Structural features of low-rank coals

Abstract

Low-rank coals are characterized by low aromaticities, small aromatic ring clusters, abundant aliphatic and hydroaromatic carbon and hydrogen, and high oxygen contents. Typically, f a values (the fraction of aromatic carbon) range between 0.5 and 0.75. The aromatic clusters contain one to three fused rings. Polymethylene bridges between aromatic systems are of various lengths, some of which may exceed ten (-CH 2-) units. The hydroaromatic units are tetralin- or dihydrophenanthrene-like structures. The oxygen content of the anthrene-like structures. The oxygen content of the lowest ranked coals can approach 25% on a dry, mineral-matter-free basis. Oxygen is contained in various functional groups, including carboxyl, phenol, and methoxyl, as well as in less well defined ethers and quinones. The carboxyl groups retain cations of the alkali and alkaline earth elements on ion-exchange sites. The phenols may contribute to the structural rigidity of the coal both by hydrogen bonding to other groups and by the incorporation of tightly held water molecules. In most low-rank coals of the United States, the organic sulfur and nitrogen contents are usually 2% or less. The functional groups incorporating these heteroatoms are not well characterized; they are usually assumed to be heterocyclic. The heteroatoms participate in the incorporation of elements such as aluminum in coordination complexes.