Effect of Composition and Rank

Pure coal consists mainly of carbon with lesser quantities of hydrogen, nitrogen, oxygen, and sulfur within the organic matrix (Chapter 10) there are also mineral impurities (Chapter 7). When coal is burned, the organic matrix is converted to gaseous combustion products and the mineral impurities generally remain as an ash residue. 

It is important to note here that the ash produced by combustion is not the mineral matter originally present in the coal, but the residues remaining after the minerals have been converted to the various elemental oxides. 

Changes resulting from decomposition of mineral matter in this manner do not make any contribution to the calorific value of the coal but usually detract from it and where calorific values must be measured accurately, require corrections of the measured values. These corrections are not large and it is often considered sufficient (but not quite precise) to use the ash content, which is easily and rapidly determined, as a measure of the mineral impurities present in a coal. 

When coals are pyrolyzed, this process is continued by the near complete elimination of hydrogen and oxygen. The carbon residue that remains, however, still contains small quantities of nitrogen and sulfur. Typical values for carbon, hydrogen, volatile matter (i.e., low-molecular-weight products of the pyrolysis), and residual (fixed, nonvolatile) carbon of the various classes of coal (Table 6.1) provide the basis of a system for describing coal rank (Chapters 2 and 8). 

The chemical structure of coal is complex (Chapter 10), but there are strong indications (and little, or no, evidence to the contrary) that the nitrogen is entirely contained within the organic matrix. On the other hand, oxygen and sulfur occur in both organic and inorganic combinations. 

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