Water vapour adsorption on lignin-based activated carbons

Lignocellulosic wastes are interesting precursors for carbon products. The high amount of Na observed in kraft lignin makes it a promising precursor for the preparation of activated carbons for desiccant applications. Water adsorption capacity and kinetics of kraft lignin-based chars and activated carbons with different burn-off and inorganic matter content have been studied. CO2 partial gasification of lignin char develops a wide porous structure. An increase of the micropore volume can be observed at low to medium burn-offs. At degrees of higher activation the mesoporous structure develops. For very high burn-off the porous structure is destroyed by coalescence of the pores and reduction of the carbon material. The carbons obtained show atomic surface concentrations of sodium from 7.6-15.4%, as revealed by XPS analysis. Water vapour adsorption isotherms have been obtained in a thermogravimetric system and have been fitted by a DS model, which properly represents the experimental data. The kinetics of water vapour adsorption follows a linear driving force mass transfer (LDF) model. The presence of sodium and oxygen surface groups on the carbon surface enhances water vapour adsorption at low relative pressure. Activated carbon produced at 41% burn-off shows the highest water vapour adsorption at low relative pressures, as a consequence of the high sodium dispersion on its surface. The sodium dispersed over the carbon surface undergoes clustering as gasification proceeds, decreasing the number of active centres. For burn-off higher than 41%, this behaviour produces a decrease in the water adsorbed at low relative pressures. (c) 2007 Society of Chemical Industry.