Burn-off characteristics and pore structure evolution of peat-based activated carbon during steam activation

Porosity adjustment is a core of the activated carbon preparation research,and the carbon burn-off accompanies and determines the pore structure evolution. In order to explore the relationship between the burn-off and the pore structure evolution of activated carbon from peat,the peat from Bijie,Guizhou province was sampled,crushed,ground,thermogravimetric analyzed in a nitrogen environment,and briquetted without binder,then a series of activated carbon samples were prepared by steam activation at different temperatures and times following carbonization. The adsorption indexes including iodine value,methylene blue value and caramel decolorization rate were determined,and the pore structure,carbonaceous structure and micro morphology of activated carbon samples were characterized by gas adsorption instrument,Raman spectroscopy and scanning electron microscope with the aim to study the relationship among the adsorption properties,pore structure,and carbonaceous structure. The results show that with the increase of the activation temperature,the pore structure evolution of activated carbon undergoes four stages,namely,pore forming(750-800 ℃ ),pore expanding(800-850 ℃ ),pore collapsing(850-900 ℃ ) and char surface ablating(900-950 ℃ ),respectively,and these stages are dominated correspondingly by the burn-off of amorphous carbon(D3 ) and scattered graphite layer structure(D1 ),burn-off of scattered graphite layer structure(D1 ) and irregular layers among parallel graphite layers(D2 ),burn-off of carbon in the active sites(D4 ),and burn-off of regular graphite carbon crystallite structure(G). Meanwhile,with the increase of the activation time,the pore structure evolution of activated carbon undergoes two stages,namely,pore fully development(60-120 min) and pore over development(120-150 min) respectively,are dominated correspondingly by burn-off of amorphous carbon(D3 ) and carbon in the active sites(D4 ). Thus, the pore structure of peat based activated carbon might be efficiently adjusted prior to char surface ablation or pore over development in activation process by removing disordered carbon in the whole course,consuming a portion of defective microcrystalline carbon,and activating a small amount of carbon at active sites. In other words,an elaborate control of burn-off,from internal surface mainly to inside-out and then to outside-in,and the surface pores appear to be formation,melting and collapse successively.