Water Clusters in Lignite and Desorption Energy Calculation by Density Functional Theory

Abstract

The interaction of water and hydrophilic sites with hydroxyl, carboxyl, and multiple oxygen-containing functional groups (OFGs) in lignite molecules was studied by density functional theory. The adsorption of water molecules on the lignite surface initially resulted in the formation of hydrogen bond-driven stable rings by three to four water molecules, followed by the formation of three-dimensional water clusters like a ″patchwork″. Aqueous layer thickness obtained from the water cluster size was 0.4–0.6 nm, which was consistent with the experimental data. Thus, pore-filling water beyond this range was less affected by the OFGs on the surface. Calculation of the adsorption energy predicts that the water clusters were primarily formed in the hydrophilic sites with three OFGs (site 1, including a carbonyl group, an alcoholic hydroxyl group and an etheroxy group in tetrahydropyran), then in COOH, and in O–H. For isolated hydroxyl groups, the interaction between the hydroxyl group and water molecules was weaker than that between the water molecules. When the water cluster was located at the hydrophilic sites with two or more OFGs, the adsorption energy of lignite–water interaction was higher than that of water–water interaction. Investigating the thermodynamics of the adsorption process at a molecular scale will help in understanding both drying and resorption process of dried lignite during industrial production.

Chemical Structure Characteristics and Model Construction of Coal with Three Kinds of Coalification Degrees

“…The charge distribution plays a crucial role in intermolecular interactions (such as van der Waals forces, HBs, etc.) and can effectively describe and predict molecular adsorption and chemical reactions. − The charge distribution determines the electrophilicity and nucleophilicity of atoms. This means that the amount of positive or negative charges on an atom is directly related to its ability to accept or donate electrons. , …”

Reaction Mechanism of Aldehyde Groups during Coal Self-Heating

“…According to the property of hydrogen on the aldehyde group and the dissociation energy of hydrocarbon bond, the reaction ability of hydrogen capture is between the functional group structure units of the hydroxyl group and carboxyl group. Based on previous investigations − on the reactions of hydroxyl and carboxyl groups during the process of coal spontaneous combustion, it can be known that the spontaneity of the elementary reaction E5 is relatively difficult and the activation energy is relatively high. Therefore, reaction E5 is difficult to occur in the initial stage and the transition stage of coal oxidation at low temperatures.…”

Research of New Cross-Flow Technology with a Gas Heat Carrier for Lignite Pyrolysis

“…The moisture in the raw coal will absorb significant amounts of heat during the pyrolysis process. 34 When the moisture content is lower, the heat demand for pyrolysis and the amount of heat carrier are reduced, thereby decreasing the raw gas output. At the same temperature of the gas heat carrier, lower moisture in the raw materials leads to a higher final pyrolysis temperature and lower char yield but higher yields of tar and pyrolysis water.…”

“…The experimental results for A850, B850, and C850 show that as the moisture of the raw materials decreased, the char yield decreased from 62.43 to 61.58%, the tar yield increased from 5.45 to 5.53%, the yield of pyrolysis water increased from 8.89 to 10.01%, and the output of barren gas decreased from 1205 to 805.59 Nm 3 /t. The moisture in the raw coal will absorb significant amounts of heat during the pyrolysis process . When the moisture content is lower, the heat demand for pyrolysis and the amount of heat carrier are reduced, thereby decreasing the raw gas output.…”