Fe2O3/粉煤灰载氧体化学链燃烧实验与机理研究

提出了一种以粉煤灰为载体制备的新型铁基载氧体。采用同步热重分析仪、小型流化床以及DFT分别研究了新型载氧体的活性与热稳定性,发泡剂含量与反应温度以及粉煤灰主要组分之间的协同作用对新型载氧体性能的影响。研究结果表明,新型载氧体在以CO为燃料的化学链系统中具有较高的活性;新型载氧体较大的孔隙率以及粉煤灰多组分间的协同作用促使850℃下发泡剂含量为10.0%(质量)的新型铁基载氧体的最大转化率(84.9%)比Fe₂O₃/Al₂O₃的最大转化率(54.3%)高30%,且新型铁基载氧体在30个循环测试中表现出良好的热稳定性。载体制备采用的发泡剂含量以及反应温度对新型铁基载氧体性能影响很大,适当的发泡剂含量(约10%(质量))可提高新型载氧体性能。此外,高温下会造成载氧体的烧结现象。最后,采用密度泛函理论(DFT)研究了粉煤灰与载氧体之间的界面作用以及协同氧化CO的电子特性。计算结果表明,粉煤灰和Fe₂O₃之间的界面电荷转移使Fe₂O₃为电正性,促使CO在表面的相互作用,载体和活性组分之间的协同作用降低了载氧体与CO前线轨道能量差,进而促进了CO与Fe₂O₃的反应。     

不同负载铁基载氧体的制备与性能研究

采用溶胶凝胶法制备了负载Al2O3、ZrO2和MgAl2O4的铁基载氧体,其中活性组分含量为70%,惰性载体含量为30%。高温煅烧后的铁基载氧体中活性组分为Fe2O3,相应的惰性载体物相分别为Al2O3、ZrO2和MgAl2O4,活性组分未与惰性负载发生固相反应。采用热重分析仪对铁基载氧体的还原反应活性和循环稳定性进行了测试。结果表明,负载Al2O3的铁基载氧体的还原反应活性最高,且在7周期循环中保持着95%以上的还原转化率和氧化转化率,是理想的载氧体材料。     

NiFeAlO4载氧体制备及煤化学链燃烧反应特性

铁基载氧体是一种具有工业应用前景的载氧体,但存在氧利用率低、在高温下易烧结等问题。虽可通过制备双金属复合载氧体或添加惰性组分改进其性能,但均存在一定缺陷。若将活性组分和惰性材料融入到一个晶体结构制备尖晶石结构载氧体,则可实现利用双金属协同作用提高载氧体活性的同时,利用Al3+提高载氧体的稳定性。采用共沉淀法和溶胶凝胶法制备了具有尖晶石结构的NiFeAlO₄载氧体,考察了制备方法、载氧体与煤质量比对NiFeAlO₄载氧体化学链燃烧特性和循环稳定性的影响,并分析了载氧体对煤转化过程的作用。结果表明,溶胶凝胶法制备的NiFeAlO₄载氧体具有更好的反应性,载氧体与煤质量比为20∶1时,碳转化率为86.7%,远高于煤单独热解时的碳转化率(34%),此时CO₂体积分数为93.6%。对反应前后NiFeAlO₄载氧体晶相结构和形貌进行分析,表明循环过程中经“还原-氧化”后生成的NiO和载氧体颗粒团聚是导致载氧体活性下降的主要原因。相较于载热作用,NiFeAlO₄载氧体在煤化学链燃烧中主要起供氧作用,其不仅会促进挥发分向煤气的转化,且NiFeAlO₄载氧体与焦炭之间也存在固-固反应,利于更多CO₂的生成。     

铁基载氧体辅助无烟煤焦富氧燃烧动力学分析

流化床铁基载氧体辅助富氧燃烧下传统石英砂床料被铁基载氧体替代,铁基载氧体扩展了传统床料的“热载体”的功能,另承担了“氧载体”的角色,为调节炉内氧分布与煤燃烧过程匹配提供了新思路。本文在热重实验平台探究了10%O₂/90%CO₂气氛下分析纯Fe₂O₃、赤铁矿及钢渣三种铁基载氧体辅助无烟煤焦燃烧特性及动力学。结果表明,相较于纯无烟煤焦燃烧,铁基载氧体辅助燃烧下无烟煤焦的燃烧特性得到显著改善,其中燃烧速率提高29%以上,燃尽温度降低65℃以上,综合燃烧指数提升2倍以上,活化能与指前因子同步增加且表现出“补偿效应”。三种铁基载氧体中分析纯Fe₂O₃对无烟煤焦燃烧特性的改善略优于赤铁矿和钢渣,钢渣可作为流化床铁基载氧体辅助富氧燃烧的床料替代石英砂。     

Ce1-xNixOy氧载体在化学链甲烷重整耦合CO2还原中的应用

化学链甲烷重整耦合CO₂还原技术既能生产合成气还可以还原CO₂生成CO。采用共沉淀法制备不同Ce/Ni摩尔比的系列Ce_1-xNi_xO_{y（x = 0,0.2,0.4,0.6,0.8,1）氧载体。通过XRD、BET、XPS及CH4-TPR等表征对氧载体的理化性质进行了研究。系统考察了Ce1-xNixOy氧载体在化学链甲烷重整耦合CO2还原反应中的反应性能。与单一金属氧化物NiO和CeO2相比,Ce1-xNixOy复合氧载体在该反应中具有更高的活性和热稳定性。在甲烷部分氧化阶段,Ce0.2Ni0.8Oy和Ce0.4Ni0.6Oy氧载体具有较高的CH4转化率。经历了20次redox循环实验,Ce0.2Ni0.8Oy氧载体的CO2转化率几乎保持不变,表明Ce0.2Ni0.8Oy氧载体具有较高的热稳定性。}     

具有自载体功能的CoFeAlO4载氧体化学链燃烧反应特性

采用燃烧法合成了具有尖晶石结构的CoFeAlO₄载氧体材料,通过表征手段和实验研究考察了不同温度下CoFeAlO₄载氧体的化学链燃烧反应特性和循环稳定性,并对CoFeAlO₄载氧体晶相结构和表观形貌的变化规律进行了分析。结果表明,温度升高有利于提高CoFeAlO₄载氧体转化还原性气体CO的能力,使得还原反应速率更快,但高温下经“还原-氧化”会造成CoFeAlO₄载氧体相态分离,难以保持稳定的自载体尖晶石结构。对反应前后CoFeAlO₄载氧体晶相结构的分析表明,高温条件下经过“还原-氧化”后生成的CoFe₂O₄和CoAl₂O₄是导致CoFeAlO₄载氧体烧结和循环稳定性下降的主要原因。     

酸改性复合氧载体CaSO4-CaO的反应性能

将CaO添加到经不同酸改性后的CaSO₄氧载体中来制备CaSO₄-CaO复合氧载体。采用XRD、H₂-TPR、CO小型卧式管式炉开展恒温反应和循环实验,对氧载体进行表征和评价。主要研究了酸的种类对CO₂生成、气体硫化物的释放及复合氧载体循环反应性能的影响。结果表明：酸化的复合氧载体的氢气还原活性均明显优于未经改性过的CaSO₄,相比无酸化处理的CaSO₄氧载体,酸化后的CaSO₄-CaO复合氧载体,特别是HCl酸化,主要是提高了CaSO₄竞争还原副反应的选择性,导致主要气体硫化物SO₂释放显著增加,而COS释放略有降低。以CaCO₃煅烧得到的CaO与经HNO₃改性的CaSO₄进行机械混合制得的复合氧载体具有较好的反应活性,但是经过6次循环反应后,还原反应活性下降;循环反应中,气体硫化物主要来自于CaSO₄在还原阶段释放出的SO₂。     

氧载体NiFe2O4与褐煤的化学链燃烧反应特性实验研究

本实验制备了氧载体NiFe₂O₄,并通过热重分析试验系统研究了其对褐煤的氧化还原性能。使用扫描电镜、比表面积和X射线衍射实验表征了表面结构,与NiO和Fe₂O₃相比,NiFe₂O₄中Ni的存在增大了比表面积和孔隙体积,提高了褐煤化学链燃烧过程中的反应活性和氧传递能力。此外,循环实验证实了NiFe₂O₄高反应活性的稳定性。     

铁基复合载氧体煤化学链气化反应特性及机理

以水蒸气作为气化/流化介质,在流化床中研究了两种铁基复合载氧体的化学链气化反应特性及循环特性,并对气化过程中的反应机理、动力学方程进行了推断。结果表明:温度为920℃时,添加不同修饰物的铁基复合载氧体与煤焦气化的反应活性依次为Fe4Al6K1>Fe4Al6>Fe4Al6Ni1。在多次循环实验过程中,合成气成分保持稳定,表明Fe4Al6K1复合载氧体循环特性良好。XRD谱图分析表明,六次氧化还原实验后的铁基载氧体氧化态仍为Fe₂O₃。K⁺主要以铁酸钾形态存在,该结构有利于促进化学链气化反应。利用高斯函数对气化反应速率进行了峰拟合,拟合结果表明化学链气化主要分为3个阶段:化学链作用阶段、煤气化阶段以及Fe₃O₄向FeO转变的气化阶段。     

化学链燃烧过程Fe2O3/Al2O3载氧体表面CH4反应：ReaxFF-MD模拟

借助ReaxFF-MD方法,对化学链燃烧过程Al₂O₃负载Fe₂O₃载氧体（Fe₂O₃/Al₂O₃）表面CH₄反应过程模拟,探究Al₂O₃惰性载体对Fe₂O₃-CH₄体系燃烧过程的调控机制。研究发现添加Al₂O₃惰性载体改变了化学链燃烧过程中Fe₂O₃载氧体反应性和Fe₂O₃/Al₂O₃-CH₄反应体系的热力学和动力学行为。主要是促进了Fe₂O₃载氧体表面CH₄氧化,并对CH₄反应过程、中间体、产物及其反应速率和放热量等均具有显著促进和调控作用。原因在于Al₂O₃惰性载体对Fe₂O₃活性相中晶格氧的活化作用促进了晶格氧的迁移-扩散-释放。添加惰性载体增强了Fe₂O₃载氧体在化学链燃烧过程晶格氧释放速率和释放量,有利于CH₄氧化燃烧向合成气的高效、清洁转化,强化了化学链燃烧过程,满足当前能源高效转化和碳减排目标。     

边界层对三氯氢硅-氢气系统中多晶硅化学气相沉积的影响

This paper presents the numerical investigation of the effects of momentum, thermal and species boundary layers on the characteristics of polycrystalline silicon deposition by comparing the deposition rates in three chemical vapor deposition (CVD) reactors. A two-dimensional model for the gas flow, heat transfer, and mass transfer was coupled to the gas-phase reaction and surface reaction mechanism for the deposition of polycrystalline silicon from trichlorosilane (TCS)-hydrogen system. The model was verified by comparing the simulated growth rate with the experimental and numerical data in the open literature. Computed results in the reactors indicate that the deposition characteristics are closely related to the momentum, thermal and mass boundary layer thickness. To yield higher deposition rate, there should be higher concentration of TCS gas on the substrate, and there should also be thinner boundary layer of HCl gas so that HCl gas could be pushed away from the surface of the substrate immediately.     

Effect of temperature on reduction reactivity of oxygen carrier particles in a fixed bed chemical-looping combustor

In a chemical-looping combustor (CLC), gaseous fuel is oxidized by metal oxide particle, e.g. oxygen carrier, in a reduction reactor (combustor), and the greenhouse gas CO₂ is separated from the exhaust gases during the combustion. In this study, NiO/bentonite particle was examined on the basis of reduction reactivity, carbon deposition during reduction, and NO_x formation during oxidation. Reactivity data for NiO/bentonite particle with methane and air were presented and discussed. During the reduction period, most of the CH₄ are converted to CO₂ with small formation of CO. Reduction reactivity (duration of reduction) of the NiO/bentonite particle increased with temperature, but at higher temperature, it is somewhat decreased. The NiO/bentonite particle tested showed no agglomeration or breakage up to 900 ‡C, but at 1,000 ‡C, sintering took place and lumps of particles were formed. Solid carbon was deposited on the oxygen carrier during high conversion region of reduction, i.e., during the end of reduction. It was found that the appropriate temperature for the NiO/bentonite particle is 900 ‡C for carbon deposition, reaction rate, and duration of reduction. We observed experimentally that NO, NO₂, and N₂O gases are not generated during oxidation.     

载料气体对化学气相沉积合成石英玻璃沉积速率的影响

采用化学气相沉积工艺合成了石英玻璃,研究了沉积速率与载料气体之间的关系.利用扫描电子显微镜观察火焰中SiO2粒子微观形态和尺寸.通过对SiCl4水解、氧化反应速率的计算表明:在一定温度下,当同时存在充足的H2O和O2时,SiCl4氧化反应速率远高于水解反应速率.当载料气体为H2,Sicl4流量为25g/min时,观察火焰中SiO2粒子的微观形态发现存在大量非球状无定形聚集体,表明SiCl4未反应完全,沉积速率较低,约为220～240 g/h.同样SiCl4流量下.采用O2作为载料气体时火焰中颗粒均为球状,SiCl4全部反应,沉积速率较高,达到300～350 g/h.当SiCl4流量为15 g/min时,载料气体的改变对SiO2粒子形态和尺寸没有影响,SiCl4可以全部反应,沉积速率基本相同.当SiCl4流量较高(25 g/min)时,载料气体对反应机理和沉积速率有显著影响,若要SiCl4在极短时间内完全反应生成球状SiO2粒子,需选择O2作为载料气体,充足的O2保证SiCl4可以通过氧化反应全部反应完毕,沉积速率相应提高.     

煤/生物质流态化富氧燃烧的CO2富集特性

流化床富氧燃烧是具有重要应用前景的燃烧中碳捕集技术。为更深入认识固体燃料的流态化富氧燃烧行为,构建了微型流态化反应-质谱联用实验系统,反应器直径10 mm,燃烧温度700～900℃,探索了基于在线质谱分析的流态化燃烧过程特性表征方法,以烟煤和花梨木为对象,研究了煤、生物质及其混合物在富氧气氛和流态化条件下的燃烧行为,重点考察了氧浓度、燃烧温度、煤与生物质质量比对CO₂谱峰曲线形态、反应总时间、起始反应时刻、烟气中富集CO₂体积分数、颗粒燃烧产生CO₂量、CO₂相对生成率等特性的影响。结果表明,在O₂/CO₂燃烧气氛下,随着氧体积分数增加,燃烧总反应时间缩短,颗粒燃烧产生的CO₂量和生成速率均增加,但烟气中富集的CO₂体积分数减小;提高燃烧温度,缩短了燃烧过程所需的时间,可以促进CO₂的富集,烟气中CO₂浓度、颗粒燃烧产生的CO₂量和生成速率均增加;生物质比例增大,起始反应时间提前,燃烧反应所需总时间减少,烟气中富集的CO₂浓度和颗粒燃烧产生的CO₂均减少,但CO₂生成速率增加。     

焙烧温度对双金属催化剂甲苯加氢重整反应催化性能的影响

以类层柱Ni-Co/Mg(Al)O水滑石为前驱体,经不同温度焙烧制备了系列焦炉煤气中焦油模型化合物甲苯加氢重整制合成气催化剂. 在常压、反应温度800℃、水/碳摩尔比0.7和体积空速12000 h-1的条件下,在35 h评价时间内850℃焙烧的催化剂可完全转化甲苯,CH4和CO的平均产率分别为34%和66%,而低于750和高于950℃焙烧的催化剂活性较差. 850℃焙烧时催化剂比表面积较大,形成了尖晶石和固溶体,活性金属与载体间的相互作用较强,还原后活性金属颗粒小且均匀分布. 催化剂上有少量须状碳生成,绝大部分积碳可被H2消除,积碳是可逆过程.     

Kinetics of decarburization of molten fe-c by gaseous hydrogen

Hydrogen was directed onto the surface of Fe-4%C held in an Al₂O₃ crucible at 1327°C, and the rate of formation of CH₄ was measured by mass-spectrometric analysis of the effluent. The measured rate approached the theoretical limit of mass transfer of CH₄ in the gas phase, provided that the concentration of other gases evolved by side-reactions was small. Deviations from predicted values appeared to be caused by carbon monoxide coming from the reaction of dissolved carbon with the crucible, and water coming from the reaction of hydrogen with the refractories. The gas mass transfer coefficient for the particular geometry used was determined in separate experiments where the carbon was oxidized by CO₂.     

焦炭颗粒在不同控制区域中的燃烧特性

针对焦炭颗粒在煤粉炉内的复杂燃烧过程,建立相互耦合的导热、气相传质模型及改进的随机孔模型. 通过FORTRAN编程,研究了焦炭颗粒在不同控制区域中的燃烧特性,应用改进的随机孔模型,研究了焦炭颗粒在扩散-化学反应动力学控制区中的燃烧特性. 结果表明,在扩散-化学反应动力学控制区,碳基上的小孔内部存在O2浓度梯度,焦炭颗粒的转化速率在转化率约为0.39时出现最大值,随燃烧过程进行先增大后减小.     

Plasma polymerization of tetrafluoroethylene in a field-free zone

Plasma polymerization of tetrafluoroethylene by itself and mixed with inert gases has been studied in the field free zone inside a Faraday cage. The chemical structure was analyzed by ESCA, revealing both linear and branched products. Linear products are formed by less energetic plasmas and at low monomer residence times. Lower energy plasmas result from the use of lower powers and lower percentages of helium in the feed. Production of linear products under these conditions is probably due to lower rates of free radical and metastable formation in the plasma. Through a combination of kinetic and mass transfer effects, shorter monomer residence times under high flow rates and within short distances from the front edge of the electrode give rise to a lower concentration of free radicals at the electrode surface, producing a more linear polymer. Linear products were also formed at very high powers. This latter result is quite unexpected and is probably due to rapid gas phase polymerization. The chemical structure was not affected significantly by the substrate temperature or by hydrodynamics in this work. All of the evidence indicates that the gases were well mixed in the reaction zone for the range of process variables and for the gases studied. The deposition rate was also studied as a function of the reaction conditions. It was affected by the concentration of free radicals, the concentration of the monomer, and the substrate temperature. The observed deposition rate profile across the electrode is consistent with mass transfer and kinetic considerations governing free radical and monomer concentrations on the electrode surface. The deposition rate is greater at the lower substrate temperature used, probably due to enhanced adsorption process.     

Reactive sputter-deposition of oxygenated amorphous carbon thin films in Ar/O2

Oxygenated amorphous carbon thin films were deposited by DC magnetron sputtering using various argon and oxygen process gas mixtures. The X-ray diffraction data indicated that the predominantly amorphous films had more defined peaks with a higher partial pressure of oxygen. Results indicated that use of oxygen in the working gas enhanced the crystalline nature of the films. Scanning electron and atomic force microscopy revealed that the surface roughness and film topography differed with the oxygen process gas variations. X-ray photoelectron spectroscopy revealed increased surface oxygen content with higher oxygen concentration in the working gas. Raman spectroscopy results suggested that the increased oxygen in the films may have led to a higher percentage of sp³-bonded carbon atoms. The growth rate (deposition rate) of the films decreased as the amount of oxygen increased. This decreased deposition rate was associated with an oxygen etching of the film.     

Simulation of SiO2/S coating deposition in a pilot plant set-up for coking inhibition

The anti-coking SiO₂/S coating was prepared on the inner surface of HK40 alloy tube in a pilot plant set-up by atmospheric pressure chemical vapour deposition (APCVD). The coating deposition was simulated using the computational fluid dynamics code, Fluent. The reaction parameters of the surface reaction for SiO₂ formation were determined based on the comparison between the experimental and the calculated values. Further, the influences of the inlet flow rate and mass concentration of source materials on the coating deposition rate were investigated. The simulated results showed that an increase of inlet flow rate led to the decrease of mass conversion of gas intermediates. The coating deposition rate along the reactor tube increased by 1–5 times as the inlet flow rate increased from 10 to 80 g min⁻¹. The mass conversion rate of the gas intermediate, Si(OH)₄, changed little at different inlet mass concentrations of source materials when the inlet flow rate was 30 g min⁻¹, and it had an increase for sulphide intermediates. The coating deposition rate along the reactor tube increased by about 10 times with increasing the inlet mass concentration from 0.2% to 2%. In the conditions we studied, SiO₂/S coating deposition was surface reaction rate limited. When the inlet flow rate was 30–40 g min⁻¹ with the resource material concentration of 1–1.6%, the SiO₂/S coating was about 15 μm at the tube outlet with the silicon-containing intermediate conversion rate of above 30% and a good uniformity of S along the reactor. This work provides a theoretical basis for optimisation of operational parameters of the anti-coking SiO₂/S coating preparation in the pilot plant set-up.