Biomass gasification under high solar heat flux: Experiments on thermally thick samples

• Beech wood is exposed to radiative heat flux higher than 1000 kW/m2.
• 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.

In this study, thermally thick samples of beech wood are exposed to radiative heat flux above 1 MW/m2 (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.