Activated carbons from bituminous coal: effect of mineral matter content

Abstract

A bituminous coal from Puertollano Basin (Spain) was selected on the basis of previous study as the most suitable raw precursor to prepare activated carbons. In fact, this coal gave the best results among a total of 10 different Spanish coals that covered a wide range of coal rank. Its initial ash level (19 wt%) was decreased in the coal mine facilities by different intensive hydropneumatic washing, yielding fractions with 12 and 8 wt% ash. Another fraction was also obtained from the original coal with 5 wt% ash by an HCl/HF washing treatment. These four coals were carbonised at 850°C and activated at 850°C for different activation times in steam and carbon dioxide, to analyse the effect of ash level on the porosity development of the resulting carbons. Contrary to what might be expected (catalysis effect, pore blocking and impediment of suitable porosity development), the initial ash level does not affect the porosity development. Those samples with a lower ash (at the same burn-off degree) yield higher micropore volumes. But when the results are presented in terms of dry ash free basis, the same porosity development is attained regardless of the initial ash level. Comparing the behaviour of steam and carbon dioxide as activating agents, the pore structure and pore volumes attained are quite similar, though the pore sizes of steam samples are slightly wider than those of carbon dioxide samples. The Dubinin–Stoeckli equation applied to experimental adsorption isotherm was used to follow the porosity development during activation for each reactive gas.