Water sorption on coal: effects of oxygen-containing function groups and pore structure

Ang Liu,Peng Liu,Shimin Liu,Kai Wang

doi:10.1007/s40789-021-00424-6

Abstract

Coal-water interactions have profound influences on gas extraction from coal and coal utilization. Experimental measurements on three coals using X-ray photoelectron spectroscopy (XPS), low-temperature nitrogen adsorption and dynamic water vapor sorption (DVS) were conducted. A mechanism-based isotherm model was proposed to estimate the water vapor uptake at various relative humidities, which is well validated with the DVS data. The validated isotherm model of sorption was further used to derive the isosteric heat of water vapor sorption. The specific surface area of coal pores is not the determining parameter that controls water vapor sorption at least during the primary adsorption stage. Oxidation degree dominates the primary adsorption, and which togethering with the cumulative pore volume determine the secondary adsorption. Higher temperature has limited effects on primary adsorption process.The isosteric heat of water adsorption decreases as water vapor uptake increases, which is found to be close to the latent heat of bulk water condensation at higher relative humidity. The results confirmed that the primary adsorption is controlled by the stronger bonding energy while the interaction energy between water molecules during secondary adsorption stage is relatively weak. However, the thermodynamics of coal-water interactions are complicated since the internal bonding interactions within the coal are disrupted at the same time as new bonding interactions take place within water molecules. Coal has a shrinkage/swelling colloidal structure with moisture loss/gain and it may exhibit collapse behavior with some collapses irreversible as a function of relative humidity, which further plays a significant role in determining moisture retention.

Highlights

Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Water is the most common solvent in nature
Higher temperature has limited effects on primary adsorption process.The isosteric heat of water adsorption decreases as water vapor uptake increases, which is found to be close to the latent heat of bulk water condensation at higher relative humidity
The results confirmed that the primary adsorption is controlled by the stronger bonding energy while the interaction energy between water molecules during secondary adsorption stage is relatively weak

Summary

Introduction

Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Water is the most common solvent in nature. The functional groups have slight variation in the trend following with coal rank, the abundance of oxygen-containing functional groups increases towards to low rank coals and enhances the hydrophilicity of coal surfaces to increase the water holding capacity (Stach 1982; Ogunsola 1993; Murata et al 2000; Ahamed et al 2019). The carboxylic groups and hydroxyl groups are the most preferred hydrophilic sites for water adsorption (Ahamed et al 2019) These preferred oxygen functional groups act as primary adsorption sites for water molecules through hydrogen bonds The sigmoid shape can be generally attributed to the combined effects of three separate sorption processes including monolayer sorption, multilayer condensation and capillary condensation (Allardice and Evans 1971)

Methods

Results

Conclusion