Biomass devolatilization at high temperature under N2 and CO2: Char morphology and reactivity

• Effect of high CO2 content in an oxy-fuel devolatilization atmosphere was studied.
• Biomass chars were obtained at high temperature under N2 and CO2 atmospheres.
• The apparent volatile yield was higher when devolatilizing in CO2 than in N2.
• Smaller particles and lower reactivity were obtained after devolatilization in CO2.
• The char-CO2 reaction probably occurred during devolatilization in CO2 at 1300 °C.

Oxy-fuel combustion is usually performed in pf reactors under an enriched O2 atmosphere of CO2 to obtain a high CO2 content in the flue gases. The effect of the differences in thermal properties of N2 (conventional air combustion) and CO2 (oxy-fuel combustion) on the devolatilization process needs to be evaluated. The morphology and reactivity of biomass chars obtained by devolatilization in an EFR (entrained flow reactor) at 1300 °C under N2 and CO2, simulating air and oxy-fuel combustion atmospheres, were studied. Four biomasses were selected: PIN (pine sawdust), OW (olive waste), OS (olive stones) and CW (coffee waste). The apparent volatile yield under CO2 was greater than under N2. The morphology of the chars was assessed using SEM (scanning electron microscopy). The higher mass loss and the lower char particle size obtained during CO2 devolatilization indicate that a char-CO2 reaction occurred. The reactivity indices indicate a lower reactivity of the CO2-chars than the N2-chars. The devolatilization atmosphere had a significant effect on the biomass chars, suggesting that gasification had occurred during CO2 devolatilization. The OW, OS and CW chars showed a very high reactivity up to intermediate conversion levels, probably due to the catalytic effect of inherent alkali metals.