Evolution of coke microcrystalline structure during calcination process of coal-based needle coke

The evolution of the microstructure of needle coke during calcination process has been determined by FT-IR, XRD, and Raman spectroscopy, in which the needle cokes were obtained by calcination at heating rates of 2 and 5 ℃/ min with the coal-based green needle coke as the raw materials, respectively. The results show that the diameter of carbon microcrystal L_a, the height of carbon microcrystal L_c, the lamella content in the crystal (N), the average carbon number in each layer (n), and the content of tending regular graphite micro crystals (I_g) in the needle coke increase gradually with the rising of the calcination temperature. However, the value of L_c appears an "inflection point" due to the escape of volatiles and shrinkage of green needle coke. The layer spacing d₀₀₂ fluctuates due to the random "layer fault" between the new layer and the original layer. The higher the heating rate, the smaller the characteristic carbon microcrystal parameters (L_a, L_c, N, and n) of needle coke, and the latter the temperature of "inflection point" for L_c appears. Also, the content of perfect graphite microcrystallite (I_G/I_all) increases gradually with the increasing of temperature, and the defective graphite microcrystallites transforms to each other continuously during calcination process, finally being developed into the graphite microcrystals. The average bond length α of C-C bond in the carbon planes would become larger with the increase of calcination temperature.