Chemical Fractionation

Chemical fractionation is used to determine the association of inorganic elements present in coals. It is especially valuable when determining the abundance of organically-associated inorganic components found in lignitic and sub-bituminous coals present as the salts of carboxylic acid groups. In high clay-containing coals, inorganic elements, such as Na, K, Ca, and Mg, are ion-exchangeable with the clay structure. If this is the case, the use of Computer-Controlled Scanning Electron Microscopy (CCSEM analysis) to determine abundance and type of clay minerals is recommended.

The organically-associated elements (such as Na, Ca, and Mg) often form small particles and low melting-point phases during combustion. Often, it is not just how much sodium or calcium is present that is important, but whether they are present as minerals or as salts or organic acid groups (organically associated). This information is extremely important in predicting low temperature deposition processes in coal-fired boilers.

The procedure involves successive extractions of H2O, 1 molar NH4OAc, and 1 molar HCl. The elements that are extracted by NH4OAc are present in an ion-exchangeable association. HCl-extractive elements are present in the coal as acid-soluble minerals, such as carbonates or sulfates, or in organic coordination complexes.