Production and examination of oxygen‐carrier materials based on manganese ores and Ca(OH)2 in chemical looping with oxygen uncoupling

This study concerns production of oxygen‐carrier particles using six different manganese ores. The ores were made to react with Ca(OH)₂ at elevated temperature, forming calcium manganite. The method utilized to manufacture particles was extrusion. Methane and syngas conversion and oxygen release of the samples in inert atmosphere were investigated. The oxygen carrier based on South African (B) manganese ore, showed good methane conversion and was able to transfer oxygen corresponding to 1.5% of its mass during reduction with gaseous fuel. All examined oxygen carriers were capable of converting syngas completely. The ability to release gaseous oxygen was examined by adding wood char in a stream of nitrogen for four selected samples sintered at 1300°C/6 h. These samples released an amount of oxygen corresponding to 0.37–0.68% of their mass. The reactivity of all the ores was improved after the proposed treatments. Reactivity results of the oxygen carrier made from South African (B) ore and Ca(OH)₂, sintered at 1300°C for 6 h were the most promising. Attrition measurements with a jet cup of the oxygen carriers sintered at 1300°C/6 h showed that all the samples made from ores were at least three times more resistant to mechanical attrition compared to particles made from synthetic Mn₂O₃. Producing feasible oxygen carriers directly from ores could potentially cut the cost of chemical looping with oxygen uncoupling and have a significant impact on its competitiveness among other carbon capture technologies. © 2013 American Institute of Chemical Engineers AIChE J 60: 645–656, 2014     

Fe2O3 on Ce‐, Ca‐, or Mg‐stabilized ZrO2 as oxygen carrier for chemical‐looping combustion using NiO as additive

Oxygen‐carrier particles for chemical‐looping combustion have been manufactured by freeze granulation. The particles consisted of 60 wt % Fe₂O₃ as active phase and 40 wt % stabilized ZrO₂ as support material. Ce, Ca, or Mg was used to stabilize the ZrO₂. The hardness and porosity of the particles were altered by varying the sintering temperature. The oxygen carriers were examined by redox experiments in a batch fluidized‐bed reactor at 800–950°C, using CH₄ as fuel. The experiments showed good reactivity between the particles and CH₄. NiO was used as an additive and was found to reduce the fraction of unconverted CH₄ with up to 80%. The combustion efficiency was 95.9% at best and was achieved using 57 kg oxygen carrier per MW fuel. Most produced oxygen carriers appear to have been decently stable, but using Ca as stabilizer resulting in uneven results. Further, particles sintered at high temperatures had a tendency to defluidize. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Multicycle investigation of a sol‐gel‐derived Fe2O3/ATP oxygen carrier for coal chemical looping combustion

Fe‐based oxygen‐carrier particles with attapulgite (ATP) as a support material for coal chemical looping combustion (CLC) have been prepared using a sol‐gel approach. The multiredox characteristics of the prepared Fe4ATP6 (Fe₂O₃ to ATP mass ratio of 40:60) were experimentally examined in a batch fluidized bed reactor at 900°C. The experimental results indicated that the synergistic reactions between ATP and Fe₂O₃ increased the coal conversion. Fe4ATP6 exhibited high reactivity, particularly for low‐rank coals, in the CLC process. The improved pore structure and surface area were responsible for the high reactivity of Fe4ATP6. In 60 redox cycles, H₂ was mainly generated in the outlet gas as the carbon conversion efficiency had reached 95%, and both the coal combustion efficiency and CO₂ capture efficiency were greater than 95%. © 2015 American Institute of Chemical Engineers AIChE J, 62: 996–1006, 2016     

The reaction of NiO/NiAl2O4 particles with alternating methane and oxygen

The reduction and oxidation behaviour of oxygen carrier particles of NiO and NiAl₂O₄ has been investigated in a fluidized bed reactor as well as a thermogravimetric analyzer (TGA). The particles showed high reactivity and gas yield to CO₂ with methane in the temperature interval 750–950°C. In the fluidized bed the yield to CO₂ was between 90 and 99% using bed masses corresponding to 16–57 kg/MW_fuel. Complementary experiments in a TGA at 750 and 950°C showed a clear reaction of the NiAl₂O₄ with CH₄ at the higher temperature. There was methane released from the reactor at high degrees of solid oxidation, which is likely associated with the lack of Ni‐sites on the particles which can reform the methane. There was some carbon formation during the reduction, although the amount was minor when the gas yield to carbon dioxide and degree of oxidation of the solid was high. A simple reactor model using kinetic data from a previous study predicted the gas yield during the reduction in the fluidized bed experiments with reasonable accuracy. The oxygen carrier system investigated in this work shows high promise for use in a real CLC system, provided that the particle manufacturing process can be scaled up with reasonable cost.     

Fe‐substituted Ba‐hexaaluminates oxygen carrier for carbon dioxide capture by chemical looping combustion of methane

Fe‐substituted Ba‐hexaaluninates (BFA‐x (x = 1–3), x indicates Fe content) oxygen carrier (OC) were found to exhibit excellent sintering‐resistance under cyclic redox atmosphere at 800°C thanks to the reservations of the structure during the CH₄ reduction step, thus preventing the agglomeration of particles during the subsequent reoxidation step. Lattice oxygen highly active for the total combustion of CH₄ was observed in the hexaaluminate structure and its chemical state was influenced by Fe content. The highest amount of active O coordinated with Fe³⁺ in the mirror plane (O‐Fe³⁺(M)) for the total combustion was reacted (0.77 mmol/g) for BaFe₃Al₉O₁₉ hexaaluminate OC. As a result, it exhibited the best reactivity with the CH₄ conversion of 83% and CO₂ selectivity of 100%. Moreover, superior regeneration and recyclability was also obtained, which originated from the fully recovery of O‐Fe³⁺(M) in the hexaaluminate structure. © 2015 American Institute of Chemical Engineers AIChE J, 62: 792–801, 2016     

High performance of la‐promoted Fe2O3/α‐Al2O3 oxygen carrier for chemical looping combustion

Iron oxide supported oxygen carrier (OC) is regarded to a promising candidate for chemical looping combustion (CLC). However, phase separation between Fe₂O₃ and supports often occurs resulted from the severe sintering of supports during calcination, which leads to the sintering and breakage of Fe₂O₃ thus the decrease of redox reactivity. In this article, La‐promoted Fe₂O₃/α‐Al₂O₃ were used as OCs for CLC of CH₄ and for the first time found that the OC with the addition of 18 wt % La exhibited outstanding reactivity and redox stability during 50 cycles of CLC of CH₄. Such a superior performance originated from the formation of LaAl₁₂O₁₉ hexaaluminate (La‐HA) phase with not only small particle size but also excellent thermal stability at CLC conditions, which worked as a binder to prevent the phase separation thereby the sintering and breakage of active species α‐Fe₂O₃ were avoided during reaction. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2827–2838, 2017     

Chemical looping combustion characteristics and kinetic behaviour of Sr-doped perovskite-type CaFeO3 oxygen carriers: Theoretical and experimental investigations

Oxygen carriers (OCs) with typical perovskite structures have attracted attention for use in chemical looping combustion (CLC) owing to their unique tunable structures and excellent performance. Thus, a further improvement in the reactivity and a deep understanding of the kinetic behaviour in CLC are highly desirable for such perovskite OCs. In this study, a series of Sr-doped perovskite-structured CaFeO₃ OCs (denoted as Sr_xCa_1−xFeO₃) were synthesized. The CLC characteristics, kinetic behaviour, and doping mechanism were systematically investigated via experiments and density functional theory (DFT) calculations. The activation energies of Sr_xCa_1−xFeO₃ OCs with various Sr contents were found to be in the range of 36.6–40.1 kJ/mol and lower than that of CaFeO₃ (62.7 kJ/mol), indicating that the Sr doping enhanced the reactivity of CaFeO₃. Among the OCs, Sr_0.4Ca_0.6FeO₃, which had the lowest activation energy and the fastest release of lattice oxygen, was regarded as the optimum OC. DFT calculations indicated that the reaction energy barrier of Sr_xCa_1−xFeO₃ (0.73–1.06 eV) was lower than that of CaFeO₃ (2.18 eV). This suggests that Sr doping and the regulation of the reaction pathways are essential drivers for enhancing the reactivity of Sr_xCa_1−xFeO₃, which affects the release of lattice oxygen and the morphological properties of OC particles.     

On the high‐gasification rate of Brazilian manganese ore in chemical‐looping combustion (CLC) for solid fuels

The high rate of char gasification observed when using a Brazilian manganese ore as compared to ilmenite is investigated in a batch fluidized‐bed reactor. Experiments were carried out at 970°C using petroleum coke, coal and wood char as fuel with a 50% H₂O in N₂ as fluidizing gas. A manufactured manganese oxygen carrier was also used, however, which presented a slower char conversion rate than the manganese ore. It is concluded that decrease in H₂ inhibition and oxygen release are unlikely to be the main responsible mechanisms for the ore's unexpected gasification rate. The ore was also mixed in different ratios with ilmenite and it was observed that the presence of even small amounts of ore in the bed resulted in increased gasification rate. Thus, the high‐gasification rate for the manganese ore could be due to a contribution from the impurities in the ore by catalyzing the gasification reaction. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4346–4354, 2013     

Oxygen uncoupling behaviour for ilmenite ore oxygen carrier generated from a calcination treatment mixed with natural manganese ore

Chemical looping with O₂ uncoupling aims at using an oxygen carrier (OC) with O₂ uncoupling behaviour to promote fuel conversion. Natural ilmenite ores have been considered highly promising OCs for chemical looping technology; however, they do not possess any O₂ uncoupling behaviour. Mn-modified ilmenite ores as OCs are capable of O₂ uncoupling, while most of them are synthesized via complicated procedures by using costly chemicals. In this study, a strategy of calcination treatment on ilmenite ores mixed with manganese ores has been established to introduce Mn into the ilmenite OCs, endowing them with O₂ uncoupling behaviour in a simple and low-cost manner. The O₂ uncoupling behaviour from Mn-modified ilmenite ores is mainly due to the newly formed (Fe_1−xMn_x)₂O₃/(Fe_1−xMn_x)₃O₄ crystal phases generated during the calcination treatment, which also alleviate the thermodynamic limit of the Mn₂O₃-Mn₃O₄ redox pair. As a result, the Mn-modified ilmenite ore OCs can release O₂ at high temperatures when decreasing the oxygen partial pressure. But more importantly, the reduced OCs can be restored in the air isothermally. This established simple calcination treatment method can be used as a low-cost strategy for producing ilmenite-based OCs with O₂ uncoupling behaviour. The O₂ uncoupling behaviour is expected to be beneficial to chemical looping combustion of fuels, promote fuel conversion, minimize OC loading, and reduce energy consumption.     

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

A 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, Ce₉Zr₁O_δ, and 5 wt.% CNTs Ce₉Zr₁O_δ were 223.92, 170.96, and 128.60 kJ/mol, respectively, which also correspond with the reactivity sequence of the oxygen carriers.     

Pyrolysis of polypropylene in the presence of oxygen

The polypropylene (PP) was coated on porous α-alumina particles and then pyrolyzed in a flow of helium or a mixture of helium–oxygen at atmospheric pressure. The mass release from PP was dramatically enhanced in the presence of oxygen at temperatures in the range of 200–300°C. The temperature for the 50% mass release was ca. 250°C at an oxygen partial pressure (P_O2) of 16.7 kPa and was lowered by 190°C as compared with a system which contained no oxygen. When P_O2 was higher than 4.2 kPa, the mass release rate obeyed first-order kinetics with respect toP_O2, and the activation energy was calculated and found to be 65–75 kJ/mol. The activation energy was considerably lower than that for pyrolysis in the absence of oxygen (230 kJ/mol) and agreed well with the value for formation of peroxides on tertiary carbons. When the pyrolysis was conducted at 250°C under P_O2=16.7 kPa, the carbon-based yield of volatiles exceeded 90%, and the yields of CS₂-soluble oil and wax were 76% and 6.0%, respectively. The carbon-based yields of other products were: Acetone, 2.5%; methanol, 1.5%; CO, 3.0%; CO₂, 2.2% and solid residue; 10%. Proton NMR (nuclear magnetic resonance) analysis showed that the CS₂-soluble oil possessed an elemental composition of C₁₀₀H₁₈₇O₁₁. The average number of carbon atoms per molecule of the oil was approximately 10, and –CH₂–OH, –C(CH₃)CO and CH₂ were typical end groups. Their formation is explained by the decomposition of peroxides formed on tertiary carbons.     

铁基载氧体对煤中汞释放行为的影响

采用Ontario-Hydro方法,在管式炉中考察了煤化学链燃烧/气化过程中Fe₄Al₆载氧体对煤中汞释放率、气态汞形态分布及汞在两反应器内释放行为的影响。结果表明,载氧体对煤中汞释放率具有显著的影响,在500~700℃,与无载氧体相比,化学链燃烧过程煤中汞释放率减少,化学链气化过程煤中汞释放率增大,而在900℃时,无论化学链燃烧过程还是化学链气化过程,煤中汞释放率均减小。Fe₄Al₆载氧体能够显著增加燃料反应器出口气态Hg²⁺的相对含量,其含量随温度的升高而逐渐升高。燃料反应器的温度也是影响煤中汞在两反应器中的分布以及空气反应器中不同价态汞百分含量的重要因素。此外,相同条件下不同煤种的汞释放率不同,主要与煤的组成不同有关。该研究对揭示载氧体对煤中汞迁移的影响机理以及煤化学链转化过程汞的控制提供了实验依据。     

高铝耐火砖负载CuO氧载体的化学链氧解耦特性实验

耐火砖常用作高温炉膛材料,具有较高的破碎强度和较强的抗烧结性能。以耐火砖颗粒为惰性载体,采用连续浸渍法制备CuO质量分数为10%～30%的Cu10RefBri、Cu25RefBri和Cu30RefBri氧载体样品,并在热重分析仪（TGA）和批次进料流化床反应器中900～950℃下进行了化学链氧解耦燃烧过程氧载体的释氧-吸氧循环实验测试。结果显示,在上述氧载体样品中Cu25RefBri的释氧速率最高,可达9×10^-5kgO₂/(s?kgOC),流化床中稳定的O₂体积分数可达1.1%。然而,随着循环次数增加,Cu25RefBri的释氧速率逐渐降低至2.0×10^-5kgO₂/(s?kgOC),同时流化床尾气中O₂体积分数降低至0.7%,该值远低于对应温度下的平衡O₂浓度值。氧浓度和释氧速率降低的主要原因在于：循环过程氧载体中形成的低释氧活性的CuAl₂O₄尖晶石含量逐渐增加,导致氧载体总体活性下降。此外,在950℃流化床实验过程,还检测到氧载体颗粒的烧结现象。     

Auto-thermal chemical looping combustion of sulphur with Fe2O3 oxygen carriers for sulphuric acid production: Thermodynamics

Chemical looping is a novel fuel conversion and material separation technology. It can be applied to obtain sulphur through selective oxidation of H₂S. Further, chemical looping combustion (CLC) of sulphur can generate SO₂ with a high concentration without NO_x formation. The high SO₂ concentration is adjustable and facilitates large-scale H₂SO₄ production. In this study, we examined the thermodynamics of the CLC of sulphur for H₂SO₄ production, which has not been reported previously. We analyzed the effects of reactor temperature and sulphur to Fe₂O₃ oxygen carrier (OC) ratios on sulphur allotrope transformations and on the distributions of reaction products. Moreover, the reactors were operated auto-thermally. Based on this design, we examined the effects of fuel reactor (FR) and air reactor temperatures on the minimum recirculation of the OC, as well as the gas and solid products and heat released from the air reactor. Our results showed that the CLC of sulphur with Fe₂O₃ OC could occur through an auto-thermal process. The FR in a sulphur CLC system should be operated over a temperature range of 800–950°C, with an Fe₂O₃ OC recirculation between 45 and 143 kg/kg_S(s). Furthermore, when the FR was operated in the auto-thermal mode, we achieved 100% SO₂ conversion. The findings of this study may be applied to reactor design for large-scale H₂SO₄ production through CLC of sulphur.     

Investigation into sulfur release in reductive decomposition of calcium sulfate oxygen carrier by hydrogen and carbon monoxide

Chemical looping combustion (CLC) is a promising technology with the inherent property of separating CO₂ from flue gas. For calcium sulfate (CaSO₄) oxygen carrier, the inhibition of the produced sulfurous gases in the reduction of CaSO₄, including sulfur dioxide (SO₂), hydrogen sulfide (H₂S) and carbonyl sulfide (COS), is the key for a CLC system. In this paper, the sensitivities of reacting temperature, oxygen ratio number (defined in this paper) and the mole fraction of both carbon monoxide (CO) and hydrogen(H₂) in the syngas to the sum of the amounts of released SO₂, H₂S and COS are discussed respectively. Thermo-gravimetric analysis (TGA) tests demonstrated that the amount of the produced sulfurous gases is greatly dependent on the partial pressure of H₂ or CO in the reduction of CaSO₄. When the partial pressure of H₂ or CO is higher than 40 kPa, the production of sulfurous gases, indicating the deterioration of the recyclability of CaSO₄, can be prevented completely even if the reacting temperature is as high as 1000 °C. A new kind of CaSO₄/CaCO₃ oxygen carrier is prepared using a mechanical mixing method. The addition of CaCO₃ nanoparticles largely improves the recyclability of the oxygen carrier in comparison with the fresh CaSO₄ oxygen carrier, without CaCO₃ nanoparticles, in a multi-cycle TGA test.     

Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

In this study, La_0.4Sr_0.6CoO_3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (D_v) and surface exchange coefficient (K_ex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La_0.4Sr_0.6CoO_3‐δ membrane was ～4.0 × 10^?7 mol cm^?2 s^?1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10^?5 mol cm^?2 s^?1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

基于NiO载氧体的煤化学链燃烧实验

采用流化床反应器并以水蒸气作为气化-流化介质,研究了以NiO为载氧体在800～960℃内的煤化学链燃烧反应特性。实验结果表明,载氧体与煤气化产物在反应器温度高于900℃体现了高的反应活性。随着流化床反应器温度的提高,气体产物中CO₂的体积浓度(干基)呈单调递增;CO、H₂、CH₄的体积浓度(干基)呈单调递减;煤中碳转化为CO₂的比率逐渐递增,碳的残余率逐渐递减。反应器出口气体CO₂、CO、H₂、CH₄的生成率随反应时间呈单峰特性,H₂生成率的峰值远小于CO的峰值;且随反应器温度升高,CO₂生成率升高,CO、H₂、CH₄的生成率降低。反应温度高于900℃时,流化床反应器NiO载氧体煤化学链燃烧在9 min之内就基本完成,CO₂含量高于92%。     

Preparation and oxygen permeation of U‐shaped perovskite hollow‐fiber membranes

The oxygen permeation of dense U‐shaped perovskite hollow‐fiber membranes based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ prepared by a phase inversion spinning process is reported. The perovskite hollow fibers with totally dense wall were obtained with the outer diameter of 1.147 mm and the inner diameter of 0.691 mm. The dependences of the oxygen permeation on the air flow rate on the shell side, the helium flow rate on the core side, the oxygen partial pressures, and the operating temperatures were experimentally investigated. According to the Wagner theory, it follows that the oxygen transport through the U‐shaped hollow‐fiber membrane is controlled by both surface reaction and bulk diffusion at the temperature ranges of 750–950°C. High oxygen permeation flux of 3.0 ml/(min cm²) was kept for about 250 h at 950°C under the conditions of the air feed flow rate of 150 ml/min and the helium flow rate of 50 ml/min. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Use of CuO/MgAl2O4 and La0.8Sr0.2FeO3/γ‐Al2O3 in chemical looping reforming system for tar removal from gasification gas

Biomass gasification gas contains condensable hydrocarbons usually referred to as “tars.” The use of chemical‐looping reforming (CLR) has been proposed as a downstream technology for tar removal. The tar removal capabilities and the regeneration properties of two particularly promising bed materials, CuO/MgAl₂O₄ and La_0.8Sr_0.2FeO₃/γ‐Al₂O₃, were investigated using C₂H₄, C₆H₆, and C₇H₈ as tar surrogates. The material La_0.8Sr_0.2FeO₃/γ‐Al₂O₃ exhibited high levels of conversion of all tar surrogates investigated, whereas CuO/MgAl₂O₄ showed less promising behavior. For this material, the C₂H₄ conversion in the absence of aromatic compounds was very high, but in the presence of monoaromatic compounds, the conversion of aromatics and C₂H₄ was poor. This indicates that monoaromatic compounds hinder the conversion of C₂H₄ effectively. Therefore, C₂H₄ may not always be a good choice as a tar surrogate and its suitability may depend on the mechanism of hydrocarbon conversion on the bed material surface in question. © 2015 American Institute of Chemical Engineers AIChE J, 62: 38–45, 2016