Haihui Wang

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.

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.

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.

Role of inherent water in low-temperature oxidation of coal

The role of water content in coal oxidation was studied using an isothermal flow reactor at atmospheric pressure and temperatures below 100°C. Transient rates of consumption of oxygen and production of CO 2 and CO were measured during oxidation experiments, by means of an online dual-column micro gas chromatograph and an oxygen analyzer. Experiments were carried out with a bituminous coal at three levels of initial water content, i.e., 0.8, 2.0, and 3.0%. Comparisons of the rates of production of carbon oxides during the oxidation experiments indicated that inherent water plays a roleinchemicalreactions occurring during coal oxidation. It was also found that the rateof oxygen consumption decreases with increasing water content of a sample. The current observations suggest that inherent water present in coal pores may react with carbonyl species to form carboxyl species during the oxidation process.

Oxygen consumption by a bituminous coal: Time dependence of the rate of oxygen consumption

Oxygen consumption of a bituminous coal was examined using the isothermal flow reactor technique. The experiments were undertaken at atmospheric pressure and at temperatures between 333 and 363 K. The gaseous oxidizing medium contained oxygen at concentrations of 10.9%, 20.9% and 39.9%. Experiments under various conditions showed that the rate of oxygen consumption of the coal decreases sharply in the first hour, followed by progressive decrease during the later time of the experiments. Analysis indicated that the Elovich equation is inapplicable to the current experimental data. Instead, it was found that an empirical equation in the form of an exponential function of the reciprocal time provides a very good fit to the experimental data. The rate of oxygen consumption can be further described as the sum of one constant and two exponential functions of time, reflecting two reaction sequences occurring in the oxidation process.

Low-temperature oxidation of coal at elevated pressures

A mathematical model is presented to describe steady-state mass transfer and oxidation processes in coal at low temperatures in a fixed-bed flow reactor. The model incorporates the effects of partial pressure of oxygen, temperature and coal particle size, and accounts for the rate of coal oxidation and the composition of oxygenated products at high pressures. This is an important development since previous models did not include the effect of pressure in their formulation. It is found that, when the partial pressure of oxygen increases the rate of oxygen consumption increases accordingly. However, the influence of partial pressure of oxygen on the rate of oxidation is less pronounced when the pressure surpasses 1 MPa. In addition the model predicts that, for a constant partial pressure of oxygen, higher rates of oxygen consumption occur at lower total pressures. The same trends are also found for the concentration of oxygenated products at the reactor outlet. It is suggested that, the variation of partial pressure of oxygen leads to different concentration levels of oxygen at the surface and within the pores of coal particles, substantially affecting the rate of oxidation.

Analytical Model for Determining Thermal Radiance of Fire Plumes with Implication to Wildland Fire

The present work aims at developing an analytical model for evaluating the thermal radiation of a fire plume formed by burning wildland fuels at a given burning intensity and crosswind. By resolving the equations governing gas continuity, momentum, and energy conservation for a convective (buoyancy) plume, gas velocity and temperature profile along the central line of the plume were determined, which enables a quantification of radiation intensity of flame and buoyancy plume zones. Model features are examined, and the predicted results are compared with the available experimental data. It is found that for a fire with burning intensity exceeding 600 kW m−1, the radiant heat emittable from its plume is not only contributed by the luminous flame zone but also by the buoyancy plume significantly. This indicates, for the first time, a necessity of inclusion of the contribution of the buoyancy plume in evaluating thermal impact of a fire plume on a receiver in the environment. The current work provides an important basis for convenient but reliable assessment of the fire risk of buildings raised by the occurrence of a wildland fire in adjacent areas.

Tests for spontaneous ignition of solid materials

This chapter addresses the flammability regulations for transport category airplanes, i.e., commercial airplanes used by airlines for transport of goods and people. Although flammability regulations for other aircraft types such as general aviation, commuters, agricultural, etc., are similar but not as comprehensive as those for transport category airplanes, they are beyond the scope of this chapter. In the United States, the Federal Aviation Administration (FAA) has the responsibility for establishing and enforcing all regulatory requirements for civil aviation. FAA fire safety regulations on transport category airplanes are quite extensive and implementation and enforcement processes are considerably more intricate and involved than those imposed by other regulatory agencies on land-based and water-based transport vehicles. Passenger cabin and engine compartment components are subject to one or more of over a dozen tests. Beyond the USA, FAA regulations and FAA regulatory changes are commonly adopted by almost all national aviation authorities. Hence, FAA regulations are essentially used worldwide, and for this reason this chapter is limited to FAA flammability requirements for transport category airplanes. A brief history of the evolution of FAA flammability regulations is provided. The original flammability requirements are described. Over the years, the FAA has greatly increased the stringency of airplane flammability requirements as the state-of-the-art of available materials advanced, and/or as existing fire threats based on large-scale testing were better understood and steps were taken to mitigate them. The development of regulatory flammability requirements in the 1980s were dynamic, and are described. This chapter also covers FAA processes for approval of design and production of airplanes. These processes or their non-USA equivalents are also used by almost all national regulatory authorities. These are often more of a challenge to applicants for regulatory approval than the tests themselves.