ACTIVATED CARBONS FROM SPANISH COALS .1. 2-STAGE CO2 ACTIVATION

A two-stage (pyrolysis and CO2 activation) treatment of Spanish coals has been studied in relation to the preparation of activated carbons. The paper analyzes three different aspects: (i) coal precursor effect (from anthracite to high-volatile bituminous), (ii) mineral matter content, and (iii) pore volume and surface area development upon activation. The coal rank influences the pyrolysis behavior, the characteristics of the char (e.g., porosity), the rate of activation, and the characteristics of the resulting activated carbon. The pyrolysis and combustion behavior of the chars, deduced from TG-DTA curves, have been related to the subsequent porosity development upon activation. Chars showing a uniform combustion are better precursors than those presenting a bifurcated DTA peak and two TG steps. Bituminous coals are more adequate precursors for the preparation of activated carbons. The mineral matter, abundantly present in Spanish coals, does not affect the porosity development, although, as expected, it induces an undesirable effect since it increases the inert part of the sample. Both adsorption capacity and pore volume development with the extent of activation are evaluated from CO2 and N2 adsorption isotherms (at 273 and 77 K, respectively) and by mercury porosimetry. The results show a gradual development of char porosity with burn-off; in addition, the characteristics of the resulting activated carbons are very dependent on the starting porosity of the char precursor. In higher rank coals, there is a decrease in char porosity upon carbonization, whereas in the lower rank coals studied an increase is found.