New Granular Model Medium To Investigate Smoldering Fronts Propagation—Experiments

Abstract

Smoldering is involved in a variety of natural situations such as forest fires and also in man-controlled processes such as oil recovery and gas production from oil shale. A general feature of these situations is that a heat wave is propagating through the solid porous medium, powered by the flameless combustion of a solid. In numerous applications, this heat supply results from the partial oxidation of the carbon left after the devolatilization of the medium as the hot wave is approaching. The process of carbon oxidation in the complex geometry of a porous medium with forced air flow is not yet fully understood. In particular, the amounts of CO and CO 2 produced, that strongly impact the velocity and the temperature of the front, remain unpredictable to date. In this work, a new model porous medium has been produced by adding pyrolytic carbon into inert porous particles, and it has been characterized in detail, aiming at experiments in situations as simple as possible: the oxidation of carbon deposited at the surface of an inert solid matrix. Two of the main parameters that influence the frontthe carbon content and the fed air velocitywere varied over wide ranges. During experiments, carbon was always totally oxidized whereas oxygen was not totally consumed at low carbon content and high air velocities. Depending on the situations, the fraction of carbon oxidized to CO (and not to CO 2) varied between 23 and 37%. It was clearly established that the combustion of a particle is limited by internal mass transfer. The thickness of the combustion front is clearly observable, and it is shown to vary drastically depending on the operating parameters. These results are intended to provide a benchmark for the validation of a numerical model in a future work.

Darcy Scale Modeling of Smoldering: Impact of Heat Loss

“…It was observed that all of the carbon was oxidized, after the passage of the front, for all of the experiments. It was also observed in Baud et al (2015) that the temperature at the axis of the combustion cell is significantly higher than close to the walls (1 cm away from the walls). This indicates that significant heat losses occur at the cell walls.…”

“…The combustion cell used to obtain the experimental data bank is described in detail in Baud et al (2015; Figure 1). Briefly, it consists of a stainless steel tube (91 mm in diameter, 600 mm long) filled with a granular reactive porous medium.…”

“…We use porous alumina spheres with diameters ranging from 1.25 mm to 3.15 mm. They were enriched with solid carbon in controlled amounts thanks to a procedure described in Baud et al (2015).…”

“…In the first three experiments, the carbon content is varied from 2.30% to 3.58% in mass. These values range from near extinct carbon content to the highest temperature the combustion cell was designed for (Baud et al, 2015). In the other experiments the air flux is varied between 21 mm/s and 210 mm/s above the bed at 20°C.…”

“…• A set of 12 experimental results was first established in a previous work (Baud et al, 2015). A situation as simple as possible was aimed for, developing a model medium containing only carbon in an inert matrix of alumina.…”