Thermochemical conversion of millimeter-sized single char particle in steam dominated environments under varying temperature, reactant composition and flux—Experimental and numerical analysis

Abstract

This work reports experimental and numerical investigations of biomass char conversion in H2O-dominated thermal environment. Temperature parametric analysis establishes 730 °C as the temperature beyond which char conversion rate becomes practically relevant — 0.4 mg min−1 °C−1. The mass loss rate of char particles of different sizes (8–20 mm) is studied under a wide range of temperatures (800–1000 °C), H2O concentrations (10%–100%, bal. N2/H2/CO, at varying ratios), and combinations thereof. Increasing the temperature/reactant concentration by 10% reduces the conversion time scales by up to 2 times. Temperature and reactant concentration in combination control the conversion regime (kinetic limit to diffusion limit) and can be used as control parameters. It is observed that the presence of H2 inhibits the char–steam reaction, attributed to adsorption of H2 on the char surface: 25% H2 in feed reduces the reaction rate by 50%. The CO+H2O reaction is found to play a pivotal role in controlling the char conversion by virtue of being the source of both H2 (blocking active sites) and CO2 (curtailing the forward reaction rate), and the sink for H2O (reduced reactant concentration). A novel method to identify the conversion regime based on temperature and reactant concentration is hypothesized and validated.