Mechanism of O2 evolution on TiO2 photoanodes

The mechanism of O₂ evolution over titanium oxide photoanodes has been studied by relating structural properties, derived from XRD and XPS, and electrochemical properties currents, (cyclic voltammetry and flatband potentials) to the performance in PEC cells of five specimens of lowest and highest photoelectrochemical efficiency.     

Detailed investigation of a pulverized fuel swirl flame in CO2/O2 atmosphere

A novel approach to oxycoal flame stabilization has been developed at the Institute of Heat and Mass Transfer at RWTH Aachen University [D. Toporov, M. Förster, R. Kneer, in: Third Int. Conf. on Clean Coal Technologies for Our Future, Cagliari, Sardinia, Italy, 15-17 May 2007]. The swirl burner design and its operating conditions have been adjusted in order to enforce CO formation thus stabilizing the flame and obtaining a full burnout at levels of O₂ content in the O₂/CO₂ mixture similar to those in air. The paper presents results of detailed numerical and experimental investigations of a stable oxy-fired pulverized coal swirl flame (type-2) obtained with a 21 vol% O₂ concentration. The combustion tests were performed in a vertical pilot-scale furnace (100 kW_th) in the framework of the OXYCOAL-AC research project aiming to develop a membrane-based oxyfuel process. The experimental results concerning gas velocities, gas and particle temperatures, and gas compositions are presented and discussed, focusing on the underlying mechanisms as well as on the aerodynamics of the oxycoal flame. A comparison between measurements and simulations has shown the validity of the numerical method used. The reported data set can be used for validation of numerical models developed for prediction of oxyfuel combustion.     

An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure

An experimental and modeling study of 11 premixed NH₃/CH₄/O₂/Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt-13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios (R) of NH₃/CH₄ (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH₄/NH₃ mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H₂O, NO and N₂ increase while those of H₂, CO, CO₂ and NO₂ have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH₄ and NH₃ oxidation and their mutual interaction.     

Laminar burning velocities and transition to unstable flames in H2/O2/N2 and C3H8/O2/N2 mixtures

Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H₂/O₂/N₂ mixtures at values of flame stretch up to 7600 s⁻¹, and existing measurements for C₃H₈/O₂/N₂ mixtures at values of flame stretch up to 900 s⁻¹. Laminar burning velocities varied linearly with increasing Karlovitz numbers—either decreasing or increasing at stable or unstable preferential-diffusion conditions—yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion coditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H₂/O₂/N₂ mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.     

Preparation and dehydrogenation properties of two cobalt-based ammine borohydrides: CoCl3·3NH3/3LiBH4 and CoCl2·3NH3/2LiBH4

Two new cobalt-based ammine borohydrides were prepared via ball milling of LiBH₄ and CoCl_n·3NH₃ (n = 3, 2) with molar ratios of 3:1 and 2:1, respectively. X-ray diffraction (XRD) results revealed the as-prepared composites having amorphous state. Thermogravimetric analysis-mass spectrometry (TG-MS) measurements showed that the two composites mainly release H₂, concurrent with the evolution of a small amount of NH₃. Further results showed that the excessive addition of LiBH₄ can suppress the liberation of NH₃, resulting in the release of H₂ with a high purity (>99 mol.%). By combination with the temperature-programmed-desorption (TPD) results, the CoCl₃·3NH₃/4LiBH₄ and CoCl₂·3NH₃/3LiBH₄composites can release 7.3 wt.% (4.2 wt.% including LiCl) and 4.2 wt.% (2.0 wt.% including LiCl) pure hydrogen, respectively, in the temperature range of 25–300 °C. Isothermal dehydrogenation results reveal that CoCl₃·3NH₃/3LiBH₄ shows favorable dehydrogenation rate at low temperatures, releasing about 5.2 wt.% (2.9 wt.% including LiCl) of hydrogen within 45 min at 80 °C.     

rGO supported PdNi-CeO2 nanocomposite as an efficient catalyst for hydrogen evolution from the hydrolysis of NH3BH3

The hydrogen economy is a proposed system that utilizes hydrogen to deliver energy. For the realization of this concept, how to safely, controllably and reversibly store and release hydrogen are critical problems which must be resolved. Metal alloys combined with suitable support materials are widely applied to various catalytic reactions. Here palladium nickel bimetallic nanoparticles doped with cerium oxide on a reduced graphene oxide (rGO) support were prepared by combining metal ion precursors and graphene oxide in a one-pot co-reduction approach. The as-received catalysts were characterized by XRD, TEM, SEM, XPS and ICP-OES, and the results revealed that PdNi-CeO₂ nanoparticles were uniform dispersal on rGO. The as-synthesized PdNi-CeO₂/rGO had been adopted as a heterogeneous catalyst for the hydrogen evolution from the hydrolysis of ammonia borane (NH₃BH₃, AB) at room temperature. Kinetically, the hydrogen-release rate was first-order with the increased concentration of catalysts. The optimized catalyst of Pd_0.8Ni_0.2-CeO₂/rGO with the CeO₂ content of 13.9 mol% exhibited an excellent activity with a turnover frequency value of 30.5 mol H₂ (mol catalyst)^?1 min^?1 at 298 K, and a low apparent activation energy (E_a) of 37.78 kJ mol^?1. The robust catalytic performance of the Pd_0.8Ni_0.2-CeO₂/rGO is attributed to the uniform controlled nanoparticle size, the synergic effect between the nanoparticles bimetallic properties, and the effective charge transfer interactions between the metal and support.     

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.     

Improved low-temperature hydrogen generation from NH3BH3 and TiO2 composites pretreated with water

Ammonia borane (AB, NH₃BH₃) is nontoxic easily transportable solid hydride with high stability in air. In this work we demonstrate that simple mixing of AB with TiO₂ (anatase) allows for hydrogen gas to be generated at temperatures as low as 80 °C. No losses of hydrogen have been observed during preparation of hydride-containing composites. It was shown that the adsorption of water vapor on TiO₂ and the increase of TiO₂ loading considerably accelerated the rate of AB decomposition. The experimentally observed formation of B–O chemical bonds and the elevated heat emission suggest strong interaction of AB with the adsorbed water species on TiO₂ surface. It has been found that this interaction proceeds at a higher rate comparing with binary AB/H₂O systems. The heat being released in the process is thought to contribute to overcoming the activation barrier in the dehydrogenation of ammonia borane to produce hydrogen gas.     

Synthesis and dehydrogenation of LiCa(NH2)3(BH3)2

We report the synthesis of a new hydrogen storage material with a composition of LiCa(NH₂)₃(BH₃)₂. The theoretical hydrogen capacity of LiCa(NH₂)₃(BH₃)₂ is 9.85 wt.%. It can be prepared by ball milling the mixture of calcium amidoborane (Ca(NH₂BH₃)₂) and lithium amide (LiNH₂) in a molar ratio of 1:1. The experimental results show that this material starts to release hydrogen at a temperature as low as ca. 50 °C, which is ca. 70 °C lower than that of pure Ca(NH₂BH₃)₂ possibly resulting from the active interaction of NH₂⁻ in LiNH₂ with BH₃ in Ca(NH₂BH₃)₂. ca. 4.1 equiv. or 6.8 wt.% hydrogen can be released at 300 °C. The dehydrogenation is a mildly exothermic process forming stable nitride products.     

N2 CARS thermometry and O2 LIF concentration measurements in a flame under electrically induced microbuoyancy

Microgravity combustion is fundamentally characterized by the absence of buoyancy driven flows. To facilitate a large range of diagnostics on microgravity flames, it is useful to create an equivalent microbuoyant condition in an earth-based laboratory. This experiment simulates microbuoyancy using electric fields to balance local convection in a region of the flame. Previous studies used N₂ coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements to show that a region exists in which the temperature profile corresponds to that of a spherically symmetric diffusion profile, as would be expected in a true microgravity environment. The current study utilizes CARS thermometry and laser induced fluorescence (LIF) to examine the temperature and O₂ concentration profiles in a region below the flame. The results show that the electrically balanced flame apparatus produces a spherically symmetric wedge of microbuoyant flame extending 10° from the vertical axis of the capillary flame arrangement; thereby quantifying the extent of the microbuoyant region.     

Industrial scale modelling of the thermochemical energy storage system based on CO2 + 2NH3 ↔ NH2COONH4 equilibrium

The thermochemical storage of energy by the system carbon dioxide, ammonia and ammonium carbamate is studied in detail. In particular, the kinetics and the thermodynamics of the reversible reaction is studied. We give two industrial models for the operation of this system. In the first, the separation of the gases NH₃ and CO₂ is achieved by compression and liquefaction of NH₃. In the second, a method of separation of the gases is proposed which is based on the solubility of NH₃ in ethanol while CO₂ is practically insoluble. The operation of this system is examined both in closed form and in the case in which CO₂ is rejected in the atmosphere, and it is taken from alcoholic fermentation or from the combustion gases of power plants burning lignite. The mass and energy balance is given, for each case, and the amount of energy losses by the use of this storage system is calculated. Finally, we give some estimates for the area of solar collectors and the amount of chemicals which are required in order to cover the energy needs of a community.     

NH3BH3/LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation

The significantly enhanced dehydrogenation performance of binary complex system, NH₃BH₃/LiBH₄·NH₃, were achieved through a chemical modification of LiH to form ternary composites of x (LiH–NH₃BH₃)/LiBH₄·NH₃. Among the studied composites, 3LiH–3NH₃BH₃/LiBH₄·NH₃ released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 °C with fast kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile byproducts, were released from 3NH₃BH₃/LiBH₄·NH₃ at the same conditions. Further investigations revealed that the hydrogen emission from x (LiH–NH₃BH₃)/LiBH₄·NH₃ composites is based on the combination mechanism of H^δ+ and H^δ− through the interaction between LiH–NH₃BH₃ and NH₃ group in LiBH₄·NH₃, in which the controllable protic hydrogen source from the stabilized NH₃ group played a crucial role in providing optimal stoichiometric ratio of H^δ+ and H^δ−, and thus leading to the significant improvement of dehydrogenation capacity and purity.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

A study on municipal solid waste (MSW) combustion in N2/O2 and CO2/O2 atmosphere from the perspective of TGA

In this paper, the combustion behavior of municipal solid waste (MSW) is carried out in a thermogravimetric analyzer under different N₂/O₂ and CO₂/O₂ atmospheres with temperature ranging from 100 °C to 1000 °C. TG (thermogravimetric) and DTG (derivative thermogravimetric) curves are analyzed. The nth order reaction fitting model is used to yield the activation energy of reduction process according to the degree of conversion. The results indicate that all samples lose most their weight between 200 °C and 540 °C. As the oxygen concentration increased, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape. At the same oxygen concentration, the peak values in CO₂/O₂ atmosphere are smaller than those in N₂/O₂ atmosphere, indicating that CO₂ has a higher inhibitory effect than N₂ on MSW combustion. After 600 °C, the weight loss peak appears much later in CO₂/O₂ atmosphere than it does in N₂/O₂ atmosphere. With the increase of heating rate, the maximum weight loss rates of samples increase obviously. The three-step reaction of nth order reaction model fits the weight loss very well.     

N-desorption or NH3 generation of TiO2-loaded Al-based nitrogen carrier during chemical looping ammonia generation technology

As a terrific hydrogen carrier, NH₃ can be wildly used as non-carbon gas fuel in energy area, because NH₃ can store a high density of H₂ and can be liquefied easily in ambient temperature and pressure. Chemical Looping Ammonia Generation (CLAG) is known to be a novel, efficient and environmentally friendly synthesis method for ammonia, in which the N-sorption/desorption of nitrogen carrier are included. The N-sorption step, which is known as the carbothermal reduction, is well studied while there are few reports on N-desorption step. Therefore, in this paper, stationary bed reactor and thermo-gravimetric analyzer (TGA) were used to study the N-desorption performance of the TiO₂-loaded Al-based N-carrier and the corresponding NH₃ generation during the N-desorption reaction. The results showed that, TiO₂ performed a good catalytic effect on the N-desorption reaction, which is due to the good dissociative adsorption performance of H₂O on TiO₂ surface, and the released hydroxyl (OH) plays an important role in the N-desorption reaction. The conversion amount of aluminum nitride and the yield of NH₃ increased with the TiO₂ loading in the reactants, the reaction temperatures and the steam concentrations in the atmosphere, while the conversion efficiency of NH₃, which is significantly affected by temperature, didn't affected by the TiO₂ loadings.     

The dependence of flame propagation in H2O2N2 mixtures on temperature,pressure, and initial composition

Hydrogen combustion in premixed flames is simulated with aid of a reaction mechanism consisting of 25 elementary reactions. Concentration-, pressure-, and temperature-dependence of flame velocities in H₂O₂N₂ mixtures are described in a wide range of conditions. Furthermore, NO formation in H₂-air flames is reproduced. A sensitivity analysis is presented demonstrating the influence of the elementary reactions involved in the mechanism.     

The photoinduced evolution of O2 and H2 from a WO3 aqueous suspension in the presence of Ce/Ce

This is a report on the production of O₂ and H₂ from photocatalytic and photochemical processes in the WO₃–H₂O–Ce⁴⁺_aq–hν system. The photoproduction of O₂ and H₂ was studied over the range of WO₃ concentrations from 2 to 8 g dm⁻³, and conduction band electron scavenger concentrations 1–20 mM Ce_aq⁴⁺. Medium and high concentrations of the electron scavenger gave mainly O₂ as the main product. Dilute solutions of [Ce_aq⁴⁺]< 2 mM initially produced dioxygen, and then hydrogen after an induction period of 3–4 h. Yields of 140–250 μmol O₂  h⁻¹ and 1–7 μmol H₂ h⁻¹ were obtained and were found to depend on the physical properties and content of WO₃, the concentration of the electron scavenger, illumination period and wavelength, and the radiation geometry. The photoactivity of the suspension was correlated to the level of crystallinity of WO₃ powders. The studied system utilizes WO₃ to accomplish the initial light absorption, charge separation, and production of O₂ and H⁺ from the interaction of water molecules with photogenerated WO₃ valence band holes, in the presence of Ce⁴⁺_aq species as a scavenger of conduction band electrons. This is followed by the evolution of H₂ from a homogeneous photochemical reduction of H⁺ and/or H₂O by photoexcited Ce³⁺_aq, formed from the earlier reduction of Ce⁴⁺_aq. The obtained results show that, with an appropriate design, tungsten trioxide is a promising material that can be used as a photoactive component in energy conversion systems or in environmental photocatalysis, using artificial or solar light.     

Comparative studies of reaction rates of NH3 with MgH2 and LiH

The reaction rate of MgH₂ with NH₃ is studied using a two-layered structure containing a top MgH₂ layer and a bottom LiNH₂ layer. Quantification of the effluent gas composition from the two-layered structure indicates substantial NH₃ emission, while the X-ray diffraction analysis reveals little formation of the reaction products between MgH₂ and NH₃. In contrast, the study of the two-layered structure containing a top LiH layer and a bottom LiNH₂ layer reveals that the reaction between LiH and NH₃ is much faster than that between MgH₂ and NH₃.     

Experimental and numerical investigation of low-pressure laminar premixed synthetic natural gas/O2/N2 and natural gas/H2/O2/N2 flames

The oxidation of laminar premixed natural gas flames has been studied experimentally and computationally with variable mole fractions of hydrogen (0, 20, and 60%) present in the fuel mixture. All flames were operated at low pressure (0.079 atm) and at variable overall equivalence ratios (0.74<?<1.0) with constant cold gas velocity. At the same global equivalence ratio, there is no significant effect of the replacement of natural gas by 20% of H₂. The small differences recorded for the intermediate species and combustion products are directly due to the decrease of the amount of initial carbon. However, in 60% H₂ flame, the reduction of hydrocarbon species is due both to kinetic effects and to the decrease of initial carbon mole fraction. The investigation of natural gas and natural gas/hydrogen flames at similar C/O enabled identification of the real effects of hydrogen. It was shown that the presence of hydrogen under lean conditions activated the H-abstraction reactions with H atoms rather than OH and O, as is customary in rich flames of neat hydrocarbons. It was also demonstrated that the presence of H₂ favors CO formation.