Low-temperature coal weathering: its chemical nature and effects on coal properties

Abstract

A systematic study was performed in which both hvAb and mvb coals were weathered for up to 500 days at 25-80 °C under closely controlled conditions in the laboratory. Periodic samples were analyzed by a wide range of elemental, spectroscopic, physical, and empirical methods. These systematic and closely controlled experiments confirmed many conclusions reached by others using less rigorous experimental designs. Gieseler plastometry and Audibert-Arnu dilatometry were found to be the most responsive to the early stage of weathering, thus demonstrating that moderate weathering will destroy coal thermoplasticity. The higher rank coal weathered significantly more slowly. The weathering rate is highly dependent on temperature. This study also allowed several new conclusions regarding coal weathering. Rates of change with time of log Gieseler fluidity, heating value, and carbon and oxygen contents show similar dependencies on temperature. The principal chemical changes from organic matrix weathering were the loss of aliphatic groups and the production of carbonyl groups. These reactions occurred rapidly at 80 °C, moderately fast at 50 °C and slowly, if at all, at 25 °C. Froth flotation tests, performed on the hvAb coal, showed that losses in flotation recovery from weathering are strongly affected by pyrite oxidation at 25 °C and by organic matrix oxidation at 80 °C. The different temperature dependencies of pyrite and organic matrix oxidation underscore the importance of using realistic temperatures to study natural weathering.

Comparison of drying methods on physical and chemical properties of Shengli lignite

“…[22] Under the hot air drying condition, the active functional groups on coal reacted with oxygen in air that caused an increase in oxygen-containing functional groups with a simultaneous decrease in aliphatic hydrogen content, that is why HAD has a higher surface oxidation degree than VD. These observations are in agreement with results from previous studies [43][44][45] . During microwave drying process some unstable groups such as C-O may be further release when a major portion of the water had already been removed.…”

Effects of oxidative weathering on the composition of organic matter in coal and sedimentary rock

“…Previous studies also reported a decreasing trend of alkyl-C content in bitumen, kerogen and surface organics of coals (Huffman et al, 1985;Wu et al, 1988;Clemens et al, 1991;Pisupati and Scaroni, 1993;Martínez and Escobar 1995;Xia et al, 2014) and sedimentary rocks (Moss et al, 1988;Faure et al, 1999;Petsch et al, 2000Petsch et al, , 2001b with oxidative weathering. This loss of alkyl-C may be related to the oxidative cleavage of labile alkyl moieties, such as olefinic and benzilic carbon (Rhoads et al, 1983;Wu et al, 1988;Wade, 1995;Hayashi et al, 1997;Faure et al, 1999) in SOM. Part of the fragments formed via the cleavage are likely to be classified as SMSF (< C 30 , Faure et al, 1999;Marynowski et al, 2011a,b).…”

“…For example, H/C in lignite-HA decreased from 0.92 to 0.82, while O/ C increased from 0.36 to 0.47 with the degree of sample weathering. In previous studies, decreasing and increasing trends of H/C and O/C, respectively, via oxidative weathering were reported for bulk, kerogen and surface organic composition of coals (Liotta et al, 1983;Wu et al, 1988;Pietrzak and Wachowska, 2003;Grzybek et al, 2006) and sedimentary rocks (Moss et al, 1988;Littke et al, 1991).…”

Review of experimental methods to determine spontaneous combustion susceptibility of coal – Indian context

“…Kok, 2008;Ledesma, Kalish, Nelson, Wornat, & Mackie, 2000;Liotta, Brons, & Isaacs, 1983;MacPhee, Giroux, Charland, Gransden, & Price, 2004;Mahidin, Usui, Ishikawa, & Hamdani, 2002;Ogloza & Malhotra, 1989;Painter, Snyder, Pearson, & Kwong, 1980;Rhoads, Senftle, Coleman, Davis, & Painter, 1983;Wu, Robbins, Winschel, & Burke, 1988). The changes of IR/FTIR spectra observed for a coal i.e.…”