Nitride formation during combustion of Ti-TiO2 and Ti-Al powder mixtures in air under SHS conditions

Nitride formation during SHS combustion of a micron-size titanium powder and its mixtures with additives in air was studied. It was shown that the yield of TiN Ti powder was higher for SHS combustion in air than for SHS combustion of powders of the same degree of dispersion in nitrogen. The mechanism of formation of TiN is probably determined by the reaction of the intermediate product TiO with atmospheric nitrogen. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 131–135, September–October, 2008.     

Dynamics of phase formation during combustion of Zr and Hf in air

The processes of phase formation taking place during combustion of Zr and Hf in air have been explored by time-resolved XRD (TRXRD) at a time resolution of 0.1 s. In case of Zr combustion, the final product was found to form in two stages: first the tetragonal high-temperature modification of ZrO₂ is formed (in the combustion wave) and then the latter undergoes polymorphic transition (behind the combustion wave) into its monoclinic modification. In combustion of Hf, product formation proceeds in three stages: the low-temperature monoclinic modification of α-HfO₂ is formed in the combustion wave, which is followed by its transformation into the tetragonal β-HfO₂ and backward conversion of the latter into α-HfO₂ behind the combustion wave. The obtained data were comparatively analyzed with those reported for combustion of Ti in air.      

Simulation of turbulent combustion and NO formation in a swirl combustor

A presumed probability density function (PDF) model for temperature fluctuation is proposed and formulated in this paper. It incorporates a two-step reaction mechanism for propane combustion and the thermal and prompt NO formation mechanisms. The present model, together with a new algebraic Reynolds stress model (ASM), is employed to simulate the turbulent combustion and NO formation in a swirl combustor. The calculated propane, carbon monoxide, and carbon dioxide concentrations agree with the measurement. The calculated gas temperature and oxygen and NO concentrations are in general agreement with the measured data. The simulated results show that NO forms mainly in the upstream region of the combustor. The flue gas recirculation effectively abates the nitrogen oxides (NO_x) emission in the combustor.     

Molten salt dynamic sealing synthesis of MAX phases (Ti3AlC2, Ti3SiC2 et al.) powder in air

Since the synthesis of non-oxidized ceramic and alloy powders requires both high temperature and oxygen insulation conditions, here we demonstrate a cost-efficient molten salt sealing/shielded synthesis method with dynamic gas tightness. Compared to conventional synthesis method, it can prevent the loss of reaction materials at high temperature, cut off the connection between reacting material and outside air, and does not require long-time ball milling mixing treatment or provision of applied pressing before or during heating. Only low-cost salts (e.g., NaCl, KCl), a few minutes of raw material mixing, and regular heating molds are required to obtain high-purity (>96 wt%), micron-sized Ti₃AlC₂ and Ti₃SiC₂ powders with narrow size distribution, which significantly decreased the complexity and production costs in the synthesis process. The effect of temperature and raw material content on the products were investigated. The mechanism of diffusion reaction between reactants in molten salt environment was analyzed. The new method developed here was also applicable to Ti₂AlC, V₂AlC and Cr₂AlC MAX phases, as well as provided new ideas for the preparation of other MXenes precursors with certain stoichiometric ratios, air-sensitive materials and nanopowders.     

Phase,microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)₃, carbon black and Y₂O₃ as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y₂O₃ was discussed. The results showed that Y₂O₃ reacted with Al₂O₃ to form yttrium aluminates of YAlO₃ (orthorhombic and hexagonal phases), Y₄Al₂O₉ and Y₃Al₅O₁₂ at the low temperature of 1350 °C. YAlO₃ could firstly be transformed into Y₂O₃ and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y₂O₃ again after the carbon removal at 700 °C in the air atmosphere. There were two ways generating AlN when adding Y₂O₃ and the possible mechanism was proposed. Y₂O₃ from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800 °C.     

Effects of Si source and high gravity on combustion synthesis of Ti3SiC2 in air

Abstract

Combustion synthesis of Ti₃SiC₂ was carried out in air with Si₃N₄, SiC, and Si as Si sources respectively, and the effect of Si source on the phase composition of the products was investigated. With Si₃N₄ as Si source, the major product was TiC_xN_1?x and no Ti₃SiC₂ was synthesised. When SiC and Si were used, Ti₃SiC₂ was synthesised. Such effect of Si source is thought to be connected with the formation mechanism of Ti₃SiC₂, where the presence of a Ti–Si melt is required. The combustion synthesis was also performed under high gravity condition instead of common gravity. The apparent density of the product prepared under high gravity was ～60% higher than that obtained under normal gravity. It is proposed that, the high gravity can facilitate the permeation of Ti–Si melt and enlarge the interface between the melt and carbide phases, which is helpful for the formation of Ti₃SiC₂.     

3D Nitride Frameworks with Variable Channel Sizes; Synthesis and Powder Neutron Diffraction Study of the Nitride Carbodiimdes, Ca4N2(CN2) and Ca11N6(CN2)2

The synthesis of powders of the calcium nitride carbodiimides, Ca₄N₂(CN₂) (1) and Ca₁₁N₆(CN₂)₂ (2) has been studied and the products characterised initially by powder X-ray diffraction. The latter phase (2) is unobtainable in pure bulk form and is always accompanied by the former compound. Nevertheless, definitive structures of the compounds were obtained from Rietveld refinements against time-of-flight powder neutron diffraction data. Each of the nitride carbodiimides are composed of Ca–N three dimensional networks built from edge- and vertex-sharing NCa₆ octahedra arranged to create one-dimensional channels. The size of the channels is dependent on the Ca:C (host:guest) ratio and the larger channels in (1) are occupied by twice as many guest species as those in (2). Importantly, neutron diffraction data confirm that the identity of these guest species is carbodiimide (as opposed to isoelectronic azide), that neither the framework nor the channels therefore contain excess electrons and that the compounds are hence not “nitride electrides” as per the parent subnitride, Ca₂N.     

Modelling and numerical investigation of the residual stress state in a green metal powder body

Die pressing of metal powder results in a green body. After release from the die, the green body must have enough strength to be handled, to endure transport to a sintering furnace and heating to the sintering temperature. Drilling, turning and milling, which are common operations in the green state, require a green body of high strength, with no defects and excellent mechanical properties. A plane strain finite element model is used to analyse pressing of metal powder into a rectangular bar. The powder behavior is described by a “cap” model, which is implemented as a user material subroutine in the non-linear finite element program LS-DYNA. To improve modelling of strength in the green state a new non-linear density dependent failure envelope has been used. The model is adjusted to the properties of a water atomised metal powder from Höganäs AB. To resolve the severe stress gradient at the side surface of the green body, the smallest element size was chosen to be 65 μm. The aim of this work is to numerically capture and understand the development of the residual axial stress in particular at the side surface. The influence of kinematics, friction, compacting pressure and die taper are studied. Results from the numerical study show that the thickness of the compressive stress region close to the side surface of the green body varies between 50 μm and 600 μm along the surface. Compacting pressure, “upper punch hold down” and die taper geometry all have a significant influence on the residual stress state while die wall friction has only a small influence. The numerical results are in agreement with results from X-ray and neutron diffraction measurements.     

ICP-AES法同时测定陶瓷样品中镁钙铁铝钛锆的研究

采用高频熔样,电感耦合等离子体原子发射光谱(ICP—AES)法实现了对陶瓷中的Mg、Ca、Fe、Al、Ti和Zr的同时测定。对影响其光谱测量的各种因素进行了较为详细的研究,确定了实验的最佳测定条件。结果表明,该方法的检出限为0．008-0．255μg/mL,回收率为95．14％-107．72％,RSD小于3．40％。该法准确、快速、简便,应用于陶瓷的测定,结果满意。     

ICP-AES法测定陶瓷样品

采用高频熔样、电感耦合等离子体原子发射光谱(ICP—AES)法实现了对陶瓷中的Mg、Ca、Fe、Al、Ti和Zr的同时测定。对影响其光谱测量的各种因素进行了较为详细的研究,确定了实验的最佳测定条件。结果表明,方法的检出限为0．008—0．255μg／mL,回收率为95．14％--107．72％,RSD小于3．40％。该法准确、快速、简便,应用于陶瓷的测定,结果满意。     

Regularities of the spread of the in situ combustion source with portioned delivery of water and air into the formation

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 24, No. 4, pp. 64–69, July–August, 1988.     

Numerical simulation of the influence of over fire air position on the combustion in a single furnace boiler with dual circle firing

The Computational fluid dynamics (CFD) code PHOENICS is applied to simulate and evaluate the combustion process within the furnace of a 1,000 MW dual circle tangential firing single furnace lignite-fired ultra supercritical (USC) boiler. The dependence on overfire air (OFA) positioning on the combustion process is studied. The results show that the highest temperature appears on the upside of the burner zone close to the front wall, and the high temperature zone rises with elevated OFA positions. However, the temperature field distributions are similar despite differing OFA positions. The char content near the rear wall is higher than that near the front wall, and below the furnace arch, coal particles concentrate towards the front wall. Also with elevated OFA positions, nitrogen oxide (NO _x ) concentrations at the outlet fall, but char content increases. In regard to NO _x emission and char burnout, the suggested optimal distance from the OFA center to the center of the uppermost primary air nozzle should be 6 meters. This work was presented at the 7 ^th China -Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Comprehensive kinetic characterization of the oxidation and gasification of model and real diesel soot by nitrogen oxides and oxygen under engine exhaust conditions: Measurement, Langmuir-Hinshelwood, and Arrhenius parameters

The reaction kinetics of the oxidation and gasification of four types of model and real diesel soot (light and heavy duty vehicle engine soot, graphite spark discharge soot, hexabenzocoronene) by nitrogen oxides and oxygen have been characterized for a wide range of conditions relevant for modern diesel engine exhaust and continuously regenerating particle trapping or filter systems (0-20% O₂, 0-800 ppm NO₂, 0-250 ppm NO, 0-8% H₂O, 303-773 K, space velocities 1.3 × 10⁴-5 × 10⁵ h⁻¹). Soot oxidation and NO₂ adsorption experiments have been performed in a model catalytic system with temperature controlled flat bed reactors, novel aerosol particle deposition structures, and sensitive multicomponent gas analysis by FTIR spectroscopy. The experimental results have been analyzed and parameterized by means of a simple carbon mass-based pseudo-first-order rate equation, a shrinking core model, oxidant-specific rate coefficients, Langmuir-Hinshelwood formalisms (maximum rate coefficients and effective adsorption equilibrium constants), and Arrhenius equations (effective activation energies and pre-exponential factors), which allow to describe the rate of reaction as a function of carbon mass conversion, oxidant concentrations, and temperature. At temperatures up to 723 K the reaction was driven primarily by NO₂ and enhanced by O₂ and H₂O. Within the technically relevant concentration range the reaction rates were nearly independent of O₂ and H₂O variations, while the NO₂ concentration dependence followed a Langmuir-Hinshelwood mechanism (saturation above ∼200 ppm). Reaction stoichiometry (NO₂ consumption, CO and CO₂ formation) and rate coefficients indicate that the reactions of NO₂ and O₂ with soot proceed in parallel and are additive without significant non-linear interferences. The reactivity of the investigated diesel soot and model substances was positively correlated with their oxygen mass fraction and negatively correlated with their carbon mass fraction.     

Simulation of the effects of nitrogen supply on yield formation processes in winter wheat with the model TRITSIM

An outline of the dynamic winter wheat model TRITSIM is given. The model describes in one-day steps growth, yield formation and development of a crop from post-winter tillering until harvest under various conditions of water and nitrogen supply. TRITSIM is coupled with a simple soil nitrogen model and a soil water model to describe effects of nitrogen and water on yield formation processes. Comparisons between model and experimental results for ontogenesis, grain biomass, nitrogen uptake and soil mineral nitrogen are given for a series of Dutch experiments. Simulations were satisfactory, except for the time course of soil mineral nitrogen.     

Characterizations of platinum catalysts supported on Ce, Zr, Pr-oxides and formation of carbonate species in catalytic wet air oxidation of acetic acid

Catalytic wet air oxidation (CWAO) of aqueous solution of acetic acid (78 mmol L⁻¹) was carried out with pure oxygen (2 MPa) at 200 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/oxide, oxide = CeO₂, Zr_0.1Ce_0.9O₂, Zr_0.1(Ce_0.75Pr_0.25)_0.9O₂ and ZrO₂). Platinum was loaded on oxides by impregnation (5 wt%), and then the catalysts were reduced under H₂. Homogenous dispersions of 2–3 nm metal crystallites were obtained. The catalytic activity depended on the ability of the support to resist to the formation of carbonates. Ce(CO₃)OH species, determined by FT-IR and XRD, were rapidly formed during the CWAO reaction especially on mixed oxides. These carbonates were responsible to a drastic drop in catalytic performances. Amounts of carbonate species increase with the ability of the catalyst to transfer oxygen.     

Modelling of CO2, CO, SO2, O2 and NOx emissions from the oxy-fuel combustion in a circulating fluidized bed

The paper presents a model of coal combustion in air and oxygen-enriched CFB environment. A computer program to calculate the CO₂, CO, SO₂, NO_x and O₂ emissions from the combustion of solid fuels in a circulating fluidized bed boiler was created. The validity of this program was verified by measurements on a 0.1MW_th OxyFuel-CFB Test Rig.The calculations have been carried out for air and so-called oxy-fuel conditions, i.e. when combustion runs in a gas mixture based on O₂ and N₂, with various fractions of oxygen.The comparison between measured and predicted by model CO, SO₂, NO_x and O₂ emissions is shown in this paper. The results of the calculation showed, that the kinetic equations of some reaction have to be modified. Authors propose to use the reaction surface area instead of the specific internal surface area of char in rate constant formulas as the combustion nature changes from internal-kinetic to external-diffusion controlling regime.     

Influence of the solid state properties of Pd/MOx (M = Ti, Al) catalysts in catalytic combustion of methane

The performance of Pd supported on alumina or titania, prepared by impregnation or sol–gel, for catalytic combustion of methane is studied. The addition of the Pd precursor after gelification of alumina has a beneficial effect on the catalytic properties of the catalysts. The better activity of the alumina catalysts is related to the high dispersion of the palladium particles, the low crystallite size and the high specific surface area. The catalytic activity of titania-based catalysts was only slightly affected by the preparation procedure. The addition of CO₂ during the oxidation of methane promotes oxidation for the titania-based catalysts.     

Role of ceria-supported noble metal catalysts (Ru, Pd, Pt) in wet air oxidation of nitrogen and oxygen containing compounds

Catalytic wet air oxidation (CWAO) of aniline, phenol, carboxylic acids and ammonia was carried out in a batch reactor over noble metals (Ru, Pd, Pt) supported on ceria. Ruthenium is very active for the conversion of a wide range of organic compounds and selective into carbon dioxide. The ability of ceria to transfer oxygen is essential for good performances in CWAO. However, Ru/CeO₂ is not selective for ammonia oxidation into N₂. Addition of small amount of Pd enhances both activity and selectivity of Ru in this reaction. Finally, oxidation of nitrogenous organic compounds requires moderate temperature and oxygen pressure and needs to adjust the oxidizing capacity of the catalyst.     

Effect of alkaline earth promoters (MgO,CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al2O3 in dry reforming of methane

Ni/Al₂O₃ promoted catalysts with alkaline earth metal oxides (MgO, CaO, and BaO) were prepared and employed in dry reforming of methane (DRM). The catalysts were prepared by impregnation method and characterized by XRD, BET, TPR, TPO, and SEM techniques. The obtained results showed that the addition of MgO, CaO, and BaO as promoter decreased the surface area of catalysts (S_BET). The catalysis results exhibited that adding alkaline earth promoters (MgO, CaO, and BaO) enhanced the catalytic activity and the highest activity was observed for the MgO promoted catalyst. TPR analysis showed that addition of MgO increased the reducibility of nickel catalyst and decreased the reduction temperature of NiO species. The TPO analysis revealed that addition of promoters decreased the amount of deposited coke; and among the studied promoters, MgO has the most promotional effect for suppressing the carbon formation. SEM analysis confirmed the formation of whisker type carbon over the spent catalysts.