Thermodynamic possibilities and constraints for pure hydrogen production by iron based chemical looping process at lower temperatures

Iron offers the possibility of transformation of a syngas or gaseous hydrocarbons into hydrogen by a cycling process of iron oxide reduction (e.g. by hydrocarbons) and release of hydrogen by steam oxidation. From the thermodynamic and chemical equilibrium point of view, the reduction of magnetite by hydrogen, CO, CH₄ and a model syngas (mixtures CO + H₂ or H₂ + CO + CO₂) and oxidation of iron by steam has been studied. Attention was concentrated not only on convenient conditions for reduction of Fe₃O₄ to iron at temperatures 400–800 K but also on the possible formation of undesired soot, Fe₃C and iron carbonate as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of magnetite at low temperatures requires a relatively high H₂/H₂O ratio, increasing with decreasing temperature. Reduction of iron oxide by CO is complicated by soot and Fe₃C formation. At lower temperatures and higher CO₂ concentrations in the reducing gas, the possibility of FeCO₃ formation must be taken into account. The purity of the hydrogen produced depends on the amount of soot, Fe₃C and FeCO₃ in the iron after the reduction step. Magnetite reduction is the more difficult stage in the looping process. Pressurized conditions during the reduction step will enhance formation of soot and carbon containing iron compounds.     

联合CO2捕集的生物质化学链气化制备合成气系统分析

本文提出以Fe₂O₃为载氧体、以CaO捕集CO₂的生物质化学链气化系统,利用Aspen Plus软件对该系统进行了模拟,以合成气组成（干基）、合成气氢碳比、含碳产物的碳摩尔分布、冷气效率及收率等为系统性能评价指标,重点分析了燃料反应器温度（T_FR）、载氧体Fe₂O₃与生物质碳摩尔比（Fe₂O₃/C）、水蒸气与生物质碳摩尔比（Steam/C）、CaO与生物质碳摩尔比（CaO/C）等系统参数对固体生物质化学链气化系统的影响。结果表明,在T_FR = 825℃、Fe₂O₃/C = 0.5、Steam/C = 0.71和CaO/C = 0.26条件下,合成气制备系统性能较优,合成气中H₂和CO₂含量分别为55.2%和15.4%,氢碳比为1.93,冷气效率为78.2%,被CaCO₃捕集的生物质碳为18.2%,收率（湿气基）为1.95 Nm³/kg_biomass,其中合成气中H₂和CO收率为1.24 Nm³/kg_biomass。     

Ni-Co双金属催化剂上沼气重整制氢宏观动力学

用传统湿式浸渍法制备La2O3掺杂的商业γ-Al2O3负载的沼气重整催化剂Ni-Co/La2O3-γ-Al2O3,通过对NiCo双金属催化剂上沼气重整制氢在常压下的宏观动力学分析,得出该催化剂上CH4与CO2消耗、H2与CO生成时的表观反应速率方程.通过改变进料中CH4与CO2的分压,求出各物质的反应分级数,确定总反应...     

Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles

This paper presents a process analysis of ZnO/Zn, Fe₃O₄/FeO and Fe₂O₃/Fe₃O₄ thermochemical cycles as potential high efficiency, large scale and environmentally attractive routes to produce hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemical process with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$ kg⁻¹ and 14.75$ kg⁻¹ of H₂ for, respectively a 55 MW_th and 11 MW_th solar tower plant operating 40 years.     

Studies on the reduction of iron oxide with hydrogen

Kinetic studies have been carried out on the hydrogen reduction of pure -Fe₂O₃ doped with foreign metal oxides employing a sensitive micro-gravimetric technique. The results show that the reduction of pure Fe₂O₃ proceeds by a consecutive two-step mechanism via Fe₃O₄, the overall rate being controlled by the topochemical reduction of Fe₃O₄ while that of doped oxides and hematite ore takes place by a different mechanism involving the mixed ferrite formed. In addition, the reduction of pure Fe₂O₃ is catalysed by metal additives in the presence of water vapour. This enhancement in reduction rate is attributed to a “hydrogen spill-over” effect.     

Carbothermal reduction of alumina: Thermochemical equilibrium calculations and experimental investigation

The production of aluminum by the electrolytic Hall–Héroult process suffers from high energy requirements, the release of perfluorocarbons, and vast greenhouse gas emissions. The alternative carbothermic reduction of alumina, while significantly less energy-intensive, is complicated by the formation of aluminum carbide and oxycarbides. In the present work, the formation of Al, as well as Al₂OC, Al₄O₄C, and Al₄C₃ was proven by experiments on mixtures of Al₂O₃ and activated carbon in an Ar atmosphere submitted to heat pulses by an induction furnace. Thermochemical equilibrium calculations indicate that the Al₂O₃-reduction using carbon as reducing agent is favored in the presence of limited amounts of oxygen. The temperature threshold for the onset of aluminum production is lowered, the formation of Al₄C₃ is decreased, and the yield of aluminum is improved. Significant further enhancement in the carbothermic reduction of Al₂O₃ is predicted by using CH₄ as the reducing agent, again in the presence of limited amounts of oxygen. In this case, an important by-product is syngas, with a H₂/CO molar ratio of about 2, suitable for methanol or Fischer–Tropsch syntheses. Under appropriate temperature and stoichiometry of reactants, the process can be designed to be thermo-neutral. Using alumina, methane, and oxygen as reagents, the co-production of aluminum with syngas, to be converted to methanol, predicts fuel savings of about 68% and CO₂ emission avoidance of about 91%, vis-à-vis the conventional production of Al by electrolysis and of methanol by steam reforming of CH₄. When using carbon (such as coke or petcoke) as reducing agent, fuel savings of 66% and CO₂ emission avoidance of 15% are predicted. Preliminary evaluation for the proposed process indicates favorable economics, and the required high temperatures process heat is readily attainable using concentrated solar energy.     

碳酸盐中CO2气化生物质制合成气

以杉木屑为原料,CO₂为气化剂,熔融碳酸盐Li2CO3-Na2CO3-K2CO3(LNK)为热介质和催化剂进行气化制合成气(H2+CO)的研究,考察气化剂CO₂流量、CO₂通入方式、复合熔盐体系中添加的金属氧化物种类和Cr2O3含量等因素对气体产物组成分布及产率的影响。结果表明:CO₂流量显著影响气化反应的平衡;以鼓泡法通入CO₂时生物质的气化效果优于吹扫法的情况,CO₂流量为99.8 L/h时气化效果较好,合成气含量和产率分别达到61.4%和350.2 mL/g生物质;添加的金属氧化物中Cr₂O₃对生物质气化过程的促进作用优于MgO和Fe₂O₃,随着Cr₂O₃含量的增大,合成气含量先增大后略微减小,在Cr₂O₃含量为10.0%时最高,为67.9%。     

Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Thermodynamic assessment of hydrogen production via solar thermochemical cycle based on MoO2/Mo by methane reduction

Inspired by the promising hydrogen production in the solar thermochemical (STC) cycle based on non-stoichiometric oxides and the operation temperature decreasing effect of methane reduction, a high-fuel-selectivity and CH₄-introduced solar thermochemical cycle based on MoO₂/Mo is studied. By performing HSC simulations, the energy upgradation and energy conversion potential under isothermal and non-isothermal operating conditions are compared. In the reduction step, MoO₂: CH₄ = 2 and 1020 K<T_red<1600 K are found to be most favorable for syngas selectivity and methane conversion. Compared to the STC cycle without CH₄, the introduction of methane yields a much higher hydrogen production, especially at the lower temperature range and atmospheric pressure. In the oxidation step, a moderately excessive water is beneficial for energy conversion whether in isothermal or non-isothermal operations, especially at H₂O: Mo= 4. In the whole STC cycle, the maximum non-isothermal and isothermal efficiency can reach 0.417 and 0.391 respectively. In addition, the predicted efficiency of the second cycle is also as high as 0.454 at T_red = 1200 K and T_oxi = 400 K, indicating that MoO₂ could be a new and potential candidate for obtaining solar fuel by methane reduction.     

Combustion wave speeds of nanocomposite Al/Fe2O3: the effects of Fe2O3 particle synthesis technique

Combustion wave speeds of nanoscale aluminum (Al) powders mixed with iron oxide (Fe₂O₃) were measured as a function of Fe₂O₃ synthesis technique and fuel/oxidizer composition. Three reactant synthesis techniques were examined; two focus on sol–gel processing of nanoscale Fe₂O₃ particles and the third utilizes commercially available nanoscale Fe₂O₃ powder. Nanoscale aluminum particles (52 nm in diameter) were combined with each oxidizer in various proportions. Flame propagation was studied by igniting low-density mixtures and taking data photographically with a high-speed camera. Both open and confined burning were examined. Results indicate that the combustion wave speed is a strong function of the stoichiometry of the mixture and a slightly fuel-rich mixture provides an optimum combustion wave speed regardless of oxidizer synthesis technique. Oxidizers processed using sol–gel chemistry originally contained impurities which retarded the combustion wave speeds. When the same oxidizers are annealed at moderate temperatures, the new heat-treated oxidizer shows a dramatic improvement, with combustion wave speeds on the order of 900 m/s.     

C-HRA-5和HR3C钢在650℃超临界水中的高温氧化行为

在650℃和压力为27MPa的蒸汽中对太原钢铁(集团)有限公司产C-HRA-5(简称C5)钢和HR3C奥氏体耐热钢进行了不同氧化时间的超临界水蒸气氧化试验,研究了不同氧化时间表面氧化层的形貌和结构组成,并分析了氧化层截面形貌及氧化机制.结果表明:材料在200 h之前增重较快,而氧化时间在200～2 000 h氧化速率较...     

Reduction of NiAl2O4 containing catalysts for steam methane reforming reaction

Reducibility of a NiAl₂O₄ containing catalyst was studied. On a measurement of NiAl₂O₄ concentration in a catalyst, a peak area ratio of NiAl₂O₄ in XRD analysis was verified to express the NiAl₂O₄ concentration. The reducibility of NiAl₂O₄ was confirmed to be dependent on the calcining temperature to form NiAl₂O₄, not dependent on the calcining time. The catalyst containing NiAl₂O₄ was ascertained to be reduced under convenient conditions to actual plant operations; H₂/N₂ = 30/70 at 1023K for 1 h + steam/CH₄ = 6 at 1023K for 17 h.     

Electrochemical characteristics of iron carbide as an active material in alkaline batteries

Electrochemical properties of iron carbide (Fe₃C) for use as an alkaline battery anode were investigated during charge–discharge cycles. Results of electrochemical measurements and Mössbauer spectroscopy suggested that Fe₃C is oxidized irreversibly to Fe₃O₄ during discharge processes and that the produced Fe₃O₄ is subsequently changed to Fe(OH)₂ and Fe during the charging process, raising the discharge/charge capacity in further galvanostatic cycles. In addition, the electrode particles were observed to be less than 100 nm in diameter and to be highly dispersed on the surface of carbon black. These phenomena seems to be caused by dissolution and deposition of Fe(OH)₂ and Fe via intermediate iron species, leading to exposure of a fresh Fe₃C surface to the electrolyte after the second discharge.     

Application of metal oxides-based nanofluids in PV/T systems: a review

Having the wide application of metal oxides in energy technologies, in recent years, many researchers tried to increase the performance of the PV/T system by using metal oxide-based nanofluids (NFs) as coolants or optical filters or both at the same time. This paper summarizes recent research activities on various metal oxides (Al₂O₃, TiO₂, SiO₂, Fe₃O₄, CuO, ZnO, MgO)-based NFs performance in the PV/T system regarding different significant parameters, e.g., thermal conductivity, volume fraction, mass flowrate, electrical, thermal and overall efficiency, etc. By conducting a comparative study among the metal oxide-based NFs, Al₂O₃/SiO₂-water NFs are mostly used to achieve maximum performance. The Al₂O₃-water NF has a prominent heat transfer feature with a maximum electrical efficiency of 17%, and a maximum temperature reduction of PV module of up to 36.9°C can be achieved by using the Al₂O₃-water NF as a coolant. Additionally, studies suggest that the PV cell’s efficiency of up to 30% can be enhanced by using a solar tracking system. Besides, TiO₂-water NFs have been proved to have the highest thermal efficiency of 86% in the PV/T system, but TiO₂ nanoparticles could be hazardous for human health. As a spectral filter, SiO₂-water NF at a size of 5 nm and a volume fraction of 2% seems to be very favorable for PV/T systems. Studies show that the combined use of NFs as coolants and spectral filters in the PV/T system could provide a higher overall efficiency at a cheaper rate. Finally, the opportunities and challenges of using NFs in PV/T systems are also discussed.     

Effect of Bi2O3 content on characteristics of Bi2O3–GDC systems for direct methane oxidation

Ceramic systems of Bi₂O₃ and gadolinia-doped ceria (GDC) solid mixture were prepared as catalysts for direct methane oxidation. These systems were characterized by temperature-programmed reduction using hydrogen and carbon monoxide, temperature-programmed reaction of methane, fixed-temperature direct methane oxidation, and X-ray diffraction analysis. Adding Bi₂O₃ to GDC promotes both hydrogen and CO oxidation activities, because of the presence of surface Bi₂O₃ and the high content of mobile oxygen in Bi₂O₃. The reactivity of CO with surface lattice oxygen is enhanced to a higher extent than that of H₂, and this enhanced extent shows a maximum in Bi₂O₃ content. Such a maximum also exists for the catalytic activity of direct methane oxidation. A synergistic effect occurs due to a combination of the high methane reactivity of GDC and the high content of mobile oxygen in Bi₂O₃. The CO₂ selectivity of direct methane oxidation can be modulated by varying the Bi₂O₃ content. The mixing of Bi₂O₃ with GDC also increases the self-de-coking capability of the catalyst during direct methane oxidation, which stabilizes the activity.     

New insight into effect of potential on degradation of Fe-N-C catalyst for ORR

In recent years, Fe-N-C catalyst is particularly attractive due to its high oxygen reduction reaction (ORR) activity and low cost for proton exchange membrane fuel cells (PEMFCs). However, the durability problems still pose challenge to the application of Fe-N-C catalyst. Although considerable work has been done to investigate the degradation mechanisms of Fe-N-C catalyst, most of them are simply focused on the active-site decay, the carbon oxidation, and the demetalation problems. In fact, the 2e⁻ pathway in the ORR process of Fe-N-C catalyst would result in the formation of H₂O₂, which is proved to be a key degradation source. In this paper, a new insight into the effect of potential on degradation of Fe-N-C catalyst was provided by quantifying the H₂O₂ intermediate. In this case, stability tests were conducted by the potential-static method in O₂ saturated 0.1 mol/L HClO₄. During the tests, H₂O₂ was quantified by rotating ring disk electrode (RRDE). The results show that compared with the loading voltage of 0.4 V, 0.8 V, and 1.0 V, the catalysts being kept at 0.6 V exhibit a highest H₂O₂ yield. It is found that it is the combined effect of electrochemical oxidation and chemical oxidation (by aggressive radicals like H₂O₂/radicals) that triggered the highest H₂O₂ release rate, with the latter as the major cause.     

The preparation of Fe3O4/nanocellulose aerogel nanocomposite as anodes for lithium-ion batteries and electrochemical performance

纤维素因其环境友好、价格低廉等优点受到研究者的广泛关注,近年来作为碳材料广泛应用于电化学研究中。采用碳化后的纳米纤维素气凝胶为载体,六水合氯化铁为铁源,通过溶液热法合成了四氧化三铁/纳米纤维素气凝胶复合材料。通过XRD和SEM对产物进行了结构表征和微观形貌分析,并将其作为锂离子电池的负极材料,测试了一系列电化学性能,并与纯Fe₃O₄纳米颗粒的进行对比。结果表明,碳化后的纳米纤维素气凝胶保持着疏松多孔的三维网络结构,尺寸均一的Fe₃O₄纳米粒子均匀的分布于其中。该复合材料表现出优异的循环稳定性,在100 mA/g的电流密度下,首次放电比容量为1064 mA·h/g,100次循环后仍稳定在847 mA·h/g。相比于纯Fe₃O₄纳米颗粒,材料的电化学性能得到大幅度提高。本文有助于推动纤维素基碳材料在电化学领域中的进一步应用,为复合电极材料的发展提供一定的实验依据。     

铁系无机材料用于新能源电催化硝酸盐还原制氨

硝酸盐是一类普遍存在的环境污染物,而其对应的还原态氨却是重要的化工原料和农业肥料。因此,探索将两者直接结合的化学转化具有重大的技术及经济战略意义,尤其是对当今“双碳”战略驱动下的高碳排放合成氨的工艺变革以及氨可能成为下一代载氢燃料。通过电催化技术,将硝酸盐还原合成氨过程与可再生能源电力结合,构建绿色低碳的含氮化学品人工“氮循环”的新循环技术与经济体系,是解决目前合成氨工业对化石能源高度依赖、高碳排放问题以及开发新氢能的有力途径。借鉴传统合成氨催化广泛应用、具有成本优势的铁系催化机制,分别选取单质Fe、Fe₂O₃和Fe₃O₄作为电催化材料,探索并揭示电催化硝酸盐还原合成氨催化反应的化学形态。结果表明：在相对于可逆氢电极电势为 -0.53 ~ -0.93 V区间内,Fe₂O₃表现出了最优异的催化活性,其生成氨的法拉第效率（FE_NH3）最高可达88%,对应的生成氨电流密度（J _NH3）为43.1 mA/cm²、氨生成速率（r_NH3）为0.20 mmol/(cm²·h)。此外,从控制硝酸盐转化率选择性获得氨和硝酸铵两种不同产物的技术路线,分析对比了相应的用电成本以及产品市场价格,充分说明了路线的经济性,说明铁系无机材料在电催化硝酸盐还原合成氨方面具有非常巨大的市场化潜力。     

铁基载氧体的污泥化学链气化过程中氮迁移热力学模拟与实验研究

基于吉布斯自由能最小化原理,采用HSC Chemistry 6.0软件,对污泥化学链气化过程中NO_x前驱物（NH₃和HCN）与Fe₂O₃载氧体的氧化还原行为进行了热力学模拟。基于污泥热解实验中NO_x前驱物的含量,计算载氧体与污泥的摩尔比（OC/SS）对NH₃、HCN以及NH₃和HCN混合气氧化过程的影响。热力学模拟结果表明：Fe₂O₃能显著促进NO_x前驱物的氧化和裂解,主要生成N₂,几乎无NO_x生成;当NH₃、HCN以及混合气（NH₃和HCN）分别作为还原剂时,其最优OC/SS分别为0.02、0.04和0.05;由于HCN还原性强于NH₃,其氧化速率较快。基于Fe₂O₃/Al₂O₃混合物（FeAl）载氧体,实验对比了污泥化学链气化与污泥热解过程中NO_x前驱物的释放特性,发现Fe₂O₃能显著降低烟气中NO_x前驱物的产率,NH₃和HCN产率分别下降32%和62%。实验结果与热力学模拟结果一致。     

Thermodynamic calculation on energy densities of multi-electron transfer Mg/Al batteries

镁离子电池和铝离子电池因其高能量密度、地壳储量丰富、安全等优良特性有望成为下一代新型高能量密度储能体系,是未来二次电池研究的热点之一。本文采用热力学方法计算和分析了近300种镁离子和铝离子电池体系的理论质量能量密度、体积能量密度和电压。在所得数据的基础上,以目前商业化锂离子电池正极材料钴酸锂为对比参考,综合考虑质量能量密度、体积能量密度、标准电极电位、毒性、腐蚀性、易燃性、环境友好性等诸多因素,逐步筛选出符合条件的一系列镁离子正极材料（O₂、S、MnO₂、MoO₃、Fe₂O₃、Fe₃O₄、NiO、MoO₂、CuO、Cu₂O）和铝离子的正极材料（O₂、S、MnO₂、MoO₃、NiO、CuO、Cu₂O）。