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.

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Two-stage fixed bed gasification is one of the most promising technologies for low and medium energy production from biomass. In industrial processes, control and optimisation is often based on constructor know-how rather than on an understanding of the mechanisms involved. We present a new original tool, the Continuous Fixed Bed Reactor (CFiBR), which was specifically designed and built to enable a fine understanding of the limiting stage of a gasifier: the char bed gasification zone. The reactor, the instrumentation, the operating procedure and set-up tests are described in detail. The potential of the reactor is demonstrated through the characterisation of the gasification of a continuous wood char bed. Temperature profiles and gas concentrations along the 65 cm bed were established and showed that the most reactive zone was the first 10 cm of the char bed. Accurate energy and mass balances provided relevant information regarding the contributions of the main reactions involved in the fixed char bed gasification process. (Résumé d'auteur

In fixed bed gasifiers, the char bed gasification zone where char is converted into syngas plays a major role in terms of efficiency and control of the process. This zone is particularly complex as many phenomena compete, i.e. heterogeneous and homogeneous chemical reactions, gas flow in porous medium and flow of solid particles. This paper investigates the mechanical and thermochemical behavior of the char bed gasification zone and focuses particularly on bed compaction. To achieve this, a low-density biomass char from wood chips and a high-density one from wood pellets were gasified in a pilot scale continuous fixed bed reactor. Measurements of profiles were taken along the char bed for temperature, gas species concentration, char composition, char bed density and char particle velocity using fine instrumentation and specific char and gas sampling techniques. In our operating conditions, the char bed reactive zone is 3 times longer for chips (45 cm) than for pellets (16 cm). We show that pelletization has no effect on: char bed compaction, final char conversion (about 95%) and syngas quality (16% H2 and 13% CO). Finally, we discuss char bed compaction and the main phenomena that control it in order to propose a line of inquiry for modeling. (Résumé d'auteur