Robert D. Gunn

Low-temperature oxidation of coal. 1. A single-particle reaction-diffusion model

In this paper, the first of a three-part series, a kinetic model for low-temperature oxidation is developed. The formulation, based on a single isothermal particle, permits assessment of the relative importance of various mass transfer resistances. Through inclusion of transport resistances, the model accounts for the dependence of the observed reaction rates on particle size and specific external surface area over the full range of particle sizes, with smooth transition between different reaction regimes for dry as well as wet low-rank coal. The approach also permits development of a new model which considers the effect of bulk and surface moisture in the coal on the rate of low-temperature oxidation. This new model is expected to apply at moisture levels above ∼0.1 kg kg−1 coal. It is shown that when liquid water fills the porous matrix, in situ moisture in fresh coal must exert a very significant retarding influence on the rate of low-temperature oxidation at constant temperature. Further, the influence of particle size on reaction rate is pronounced even at low temperatures and small particle sizes. For dried coal on the other hand, the influence of particle size becomes important only at significantly higher temperatures or for considerably larger particle sizes. Results of the extension of the model to (1) the interpretation of rate data obtained in isothermal batch and fixed-bed reactors and (2) the modelling of spontaneous combustion in coal stockpiles are reported in parts 2 and 3 respectively.

Low-temperature oxidation of coal. 3. Modelling spontaneous combustion in coal stockpiles

In the first two papers of this series, a reaction-diffusion model for low-temperature oxidation of coal was developed and used to extract kinetic rate parameters for a Wyoming subbituminous coal. In this paper, this model is incorporated in a two-dimensional model for spontaneous combustion of open coal stockpiles where the influence of moisture migration can be neglected. Wind-driven forced convection is found to be the dominant mechanism for flow of air within the open stockpile. The model calculations are in excellent qualitative agreement with observations reported from large-scale tests. The roles of bed porosity, side slope, wind velocity, coal reactivity and bed particle size are examined in detail. A correlation is also developed for long-term safe storage of coal in open stockpiles.

Geometrical Aspects of Sorted Patterned Ground in Recurrently Frozen Soil

A model for sorted patterned ground shows that some types arise from density-driven Rayleigh free convection that occurs during thawing of water-saturated recurrently frozen soils. The regularly spaced convection cells result in an uneven melting of the underlying ice front. Frost action causes stones to be upthrusted and to form in a pattern on the ground surface that mirrors the corrugation in the underlying ice front. The implications of the water circulation direction in the cells on the sorting process are considered.

Low-temperature oxidation of coal. 2. An experimental and modelling investigation using a fixed-bed isothermal flow reactor

Results of an experimental and theoretical modelling investigation on the low-temperature oxidation of fresh, wet, low-rank coals are reported. The rates of oxygen consumption and carbon dioxide formation were measured for Wyoming subbituminous coal, using an isothermal fixed-bed flow reactor. The measurements are interpreted with a one-dimensional continuum model, including the single-particle diffusion-reaction model developed in Part 1, for the fixed-bed reactor. The model calculations are also compared with other data reported in the literature. The results indicate that the model provides a satisfactory explanation for the dependence on particle size and temperature of the low-temperature oxidation of dry and fresh wet coal. Extension of this formulation to model spontaneous combustion in coal stockpiles is reported in Part 3.

Reverse combustion instabilities in tar sands and coal

A linear stability analysis shows that reverse combustion in coal and tar sands is only conditionally stable for mobility ratios less than one. However, high air flow rates and gas generation at the combustion front can be stabilizing influences. For unstable operation, an estimate of the size of the reverse combustion channel may be obtained from the curve for the most highly amplified wave-length This in turn provides a method for calculating air flux and combustion front velocity. Recently, the Laramie Energy Research Center and Sandia Laboratories have obtained pertinent experimental data concerning reverse combustion from a field test of in situ coal gasification at Hanna, Wyo. These data show that 9.7 days were required for the development of a reverse combustion path 68 to 70 ft in length. The stability theory developed in this work predicts a length of 68 ft for this same 9.7-day period. In addition to quantitative predictions, stability theory provides an explanation of certain puzzling quantitative observations concerning reverse combustion. (17 refs.)

Stability Of Forward And Reverse Wet Combustion Fronts In Porous Media

A linear stability analysis shows that reverse wet combustion is unstable for nearly all physically realizable operating conditions for the special case of coincident steam and combustion fronts. Expansion effects due to gas generation from combustion and vaporization are found to have a stabilizing influence on reverse wet combustion. Forward wet combustion also is found to be conditionally stable. Significant expansion effects at the combustion front can overwhelm the stabilizing influence of a favorable mobility ratio and effect destabilization with respect to oscillatory long wavelength modes.