惰性载体对CaSO4载氧体化学链燃烧性能的影响

为提高化学链燃烧中CaSO₄载氧体的机械强度和反应活性,采用机械混合法制备了负载甘油、硅溶胶以及拟薄水铝石惰性载体的CaSO₄载氧体,并对其机械强度、反应特性等进行了实验研究。结果表明,甘油和硅溶胶的加入可显著提高CaSO₄载氧体的机械强度,而拟薄水铝石作用较小。热重还原实验表明,甘油和硅溶胶的加入可加快载氧体与甲烷的反应速率,缩短反应时间。XRD分析表明,惰性载体的加入不会影响CaSO₄向CaS转化,CaSO₄接近完全转化。     

LaCoO_3载氧体在化学链燃烧中的性能研究

采用柠檬酸络合法制备了LaCoO_3氧化物,TPR表征结果表明该氧化物具有吸附氧和晶格氧,可以作为化学链燃烧的载氧体。化学链燃烧反应性能评价结果表明,以CO为燃料、反应温度为400℃和常压条件下,连续循环10次CO都全部转化,但是由于发生歧化反应,CO_2选择性没有达到100%。采用XRD,SEM和XPS等手段对新鲜LaCoO_3载氧体和循环10次后的LaCoO_3载氧体进行了表征,结果表明:连续循环10次后载氧体仍保持钙钛矿结构,由于钙钛矿氧化物具有丰富的吸附氧和晶格氧,所以载氧体保持了很好的稳定性。     

Cu修饰的Fe_2O_3/Al_2O_3氧载体的循环反应稳定性研究

采用冷冻干燥法制备了经Cu修饰(10%)的Fe2O3/Al2O3氧载体。利用热重分析仪分别在850、900和950℃等温环境下,使氧载体交替接触还原气体和氧化气体,来模拟氧载体在化学链燃烧中的循环过程。结果表明,经Cu修饰的Fe2O3/Al2O3氧载体在850和900℃下的等温循环过程中反应性能都很稳定,在950℃时的循环反应前期有微量烧结,但在循环后期反应性能也很稳定。随着反应温度的升高,氧载体氧化速率增大,还原速率和载氧率先减小后增大。与未经修饰的Fe2O3/Al2O3氧载体相比较,在900℃下作等温循环实验,经Cu修饰的Fe2O3/Al2O3氧载体具有较高的载氧能力和还原速率,但氧化速率较低;两者都具有较好的循环稳定性。     

Cu修饰的Fe2O3/Al2O3氧载体的循环反应稳定性研究

采用冷冻干燥法制备了经Cu修饰（10%）的Fe₂O₃/Al₂O₃氧载体。利用热重分析仪分别在850、900和950 ℃等温环境下,使氧载体交替接触还原气体和氧化气体,来模拟氧载体在化学链燃烧中的循环过程。结果表明,经Cu修饰的Fe₂O₃/Al₂O₃氧载体在850和900 ℃下的等温循环过程中反应性能都很稳定,在950 ℃时的循环反应前期有微量烧结,但在循环后期反应性能也很稳定。随着反应温度的升高,氧载体氧化速率增大,还原速率和载氧率先减小后增大。与未经修饰的Fe₂O₃/Al₂O₃氧载体相比较,在900 ℃下作等温循环实验,经Cu修饰的Fe₂O₃/Al₂O₃氧载体具有较高的载氧能力和还原速率,但氧化速率较低;两者都具有较好的循环稳定性。     

煤和生物质化学链气化中铜基载氧体与灰分的碳热反应研究

从反应温度、灰的种类和灰的比例三个方面,对煤和生物质化学链气化过程中铜基载氧体与灰中矿物的碳热反应过程进行了研究;通过往复切换固定床的氧化还原气氛模拟化学链气化的循环过程,利用XRD和SEM-EDS等表征手段并结合热力学计算对产物进行分析。结果表明,灰中Fe_2O_3和Al_2O_3易与CuO/Cu_2O反应形成CuAl_2O_4、Cu_2Fe_2O_4和CuFe_2O_4等尖晶石结构的物质,而CaO能够通过阻碍Cu-Al和Cu-Si复合化合物的形成来缓解铜基载氧体的烧结。温度升高促使CuO极易与CaSiO_3和MgSiO_3等硅酸盐矿物发生固-固反应,生成CaCuSi_2O_6和CuMgSi_2O_6等而降低铜基载氧体的反应活性。随着灰分比例的增加,Ca~(2+)和Fe~(3+)等离子富集所生成的Ca_2Fe_9O_(13)易与SiO_2发生反应生成高熔点的CaFeSi_2O_6三相共熔体,与铜基载氧体共熔并覆盖在载氧体表面,阻碍其释氧性能。     

化学链燃烧中LaNixFe1-xO3载氧体的性能研究

采用柠檬酸络合法制备了不同Ni含量的LaNixFe1-xO3系列复合氧化物载氧体。采用热分析、X射线衍射、程序升温还原和扫描电境等手段对载氧体进行了表征,并在化学链燃烧反应中进行性能评价。XRD表征结果表明不同Ni含量的LaNixFe1-xO3均能形成钙钛矿结构,TPR表征结果表明随Ni含量增加,LNixaFe1-xO3上氧数量增加,还原能力增强。在连续十次化学链燃烧反应中,95%一氧化碳在LaFe0.5Ni0.5O3上被氧化,扫描电境照片发现部分载氧体颗粒出现破碎、烧结现象。     

Mn-Fe_2O_3载氧体作用下CO化学链燃烧特性及动力学研究

采用共沉淀法制备不同物质的量比Mn掺杂的铁基载氧体(Mn-Fe2O3),并进行XRD、BET和TEM表征。开展不同温度下Mn-Fe2O3与CO的化学链燃烧实验,研究载氧体的反应特性,确定较优的掺杂量和反应温度。结果表明,适量的Mn掺杂有助于改善铁基载氧体的反应活性,Fe∶Mn物质的量为50∶1时燃烧反应转化率最高。多循环化学链燃烧实验证实了载氧体稳定性较好。不同升温速率(30、40、50℃/min)下反应动力学分析表明,Mn-Fe2O3与CO的化学链燃烧还原反应均属于随机成核和随后生长的Avrami-Erofeev方程模型,并依据模型分别计算出了该模型的活化能和频率因子。     

Ca-Fe/bentonite载氧体煤化学链燃烧反应特性

以天然石膏粉、膨润土(bentonite)和Fe(NO3)3·9H2O为原料,通过机械混合造粒法制备了钙基复合载氧体。在小型流化床反应器中,水蒸气作为气化-流化介质,研究了温度、活性组分含量及循环次数对复合载氧体反应活性的影响,同时考察了不同煤种化学链燃烧反应特性。实验结果表明,CaSO4含量为60%,Fe2O3为活性助剂的CaSO4-Fe2O3/ben(Ca-Fe/ben)载氧体平均磨损速率为0.089%/h。反应温度为900℃时,碳转化率达到95%所需的时间为20.8min,CO2平均干基浓度为95.99%,表现高的反应活性。10次氧化/还原反应后,CO2平均干基浓度保持在80%,载氧体保持良好的循环反应活性。同时,实验发现高挥发分高灰分的煤种更适于煤的化学链燃烧,且CO2浓度均保持在90%以上。粒径分布曲线表明循环反应中载氧体表现强的抗磨损能力。     

表面形貌控制增强铁基载氧体与褐煤化学链燃烧反应活性

为探究载氧体形貌控制获取适用于化学链燃烧的高活性表面结构载氧体的可行性,以Fe2O3作为模型载氧体,从理论上对比研究Fe2O3的高弥勒指数晶面(104)和Fe2O3自然裸露的最主要晶面之一(001)的表面电子特性,结果表明,Fe2O3(104)的电子结构更有利于表面与煤模型分子的相互作用.基于理论分析结果,从实验上控制制备了单晶载氧体Fe2O3(104)/Al2O3,研究了该载氧体与褐煤的化学链燃烧反应特性.Fe2O3(104)/Al2O3比传统浸渍法制备的载氧体Fe2O3/Al2O3具有更高的反应活性,与理论计算结果一致.元素分析表明,Fe2O3(104)/Al2O3与褐煤反应的积碳量远少于Fe2O3/Al2O3与褐煤反应的积碳量.对比新鲜载氧体及再生后载氧体的结构发现,Fe2O3(104)/Al2O3在反应过程中不断进行氧化还原反应而发生结构弛豫后,仍然能通过氧化再生.这表明形貌控制制备可为化学链燃烧技术开发新型高效载氧体提供新思路.     

K_xFe_yO_z对煤化学链催化燃烧性能影响研究

以廉价的钙铝水泥作为载氧体制备过程的载体,并以机械混合-挤压成型造粒法制备了基于Fe2O3为载氧体活性相、钙铝水泥为载体的新型载氧体。在单流化床反应器上研究了钙铝水泥添加比例以及钾添加剂对合成载氧体的化学链燃烧性能的影响。研究结果表明,合成载氧体中载体以Ca2Al2SiO7形式存在,钾的添加显著提高了煤气化反应速率以及煤转化速率,钾在稳定相中以K2Fe22O34存在。K2Fe22O34在煤化学链燃烧过程的催化性能体现在其作为储钾相与KFeO2相的形态转变过程中。     

Effective direct chemical looping coal combustion with bi-metallic Fe–Cu oxygen carriers studied using TG-MS techniques

This paper contains the results of research on a promising combustion technology known as chemical looping combustion (CLC). The noteworthy advantage of CLC is that a concentrated CO₂ stream can be obtained after water condensation without any energy penalty for CO₂ separation. The objective of this work was to prepare novel bi-metallic Fe–Cu oxygen carriers and to evaluate the performance of these carriers for the CLC process with hard coal/air. One-cycle CLC tests were conducted with supported Fe–Cu oxygen carriers in thermogravimetric analyzer (TG) utilizing hard coal as a fuel. The effects of the oxygen carrier chemical composition, particle size, and steam addition on the reaction rates were determined. The fractional reduction, fractional oxidation, and the reaction rates were calculated from the TG data. Notably, the support had a considerable effect on the reaction performance. Moreover, bi-metallic Fe–Cu oxygen carriers exhibited significantly improved reactivity compared with monometallic Fe oxygen carriers. Furthermore, the addition of a second reactive metal oxide stabilized the oxygen carrier structure. The oxidation reaction was significantly faster than the reduction reaction for all supported Fe–Cu oxygen carriers. The TG data indicated that these oxygen carriers had stable performances up to 900 °C and may be effectively used for direct coal CLC reactions.     

Copper-based oxygen carriers supported with alumina/lime for the chemical looping conversion of gaseous fuels

Copper(Ⅱ) oxide in varying ratios was combined with either an alumina-based cement(Al300), or CaO derived from limestone as support material in a mechanical pelletiser. This production method was used to investigate its influence on possible mechanical and chemical improvements for oxygen carriers in chemical looping processes. These materials were tested in a lab-scale fluidised bed with CO or CH_4 as a reducing gas at 950 °C. As expected, the oxygen carriers containing a greater ratio of support material exhibited an enhanced crushing strength. Oxygen carriers comprised of a 1:3 ratio of support material to active CuO exhibited increased crushing strength by a minimum of 280% compared to pure CuO pellets.All oxygen carriers exhibited a high CO conversion yield and were fully reducible from CuO to Cu. For the initial redox cycle, Al300-supported oxygen carriers showed the highest fuel and oxygen carrier conversion. The general trend observed was a decline in conversion with an increasing number of redox cycles.In the case of CaO-supported oxygen carriers, all but one of the oxygen carriers suffered agglomeration.The agglomeration was more severe in carriers with higher ratios of CuO. Oxygen carrier Cu25Al75(75 wt% aluminate cement and 25 wt% CuO), which did not suffer from agglomeration, showed the highest attrition with a loss of approximately 8% of its initial mass over 25 redox cycles. The reducibility of the oxygen carriers was limited with CH_4 in comparison to CO. CH_4 conversion were 15%-25% and 50% for Cu25Ca75(25 wt% CuO and 75 wt% CaO) and Cu25Al75, respectively. Cu25Ca75 demonstrated improved conversion, whereas Cu25Al75 exhibited a trending decrease in conversion with increasing redox cycles.     

Study on modification of Cu-based oxygen carrier for chemical looping combustion

Chemical looping combustion (CLC) is a promising technology for segregation of carbon dioxide. CLC uses a metal oxide as an oxygen carrier, which transfers oxygen from the air to the fuel avoiding direct contact between them, thus separating the carbon dioxide and nitrogen. Cu-based oxygen carriers are excellent mediums due to high reactivity, environmental friendliness, and favorable thermodynamics. However, there are agglomeration issues due to low melting point of Cu. To solve this issue, a new preparation method as well as a dispersion reagent and a thermal durability-enhanced reagent were applied simultaneously to the oxygen carrier. The carriers were synthesized using both wet and dry impregnation methods. Based on the initial oxygen loading capability tests, the dry impregnation method received additional investigation. The characterizations of the oxygen carriers were evaluated using thermogravimetric analyzer (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), and surface area analyzer. TG results demonstrate that the enhanced dry impregnation was an effective preparation method, where the mass loss of the oxygen carrier was typically 3.4 %, correlating to almost 17 % loaded CuO. XRD results indicate a new phase, CuAl₂O₄ spinel, formed after the first few redox cycles, which is responsible for promoting the thermal stability of the oxygen carriers. SEM results show that the addition of the dispersants decreased the agglomeration and the enhanced reagent chemicals greatly improved the strength of the carriers. However, the surface area of the oxygen carriers decreased with the addition of the additives. In addition, with the increasing redox cycles, the surface area also decreased while the pore size increased, indicating that small pores were crushed, but the reactivity of the oxygen carriers did not decrease. In conclusion, the oxygen carriers produced in this manner are suitable for multi-cycle tests, and a major hurdle toward reducing greenhouse gases has been achieved.     

Thermal characteristics of Cu-based oxygen carriers

Chemical looping combustion (CLC) is a novel combustion technology with the capability for segregation of exhaust products (i.e., carbon dioxide/H₂O or N₂/O₂). The combustion is performed in two interconnected reactors with a solid oxygen carrier circulating between them, transferring oxygen from the air to the fuel. The feasibility of a successful CLC system depends on the selection of an appropriate oxygen carrier. Cu-based oxygen carriers are good oxygen carriers due to high reactivity. However, it faces low melting point, agglomeration problems in fluidized bed. In this study, a circular reduction?Coxidation reaction simulated to the cyclic operation of the Cu-based oxygen carrier was conducted on the thermogravimetric analyzer (TG). The thermal behaviors of the potential Cu-based oxygen carrier were investigated by using an X-ray diffraction (XRD), scanning electron microscope (SEM), and surface analyzer. Multiple TG results show that the weight loss was 3.4%, indicating that the loading CuO amount was 17%. Moreover, the weight loss and weight gain was equal during 73 redox cycles, suggesting the good thermal stability of the oxygen carrier. The conversion rate of reduction and oxidation for each redox cycle remained constant even after 73 redox cycles. XRD results show the new phase formation of CuAl₂O₄ during redox cycles, which promotes the thermal stabilization of the oxygen carrier. The surface area of the oxygen carrier decreased from 105 to 13?m^2?g^?1 after 73 redox cycles and the particle size distribution shifted from 5?C15?nm to 15?C30?nm, suggesting that the micorpores were blocked or collapsed. However, the reactivity of the oxygen carrier didn??t decrease. SEM results show that CuO was evenly distributed on the surface of Al₂O₃ after 73 redox cycles. Overall, these results suggested that the Cu-based oxygen carrier was ready for fluidized bed tests.     

Sewage sludge ash as an alternative low-cost oxygen carrier for chemical looping combustion

In this paper, novel low-cost oxygen carriers containing Fe₂O₃ are evaluated for use in chemical looping combustion. Sewage sludge ashes and reference samples were prepared and used in cyclic reduction and oxidation experiments in a thermogravimetric analyzer (TG). A gaseous (3 % H₂) fuel and a solid fuel (hard coal) were tested. Three-cycle CLC tests were carried out in the 600–800 °C temperature range and long-term testing was performed at 950 °C. A reactivity study showed that the natural sewage sludge ash sample was stable during the cycling TG tests when hydrogen was used as a fuel at all of the temperatures investigated. Strong temperature effects on the oxygen transport capacity were observed. An one-cycle test at 900 °C showed also that the sewage sludge ash successfully reacted with coal. The oxygen released was fully used for coal combustion, with appreciable reaction rate at temperature of ~750–800 °C, that is significantly lower than that obtained for pure Fe₂O₃-based oxygen carrier. The oxidation reaction was much faster than the reduction reaction. Moreover, the sewage sludge ash showed a low tendency toward agglomeration in the cyclic test, which was superior to the behavior of synthetic materials. The sewage sludge ash exhibited also high mechanical strength, an attrition index of 1 % and a high-temperature resistance of 1,170 °C in a reducing atmosphere. We conclude that sewage sludge ash can be effectively used as a low-cost, valuable oxygen carrier in practical application in chemical looping combustion technology for power generation.     

Application of Fe2O3/Al2O3 Composite Particles as Oxygen Carrier of Chemical Looping Combustion

Chemical looping combustion (CLC) of carbonaceous compounds has been proposed, in the past decade, as an efficient method for CO2 capture without cost of extra energy penalties. The technique involves the use of a metal oxide as an oxygen carrier that transfers oxygen from combustion air to fuels. The combustion is carried out in a two-step process: in the fuel reactor, the fuel is oxidized by a metal oxide, and in the air reactor, the reduced metal is oxidized back to the original phase. The use of iron oxide as an oxygen carrier has been investigated in this article. Particles composed of 80 wt% Fe2O3, together with Al2O3 as binder, have been prepared by impregnation methods. X-ray diffraction (XRD) analysis reveals that Fe2O3 does not interact with the Al2O3 binder after multi-cycles. The reactivity of the oxygen carrier particles has been studied in twenty-cycle reduction-oxidation tests in a thermal gravimetrical analysis (TGA) reactor. The components in the outlet gas have been analyzed. It has been observed that about 85% of CH4 converted to CO2 and H2O during most of the reduction periods. The oxygen carrier has kept quite a high reactivity in the twenty-cycle reactions. In the first twenty reaction cycles, the reaction rates became slightly higher with the number of cyclic reactions increasing, which was confirmed by the scanning electron microscopy (SEM) test results. The SEM analysis revealed that the pore size inside the particle had been enlarged by the thermal stress during the reaction, which was favorable for diffusion of the gaseous reactants into the particles. The experimental results suggested that the Fe2O3/Al2O3 oxygen carrier was a promising candidate for a CLC system.     

Cycle performance of Cu-based oxygen carrier based on a chemical-looping combustion process

The cycle life of oxygen carrier(OC) is crucial to the practical applications of chemical looping combustion(CLC). Cycle performance of Cu/SiO_2 prepared with a mechanical mixing method was evaluated based on a CLC process characterized with an added methane steam reforming step. The Cu/SiO_2 exhibited high redox reactivity in the initial cycles, while the performance degraded with cycle number. Through characterization of the degraded Cu/SiO_2, the performance degradation was mainly caused by the secondary particles' fragmentation and the fine particles' local agglomeration, which worsened the distribution and diffusion of the reactive gases in the packed bed. A regeneration method of the degraded OC based on re-granulation has been proposed, and its mechanism has been illustrated. With this method, the performance of the degraded OC through 420 redox cycles was recovered to a level close to the initial one.     

煤化学链燃烧Fe2O3载氧体的反应性研究

利用流化床反应器并以水蒸气作为气化-流化介质,研究了温度、反应时间、循环数对Fe₂O₃载氧体反应性的影响。实验表明,载氧体与煤气化产物的反应性随温度升高而增强,且温度越高,反应受化学反应控制时间越短。当温度高于900℃时,煤中碳转化为CO₂的比率大于90%,载氧体体现了很好的反应性,但反应温度低于850℃时,比率小于75%。反应温度900℃时,CO₂干基浓度随循环数而逐渐降低,CO、CH₄浓度增加,且CH₄浓度值大于CO。利用XRD、SEM分析了固体反应产物成分与微观形态结构。分析表明,Fe₂O₃的还原产物为Fe₃O₄,载氧体颗粒随循环数增加而逐渐烧结。     

Cu-橄榄石载氧体煤焦化学链气化实验研究

用浸渍法以橄榄石为载体负载CuO制备了Cu-橄榄石载氧体,并在固定床上开展了载氧体存在下煤焦的水蒸气气化实验。结果表明,提高气化温度和水碳物质的量比可以提高煤焦气化的碳转化率和合成气产率;增加载氧体CuO负载量和载氧体/煤焦质量比能提高碳转化率和水转化率,但会消耗产气中的合成气,导致合成气产率降低。高温再生后载氧体依然能保持活性,表明橄榄石载体抑制了Cu/CuO的烧结。载氧体经950℃再生八次后用于煤焦气化,实现碳转化率42.3%,水转化率57.3%,合成气产率2.12L/(g·h)。