Characterization of coal and biomass chars from high temperature pyrolysis in CO₂ atmosphere

Understanding the initial stage of solid fuels conversion is essential to mastering their subsequent chemical and physical transformations with relevance in the design, modeling and improvement of commercial and lab scale solid fuels thermochemical conversion systems. In this regard, coal and biomass chars produced during pyrolysis in an entrained flow reactor in CO₂ atmosphere at 1300 °C, 1400 °C, and 1500 °C are characterized using various analytical techniques. The conversion level of the two fuels in the entrained flow reactor is significantly different, varying from 50 to 64% for coal and 87 to 91% for biomass and increasing with the operating temperature. This level of conversion is directly reflected to the particle densities as coal chars display a vertical parabola trend, meaning that chemical and physical transformations are still taking place. Biomass chars on the other hand display an increasing linear trend, which suggests that these chars are approaching their optimum structural changes. This hypothesis is confirmed by the BET surface area, which is very close for all biomass chars. The BET surface area of coal chars increases slightly from 1300 °C to 1400 °C but drops significantly at 1500 °C. A plausible justification of this observation is the coalescence of the pore structure, typical to entrained flow reactor chars. This hypothesis is substantiated by the SEM images of coal char showing visible structure collapse at 1500 °C. Coal chars generated at 1400 °C and 1500 °C have comparable reactivities to CO₂ at 800 °C and 900 °C, significantly lower that the reactivity of coal char generated at 1300 °C. A similar trend is found with biomass chars suggesting that the deactivation of reactive sites for heterogeneous reactions is initiated after 1300 °C.