Texts for educational purposes Types of burner

Bunsen burner is a laboratory gas burner having a vertical metal tube into which the gas is led, with a hole in the side of the base of the tube to admit air. The amount of air can be regulated by a sleeve on the tube. When no air is admitted the flame is luminous and smoky. With air, it has a faintly visible hot outer part (the oxidizing part) and an inner blue cone where combustion is incomplete (the cooler reducing part of the flame). The device is named after Robert Bunsen, who used a similar device (without a regulating sleeve) in 1855.

Oxyacetylene burner is a welding or cutting torch that burns a mixture of oxygen and acetylene (ethyne) in a specially designed jet. The flame temperature of about 3300°C enables all ferrous metals to be welded. For cutting, the point at which the steel is to be cut is preheated with the oxyacetylene flame and a powerful jet of oxygen is then directed onto the steel. The oxygen reacts with the hot steel to form iron oxide and the heat of this reaction melts more iron, which is blown away by the force of the jet.

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Catalytic reactions

Catalysis is the process of changing the rate of a chemical reaction by use of a catalyst.

Catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Catalysts that have the same phase as the reactants are homogeneous catalysts (e.g. enzymes in biochemical reactions or transition-metal complexes used in the liquid phase for analyzing organic reactions). Those that have a different phase are heterogeneous catalysts (e.g. metals or oxides used in many industrial gas reactions). The catalyst provides an alternative pathway by which the reaction can proceed, in which the activation energy is lower. It thus increases the rate at which the reaction comes to equilibrium, although it does not alter the position of the equilibrium. The catalyst itself takes part in the reaction and consequently may undergo physical change (e.g. conversion into powder). In certain circumstances, very small quantities of catalyst can speed up reactions. Most catalysts are also highly specific in the type of reaction they catalyse, particularly enzymes in biochemical reactions. Generally, the term is used for a substance that increases reaction rate (a positive catalyst). Some reactions can be slowed down by negative catalysts.

Catalytic converter is a device used in the exhaust systems of motor vehicles to reduce atmospheric pollution. The three main pollutants produced by petrol engines are: unburnt hydrocarbons, carbon monoxide produced by incomplete combustion of hydrocarbons, and nitrogen oxides produced by nitrogen in the air reacting with oxygen at high engine temperatures. Hydrocarbons and carbon monoxide can be controlled by a higher combustion temperature and a weaker mixture. However, the higher temperature and greater availability of oxygen arising from these measures encourage formation of nitrogen oxides. The use of three-way catalytic converters solves this problem by using platinum and palladium catalysts to oxidize the hydrocarbons and the CO and rhodium catalysts to reduce the nitrogen oxides back to nitrogen. These three-way catalysts require that the air-fuel ratio is strictly stoichiometric. Some catalytic converters promote oxidation reactions only, leaving the nitrogen oxides unchanged. Three-way converters can reduce hydrocarbons and CO emissions by some 85%, at the same time reducing nitrogen oxides by 62%.

Texts from scientific articles

Journal: Combustion and flame

Gas phase chemistry in catalytic combustion of methane/air

mixtures over platinum at pressure of 1 to 16

Abstract

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The gas-phase combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in a laminar channel-flow catalytic reactor at pressures 1 bar£p£16 bar. In situ, spatially resolved one-dimensional Raman and planar laser induced fluorescence (LIF) measurements over the catalyst boundary layer were used to assess the concentrations of major species and of the OH radical, respectively. Comparisons between measured and predicted homogeneous (gaseous) ignition distances have led to the assessment of the validity of various elementary gas-phase reaction mechanisms. At low temperatures (900£T£1400^oK) and fuel-to-air equivalence ratios (0.05£j£ 0.50) typical to catalytic combustion systems, there were substantial differences in the performance of the gaseous reaction mechanisms originating from the relative contribution of the low- and the high-temperature oxidation routes of methane. Sensitivity analysis has identified the significance of the chain-branching reaction CHO+M=CO+H+M on homogeneous ignition, particularly at lower pressures. A gas-phase reaction mechanism validated at 6 £p£ 16 bar has been extended to 1 bar £p£ 16 bar, thus encompassing all catalytic combustion applications. A reduced gas-phase mechanism was further derived, which when used in conjunction with a reduced heterogeneous (catalytic) scheme reproduced the key catalytic and gaseous combustion characteristics of the full hetero/homogeneous reaction schemes.

Experimental

Four different C1/H/O gas-phase mechanisms were investigated, which included the part of C2 chemistry that led to recombination of C1 radicals to C2 species. The mechanisms are further denoted as Warnatz-Maas, Warnatz et.al. and Leeds. The species transport properties were calculated from the Chemkin database. Each of the mechanisms in Refs. [22,35,36] was provided with its own thermodynamic data had the same thermo data as Warnatz. It is emphasized, however, that the discrepancies in the predictions with the above schemes (see next section) predominantly reflected kinetic and not thermodynamic differences; this was verified by interchanging the thermodynamic databases of the different mechanisms. Gas-phase and surface reaction rates were evaluated using Chemkin and Surface-Chemkin respectively. A set of

hetero/homogeneous schemes will be further denoted by the assigned names of its components, for example, Deutschmann/Warnatz schemes. Finally, the prefixes S and R will denote a surface and a gaseous reaction, respectively.

Conclusions

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The homogeneous ignition of fuel-lean ( 0.31 £ j £ 0.40) methane/air mixtures over platinum was investigated in the pressure range 1 bar £ p £ 16 bar, which encompasses all practical catalytic combustion systems. In situ nonintrusive measurements of major species and trace species concentrations over the catalyst boundary layer of a channel-flow reactor were compared against detailed numerical predictions with elementary hetero/homogeneous chemical reaction schemes. The following are the key conclusions of this study. It was also shown that crucial in the performance of the gaseous schemes was the correct prediction of the minimum equivalence ratio above which the self-inhibited ignition behavior of methane was maintained. Finally, notwithstanding the ultra fuel-lean conditions, C2 chemistry could not be ignored. In particular, the inclusion of the radical recombination reaction CH₃+CH₃=C₂H₆ was necessary for accurate homogeneous ignition predictions.