Abstract
Painan coals of West Sumatra were selected as semi-anthracitic coal sample for studying the physicochemical properties such as measurement, evaluation and description of the changes of surface characteristic of coal sample and their oxidation in the atmospheric air at a temperature ranging from 105 to 400 °C for 30 min. Several methods are adopted to analyze and discuss several phenomena of the oxidized Painan coal surface during oxidation process for the change in the physicochemical properties as determined by Atomic Force Microscope (AFM), contact angle, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Thermal Analysis (TG–DTA) analyses as well as other supporting analytical equipment. AFM analyses revealed some changes in adhesion force and surface morphology with more adhesion force available between 0.6 and 8.6 nN on polished coal surfaces due to the increased oxidation temperature. The study revealed that the extent of hydrophobicity of coal surface decreased with the increased of oxidation temperature expressed as contact angles at about 80° and 20°. Another phenomenon occurred during the experiment was hydrophilicity index of coal surface increase at approximately 1.3 and 2.9. Oxidation of coal that occurred with increased temperature also indicated an increase in oxygen content from 3.8% to 22.9 wt%. Increased oxygen functional group also noted that oxidation of coal took place during the treatment. We also found that oxidation treatment also affected the combustion properties of coal: decreasing ignition temperature between 452.9 and 317.6, lowering the reactivity of coal at maximum combustion rate temperature, and reflecting their char characteristics as burnt out, ranging from 652.3 to 648.5 °C.
Highlights
Understanding of coal surface physic and chemical properties is the prerequisite to better know the unique characteristic of coal that is affected by the oxidation process
The study revealed that the extent of hydrophobicity of coal surface decreased with the increased of oxidation temperature expressed as contact angles at about 80° and 20°
The oxygen content increased with the oxidation temperature
Summary
Understanding of coal surface physic and chemical properties is the prerequisite to better know the unique characteristic of coal that is affected by the oxidation process. Even though the observing range and area of AFM is limited, its wide range of applications on coal pore sciences and physical structure has been well studied (Yordan and Yoon 1988; Drelich et al 1997; Lawrie et al 1997; Cohen et al 1998; Datta et al 2000; Bruening and Cohen 2005; Ozdemir et al 2009; Liu et al 2010; Morga 2011; Wu et al 2014; Xia and Xie 2014; Xia and Yang 2014; Pan et al 2015). The last results did not report adhesion force phenomenon resulting from coal surface during oxidation as valuable information for further study of coal surface contact angle and its chemical properties
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