熔融盐中铈基氧载体部分氧化甲烷制取合成气的试验研究

【作者】 魏永刚；

【导师】 王华；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2007， 硕士

【摘要】 随着世界经济的发展，全球性石油资源日益枯竭，储量丰富的天然气将成为最具希望的替代能源之一。勘探表明，我国天然气远景储量达到43万亿m³，占世界天然气总储量的10％。因此，我国对天然气的转化催化技术进行研究具有重大的战略意义。从已有的天然气化工利用技术来看，甲烷的转化利用途径可以分为两类：直接转化和间接转化。目前其工业大规模应用主要集中在甲烷的间接转化中，而甲烷的间接转化利用过程中合成气的制备又是其中重要的组成部分。当前已经工业化和正在开发的合成气制备工艺主要包括：甲烷蒸汽重整、甲烷—二氧化碳重整、甲烷部分氧化、联合重整以及自热重整等技术。甲烷的水汽重整已经工业化应用，但存在能耗及H₂／CO比例高等不足；传统甲烷部分氧化技术虽然克服前者存在的不足，但又需要消耗纯氧，增加了生产成本。为了解决这些矛盾，作者所在的课题组提出了一种全新的合成气制取技术——熔融盐中利用晶格氧部分氧化甲烷制取合成气技术。该技术采用变价金属氧化物中的晶格氧实现甲烷的部分氧化，较大地降低了合成气的生产成本，同时可以避免传统反应过程中的催化剂床层热点、爆炸等问题。作者在对氧化铁、钙钛矿以及稀土金属氧化物等氧载体进行了部分氧化甲烷制气的预研究后，最终确定了CeO₂为氧载体的活性组分，并采用浸渍法制备了不同担载量的系列铈基氧载体，在熔融盐（Na₂CO₃与K₂CO₃质量比为1∶1）中进行了部分氧化甲烷制取合成气的试验研究。利用Outokumpu公司生产的HSC Chemistry5.1化学计算软件，根据系统自由能最小原则，对反应体系中可能发生的化学反应的Δ_rG、Δ_rH和logK进行了理论计算；绘制了不同CH₄与CeO₂反应量的平衡组成图，结果表明在甲烷过量的情况下，碳在高温区一直处在一个较高的比例，甲烷量的增加导致未反应的甲烷出现了较多的裂解发生，CO的量虽随温度升高时有所增加，但增加的速率较慢，H₂的量随温度升高增加很快，导致产物气中H₂／CO远大于2；而CeO₂的适当过量，随温度的升高碳及H₂O、CO₂量明显减少，CO和H₂的量不断升高，说明较高的温度更有利于获得H₂／CO比例合适的合成气。以比表面积较大、热稳定性较好和抗压碎能力较强的γ-Al₂O₃为载体，采用浸渍法制备了担载量分别为5％、10％、20％、30％和40％的系列铈基氧载体；通过XRD、SEM、TG、O₂-TPD、H₂-TPR等检测手段对氧载体的性能进行了表征；利用热重反应器研究了氧载体的循环反应性能；在石英管反应器中，对0.462×10⁵、0.924×10⁵、1.386×10⁵h^-1三种空速，反应温度在750～900℃下进行了CeO₂氧载体部分氧化甲烷制合成气的性能实验，结果表明：在所研究的空速和温度范围内，0.924×10⁵h^-1的空速和850～900℃的温度范围比较适合合成气的制备；在石英反应器中进行了氧载体的循环实验，发现循环后的氧载体出现了少量结聚现象，分散度有所降低，近而使其在部分氧化甲烷过程中甲烷转化率和CO选择性出现了少量的降低。在熔融盐体系中进行了纯氧载体与不同担载量氧载体的反应活性研究。发现当温度高于865℃以后，担载量为10％的铈基氧载体无论在甲烷转化率还是在H₂、CO选择性上都表现出了最高的活性，甲烷转化率、H₂、CO选择性最高时分别达到了79％、96％和97％；对10％CeO₂／γ-Al₂O₃氧载体与甲烷反应后产品气中H₂与CO摩尔比随温度变化情况进行了分析发现，当温度在870℃左右时，H₂与CO比接近与理论值2。通过这些实验说明：利用氧化铈中的晶格氧能够实现部分氧化甲烷制取合成气，以其为活性组分选择合适的载体进行担载的确能够提高氧载体的反应活性，但是活性组分在载体上有一个最佳的担载量，而且在反应过程中也有一个最佳的反应温度范围。总之，熔融盐中利用晶格氧部分氧化甲烷制取合成气技术是一项很有实用价值的能源利用新技术。在反应器设计、氧载体性能优化、反应机理等方面还需要进行更深入的研究。更多还原

【Abstract】 With the development of the world economy, the oil will dry up increasingly in the world, and the abundant natural gas will become the most promising substitute of oil. The prospecting making known, the natural gas reserves reach 43 trillion steres in our country, which is 10% in the total natural gas reserves of the world. We can see from the existent technology of the natural gas transform, the routs of methane transform include two kinds: direct transform and indirect transform. The large-scale applications mainly centralized in the methane indirect transform at present, and in the methane indirect transform preparation syngas is the important part. The technologies of the steam reforming of methane, the carbon dioxide of methane, partial oxidation of methane, unite reforming of methane, and self-thermal reforming are included in the preparation technologies of syngas which have been industrialization or being developed currently. Although the technology of the steam reforming of methane has been industrialization, it had some deficiencies for highly energy consumption and ratio of H₂/CO.The conventional technology of partial oxidation of methane to syngas has overcome the deficiencies of SRM, while it needs to consume pure oxygen and add the production cost. In order to solve these contradictions, a novel technology of production syngas, partial oxidation of methane to syngas using lattice oxygen in molten salt has been brought forward by our research group. In the technology methane is oxidized by lattice oxygen in metal oxides which can change their valences, through the technology the production cost can be reduced, at the same time, the heat- point existed in the catalyst bed and blast and so on in the conventional technology can be avoidedFerric oxide, perovskite-type oxides and rare earth metal oxides as the oxygen carriers are beforehand researched for partial oxidation of methane to syngas, and finally CeO₂ is used as active component of oxygen carrier. A series of ceria-based oxygen carriers are prepared by incipient wetness impregnation, and are experimented for partial oxidation of methane to syngas in molten salt （1: 1 weight ratio of Na₂CO₃ and K₂CO₃） .The Δ_rG, Δ_rH and logK in some most possiblereactions are calculated in theory based the principle of the minimal Gibbs free energy change by using HSC Chemistry 5.1 produced by Outokump. Charts of equilibrium compositions of CH₄ and CeO₂ at different reaction amounts were drown by the same software. When the methane is excessive the carbon has the higher ratio at the high temperature area, at the same time, the unreacted methane occurs pyrogenation. Although the amount of CO has some increase with temperature, the increasing speed is slow, and the amount of H₂ has a sharp increase when temperature increasing, so the H₂/CO ratio is more than 2.If the CeO₂ is excessive, the amounts of carbon, H₂O and CO₂ have obvious decrease, and the amounts CO and H₂ continuously increase with the temperature, the results show that the higher temperature is favorable for production synthesis gas with appropriate H₂/CO ratio.Using the γ-Al₂O_.3 which has a bigger BET surface, heat-stability and resistance smash capability as a carrier, The 5%、 10% 、 20% 、 30% 、 40% CeO₂ carried y-Al₂O₃ are prepared by incipient wetness impregnation;The oxygen carriers were characterized by means of XRD, SEM, TG, O₂-TPD and H₂-TPR, etc... The circulation performances of oxygen carriers are investigated by TG experiment; In the quartz tube reactor the three space velocities of 0.462×10⁵, 0.924×10⁵, 1.386×10⁵ h^-1 are experimented from 750 to 900℃, respectively. The results show that the space speed of 0.924×10⁵ h^-1 and the temperature of 850 to 900’C are suitable for making syngas; When the oxygen carrier is used in circulation some agglomerations are found, which result in a little decrease of methane conversion and CO selectivity. The pure and different mass of CeO₂ carried γ-Al₂O₃ as oxygen carriers are investigated in molten salt.When the temperature is higher than 865℃, the 10% CeO₂/γ-Al₂O₃ catalyst has the higher methane conversion, H₂ and CO selectivity, which are 79% 、 96% 和 97% at tiptop, respectively; When temperature is 870℃ the H₂/CO ratio reaches 2, and it is close to the theory value. These experimentations indicate that the partial oxidation of methane to syngas using lattice oxygen of CeO₂ in molten salt is feasible; the activity of oxygen carriers could be improved by carried γ-Al₂O₃, but the amount of CeO₂ has a optimal value and the reaction temperature also has a optimal range.The technology of partial oxidation of methane to syngas using lattice oxygen in molten salt is a good energy utilization and practicality new method. There aresome aspects which need to in-depth investigation for instance the design of reactor, the performances optimization of oxygen carriers and the reaction mechanism.更多还原