空气污染组分H2O和CO2对乙烯燃烧性能的影响(II)——反应机理和动力学模拟

超燃冲压发动机在高空工作时, 以高温高速纯净空气作氧化剂使燃料燃烧. 但在地面实验中, 高温空气往往通过燃烧加热方式获得, 会使空气中含有H2O和CO2等污染组分. 本文用活塞流反应器进行动力学模拟, 研究在不同初温、压强和燃气比的条件下, H2O和CO2污染组分对乙烯燃烧的温度、压强和点火延迟时间等特性的影响. 模拟结果表明: 乙烯在含有H2O/CO2污染物的空气中燃烧, 相比纯净的空气而言, H2O对乙烯的点火有一定的促进作用, 而CO2有一定的抑制作用; 空气中含H2O和CO2污染物使乙烯燃烧的平衡温度和压强降低, 在污染物浓度相同时, CO2引起的下降幅度比H2O的大. 模拟结果能较好地解释现有的实验现象.     

空气污染组分H_2O和CO_2对乙烯燃烧性能的影响(Ⅱ)——反应机理和动力学模拟

超燃冲压发动机在高空工作时,以高温高速纯净空气作氧化剂使燃料燃烧.但在地面实验中,高温空气往往通过燃烧加热方式获得,会使空气中含有H2O和CO2等污染组分.本文用活塞流反应器进行动力学模拟,研究在不同初温、压强和燃气比的条件下,H2O和CO2污染组分对乙烯燃烧的温度、压强和点火延迟时间等特性的影响.模拟结果表明:乙烯在含有H2O/CO2污染物的空气中燃烧,相比纯净的空气而言,H2O对乙烯的点火有一定的促进作用,而CO2有一定的抑制作用;空气中含H2O和CO2污染物使乙烯燃烧的平衡温度和压强降低,在污染物浓度相同时,CO2引起的下降幅度比H2O的大.模拟结果能较好地解释现有的实验现象.     

空气污染组分H2O和CO2对乙烯燃烧性能的影响

超燃冲压发动机在高空工作时, 以高温高速纯净空气作氧化剂使燃料燃烧. 但在地面实验中, 高温空气往往通过燃烧加热方式获得, 从而使空气包含了H2O和CO2污染组分. 本文用电阻加热来流空气并添加污染组分的方法, 研究了燃烧室模型中乙烯的燃烧状态和壁面压力受污染组分的影响. 用化学反应动力学模拟的方法, 在绝热刚性反应器模型中用H2O和CO2取代空气中的N2, 研究了污染组分对点火延迟和燃烧温度的影响, 并从链反应机理的角度讨论了实验和动力学模拟结果.     

Pd/NiAl_2O_4催化剂上甲烷燃烧反应的红外光谱研究

采用原位红外光谱研究了Pd/NiAl2O4催化剂上甲烷燃烧反应机理,考察了Pd在载体上的氧化-还原状态对催化剂性能的影响.结果表明,甲酸盐向碳酸盐的转化是反应的控制步骤.经预还原处理的催化剂在贫燃富氧条件下反应一段时间后,活性组分仍为Pd-PdO混合形态;Pd单质的存在使O2在其上吸附形成活性O2-物种,从而促进了甲酸盐向碳酸盐的转化.     

H2O、CO2组分对NOx存储-还原性能的影响

NOx存储-还原技术是控制汽车稀燃NOx排放的重要手段之一,在汽车尾气中H2O、CO2组分含量均相对较高,有必要弄清这些组分对NOx存储-还原特性的影响。论文以MnOx改性Pt/Ba/Al2O3催化剂为研究对象,评价在不同气氛下的NOx存储能力和催化还原性能。结果表明：CO2、H2O组分均抑制催化剂的NOx存储性能,H2O的抑制作用主要表现在低温区,CO2抑制NOx存储的现象在高温区更为显著。CO2对NOx存储速率的抑制作用较H2O更为明显,且其NOx存储速率随着温度的升高表现的差异性更为明显。对于NOx催化还原过程,CO2、H2O或CO2 H2O添加均导致N2选择性降低,其N2选择性按CO2 > H2O > CO2 H2O的顺序降低。     

两段法甲烷催化氧化制合成气研究

提出了一种将甲烷低温催化燃烧和部分氧化相结合制取合成气的新方法 ,考察了反应条件对Pd Pt催化剂上的甲烷低温燃烧反应性能以及Ni-La2 O3 MgAl2 O4 -Al2 O3催化剂上甲烷催化氧化制合成气反应性能的影响。结果表明 :采用两个串联固定床反应器和分段进氧 ,不仅可以使反应原料偏离爆炸极限 ,确保过程的安全操作 ;而且一段反应器采用低温进料 ,通过少量甲烷催化燃烧 ,为二段反应提供含有少量CO2 、H2 O等氧化产物的反应原料。在二段反应器中 ,放热的甲烷部分氧化反应和吸热的蒸汽重整及CO2 重整反应同时进行 ,可避免催化剂床层飞温 ,使反应基本上在绝热恒温条件下进行 ,可用两个串联的固定床反应器实现甲烷部分氧化制合成气反应。在适合的反应条件下 ,甲烷转化率可达 93% ,H2 和CO选择性分别为 97%和 98%。     

Fe203中的晶格氧用于甲烷在熔融盐中的催化燃烧(Ⅰ)催化剂的制备及性能表征

制备了新鲜的Fe2O3甲烷燃烧催化剂,利用XRD、SEM和TG等分析手段对催化剂的性能进行了表征.并在质量比为1:1的Na2CO3和K2CO3熔融盐中研究了Fe2O3中的晶格氧对甲烷的催化燃烧性能.TG分析表明,Fe2O3与CH4的反应分为两个阶段,第一个阶段从550℃开始,700℃结束,这一阶段里Fe2O3主要被还原成了Fe3O4,第二个阶段从800℃开始,Fe3O4被还原成了FeO和Fe;新生成的铁和低价铁氧化物与空气反应的TG曲线只出现一个增重阶段,从350℃开始质量开始显著增加,最后又重新生成了Fe2O3.在熔融碳酸盐体系进行的切换反应表明,在熔融盐中Fe2O3中的晶格氧能有效地对甲烷的燃烧过程进行催化,并且失去晶格氧的Fe2O3能与空气反应重新恢复其晶格氧,并且整个反应过程放出的热被储存在熔融盐热载体中,这更有利于甲烷的完全燃烧或部分氧化制合成气.因此,控制合适的反应条件,利用Fe2O3作为催化剂在熔融盐体系里使甲烷完全燃烧或部分氧化制合成气是可能的.     

壳聚糖在水溶液中的辐射降解反应

研究了壳聚糖在CH3COOH/NaCl缓冲溶液均相体系下的辐射降解反应,给出了H2O2、异丙醇、pH、样品初始分子量等因素对壳聚糖降解的影响,探讨了实验条件下溶液中不同自由基对壳聚糖降解的作用,并对辐照前后壳聚糖的结构进行了表征.结果表明,酸性条件下,壳聚糖的降解主要由.H和.OH自由基共同作用引起,加入H2O2或者通入N2O都能够略微提高.OH自由基浓度,对壳聚糖的降解有促进作用.加入异丙醇后,由于同时降低了.H和.OH自由基浓度,导致壳聚糖降解缓慢.当溶液的pH接近中性后,对壳聚糖的降解起主要作用的为.OH自由基,加入H2O2或者通入N2O都会增加.OH自由基的浓度,从而明显提高壳聚糖的降解速率.此外,研究发现低分子量的壳聚糖具有较快的降解速率.样品的UV、FTIR分析表明,辐照后除在壳聚糖分子链端生成羰基外,壳聚糖主链结构未见变化,脱乙酰度也没有显著改变,显示出辐射降解是一种有效的控制壳聚糖分子量方法.     

Mo2C/Al2O3催化剂的制备条件对POM反应的影响

对不同碳化终温和经过H2、CO2预处理的Mo2C／Al2O3催化剂进行了POM催化活性评价与结构表征．结果表明，高碳化终温的Mo2C／A12O3催化剂对于POM反应制合成气有高的甲烷转化率，CO和H2选择性、在850℃时H2预处理比CO2预处理的催化剂具有好的甲烷转化率和好的CO、H2选择性，但H2预处理时间过长会使CO和H2的选择性有所降低，CO2预处理会导致低的甲烷转化率和极低的CO和H2的选择性．XRD结果表明，在700-850℃之间碳化的样品均为β-Mo2C，BET比表面的测定结果表明随碳化终温的增加，Mo2C/Al2O3催化剂的比表面有所增加．     

溶液燃烧法制备的Ni基催化剂及其浆态床甲烷化催化性能

分别以硝酸铝、硝酸氧锆、硝酸镧和硝酸铈为载体前驱体,与硝酸镍和尿素配制水溶液,采用溶液燃烧法制备了Ni-Al2O3、Ni-Zr O2、Ni-La2O3和Ni-Ce O2催化剂,研究了浆态床CO甲烷化催化性能,并进行了低温N2吸附-脱附、XRD、SEM、TEM、H2-TPR和H2化学吸附等表征分析。结果表明,以硝酸铝为前驱体制备Ni-Al2O3催化剂时燃烧火焰稳定且持续时间长,达23 s,样品比表面积(468 m2·g-1)和金属Ni表面积(10 m2·g-1)均较大、Ni粒径小(3~5 nm)且分散度高,CO甲烷化催化活性和稳定性好,CO转化率和CH4选择性分别达到94%和95%,在100 h的甲烷化反应中未出现明显失活;以硝酸氧锆和硝酸镧为前驱体制备样品时未出现明显的燃烧火焰,持续时间仅为12 s和5 s,催化剂比表面积、金属表面积及催化活性均较低;以硝酸铈为前驱体制备样品时燃烧过程迅速而剧烈,样品比表面积(22 m2·g-1)和金属Ni表面积(5 m2·g-1)小、Ni粒径大且分散性差,甲烷化催化性能最差,CO转化率仅为41%,CH4选择性仅为89%。     

Experimental and numerical simulation of effects of CO2/N2 concentration and initial temperature on combustion characteristics of biomass syngas

Biomass syngas is a form of renewable energy with very broad application prospects, and it has different combustion characteristics according to the fuel composition and processing technology of biomass syngas. The influence of combustion composition, diluent and temperature variation on combustion characteristics were studied in this paper. The FFCM-1 mechanism was used to investigate the combustion characteristics of CO/CH₄/H₂ under varied diluents CO₂/N₂ and temperature by using spherical expansion flame method and ANSYS CHEMKIN-PRO. The experimental laminar burning velocity was compared with the simulation results of FFCM-1 mechanism. The results reveal that the experimental data are in good agreement with the simulation results, which are somewhat different under the condition of rich fuel. The laminar burning velocity decreases significantly with the increase of diluent CO₂/N₂, with the effect of diluent CO₂ being more significant. The laminar burning velocity increase dramatically with the increase of initial temperature, and the adiabatic flame temperature also decreases with the increase of diluent. The reduction caused by diluent CO₂ is much larger than that caused by diluent N₂. The change of initial temperature also affects the adiabatic flame temperature, but the range of variation is not as pronounced as that of diluent. Not only was the interaction between the combustion characteristics of CO/CH₄/H₂ under different diluents and temperature changes explored in this paper, but the influence mechanism was also revealed in depth.     

The radiolysis of mixtures of carbon dioxide and hydrogen

The self-radiolysis of CO₂ in excess tritium (³H₂) has been studied at pressures of 0.1 to 1.0 atm, temperatures of ?80° to +100°C, and in the presence of added H₂O, He, or Ar. The primary products of decomposition are CO and ³H₂O. Secondary products are C³H₄, C₂³H₄, and a white polymer. The rates of disappearance of CO₂ and formation of products and G-;values were measured. The disappearance of CO₂ initially obeys first-order kinetics, then slows down with time at a rate depending upon the initial pressure of ³H₂. The initial rates are proportional to pressures of CO₂ and ³H₂. They are independent of temperature, decreased by addition of H₂O vapor, and increased by addition of He or Ar. The proposed mechanism of decomposition of CO₂ and formation of products involves ionization of CO₂ followed by dissociative recombination forming CO and O. Then the O reacts with a hydrogen-containing species forming OH and H₂O, and a back reaction forms CO₂ from CO and OH.     

Filtration Combustion of Hydrocarbon Fluids in a Moving Bed of Inert Heat-Carrying Agent

Procedure for combustion of hydrocarbon fluids in the filtration mode under the conditions of a mobile bed of an inert heat-carrying agent was suggested and tested. This method can be implemented both with an excess amount of the oxidizing agent and combustible fluids burned to CO₂ and H₂O and at a deficiency of the oxidizing agent when the starting fuel is converted to the synthesis gas mostly containing CO and H₂. The fundamental aspects of the filtration combustion were experimentally studied for the example of a model fuel (isopropanol) at various ratios between the amounts of oxidizing agent and fuel.

     

Reductive and oxidative combustion of polyethylene bags: Characterization of carbonaceous and nitrogenous species

This study aims to experimentally characterize the gaseous carbonaceous and nitrogenous species from the reductive and oxidant combustion of polyethylene plastic bags. The experimental device used is the tubular furnace, coupled to two gas analyzers: a Fourier transform infrared analyzer (FTIR) and a non dispersive infrared analyzer (NDIR). The gaseous products analyzed are: CO, CO₂, CH₄, C₃H₈, C₂H₄, C₂H₂, C₆H₆, HCN, N₂O, NO, NO₂ and NH₃. The experiments were conducted at temperatures ranging from 800 to 1000 °C. The results obtained allow us to note that carbonaceous compounds are mainly emitted as carbon oxides (CO and CO₂) whether you are reductive combustion or oxidative combustion.In addition:

• -Under reductive conditions, combustion is controlled by oxygen. The hydrocarbon most active in the formation of carbon monoxide is ethylene (C₂H₄) and to a lesser extent, from 900 °C, acetylene (C₂H₂). The extents we have made show that ammonia seem to be emitted during combustion with 10% of oxygen.
• -In an oxidative environment, there is production of C₆H₆ in substantial quantities, which partly explains the presence of soot and tar in the smoke exhaust ducts. The C₂H₄, CH₄ and C₂H₂ are hydrocarbons most active in the formation of CO and CO₂. Increasing of concentration of local oxygen from 10 to 21% for the combustion of plastic bags, favors an increase in efficiency of carbon conversion about 30%. About 99% of the carbon of the fuel is found to be converted to carbon oxides or hydrocarbons. Nitrogen monoxide (NO) is the major component among the gases measured with a conversion rate of nitrogen about 20%, three times larger than that obtained during the reductive combustion of plastic bags with 10% oxygen.

     

Fabrication of mesoporous ZnO nanosheets from precursor templates grown in aqueous solutions

Mesoporous ZnO nanosheets were successfully prepared by pyrolytic transformation of zinc carbonate hydroxide hydrate, Zn₄CO₃(OH)₆·H₂O. The nanosheets were initially formed as assemblies on glass substrates during chemical bath deposition (CBD) in aqueous solutions of urea and zinc acetate dihydrate, zinc chloride, zinc nitrate hexahydrate, or zinc sulfate heptahydrate at 80°C. It was key to induce heterogeneous nucleation of Zn₄CO₃(OH)₆·H₂O by promoting a gradual hydrolysis reaction of urea and controlling the degree of supersaturation of zinc hydroxide species. Morphology of Zn₄CO₃(OH)₆·H₂O was largely influenced by the anions present in the CBD solutions. The Zn₄CO₃(OH)₆·H₂O nanosheets were transformed into wurtzite ZnO by heating at 300°C in air without losing the microstructural feature.     

Comparison of pulverized coal combustion in air and in O2/CO2 mixtures by thermo-gravimetric analysis

Thermo-gravimetric technique was used to study the combustion characteristics of pulverized coal in different O₂/CO₂ environments. The effects of combustion environment, oxygen concentration, particle size and heating rate were considered and the differences of pulverized coal pyrolysis, combustion and gaseous compounds release under two environments were analyzed. Results show that the coal pyrolysis in CO₂ environment can be divided into three stages: moisture release, devolatilization and char gasification by CO₂ in higher temperature zone. In the lower temperature zone, the mass loss rate of coal pyrolysis in CO₂ environment is lower than that in N₂ environment. The burning process of pulverized coal in O₂/CO₂ environment is delayed compared with that in O₂/N₂ environment for equivalent oxygen concentrations. With the oxygen concentration increase or the coal particle size decrease, the burning rate of coal increases and burnout time is shortened. As the heating rate increases, coal particles are faster heated in a short period of time and burnt in a higher temperature region, but the increase in heating rate has almost no obvious effect on the combustion mechanism of pulverized coal. During the programmed heating process, species in flue gas including H₂O, CO₂, CO, CH₄, SO₂ and NO were determined and analyzed using the Fourier-transform infrared (FTIR) spectrometer. Compared with pulverized coal combustion in O₂/N₂ environment, much more CO is produced in O₂/CO₂ coal combustion process, but the releases of SO₂ and NO are less than those released in O₂/N₂ environment. The present results might have important implications for understanding the intrinsic mechanics of pulverized coal combustion in O₂/CO₂ environment.     

Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2(H2O)n−, n=1–10, in the C−O Stretch Region

We investigate anionic [Co,CO₂,nH₂O]⁻ clusters as model systems for the electrochemical activation of CO₂ by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm⁻¹ using an FT-ICR mass spectrometer. We show that both CO₂ and H₂O are activated in a significant fraction of the [Co,CO₂,H₂O]⁻ clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)₂⁻. However, calculations find Co(HCOO)(OH)⁻ as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm⁻¹ are most likely due to higher lying isomers of the composition Co(HOCO)(OH)⁻. Upon additional hydration, all species [Co,CO₂,nH₂O]⁻, n≥2, undergo IRMPD through loss of H₂O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm⁻¹, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO⁻ ligand around 1580–1730 cm⁻¹, and the symmetric C−O stretching mode of the HCOO⁻ ligand around 1300 cm⁻¹. A weak feature above 2000 cm⁻¹ is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.     

Relative rates of reaction of hydroxyl radicals with O(3P) atoms and CO molecules

The rates of decay of O(³P) atoms in H₂/CO/N₂ mixtures in a discharge flow system have been measured, using O + CO chemiluminescence. The mechanism is: O + H₂ → OH + H (1), O + OH → O₂ + H (2), CO + OH → CO₂ + H (3). At 425 K, k₂/k₃ = 260 ± 20; literature values of k₃ combine to yield k₂ = (2.65 ± 0.52) × 10¹⁰ dm³ mol^?1 s^?1.     

Theoretical Efficiency Limits of Photoelectrochemical CO2 Reduction: A Route-Dependent Thermodynamic Analysis

Solar-fuel formation via photoelectrochemical (PEC) routes using water and CO₂ as feedstock has attracted much attention. Most PEC CO₂ reduction studies have been focused on the development of novel photoactive materials; however, there is still a lack of understanding of the key limiting factors of this process. In this study, the theoretical limits of Solar-to-Fuel (STF) efficiencies of single- and dual-junction photo-absorbing materials are illustrated for single-step multi-electron CO₂ reduction into fuels including HCOO⁻, CO, CH₃OH and C₂H₅OH. It is also highlighted that STF efficiency depends on the route of two-step PEC CO₂ reduction process using CH₃OH as a model fuel. Finally, it is illustrated the beneficial role of alternative strategies such as dual-junction photo-absorbing electrodes, externally applied bias and subsequent reactor chambers on the maximum theoretical efficiencies of PEC CO₂ reduction.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissociation. The H2 was adsorbed first, and activated to form H＊ over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH^＊, which reacted with adsorbed CO subsequently via CO^＊＋OH^＊ → CO2^＊＋H^＊; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO on the surface via CO^＊＋2H^＊ → CH^＊＋OH^＊. The formation of the surface species （CH） from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.