Thermochemical splitting of CO_(2) on perovskites for CO production: A review

Energy supply dominated by fossil energy has been and remains the main cause of carbon dioxide emissions,the major greenhouse gas leading to the current grave climate change challenges.Many technical pathways have been proposed to address the challenges.Carbon capture and utilization(CCU) represents one of the approaches and thermochemical CO_(2) splitting driven by thermal energy is a subset of the CCU,which converts the captured CO_(2) into CO and makes it possible to achieve closed-loop carbon recirculation.Redox-active catalysts are among the most critical components of the thermochemical splitting cycles and perovskites are regarded as the most promising catalysts.Here we review the latest advancements in thermochemical cycles based on perovskites,covering thermodynamic principles,material modifications,reaction kinetics,oxygen pressure control,circular strategies,and demonstrations to provide a comprehensive overview of the topical area.Thermochemical cycles based on such materials require the consideration of trade-off between cost and efficiency,which is related to actual material used,operation mode,oxygen removal,and heat recovery.Lots of efforts have been made towards improving reaction rates,conversion efficiency and cycling stability,materials related research has been lacking-a key aspect affecting the performance across all above aspects.Double perovskites and composite perovskites arise recently as a potentially promising addition to material candidates.For such materials,more effective oxygen removal would be needed to enhance the overall efficiency,for which thermochemical or electrochemical oxygen pumps could contribute to efficient oxygen removal as well as serve as means for inert gas regeneration.The integration of thermochemical CO_(2) splitting process with downstream fuel production and other processes could reduce costs and increase efficiency of the technology.This represents one of the directions for the future research.

Occurrence of the Bunsen side reaction in the sulfur-iodine thermochemical cycle for hydrogen production

This study aimed to establish a closed-cycle operation technology with high thermal efficiency in the thermochemical sulfur-iodine cycle for large-scale hydrogen production.A series of experimental studies were performed to investigate the occurrence of side reactions in both the H2SO4 and HI x phases from the H2SO4/HI/I2/H2O quaternary system within a constant temperature range of 323-363 K.The effects of iodine content,water content and reaction temperature on the side reactions were evaluated.The results showed that an increase in the reaction temperature promoted the side reactions.However,they were prevented as the iodine or water content increased.The occurrence of side reactions was faster in kinetics and more intense in the H2SO4 phase than in the HI x phase.The sulfur or hydrogen sulfide formation reaction or the reverse Bunsen reaction was validated under certain conditions.

A review of recent researches on Bunsen reaction for hydrogen production via S–I water and H2S splitting cycles

The Bunsen reaction is the center reaction for both the sulfur–iodine water splitting cycle for hydrogen production and the novel hydrogen sulfide splitting cycle for hydrogen and sulfuric acid production from the sulfur-containing gases.This paper reviews the research progress of the Bunsen reaction in recent 10–15 years.Researches were initially focused on the optimization of the operating conditions of the conventional Bunsen reaction requiring excessive water and iodine to improve the products separation efficiency and to avoid the side reactions and iodine vapor deposition.Alternative methods including electrochemical methods,precipitation methods,and non-aqueous solvent methods had their respective advantages,but still faced challenges.In development of the technology of H2S splitting cycle,dissolving iodine in toluene solvent could render the Bunsen reaction to occur with the flowable I2 stream at ambient temperature such that the side reactions and iodine vaporization can be avoided and the corrosion hazard lessened.It also prevented the Bunsen reaction from using excessive iodine and water.The products from the Bunsen reaction including HI,H2SO4,H2O,and toluene could be directly electrolyzed.

Evolution of stress fields during the supercontinent cycle

We investigate the evolution of stress fields during the supercontinent cycle using the 2D Cartesian geometry model of thermochemical convection with the non-Newtonian rheology in the presence of floating deformable continents.In the course of the simulation,the supercontinent cycle is implemented several times.The number of continents considered in our model as a function of time oscillates around 3.The lifetime of a supercontinent depends on its dimension.Our results suggest that immediately before a supercontinent breakup,the over-lithostatic horizontal stresses in it(referring to the mean value by the computational area)are tensile and can reach-250 MPa.At the same time,a vast area beneath a supercontinent with an upward flow exhibits clearly the over-lithostatic compressive horizontal stresses of 50-100 МРа.The reason for the difference in stresses in the supercontinent and the underlying mantle is a sharp difference in their viscosity.In large parts of the mantle,the over-lithostatic horizontal stresses are in the range of±25 MPa,while the horizontal stresses along subduction zones and continental margins are significantly larger.During the process of continent-to-continent collisions,the compressive stresses can approximately reach 130 MPa,while within the subcontinental mantle,the tensile over-lithostatic stresses are about-50 MPa.The dynamic topography reflects the main features of the su-percontinent cycle and correlates with real ones.Before the breakup and immediately after the disin-tegration of the supercontinent,continents experience maximum uplift.During the supercontinent cycle,topographic heights of continents typically vary within the interval of about±1.5 km,relatively to a mean value.Topographic maxima of orogenic formations to about 2-4 km are detected along continent-to-continent collisions as well as when adjacent subduction zones interact with continental margins.

In-Depth Energy and Irreversibility Analysis in the Solar Driven Two-Step Thermochemical Water Splitting Cycle for Hydrogen Production

Hydrogen production via a two-step thermochemical cycle based on solar energy has attracted increasing attention.However,the severe irreversible loss causes the low efficiency.To make sense of the irreversibility,an in-depth thermodynamic model for the solar driven two-step thermochemical cycles is proposed.Different from previous literatures solely focusing on the energy loss and irreversibility of devices,this work decouples a complex energy conversion process in three sub-processes,i.e.,reaction,heat transfer and re-radiation,acquiring the cause of irreversible loss.The results from the case study indicate that the main irreversibility caused by inert sweeping gas for decreasing the reduction reaction temperature dominates the cycle efficiency.Decreasing reduction reaction temperature without severe energy penalty of inert sweeping gas is important to reducing this irreversible loss.A favorable performance is achieved by decreasing re-oxidation rate,increasing hydrolysis conversion rate and achieving a thermochemical cycle with a lower equilibrium temperature of reduction reaction at atmosphere pressure.The research clarifies the essence of process irrrversibility in solar thermichemical cycles,and the findings point out the potential to develop efficient solar driven two-step thermochemical cycles for hydrogen production.

Thermogravimetric Analysis of Zirconia-Doped Ceria for Thermochemical Production of Solar Fuel

Developing an efficient redox material is crucial for thermochemical cycles that produce solar fuels (e.g. H2 and CO), enabling a sustainable energy supply. In this study, zirconia-doped cerium oxide (Ce1-xZrxO2) was tested in CO2-splitting cycles for the production of CO. The impact of the Zr-content on the splitting performance was investigated within the range 0 ≤ x < 0.4. The materials were synthesized via a citrate nitrate auto combustion route and subjected to thermogravimetric experiments. The results indicate that there is an optimal zirconium content, x = 0.15, improving the specific CO2-splitting performance by 50% compared to pure ceria. Significantly enhanced performance is observed for 0.15 ≤ x ≤ 0.225. Outside this range, the performance decreases to values of pure ceria. These results agree with theoretical studies attributing the improvements to lattice modification. Introducing Zr4+ into the fluorite structure of ceria compensates for the expansion of the crystal lattice caused by the reduction of Ce4+ to Ce3+. Regarding the reaction conditions, the most efficient composition Ce0.85Zr0.15O2 enhances the required conditions by a temperature of 60 K or one order of magnitude of the partial pressure of oxygen p(O2) compared to pure ceria. The optimal composition was tested in long-term experiments of one hundred cycles, which revealed declining splitting kinetics.

热化学硫碘制氢系统中硫酸分解反应器的换热性能

硫酸分解反应器作为热化学硫碘制氢系统中的重要设备,其换热需要匹配系统的产氢量换热需求。为研究硫酸分解反应器不同结构对换热的影响,使反应器的换热满足系统需求,同时满足制造工艺的限制。通过试验对硫酸分解反应进行反应动力学参数标定并建立反应动力学模型,用gPROMS软件对反应器进行仿真,得到反应器内的压力、温度、流量和各组分浓度等参数。结果表明:反应器总长度不变,调整预热段和反应段长度比或提高填充颗粒热导率无法提升总转化率。反应器增加预热段长度可显著增加总转化率,关键原因是预热段长度决定了反应器内温度能否达到SO3分解反应所需最佳温度850℃。减小反应器直径并不能增加总转化率,虽然反应器的直径减小有利于传热,但由于整体入口流量不变,流体流速显著提高,减少了反应物的停留时间,同时还会显著增加反应器流阻。采用套筒环腔内、外加热结构作为反应器预热段可有效提高总转化率。采用内外同时加热时,增加了换热面积,有利于缩短预热段长度,预热段长度仅需900 mm左右反应器出口温度即可达到850℃,得到了符合要求的反应器结构设计。

Using a simulation software to perform energy and exergy analyses of the sulfur-iodine thermochemical process

The objective of this work is to demonstrate the utilization of the power of simulation tools to perform an exergy analysis of a process.Exergy analysis,being a new and useful thermodynamics tool,will be applied to one of the newest research fields in hydrogen production.One of the many advantages of computer simulation is elimination of the need to construct a pilot plant.Presently,extensive research is underway to come up with the production and use of clean fuels.The research entails performing pilot studies and proof of concept experiments using validated models.The research is further extended to various analyses such as safety,economic sustainability and energy efficiency of the processes involved.The production of hydrogen through thermochemical water splitting processes is one of the newest technologies and is expected to compete with the existing technologies.Among a wide range of thermochemical cycles,the sulfur-iodine(SI)thermochemical cycle process has been proposed as a promising technology for the production of hydrogen.In this research,we demonstrate how a commercial simulator can be used to perform an energy and exergy analysis of the SI water splitting process.Using a commercial simulator,a process flowsheet is developed based on research findings presented by other authors and an energy-exergy analysis is carried out on the process.The method of energy–exergy analysis used in this presentation indicates that an energy and exergy efficiency of 17%and 24%can be attained,respectively,in the conceptual design of the SI cycle.

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<Tred<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 Tred=1200 K and Toxi=400 K,indicating that MoO2 could be a new and potential candidate for obtaining solar fuel by methane reduction.

A simple and novel effective strategy using mechanical treatment to improve the oxygen uptake/release rate of YBaCo_(4)O_(7+δ) for thermochemical cycles

In recent years,oxygen storage materials(OSMs)have been widely used in many fields.It would be particularly important for researchers to design high-oxygen-uptake/release-rate materials.In this study,various synthesis processes were used to successfully synthesize YBaCo_(4)O_(7+δ)and comprehensively investigate their potential applications.Compa red with traditional solid-state reaction method and co-precipitation method,the results demonstrated that the utilization of mechanical ball milling treatment on co-precipitated precursors could lead to samples with reversible oxygen uptake/release under an oxidative atmosphere at low temperatures.The resultant materials exhibited fast oxygen absorption/desorption rate that could uptake/release oxygen directly to the equilibrium state within 9 min and20 min,respectively.The mechanochemically ball-milled sample possessed outstanding oxygen sto rage performance,which could be attributed to their small particle size,the active outer surface of particles,large specific surface area,and relatively low activation energy.Moreover,the ball-milled sample also exhibited excellent cycling stability during relatively short time spacing.TG results also demonstrated that the ball-milled samples could reversibly uptake/release 2.90 wt.%of excess oxygen(while only 0.70 wt.%for solid-state samples)by adjusting the ambient temperature under pure O_(2) atmosphere,which would make them promising candidates in various applications.This research demonstrated that mechanical treatment could be an effective strategy to tune the properties and oxygen storage capacity(OSC)performances of YBaCo_(4)O_(7+δ).

热化学硫碘循环水分解制氢研究进展

随着化石能源日益短缺和环境污染日益严重,寻求高效、清洁、可再生能源迫在眉睫。氢能作为现代能源体系新密码,因清洁、可再生、可存储、用途广泛等优点,近年来受到广泛关注。目前制氢方式众多,但大规模、高效、低成本、绿色的制氢方式才是未来氢能经济的基础。其中,热化学硫碘循环(也称碘硫循环)水分解制氢因具备上述优点被认为是最具发展潜力的制氢方法之一。从Bunsen反应、H2SO4分解和HI分解3方面综述了热化学硫碘循环水分解制氢的基础研究情况,对目前各国已建立的硫碘循环系统进行总结,着重介绍了核能耦合硫碘循环制氢流程的最新进展。从经济、环保和安全方面对目前核能耦合硫碘制氢进行分析讨论,以期为未来研究和发展提供新思路。针对Bunsen反应部分,寻找一种高效分离反应物的新方法至关重要;而对H2SO4分解和HI分解部分,现阶段主要研究仍集中在开发稳定、高效、低成本的催化剂。经过几十年研究,热化学硫碘循环水分解制氢技术已取得长足发展。在实际产氢中,苛刻的高温高腐蚀环境、复杂的耦合流程是限制其规模化、工业化的主要原因,开发由工业结构材料制成的耐腐蚀和耐热系统,继续对耦合流程进行优化和模拟是未来热化学硫碘循环水分解制氢技术的发展方向。

核能制氢与高温气冷堆

氢是环境友好的能源载体,利用核能制氢引起了广泛的研究兴趣。本文对核能制氢的工艺,包括甲烷蒸气重整、高温电解和热化学循环进行了综述;提出了制氢过程对反应堆的要求,并介绍了高温气冷堆用于核能制氢的优势。

太阳能制氢研究现状及展望

综述了国内外制氢研究现状。对常用的太阳能制氢方法:直接热分解法、热化学循环法、光电化学分解法(PEC)以及光催化法进行了分析,指出了各种方法的研究难点和重点。并结合我国的现状提出目前我国应该把光电化学分解法和2步热化学循环法作为研究的重点。

核能制氢技术的发展

氢是清洁能源,有非常好的应用前景。但氢是二次能源,需要利用一次能源来生产。以可持续的方式(原料来源丰富、无温室气体排放)实现氢的大规模生产是实现氢广泛利用的前提。核能是清洁的一次能源,核电已经成为世界电力生产的主要方式之一。正在研发的第四代核能系统除了要使核电生产更经济和更安全之外,还要为实现核能在发电之外的领域的应用开辟途径。核能制氢就是以来源丰富的水为原料,利用核能实现氢的大规模生产。热化学循环工艺和高温蒸汽电解都是有望与核能耦合的先进制氢工艺,世界上许多国家,如美国、日本、法国、加拿大和中国,都在大力开展核能制氢技术的研发工作。中国正在积极发展核电,在大力开展核电站建设的同时,也非常重视核氢技术的发展。可以提供高温工艺热、最适合用于制氢的高温气冷堆示范电站的建设已经列入国家重大专项;在进行示范电站建设的同时,正在开展制氢工艺的研发工作。在2009年,清华大学核能与新能源技术研究院成功进行了对硫碘热化学循环和高温蒸汽电解的实验室规模工艺验证。

大容量热化学吸附储热原理及性能分析

储能技术是提高能源利用效率的一种有效手段,可有效调配能量供给与需求在时间、空间和强度上的匹配关系,传统显热储存技术和相变潜热储存技术的储热密度一般在100～200kJ/kg,储热能力较低不利于规模化应用.本工作提出一种基于固-气化学反应的大容量热化学吸附储热方法,利用吸附工质对在化学反应过程中热能与化学吸附势能的相关转化实现热量的储存和释放,具有高效储热的显著优点.采用4种典型温区的吸附储热工质对为例进行了热化学吸附储热热力循环特性及工作性能的理论研究,在此基础上对不同温区吸附储热工质对（50～280℃）的热化学物性参数、储热温度、储热密度进行了分析比较,以期实现不同温度品位的热量储存.结果表明：热化学吸附储热技术的反应盐储热密度高达2000 kJ/kg以上,其储能密度约为传统显热储存技术和相变潜热储存技术的10～20倍,是一种具有发展潜力的大容量、高性能储热方法,该新技术可为规模化工业储热应用及太阳能等可再生能源的高效利用提供技术支撑.

硫碘制氢中碘化氢分解的化学模拟及实验研究

对热化学硫碘制氢中的碘化氢分解反应进行了化学热力学、动力学模拟以及实验研究,同时利用热力学的方法研究了氢气选择性膜对碘化氢分解率的影响。化学热力学模拟中,500℃下,HI的分解率只达到22.8%。但利用膜分离只移走氢气和同时移走氢气和碘蒸气的情况下,分解率分别增长了30.3%和54.8%。化学动力学模拟中,随温度的升高,HI浓度降低曲线呈现一定的线形规律,该分解反应对温度的敏感性较高。通过实验结果和动力学模拟结果的对比,该动力学模型能较好地描述HI分解的化学反应历程。

太阳能金属热化学循环制氢现状

介绍太阳能电解水制氢、直接分解水制氢、金属热化学循环制氢气的优缺点,说明两步热循环制氢具有优良的发展前景。重点介绍了Fe,Mg,Al,Zn等金属在两步热化学循环制氢中的反应温度、动力学、能量利用率及副产品等指标,指出ZnO/Zn最适合热化学循环制氢,并介绍ZnO/Zn两步热循环制氢的最新设备。展望热化学循环制氢未来发展方向。

热化学硫碘制氢Bunsen反应两相分离特性的实验研究

为确定热化学硫碘闭路循环运行条件及提高流程热效率,在291～358K温度范围内,实验研究了Bunsen反应中H2SO4-HI-I2-H2O四元混合溶液在摩尔比约为1/2/1.6/12时发生液-液分离后的两相分离特性。经过离子平衡、质量平衡计算,结果表明:345～358K温度范围内发生Bunsen反应有利于两相分离特性的改善,从而提高系统热效率及优化循环条件。

水量对于Bunsen反应分层特性影响的实验研究

以Bunsen反应产物不同酸相溶液的分层特性为研究对象,通过分析H2SO4/HI/I2/H2O四元混合溶液的分层现象,实验研究水量对两相溶液体系分层的具体影响。实验结果表明:过量水的加入导致两相溶液中的杂质增加,但能有效抑制副反应的发生,过量的水更易移动至H2SO4相;两相溶液的分层特性随水量的增加而变差,当H2O/H2SO4物质的量之比为16时,两相溶液中的I2物质的量分数分别约0.07和0.50;水量是影响分层特性的最重要因素之一。

光热驱动多孔氧化铈热化学循环解水制氢非热质平衡模型

热化学循环太阳燃料技术过程所涉及的多孔介质中复杂反应及热质传递过程,尚未建立较为完善的数学模型。以多孔氧化铈热化学循环解水过程为研究对象,将颗粒尺度的氧输运与宏观尺度的热质输运相耦合,提出完整的光热驱动条件下多孔介质非热质平衡模型,实验数据对比验证了动力学及热质输运模型的可靠性,分析了两种尺度(颗粒及床层)下,非热平衡效应、入射辐射热流、反应物浓度对动态过程的影响。入射辐射在床层的体积效应下,轴向的温度梯度使得缺陷反应的热力学平衡控制最大氧空位浓度出现在床层前侧,在缺陷反应的动态过程中,氧化过程相较于还原反应更快,提高多孔载氧体反应器的产物H2浓度应主要从还原阶段中反应过程及条件出发。可为该类问题的建模和过程设计提供较为完整的理论基础和参考路径。