Eric M. Kennedy

Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling

Coal oxidation at low temperatures (i.e. <100 °C) is the major heat source responsible for the self-heating and spontaneous combustion of coal and is an important source of greenhouse gas emissions. This review focuses on the chemical reactions occurring during low-temperature oxidation of coal. Current understanding indicates that this process involves consumption of O 2, formation of solid oxygenated complexes, thermal decomposition of solid oxygenated complexes and generation of gaseous oxidation products. Parameters, such as mass change, heat release, oxygen consumption, and formation of oxidation products in the gas or solid phase, have been used to qualitatively and quantitatively describe the oxidation process. Reaction mechanisms have been proposed to explain the characteristics of consumption of O2, and formation of oxidation products in the gas and solid phases. Various kinetic models have also been developed to describe the rate of oxygen consumption and the rates of formation of gaseous oxidation products in terms of the rate parameters of the relevant reactions, oxidation time, temperature, and initial concentration of oxygen in the oxidising medium. Further research emphasis should be placed on the formation of the complete reaction pathways proceeding in the oxidation process and on the development of kinetic models applicable for predicting the self-heating and gas emission in a coal seam or stockpile.

Analysis of the mechanism of the low-temperature oxidation of coal

The mechanism of the oxidation of coal at low temperatures, i.e., below 100°C, was examined using measurements of the gases emitted from a bed of coal in an isothermal flow reactor. Employing an online two-column micro gas chromatograph, transient rates of production of CO2 and CO were monitored during desorption and oxidation experiments. A bituminous coal was milled into three nominal top size classes: 0-0.5 mm, 0-1 mm, and 0-2 mm. Desorption experiments with unoxidized coal samples at 20-70°C indicated that even an unoxidized coal incorporates oxygenated complexes in its structure. The threshold for thermal decomposition of these oxygenated species was found to be between 50 and 70°C. Carbon oxides liberated from oxidizing coal were compared with those from the thermal decomposition of coal oxidized at the same temperature, suggesting that two parallel reaction sequences contribute to the emission of carbon oxides during oxidation. A multi-step reaction mechanism was also proposed to describe low-temperature oxidation of coal and to explain the phenomena observed during the desorption and oxidation experiments.

Comparison of the oxidative dehydrogenation of ethane and oxidative coupling of methane over rare earth oxides

Abstract The characteristics of the oxidative dehydrogenation of ethane and the oxidative coupling of methane have been compared over four high purity rare earth oxides--La 2 O 3 , CeO 2 , Sm 2 O 3 and Pr 6 O 11 . With each oxide ethane reacts approximately four times as fast as methane and the activity order per g is Pr 6 O 11 〉Sm 2 O 3 〉La 2 O 3 〉CeO 2 . In terms of unit area is La 2 O 3 〉Sm 2 O 3 ≥Pr 6 O 11 ≫CeO 2 . The selectivity to hydrocarbons increases with temperature for both reactions but the absolute values are not only much higher for ethane than for methane but also much less catalyst dependent. For ethane at 750°C the order in selectivity to ethylene is La 2 O 3 (74%)〉Sm 2 O 3 (68%)〉CeO 2 (57%)P˛r 6 O 11 (53%). Under identical conditions, (13% hydrocarbon, 3.5% oxygen, balance helium) the selectivity of the oxidative coupling of methane to ethane plus ethylene together is about 40% for La 2 O 3 and 33% for Sm 2 O 3 but less than 3% for CeO 2 and Pr 6 O 11 , the oxides with variable valence state. This selectivity pattern can be explained in terms of two interacting factors, the average lifetime of alkyl and alkylperoxy species in the gas phase and the tendency of the oxides to oxidise alkyl radicals. The higher oxidation power of CeO 2 and Pr 6 O 11 relative to Sm 2 O 3 and La 2 O 3 , also manifests itself in a low yield of carbon monoxide relative to carbon dioxide and a lesser ratio of hydrogen to water. The water-gas shift reaction is not at equilibrium for the former pair of oxides. The yield of minor products, propene during methane coupling and methane and butene during ethane oxidation, increases with temperature. The more selective oxides, La 2 O 3 and Sm 2 O 3 , also show a small maximum in methane production at about 650°C which may be due to carbon-carbon bond cleavage in an adsorbed ethoxy species.

Thermal decomposition of solid oxygenated complexes formed by coal oxidation at low temperatures

Solid oxygenated complexes formed by coal oxidation play an important role in low-temperature oxidation of coal. Using an isothermal-flow reactor, the decomposition behaviour of solid oxygenated complexes was examined under pure nitrogen, at temperatures between 60 and 110 °C. The production of CO2 and CO during thermal decomposition of the complexes was quantified by an on-line dual-column micro GC. Experiments show that the production rates of CO2 and CO depend on temperature, but are independent of the particle size of the samples, indicating that the thermal decomposition process is dominated by chemical kinetics rather than diffusion. It was also found that the rates of formation of carbon oxides follow the Elovich equation and the activation energies for the production of CO2 and CO are 52.1 ± 6.3 and 72.0 ± 5.8 kJ/mol, respectively, indicating two separate reaction pathways proceeding in the decomposition of solid oxygenated complexes.

Theoretical analysis of reaction regimes in low-temperature oxidation of coal

This paper examines the low-temperature oxidation of coal using pore model resembling ordinary tree structures, where the trunk of each effective pore reaches the exterior of the coal particle. Theoretical analysis shows that, at low temperatures and atmospheric pressure, the mean diffusivity of oxygen in a coal particle is related to the porosity and particle size, and varies between 10-8 and 10-6 m2/s. When the particle size is large (more than 1 mm in diameter), the coal oxidation is controlled by continuum diffusion, while for a very fine particle the reaction regime switches to Knudsen-diffusion controlled (for active coal) or kinetically controlled (for less active coal). With increasing porosity of fine coal particles, the trend for the reaction regime to be kinetically controlled becomes more significant. For the less active coal with high porosity and particle size of several tens of microns, the reaction regime is almost entirely kinetically controlled. The rate of oxygen consumption of coal usually shows a dependence on particle size, but in the case of the less active coal and a particle size of a few tens of microns, the rate of oxygen consumption is virtually independent of the particle size. The independence of the rate of oxygen consumption of the particle size is also observed for larger particles (even around 500 μm in radius), when the coal reactivity is sufficiently low. The predictions from the present model are in agreement with published experimental findings, and have application to the modelling of spontaneous combustion of coal.

Oxidative dehydrogenation of ethane and the coupling of methane over sodium containing cerium oxides

Abstract The oxidation of ethane over a Ce2(CO3)3 derived catalyst showing much better selectivity than pure ceria for methane coupling has been investigated. XPS and bulk analyses show that the presence of sodium is responsible for the improved selectivity. The effect can be duplicated by the addition of sodium to pure CeO2. These catalysts also show better selectivity than pure CeO2 for the oxidation of ethane to ethylene but the enhancement due to sodium is less dramatic than for methane coupling. Using a feed stream comprising 15% ethane and 4% oxygen a selectivity over 70% can be achieved at 775°C.A selectivity over 90% is possible at near complete oxygen conversion when using an ethane to oxygen ratio of fifteen. The improvement in selectivity relative to pure ceria is achieved at the expense of a three fold drop in activity. The overall kinetic order for sodium containing catalysts is higher in ethane than in oxygen indicating that the reaction is more likely to be limited by a reaction between the hydrocarbon and the catalyst than by resupply of oxygen to the catalyst. The kinetic order for ethylene production is higher in ethane and lower in oxygen than is carbon dioxide formation. Thus selectivity to ethylene improves with increase in ethane/oxygen ratio as is also true for the methane coupling reaction. It is suggested that for both reactions sodium acts by blocking sites which cause oxidation of hydrocarbon radicals liberated into the gas phase by the primary reaction.

Kinetic modeling of low-temperature oxidation of coal

A kinetic model has been developed for determining the rate of oxygen consumption and production of carbon oxides during the oxidation of coal at low temperatures (i.e. <100°C), based on current understanding of the mechanism of coal oxidation. The chemical reactions considered in the model include two parallel sequences consuming oxygen and two thermal decomposition pathways producing carbon oxides. The resulting rate expressions reflect the contributions of various reactions consuming oxygen and producing carbon oxides and predict the effects of temperature, oxidation time and [O2] in the gas phase. The general form of the rate expressions confirms that chemisorption is relatively fast, only playing an important role at the early stage of coal oxidation. With the formation of stable and unreactive oxygenated complexes in a coal’s structure, the oxidation of coal is dominated by thermal decomposition of oxygenated complexes.

Low temperature oxidation of linseed oil: a review

This review analyses and summarises the previous investigations on the oxidation of linseed oil and the self-heating of cotton and other materials impregnated with the oil. It discusses the composition and chemical structure of linseed oil, including its drying properties. The review describes several experimental methods used to test the propensity of the oil to induce spontaneous heating and ignition of lignocellulosic materials soaked with the oil. It covers the thermal ignition of the lignocellulosic substrates impregnated with the oil and it critically evaluates the analytical methods applied to investigate the oxidation reactions of linseed oil.Initiation of radical chains by singlet oxygen (1Δg), and their propagation underpin the mechanism of oxidation of linseed oil, leading to the self-heating and formation of volatile organic species and higher molecular weight compounds. The review also discusses the role of metal complexes of cobalt, iron and manganese in catalysing the oxidative drying of linseed oil, summarising some kinetic parameters such as the rate constants of the peroxidation reactions.With respect to fire safety, the classical theory of self-ignition does not account for radical and catalytic reactions and appears to offer limited insights into the autoignition of lignocellulosic materials soaked with linseed oil. New theoretical and numerical treatments of oxidation of such materials need to be developed. The self-ignition induced by linseed oil is predicated on the presence of both a metal catalyst and a lignocellulosic substrate, and the absence of any prior thermal treatment of the oil, which destroys both peroxy radicals and singlet O2 sensitisers. An overview of peroxyl chemistry included in the article will be useful to those working in areas of fire science, paint drying, indoor air quality, biofuels and lipid oxidation.

Theoretical study of unimolecular decomposition of catechol

This study develops the reaction pathway map for the unimolecular decomposition of catechol, a model compound for various structural entities present in biomass, coal, and wood. Reaction rate constants at the high-pressure limit are calculated for the various possible initiation channels. It is found that catechol decomposition is initiated dominantly via hydroxyl H migration to a neighboring ortho carbon bearing an H atom. We identify the direct formation of o-benzoquinone to be unimportant at all temperatures, consistent with the absence of this species from experimental measurements. At temperatures higher than 1000 K, water elimination through concerted expulsion of a hydroxyl OH together with an ortho H becomes the most significant channel. Rice-Ramsperger-Kassel-Marcus simulations are performed to establish the branching ratio between these two important channels as a function of temperature and pressure. All unimolecular routes to the reported major experimental products (CO, 1,3-C(4)H(6) and cyclo-C(5)H(6)) are shown to incur large activation barriers. The results presented herein should be instrumental in gaining a better understanding of the decomposition behavior of catechol-related compounds.

Quantum chemical and kinetic study of formation of 2-chlorophenoxy radical from 2-chlorophenol: Unimolecular decomposition and bimolecular reactions with H, OH, Cl, and O2

This study investigates the kinetic parameters of the formation of the chlorophenoxy radical from the 2-chlorophenol molecule, a key precursor to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCCD/F), in unimolecular and bimolecular reactions in the gas phase. The study develops the reaction potential energy surface for the unimolecular decomposition of 2-chlorophenol. The migration of the phenolic hydrogen to the ortho-C bearing the hydrogen atom produces 2-chlorocyclohexa-2,4-dienone through an activation barrier of 73.6 kcal/mol (0 K). This route holds more importance than the direct fission of Cl or the phenolic H. Reaction rate constants for the bimolecular reactions, 2-chlorophenol + X --> X-H + 2-chlorophenoxy (X = H, OH, Cl, O2) are calculated and compared with the available experimental kinetics for the analogous reactions of X with phenol. OH reaction with 2-chlorophenol produces 2-chlorophenoxy by direct abstraction rather than through addition and subsequent water elimination. The results of the present study will find applications in the construction of detailed kinetic models describing the formation of PCDD/F in the gas phase.