A SAXS study of the pore structure evolution in biochar during gasification in H₂O, CO₂ and H₂O/CO₂

Gasification of biomass allows for its efficient utilisation as a renewable fuel through syngas production. This work presents the different effects of gasifying agents (H₂O, CO₂ and H₂O/CO₂) on the pore structure evolution in biochar during gasification. The effects of temperature (700, 800 and 900 °C) and biomass particle size (up to 5.6 mm) were also studied. The pore structure of biochar was characterized using synchrotron small angle X-ray scattering (SAXS). The pore development in biochar during gasification in H₂O/CO₂ was close to that in H₂O. Carbon removal is more selective in CO₂ than H₂O and the derived biochar displayed pore fractal features, whereas the biochars gasified in H₂O and H₂O/CO₂ exhibited a surface fractal network due to the less selective carbon removal in the presence of H₂O. The pore structure development produced by various gasifying agents was paralleled by the evolution of the aromatic structures characterized by Raman spectroscopy. The different pore structure features result from the different reactivity of carbon sites with H₂O and CO₂, which can be attributed to the different amounts of O-containing groups in biochar, as well as the different reactivity of H₂O and CO₂. Increasing temperature reduced the differences in pore structure between biochars gasified in H₂O and CO₂. Biomass particle size had little impact on the pore structure of biochar.