Exothermicity in wood torrefaction and its impact on product mass yields : from micro to pilot scale.
Résumé
This paper is focused on the effects of exothermic reactions during torrefaction, a mild heat treatment process in the temperature range 200 to 300°C. Three different scalcs are considered, the micro-particle (podwer scale), the macro-particle (woodchips or larger) and the thick fixed bed (pilot reactor) together with three wood types, spruce, beech and locust. At the powder scale, TGA-DSC tests indicate that exothermic reactions are noticeable principally during the first stages of torrefaction. The mass loss kinetics are used to evaluate parameters for a DAEM (Distributed Activation Energy Method) model. At the macro-particle scale, temperature measurements within wood planks heated in an oven depict the presence of temperature overshoots due to the exotherrnic reactions that lead to unevenly treated particles. At the reactor scale, a large fixed bed of wood chips is heated in the same aven by an up-flowing recirculated mixture of inert gas and volatiles. The exothermic reactions are found to generate a heat wave that propagates up the bed. Overall mass losses are found to largely exceed those preclicted with the DAEM model based on recorded bed temperatures. This means that, in order to reach the measured product yields, the inner core temperatures of the wood chips must be higher than either their outer surface or the gas flow. Multi-scale modelling approaches are therefore required to take into account the combined exothemicity and diffusional limitations within the wood chips or at their exchange surfaces.