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Combustion of char has conventionally been reported to be diffusion controlled. Analytically, the process is reported to follow second order initial diametric ( d 0 ) dependence (d 0 β ; β = 2) for both single-film (no CO combustion) and two-film models (CO burns in a concentric sphere over the particle). However, experimental investigations indicate deviation from classical diffusion limit with β exceeding 2.00 and going as high as 2.37. Videography investigations depict luminous film engulfing the particle for certain Temperature-Oxygen concentration-Particle diameter combinations (for which, β ≥ 2 ). The observed deviation is hypothesized to convective resistance offered by the CO generated on the surface to motion of C O 2 towards the surface. This results in reduced C O 2 concentration at the surface with enhanced conversion time being the implication (hence, β > 2 ). Such convective resistance remains unaccounted for in the prevailing analytical models. The CO dominated film thickness is enhanced with temperature and reactant concentration, increasing the convective resistance, and further deviating from d 0 2 behaviour. The analytical solution shows that in presence of a convectively expanding CO film, total conversion time is a function of film diameter while also being dependent on d 0 2 . The hypothesis is validated by comparing analytical estimates with experimentally observed film diameter and conversion time. [ABSTRACT FROM AUTHOR]

This work reports experimental and numerical investigations of biomass char conversion in H 2 O-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), H 2 O concentrations (10%–100%, bal. N 2 /H 2 /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 H 2 inhibits the char–steam reaction, attributed to adsorption of H 2 on the char surface: 25% H 2 in feed reduces the reaction rate by 50%. The CO+H 2 O reaction is found to play a pivotal role in controlling the char conversion by virtue of being the source of both H 2 (blocking active sites) and CO 2 (curtailing the forward reaction rate), and the sink for H 2 O (reduced reactant concentration). A novel method to identify the conversion regime based on temperature and reactant concentration is hypothesized and validated. • Thermochemical conversion of char in H 2 O and thermally aided ambient is investigated. • Impact of particle size, density and Reynolds number on char–H 2 O conversion addressed. • Char–steam gasification inhibited by product species — H 2 (chemisorption) and CO (reactant sink). • Hypothesis to identify conversion regime for given conditions; proposed and validated. • In-house 1-D numerical model establishes intraparticle mass and energy gradients. [ABSTRACT FROM AUTHOR]