Large Char Particle High-Pressure Oxidation Rates and Mechanisms

Abstract

The main objective of this study was to investigate the high-pressure oxidation rates of large (6-9 mm diameter) char particles for a wide range of five experimental conditions, with particular focus on the role of the ratio of the carbon-oxide combustion products at the reaction surface and on its analytical consequences on the Stefan flow (BLD) behavior. The char particles were obtained by pyrolytic heating of a Pittsburgh HV bituminous coal in nitrogen. The experimental conditions and ranges were gas temperature (825-1200 K), gas (oxidizer) velocity (0.08-1.28 m/s), particle mass (0.056-0.16 g), oxygen mole fraction (0.04-0.21), and total pressure (1-5 atm). This allowed for a suite of 13 different permutations of operating conditions, with each repeated five times for a total of 65 experiments, generating about 1000 experimental data points. The experiments were conducted in a high-pressure, controlled-profile (HPCP) laboratory reactor with a cantilever balance attachment. The particles were suspended from the balance attachment in a wire basket for continuous weighing to obtain the oxidation rates. The reaction was found to be mostly governed (controlled or dominated) by boundary layer diffusion (BLD), as broadly expected and predicted for particles of this size from the Stefan flow BLD analysis, but results also showed numerical variabilities in the carbon oxides ratio (CO/CO 2 ) that are believed to be original in this work. In sum, reaction rate increased with increasing temperature, oxygen concentration, and ambient velocity, in agreement with past work. Burn-out time was found to be independent of total pressure, in agreement with prediction where the reaction rate is controlled or dominated by boundary layer oxidizer diffusion. The particles burned in two stages: stage I extended through 85% to 90% of the char mass loss at which point there was a temperature drop or (nominal) "extinction" accompanied by slower reaction in the subsequent stage II. The primary focus in this study was on the stage I where comparison with rigorous theory established that the reaction was diffusion-controlled for nine of the 13 sets of experimental conditions. In the remaining four sets (high velocity, low pressure, and low oxygen concentration), there was a measurable chemical reaction resistance; however, diffusion, though not controlling, was still dominant. For eight of the 13 sets of test conditions, analysis indicated full conversion of CO to CO 2 at the particle surface. For the other five sets of test conditions (low oxygen, low temperature, and low velocity), however, CO accounted for 14% to 79% of the product gas leaving the surface, with the balance being CO 2 . The effects of the test variables were quantitatively accounted for by incorporating Stefan flow behavior in a rigorous theory of particle burning. IntroductionIn this paper, we report results of experimental and analytical studies on high-pressure oxidation rates of large char particles, obtained from a bituminous coal,

Construction Strategy for Atomistic Models of Coal Chars Capturing Stacking Diversity and Pore Size Distribution

“…While extensive atomistic simulations have been created for coals (>134) , and a greater number for carbon, − representations of coal char are few and generally of limited applicability. Despite the broad interest in coal structure, it is the char that has the greatest influence on combustion reactivity. − …”

Study on the Anisotropy of Mass Transfer for Oxygen in the Ash Layer of Shale Char Particles

“…Guedes de Carvalho et al showed that, under diffusion-controlled conditions, the mechanism of combustion is carbon oxidation to CO at the particle surface followed by combustion of CO far from the particle. Mathias et al investigated the high-pressure oxidation rates of large char particles for a wide range of experimental conditions, with particular focus on the role of the ratio of the carbon/oxide combustion products at the reaction surface. In summary, the reaction rate increased with an increasing temperature, oxygen concentration, and ambient velocity.…”