Porosity development during CO2 and steam activation in a fluidized bed reactor

Steam- and CO2-activated carbons from a Spanish HV bituminous coal char were obtained at 800-900 degrees C in a batch laboratory scale fluidized bed reactor. The N-2 and CO2 adsorption isotherms and scanning electron microscopy allow for sample characterization. The development of porosity upon activation, using a fluidized bed reactor, is different depending on the activating agent. A much better activation development is attained with steam. Thus, total micropore volume linearly increases with the activation degree, attaining an apparent specific surface area of 1150 m(2)/g at 50% burnoff for steam-activated carbons. Although CO2 activation also produces a linear increase vs burnoff, much lower pore volume values are obtained, reaching an equivalent specific surface area of only 800 m(2)/g at 50% burnoff. SEM analysis indicates that CO2 produces external burning of the carbon particles. This different behavior of CO2 and steam activation was also observed for several runs carried out with almond shell char. The explanation of this behavior may be to a partial diffusion control in the CO2 gasification reaction, under fluidized bed conditions.