Kinetic analysis on water adsorption of thermally upgraded lignite

Abstract

An in-situ water adsorption analyzer was designed and set up for obtaining water dynamic adsorption process of thermally upgraded lignite under relative humidity (RH) of 55%–85% at 20 °C and 30 °C. Four upgraded samples with different pore structures and oxygen functional groups were prepared at 120, 160, 240 and 320 °C. Grey correlation degree analysis method was used to study the correlation between water adsorption capacity and its influencing factors. The results showed that upgrading temperature was the dominant factor influencing water adsorption of upgraded coals compared to RH and environmental temperature. The dynamic adsorption process of water on coal could be well described by Intra-particle diffusion model, and it could be divided into surface adsorption stage, pore diffusion stage, and dynamic equilibrium stage. The rate-controlling factors of surface adsorption stage were oxygen functional groups and specific surface area, while those of pore diffusion stage were pore volume and pore size. During adsorption under RH 55%, water was firstly adsorbed on coal surface through oxygen functional groups with a relatively fast rate, which was orderly followed by formation of water clusters and capillary condensation of water in pores with a relatively slow rate.