Chemical Looping Reforming Of Methane Research Articles

Pressure-Induced Enhancement in Chemical Looping Reforming of CH4: A Thermodynamic Analysis with Fe-Based Oxygen Carriers.

Chemical looping reforming of methane (CLRM) with Fe-based oxygen carriers is widely acknowledged as an environmentally friendly and cost-effective approach for syngas production, however, sintering-caused deactivate of oxygen carriers at elevated temperatures of above 900 °C is a longstanding issue restricting the development of CLRM. Here, in order to reduce the reaction temperature without compromising the chemical-looping CH4 conversion efficiency, we proposed a novel operation scheme of CLRM by manipulating the reaction pressure to shift the equilibrium of CH4 partial oxidation towards the forward direction based on the Le Chatelier's principle. The results from thermodynamic simulations showed that, at a fixed reaction temperature, the reduction in pressure led to the increase in CH4 conversion, H2 and CO selectivity, as well as carbon deposition rate of all investigated oxygen carriers. The pressure-negative CLRM with Fe3O4, Fe2O3 and MgFe2O4 could reduce the reaction temperature to below 700 °C on the premise of a satisfactory CLRM performance. In a comprehensive consideration of the CLRM performance, energy consumption, and CH4 requirement, NiFe2O4 was the Fe-based OCs best available for pressure-negative CLRM, especially for an excellent syngas yield of 23.08 mmol/gOC. This study offered a new strategy to address sintering-caused deactivation of materials in chemical looping from the reaction thermodynamics point of view.

Oxygen Species Involved in Complete Oxidation of CH4 by SrFeO3-δ in Chemical Looping Reforming of Methane.

Compared with conventional methane reforming technologies, chemical looping reforming (CLR) has the advantages of self-elimination of coke, a suitable syngas ratio for certain down-stream processes, and a pure H2 or CO stream. In the reduction step of CLR, methane combustion has to be inhibited, which could be achieved by designing appropriate oxygen carriers and/or optimizing the operating conditions. To gain a further understanding of the combustion reaction, methane oxidation by perovskite (SrFeO3-δ) at 900 °C and 1 atm in a pulse mode was investigated in this work. The oxygen non-stoichiometry of SrFeO3-δ prepared by a Pechini-type polymerizable complex method is 0.14 at ambient conditions, and it increases to 0.25 and subsequently to 0.5 when heating from 100 to 900 °C in argon that contains 2 ppmv of molecular oxygen. The activation energies of the first and second transitions are 294 and 177 kJ/mol, respectively. The presence of 0.99 vol.% hydrogen in argon significantly reduces the amount CO2 produced. At a pulse interval of 10 min, the amount of CO2 produced in the absence of hydrogen is one order of magnitude greater than that in the presence of hydrogen. In the former case, the amount of CO2 produced dramatically decreases first and then gradually approaches a constant, and the oxygen species involved in methane combustion can be partially replenished by extending the pulse interval, e.g., 82.5% of this type of oxygen species is replenished when the pulse interval is extended to 60 min. The restored species predominantly originate from those that reside in the surface layer or even in the bulk.

Optimizing the sulfur-resistance and activity of perovskite oxygen carrier for chemical looping dry reforming of methane

Perovskite oxides has been attracted much attention as high-performance oxygen carriers for chemical looping reforming of methane, but they are easily inactivated by the presence of trace H2S. Here, we propose to modulate both the activity and resistance to sulfur poisoning by dual substitution of Mo and Ni ions with the Fe-sites of LaFeO3 perovskite. It is found that partial substitution of Ni for Fe substantially improves the activity of LaFeO3 perovskite, while Ni particles prefer to grow and react with H2S during the long-term successive redox process, resulting in the deactivation of oxygen carriers. With the presence of Mo in LaNi0.05Fe0.95O3−σ perovskite, H2S preferentially reacts with Mo to generate MoS2, and then the CO2 oxidation can regenerate Mo via removing sulfur. In addition, Mo can inhibit the accumulation and growth of Ni, which helps to improve the redox stability of oxygen carriers. The LaNi0.05Mo0.07Fe0.88O3−σ oxygen carrier exhibits stable and excellent performance, with the CH4 conversion higher than 90% during the 50 redox cycles in the presence of 50 ppm H2S at 800 °C. This work highlights a synergistic effect in the perovskite oxides induced by dual substitution of different cations for the development of high-performance oxygen carriers with excellent sulfur tolerance.

Performance evaluation of carbon-negative syngas production via chemical looping reforming of methane from biomass pyrolysis volatiles

To realize the clean and efficient directional conversion of biomass, chemical looping reforming of biomass volatile CH4 based on a decoupling strategy is expected to carry out carbon-negative production of hydrogen-rich syngas. In this paper, a multi-functional oxygen carrier (NiFe2O4@SBA-16) for encapsulating the active metal oxide component NiFe2O4 with a three-dimensional interconnecting pore molecular sieve SBA-16 was designed. The experimental results showed that the modification strategy of composite metal oxide encapsulation improved syngas selectivity and CH4 conversion based on the fixed-bed reactor. 20 %NiFe2O4@SBA-16 showed 22.1 % CH4 conversion and 92.6 % CO selectivity under 750 °C, and it maintained good cycle stability after 15 cycles. The H2/CO ratio was effectively improved by adding part of CO2 into the feed gas, and the CH4 conversion reached over 35 %. This study provides essential data for the clean preparation of hydrogen-rich syngas from biomass and the development and design of multifunctional oxygen carriers.

Solution Combustion Synthesis of Ce‐based Composite Oxygen Carriers for Chemical Looping Reforming of Methane

AbstractEfficient activation and the conversion of methane to value‐added products have long attracted interest. Chemical looping reforming has become a competitive technology to transform methane into syngas that can act as raw material for various chemicals. In this paper, the methane chemical looping reforming reactivity of the Ce−Co composite oxygen carriers fabricated with different preparation methods, including solution combustion, ball‐milling, hydrothermal, and co‐precipitation method, are investigated on a fixed‐bed. Various bulk/surface characterizations (TEM, XRD, BET, XPS, Raman, etc.) were applied to reveal the effect of preparation methods on the properties of these oxygen carriers. The results indicate that the oxygen carrier prepared by the solution combustion method exhibits excellent methane conversion, high syngas yield, and desirable H2/CO ratio, as well as stable regenerability because of its relatively small crystal size, large specific surface area, high oxygen vacancy concentration and Ce3+/Ce4+ ratio. This work may be helpful in the preparation of highly active and stable oxygen carriers for chemical looping reforming reaction of methane.

Dopant-Enhanced harmonization of α-Fe2O3 oxygen migration and surface catalytic reactions during chemical looping reforming of methane

The rate-matching between oxygen migration and surface catalytic reaction of oxygen carrier (OC) is a critical determinant for the reaction selectivity of the chemical looping reforming (CLR) of methane. However, the theoretical mechanism behind this rate-matching relationship is still unclear. Herein, we propose to use density functional theory (DFT) combined with microkinetic simulations to establish a model integrating the methane oxidation reactions with oxygen migration kinetics on α-Fe2O3 oxygen carrier. Our calculated results reveal that the transition metals (TMs)-doping such as Co, Ni, and Cu after Fe element in the periodic table can accelerate the oxygen migration rate as it promotes the upshift of the O p-band center. Moreover, a suitable oxygen migration energy barrier of about 1.0 eV is helpful to enhance the harmonization of α-Fe2O3 oxygen migration and surface catalytic reactions. Based on this, Ni-doping in α-Fe2O3 is predicted to be able to enhance the oxygen migration rate to match the rate of first-step methane dissociation, thereby simultaneously improving the reaction rate and the selectivity in the CLR of methane to syngas. These results could help to understand the relationship between oxygen migration and surface catalytic reaction rates and pave the way for the rational design of metal oxide oxygen carriers.

Syngas production through multi-cycle chemical looping of chromite mines waste: A sustainable approach to mitigate CO2 emissions

One of the major contributors to global warming is the CO2 emissions from the steel industry. In the primary steel making process, coal/coke is used for reduction purposes that lead to CO2 emissions. Replacing coke with other reducing agents like syngas (H2 and CO) could be a possible approach to reduce CO2 emissions. One effective route for the generation of syngas in a sustainable way is the chemical looping method. In this work, we have investigated the utilization of laterite ore (a mining waste) as an Oxygen carrier for the chemical looping reforming of methane. Multiple redox cycles in the presence of methane and air as reducing and oxidizing agents were performed in DSC-TG to determine the performance of the oxygen carrier. The reacted samples formed in the reaction were analyzed using XRD and SEM. The product gases generated during the reaction were also analyzed using gas chromatography. The results of the reactivity analysis showed that H2 and CO gases were continuously formed during the reforming reaction of methane. The phase analysis of reacted samples in XRD and SEM clearly showed the formation of stable chromite and reduced iron oxide phases. The oxygen carrier showed good stability, and a total hydrogen yield of 203.9 ml/g of OC after 20 cycles of redox reaction were observed. In addition, the theoretical yield of the reacted sample after the first cycle was calculated and compared with the experimental H2 yield at different cycles. A thermodynamic-based simulation study is also reported in this work. The syngas generated from the reforming reaction is used to pre-reduce the Iron bearing ore materials in a blast furnace. The results showed a significant decrease in the CO2 emissions and coke rate could be achieved in primary steelmaking by integrating a chemical looping process with a blast furnace. This study effectively shows the utilization of laterite ore as an oxygen carrier for generating syngas through a chemical looping reforming process to mitigate CO2 emissions.

Carbon-neutral and carbon-negative chemical looping processes using glycerol and methane as feedstock

Carbon-negative and neutral methods to produce H2 and other syngas-derived chemicals are tested and demonstrated in this study through chemical looping reforming of methane or glycerol. A chemical looping reactor provides the heat required to reform the glycerol or methane while having inherent CO2 capture. This is achieved using dynamically operated packed beds. If the glycerol or methane is from a biological source this gives the system the potential for negative emissions. To evaluate the potential of this system, 500 g packed bed of oxygen carriers were cyclically reduced, oxidized, and used to carry out reforming experiments. The reforming process was tested at various pressure (1 – 9 bar) and temperature (600 – 900 °C). These conditions were tested at this scale for the first time. Complete conversion of glycerol is achievable with only small quantities of CH4 slip. The maximum H2 production was achieved at 1 bar and 700 °C producing a H2/CO ratio of 10, this lowered to 9 when the temperature was increased to 900 °C. Adding CO2 to the feed stream along with H2O allows for a H2/CO ratio suitable for the Fischer Tropsch (FT) synthesis. Chemical looping reforming of CH4 with steam was successfully demonstrated in a lab reactor setup at 1 and 5 bar for multiple cycles with CH4 conversion > 99% and controlled heat losses. The temperature and concentration profiles provided identical results for consecutive cycles verifying the continuity and the feasibility of the process.

Chemical looping reforming characteristics of methane and toluene from biomass pyrolysis volatiles based on decoupling strategy: Embedding NiFe2O4 in SBA-15 as an oxygen carrier

To realize the directional conversion of biomass to produce hydrogen-rich syngas, biomass gasification with a complex reaction system was decoupled into two sub-processes. NiFe2O4@SBA-15 was prepared by an embedding strategy via the impregnation method to achieve high reactivity during the volatile chemical looping reforming process. The chemical looping reforming of methane and toluene was investigated based on a fixed-bed reactor and on-line mass spectrometry and gas chromatography. It was found that the selectivity and conversion were improved after embedding, and the conversion of toluene reached 97.5 % at 750 °C. The NiFe2O4@SBA-15 maintained good stability after 15 cycles, and the reaction performance had not obviously decreased. Real-time reaction performance showed that the conversion of lattice oxygen could reach 100% above 750 °C. The reaction system belonged to the phase boundary-controlled mechanism based on the conversion of lattice oxygen, and the activation energy was 43.10 kJ·mol−1. In addition, in-situ diffuse reflectance infrared spectroscopy(DRIFTS) study had shown that the dissociation of methane and toluene and the further conversion of formate might be the reaction rate-limiting steps. Although methane and toluene competed in adsorption, the reaction processes were independent.

Rational design and reduction kinetics of efficient Ce-Co oxygen carriers for chemical looping reforming of methane

One of the most significant topics in chemical looping reforming technology is to design and prepare appropriate oxygen carriers with high reactivity and excellent stability. Starting from preparation strategy and doping engineering, this work focuses on the study for chemical looping reforming of methane over the cobalt-doped Ce-based oxygen carriers synthesized via solution combustion method with the assistance of coconut shell. The reactivity of as-prepared oxygen carriers was evaluated by isothermal redox experiments, and the apparent reduction kinetics of CH4 over representative oxygen carriers was investigated on the fixed bed. A series of characterization analyses (including XRD, XPS, H2-TPR, BET, SEM, TEM and Raman) indicated that the introduction of cobalt decreases the crystallite size of composite oxygen carrier, as well increases their oxygen vacancy concentration and lattice oxygen mobility compared with the pure CeO2. Moreover, benefitting from the interaction effect between carbon material and microwave field, a moderate amount of coconut shell additive could not only further facilitate above positive changes, but enhance the interaction between Ce and Co. As consequence, for the Ce9Co1Oδ-10CS oxygen carrier, syngas with a H2/CO ratio of about 2.2 can be stably obtained, and both CH4 conversion and CO selectivity exceed 90%, and it maintains high stability in consecutive redox cycles. From the kinetic study, it has been demonstrated that the reduction of CeO2, Ce9Co1Oδ and Ce9Co1Oδ-10CS oxygen carriers are well represented by the phase-boundary controlled (contracting cylinder) mechanism, and the activation energies are calculated to be 128.73, 91.18 and 46.82 kJ/mol in turn, which also corresponds well to the reaction performance of oxygen carriers.

Utilization of laterite ore as an oxygen carrier in chemical looping reforming of methane for syngas production

Growing energy demands in various sectors have resulted in overusing fossil fuel sources and rise in greenhouse gases in the atmosphere. This necessitates the need for reducing greenhouse gases and shifting to cleaner, renewable energy sources like H2. Chemical looping is one such renewable method to produce clean H2. The efficiency of this process depends on the oxygen carrier. Generally, oxygen carriers (OC) are transition metal oxides (Fe2O3 or NiO) or some complex metal oxides like spinel or perovskites, but usage of these OCs are restricted due to their availability and redox performance. One solution for selecting OCs can be using industrial waste like slag or low-grade ores because of their composition, which consists of metal oxides. One such low-grade ore is Ni-laterite ore or chromite overburden, a mining waste found in the chromite mines of Sukinda (India). In this work, we have focused on utilizing this laterite ore as an OC for the chemical looping reforming of methane to produce syngas. The reactivity analysis of laterite ore with CH4 was performed in DSC-TG and the reaction products were analyzed in gas chromatography along with microscopy and spectroscopic techniques. Results showed the formation of H2 and CO gases along with reduced metallic phases. The total H2 yield at 750 °C is determined to 53.67 (±1.09) ml/g of OC which is comparable or even higher than existing CeO2 based OCs. Further, thermodynamic calculations are presented to calculate the theoretical yield for our process and compared with the experimental H2 yield. This study effectively demonstrates the performance of laterite ore as an OC for generating clean and renewable energy through chemical looping technique.

Kinetic investigation of solar chemical looping reforming of methane over Ni–CeO2 at low temperature

Chemical looping reforming of methane utilizing the oxygen exchange material CeO2 is catalytically enhanced with metallic Ni to improve reaction rates at low operating temperatures. A kinetic study and comparison to noncatalyzed CeO2 is presented.

Enhanced syngas selectivity and carbon utilization during chemical looping reforming of methane via a non-steady redox cycling strategy

A cycling strategy is presented for simultaneous improvement of conversion and syngas selectivity during chemical looping reforming. Periodic recharging is used to re-establish a favorable nonstoichiometry profile during non-steady cycling.

DFT studies of CO reaction behavior on α-Fe2O3(001) oxygen-vacancy surface in chemical looping reforming

Chemical looping reforming of methane to syngas (CO and H2) is one of the most promising routes for methane utilization, where the further reaction of CO on oxygen carrier surfaces is a primary determinant of CO selectivity. In this work, the effects of oxygen vacancy (Vo) on CO desorption, CO oxidation, and CO dissociation are systematically studied by using density functional theory calculattions. Our calculated results reveal that increasing Vo concentration can weaken CO desorption at Fe sites due to the enhanced localization of electrons in the Fe atoms. Also, the increase in Vo concentration from 1/12 ML to 1/6 ML leads to a dramatic increase of activation energy in the CO oxidation from 0.64 eV to 1.10 eV. Moreover, the increase in Vo concentration is conducive to CO dissociation, but the dissociation is still almost impossible due to the high reaction energies (large than 3.00 eV). Considering these three reaction paths, CO desorption can proceed spontaneously at reaction temperatures above 900 K. Increasing Vo concentration can improve the selectivity of syngas production due to the less favorable CO oxidation compared with CO desorption at high Vo concentrations (1/6 ML). This work reveals the microscopic mechanism that CO selectivity rises in the CLRM as the degree of Fe2O3 reduction increases.

Solar-driven chemical looping reforming of methane over SrFeO3-δ-Ca0.5Mn0.5O nanocomposite foam

Strontium ferrite (SrFeO3-δ) is a very attractive oxygen transfer agent for chemical looping reactions and hydrogen-rich syngas generation. Dispersing SrFeO3 in a medium such as Ca0.5Mn0.5O could enhance the activity and cyclability. In this study, SrFeO3-δ-Ca0.5Mn0.5O (30 wt% SrFeO3-δ) nanocomposite with a reticulated foam structure was explored as the oxygen carrier for chemical looping reforming of methane in a solar tubular reactor. The foam nanocomposite was prepared by a hard-templating method. The performance was investigated at temperatures of 850–1000 °C and methane flowrates of 25–250 STP mL/min, and the oxidative gas was either CO2 or H2O in the oxidation step. In the reduction step of 27 successive redox cycles, the production rate of CO changed marginally and CO yield maintained at about 1.9 mmol/g, even though sintering occurred. The productivity of H2 decreased first and then tended to be stable at 3.8 mmol/g (i.e., twice the CO yield) as the cycling number increased (the average oxygen storage capacity of the material was ∼1.95 mmol/g). Microscopic and X-ray diffraction investigations suggested that the element distribution pattern and crystalline phase of the foam nanocomposite remained almost unchanged after 27 redox cycles, confirming material stability. The maximum solar-to-fuel efficiency for the foam nanocomposite was 5.68%, which was 21.4% higher than that for the powder nanocomposite. To increase syngas productivity and solar-to-fuel efficiency, it is required to conduct the reforming reaction at high temperatures and methane flowrates. However, the energy upgrade factor will decrease as methane flowrate increases.

Theoretical insights into the oxygen supply performance of α-Fe2O3 in the chemical-looping reforming of methane

The chemical looping reforming of methane to syngas is of great significance to the sustainable development of energy and environment, where the oxygen carrier (OC) is the core element of this technology. To study the oxygen migration properties of OCs and the matching problem of oxygen migration rate with the surface catalytic reaction rate, we present a theoretical study by taking the abundant and cheap α-Fe2O3 as the model OC. It is found that the migration rate of lattice oxygen and the dissociation of methane will be enhanced as the concentration of surface oxygen vacancy increases. By comparing the oxygen migration rate with the methane dissociation rate, we reveal that the two rates can match well at a low surface oxygen vacancy concentration, which is the key to the selective production of syngas. Lower or higher surface oxygen vacancy concentration will lead to the products towards carbon dioxide or coke.

Influence mechanism of inert carrier on the anti-carbon deposition effect of nickel-based oxygen carrier in chemical looping methane reforming process

In this paper, density functional theory (DFT) calculations are applied to investigate the reaction mechanism of syngas production by chemical looping reforming of methane on NiO-based surfaces. The work mainly focused on the basic reaction steps, including the catalytic dehydrogenation of CH4, the generation of H2 and H2O, and the formation of CO. The mechanism of formation and removal of carbon deposits is analyzed emphatically. Firstly, it is found that NiO fragments are very stably attached on the surface of ZrO2 and Al2O3 and carriers. For the methane dehydrogenation step, it is shown that CH4 → CH3 + H is the rate-limiting step on both surfaces. Moreover, the calculation results showed that the formation of CO and H2O molecules was also more favorable on the surface of NiO/Al2O3, indicating that the Al-Ni structure owns better lattice oxygen transferring ability. For the key step of the H2 molecule generation, the surface of NiO/ZrO2 shows better reaction performance and lower activation energy. The NiO fragments loaded on the surface of Al2O3 is able to exhibit better anti-carbon properties, indicating that the introduction of Al2O3 can well enhance the cyclic stability of oxygen carriers.

Controlling the reaction microenvironments through an embedding strategy to strengthen the chemical looping reforming of methane based on decoupling process

Aiming at the complexity of traditional biomass gasification technology with low conversion efficiency and high tar content and realizing the directional conversion of biomass into syngas, this paper proposed a new decoupling process of biomass pyrolysis and chemical looping reforming of volatile components. Designing a high activity and selective oxygen carrier is the key to the chemical looping reforming process. This study used an embedding strategy to develop the oxygen carrier. NiFe2O4 was embedded in SBA-15 to control the reaction microenvironment. The reaction performance of NiFe2O4@SBA-15 under different conditions was investigated via a combination of fixed-bed reactor and on-line mass spectrometry. The results showed that the CO selectivity was close to 100%. The CH4 conversion and CO selectivity of NiFe2O4@SBA-15 were increased by 121.29% and 114.40% than NiFe2O4, respectively. The optimal embedding ratio (20%) and reaction temperature (900 °C) were determined during the experiment, which was consistent with the thermodynamic simulation results. The gas–solid reaction model was used to solve the reaction kinetics of the oxygen carrier, and the activation energy was 118.23 kJ·mol−1. This study gives a research basis for screening oxygen carrier and the design of reactor in the directional conversion of biomass pyrolysis full volatiles into syngas via chemical looping reforming.

Production of hydrogen by chemical looping reforming of methane and biogas using a reactive and durable Cu-based oxygen carrier

The objective of this work was to assess the suitability of a synthetic Cu-based oxygen carrier in a continuous pilot plant for the production of blue and green hydrogen through the autothermal Chemical Looping Reforming (CLRa). In CLRa, methane is converted to a H2 + CO mixture through partial oxidation and reforming reactions in the fuel reactor. The degree of the partial oxidation of methane was defined by controlling the oxygen flow in the air reactor. Steam was used as reforming gas in natural gas to produce blue H2, but the existing CO2 in biogas was the reforming gas to produce green H2. Operating at 950 °C in the fuel and air reactors, CH4 conversion and H2 yield parameters were 96 % and 2.60 mol of H2 per mole of CH4, respectively. These experimental results were close to the theoretical values that could be achieved in the CLRa process. Furthermore, the physicochemical characterization of the samples extracted from the pilot plant throughout the experimental campaign revealed that the Cu-based oxygen carrier maintained its mechanical integrity and chemical stability under harsh operating conditions. Therefore, it can be concluded that Cu-based oxygen carriers can be considered a promising alternative to Ni-based materials for the production of blue and green hydrogen through the CLRa process.

Production of Blue and Green Hydrogen by Chemical Looping Reforming of Methane and Biogas Using a Highly Reactive Cu-Based Oxygen Carrier

Production of Blue and Green Hydrogen by Chemical Looping Reforming of Methane and Biogas Using a Highly Reactive Cu-Based Oxygen Carrier