chemical looping methane partial oxidation provides an energy and cost effective route for methane utilization. However, there is considerable CO2 co-production in current chemical looping systems, rendering a decreased productivity in value-added fuels or chemicals. In this work, we demonstrate that the co-production of CO2 can be dramatically suppressed in methane partial oxidation reactions using iron oxide nanoparticles embedded in mesoporous silica matrix. We experimentally obtain near 100% CO selectivity in a cyclic redox system at 750–935 °C, which is a significantly lower temperature range than in conventional oxygen carrier systems. Density functional theory calculations elucidate the origins for such selectivity and show that low-coordinated lattice oxygen atoms on the surface of nanoparticles significantly promote Fe–O bond cleavage and CO formation. We envision that embedded nanostructured oxygen carriers have the potential to serve as a general materials platform for redox reactions with nanomaterials at high temperatures.

Chemical Looping Partial Oxidation of Methane for Co‐Production of Syngas and Electricity: Process Modeling and Systems Analysis

Chemical looping partial oxidation of methane (CLPOM) to synthesis gas is studied as an alternative to conventional auto‐thermal reforming (ATR). For comparative analysis, performance models are developed based on mass and energy balances of the processes as functions of process design, operating parameters, and oxygen carrier (OC) properties. Sensitivity analyses are performed at the process and systems levels for CLPOM incorporated into a gas‐to‐liquids (GTL) plant and a preliminary cost analysis is conducted. Process‐level and systems‐level results show that while CLPOM with currently available OCs is less fuel‐efficient than ATR (requiring ≈48% more fuel for a 50 000 barrels per day GTL reference plant), it is economically competitive because of its potential to co‐produce electricity (≈240 MW). The comparative economic advantage of CLPOM can be significantly increased through improvements in OC performance.

Chemical looping methane partial oxidation: The effect of the crystal size and O content of LaFeO3

The site requirements for the methane total combustion, partial oxidation, and decomposition on LaFeO3 perovskite catalysts during the chemical looping process were addressed by a kinetic study through varying the O content and the crystal size. The exchange between the surface oxygen and lattice oxygen is rapid, and the active sites of Fe associated with the surface oxygen are highly dynamic in nature, which depends on the amount of O in the perovskite during reaction and the O binding energy. Three types of active sites are identified where Fe atoms highly coordinated with O lead to total combustion of methane, Fe moderately coordinated with O leads to partial oxidation, and Fe highly coordinated with vacancy sites leads to carbon formation. The crystal size of LaFeO3 perovskite plays a significant role in determining the O binding energy and their catalytic properties on the partial oxidation of methane by the chemical looping process. Different crystal sizes of perovskites have been obtained using various preparation methods. Larger crystal has a high selectivity to synthesis gas and high capacity of O removal, due to a lower O–Fe bond energy, as indicated by a smaller band gap measured by UV–Vis absorption spectroscopy. Nearly pure synthesis gas was produced by methane partial oxidation using chemical looping on LaFeO3 by tuning the surface O coverage through controlling the oxidation degree in the oxidation step.