Sylvain Salvador

Combustion and gasification characteristics of chars from raw and torrefied biomass.

Torrefaction is a mild thermal pretreatment (T<300°C) that improves biomass milling and storage properties. The impact of torrefaction on the gasification and oxidation reactivity of chars from torrefied and raw biomass was investigated. Thermogravimetric analysis was used to study the differences in O(2) and steam reactivity, between chars prepared from torrefied and raw willow, under both high- and low-heating-rate conditions. High-heating-rate chars were prepared at 900°C with a residence time of 2s. Low-heating-rate chars were prepared with a heating rate of 33°C/min, a maximum temperature of 850 or 1000°C, and a residence time of 30 min or 1h, respectively, at the maximum temperature. Pretreatment by torrefaction consistently reduced char reactivity. Torrefactions impact was greatest for high-heating-rate chars, reducing reactivity by a factor of two to three. The effect of torrefaction on a residence time requirements for char burnout and gasification was estimated.

Darcy Scale Modeling of Smoldering: Impact of Heat Loss

ABSTRACT Modeling the propagation of smoldering fronts with forced air feeding in a porous medium remains a challenge to science. One of the main difficulties is to describe the carbon oxidation reaction that supports this self-sustained process. Pore scale approaches are required to tackle this complex coupled heat and mass transfer problem with chemistry. They, nevertheless, require high computation effort and still miss experimental validation. Furthermore, the heat loss at the walls of the cells inherent to every laboratory scale system adds another level of complexity in the understanding of the coupling between the phenomena at stake. Indeed, it induces a nonhomogeneous temperature field throughout the system. In this article, a 2D Darcy scale model is developed and validated by confrontation with experimental results from the literature, covering wide ranges of carbon content of the medium and forced air velocity. A reasonable description of the front temperature, velocity, and non-consumption oxygen amount is reached. The model finally enables understanding of the impact of heat loss, which controls the front shape and stability near the system walls.