Study On Pore Structure Changes Of Lignite And Their Effects On Moisture Re-adsorption Of Drying Lignite

As the continuous increase of primary energy consumption, the reserve of high quality coal has dropped significantly. Low-rank coals constitute a major energy source for the future gradually because of its relatively rich reserves. Lignite, as a kind of low-rank coal, constitutes significant supplies for both energy and chemical feedstock because of its abundance, easy access, and low mining cost. Since high moisture content of lignite entails high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in combustion of such coals. Hence, it is necessary to reduce the moisture content before its utilize in a large scale. However, upgraded lignite will re-adsorb moisture again due to the porous physical structure and abundant hydrophilic functional groups in lignite. Therefore, it is necessary to explore the interaction between pore structure or hydrophilic functional groups and moisture re-adsorption of upgraded lignite, and then control the moisture re-adsorption capacity content of upgraded lignite. It would theoretically guide the upgrading of lignite.A lignite in Inner Mongolia was selected and treated with thermal upgrading process in this thesis. In order to investigate the influence of drying conditions on dewatering performance and re-adsorption behavior, lignite was upgraded at different time, pressure and temperature. The characteristics of pore structure, content of hydrophilic functional groups, ash and moisture re-adsorption behavior of the upgraded lignite were investigated. The effect of pore structure and hydrophilic functional groups on moisture re-adsorption content of lignite were analyzed. Moisture adsorption types of upgraded lignite were also studied. The main findings are list as follows:(1) The increase of drying temperature, pressure and time can promote the removal of moisture in lignite. The effects of three drying factors on removing moisture in lignite are different. The effect of temperature is the most obvious, followed by pressure, and the impact of drying time is minimized.(2) Under the same re-adsorption conditions, the increasing of total pore volume in upgraded lignite leads to the increase of moisture adsorption content. Lignite meso-porous volume have the same effect.(3) Some inorganic substances in lignite can be seen as the hydrophilic groups. At low relative humidity, the inorganic substances could promote moisture adsorption of upgraded lignite.(4) At low relative humidity, moisture adsorption content of upgraded lignite is mainly related to the concentration of hydrophilic functional groups exposed on the upgraded lignite surface. At high relative humidity, moisture adsorption content of upgraded lignite is mainly related to pore volume.(5) During moisture re-adsorption of drying lignite, moisture re-adsorption content increase gradually with the increasing of relative humidity. Adsorption/desorption rate curves can be divided into three stages: decreasing rate stage, constant rate stage and increasing rate stage. Moisture type re-adsorbed in decreasing rate stage mainly is monolayer adsorption moisture; It in constant rate stage mainly is multilayer adsorption moisture; It mainly is capillary condensation moisture in increasing rate stage.(6) Moisture re-adsorption content and rate are large when relative pressure p/p0 < 0.1 or p/p0 > 0.8. Moisture re-adsorption content and rate are small when relative pressure 0.1 < p/p0 < 0.8.