Carbon Monoxide-carbon Dioxide Research Articles

Correlation between the critical viscosity and ash fusion temperatures of coal gasifier ashes

Coal gasification yields synthesis gas, an important intermediate in chemical manufacturing. It is also vital to the production of liquid fuels through the Fischer-Tropsch process and electricity in Integrated Gasification Combined Cycle power generation. Minerals naturally present in coal become molten in entrained-flow slagging gasifiers. Molten coal ash slag penetrates and dissolves refractory bricks, leading to costly plant shutdowns. The extent of coal ash slag penetration and refractory brick dissolution depends on the slag viscosity, the gasification temperature, and the composition of slag and bricks. We measured the viscosity of several synthetic coal ash slags with a high-temperature rotary viscometer and their ash fusion temperatures through optical image analysis. All measurements were made in a carbon monoxide-carbon dioxide reducing atmosphere that approximates coal gasification conditions. Empirical correlation models based on ash fusion temperatures were used to calculate critical viscosity temperatures based on the coal ash compositions. These values were then compared with those obtained from thermodynamic phase-transition models. An understanding of slag viscosity as a function of ash composition is important to reducing refractory wear in slagging coal gasifiers, which would help to reduce the cost and environmental impact of coal for chemical and electricity production.

Theoretical study of carbon dioxide-carbon monoxide conversion by La +, Hf + and Ta +

The entire reaction mechanism for the gas-phase CO 2–CO conversion by early transition metal ions, La +, Hf +, and Ta +, are studied using density functional theory (DFT). The results indicate that the lowest energy path corresponds to the η 2-O coordination of CO 2 followed by the insertion of M + into the C–O bond. The reactions are all exothermic due to the participation of the metal ions, to be compared with the strong endothermic process of the unimolecular CO 2 decomposition. Crossing points (CPs) are localized, and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate (IRC) approach.

Design and operation of a variable high-temperature oxygen partial-pressure probe device for solid-state NMR

For most oxide systems, the stoichiometry at elevated temperatures is sensitive to the external oxygen partial pressure. Nuclear magnetic resonance is a powerful technique for studying the dynamics of defects caused by the nonstoichiometry in these solids, since NMR provides microscopic information about the system ( 1). For example, jump rates can be determined from the temperature dependence of nuclear spin-lattice relaxation (NSR) rates (Zeeman NSR rate 1 / T, ; rotating-frame NSR rate 1 / T,,) using standard nuclear spin relaxation theories (2). Also, it may be possible to measure the chemical diffusion of these systems. If the defects responsible for nonstoichiometry cause an additional NSR rate ( 3)) the change of the NSR rate at a constant temperature after a stepwise PO2 change would indicate chemical diffusion. To perform NMR diffusion studies on oxide systems the following capabilities are necessary: the operating temperature must be as high as possible, while the probe’s outer temperature must be maintained close to room temperature. This allows the probe to be used in a standard wide-bore superconducting NMR magnet. Further, the sample area must be isolated from the heater wires to allow the use of corrosive gas mixtures, which provide a wide range of oxygen partial pressures. Reduced oxygen partial pressures down to 1O-6 atm are attainable with argon-oxygen gas mixtures, and oxygen partial pressures between lo-* and lo-‘* atm are attained with carbon monoxide-carbon dioxide gas mixtures. In the latter case, the PO2 is determined by the equilibrium of the chemical reaction 102 + CO P CO2 (2). Here, we report the technical details of an NMR probe system which has the capabilities described above and has been successfully used for performing NMR measurements at temperatures between 150 and 1500 IS in r-utile ( TiOzmX) at different oxygen partial pressures (3). (The actual temperature range is limited by the physical properties of r-utile.) Many of the components used in this system are commercially available; only the oxygen partial-pressure measuring instruments ( ZrOz cells) and the NMR probe itself are homebuilt.

Tungsten trioxide reduction-carburization with carbon monoxide-carbon dioxide mixtures: Kinetics and thermodynamics

The production of tungsten carbide (WC) starting from tungsten trioxide (WO 3) by means of COCO 2 mixtures (61, 78 and 100% v/v CO) in the temperature interval from 700° to 1100°C was studied with the use of scanning electron microscopy, thermogravimetry and X-ray diffraction techniques. It was observed that between 700° and 800°C the process was controlled by the nucleation and growth of the lower oxide W 20O 58, whereas between 800° and 1100°C the process was controlled by the elementary reaction WO 2→W which presented the Arrhenius equation k=11.36 exp(−18000/ RT). A second order dependence of the initial rate constant with respect to the partial CO pressure was estimated. The thermochemical data used for the W-C-O system were consistent with the experimental results. It was concluded that the most appropriate conditions for the reduction-carburization of tungsten trioxide by means of carbon monoxide-carbon dioxide mixtures are: temperature = 900°C; gaseous mixture compostion = 100% v/v CO; volumetric flow rate = 1450 ml (STP)/min. and time = 6 h. With these conditions, a tungsten carbide powder with an average particle diameter of 0.40 μm was obtained.

Influence of potassium on the nucleation and early growth of magnetite in the reduction of hematite with carbon monoxide-carbon dioxide

The enhancement of the reduction rate of hematite to magnetite in the presence of potassium oxide is well known. On the other hand, the nucleation of magnetite and its first growth steps have recently been investigated. It was therefore of interest to study, on the same scale, the influence that potassium may have on these stages and to understand the reasons. A series of samples were therefore prepared with reaction times ranging from 0.5 to 45 min during the reduction of hematite single crystals (prepared by chemical vapour transport) at 600 ° C with CO-CO 2 (2:98). Also the morphology and growth rate of samples with and without potassium were compared with a scanning electron microscope. The observation of a strong influence of potassium on the number and growth rate of the nuclei, which are often anisotropic, is reported. The interpretation is based on the transient formation of KFe 11O 17 which, owing to its magnetoplumbite structure, can easily form an epitaxic layer on Fe 2O 3; a detailed mechanism of reduction and nuclei formation is suggested. The following stages in the reduction of KFe 11O 17 are also discussed with the help of a proposed iron-potassium-oxygen diagram.

Oxidation behaviour of vanadium nitride in carbon dioxide-carbon monoxide mixtures

The oxidation of vanadium nitride by CO 2-CO mixtures, at a total pressure of 1 atm, for ratios 0.54 ⩽ P co 2 ,/ P co ⩽ 4 leads to the formation of a porous V 2O 3 scale between 1050 and 1200°C. The kinetics are continuously decelerated. Their shape, intermediate between linear and parabolic, is well represented by a model based on the hypothesis of two parallel processes: a reaction regime slowed by the progressive closure of the open porosity of the scale and a diffusion process of the reactants through lattice defects or, more probably, low-resistance paths in V 2O 3. The effect of the nature of the substrate and atmosphere is discussed.

Additional experiments on and interpretation of the role of potassium in the oxygen exchange between wustite and a carbon monoxide-carbon dioxide atmosphere

We have previously shown that, in the redox interaction between wustite and a CO-CO 2 atmosphere, K + accelerates the oxygen exchange in both directions. In view of the numerous interpretations suggested by previous workers as regards the role of potassium in the reduction of iron oxides and ores, the action of K +, Ba 2+ and Li + has been compared in order to establish the role of size and charge of the additive ions. Dealing with small wustite single crystals at 890 °C it was established that Ba 2+ is nearly as active as K +, whereas Li + is inactive (but this may also be due to its volatility). Two possible mechanisms are proposed to explain the potassium activity: it operates as a catalyst through the formation of a kind of activated complex, and/or it increases the oxygen ion mobility.

High‐Temperature Thermoelectric Power Measurements in Wustite

High‐temperature thermoelectric power was measured on single‐crystal samples of wustite in equilibrium with a carbon dioxide‐carbon monoxide atmosphere. These measurements were made using a “heat pulse” technique which allowed the thermoelectric power of a sample to be determined in seconds. The short time required for this measurement precluded any possibility of an ionic contribution to the measured thermoelectric power. The results of this study are similar to those of earlier workers in that the measured thermoelectric power changed sign from positive to negative with increasing defect concentration. A defect model for wustite is proposed to explain these results. This model assumes a defect structure dominated by clusters of four vacancies coordinated around a trivalent iron cation in a tetrahedral position. Due to the high negative charge of such a cluster, electron holes are trapped in octahedral sites adjacent the cluster vacancies. Conduction occurs through the thermally activated hopping of these trapped holes between cluster near‐neighbor sites. The high defect concentrations in wustite result in these near‐neighbor sites being shared between different clusters. It is therefore possible for a given electron hole to hop through the crystal on a continuous path of near‐neighbor sites. A modified Heikes‐type equation is used to show that such a model is consistent with the measured values of the thermoelectric power. The proposed model is qualitatively consistent with other studies using X‐ray and neutron diffraction, electrical conductivity, and diffusion.

Electrical properties of growing alumina scales

Scales growing on platinum 22 wt.% aluminum specimens at 1100°C were investigated by the following electrical techniques: open-circuit potential and short-circuit current as a function of time and oxygen pressure; current-voltage relationship at several oxygen pressures; transients in the short-circuit current on rapidly changing the oxygen pressure; changes in the growth rate with applied potential. The growth characteristics of the scale were also investigated using kinetic and metallographic techniques. The apparent parabolic rate constant of oxidation decreases in the first hours of oxidation and stabilizes after 20–30 hr at 2 × 10−13 g2·cm−4·sec−1. Transport in the scale is by lattice and grain-boundary diffusion and the changing kinetics are a consequence of changes in the oxide grain size. It was also determined that the electrical characteristics of the scale were affected by the hydrogen-water or carbon monoxide-carbon dioxide atmospheres used to obtain low oxygen pressures. Due to these complexities and to the difficulty in making a satisfactory oxide-gas electrode only qualitative observations could be made from the electrical measurements. These measurements are compatible with the scale having constant ionic conductivity in the pressure range 1 to 10−15 aim and predominantly electronic conductivity at lower oxygen pressures. The rate of oxidation can be accelerated or retarded by passing a current through the scale.

The effects of carbon monoxide-carbon dioxide mixtures on operant behavior in the rat

Ten rats conditioned to a positive reinforced FR-15 bar press task were exposed to 1000 ppm carbon monoxide (CO); 2.5%, 5.0% and 7.5% carbon dioxide (CO 2); and CO-CO 2 mixtures. CO reduced the bar press responses from a mean of 90.44/min to 3.89/min. Carbon dioxide in concentrations of 2.5%, 5.0% and 7.5% in the presence of 1000 ppm CO increased the rats' performance to means of 16.74/min, 41.94/min and 44.50/min, respectively. Concentrations of CO 2 as high as 7.5% alone had little effect on the operant behavior of these rats. These results indicate that CO 2 can partially antagonize the central nervous system depressant effects of CO.

The oxidation of iron at controlled oxygen partial pressures-—II. Carbon monoxide-carbon dioxide

Pure iron has been oxidized, at atmospheric pressure, in a range of CO CO 2 mixtures at 500°C, and in 1·5%CO CO 2 over the temperature range 350–600°C. Weight gains, measured in situ on an electronic microbalance, fitted a parabolic law for the duration of the experiments (∼ 150 h) under all conditions, except when the proportion of CO exceeded 30%. The parabolic rate constant can be expressed as k = 1 ⋅ 47 × 10 5 ( P c o P c o 2 ) − 0.29 exp ⁡ ( − 30 , 000 R T ) g 2 m − 4 s − 1 . At higher levels of CO (⩾ 40%) significant carbon deposition occurs. It is shown that in 40%CO this causes a change in the oxidation kinetics from parabolic to linear. It is suggested that the breakaway corrosion of mild steel in CO 2 at high pressure can be accounted for by the concentration of CO within a porous oxide.

Internal oxidation and decarburization properties of an Fe-1 w/o Mn and Fe-1 w/o Mn-1 w/o C alloys in carbon dioxide-carbon monoxide atmospheres at 1OOO°C

Internal oxidation and decarburization properties of an Fe-1 w/o Mn and Fe-1 w/o Mn-1 w/o C alloys in carbon dioxide-carbon monoxide atmospheres at 1OOO°C

?-Radiation induced isotope exchange in the carbon monoxide-carbon dioxide system. Studies in silica vessels

The behaviour of the isotopes 13C, 14C and 18O in the CO—CO2 system in silica vessels has been studied, and the usefulness of the system as a radiation dosimeter investigated. Each insotope has been employed under identical conditions. For the exchange of an oxygen atom between CO and CO2 the established G-value of 4.2 has been confirmed. Apparent anomalies between results involving carbon- and oxygen-isotope labelling have been explained by a schematic initial reaction scheme involving participation of the silica: [graphic omitted] (1), (18O)+ C16O→C16O18O (2), [graphic omitted] (3a), (C18O +16O)ads.Si16O18O→Si16O18O + C18O +(16O)(3b), (16O)+ C16O→C16O16O (4), (16O)+ C18O→C16O18O (5).

The development of the wustite-fayalite scale on an iron-1.5 wt.% silicon alloy at 1000�C

An investigation has been carried out on the morphological development and structure of the wustite-fayalite scale formed on a ferritic Fe-1.5 wt.% Si alloy exposed to carbon dioxide-carbon monoxide atmospheres at 1000°C. The amorphous silica film formed on the metallographically polished specimens crystallized to β-cristobalite at the reaction temperature. Wustite and fayalite developed within nodules which grew laterally to cover the alloy surface. Concurrent with growth of the nodules externally, oxygen diffusion into the underlying alloy led to precipitation of silica as α-tridymite. This internal oxidation zone was sufficiently depleted in silicon for its transformation to an austenitic phase. A fully developed scale was composed of an external wustite layer and an inner wustite-fayalite conglomerate layer, interspersed with discontinuous fayalite bands.

Kinetics of wustite-fayalite scale formation on iron-silicon alloys

A superficial duplex scale consisting of an inner wustite-fayalite conglomerate and an outer layer of wustite was formed on an iron-1.5wt.% silicon alloy upon its exposure in carbon dioxide-carbon monoxide atmospheres at 890and 1000°C. Variations in the oxidation curves before onset of linear reaction kinetics could be correlated with appearance of fayalite bands in the scales. The linear rate constants for the overall reaction kinetics showed a proportional dependence on the pressure of carbon dioxide due to reaction control by surface reaction steps for dissociation of carbon dioxide and incorporation of chemisorbed oxygen into the wustite lattice. Growth of the wustite-fayalite conglomerate was not dependent upon the oxidizing potential of an atmosphere. Wustite growth involved iron migration while the growth of the inner wustite-fayalite conglomerate layer involved inward migration of oxygen resulting from a dissociative reaction of wustite at its inner surface.

Oxidation of Graphite by Carbon Dioxide-Carbon Monoxide Gas Mixtures

Oxidation of Graphite by Carbon Dioxide-Carbon Monoxide Gas Mixtures

The kinetics of wustite scale formation on iron-nickel alloys

Iron and iron-nickel alloys containing up to 40 wt. % nickel were oxidized in carbon dioxide-carbon monoxide atmospheres at 1000°C. Wustite was the only oxide formed in the superficial scales and subscales which exhibited preferential formation at alloy grain boundaries. The linear rate constants for the overall reaction kinetics showed a proportional dependence on the partial pressure of carbon dioxide. Equations based upon detailed balance of the surface reaction steps for dissociation of carbon dioxide and incorporation of chemisorbed oxygen into the wustite lattice accounted for the dependence of the reaction kinetics on the atmospheric and alloy compositions.

The Oxidation and Carburisation of a 20/25/Nb Steel in Carbon Dioxide, in Carbon Monoxide and in Carbon Dioxide-Carbon Monoxide Mixtures

The Oxidation and Carburisation of a 20/25/Nb Steel in Carbon Dioxide, in Carbon Monoxide and in Carbon Dioxide-Carbon Monoxide Mixtures

The Oxidation and Carburisation of a 20/25/Nb Steel in Carbon Dioxide, in Carbon Monoxide and in Carbon Dioxide-Carbon Monoxide Mixtures

AbstractThe oxidation and carburisation of a 20% Cr/25% Ni/Nb austenitic steel has been investigated between 600° and 850° in carbon dioxide, carbon monoxide and their mixtures at pressures within the range 0·04–760 cm.Under all conditions the oxide formed markedly inhibits the attack of the steel. In carbon dioxide, the major reaction is M + CO2 = MO + CO with the reaction 2M + CO2 = 2MO + C contributing less than 7% to the total weight gain. All the carbon deposition appears to take place within the first few hours of the oxidation. Detailed studies show that the deposited carbon diffuses into the steel.In carbon monoxide, the reaction is essentially M + CO = MO + C, the carbon passing through the oxide film into the steel. Carbon is also deposited in varying amounts on the surface of the oxide by the reaction 2CO = C + CO2. This carbon is present as a filamentary growth and diffuses only slowly into the steel.In carbon dioxide-carbon monoxide mixtures containing up to 75 vol.-% carbon monoxide, the ste...

The high temperature oxidation of beryllium: Part V In moist carbon dioxide and moist carbon monoxide

The chemistry and the kinetics of the oxidation of electrolytic flake beryllium in moist carbon dioxide and in moist carbon monoxide (either gas, total pressure 10 cm of mercury; partial pressure of water, 1.2 cm) have been investigated at temperatures in the ranges 500° to 750°C and 600° to 700°C respectively. In moist carbon dioxide, the reaction with water vapour predominates over that with carbon dioxide in the proportions of approximately 4:1; the reactions are: Be + H 2 O = BeO + H 2; Be + CO 2 = BeO + CO and 2 Be + CO 2 = 2 BeO + Be 2 C. The oxidation is protective up to 600° C and non-protective at 650° C and above; the reaction after breakaway takes place at the particle boundaries giving rise to an interpenetrating region of oxide and unattacked metal. In moist carbon monoxide, the principal reaction is with water vapour and the reaction Be + CO = BeO + C only contributes about 3 % to the total weight gain. An investigation of the oxidation in a moist carbon monoxide-carbon dioxide mixture (partial pressures: water, 1.2 cm; carbon monoxide, 0.5 cm; carbon dioxide, 8.3 cm) at 700° C shows that the kinetics are the same as in moist carbon dioxide, the reaction with carbon monoxide only contributing 0.2 % to the total weight gain.