空气气氛下PbCl2高温挥发的热力学和动力学分析

针对我国垃圾焚烧飞灰高氯含量的特点,以PbCl2为研究对象,首先利用热力学Gibbs自由能判据理论对空气气氛下PbCl2高温挥发反应的主要反应和热力学平衡进行分析,然后利用高温热重差热分析仪(TG-DTA)研究升温速率(10、15、20 ℃/min)对其影响,并结合热力学分析建立PbCl2高温挥发的本征动力学模型.热力学分析表明:PbCl2的高温挥发主要是PbCl2离子晶体的相变过程,PbO和Pb的生成量很小;实验结果表明:随着升温速率的升高,挥发反应TG曲线向高温方向移动,挥发反应的各特征温度都有所升高;动力学分析表明:零级反应动力学可以很好地描述PbCl2的高温挥发反应.     

生物质成型燃料的热重分析及动力学研究

对三种生物质成型燃料在不同气氛下和不同升温速率下进行热重实验,研究反应条件对生物质成型燃料失重特性的影响规律,并对其空气气氛下的动力学特性进行了分析。研究结果表明,生物质在空气气氛下的挥发分析出速率比N₂气氛下高,随着温度升高,N2气氛下主要是纤维素、半纤维素以及木质素的分解,而空气气氛下还伴随有其分解产物的燃烧。生物质中挥发分含量较高时,反应活性也比较高。实验温度由室温升至800℃时,在升温速率为10℃/min ~ 25℃/min范围内,随着升温速率的升高,松木热重曲线先向低温区移动再向温度较高的一侧移动,最大失重速率对应的温度也表现出相同规律,当升温速率为20℃/min时最大失重速率对应的温度最低,升温速率为25℃/min时失重峰值最大。动力学特性分析表明,采用2组分动力学模型可以较好地表征生物质在空气中的失重特性,计算结果与实验结果吻合度较高。     

生物质热重实验及动力学分析

采用热重分析方法对黄桷树的热解行为及其动力学规律进行了研究。分析了试样在不同粒度(0.83,0.12,0.075mm)和不同升温速率(10,15,20,25℃/min)下的实验结果。结果表明:样品的失重过程由干燥和初挥发段、升温段、热解段和炭化段4个阶段组成;在升温速率一定的情况下,随着试样粒度的减小,试样在干燥和初挥发段失水明显、热解起始温度降低、有利于热解进行;随着升温速率的升高,各个阶段的起始和终止温度向高温侧偏移,且主反应区间也增加。采用Flynn-Wall-Ozawa对试样热解过程进行了动力学分析研究,得到了表观活化能。     

芦苇热解过程的动力学模型研究

采用热重实验对芦苇的热解特性进行了研究,并采用2种动力学模型对不同升温速率(10、20、30℃/min)下芦苇热解过程进行了动力学研究。实验结果表明,芦苇热解主要分为水分析出阶段(30~120℃)、解聚过程阶段(120~237℃)、挥发分脱除阶段(237~369℃)及无机物和残留有机物的分解阶段(369~682℃),并且随着升温速率的增大,热解温度特征点向高温侧偏移。模型计算结果表明,n级单一反应模型在n=1时拟合程度最高,主要遵循一级反应,活化能分别为30.70、34.60、33.01 k J/mol;分布式活化能模型计算得出的活化能处于30~116 k J/mol之间。通过对比2种模型的计算结果,得出分布式活化能模型能更好地反映芦苇的热解过程。     

基于分布式活化能模型的污泥热解特性研究

文中通过非等温热重实验,并利用分布式活化能模型,对污泥热解特性与动力学参数随热解温度及升温速率的变化规律进行分析。实验结果表明:污泥热解过程主要分为脱水(室温~205℃)、脱挥发分(205~550℃)及碳类无机物热分解(550~800℃)3个阶段,并随升温速率升高(15、20、50℃/min),挥发分失重速率增大,且热解特征值向高温侧移动;模型计算结果表明:污泥热解活化能分布在70~180 k J/mol之间,随转化率升高,活化能与指前因子均呈先增加再降低的趋势,二者存在一定的补偿效应。     

废弃柞木段热解特性及其反应动力学研究

以废弃柞木段为研究对象,进行了不同升温速率(5,15,25℃/min)下的热解失重实验以及TG和DTG曲线分析,采用分布活化能模型(DAEM)和一级反应模型研究其反应动力学特性。结果表明,脱水干燥的废弃柞木段热解过程主要分为过渡、挥发分析出和碳化3个阶段,随着升温速率的提高,DTG曲线有向高温侧移动的趋势,不同升温速率下的最大热解速率所对应的温度在360~380℃;采用DAEM得到的主热解阶段活化能为210~260 k J/mol,一级反应模型得到的主热解阶段活化能约为62 k J/mol,两种模型都能够较好地描述废弃柞木段主热解阶段,而DAEM模型更为全面。     

升温速率对煤热解特性影响的TG/DTG分析

利用热重法对我国四种典型动力用煤在不同升温速率(20℃/min、35℃/min、50℃/min、75℃/min、100℃/min)下的热解过程进行了研究.由各工况热重曲线得到热解特征参数,利用微分法与积分法相结合的方法求得了各煤样的动力学参数以及机理函数.分析比较了不同煤种、不同升温速率下的热解特性.结果表明,中温段热解反应最剧烈,活化能也最大;随着升温速率的提高,各煤样平均活化能增加.     

猪粪热解特性及其动力学研究

在程序控温热重分析仪上进行了不同升温速率(10,20,30,50℃/min)的猪粪热解失重试验,获得了猪粪热解特性参数;采用分布活化能模型(DAEM)进行动力学分析,计算得到整个热解过程的活化能和频率因子的分布规律。结果表明,猪粪热解过程呈现失水干燥段、热解过渡段、挥发分析出段和碳化段,升温速率对猪粪的热解有一定的影响,表现为随升温速率的升高,DTG曲线向高温侧移动;动力学分析表明,猪粪热解活化能在52~113 kJ/mol变化,低于锯末、稻壳、稻秆、椰壳热解的活化能,说明猪粪较其他生物质易受热分解;同时猪粪热解的活化能和频率因子之间存在动力学补偿关系,但整个热解过程中这种补偿关系呈分段趋势。     

热重-红外联用技术分析桉树热解动力学及挥发产物对结渣影响研究

基于热重-红外联用技术(TG-DTG-FTIR),研究升温速率为10、30、60℃/min下桉树的热解动力学活化能E及挥发分产物吸收带和特征峰。实验结果表明:桉树热解过程可分为水分挥发阶段(0~265℃)、挥发分析出阶段(265~605℃)、炭化阶段(605~700℃),共3个阶段,并且随着升温速率的增大热解最大速率增大,起始热解温度(T_(in))、终止热解温度(T_h)和最大峰值温度(T_(max))的热解曲线均向高温处稍微移动。在不知反应机理函数的情况下,利用Flynn-Wall-Ozawa法在转化率α∈[0.2,0.8]下计算桉树的热解动力学活化能E值,结果在33.25~60.12 k J/mol之间,与Coats-Redfern法求解的结果相近。利用FTIR谱图分析各热解阶段的产物,同时验证与之相应的TG-DTG曲线。     

厨余垃圾及其水热炭燃烧特性与动力学研究

采用热重分析法研究厨余垃圾及其水热炭的燃烧特性与反应动力学。对比分析厨余垃圾及其水热炭在3种不同升温速率(10、20、40℃/min)下的燃烧特性,分别采用KAS(Kissiger-Akahira-Sunose)法和FWO(Flynn-Wall-Ozawa)法计算燃烧过程中反应动力学参数。结果表明:20℃/min升温速率下,厨余垃圾与水热炭呈现不同的燃烧特性,厨余垃圾微分热重(DTG)曲线呈明显的双峰结构,而随着炭化温度的升高,水热炭DTG曲线第1个峰逐渐变缓,最后消失。随着升温速率的增大,各样品DTG曲线整体向高温侧偏移,着火温度和燃尽温度升高,燃烧特性指数增大。KAS法和FWO法求得的各样品燃烧活化能均具有相似变化趋势,因挥发分含量减少及固定碳含量增加,厨余水热炭热值增大,燃烧过程中平均活化能高于厨余垃圾。     

玉米芯水解残渣热解焦ZnCl2活化动力学分析

为优化玉米芯水解残渣低温热解、ZnCl2活化制备活性炭的工艺过程,获取活化过程的优化工艺参数,在热分析仪上对残渣热解焦的ZnCl2活化过程进行了研究。通过对比ZnCl2溶液浸渍前后残渣热解焦的热重特性,分析了ZnCl2活化机理,采用分布活化能模型(DAEM)对浸渍热解焦活化过程进行了动力学分析。结果表明,经ZnCl2浸渍后的残渣热解焦热失重区间集中在450~650℃,且失重峰明显增强,固体残留质量大幅降低。DAEM计算结果表明,浸渍热解焦在活化过程中的活化能分布为190~280 kJ/mol,在E=210 kJ/mol时活化能分布函数达到最大值。     

O_2_CO_2气氛下痕量元素迁移特性试验研究

在管式炉上进行徐州烟煤的燃烧试验,采用电感耦合等离子体光谱质谱联用仪(ICP-MS)对燃烧剩余灰渣进行测定.研究了不同温度和不同燃烧气氛(空气气氛和O2/CO2气氛)下痕量元素的迁移特性.结果表明,煤燃烧过程中,痕量元素Cr、Mn、Ni、Zn、Cd、Pb在灰渣中富集,As挥发率达70%以上.随着温度的升高,大部分痕量元素在灰渣中的含量降低,Mn、Ni、Cr在灰渣中的含量相对比较稳定.O2/CO2气氛下,各元素随温度的变化趋势并未受到影响,但整体上O2/CO2气氛下各痕量元素在灰渣中的含量要大于空气气氛下的,随着温度的升高,O2/CO2气氛和空气气氛下痕量元素在灰渣中的含量越来越接近,说明燃烧气氛对痕量元素迁移的影响随温度升高而减弱.     

不同形态硫对城市生活垃圾热过程中重金属迁移特性的影响

采用管式实验炉研究了不同形态硫化物对城市生活垃圾中Cd、Cu、Cr、Pb、Ni、Zn六种元素在热过程中的迁移特性的影响,使用原子吸收光谱仪和扫描电子显微镜等仪器对重金属浓度、灰渣表面形貌等进行了分析。结果表明,在热过程中,无机硫相对于有机硫能更有效地将重金属固定在底渣中。含硫化合物的存在可以减少Cd、Cu、Pb在热过程中的挥发;其中无机硫可以减少Cr、Ni的挥发,而有机硫则会增大Cr、Ni的挥发。但在还原性气氛中,含硫化合物的存在会增大重金属的挥发。     

Electrochemical synthesize and characterization of ZnO/ZnS nanostructures for hydrogen production

In present study, zinc oxide/zinc sulfide (ZnO/ZnS) nanostructures were fabricated by anodization of Zn. The morphological, structural and compositional properties of ZnO/ZnS were studied by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). From FESEM results, well-defined and smooth spherical-like ZnO/ZnS nanostructures were obtained with tube-like ZnO nanostructures underneath the compact layer. XRD patterns revealed that achieved materials offer high crystallinity hexagonal ZnO as dominant with hexagonal close-packed polycrystalline Zn. Moreover, measure of water contact angle was realized to learn about wetting property of the electrodes. Electrochemical impedance spectroscopy (EIS) was recorded to examine electrocatalytic performance of electrodes against hydrogen evolution reaction (HER) in 1 M KOH solution. The values of energy consumption and energy efficiency were calculated as 571.9 kJ mol⁻¹ and 49.5% at current density of 50 mA cm⁻² for the HER on 40-ZnO/ZnS/Zn electrode at 25°C.     

Large area (9 × 9 cm) electrostatically sprayed nanocrystalline zincite thin films for hydrogen production application

Synthesis of single phase nanocrystalline zinc ferrite (zincite) thin films that provides high surface area for active redox reactions is reported. Electrostatic spray method is employed for obtaining these ferrite films onto a stainless steel substrate of 9 × 9 cm² area using zinc acetate and iron nitrate precursor solutions, mixed in 1:1.5 proportions in triply-distilled water. Single zinc ferrite phase in films, confirmed from the X-ray diffraction pattern, reveals the aggregation of several nanometer-sized spherical grains. About 4 nm average roughness is obtained from the 3D AFM image. Zinc ferrite film deposited onto a glass substrate shows both direct (2.96 eV) and indirect (1.92 eV) band gap energies. Hydrophilic behavior that might facilitate easy reaction kinetics in a water splitting process is noticed. Finally, 450 mL/h hydrogen production rate is confirmed when zinc ferrite film electrode was used as an anode in presence of Pt mesh as a cathode.     

Performance improvements of alkaline batteries by studying the effects of different kinds of surfactant and different derivatives of benzene on the electrochemical properties of electrolytic zinc

Electrolytic zinc powders were prepared in 12 M KOH, 4 wt.% zinc oxide solutions in the presence of different kinds of surfactant and organic additives using the galvanostatic technique. Then the electrochemical behavior of zinc was investigated using the sweep voltametry technique. Zinc samples electrolyzed in the presence of cationic cetyl trimethyl ammonium bromide (Zn-CTAB), have maximum corrosion rate. Furthermore, scanning electron microscopy revealed the highest surface area. Zinc deposited with anionic surfactants, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS), have high dendritic and secondary growth. More zinc ions electrolyzed on the cathode electrode in the presence of SDBS compared with SDS. We suppose the Benzene molecule in SDBS changes morphology, thus effects of the benzene molecule is investigated by utilizing several organic compounds during zinc electrodeposition. Naphthalene with 10 pi electrons at two fused rings decreases corrosion rate and needle growth of zinc deposited, compared to benzyl chloride which has 6 pi electrons. Enhanced delocalization of pi electrons by strongly activating group (–NH₂) in the aniline molecule increases the corrosion rate and dendrites compared with benzyl chloride, which has the weakly activating group (–CH₂Cl). The addition of chloro benzene with inactivating and electrodrawing group (–Cl) creates high surface area without any dendritic growth. The effects of electrolyte additives on the electrochemical capacity of AA-sized alkaline Zn-MnO₂ batteries are verified. The addition of Triton X-100 in anode gel resulted in maximum electrical capacity. Anionic (SDBS and SDS) additives gave higher electrical capacity than cationic (CTAB). Also, the reaction mechanism for zinc electrodeposition in alkaline electrolytes and its dependence upon the presence of organic additives are discussed in detail.     

CVD of CuGaSe2 for thin film solar cells with various transport agents

Chemical vapor deposition (CVD) in an open tube system was employed to deposit single-phase CuGaSe₂ thin films on plain and Mo-coated glass substrates. The use of HCl and ternary CuGaSe₂ source material resulted in non-stoichiometric volatilization of the source material. The use of binary source materials – Cu₂Se and Ga₂Se₃ – in combination with I₂ and HCl as the respective transport agents yielded single-phase CuGaSe₂ thin films while the source materials were volatilized stoichiometrically. Mo/CuGaSe₂/CdS/ZnO devices were fabricated from these samples exhibiting an open-circuit voltages up to V_oc=853 mV.     

Effect of minor nickel alloying with zinc on the electrochemical and corrosion behavior of zinc in alkaline solution

The electrochemical and corrosion behavior of pure zinc and Zn-0.5Ni alloy in strong alkaline solution (7 M KOH) was investigated by Tafel plot, potentiodynamic, potentiostatic and electrochemical impedance spectroscopy (EIS) methods, and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Measurements were conducted under different experimental conditions. The results of both Tafel plot extrapolation and the electrochemical impedance spectroscopy (EIS) measurements exhibited the same trend, which the cathodic and anodic processes on the alloy surface are less significant compared with those on the pure zinc. The results revealed that, the shift in steady state of open-circuit potential (E_corr) to more negative potential in the case of the studied alloy compared with that of pure zinc has a positive effect on both charge efficiency and self-discharge.The anodic potentiodynamic measurements demonstrated that the polarization curves exhibited active/passive transition. The active dissolution of both pure zinc and its alloy increases with increasing temperature and scan rate. The activation energy (E_a) value of active region and peak current (I_AI) of the two studied electrodes in the investigated alkaline solution is calculated and compared. In the case of alloy, the results obtained at certain positive potential (+425 mV vs. SCE), exhibited high current density indicating that the most passive layer was destroyed. This indicates that the addition of small amount from Ni to Zn promotes the electrochemical reaction (in the passive region), acting as so-called self catalysis. Accordingly, one can conclude that, the electrochemical behavior of the investigated alloy in strong alkaline solution contributes to suppression of hydrogen gas evolution and increases the corrosion resistance. In addition, reactivation of the alloy surface takes place in the passive region.     

DIRECT SOLAR THERMAL DISSOCIATION OF ZINC OXIDE: CONDENSATION AND CRYSTALLISATION OF ZINC IN THE PRESENCE OF OXYGEN

The solar thermal production of zinc from zinc oxide is part of a two-step water splitting cycle. Zinc oxide thermally dissociates into zinc vapour and oxygen at elevated temperatures;

ZnO(s)=Zn(g)+0.5O₂

In practice, the yield of zinc depends on the kinetics of dissociation and the technical feasibility of quenching its gaseous products fast enough to avoid reoxidation. As the gaseous products cool, the rate of zinc oxidation varies as the zinc changes its phase. The condensation of zinc in the presence of oxygen was studied by fractional crystallisation in a temperature gradient tube furnace. It was observed that the oxidation of zinc vapour is a heterogeneous process. In the absence of nucleation sites, zinc vapour and oxygen can coexist in a meta-stable state. The different stages and forms of nucleation, crystal growth and oxidation were deduced from the morphology of the quenched products, as determined by scanning and high-resolution transmission electron microscopy. Zinc morphology depends on whether it crystallised at above or below the melting point.     

Nickel volatilization phenomenon on the Ni-CGO anode in a cathode-supported SOFC operated at low concentrations of H2

Rapid degradation phenomenon is generally occurred when Ni-based anode on a cathode-supported SOFC is operated in low concentrations of hydrogen at high current density. In order to clarify this phenomenon, homogenous NiO-Ce_0.8Gd_0.2O_1.9 (CGO) composites powder with fixed weight ratio of Ni:Ce was synthesized using a nitric-citrate sol-gel method, and coated on LSM-CGO cathode-supported SOFC using slurry coating method. As-prepared fuel cells exhibited good performance when they were operated at pure H₂. However, rapid degradation phenomenon on Ni-CGO anode usually happened when low concentration of H₂ was used as fuel at high current density. Obvious microstructure damage and sintering of Ni were observed in SEM micrographs of Ni-CGO anode after repeated degradation process in 5.66% of H₂ at high current density. Furthermore, the decrease in Ni amount in Ni-CGO anode was also found via EDX analysis when this degradation process was repeated for several times. It is inferred that the volatilization of nickel hydroxide should happen at triple-phase boundaries of Ni-CGO anode when high partial pressure ratio of H₂O and H₂ appeared in this case.