Structure and oxidation reactivity of char: Effects of pyrolysis heating rate and pressure

Two different ranks of coals (Ximeng lignite, L and Xishan bituminous coal, B) and biomass (cornstalk, C) were selected to investigate effects of heating rate and pressure on structure and oxidation reactivity of chars from pyrolysis. The chars were prepared in a two-step pressurized fixed bed reactor at 900℃ from slow pyrolysis and fast pyrolysis under a range of pressures (0.1-2.0 MPa), which were marked as SC and FC, respectively. Specific surface area, surface morphology, and aromaticity of chars were characterized. Isothermal thermogravimetry was performed to study oxidation reactivity of chars. Results indicate that the char yields of three samples are distinctly varied and pyrolysis pressure and heating rate influence them through different residence time and diffusion rate of volatiles varying with different raw materials. The yields of chars from fast pyrolysis are all lower than those from slow pyrolysis and they increase slightly with increasing pyrolysis pressure from atmospheric pressure to 2.0 MPa. The specific surface areas of L-FC and B-FC are larger than that of L-SC and B-SC. However, the specific surface area of C-FC is smaller than that of C-SC. The morphology of FC is all rougher than that of SC. The aromaticity of B-SC is higher than that of B-FC, while that of L-SC and C-SC is lower than that of L-FC and C-FC derived from pressurized pyrolysis. The evolution of char structure at low heating rate and high pressure results generally in poor reactivity of char. The reactivity of cornstalk char is much higher than that of coal char, which should be correlated to the dispersion and concentration of inorganic matters in the char which is affected by heating rate and pressure.