350 MW超临界循环流化床锅炉动态特性试验研究

随着CFB(循环流化床)技术的发展,流化床机组在电网中占比越来越高。然而其热惯性大,动态调峰能力差,使得要求随时吸纳新能源发电的电网负荷调度越来越困难。为了研究切实可行的技术措施,对某350 MW超临界循环流化床发电机组,进行阶跃扰动试验,分析了给煤量和燃烧率阶跃时床温和负荷的动态响应过程,试验结果表明:CFB机组负荷响应的延迟时间约为100 s,负荷响应时间约为15 min;燃烧率阶跃时的负荷响应速度为0.56~0.6MW/min(负荷每分钟变化0.16%~0.17%),与燃煤机组负荷变化率相差很大。大热惯性是CFB机组的固有属性,改善其负荷调节能力须协同其它调节方式,才能满足电网负荷调度要求。     

Low shear rheological behaviour of two-phase mesophase pitch

The low shear rate rheology of two phase mesophase pitches derived from coal tar pitch has been investigated. Particulate quinoline insolubles (QI) stabilised the mesophase spheres against coalescence. Viscosity measurements over the range 10–10⁶ Pa s were made at appropriate temperature ranges. Increasing shear thinning behaviour was evident with increasing mesophase content. At low mesophase contents the dominant effect on the near Newtonian viscosity was temperature but at higher contents it was the shear rate; temperature dependence declined to near zero. The data indicated that agglomeration could be occurring at intermediate mesophase volume fractions, 0.2–0.3. The Krieger–Dougherty function and its emulsion analogue indicated that in this region the mesophase pitch emulsions actually behaved like ‘hard’ sphere systems and the effective volume fraction was estimated as a function of shear rate illustrating the change in extent of agglomeration. At the higher volume fractions approaching the maximum packing fraction, which could only be measured at higher temperatures, the shear thinning behaviour changed in character and it is considered that this is possibly due to shear induced deformation and breakup of dispersed drops in the shear field.     

Physical characteristics of AFEX-pretreated and densified switchgrass,prairie cord grass,and corn stover

The goal of the study was to evaluate and compare the physical properties of control, pretreated and densified corn stover, switchgrass, and prairie cord grass samples. Ammonia Fiber Expansion (AFEX) pretreated switchgrass, corn stover, and prairie cord grass samples were densified by using the comPAKco device developed by Federal Machine Company of Fargo, ND. The densified biomass were referred as “PAKs” in this study. All feedstocks were ground into three different grind size of 2, 4 and 8 mm prior to AFEX pretreatment and the impact of grinding on pellet properties was studied. The results showed that the physical properties of AFEX-PAKed material were not influenced by the initial grind size of the feedstocks. The bulk density of the AFEX-PAKed biomass increased by 1.2–6 fold as compared to untreated and AFEX-pretreated materials. The durability of the AFEX-PAKed materials were between 78.25 and 95.2%, indicating that the AFEX-PAKed biomass can be transported easily. To understand the effect of storage on the physical properties of these materials, samples were stored in the ambient condition (20 ± 2 °C and 70 ± 5% relative humidity) for six months. After storage, thermal properties of the biomass did not change but glass transition temperature decreased. The water absorption index and water solubility index of AFEX-treated and AFEX-PAKed biomass showed mixed trends after storage. Moisture content decreased and durability increased upon storage.     

Dimethyl carbonate synthesis from methyl nitrite and CO over activated carbon supported Wacker-type catalysts: The surface chemistry of activated carbon

A series of supported Wacker-type catalysts under treatment of nitric acid were prepared and utilized in the synthesis of dimethyl carbonate from CO and methyl nitrite. Nitrogen adsorption, Boehm titration, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and H₂-temperature-programmed reduction (H₂-TPR) were employed to explore the relationship between the physicochemical properties and reactivity of catalysts. Correlated with the results of Boehm titration, it was found that catalytic activities increased monotonically with the number of phenolic group on activated carbon surface. XPS spectra showed that the catalysts, which had much more amount of phenolic group, were possessed of large amount of Cu^2 + species. H₂-TPR results indicated that the phenolic group contributed to the easier reducibility of Cu^2 + species, further facilitating the oxidation cycle between Pd and Cu species and consequently the catalytic performance.     

High coverage H2 adsorption and dissociation on fcc Co surfaces from DFT and thermodynamics

Hydrogen adsorption, desorption and dissociation on the Co(100), (311), (111) and (110) surfaces at different coverage have been systematically studied using density functional theory and ab initio atomistic thermodynamics. On the basis of the computed stepwise H₂ adsorption energies, the saturated coverage on the Co(311), (111), (110) surfaces are 8/18, 3/9 and 6/18 ML, respectively, revealing that these surfaces have different potential hydro-treating abilities and activity. The lateral repulsive interactions can affect the adsorption structures and become stronger with the increasing of H₂ coverage, which lead to H₂ migration over Co surfaces. Comparison of dissociation energies and corresponding desorption energies, it can be concluded that H₂ molecules prefer dissociation rather than desorption both kinetically and thermodynamically at different coverage. Hydrogen stable coverage on the surfaces highly depends on temperatures and H₂ partial pressure. These results should provide important information about morphology of Co-catalysts under Fischer-Tropsch synthesis reduction conditions.     

Effect of methanol addition on higher alcohol synthesis over modified molybdenum sulfide catalysts

One of the main problems in higher alcohol synthesis is the poor product distribution. Cofeeding of methanol, together with the synthesis gas, has been suggested in order to increase the yield of ethanol and higher alcohols. In this work, the effect of methanol addition on K–MoS₂ and K–Ni–MoS₂ catalysts was studied at 71 bar, 340 °C and GHSV = 6000 N mL/h · g_catalyst. Under these conditions methanol recycle is not a viable option for boosting higher alcohol production. The main result was an increase in methane yield, while the effect in higher alcohols was negative or not significant.     

Activation of graphene aerogel with phosphoric acid for enhanced electrocapacitive performance

We present a facile and efficient route to introduce in-plane nanopores on the graphene sheets by activation of graphene aerogel (GA) with phosphoric acid (H₃PO₄). Results from N₂ adsorption and TEM images showed that H₃PO₄ activation created mesopores with pore size of 2–8 nm on the graphene sheets. With such nanopores on graphene sheets, the activated GA exhibits a specific capacitance of 204 F g⁻¹, enhanced rate capability (69% capacitance retention from 0.2 to 30 A g⁻¹), reduced equivalent series resistance (3.8 mΩ) and shortened time constant (0.73 s) when comparing with the hydrothermally-derived pristine GA and thermally annealed GA in the absent of H₃PO₄. The excellent capacitive properties demonstrate that introduction of nanopores on GA by H₃PO₄ activation not only provides large ion-accessible surface area for efficient charge storage, but also promotes the kinetics of electrolyte across the graphene two-dimensional planes.     

Comb-shaped diblock copolystyrene for anion exchange membranes

To improve the properties of diblock copolystyrene-based anion exchange membranes (AEMs), a series of AEMs with comb-shaped quaternary ammonium (QA) architecture (QA-PS_m-b-PDVPPA_n-xC where x denotes the number of carbon atoms in different alkyl tail chains and has values of 1, 4, 8, and 10 and C denotes carbon) were designed and synthesized via subsequent quaternization reactions with three different alkyl halogens (methyl iodide and N-alkane bromines (CH₃[CH₂] _x-1Br where x = 4, 8, and 10). Compared with triblock analogues quaternized with methyl iodide in our previous research, QA-PS_m-b-PDVPPA_n-xC (x = 4, 8, and 10) AEMs are more flexible with the introduction of a long alkyl tail chain; this probably impedes crystallization of the rigid polystyrene-based main chain and induces sterically adjustable ionic association. An increase in the pendant alkyl tail chain length generally led to enhanced microphase separation of the obtained AEMs, and this was confirmed using small-angle X-ray scattering and atomic force microscopy. The highest conductivity (25.5 mS cm⁻¹) was observed for QA-PS₁₂₀-b-PDVPPA₈₀-10C (IEC = 1.94 meq g^–1) at 20 °C. Furthermore, the water uptake (<30%) and swelling ratio (<13.1%) of QA-PS_m-b-PDVPPA_n-xC AEMs are less than half of these corresponding values for their triblock counterparts. The QA-PS₁₂₀-b-PDVPPA₈₀-10C membrane retained a maximum stability that was as high as 86.8% of its initial conductivity after a 40-day test (10 M NaOH, 80 °C), and this was probably because of the steric shielding of the cationic domains that were surrounded by the longest alkyl tail chains. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47370.     

Structure,spectroscopic properties and laser performance of Nd:YNbO4 at 1066 nm

We have demonstrated continuous wave (CW) laser operation of Nd:YNbO₄ crystal at 1066 nm for the first time. A maximum output power of 1.12 W with the incident power of 5.0 W is successfully achieved corresponding to an optical-to-optical conversion efficiency of 22.4% and a slope efficiency of 24.0%. The large absorption cross section (8.7 × 10⁻²⁰ cm²) and wide absorption band (6 nm) at around 808 nm indicates the good pumping efficiency by laser diodes (LD). The small emission cross section (29 × 10⁻²⁰ cm²) and relative long lifetime of the ⁴F_3/2 → ⁴I_11/2 transition indicates good energy storage capacity of Nd:YNbO₄. Moreover, the raw materials of Nd:YNbO₄ are stable, thus, it can grow high-quality and large-size by Czochralski (CZ) method. Therefore the Nd:YNbO₄ crystal is a potentially new laser material suitable for LD pumping.     

Tristable switching of the electrical conductivity through graphene quantum dots sandwiched in multi-stacked poly(methyl methacrylate) layers

Tristable switching nonvolatile memory (NVM) devices based on graphene quantum dots (GQDs) sandwiched between multi-stacked poly (methyl methacrylate) (PMMA) layers were fabricated on indium-tin-oxide (ITO)-coated glass substrates by using a solution-processed method. Current-voltage (I-V) curves at 300 K for the silver nanowire/PMMA/GQD/PMMA/GQD/PMMA/ITO/glass devices showed tristable switching currents with high-resistance, intermediate-resistance, and low-resistance states. The device's cycling endurance of the three resistance states remained stable with a distinguishable value for each resistance state over 1000 cycles, and the obtained retention results showed well-distinguished resistance states without degradation for up to 1 × 10⁴ s. Schottky emission, Poole-Frenkel emission, trapped-charge limited-current, and ohmic conduction were proposed as the dominant conduction mechanisms for the fabricated NVM devices based on the obtained I-V characteristics. The described energy-band diagrams confirm the proposed conduction band mechanisms.