Evolution of Carbon Microstructures during the Pyrolysis of Turkish Elbistan Lignite in the Temperature Range 700−1000 °C

Chars and activated carbons were produced from raw, HCl-washed, and HCl/HF-washed Elbistan lignites at 700 °C, 800 °C, 900 °C, and 1000 °C. The pyrolysis and activation reactions increased the BET areas (m 2 /g carbon) of the acid-washed samples almost 10-fold. The increase of the BET areas (m 2 /g carbon) by increasing the temperature of pyrolysis or activation from 700 °C to 1000 °C was explained with the burn-out of carbon which led to the development of porosity. The values of the stacking heights, L c of HCl/HF-washed samples seemed to increase from 1.0 nm to 1.5 nm, the average number of graphene sheets increased from 2.8 to 4.4, and the lateral size of the crystallites, L a, increased very faintly from 5.0 nm to 5.5 nm when the pyrolysis temperature was increased from 700 °C to 1000 °C. Activation reactions performed at the same temperature range did not change the stacking heights. The values of L c for activated HCl/HF-washed samples stayed almost constant in the same range as for the carbonized samples within 1.0−1.5 nm. This indicated that oxidative reactions during activation did not alter the stacking heights of the crystallites significantly in the temperature range of 700−1000 °C. The results presented in the present work can be considered as indications for the development of turbostratic (fully disordered) structures in the temperature range of 700−1000 °C.