Inhibition of premixed carbon monoxide–hydrogen–oxygen–nitrogen flames by iron pentacarbonyl

This paper presents measurements of the burning velocity of premixed CO–H₂–O₂–N₂ flames with and without the inhibitor Fe(CO)₅ over a range of initial H₂ and O₂ mole fractions. A numerical model is used to simulate the flame inhibition using a gas-phase chemical mechanism. For the uninhibited flames, predictions of burning velocity are excellent and for the inhibited flames, the qualitative agreement is good. The agreement depends strongly on the rate of the CO + OH ↔ CO₂ + H reaction and the rates of several key iron reactions in catalytic H- and O-atom scavenging cycles. Most of the chemical inhibition occurs through a catalytic cycle that converts O atoms into O₂ molecules. This O-atom cycle is not important in methane flames. The H-atom cycle that causes most of the radical scavenging in the methane flames is also active in CO–H₂ flames, but is of secondary importance. To vary the role of the H- and O-atom radical pools, the experiments and calculations are performed over a range of oxygen and hydrogen mole fraction. The degree of inhibition is shown to be related to the fraction of the net H- and O-atom destruction through the iron species catalytic cycles. The O-atom cycle saturates at a relatively low inhibitor mole fraction (100 ppm), whereas the H-atom cycle saturates at a much higher inhibitor mole fraction (400 ppm). The calculations reinforce the previously suggested idea that catalytic cycle saturation effects may limit the achievable degree of chemical inhibition.     

Predictions of the optimum ternary alkali-carbonate electrolyte composition for MCFC by computational calculation

Various kinds of phase diagrams for Li–Na–K ternary carbonate systems were plotted and the vapor pressures of chemical species such as alkali cations and oxygen molecule at various compositions and various temperatures were calculated by computational manipulation of thermodynamic databases. The liquidus temperature of (Li_0.52Na_0.48)₂CO₃–(Li_0.62K_0.32)₂CO₃, (Li_0.62K_0.32)₂CO₃–(Li_0.44Na_0.30K_0.26)₂CO₃, and (Li_0.44Na_0.30K_0.26)₂CO₃–(Li_0.52Na_0.48)₂CO₃ binary systems decrease with the increase of Li₂CO₃ content without any ternary intermediate compound. Total vapor pressure of alkali metal species governed by the summation of the vapor pressure of free Na and K and the vapor pressure of alkali metal species starts to decrease abruptly when the content of (Li_0.52Na_0.48)₂CO₃ is over 70 mol% in (Li_0.52Na_0.48)₂CO₃–(Li_0.62K_0.32)₂CO₃ system while over 50 mol% in (Li_0.44Na_0.30K_0.26)₂CO₃–(Li_0.52Na_0.48)₂CO₃ system. On the contrary, the equilibrium vapor pressure of oxygen molecule abruptly increases at the same composition range.     

Solubility and electrochemical studies of LiFeO2–LiCoO2–NiO materials for the MCFC cathode application

The dissolution of the state-of-the-art lithiated NiO is still considered as one of the main obstacles to the commercialisation of the molten carbonate fuel cell (MCFC). Development of alternative cathode materials has been considered as a main strategy for solving this problem. Ternary compositions of LiFeO₂, LiCoO₂ and NiO are expected to decrease the cathode solubility while ensuring a good electrical conductivity and electrochemical activity towards the oxygen reduction.

In this work, new material compositions in the LiFeO₂–LiCoO₂–NiO ternary system were synthesised using Pechini method and investigating their electrical conductivity by the DC four probe method. Then the influence of the cobalt content in the composition was determined in terms of AC impedance analysis and solubility measurements after 200 h of immersion in Li₂CO₃–Na₂CO₃ at 650 °C. The DC electrical conductivity study reveals the ability of improving the electrical conductivity, adequate for MCFC cathode application, by controlling the Co content of the composition. A special attention was given to the evolution of the open circuit potential as a function of time and to the impedance spectroscopy characterization related to microstructure modifications. Taking into account solubility, electrical conductivity, as well as electrochemical performance in the fuel cell, this study reveals the possibility of using LiFeO₂–LiCoO₂–NiO ternary materials for MCFC cathode.     

Selective removal of CO from hydrogen-rich stream over CuO/CexSn1−xO2–Al2O3 catalysts

The solid solutions of Ce_xSn_1−xO₂ incorporated with alumina to form Ce_xSn_1−xO₂–Al₂O₃ mixed oxides, by the suspension/co-precipitation method, were used to prepare CuO/Ce_xSn_1−xO₂–Al₂O₃ catalysts for the selective oxidation of CO in excess hydrogen. Incorporating Al₂O₃ increased the dispersion of Ce_xSn_1−xO₂, but did not change their main structures and did not weaken their redox properties. Doping Sn⁴⁺ into CeO₂ increased the mobility of lattice oxygen and enhanced the activity of the 7%CuO/Ce_xSn_1−xO₂–Al₂O₃ catalyst in the selective oxidation of CO. The selective oxidation of CO was weakened as the doped fraction of Sn⁴⁺ exceeded 0.5. Incorporating appropriate amounts of Sn⁴⁺ and Al₂O₃ could obtain good candidates 7%CuO/Ce_xSn_1−xO₂–Al₂O₃(20%), 1–x=0.1–0.5, for a preferential oxidation (PROX) unit in a polymer electrolyte membrane fuel cell system for removing CO. Its activity was comparable with, and its selectivity was much larger than, that of the noble catalyst 5%Pt/Al₂O₃.     

Low-temperature catalytic partial oxidation of hydrocarbons (C1–C10) for hydrogen production

The catalytic partial oxidation of hydrocarbons to provide hydrogen for fuel cells, mobile or stationary, requires high temperatures (900°C), multireactors and incurs the highest incremental costs for the gasoline fuel processor. New experimental data between 500°C and 600°C, supported by equilibrium calculations, show that hydrogen with low carbon monoxide concentrations can be produced from liquid and gaseous hydrocarbons, thus simplifying the reactor chain. Low sulphur refinery feeds (C₄–C₆, C₄–C₁₀), simulated natural gas (C₁–C₃) and single compounds are used and safety procedures discussed. Results from laboratory reactors with 1 wt% rhodium on mixed oxide catalysts show that hydrogen rates of 43,000 lH₂/h/l reactor (power density 129 kWth/l reactor) are produced with RON=95 feeds. However, the cost and availability of rhodium limit the catalyst rhodium content to 0.1 wt% when 31,100 lH₂/h/l reactor were measured. Optimisation and reactor scale-up for heat management is in progress.     

Electrochemical performance of Nd0.6Sr0.4Co0.5Fe0.5O3−δ–Ag composite cathodes in intermediate temperature solid oxide fuel cells

The electrochemical performances of Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ–Ag composite cathodes have been investigated in intermediate temperature solid oxide fuel cells. The Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ–Ag cathodes prepared by ball milling followed by firing at 920 °C show the maximum performance (power density: 0.15 W cm⁻² at 800 °C) at 3 wt.% Ag. On the other hand, the Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ–Ag composite cathodes with 0.1 mg cm⁻² (0.5 wt.%) Ag that were prepared by an impregnation of Ag into Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ followed by firing at 700 °C (but the electrolyte–Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ assembly was prepared first by firing at 1100 °C) exhibit much better performance (power density: 0.27 W cm⁻² at 800 °C) than the composite cathodes prepared by ball milling, despite a much smaller amount of Ag due to a better dispersion and an enhanced adhesion. AC impedance analysis indicates that the Ag catalysts dispersed in the porous Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ cathode reduce the ohmic and the polarization resistances due to an increased electronic conductivity and enhanced electrocatalytic activity.     

Effect of preparation conditions of CuO–CeO2–ZrO2 catalyst on CO removal from hydrogen-rich gas

The activities of CuO–CeO₂–ZrO₂ catalysts synthesized by four methods, e.g. sol–gel, co-precipitation, one-step impregnation, and two-step impregnation, were compared for CO removal from hydrogen-rich gas. The influence of the precipitant and calcination temperature on the catalytic activity was investigated, and a series of analytical methods, such as XRD, H₂-TPR, TG-DSC, and SEM, were used to characterize the catalysts. It was indicated that CuO–CeO₂–ZrO₂ catalyst prepared by co-precipitation method exhibits the widest operation temperature range with the 99% conversion of CO and relatively high selectivity. The optimized preparation conditions were confirmed using Na₂CO₃ as a precipitant, and calcining at 500 °C. It was proposed that the high activity and selectivity result from the high dispersion of copper and strong interaction among CuO, CeO₂, and ZrO₂. The effects of precipitants on the grain size and morphology of the catalyst is larger than that of calcination temperature.     

The effect of doping rare-earth chloride dopant on the dehydrogenation properties of NaAlH4 and its catalytic mechanism

A series of rare-earth chlorides has been adopted to catalyze dehydrogenation reaction of NaAlH₄. X-ray diffraction analysis and isothermal dehydrogenation measurement have proved that these chlorides enhance the dehydrogenation kinetics and lower the decomposition temperature of NaAlH₄. The catalytic effect from high to low is in following order: SmCl₃>CeCl₃>TiCl₃>NdCl₃>GdCl₃>LaCl₃>ErCl₃. In order to reveal the catalytic mechanism of the rare-earth chlorides on dehydrogenation reactions, systems doped with LaCl₃ were investigated under different milling and dehydrogenation conditions. It has been proposed that the La cation reacts with hydrogen, which was released from NaAlH₄, and then forms some sort of La–Al alloys. This process improves the performance of NaAlH₄ dehydrogenation at a relatively low temperature. In the present research, the catalytic effect of La₂O₃ has also been investigated. Results show that La₂O₃ also have a catalytic effect on the dehydrogenation of NaAlH₄, but the effect is less obvious than that of the rare-earth chlorides.     

A new approach to deal with the hysteresis phenomenon in hydride systems TiFe1−xMx−H2(M=Mn,Ni)

Hysteresis in hydrogen storing materials during the hydriding–dehydriding cycles presents an efficiency loss which is to be minimized in order to get high performance. In this paper a new approach is presented for dealing with the hysteresis phenomena. Thus, the hydride systems TiFe_0.9Mn_0.1–H₂ and TiFe_0.8Ni_0.2–H₂ were considered in a thermal hydriding–dehydriding process performed at constant volume from room temperature up to 623 K and pressure ranging from 0.6 to 6.6 MPa. The obtained data were interpreted using a new mathematical approach aiming to optimize the hydriding–dehydriding path conditions in order to get minimum energy loss during their performance. It was found that optimum conditions are highly affected mainly by the applied starting pressure and the used hydriding–dehydriding temperature.

The experimental results of the two substitutes were compared from the view point of energy loss due to hysteresis. It was found that Mn substitute is better in this respect.     

Brown coloring electrochromic devices based on NiO–TiO2 layers

Brown coloring electrochromic 5×10 cm² windows with the configuration K-glass/NiO–TiO₂/electrolyte/CeO₂–TiO₂/K-glass have been prepared and characterized by optoelectrochemical techniques (cyclic voltammetry, chronoamperometry and galvanostatic measurements). The electrochromic layers have been prepared by the sol–gel technique. As electrolyte either a 1 M aqueous KOH solution or a newly developed starch-based gel impregnated with KOH have been used. The CeO₂–TiO₂ sol–gel layers sintered at 550 °C have been previously characterized in 1 M aqueous KOH electrolyte as a function of the thickness up to 2000 cycles and showed a highly reversible behavior without any corrosion effect. The NiO–TiO₂ sol–gel layers sintered at 300 °C have been extensively characterized in the same electrolyte up to about 7000 cycles. All windows present a deep brown color characteristic of the presence of Ni³⁺ (NiOOH) species, that is fully reversible for several thousands of cycles with a rather-fast kinetics (<30 s). The transmittance of the bleached state however slowly decreases with cycling (permanent coloration). The full-bleached condition can be nevertheless recovered by applying a negative potential for a long duration. Deeper coloration is usually obtained by cycling the windows galvanostatically with a current density of 20 μA/cm². The lifetime of the windows is however limited because of the degradation of the NiO-based layers due to the not fully reversible exchange of OH⁻ that turns the layers mechanically fragile and leads eventually to their complete removal from the substrate. Windows working satisfactorily up to 7000 and 17 000 cycles have been obtained using aqueous KOH electrolyte and starch KOH gel electrolyte, respectively. Memory tests showed that the devices bleach at the open circuit potential from T=39% (colored state) to about T=50% in 60 min.     

Nitrous oxide formation and destruction in lean, premixed combustion

Concentraion profiles of N₂O, NO, and N₂ from atmospheric pressure, flat-flame burner experiments are presented. Axial profiles of species and temperature are described for CH₄-air and H₂---O₂---Ar flames doped with either NH₃, NO, or N₂O.

Species concentrations were determined by microprobe sampling and direct analysis by gas chromatography or chemiluminescence analysis, for flames ranging in temperature from 1300 to 2000 K. Burner surface temperatures were also estimated for these flames, using heat transfer analysis and an optical method. Nitrogen-atom balances were achieved in each of the H₂---O₂---Ar flames to within experimental error and better than 6%.

Axial profiles of N₂O were similar for all flames. At sampling locations nearest to the burner (0.5–1.0 mm), N₂O concentrations were highest. Concentrations decreased monotonically in the downstream direction, with N₂O destruction essentially complete in the postflame region (height greater than 3 mm) for all flames except the one at the lowest temperature (1300 K). With NH₃ as dopant, early-flame and postflame N₂O concentration varied inversely with temperature. The lowest early-flame N₂O concentration was observed with NO as dopant, and the highest was observed with direct N₂O addition. Increased initial concentrations of NH₃ led to higher early-flame N₂O concentrations. With N₂O as dopant, product branching to NO and N₂ in the postflame region is approximately 8% and 92%, respectively. An NO removal process involving NH_i is active in lean, NO-doped flames.     

Radiation damage studies on the first wall of a HYLIFE-II type fusion breeder

The radiation damage on the first wall [made of (1) a ferritic steel (9Cr–2WVTa), (2) a vanadium alloy (V–4Cr–4Ti) and (3) SiC_f/SiC composite] of an inertial fusion energy (IFE) reactor of HYLIFE-II type is investigated. A protective liquid wall with variable thickness, containing Flibe + heavy metal salt (UF₄ or ThF₄) is used for first wall protection. The content of heavy metal salt is chosen as 4 and 12 mol%. Neutron transport calculations are performed with the aid of the SCALE4.3 System by solving the Boltzmann transport equation with the XSDRNPM code in 238 energy groups and S₈–P₃ approximation.

A flowing wall with a thickness of 60 cm can extend the lifetime of the solid first wall structure to a plant lifetime of 30 years for 9Cr–2WVTa and V–4Cr–4Ti, whereas the SiC_f/SiC composite as first wall needs a flowing wall with a thickness of 85 cm to maintain the radiation damage limit. Substantial extra revenue can be gained through the insertion of a heavy metal salt constituent into Flibe, which allows breeding fissile fuel for external reactors and increasing energy multiplication (²³³U with a value of up to 1,000,000,000 $/year or ²³⁹Pu for few 100,000,000 $/year).     

Emission spectroscopy of hot water vapor from H2–air deflagrative combustion in a closed cylinder

The water molecule is often selected as a probe of its environment to retrieve distributions of temperature and concentration in a hot gas by emission spectroscopy. A combustion chamber closed at both ends has been designed and a fast CCD camera coupled to an Ebert–Fastie spectrometer was used to experimentally validate the current spectroscopic parameters of H₂O at high pressures and temperatures. Time-resolved H₂O emission spectra from the deflagrative combustion of H₂ in air were recorded for temperatures from 2500 to 2800 K and final pressures from 2 to 40 bar in the spectral region 10,332–10,496 cm⁻¹. A time-dependent numerical model was developed; it predicts reliably the observed dynamic pressures of the H₂–air flame after initiation of combustion. Reasonably uniform distributions of temperature and [H₂O] simulated by this combustion code have been introduced into a line-by-line radiative transfer code. Good agreement between calculated and experimental spectra at the end of combustion was obtained, except for a few intense lines, which can be better reproduced by improving the parameters for each spectroscopic line.     

Degradation of volatile fatty acids in highly efficient anaerobic digestion

To improve the efficiency of anaerobic digestion, we examined the effects of C₂–C₆ volatile fatty acids (VFAs) on methane fermentation, as well as the behavior of VFAs in anaerobic digestion. The VFA concentrations and methane production in anaerobic digestion were increased by pretreatment of waste activated sludge (WAS), such as ultrasonic disintegration, thermal and freezing treatments. The major intermediate products of anaerobic digestion for untreated and pretreated WAS, such as acetate, propionate, isobutyrate, butyrate, isovalerate, valerate, isocaproate and caproate, were used as substrates and the anaerobic degradation of these was carried out under the same conditions. It was found that decomposition rates of the VFAs (C₂–C₆) with a straight chain (normal form) were greater than those of their respective isomers with a branched chain (iso form). It was shown that the decomposition rates of the iso and normal forms of butyrate were greater than those of valerate and caproate. This was caused by the isomerization between butyrate and isobutyrate which occurred during the digestion process. Anaerobic bacteria in digested sludge converted butyrate to isobutyrate and vice versa by migration of the carboxyl group to the adjacent carbon atom. In addition, inhibition of degradation of the VFAs by acetate in a digester was also examined.     

The chemistry of sodium with sulfur in flames

An experimental and analytical program of sodium/sulfur chemistry has been conducted in a series of fuel rich and lean H₂/O₂/N₂ flames, with and without added sulfur, and covering a wide range of temperatures and stoichiometries. Fluorescence measurements of OH and Na profiles together with sodium line reversal temperature profiles provided a broad data base for kinetic modeling. Analysis indicated NaSO₂ to be the only significant sodium/sulfur product formed in the lean flames. NaOS is dominant in the rich flames, coupled with small contributions from NaSO₂, NaSH, NaS and NaS₂. A bond dissociation energy of D₀(Na---SO₂) = 197 ± 20 kJ mol⁻¹ is derived. Calculations indicate that the linear or triangular structures for NaOS both co-exist in approximately equal proportions in flames. Analyses based on results developed in the study show that Na₂SO₄ formation is kinetically limited and cannot be a significant gas phase flame product at sodium levels much below 100 ppm. Na₂SO₄ induced corrosion in combustion systems must result from heterogeneously formed Na₂SO₄.     

Comparisons of computed and measured premixed charge engine combustion

Comparisons are presented of computed and measured cylinder pressure in a reciprocating engine with a pancake combustion chamber and premixed propane/air charges. Engine operating conditions range over volumetric efficiency of 30–60%; equivalence ratio of 0.87–1.1; and rpm of 1000–1500. The computations start from the actual spark times and simulate the growth of the flame kernel into a fully developed turbulent flame by taking into account the increasing influence of turbulent eddies on the growing flame kernel. A k-ε submodel is used for turbulence. The species conversion submodel assumes that the species (C₃H₈, O₂, H₂O, CO₂, CO, H₂, and N₂) concentrations approach their local thermodynamic equilibrium values with a characteristic conversion time that is a combination of a turbulent mixing time and a chemical conversion time in laminar propane---air flames. In all cases computed and measured cylinder pressure agree well in trends and magnitudes during the entire duration of combustion. The difference in magnitudes generally is much less than 8%. The main conclusion is that laminar flame processes must be explicitly accounted for in order to reproduce certain elements of premixed charge engine combustion.     

Effect of salt concentration on poly (vinyl chloride)/poly (acrylonitrile) based hybrid polymer electrolytes

Hybrid, solid polymer electrolyte films consisting of poly (vinyl chloride) (PVC), poly (acrylonitrile) (PAN) and, propylene carbonate (PC) with different concentrations of LiClO₄ are prepared by means of a using solvent-casting technique. The structure and complex formation are studied by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The temperature dependence of the ionic conductivities of the polymer films is explained in terms of a free volume model. The conductivities of PVC–PAN–LiClO₄–PC complexes are determined at different salt concentrations. The highest ionic conductivity (8.35 × 10⁻⁵ S cm⁻¹) is obtained for 8 wt.% LiClO₄ in the polymer complex at 304 K. The thermal stability of the electrolyte is examined by thermogravimetric/differential thermal analysis (TG/DTA).     

Integration of acidogenic and methanogenic processes for simultaneous production of biohydrogen and methane from wastewater treatment

Feasibility of integrating acidogenic and methanogenic processes for simultaneous production of biohydrogen (H₂) and methane (CH₄) was studied in two separate biofilm reactors from wastewater treatment. Acidogenic bioreactor (acidogenic sequencing batch biofilm reactor, AcSBBR) was operated with designed synthetic wastewater [organic loading rate (OLR) 4.75 kg COD/m³-day] under acidophilic conditions (pH 6.0) using selectively enriched acidogenic mixed consortia. The resultant outlet from AcSBBR composed of fermentative soluble intermediates (with residual carbon source), was used as feed for subsequent methanogenic bioreactor (methanogenic/anaerobic sequencing batch biofilm reactor, AnSBBR, pH 7.0) to generate additional biogas (CH₄) utilizing residual organic composition employing anaerobic mixed consortia. During the stabilized phase of operation (after 60 days) AcSBBR showed H₂ production of 16.91 mmol/day in association with COD removal efficiency of 36.56% (SDR_A—1.736 kg COD/m³-day). AnSBBR showed additional COD removal efficiency of 54.44% (SDR_M—1.071 kg COD/m³-day) along with CH₄ generation. Integration of the acidogenic and methanogenic processes enhanced substrate degradation efficiency (SDR_T—4.01 kg COD/m³-day) along with generation of both H₂ and CH₄ indicating sustainability of the process.     

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.