Analysis of CO2 sorption/desorption kinetic behaviors and reaction mechanisms on Li4SiO4

The CO₂ sorption/desorption kinetic behaviors on Li₄SiO₄ were analyzed. The theoretical compositions of the sorption/desorption reactions were calculated using FactSage. The sorption/desorption process on Li₄SiO₄ was investigated by comparing the shrinking core, double exponential, and Avrami–Erofeev models. The Avrami–Erofeev model fits the kinetic thermogravimetric experimental data well and, together with the double‐shell mechanism, clearly explains the sorption/desorption mechanism. The sorption process is limited by the rate of the formation and growth of the crystals with double‐shell structure consisting of Li₂CO₃ and Li₂SiO₃. The whole desorption process is found to be controlled by the rate of the formation and growth of the Li₄SiO₄ crystals. Furthermore, the influence of steam on the CO₂ sorption process was analyzed. It has been observed that the presence of steam enhance the mobility of Li⁺ and, therefore, the rate of diffusion control stage. © 2012 American Institute of Chemical Engineers AIChE J, 59: 901–911, 2013     

Parametric study and effect of calcination and carbonation conditions on the CO2 capture performance of lithium orthosilicate sorbent

The world is currently facing the challenges of global warming and climate change. Numerous efforts have been taken to mitigate CO_2 emission, among which is the use of solid sorbents for CO_2 capture. In this work, Li_4SiO_4 was synthesised via a sol–gel method using lithium nitrate(LiNO_3) and tetraethylorthosilicate(Si C8 H20 O4) as precursors. A parametric study of Li:Si molar ratio(1-5), calcination temperature(600–800 °C) and calcination time(1–8 h) were conducted during sorbent synthesis. Calcination temperature(700–800 °C) and carbonation temperature(500–700 °C) during CO_2 sorption activity were also varied to confirm the optimum operating temperature. Sorbent with the highest CO_2 sorption capacity was finally introduced to several cyclic tests to study the durability of the sorbent through 10 cycles of CO_2 sorption–desorption test. The results showed that the calcination temperature of 800 °C and carbonation temperature of 700 °C were the best operating temperatures, with CO_2 sorption capacity of 7.95 mmol CO_2?(g sorbent)-1(93% of the theoretical yield). Throughout the ten cyclic processes, CO_2 sorption capacity of the sorbent had dropped approximately 16.2% from the first to the tenth cycle, which was a reasonable decline. Thus, it was concluded that Li_4SiO_4 is a potential CO_2 solid sorbent for high temperature CO_2 capture activity.     

Novel Li4SiO4-based sorbents from diatomite for high temperature CO2 capture

Using inexpensive porous diatomite as silicon source, novel Li₄SiO₄-based sorbents for high temperature CO₂ capture were prepared through the solid-state reaction method at lower temperature (700 °C). Effect of different raw material ratios on CO₂ absorption capacity was investigated. The results showed that CO₂ absorption capacity was dependent on the raw material ratio. When the raw material ratio was 2.6:1, the CO₂ absorption capacity reached 30.32 wt% (83% of the theoretical absorption capacity) in the atmosphere (50 mL/min N₂ and 50 mL/min CO₂). Meanwhile, it was found that the as-prepared Li₄SiO₄-based sorbents from diatomite exhibited good absorption–desorption performance.     

Behaviors and kinetic models analysis of Li4SiO4 under various CO2 partial pressures

The absorption behaviors of Li₄SiO₄ sorbent under various CO₂ partial pressures and temperatures were investigated through numerical and experimental methods. It was found that Li₄SiO₄ showed poor absorption capacity at high temperatures (>525°C) under CO₂ partial pressure of 5066 Pa. This phenomenon was explained by the thermodynamic results from FactSage5.5 software. Meanwhile, a modified Jander‐Zhang model based on the double‐shell structure of the Li₄SiO₄ sorbent was developed to describe the absorption kinetic behaviors of CO₂ on Li₄SiO₄. The results showed that the modified Jander‐Zhang model could fit the kinetic experimental data well. Furthermore, the influence of steam on CO₂ absorption was also analyzed by the modified Jander‐Zhang model. The results showed that the activation energy in the absorption process with steam was smaller than that without steam, which indicated that the presence of steam could promote the CO₂ diffusion in product layer, therefore, improving the sorption capacity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2153–2164, 2017     

K-doping effect in the kinetics of CO2 capture at high temperature over lithium silicates obtained from rice husks: In situ/operando techniques

K-doped lithium silicates were synthesized as CO₂ sorbents using rice husks as silica source. Two different doping methods were used to prepare the solids, incipient wetness impregnation and co-impregnation. Noticeable differences were observed between the sorbents obtained by each route. Unlike the co-impregnated sample, a KLi₃SiO₄ phase was detected by XRD in the doped-samples prepared by incipient wetness impregnation. This phase was associated with a porous layer deposition over the sorbent particle surface, which was observed by SEM. The effect of CO₂ partial pressure and sorption temperature was evaluated for the solids. A remarkable improvement of the capture efficiency was observed in the doped-samples, especially at low temperature and CO₂ partial pressure. The kinetics of the samples was studied using the double exponential model and operando Raman and DRIFT spectroscopies. Different reactivity properties were found depending on the synthesis method employed. The complete analysis of the results allowed us to propose a transformation mechanism for the solids during the sorption step, highlighting the improvement of surface reaction and volumetric diffusion processes in the CO₂ capture step.     

TiO2掺杂对Na2CO3/Al2O3吸收剂CO2捕捉性能的影响

钠基固体吸收剂脱除燃煤烟气CO₂技术具有反应温度低、能耗低等优点,日益受到学术界的关注。该技术的主要不足是吸收剂的活性成分碳酸钠与CO₂的反应（碳酸化反应）活性较低。针对这一问题,本文旨在研制一种新型改性钠基固体吸收剂,采用活性氧化铝作为载体、TiO₂作为掺杂剂进行改性,利用热重分析装置、XRD、SEM和氮吸附仪研究钠基固体吸收剂的CO₂捕捉性能。结果表明：掺杂TiO₂后,钠基固体吸收剂与CO₂的反应速率加快,CO₂捕捉量增加;反应前后除TiO₂外无其他含Ti化合物生成;碳酸化反应产物为NaHCO₃和Na₅H₃（CO₃）₄;然而TiO₂掺杂过多会堵塞吸收剂的微观孔道,不利于甚至阻碍碳酸化反应的进行,因此,TiO₂的掺杂量应控制在一定的范围内。     

A kinetic model involving oxidation and volatilization in SiC-B4C-xAl2O3 ceramics system

In this paper, a new kinetic model considering both oxidation and volatilization kinetics is established and applied to analyze the oxidation of SiC-B₄C-xAl₂O₃ ceramics and other systems in various oxidation conditions. The effects of diffusion area and volume changes during the oxidation process are considered in this model. The physical meaning of each parameter in this model is explicit and simple. According to this model, the diffusion coefficient of species and the corresponding diffusion activation energy are easily available. The practicability of this model is well verified by the experimental data of SiC-B₄C-xAl₂O₃ and other systems oxidized under different conditions. In addition, the practice shows that the model is applicable not only to the systems where oxidation and volatilization coexist, but also to the system where only oxidation plays a major role. We hope the model proposed in this work can be used in other materials with more complex environments.     

Synthesis,kinetic study,and reaction mechanism of Li4SiO4 with CO2 in a slurry bubble column reactor

Abstract

This study was performed to investigate the synthesis, kinetic and reaction mechanism of Li₄SiO₄ with CO₂ in a slurry bubble column reactor. The Li₄SiO₄ powder sample was prepared via a solid-state reaction. The sample was characterized via X-ray diffraction (XRD) analysis and verified as a single phase. The median diameter of the sample was measured using the laser diffraction and scattering method as about 20?μm. The synthesized sample was suspended in binary molten carbonate of Li₂CO₃–K₂CO₃ having a molar ratio of 38:62. The experimental results show that Li₄SiO₄ in the slurry bubble column absorbed approximately a stoichiometric amount of CO₂. The kinetic study shows that the CO₂ reaction behavior on the Li₄SiO₄ surface was fitted to a double exponential model and the limiting step of the reaction was lithium diffusion. The mass transfer coefficient of CO₂ and rate constant of reaction with Li₄SiO₄ were studied to understand the overall absorption mechanism in the reactor. The resistance for the direct reaction of CO₂ on the Li₄SiO₄ was much smaller than the resistance for the mass transfer of CO₂ to the Li₄SiO₄. We can conclude that the direct contact of CO₂ with Li₄SiO₄ was the main path for the reaction.     

Synthesis of waste bagasse-derived Li4SiO4-based ceramics for cyclic CO2 capture: Investigation on the effects of different pretreatment approaches

The bagasse, which is a typical agricultural and forestry waste, was employed as the silicon source to produce Li₄SiO₄-based ceramics with superior CO₂ sorption performance. It is found that the presence of impurities elements in the bagasse ash will bring complex side reactions and form side products during the sorbent preparation process, which are extremely harmful to the performance of the synthetic sorbent. Pretreatment has been considered as an effective approach to reduce the impurities elements. In this work, the effects and mechanisms of water/acid washing pretreatments on bagasse ash were firstly detailed investigate and compared. The results indicate that the hydrochloric acid washing pretreatment can effectively remove the residual metals (Ca, Fe, etc.) in the sample in addition to the water-soluble impurities (K, P, S, etc.). The maximum sorption capacity of sorbent synthesized by the acid washing pretreatment is about 0.32 g/g sorbent and the superior performance can be well maintained over 10 cycles. A novel technical route utilizing the ash from biomass power plants for in-situ CO₂ capture has been developed, which has great potential for carbon capture in the future.     

新型Li_4SiO_4高温吸收CO_2的动力学研究

高温固体CO2吸收剂硅酸锂材料以其较高的吸收容量、优良的循环吸收稳定性成为研究热点。文中以廉价的、具有丰富孔结构的硅藻土和碳酸锂为原料,采用高温固相法于600℃下合成了可在高温直接吸收CO2的硅酸锂材料。XRD结果分析表明所制备的材料由Li4SiO4和少量的LiAlSi2O6相组成,用同步热重分析仪(TG-DSC)研究了在等温条件下硅酸锂材料吸收CO2的性能。用双指数模型拟合了硅酸锂材料吸收CO2的过程。结果表明:吸收CO2的温度不同,硅酸锂材料吸收CO2反应的控制步也不相同。表面反应速率常数与扩散速率常数的相对大小在很大程度上影响了硅酸锂材料吸收CO2的性能。     

湿法再生CO2空气捕集材料的能耗与性能优化

湿法再生阴离子交换树脂膜材料,可通过调控湿度驱动CO₂的吸附与脱附,材料再生成本极低。该材料需经过高温水热预处理张开孔结构,增强气体扩散速率,能耗较高;此外,利用液态水润湿材料以驱动脱附时,材料的解吸比（脱附量/吸附量）只有~30%。通过系统研究不同预处理水温及时间下膜材料的CO₂吸附/脱附性能,发现采用常温水浸泡预处理亦可获得良好的材料微观孔结构以及碳捕集性能,显著降低预处理能耗;更重要的是,基于微观尺度的气体吸附和液体浸润相关理论,实验发现超声雾化所获得的微米级水颗粒,由于更易扩散进入孔隙,可将解吸比从~30%提升到~60%,极大提升了树脂膜材料的再生性能。这些预处理能耗与脱附性能的优化,为大规模空气捕集的工程化实施提供了有利条件。     

循环捕集CO2后煅烧石灰石的硫化特性

ZEC(zero emission coal)系统中,粗煤气进入碳酸化/重整炉前需先脱除H₂S,提出利用经过多次碳酸化/煅烧捕集CO₂循环的煅烧石灰石(CaO)脱除H₂S,并研究循环碳酸化/煅烧次数、硫化温度、H₂S浓度和微观结构对循环CaO硫化特性的影响。结果表明,多次循环碳酸化/煅烧捕集CO₂后CaO仍具有较高H₂S吸收性能。前20次循环,CaO硫化转化率随循环次数增加迅速降低;20次循环后,CaO硫化转化率缓慢下降。硫化120 min后,未循环CaO的硫化转化率接近100%,而经历1、20和100次循环后CaO的硫化转化率分别为94%、81%和74%。H₂S浓度对循环CaO硫化性能影响较大。硫化温度(800～1000℃)对循环CaO的硫化性能影响较小,最佳硫化温度为900℃。随循环次数增加,CaO颗粒发生高温烧结,导致比表面积降低和20～150 nm内孔隙减少,而这是与H₂S吸收密切相关的孔隙,导致CaO硫化转化率降低。     

高岭土-硅酸锂的制备及其吸附CO2动力学

以煤系高岭土为硅源,采用浸渍沉淀法制备硅酸锂吸附剂。采用X射线衍射仪、扫描电子显微镜分别表征和分析了样品的结构特征和形貌特征,并使用差热-热重联用分析仪研究了硅酸锂吸附CO₂的性能。实验结果表明,与以市售-SiO₂为硅源制备的硅酸锂相比,高岭土-硅酸锂在较低温度下有明显的吸附CO₂优势;采用浸渍沉淀法可成功制备出纯度较高的硅酸锂,最大吸附量为35.7%(704℃)(文中吸附量均为质量分数),高出固相法5.9%;采用双指数模型对硅酸锂吸附CO₂进行动力学分析,温度低于600℃时,表面化学反应过程是整个反应过程的控制步骤。高于650℃,锂迁移成为控制步骤。在吸附CO₂的过程中,浸渍沉淀法制备的样品锂迁移速率是固相法的1.5～2.2倍,低于650℃时的吸附,浸渍沉淀法的样品表面化学反应速率皆高于固相法。     

胶束催化作用及其在动力学分析中的应用

研究表明，适量的胶束可催化化学反应，在动力学分析中大有潜力，本文介绍了胶束催化作用，并就其在动力学分析中的最新研究进展及时予以评述。     

变温吸附碳捕集系统能效性能实验研究

燃烧后CO₂捕集技术（PCC）因易于与既有电厂结合而被认为是一项减少二氧化碳排放的重要技术。化学吸收、吸附和膜分离是PCC的主流技术。在CO₂吸附技术类中,变温吸附（TSA）是一种有效的吸附方法。近年来,TSA技术的能源消耗和能源转换效率问题成为人们对其大规模部署的关注焦点。然而,大多数的研究都是将数学模型和仿真方法应用于TSA的性能评估,缺乏足够的实验研究支持。为了对TSA系统的能源转化效率进行实验分析,开发了一套四步法TSA系统,能效性能是基本分离性能外的主要考察指标。实验采用沸石13X-APG作为吸附剂材料,根据实验测得的两组吸附等温线,计算了CO₂/N₂的吸附选择性系数。通过进气CO₂浓度、解吸时间、吸附温度和解吸温度对纯度、回收率、单位能耗和第二定律效率的影响分析,得到了4组实验结果。结果表明,第二定律效率的范围为3.24%～9.23%,回收率和纯度最高分别为83.97%和94.70%。解吸温度和进气CO₂浓度的升高,吸附温度的降低有利于分离及能效性能提升。延长解吸时间有利于分离和能效提升,但过长的操作时间反而使得效果变差,这会对工程中的运行策略优化产生积极的指导意义。     

石灰石和白云石高温循环脱除CO2过程分析

在N2气氛和高浓度CO2气氛两种典型锻烧气氛下,对石灰石和白云石在循环煅烧/碳酸化捕集CO2过程中的主要系统参数包括长周期循环碳酸化转化率、平均碳酸化转化率、CO2捕集效率和煅烧炉能量需求进行了实验研究和计算分析.结果表明,吸收剂补充流率和吸收剂循环流率对平均碳酸化转化率、CO2捕集效率和煅烧炉所需能量具有直接影响.在...     

化学吸收法CO2捕集解吸能耗的分析计算

推导出了化学吸收法CO2捕集工艺解吸总能耗的准确计算公式,根据年产量100万吨CO2的捕集工艺流程的节点参数,分析计算了解吸能耗中各项能耗的大小,并与Leites、晏水平、王海波等能耗估算公式结果进行了对比。分析表明,本文推导的能耗计算公式适用于设计后或运行时的能耗计算,在捕集工艺设计前可采用Leites、晏水平、王海波等公式估算解吸能耗。其中晏水平公式考虑了循环倍率,因此能耗估算公式的适用性强,循环倍率和比热容取值偏差不大时,估算精度较高。通过对推导出的解吸能耗公式的分析,即使吸收剂质量浓度、溶液的再生度、贫富液换热器的性能一定,仍能进一步降低解吸总能耗,并提出了进一步降低解吸总能耗的有效措施。     

Correlation of electrical properties and performance of OCM MOx/Na2WO4/SiO2 catalysts

The electrical conductivity and catalytic performance of MO_x/Na₂WO₄/SiO₂ catalysts in oxidative coupling of methane (OCM) are measured and correlated. M is V, Cr, Mn, Fe, Co or Zn. In this study, for the first time, the conductivity of the catalyst powder was measured under the OCM conditions as well as in oxygen. A definite correlation between the catalytic performance, the electrical conductivity under the OCM conditions, and the band gap of metal oxide constituent of the catalysts is observed. Manganese oxide (MnO_x with the lowest band gap) on Na₂WO₄SiO₂ with the highest electrical conductivity shows the best catalytic performance.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

变温吸附碳捕集系统能效性能对标分析

准确且合理的能耗分析对碳捕集技术规模化发展至关重要,其既是碳捕集技术节能降耗的必要前期准备,也是碳捕集过程开展绿色化、清洁化性能评价的重要数据基础。因此,在对碳捕集过程的能源转换共性展开探索的同时,迫切需要面向工程实践的需求,形成易于操作的能效性能对标分析方法,从而保证对类型技术的性能认知可以在合理且统一的评价平台上进行归纳与比较。本工作基于对标分析(Benchmarking Analysis)方法,对变温吸附碳捕集(TSA)过程的能耗分析方法进行了阐述,包括流程、参数、模型等。研究了吸附温度和解吸温度对TSA能效性能结果的影响,演示并量化了该方法的可行性,重点对边界变化对评价结果的影响进行了讨论。提出的对标分析方法对碳捕集技术的能效性能对标评估给出了较具体的指导。