Activated carbons from Spanish coals .2. Chemical activation

Activated carbons from Spanish coals have been prepared by chemical activation with alkali and alkaline-earth hydroxides, The paper analyzes the following aspects: presence of water during pyrolysis, extension of the impregnation and drying processes, activating agent, temperature and time of pyrolysis, activating agent/coal ratio, coal rank, and mineral matter content. KOH and NaOH activation provides very successful results in which N-2 apparent surface areas of 2500 m(2)/g are obtained; contrarily, Ca(OH)2 produces a small porosity development. A pyrolysis temperature of 700 degrees C yields activated carbons with a wider pore size distribution than those prepared at 500 degrees C. The pyrolysis time does not affect the microporosity development but an important reduction in the ash content is observed when the time is increased. The use of an activating agent/coal ratio of 2/1 compared to 1/1 has favorable effects: (i) a decrease in the ash content and (ii) a wider pore size distribution. The addition of a drying step before pyrolysis greatly increases the nitrogen surface area and decreases the ash content. Porosity development depends on the coal rank. Low rank coals yield activated carbons with wider pore size distributions. The chemical activation process presents the great advantage, compared to physical activation, of removing the inherent mineral matter of the coal. Activated carbons obtained by chemical activation exhibit much lower ash content than the corresponding original coals. Ash reduction, depending on the coal used, could be very noticeable; the highest reduction observed corresponds to coal UA10 (11.3% ash) that, after NaOH activation, results in an activated carbon with only 0.5% ash content.