This work was carried out in order to quantify the impact of the pyrolysis heating rate both on the properties of the residual charcoal and on the behaviour during gasification by H 2 O of the charcoal. The experiments were conducted on 10 mm diameter beech wood spheres, pyrolysed at atmospheric pressure under heating rates covering the range from very slow, 2.6 K min K1 , to very rapid, over 900 K min K1 , i.e. the highest value that can be reached. When charcoal is submitted to gasification at 20% H 2 O i nN 2 at 1200 K, the ratio of the times for complete conversion reach 2.6. Such a difference is considerable as far as an industrial application is concerned. The initial properties of the charcoal such as apparent density, porosity, and pore surface area obtained by N 2 or Ar adsorption were measured in order to explain the differences in gasification kinetics within the charcoal. The charcoal particles exhibit densities as different as 219-511 kg m K3 and porosities between 87 and 70% for charcoal prepared at 900 and 2.6 K min K1 respectively. The specific surface area is higher than 600 m 2 g K1 for three charcoals. Influence of ash content of the initial charcoals, at 1.6-2.7%, is also regarded with particular attention to explain the observed differences in gasification kinetics.

The present work deals with a study coupling experiments and modeling of charcoal gasification by steam at large particle scale. A reliable set of experiments was first established using a specially developed "macro-TG" apparatus where a particle was suspended and continuously weighed during its gasification. The main control parameters of a fixed-bed process were modified separately: steam gasification of beech charcoal spheres of different diameters (10 to 30 mm) was studied at different temperatures (830 to 1030 • C), different steam partial pressures (0.1 to 0.4 atm H 2 O), and different gas velocities around the particle (0.09 to 0.30 m/s). Simulations with the particle model were performed for each case. Confrontations with experimental data indicate that the model predictions are both qualitatively and quantitatively satisfactory, with an accuracy of 7%, until 60% of conversion, despite the fact that the phenomena of reactive surface evolution and particle fracturing are not well understood. Anisotropy and peripheral fragmentation make the end of the process difficult to simulate. Finally, an analysis of the thermochemical situation is proposed: it is demonstrated that the usual homogeneous or shrinking core particle models are not satisfying and that only the assumption of thermal equilibrium between the particle and the surrounding gas is valid for a model at bed scale.

In wood gasification, oxidation of char particles by H 2 O, CO 2 or O 2 plays a major role in the performance and efficiency of air gasifiers. These reactions are generally analyzed under carefully design and controlled laboratory conditions, using either micro-samples to focus on the reaction kinetics or large spherical particles, but rarely using the real shape encountered in industrial processes. The objective of this work was to conduct a complete parametric study on char gasification kinetics at particle scale in operating conditions like those of industrial applications. Experimental results from a macro-Thermo Gravimetric reactor are compared to those from a char particle model, which analyzes reactivity versus conversion through the surface function F(X). We first show that particle thickness is a representative dimension of a char particle with respect to its apparent kinetics. Second, considering the three reactions independently, we compared the influence of temperature (800-1050 1C) and reacting gas partial pressure (0.03-0.4 atm) and determined the intrinsic kinetic parameters and surface function F(X). Simulations provided profiles of temperature and gas concentrations within the particle, mainly revealing internal mass diffusion limitation. The experimental data base proposed and the model results improve our understanding of the gasification reaction and support the elaboration of process models.