Carbon deposition from irradiated methane

Carbon-14 tracer techniques have been used to measure directly the carbon deposition resulting from the radiolysis of methane by itself and in the presence of carbon dioxide, oxygen and graphite. It is shown that carbon deposition is increased in the presence of carbon dioxide and decreased in the presence of oxygen. at high carbon dioxide pressures (40 cm Hg) and high doses (>100 Mrad), the majority of the methane is decomposed to deposit. Irradiation of methane by itself produces less deposit than in the presence of graphite and there is an enhancement of the surface deposit compared with that within the bulk of the graphite. The results of the analyses for light hydrocarbons produced during methane radiolysis over a range of conditions are also briefly discussed.     

Optical emission spectrum in combustion with formation of condensed reaction products

Broadband far ultraviolet (UV) emission (up to 200 nm) was recorded during combustion of heterogeneous systems with the formation of condensed reaction products. It was shown that UV emission occurred during combustion in various gases (He, Ar, and N₂) and had the highest intensity in helium at a pressure of 25 kPa. This emission is attributed to the chemiionization of gas, separation of charges in combustion products, and subsequent microbreakdowns.     

Submicron ash formation from coal combustion

In recent years, fine particles have been found to be the cause of various harmful effects on health, and many countries have imposed restrictions on emission of these particles. Fine ash particles are formed during coal combustion in power stations and, if not collected in the air pollution control devices, are emitted into the atmosphere. The fine ash particles can remain airborne for long periods and can result in deleterious health effects when inhaled and deposited in the lungs.Previous studies have shown that combustion of coals of different rank can result in differences in the amount and chemistry of the submicron ash particles. This study examines the variability occurring between the submicron ashes formed from coals of similar rank. Five Australian bituminous coals were burned in a laminar flow drop tube furnace in two different oxygen environments to determine the amount and composition of submicron ash formed. The experimental setup is described and the repeatability of the experiments is discussed. The variability in the submicron ash yield as a percentage of the total ash collected and the submicron ash composition are presented and discussed. This paper presents experimental results rather than a detailed discussion on its interpretation. However, the results indicate that the condensation of evaporated species is responsible for the formation of ash particles smaller than 0.3 μm.     

Pulmonary effects of combustion products from nickel-coated polycarbonate

The pulmonary effects of acute (30 min) and chronic (7, 14, or 21 days; 30 min day⁻¹) exposure to smoke from white polycarbonate structural foam, or a Ni-urethane coated version of this plastic, were analyzed by exposing groups of 4 mice in a dome chamber apparatus. The lungs of test fatalities were consistently inflamed and hemorrhagic, and characterized histologically by areas of atelectasis and hyperinflation. The Ni-coated material was more toxic than the uncoated material, and produced greater intrapulmonic hemorrhage. A histological method was developed for assessing the proliferation of type II pneumocytes as an index of damage to the alveolar epithelium. Examination of lungs from animals sacrificed at 2, 4, 8, 14, 21, or 28 days following acute exposure revealed that only the 8-day animals in the Ni-exposure group had significantly more type II pneumocytes than controls (P<0.01). Similar examination of lungs from chronically exposed animals sacrificed at 1 day following the last exposure revealed no significant differences between experimental animals and controls. The Ni content of coated samples and the ash following thermal decomposition was determined by atomic absorption spectrophotometry. The original samples were 2.5% Ni. Nickel lost during pyrolysis could account for the increased toxicity of the coated material through production of toxic Ni compounds. These results indicate that pulmonary irritants produced by these plastics affect the vascular elements of the lungs more than the alveolar epithelium, and that the pulmonary damage produced in mice under these conditions does not persist in survivors beyond 1 day post-exposure. The acute combustion toxicity of Ni-coated plastics may reflect the formation of toxic Ni compounds. © 1997 John Wiley & Sons, Ltd.     

Catalytic combustion of methane over perovskites

Perovskite-type oxides of the series La_{1− x}A_xMnO₃ (A Sr, Eu and Ce) were prepared by the amorphous citrate process, leading to high surface area catalysts (up to 45 m² g⁻¹). They were tested in a flow reactor for the total combustion of methane. Complete conversion was obtained over all of the catalysts between 500 and 600°C and catalyst performance did not change significantly after 100 h on-stream. Specific activity was found to decrease monotonically with increasing the temperature of the O₂ TPD desorption peak maximum. The rate of methane combustion was low below 500°C, then grew very fast, showing that two kinds of oxygen are active in these catalysts: an adsorbed oxygen species, that reacts at low temperature, and a lattice oxygen species, that becomes available at high temperature, boosting the catalytic activity.     

Catalytic combustion of methane over barium hexaferrites

Barium hexaferrite BaFe₁₂O₁₉ and iridium-containing barium hexaferrites have been prepared by the citrates gel method. Their catalytic activity in methane combustion has been evaluated. BaFe₁₂O₁₉ is an efficient catalyst for this reaction, and the introduction of iridium in the hexaferrite structure does not improve this activity. Mössbauer spectroscopy suggests that a part of the iridium ions are incorporated in the hexaferrite structure, however in crystallographic sites where they cannot interact with the gas phase. Infrared study of CO adsorption reveals the presence of two types of iridium particles in the surface: small Ir particles, in strong interaction with the hexaferrite structure, and some larger Ir particles which were not incorporated into the lattice.     

Catalytic combustion of methane on novel catalysts derived from Cu-Mg/Al-hydrotalcites

Novel Cu-Mg/Al mixed oxides (designated as i-CMAO-800) were prepared by calcinations of Cu-Mg/Al hydrotalcites [(Cu²⁺ +Mg²⁺)/Al³⁺= 3] at 800 °C. Their performance for the catalytic combustion of methane was investigated. The oxides and their precursors were characterized by XRD, TG-DSC, TPR and N₂ adsorption/desorption techniques. The results showed that BET surface areas and the stability of the resultant oxides were greatly influenced by the copper contents in hydrotalcite precursors, bringing about difference in their activities for methane catalytic combustion. XRD results indicated that Cu was highly dispersed in hydrotalcite precursors in case of low copper contents, (Cu 40 wt%). For higher Cu contents, Cu(OH)₂ was formed, and, consequently, a separate phase of CuO was detected in the oxide catalysts after calcination. As indicated by the TG-DSC results, different decomposition behaviors were observed for various hydrotalcites. Thermal calcination promoted the formation of copper aluminates and segregation of CuO from the bulk phases. TPR results showed 15CMAO-800 has the highest reduction rate, and the catalytic activities of iCMAO-800 mixed oxides depend on both the reduction rates and the amounts of copper ions in mixed oxides. The catalyst 15-CMAO-800 showed the best performance.     

Effects of hydrogen addition on methane combustion

In this study, the effect of hydrogen on methane combustion characteristic was tested. The ignition temperature (T₁₀) and burn off temperature (T₉₀) was carried out in a quartz reactor at atmospheric pressure with the mixture flow rate of 800 mL/min. The compositions of outlet gas were measured online by Gasmet DX4000 FTIR gas analyzer. The results showed that hydrogen enhanced the activity of methane. For all methane concentration range, the T₁₀ of methane could decrease 50 °C–70 °C with the H₂/CH₄ mole ratio at 0.1. For 1 vol.% methane combustion, when the H₂/CH₄ was equal to 0.05, the T₁₀ and T₉₀ could decrease 45 °C and 42 °C, respectively. When the H₂/CH₄ was 2.5, the T₁₀ and T₉₀ could decrease about 170 °C and 180 °C, respectively. Further more, CO generated in a wider temperature range when the hydrogen was added.     

Toxic products from the combustion and pyrolysis of polyurethane foams

Products from the thermal decomposition of four polyurethane foams heated to temperatures in the range 220 to 400 °C, in atmospheres of nitrogen, of 6% oxygen in nitrogen and of air were examined for some of the anticipated toxic materials. When phosphorus-containing inhibitors were added to or chemically incorporated in the foams, phosphorus compounds were evolved under most of the conditions employed. Other materials detected were hydrogen cyanide, isocyanate, urea, halogenated compounds and alkenes. A brief discussion is given of the hazard presented by polyurethane foams decomposing under these conditions.     

Formation of ceramic nanoinclusions in combustion products

Al₂O₃- and Al₂O₃/TiC-based cermets with a metallic binder were prepared by SHS in air. The Fe-Ti binder in combustion products was found to contain nanoinclusions of TiN and TiC, respectively. The elevated compression strength of synthesized materials was explained by dispersion-strengthening with nanosized inclusions of TiN or TiC.      

Catalytic combustion of methane over palladium exchanged zeolites

Palladium cation exchanged zeolites (ZSM-5, mordenite and ferrierite) were studied as catalysts for methane combustion. Pd-zeolites showed much higher activities than PdO/Al₂O₃. For comparable palladium loadings, PdO/Al₂O₃ requires a reaction temperature of ca. 70–80°C higher than Pd-ZSM-5 for conversions between 50–100%. The catalytic activity of Pd-ZSM-5 seems to be related to its reducibility. Temperature-programmed reduction experiments with carbon monoxide showed a lower reduction temperature (ca. 157°C) for Pd-ZSM-5 than for PdO/Al₂O₃ (225°C). Further, the positioning of the palladium by ion exchange offers a highly dispersed form of Pd^II supported on the high surface area zeolite.     

Catalytic combustion of methane on palladium single crystals

Catalytic methane combustion was studied over the palladium single crystals Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0). Under lean reaction conditions at 600 K (O₂:CH₄ = 10:1), stoichiometric palladium oxide was formed with an increase in surface area by a factor of approximately two. The oxide phase formed a “cauliflower-like” surface structure composed of approximately 4 nm sized semispherical oxide agglomerates. This oxide structure was independent of the original metal single crystal orientation. The turnover rates over the oxide structure starting with metal single crystals were 0.7 s⁻¹ on Pd(1 1 1), 0.9 s⁻¹ on Pd(1 0 0) and 0.9 s⁻¹ on Pd(1 1 0) at 600 K, 160 Torr O₂, 16 Torr CH₄, 1 Torr H₂O and N₂ balance to 800 Torr, suggesting that the methane combustion reaction is independent of the initial structure of the catalyst. Methane combustion on palladium single crystals experienced an activation period in which the initial turnover rates based on the initial Pd surface area were about 1/8–1/4 of the steady-state rates determined based on the oxide surface area. This activation period was caused by the slow oxidation of palladium single crystals and concomitant surface area increase during reaction. The increase in surface area happened mostly in the first 10 min of reaction. Carbon dissolution into the crystal was not found during methane combustion under reaction conditions in excess oxygen.