Perovskite oxygen carrier with chemical memory under reversible chemical looping conditions with and without SO2 during reduction

Oxygen carrier materials (OCM) are usually exposed to sulfur-contained gases in the fuel reactor for chemical looping combustion. This work provides both experimental and model work to understand the SO2 effect on the heterogeneous redox kinetics of a CaMn0.375Ti0.5Fe0.125O3-δ-based perovskite oxygen carrier. The cycle reactivity and redox kinetics under reducing conditions were conducted with and without SO2 in a micro-fluidized bed thermogravimetric analysis technology (MFB-TGA). The redox kinetic behaviors were simulated by a bubbling fluidized bed reactor model coupled with a two-stage kinetic model. The SO2 can react with the perovskite to increase the oxygen transfer capacity from 4 wt% to 5 wt%. When the temperature is higher than 1173 K, SO2 has almost no effect on the H2 reduction reactivity, while the oxidation reactivity decreases by 50%, but the oxidation is still fast enough to achieve 4 wt% capacity within 8 s. When the temperature is lower than 1173 K, there is a significant sulfur-poisoning effect during oxidation and reduction. The analyses of XRD, SEM-EDS, and in-situ DRIFTS indicated that most of the absorbed sulfur mainly existed in the sulfate/sulfide shell on the particle surface. The chemical kinetics and physical structure of CaMn0.375Ti0.5Fe0.125O3-δ perovskite can be completely recovered in the absence of SO2, and this perovskite oxygen carrier is chemically memorable and reversible in its solid structure. The fundamental understanding of the sulfur effect on the redox kinetics and solid structure of the perovskite oxygen carrier provides a new insight to the material development and corresponding reaction mechanisms.acceptedVersio

Industry-scale production of a perovskite oxide as oxygen carrier material in chemical looping

How to upscale the production of oxygen carrier particles from laboratory level to industrial level is still challenging in the field of chemical looping. The upscaled oxygen carrier must maintain its physical and chemical properties. In the present contribution, a spray drying granulation protocol was developed to produce a perovskite oxygen carrier (CaMn0.5Ti0.375Fe0.125O3-δ) at an industrial scale. The micro-fluidized bed thermogravimetric (MFB-TGA) experiments were performed to measure the oxygen uncoupling and redox reaction kinetics under the fluidization state with enhanced heat and mass transfer, and the obtained experimental data at different temperatures were fitted by a fluidized-bed reactor coupled with a semi-empirical kinetic model. The physical and chemical properties of granulates were compared with those of the same perovskite composition prepared at the laboratory level. The results show the volume fraction of particle size at 75–500 μm is greater than 90% for the upscaled granulats, and the particles show no degradation in reactivity and no agglomeration for more than 20 redox cycles at high temperatures. The heterogeneous reaction rates are high, especially for the oxidation, e.g. it only spent ∼ 5 s to achieve full oxidation. Low attrition index of 3.74 wt% was found after the five-hour attrition test. The industrial-scale particles possess similar chemical and physical properties as the laboratory-scale particles with regards to the reaction kinetics, attrition index, crystalline phase, etc. The required bed inventories and fan energy consumption were finally estimated and found to be lower than other oxygen carriers reported in the literature.acceptedVersio

Using potassium catalytic gasification to improve the performance of solid oxide direct carbon fuel cells: Experimental characterization and elementary reaction modeling

The performance of a solid oxide electrolyte direct carbon fuel cell (SO-DCFC) is limited by the slow carbon gasification kinetics at the typical operating temperatures of cell: 650–850 °C. To overcome such limitation, potassium salt is used as a catalyst to speed up the dry carbon gasification reactions, increasing the power density by five-fold at 700–850 °C. The cell performance is shown to be sensitive to the bed temperature, emphasizing the role of gasification rates and that of CO production. Given the finite bed size, the cell performance is time-dependent as the amount of CO available changes. A reduced elementary reaction mechanism for potassium-catalyzed carbon gasification was proposed using kinetic data obtained from the experimental measurements. A comprehensive model including the catalytic gasification reactions and CO electrochemistry is used to examine the impact of the catalytic carbon gasification process on the device performance. The power density is maximum around 50% of the OCV, where carbon utilization is also near maximum. Results show that bed height and porosity impact the power density; a thicker bed maintains the power almost constant for longer times while lower porosity delivers higher power density in the early stages.National Natural Science Foundation (China) (20776078)National Natural Science Foundation (China) (51106085)Low Carbon Energy University Alliance (LCEUA) (Seed Funding

Experimental characterization and elementary reaction modeling of solid oxide electrolyte direct carbon fuel cell

A detailed mechanistic model for solid oxide electrolyte direct carbon fuel cell (SO-DCFC) is developed while considering the thermo-chemical and electrochemical elementary reactions in both the carbon bed and the SOFC, as well as the meso-scale transport processes within the carbon bed and the SOFC electrode porous structures. The model is validated using data from a fixed bed carbon gasification experiment and the SO-DCFC performance testing experiments carried out using different carrier gases and at various temperatures. The analyzes of the experimental and modeling results indicate the strong influence of the carrier gas on the cell performance. The coupling between carbon gasification and electrochemical oxidation on the SO-DCFC performance that results in an unusual transition zone in the cell polarization curve was predicted by the model, and analyzed in detail at the elementary reaction level. We conclude that the carbon bed physical properties such as the bed height, char conversion ratio and fuel utilization, as well as the temperature significantly limit the performance of the SO-DCFC.National Natural Science Foundation (China) (20776078)National Natural Science Foundation (China) (51106085)Low Carbon Energy University Alliance (LCEUA) (Seed Funding

Simulation of Electrochemical Impedance Spectra of Solid Oxide Fuel Cells Using Transient Physical Models

A general electrochemical impedance spectroscopy ͑EIS͒ modeling approach by directly solving a one-dimensional transient model based on physical conservation laws was applied for simulating EIS spectra of an anode-supported solid oxide fuel cell ͑SOFC͒ button cell consisting of Ni-yttria-stabilized zirconia ͉Ni-scandia-stabilized zirconia ͑ScSZ͉͒ScSZ͉lanthanum strontium manganate ͑LSM͒-ScSZ multiple layers. The transient SOFC model has been solved for imposed sinusoidal voltage perturbations at different frequencies. The results have then been transformed into EIS spectra. Six parameters had to be tuned ͑three for the cathode and three for the anode͒ and have been estimated using data from a symmetric cathode cell and from a button cell. The experimental and simulated EIS spectra were in good agreement for a range of temperatures ͑750-850°C͒, of feed compositions ͑mixture of H 2 /H 2 O/N 2 ͒, and of oxidants ͑air and oxygen͒. This approach can help in interpreting EIS spectra, as illustrated by identifying the contribution of transport limitation. Fuel cell electrochemical systems are usually complex and are governed by coupled physicochemical processes such as chemical and electrochemical reactions, charge transport, and mass transport. 1,2 Because polarization curves can only provide a general description of the cell performance, electrochemical impedance spectroscopy ͑EIS͒ has become widely used in fuel cell research and development because it involves a relatively simple electrical measurement that gives detailed information about the fuel cell system, from mass-transport properties, chemical reaction rates, and dielectric properties to defects, microstructure, compositional influences, etc. 3 In this dynamic technique, usually a voltage perturbation is applied to a system and the amplitude and phase shift of the resulting current response are measured. Measurements can be conducted over a wide range of frequencies, resulting in the construction of impedance spectra. 5 Although the approach is useful and quite powerful, it often has limitations such as: 1. The approach can lead to ambiguities in data interpretations because the equivalent circuits are seldom unique except for only the simplest circuits. An equivalent circuit involving several circuit elements could often be rearranged in various configurations while still yielding the same impedance. 2. Detailed physical and chemical processes in the system cannot be predicted by equivalent-circuit models. For instance, the effects of current distributions and concentration distributions cannot be taken into account when interpreting data from equivalent-circuit models. 3. The measured system could only be approximated by circuit elements when assuming linear response of the system. The impedance is supposed to be independent of the amplitude of the applied signals. However, the electrochemical system could be highly nonlinear, especially for sinusoidal perturbations with high amplitudes. It was suggested that nonlinear EIS ͑NLEIS͒ measurements have several potential advantages. To investigate solid oxide fuel cell ͑SOFC͒ electrode reaction kinetics, Miterdorfer and Gauckler 7-9 used a state-space model ͑SSM͒, which is widely used in control theory for solving complex differential equations. Bieberle and Gauckler 5 studied in depth elementary electrochemical reactions in SOFC anode by both experimental and SSM approaches. To simulate the electrochemical impedance spectra, the models were solved directly through the SSM approach. Bessler 10 presented a computational method for simulating EIS spectra based on transient numerical simulations of the reaction system. The impedance was then calculated in the time domain from the simulated periodic response of the system, maintaining its full nonlinear response. This method has been further validated by detailed modeling studies on SOFC EIS spectra achieved from gas-transport processes. 11 Gewies et al. 12 also applied this method on Ni/yttria-stabilized zirconia ͑YSZ͒ cermet anodes. Zhu and Kee 13 developed a time-accurate model to analyze EIS spectra in anode-supported button cells with internal methane reforming. This model represented significant advantages regarding physical conservation laws, porous media transport within the electrode, and heterogeneous chemistry reactions mechanisms, all of those being solved in the time domain. However, the spatial variations of ion and electron transport throughout the electrode structures were not considered. In this paper, a general approach for EIS spectra simulation is applied by solving a comprehensive set of coupled transient models based on physical conservation laws. This simulation approach is illustrated by considering a transient model of an anode-supported SOFC button cell consisting of Ni-YSZ͉Ni-scandia-stabilized zirconia ͑ScSZ͉͒ScSZ͉LSM-ScSZ multiple layers. The simulation results of the EIS spectra were then compared to the measured EIS spectra under various conditions to prove the validity of both the transient model and the EIS simulation approach. Experimental Testing cell.-The anode-supported SOFC button cell used in this study consisted of a Ni/YSZ anode support layer ͑680 m͒, a Ni/ScSZ anode active interlayer ͑15 m͒, a ScSZ thin-film electrolyte layer ͑20 m͒, and a lanthanum strontium manganate ͑LSM͒/ ScSZ cathode layer ͑15 m͒. 14,1

Post-Combustion CO2 Capture Demonstration Using Supported Amine Sorbents: Design and Evaluation of 200 kWth Pilot

CO2 capture using supported amine sorbents is a promising post-combustion capture technology. With regard to supported amine sorbents, the most important issues exist in the regeneration process. In this paper, various regeneration strategies including thermal regeneration in pure CO2 stream, vacuum regeneration, and steam-stripping regeneration are put forward and compared from the aspect of energy consumption. Besides, to ensure the long-term thermal and chemical stability and high working capacity, a kind of supported amine sorbent with the amine groups bonded to the support was tested and employed. As to the regeneration characteristics, the sorbent can be regenerated completely in pure CO2 and has perfect cyclic stability. Finally, one 200 kWth pilot plant was designed and constructed to demonstrate the process feasibility of continuous CO2 capture from flue gases with supported amine sorbents. The selection of the type of absorber and regenerator is discussed in detail. The pilot was evaluated for the air tightness and heat transfer issues

A Multiscale Model of Oxidation Kinetics for Cu-Based Oxygen Carrier in Chemical Looping with Oxygen Uncoupling

Copper oxide is one of the promising oxygen carrier materials in chemical looping with oxygen uncoupling (CLOU) technology, cycling between Cu2O and CuO. In this study, a multiscale model was developed to describe the oxidation kinetics of the Cu-based oxygen carrier particle with oxygen, including surface, grain, and particle scale. It was considered that the solid product grows with the morphology of disperse islands on the grain surface, and O2 contacts with two different kinds of grain surfaces in the grain scale model, that is, Cu2O surface (solid reactant surface) and CuO surface (solid product surface). The two-stage behavior of the oxidation reaction of the Cu-based oxygen carrier was predicted successfully using the developed model, and the model results showed good agreement with experimental data in the literature. The effects of oxygen partial pressure, temperature, and particle structure on the oxidation performance were analyzed. The modeling results indicated that the transition of the conversion curve occurs when product islands cover most part of the grain surface. The oxygen partial pressure and particle structure have an obvious influence on the duration time of the fast reaction stage. Furthermore, the influence of the external mass transfer and the change of effectiveness factor during the oxidation reaction process were discussed to investigate the controlling step of the reaction. It was concluded that the external mass transfer step hardly affects the reaction performance under the particle sizes normally used in CLOU. The value of the effectiveness factor increases as the reaction goes by, which means the chemical reaction resistance at grain scale increases resulting from the growing number of product islands on the grain surface

Potassium-Promoted γ‑Alumina Adsorbent from K₂CO₃ Coagulated Alumina Sol for Warm Gas Carbon Dioxide Separation

This paper provided a practical synthesis method of K-promoted γ-alumina for CO₂ precombustion capture integrating adsorbent pelletting and catalytic active component promotion into one single process. The precursor of the absorbent, K-promoted pseudo-boehmite (PB), was synthesized from alumina sol and K₂CO₃ powder by mixing, aggregating, drying, and aging. Potassium-promoted γ-alumina was obtained after calcination at 400 °C from K-promoted PB. Characteristic crystallines of several stages of products in the synthesis process have been detected by X-ray diffraction. The optimal Al:K ratio in view of CO₂ capacity and cyclic performance of K-promoted γ-alumina was obtained by a thermal gravimetric analyzer. The optimal ratio of K-promoted γ-alumina showed a capacity of 0.67 mmol/g (dry base) for the first cycle and 0.34 mmol/g (wet base) after 30 pressure swing adsorption (PSA) cycles at 300 °C. K-promoted γ-alumina adsorbent could be favorable for a low energy penalty precombustion CO₂ capture application. It showed a variety of advantages such as low adsorption heat, good mechanical strength, low cost in synthesis, and fair stable capacity