Thermal Transformations of Nitrogen and Sulfur Forms in Peat Related to Coalification

Abstract

The chemical pathways for nitrogen and sulfur transformations during coalification are elucidated by comparing the chemical forms of unaltered peats, lignites, and coals and pyrolyzed peats. Nitrogen forms are characterized by a combination of X-ray photoelectron spectroscopy (XPS) and 15N nuclear magnetic resonance (NMR). In unaltered peats, the 15N NMR and XPS nitrogen (1s) spectra are consistent with the presence of amide nitrogen. When peat is pyrolyzed, the main peak in the 15N NMR spectrum broadens and shifts from −260 ppm to −245 ppm, which is consistent with the loss of some amide nitrogen and the appearance of pyrrolic nitrogen forms. The pyrolyzed peat shows a new XPS peak that appears at 398.6 eV, which is characteristic of pyridinic nitrogen. These results indicate that a thermal transformation of amide nitrogen into pyrrolic and pyridinic forms occurs after thermal stress that is roughly equivalent to lignitification. High total nitrogen levels are found in pyrolyzed peats relative to lignites and higher-rank coals, suggesting that some amides initially found in peat are lost via nonthermal pathways during coalification. Lignites contain the highest levels of quaternary nitrogen, and they are associated with protonated pyridinic structures. The relatively low levels of quaternary nitrogen in pyrolyzed peats in the presence of pyridinic nitrogen indicates that there are fewer acidic sites in pyrolyzed peats relative to lignites. Hence, most quaternary nitrogen is formed during lignitification as a result of the creation and interaction of basic nitrogen species with acidic functionalities and is lost completely during bitumenization. Sulfur X-ray absorption near-edge structure spectroscopy (S-XANES) of unaltered peats detect the presence of disulfide, mercapto, aliphatic sulfide, and aromatic forms of organically bound sulfur. The level of organic sulfur in pyrolyzed peats is comparable to that in lignites and higher-rank coals, suggesting that much of the organic sulfur in coals is derived from sulfur species incorporated during peatification. XPS and S-XANES results show that the relative level of aromatic sulfur increases as the severity of peat pyrolysis increases. The relative level of aromatic sulfur increases through the selective loss of disulfide, aliphatic sulfide, and SO3 groups and through the transformation of aliphatic sulfur forms. Aliphatic sulfur is present mostly as mercapto and disulfide species in peats pyrolyzed to an equivalence vitrinite reflectance of Ro = 0.5 and in lignites but not in higher-rank coals. These results indicate that mercapto and disulfide species are lost after lignitification. Organic sulfur in peats pyrolyzed to Ro = 1.0 exist mainly as aromatic forms, consistent with the level of aromatic sulfur increasing with the increasing degree of coalification.