Reduced Oxygen Carrier Research Articles

Novel core shell structure to preclude phase segregation of iron-based oxygen carriers for chemical looping combustion

Iron-based oxygen carriers as one of the most commonly used oxygen carriers in chemical looping combustion, shows poor cycling stability. In this work, spinel-structured, Fe2O3@MgO core–shell structure, and Fe2O3/CuO@MgO core–shell structure oxygen carriers were prepared using compression molding. The effects of deep reduction and oxidative regeneration processes on the cyclic stability of oxygen carriers with different structures were investigated using thermal gravimetric analyzer. The mechanisms for the changes in the cyclic stability were explored through a series of characterization analyses. The results showed that the reduction process of spinel oxygen carriers generated Fe-Mg solid solution, which was unfavorable to deep reduction, and phase separation occurred during 5 cycles. Due to the migration of Fe3+ ions to the surface, surface was seriously sintered, deteriorating the cyclic stability of spinel oxygen carriers. For Fe2O3@MgO oxygen carrier, the deep reduction can be realized, but the interdiffusion phenomenon of Fe2+ and Mg2+ occurred after 5 reductions, and the generation of (MgO)x(FeO)1−x solid solution impeded the deep reduction. The addition of CuO in the core of oxygen carriers to form Fe2O3/CuO@MgO could inhibit the interdiffusion of Fe2+ and Mg2+, and maintained the oxygen carrying capacity up to more than 95 % in 10 cycles. Meanwhile, CuO synergized with Fe2O3 to enhance the reduction and oxidation reactivity of the oxygen carrier. On this basis, the ion diffusion mechanism during the deep reduction and oxidation of iron-based oxygen carriers with different structures was proposed. This study provides guidance for the modification of iron-based oxygen carriers with superior cycling stability.

Study on the splitting mechanism of H2O/CO2 on the surface of reduced CaFe2O4

In the process of biomass chemical looping gasification using calcium ferrite as an oxygen carrier, H2O or CO2 can be injected into the oxidation reactor to produce H2 or CO with high purity while oxidizing the reduced oxygen carrier, which is a new technology that has attracted wide attention in recent years. However, the splitting reaction mechanism of H2O or CO2 is still unclear. Therefore, based on density functional theory (DFT) calculations, the adsorption behavior of H2O and CO2, the electronic structure, and the splitting reaction path of H2O and CO2 on the surface of the reduced CaFe2O4 were systematically studied to deeply understand the reaction mechanism. The results show that H2O and CO2 form stable complexes on the CaO(111)/Fe(110) surface by chemisorption. H2O is splitted to H2 and adsorbed O* through a two-step reaction. In this process, the step of H2O cracking to HO* and H* is the rate-determining step. CO2 is splitted into CO* and O* through a one-step reaction, in which the desorption of CO from the surface is a rate-determining step. The total charges of H2O and CO2 after adsorption are –0.313 |e| and –1.086 |e|, respectively, and the total charges of Fe atoms on the surface are 0.444 |e| and 0.265 |e|, respectively. H2O and CO2 are electron acceptors, and Fe atoms on the surface are electron donors. The splitting of H2O or CO2 and the oxidation of reduced oxygen carriers are generally exothermic reactions, which can realize the oxidation of the reduced oxygen carrier and provide heat for the system.

Multiscale CFD modelling of syngas-based chemical looping combustion in a packed bed reactor with dynamic gas switching technology

Chemical looping combustion (CLC) is a highly efficient energy conversion technology designed for burning fossil fuel with inherent carbon capture potential. The main advantage of running the CLC process in a packed bed configuration over the conventional circulating fluidized bed setup is the ability to operate at high pressure. In this work, a dynamic multiscale model was developed to simulate packed bed syngas CLC with ilmenite oxygen carrier (OC) particles, subsequently validating simulation results in terms of fuel and OC conversion, and temperature distribution using literature data for both oxidation and reduction steps. The model was used for investigation of multiple cycle operation, intraparticle fuel and OC concentration distributions, as well as effects of particle sizes and flow rates during OC reduction. Results revealed pressure drop reductions by 45% for 50% larger particle sizes and 66% for 50% lower flow rates, as well as better bed utilization for lower particle sizes and flow rates based on fuel breakthrough curves. In addition, a CFD multilayer particle model was developed to strengthen multiscale model results on pressure drops and mass transfer efficiency, and study temperature distribution with respect to particle dimensions during OC oxidation. Results indicated a positive effect on energy efficiency for smaller particles due to higher packing density and particle surface area, reaching intraparticle temperatures of 1110 ℃ and exit gas temperatures of 980 ℃ (i.e., 13% higher than base case size) during OC oxidation.

Hydrogen production and CO2 capture from Linz-Donawitz converter gas via a chemical looping concept

Linz-Donawitz converter gas (LDG), an important by-product in the steel industry with CO (55–60 vol%) and CO2 (15–20 vol%) as the main components, is usually used as fuel for heating or power generation, releasing massive of diluted CO2 with the concentration lower than 15 % that is difficult to capture. Herein, we propose a method to produce pure H2 via chemical looping water splitting by using LDG as reducing agent, which can easily concentrate CO2 for capture. Fe2O3, one of the most environmentally friendly and low-cost oxides, was chosen as the oxygen carrier to perform this concept. It was found that the addition of small amounts of Ce0.75Zr0.25O2 solid solution could strongly improve the activity of Fe2O3 for CO conversion, especially at relatively low temperatures. Compared with pure Fe2O3, the CO conversion and H2 yield were increased from 39.5 % and 0.34 mmol·g−1 to 100 % and 2.32 mmol·g−1 at 650 °C, respectively, after adding 5 wt% of Ce0.75Zr0.25O2. The strong interaction between the two oxides contributed to the enhanced performance via promoting CO adsorption and lattice oxygen releasing. Water splitting assisted by air oxidation could fully regenerate the reduced oxygen carrier, and the oxygen carrier was stable during the long-term chemical looping process, with the CO2 capture rate at ca. 95.8 %. In situ DRIFTS results showed that CO2 was producedthrough a direct reaction of lattice oxygen with CO, and the decomposition of carbonate species intermediate by adsorption of CO on the surface of oxygen carriers. This study gave full evidence that it was feasible for achieving H2 generation and CO2 capture in one step via chemical looping reforming of reducing exhaust gases (e.g., LDG) from industrial emissions.

Spatial migration of lattice oxygen for copper-iron based oxygen carriers in chemical looping combustion

Oxygen carrier (OC) is of great importance in chemical looping combustion (CLC) technology. The migration rate of bulk lattice oxygen during reduction process is detrimental in reaction rate. However, the migration path of lattice oxygen is still not clear. In this study, a thermogravimetric analyzer (TGA) was used to investigate the reaction characteristics and kinetic parameters of the OC. The oxygen migration paths of CuO and CuFe2O4 bulk lattices were studied by density functional theory (DFT) in a five-layer (2 × 1) CuO (1 1 1) and nine-layer (1 × 1) CuFe2O4 (1 0 0) plane model. The kinetic parameters, activation energy changes and relative kinetic model of the OC reduction were explored. According to the DFT results, the deeper bulk phase oxygen requires greater more energy toovercome energy barrier for migration. CuO has a lower oxygen release capacity than CuFe2O4 in the early stages, but higher in the later stage. The reaction kinetics study revealed that the reaction mechanism of the reduction of the copper-iron composite OC is a tertiary chemical reaction control mechanism. During the reaction, the activation energy of the OC changes continuously. The macroscopic reaction kinetics agree with the microscopic simulation, proving the simulation model's validity.

Sustainable assessment of an integrated energy system coupled with solar thermochemical cycle

The energy transition globally makes the development of carbon-free and low-carbon fuels obligatory as these fuels cannot only be utilized as energy carriers but also for long-term energy storage. This paper presents the energy, technoeconomic, and environmental assessment of a new hybrid renewable energy integrated system with a novel concept of the thermochemical cycle for methanol production. For this purpose, a thermochemical cycle based on CeO2/Ce2O3 oxides is integrated with the methanol synthesis system. It consists of two chemical steps: (1) oxygen carrier reduction step where metal oxide reduction Ce (IV) to Ce (III) (endothermic phase) is performed employing renewable thermal energy and chemical energy originated from ultra-high concentrated photovoltaics and fossil methane respectively and (2) waste gas dissociation step where oxidation of Ce(III) takes place with dissociation of water and carbon dioxide (exothermic phase) and resulting syngas is converted to methanol. Plant design and simulation are performed in ASPEN plus commercial software. MATLAB was employed for the programming of thermal data to perform pinch analysis. The technical feasibility of the second step was investigated comprehensively in terms of temperature, input molar ratio and the potential operating conditions that were identified (TOXI = 900 °C and H2O/CO2 = 3.7) based on methanol production performance. Overall energy efficiency, solar-to-fuel efficiency and thermal efficiency are calculated to be 77.2%, 38% and 44.1% respectively. The findings of the environmental analysis show that carbon utilization of 20.85%, carbon storage of 79.14%, carbon capture of 100% and carbon avoidance of 19.6% are achieved. Economic analysis shows that the net project capital expenditure (CAPEX) and operating expenditure (OPEX) are estimated to be $186.49M and $218.67M. It was found that the best-case scenario of natural gas (NG) price is 40$/MWh, where the breakeven point is achieved during the 5th year of the project lifetime.

Low-temperature chemical looping oxidation of hydrogen for space heating

Chemical looping combustion (CLC) is an advanced combustion process in which the combustion reaction splits into two parts; in the first reaction metal oxides are used as oxygen suppliers for fuel combustion and then in the second reaction, reduced metal oxides are re-oxidised in an air reactor. Although this technology could be applicable for the safe implication of “low-temperature oxidation of hydrogen”, there is limited understanding of oxygen carrier reduction stages and the oxidation mechanism of hydrogen throughout the process. The novelty of this research lies in its pioneering investigation of low-temperature oxidation of hydrogen through chemical looping technology as a safe and alternative heating system, using three distinct metal oxide oxygen carriers: CuO, Co3O4, and Mn2O3. The oxidation of hydrogen over these oxygen carriers was comprehensively studied in a fixed-bed reactor operating at 200–450 °C. XRD analysis demonstrates that CuO directly reduced to metallic Cu at 200–450 °C, instead of following a sequential reduction step CuO→Cu4O3→Cu2O→Cu throughout the temperature. Co3O4 was reduced to a mixture CoO and Co at 450 °C, which may refer to a sequential reduction step Co3O4→CoO→Co with increasing the temperature. Decreasing the reduction temperature led to an elevation in CoO formation. Mn2O3 can also reduce to a mixture of Mn3O4 and MnO at temperatures between 250 and 400 °C. Compared to temperature, the increase in the residence time did not show any further reduction in Mn2O3. SEM results showed that most of the metal oxide particles were evenly dispersed on the supports. Based on the experimental results, a potential reduction stage of CuO, Co3O4 and Mn2O3 was proposed for low-temperature hydrogen oxidation, which could be a potential application for space heating using safe hydrogen combustion.

TGA Study of the Reaction Kinetics of a CuFeMnAlO4+δ Oxygen Carrier for Methane Chemical Looping Combustion under High Pressure and Atmospheric Pressure Conditions

Reduction and oxidation kinetics of a CuFeMnAlO4+δ oxygen carrier for methane were studied using an atmospheric thermogravimetric analyzer (TGA) and a pressurized TGA. Important parameters such as temperature (500–900 °C), particle size distribution (150–650 μm), and partial pressures of reactants (methane and oxygen) were evaluated. Results showed that the reduction of the oxygen carrier was not affected by particle size distribution studied in this work, and that above 750 °C, the effect of the temperature on oxygen carrier reduction started to diminish. The reaction time approaches an asymptotic limit as methane partial pressure increases. For oxygen carrier oxidation, it was found that the rate of oxidation increased with increasing oxygen concentration. The effect of the oxygen concentration was more noticeable when the oxygen concentration was below 7.5%. Because of the inherent feature of the oxygen carrier, its chemical looping with oxygen uncoupling property was also assessed. A phase boundary-controlled shrinking sphere model was used to model the test data from the TGAs. Modeling results showed that the oxygen carrier reduction had a reaction order of 1 with an activation energy of ∼46 kJ/mol. Oxygen carrier oxidation demonstrated a reaction order of 1.1 with an activation energy of ∼55 kJ/mol. Additionally, the differences of the experimental results between the atmospheric pressure TGA and the pressurized TGA were discussed in detail and the limitations of the pressurized TGA were highlighted.

Fe-based oxygen carrier for the chemical looping combustion of CO, H2, and CH4 syngas in fluidized bed reactor under interruption of H2S

Chemical looping combustion (CLC) reduces the CO2 capturing cost, elevates combustion efficiency, and prevents NOx formation. This study applied Fe2O3/SiO2 and Fe2O3/Al2O3 oxygen carriers to react with H2, CO and CH4 under different operating setup and examined the interference of H2S. Examination was also conducted on the reaction kinetics, cycle test, and characterization of oxygen carriers. Reduction of oxygen carrier from Fe2O3 to between FeO and Fe occurred during the reaction. The higher syngas concentration led to higher diffusion capacity and promoted utilization of oxygen carriers. Carbon deposition was enhanced by excess concentration of CO and CH4. Involvement of H2S led to declining of performance due to the presence of carbonyl sulfide (COS) and FeS. With Fe2O3/SiO2, application of multiple redox cycles reduced the utilization value to 43 % due to formation of Fe2SiO4. Analysis of exhausted gas after reaction was applied in study of proposed reaction mechanism. Study of reaction kinetics using Type I deactivation test indicated a well fitted result with experimental data with R2 in the range of 0.9605–0.9976. Thus, compared to other studies with discussion only on CO, H2, or CH4 in CLC, this study offered a comprehensive research on interruption of H2S, characterization, and reaction kinetics.

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.

Interactions between potassium ashes and oxygen carriers based on natural and waste materials at different initial oxidation states

AbstractOne of the most essential features of an oxygen carrier is its ability to be oxidized and reduced in order to transfer oxygen in a chemical looping system. A highly reduced oxygen carrier can experience multiple performance issues, such as decreased reactivity, agglomeration, and defluidization. This is crucial for processes that require limited oxygen transfer from the air reactor to the fuel reactor. Meanwhile, biomasses as environmentally friendly fuel options contain ashes, which would inevitably react with oxygen carriers and exacerbate the performance issues. To mimic the interactions between a highly reduced oxygen carrier and biomass ash compounds, four iron‐based oxygen carriers, based on natural ores and waste materials, and three potassium salts, K2CO3, KH2PO4, and K2SO4, were investigated in a tubular reactor under an atmosphere consisting of 2.5% H2 and 10% steam in Ar and N2 at 900°C for 3 h. The results from the X‐ray diffraction (XRD) material analysis showed that both initially fully oxidized and highly reduced materials reach the same oxidation state after the experiment. Based on the scanning electron microscopy coupled with energy dispersive X‐ray spectroscopy results, K from K2CO3 and K2SO4 diffuses in the oxygen carrier particles, while K from KH2PO4 always forms a distinct layer around the particles. The initial oxidation state of an oxygen carrier surface affects the interactions with the potassium salt only to minor extents. Thus, the final state of the material and its performance in a large‐scale process are only occasionally and mildly affected by its initial oxidation state. © 2023 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Experimental and simulation investigation on acetone deoxygenation with Ce/Fe-based oxygen carrier

Bio-oil is a renewable energy and a promising alternative to oil, but its disadvantages such as high oxygen content, low higher heating value (HHV), and instability are needed upgrading. For ketones account for a relatively large proportion of bio-oil, acetone is chosen as a single model compound for hydrodeoxygenation (HDO) in this work. The acetone HDO investigation was carried out in a high pressure reactor, using the reduced Fe-based oxygen carrier1 (OC1) and Ce-doping reduced Fe/Al oxygen carrier2 (OC2). The OCs’ evolution was analyzed by XRD, and the liquid phase products were tested by GC–MS. The results showed the OCs played the dual role of carrying oxygen and catalysis and the HDO capability of OC2 on acetone was better than that of OC1. In addition, two molecular structure models of OC1* (reduced iron-based oxygen carrier) and OC2* (Ce-modified Fe/Al reduced oxygen carrier) were established, and the acetone adsorption on OCs* and the H2O dissociation process were simulated with density functional theory (DFT). The analysis showed that the oxygen vacancies generated by Ce-doping and the Al2O3 carrier changed the optimal adsorption sites of acetone and H2O, then promoted the acetone adsorption and H2O dissociation reaction, improved the OCs performance, verifying that OC2 had better HDO performance on acetone. This work provides guidance for the improvement of crude bio-oil upgrading.

High purity hydrogen production via coupling CO2 reforming of biomass-derived gas and chemical looping water splitting

Hydrogen is playing an increasingly larger role in carbon neutrality. However, efficient and sustainable hydrogen production from renewable resources suffers from unwanted side reactions and kinetic inefficiencies. Here, we study a low carbon and renewable H2 production route that integrated biomass-derived gas dry reforming couples chemical looping water splitting process (DR-CLWS). This process requires neither removal of CO2 from upstream biomass-derived gas, nor downstream H2 purification and CO2 separation, hence achieving a great level of process intensification. The reforming temperature, CH4/CO2 ratio, oxygen carrier reduction temperature, reduction behavior, types of fuel gas and oxygen carriers, H2 production, oxygen carrier regeneration and process cyclability were investigated, respectively. The coupling process resulted in higher CH4 conversion (near 100%) and inherently CO2 separation. CO-free H2 was produced due to no carbon being carried over from the reduction cycle to the H2 production cycle. Meanwhile, H2 yields up to 2.93 mmole per mmole CH4. Mass balance analysis reveals that the process can be operated auto-thermal without costly oxygen production or without mixing air with carbon-containing fuel gases. Moreover, the cyclability results showed that the oxygen carrier remained its activity over 265 redox cycles.

Potassium occurrence form in the deep reduction of potassium-modified iron-based oxygen carrier by coal char

Coal-direct chemical looping hydrogen generation (CLHG) is a new hydrogen production technology using coal as the raw material. A potassium-modified iron-based composite oxygen carrier (OC) increases the low reaction rate of coal char and OC in the deep reduction of the OC. However, potassium loss resulting from potassium migration and transformation hinders the application of complex OCs in this process. This study investigates the influence law of reaction atmosphere on the potassium occurrence form in the deep reduction of potassium-modified iron-based OC by coal char. Simulation calculations are employed to examine the effect of reaction temperature, fuel amount, and iron oxide reduction degree on the potassium occurrence form. It is found that the potassium migration form in the reduction process is mainly K2O. A relatively high temperature will accelerate the potassium migration, and potassium concentrates in the parts with a high lattice oxygen content. Fe2O3 and CO2 restrict potassium migration, while CO and C have the opposite effect. According to the simulation, without an inert carrier, volatile potassium gradually increases with the increase in the molar ratio of C/Fe2O3, reaching a maximum value under the molar ratio of 1.2 before a slight decrease. Under the C/Fe2O3 molar ratio of below 0.3, potassium mainly exists in the form of KFeO2 with no volatile potassium production. Thereafter, as the C/Fe2O3 molar ratio increases, KFeO2 slightly decreases, and 3.4% volatile potassium is produced; however, most potassium is maintained as KFeO2. As iron oxide has a relatively high reduction degree, there is more volatile potassium; however, the probability of potassium loss is considerably high.

Thermogravimetric kinetic analysis of the Ce‐Zr composite oxygen carrier prepared by ball milling

AbstractA series of zirconium‐doped cerium‐based composite oxygen carriers were prepared using the ball milling method. The results from X‐ray powder diffraction (XRD), H2‐temperature programmed reduction experiments (H2‐TPR), and Brunauer–Emmett–Teller (BET) show that a solid solution, formed from CeO2 and ZrO2, possesses a mesoporous structure and exhibits excellent reaction performance. In addition, carbon nanotubes (CNTs) with an addition amount of 5 wt.% greatly increased the specific surface area of oxygen carriers, thereby improving the activity of solid solutions. In order to further understand the performance of oxygen carriers, the kinetics of H2 reduction of oxygen carriers was studied by thermogravimetric analysis (TGA). The three‐dimensional diffusion model showed the best fitting effect among four typical models (viz., the diffusion‐controlled model, the unreacted shrinking core model, the uniform model, and the nucleation‐nuclei growth model). Finally, the activation energies of CeO2, Ce9Zr1Oδ, and 5 wt.% CNTs Ce9Zr1Oδ were 223.92, 170.96, and 128.60 kJ/mol, respectively, which also correspond with the reactivity sequence of the oxygen carriers.

Hydrogen and power co-production from autothermal biomass sorption enhanced chemical looping gasification: Thermodynamic modeling and comparative study

Calcium looping and chemical looping technologies envisage advanced solutions for H2 production. This study compared the H2 and power co-production from biomass sorption enhanced gasification (SEG) and sorption enhanced chemical looping gasification (SECLG) under autothermal conditions with thermodynamic simulation. The thermal self-sufficiency of calcination was achieved by splitting biomass for combustion and oxidizing the reduced oxygen carrier, respectively. It was found that SECLG was able to achieve higher energy efficiency (64.6%) than SEG (55.7%) at the optimized carbonator temperature. In both processes, parametric analysis illustrates that under the autothermal-available carbonator temperature range, higher carbonator temperature and fixed carbon conversion are recommended to achieve higher H2 yields and energy efficiencies, owing to lower energy penalty leading to lower requirement of combusted biomass content or Ni/C molar ratio for thermal self-sufficiency. Elevating carbonator pressure slightly improved the energy performance. Regarding CO2 capture through oxyfuel combustion in SEG and SECLG processes, the energy penalty from higher calcination temperature greatly degraded the energy performance, which was even higher than the power consumption of air separation unit. According to exergy analysis, the main exergy destruction occurred at the syngas production section (∼49%). From the view of energy performance, SECLG is a promising autothermal strategy for SEG upgrade.

Development of a novel carbon capture and utilization approach for syngas production based on a chemical looping cycle

The present work assesses the potential of reducing CO2 emissions associated with steel production through the introduction of a decarbonization process downstream of a steel mill eventually producing an alternative fuel/syngas. The analysed system is composed of a calcium looping process for CO2 separation followed by a chemical looping section for syngas production from CO2 and H2Os. The main units in the chemical looping cycle are: the oxidizer, where a flux of CO2 and H2Os reacts with an oxygen carrier to produce CO and H2; the air reactor, where the oxidation of the oxygen carrier is completed by the interaction with air; the reducer, where the reduced oxygen carrier is regenerated to the initial state (Fe2O3 or NiFe2O4 in the present case) through an endothermic reaction occurring at high temperatures. A MATLAB model was created to determine the molar flow rate of the components flowing through the thermochemical cycle and the thermal power associated with each unit at the operating conditions. The analysis is carried out focusing on the treatment of 1t/h of CO2, resulting in 7.1t/h of NiFe2O4 or 12.1t/h of Fe2O3. The syngas at the outlet from the oxidizer reactor is composed of equimolar H2 and CO with a mass flow rate of 0.05t/h and 0.64t/h, respectively. A separate MATLAB model was developed to identify the experimental conditions necessary to reach fluidization of FeO particles in a lab-scale oxidizer reactor (umf=0.162m/s). Companion CFD simulations were carried out to evaluate the hydrodynamics of the lab-scale oxidizer reactor and the associated reaction kinetics (Langmuir-Hishelwood) above minimum fluidization conditions with the aim of assessing the assumptions performed in the MATLAB in terms of conversion rates. For the imposed inlet velocity conditions of the gas mixture (2.6 times above the minimum fluidization velocity) large bubbles with low frequency are observed, while full consumption of the reactant gases is achieved during the first 15 s of simulation, due to the significant reaction rate (2.6kmol/sm3). The results of the CFD simulation and the comparison with existing literature allow to validate the assumptions on the oxidizer conversion and the overall accuracy of the model.

Effect of the Mass Conversion Degree of an Oxygen Carrier on Char Conversion and Its Implication for Chemical Looping Gasification.

Chemical looping gasification (CLG) is an emerging process that aims to produce valuable chemical feedstocks. The key operational requirement of CLG is to limit the oxygen transfer from the air reactor (AR) to the fuel reactor (FR). This can be accomplished by partially oxidizing the oxygen carrier in the AR, which may lead to a higher reduction degree of the oxygen carrier under the fuel conversion. A highly reduced oxygen carrier may experience multiple issues, such as agglomeration and defluidization. Given such an interest, this study examined how the variation of the mass conversion degree of ilmenite may affect the conversion of pine forest residue char in a fluidized bed batch reactor. Ilmenite was pre-reduced using diluted CO and then underwent the char conversion at 850, 900, 950, and 975 °C. Our investigations showed that the activation energy of the char conversion was between 194 and 256 kJ/mol, depending upon the mass conversion degree of ilmenite. Furthermore, the hydrogen partial pressure in the particle bed increased as the oxygen carrier mass conversion degree decreased, which was accompanied by a lower reaction rate and a higher reduction potential. Such a hydrogen inhibition effect was confirmed in the experiments; therefore, the change in the mass conversion degree indirectly affected the char conversion. Langmuir–Hinshelwood mechanism models used to evaluate the char conversion were validated. On the basis of the physical observation and characterizations, the use of ilmenite in CLG with biomass char as fuel will likely not suffer from major agglomeration or fluidization issues.

Parameter optimization and exergy efficiency analysis of petroleum coke-to-hydrogen by chemical looping water splitting process with CO2 capture and self-heating

The petroleum coke-to-hydrogen process is promising since the output of high-sulfur petroleum coke is increasing year by year, the pollution emissions of its combustion for power generation are serious, and the refinery needs a lot of hydrogen. Therefore, this study establishes the process model and conduct the parameter optimization of the petroleum coke direct chemical looping hydrogen production process. The optimized mass ratios of oxygen carrier (Fe2O3), steam, and air to petroleum coke are 12.71, 2.98, and 2.18; however, that are 12.71, 1.79–2.08, and 4.53–5.32 when the proportion of the reduced oxygen carrier entering steam reactor is reduced from 100% to 60–70%, which can be operated in a self-heating mode with air preheating temperature of 278.54–816.80 ℃. When the proportion is increased from 60% to 65.32% and 70%, the product exergy of hydrogen is increased from 454.58 MW to 494.92–530.36 MW with about 100% CO2 capture and the exergy efficiency is therefore increased from 59.56 to 60.75% to 62.32–63.59% and 62.65–63.91%. This paper also studies the sulfur distribution and existing forms of sulfides. Finally, this study found that the exergy efficiency of the chemical looping hydrogen process with the proportion of 65–70% is significantly higher than that of the hydrogen production process by petroleum coke gasification.

Kinetic model development and Bayesian uncertainty quantification for the complete reduction of Fe-based oxygen carriers with CH4, CO, and H2 for chemical looping combustion

Three kinetic models are developed and calibrated for the complete multi-step reduction of an Fe-based oxygen carrier (OC) particle with CH4, CO, and H2, using data from thermogravimetric analysis. The complete reduction rate profiles exhibit complex dynamics whose trajectory is significantly different depending on the reducing gas. A Bayesian model building and parameter estimation framework is applied for simultaneous parameter and model structure uncertainty quantification. The final models show excellent agreement between model predictions and calibration data, as well as new data not used for calibration (for the reduction of the OC with HC4). Parameter uncertainty is quantified by determining their joint posterior distribution, and model structure uncertainty is addressed by incorporating Gaussian process stochastic functions (represented by Bayesian smoothing splines) into the kinetic models. The final kinetic models with discrepancy functions are readily employable in equation-oriented simulation and optimization platforms.