An experimental study of thermally thick biomass (beech wood spheres) pyrolysis under high radiative heat flux was performed. The influence of sample diameter (5-20 mm), incident heat flux (60-180 kW/m 2 ) and initial moisture content (1-50 wt%) was studied. Char yields and temperature histories were monitored. Initial moisture content impact was highlighted. Indeed, steam coming from the sample core drying can gasify the external char layer, reducing therefore the char yield and increasing syngas production. This study was supported by a 2D unsteady numerical model of biomass degradation (mass, momentum and heat conservation coupled with Broido-Shafizadeh reaction scheme). This model gave more insight about phenomena occurring inside the degrading sample. It revealed that a pyrolysis front follows up a drying one. Therefore, steam is forced out of the sample through a high temperature char layer, making char steam gasification chemically possible.

gasification under high solar heat flux: Experiments on thermally thick samples. Fuel, Elsevier, 2016, 174, pp.257-266 Beech wood is exposed to radiative heat flux higher than 1000 kW/m 2 . Sample geometry evolves dramatically. Temperature higher than 1500°C are reached. Tar yield is lowered by thermal cracking and steam reforming. Initial moisture content plays key role in the biomass behavior. t r a c tIn this study, thermally thick samples of beech wood are exposed to radiative heat flux above 1 MW/m 2 (1000 suns). It was motivated by the fact that concentrated solar energy allows to achieve temperatures higher than 1200 °C where char gasification, tar thermal cracking and tar steam reforming can take place. It is achieved using a new experimental device made of an artificial sun and a new reaction chamber, that monitors the sample mass throughout a run and can trap the produced tars using a liquid nitrogen cooled tar condensing device. Thanks to this experimental device, it is possible to compute the average wood consumption rate as well as drying water, char, gas and tar production rates. The produced light gases are also analyzed using microGC. Furthermore, a radiometer is used to monitor surface temperature, which is around 1500 °C. First, a new behavior has been highlighted. Under high radiative heat flux, a char crater which mirrored incident heat flux distribution, is formed inside of the sample. Then, using this device, the impact of two major parameters was tested: wood fiber orientation relative to the solar flux and initial moisture content. Wood fiber orientation (end grain and with the grain) was shown to only have a minor impact on the production rates, gas composition and crater formation. Three initial moisture contents (0, 9 and 55 %wb) were tested. It was shown that increasing the sample moisture leads to direct drying steam gasification of the char produced by the pyrolysis. Moreover, steam also promotes tar steam reforming and therefore decreases the tar yield. Finally, form an energetic point of view, the dry samples can achieve an energetic conversion efficiency of 90%, capturing up to 72% of the incident solar power in chemical form.

This article presents a new numerical model describing the behaviour of a thermally thick wood sample exposed to high solar heat flux (above 1 MW/m 2). A preliminary study based on dimensionless numbers is used to classify the problem and support model building assumptions. Then, a model based on mass, momentum and energy balance equations is proposed. These equations are coupled with liquid-vapour drying model and pseudo species biomass degradation model. By comparing to a former experimental study, preliminary results have shown that these equations are not enough to accurately predict biomass behaviour under high solar heat flux. Indeed, a char layer acting as radiative shield forms on the sample exposed surface. In addition to this classical set of equations, it is mandatory to take into account radiation penetration into the medium. Furthermore, as biomass contains water, medium deformation consecutively to char steam gasification must also be implemented. Finally, with the addition of these two strategies, the model is able to properly capture the degradation of biomass when exposed to high radiative heat flux over a range of sample initial moisture content. Additional insights of biomass behaviour under high solar heat flux were also derived. Drying, pyrolysis and gasification fronts are present at the same time inside of the sample. The coexistence of these three thermochemical fronts leads to char gasification by the steam produced from drying of the sample, which it is the main phenomenon behind medium ablation.