The Significance of Char Morphology to the Analysis of High-Temperature Char−CO₂Reaction Rates^†

Abstract

The reaction rates of coal chars with CO2 is an important aspect of coal performance under gasification conditions. Most studies of the char−CO2 reaction are undertaken at temperatures (typically ∼1200 K or below) significantly lower than those found in entrained flow applications, to allow detailed investigations into the surface reaction processes. Application of these data to high temperatures therefore requires consideration of how these reaction rates are affected by gas diffusion through the pore structure of reacting particles, yet there are very few char−CO2 rate data at high temperatures and pressures against which such applications can be tested or verified. This paper presents results of measurements of the char−CO2 reaction rate at high pressures (2.0 MPa) and high temperatures (up to 1673 K) using an entrained-flow reactor and also presents analyses of the morphology of the chars sampled during reaction. Using an effectiveness-factor-based approach, the effects of pore-diffusion and surface reaction (intrinsic reactivity) are separated, allowing determination of the high temperature intrinsic (surface) reaction rate. By accounting for the char morphology that arises specifically from high-pressure, high-temperature pyrolysis, these high-temperature intrinsic rates can be shown to be consistent with an extrapolation to high temperatures of intrinsic rates directly measured at low temperatures. This demonstrates the importance of accounting for the morphology of high-pressure chars, and shows that traditional techniques where char particles are considered as reacting porous solids are often unsuitable.