Effect of high CO2 concentration on char formation through mineral reaction during biomass pyrolysis

O2/CO2 combustion has attracted considerable attention as a promising technology for CO2 capture. Using biomass for fuel is considered carbon neutral, and O2/CO2 biomass combustion can mitigate the deleterious environmental effect of greenhouse. In this study, the effect of CO2, the main component gas in O2/CO2 combustion, on the pyrolysis characteristics of biomass is investigated. Cellulose, lignin, and metal-depleted lignin pyrolysis experiments were performed using a thermobalance. Information on the surface chemistry of the chars was obtained by Fourier transform infrared (FTIR) spectroscopy to investigate changes in the surface chemistry during pyrolysis under different surrounding gasses. When the temperature increased to 1073 K at heating rate of 1 K s−1, the char yield of lignin in the presence of CO2 increased by about 10% compared with that under Ar. However, for cellulose and metal-depleted lignin, no significant difference appeared between pyrolysis under CO2 and that under Ar. FT-IR showed that a strong peak corresponding to carbonate ions appeared in the char derived from lignin under CO2. Therefore, salts such as Na2CO3 or K2CO3 formed during the lignin pyrolysis under CO2. At around 1650–1770 cm−1, a significant difference appeared in the FTIR spectra of chars formed under CO2 and those formed under Ar. C=O groups not associated with an aromatic ring were found only in chars formed under CO2. It was suggested that these salts affected the char formation reaction, in that the char formed during lignin pyrolysis under CO2 had unique chemical bands that did not appear in the lignin-derived char prepared under Ar.