【作者】 刘少文；

【导师】 李永丹；

【作者基本信息】 天津大学 ， 工业催化， 2007， 博士

【摘要】 矿物燃料的使用向大气排放大量的CO2,产生全球气候温室气体效应,迫切需要发展矿物燃料脱碳技术。燃料电池因其较高的能量转换效率而受到广泛关注,低温燃料电池由于能够满足汽车燃料电池与小规模的制氢需要而成为研究热点。但低温燃料电池容许的COx含量较低,传统的制氢过程要达到上述要求是不经济的。甲烷裂解制氢可以脱除天然气中的碳,制得的氢气不含CO,而且氢气过程得以简化,是很有前景的制氢过程。因此,研究甲烷裂解制氢具有十分重要的意义。甲烷在催化剂上裂解生长碳使催化剂失活,要实现连续制氢就必须对催化剂进行再生。选用25Ni/Cu-Al2O3和75Ni/Cu-Al2O3两种催化剂,分别在500℃和650℃进行甲烷裂解制氢与催化剂再生,反应与再生过程的温度与时间保持相同,催化剂再生时采用空气进行再生,气体流量均为370 cm3/min(STP),考察催化剂活性与稳定性的影响因素。实验结果表明低镍含量的催化剂表现出较好的稳定性,温度增加时其稳定性降低。甲烷转化率随操作周期呈下降趋势,但25Ni/Cu-Al2O3催化剂在500℃制氢与再生操作达到第五周期时,其甲烷转化率趋于稳定。对切换时间的影响研究表明切换时间存在一个最佳值,即5分钟的切换时间甲烷裂解制氢效率最高。对生成碳产品进行了XRD和TEM表征,并对实验结果进行了讨论。甲烷裂解同时制氢和碳纳米管工艺过程能有效降低制氢成本。本文重点考察催化剂组成、反应温度、甲烷浓度对碳纳米材料的规模生产与性能的影响。研究发现镍铝催化剂(3Ni1Al)在600 oC时纯甲烷裂解制氢的浓度较高,达到～55%,积碳量达9mgC/mgcat。铜修饰后催化剂的活性温区上移,650 oC时纯甲烷裂解制得的氢气浓度较高,达到63%,相应的积碳量达8 mgC/mgcat。TPO技术定量表征了碳产品的组成, Raman光谱和TEM(HRTEM)表征了碳产品的石墨化程度。压汞法对碳纳米管的孔结构进行了表征,适合用作气体吸附剂、催化剂载体与催化剂。反应器操作模式能影响甲烷催化裂解制氢过程。以纯甲烷为原料,分别考察了75Ni10Cu15Al和2Co1Al(原子比)催化剂上流化床与固定床操作模式下甲烷裂解制氢反应,结果表明流化床中的甲烷裂解反应速率较高。流化床操作的高表观速率主要是因为此模式下有效消除了外扩散,同时极大减少了内扩散阻力。不同温度下催化剂上生长的碳的TEM表征发现,金属颗粒尺寸随反应温度增加而增加,表明催化剂烧结是失活原因之一。但相同温度下固定床中催化剂金属颗粒尺寸明显大于流化床中的金属颗粒尺寸,且金属颗粒尺寸分布变宽,这说明流化床反应器有利于阻止金属颗粒的烧结。通过对甲烷裂解催化剂失活原因的分析发现流化床中催化剂颗粒的流态化有利于延长催化剂活性寿命。反应条件是影响催化剂活性的重要因素。采用15Ni3Cu2Al(原子比)复合氧化物催化剂,用氮气稀释的甲烷为原料,在流化床中对甲烷催化裂解制氢进行了研究。初始甲烷浓度范围为20%-50%,反应温度控制在500-680oC,临界流化高度为10-30mm。600oC时反应气体流量控制在250-360 ml/min (标准状态)之间,流化床稳定操作可以在一定的反应时段内实现。当初始甲烷浓度为48%,反应温度600oC,产物氢气浓度达42%,且可以稳定维持在30min以上,能实现氢气的稳定生产。对实验动力学数据进行数学模拟,分别提出了碳生长反应初期和平稳期的反应动力学模型,计算结果表明误差小于2%。流化床中催化剂颗粒的流化状态影响反应效率,而催化剂颗粒尺寸及其分布是影响流化状态的最重要因素之一。采用75Ni-15Cu-10Al (原子比)催化剂,反应气体为纯甲烷,气体空速为0.099 m3/gh (STP),反应温度550℃,反应时间为1小时。不同的催化剂颗粒尺寸及其分布得到相应的积碳量,并进行回归处理。为了探讨催化剂颗粒尺寸及其分布对甲烷裂解反应的影响程度,选择了两种回归方案,一是以催化剂颗粒尺寸、颗粒分布宽窄(称之为分布因素1)为自变量的回归方案,另一个则是以颗粒尺寸、分布因素1和主次分布区域比例(称之为分布因素2)为自变量的回归方案。两者的回归结果分析表明模型与参数估值的可靠性较高,准确性也能满足要求。但在进行优化设计的时候发现后者与实际情况较符合,这说明颗粒分布对流化状态的影响不能忽略。更多还原

【Abstract】 It has been understood that production of hydrogen from fossil and carbonaceous fuels with reduced CO2 emission to the atmosphere is key to the production of hydrogen-rich fuels for mitigating the CO2 greenhouse gas climate change problem. The intense interest in fuel cell technology stems from the fact that fuel cells are environmentally benign and extremely efficient. The stringent COx-free hydrogen requirement for the current low temperature fuel cells has motivated the development of COx-free hydrogen production alternatives to the conventional hydrogen production technologies. It is therefore, desirable to explore other avenues for hydrogen production with specific applications for fossil fuel decarbonization and current fuel cells. Methane catalytic decomposition significantly simplifies the conventional hydrogen production process and makes it particularly attractive for fuel cell application.In the present paper, we report the results of catalytic behaviour of 25Ni/Cu-Al2O3 and 75Ni/Cu-Al2O3 catalysts and characterization of formed carbon by XRD and TEM over them during the catalytic decomposition of methane and catalyst regeneration. CO-free H2 was produced intermittently by methane decomposition and the formed carbon combustion in a fluidized bed reactor in cyclic manner. The process involved two reactions: first, catalytic decomposition of methane to H2 and carbon (deposited on the catalyst), and second, gasification of the carbon deposited on the catalyst by air to CO2. The two reactions were carried out separately in cyclic manner by switching a methane-containing feed and a air-containing feed at a predecided interval of time. 25Ni/Cu-Al2O3 catalyst behaves higher stability than 75Ni/Cu-Al2O3 catalyst in the cycle operation. And it reduces with operation temperature increase. The process shows best performance at an optimum value (5min) of the feed switchover time.With the thorough progress of research and application of carbon nanotubes, the Simultaneous process for hydrogen and carbon nanotubes got extensive attention in the world. But some troubles of the operation in a fixed bed reactor such as volume expansion, catalyst loading and carbon product unloading, etc., baffled the progress of the process. So the process for hydrogen and carbon nanotubes from methane decomposition in a fluidized bed reactor was investigated. The effects of some factors such as catalyst composition, reaction temperature, and initial methane concentration on hydrogen yield, hydrogen content in product, carbon nanotube composition and configuration were studied. It was discovered that hydrogen content in product and carbon yield reached ~55%, 9mgC/mgcat respectively at 600oC over 3Ni1Al catalyst. When the catalyst was deposited by metal copper, the reactive temperature range of the catalyst ascended. The corresponding values arrived at 63%and 8mgC/mgcat at 650oC respectively. Carbon formed on catalysts was characterized by TPO, Raman, TEM (HRTEM) and pore measurement. It was found that the quantitative result from TPO was basically consistent to the experimental data. Therefore the quantitative characterization of carbon by TPO may be considered feasible.In fact, reactor type would affect the process for hydrogen production from methane decomposition. So in the next work the processes from methane decomposition in fluidized and fixed bed reactors were compared. Hydrogen production was investigated over 75Ni10Cu15Al, 2Co1Al (atomic ratio) catalysts in two types of reactors respectively. Pure methane was used as reaction gas. It was displayed that the performance in a fluidized bed was obviously higher than that in a fixed bed. The carbon formed at 600oC and 700oC respectively was characterized by TEM. It appeared that the size of metal particle increased with reaction temperature growth. This means that sinter between metal particles had taken place. But at the same reaction temperature it in a fixed bed was bigger than that in a fluidized bed and size distribution was wider. It may be considered that fluidized bed reactor was beneficial to prevent the sinter. The analysis of catalyst deactivation reasons indicated that throwback could prolong the lifetime of catalysts. It may be concluded that the main reason of high performance in a fluidized bed was throwback and relative prevention of sinter between metal particles.The effect of reaction conditions on hydrogen production from methane decomposition and its kinetics were investigated. In the same way, 15Ni3Cu2Al(atomic ratio)was used as the catalyst for methane decomposition. Diluted methane was used as reaction regent, reaction temperature was controlled between 500-680oC and gas volume was selected between 250-360 ml/min. fluidization may be maintained for some trivial. When the initial methane concentration was 48% and reaction temperature was 600oC, hydrogen content in product gas reached 42% and also maintained for over 30 minutes. According to the kinetic experimental data, kinetic models for growing and stability period of carbon growth were posted respectively. The error of theoretical values was less than 2%.Fluidization was affected by the size of catalyst particles and their distribution and the change of methane conversion took place. In order to make the mathematical relationship between them, the quantitative characterization of particle distribution must be made. But it was usually characterized by distribution diagram. In the text two distribution variables were used to characterize the distribution, distribution variable 1 that means width degree of particle distribution and distribution variable 2 that means proportion of particle weight in the secondary distribution region to that in the main distribution region. And then regression analysis was done. Reaction conditions were 15Ni3Cu2Al(atomic ratio)catalyst, reaction temperature 550 oC and gas volume 340 ml/min. Two regression schemes were selected: one in which particle size and distribution factor 1 were choosed as independent variables and another one in which particle size, distribution factor 1 and distribution factor 2 were choosed as independent variables. Regressive results indicated that reliability and veracity were satisfactory. But in the next step for their optimization design, it was found that the last situation was more accorded to the experiment than the first one. This implied that the effect of particle distribution on fluidization was complicated. The above results will be of instructional meaning to the next implication.更多还原