Functional Forms of Nitrogen and Sulfur in Coals and Fate of Heteroatoms during Coal Carbonization

Abstract

The functionalities of the nitrogen (coal-N) and sulfur (coal-S) present in three caking coals with carbon contents of 76-81 wt%-dry and the fate of these heteroatoms during coal carbonization have mainly been studied by the X-ray photoelectron spectroscopy (XPS) and with a fixed bed quartz reactor, respectively. The XPS measurements exhibit that about 80-90 % of the coal-N exists as pyrrolic-N and pyridinic-N, and the rest can be assigned to quaternary-N (pyridinic-N associated with the hydroxyl groups from phenols or carboxylic acids), whereas the coal-S consists of thiophenic-S, alkyl-S and FeS₂, and the proportion of the former is as high as 60-70 %. When the coal sample is carbonized in He at a heating rate of 3°C/min, yield of N-containing species observed at 1000°C increases in the order of HCN ≤ tar-N < NH₃ < N₂ < oil-N < coke-N, regardless of the type of coal, and the latter two are present predominantly as NH₄⁺ and in polynuclear aromatic structures, respectively. On the other hand, yield of S-containing species at 1000°C increases in the order of COS < CS₂ < tar-S < oil-S < H₂S < coke-S with all coals, and more than 95 % of the coke-S can be regarded as thiophenic-S. These tendencies are also observed with physical mixtures of each coal investigated. Possible mechanisms for the formation of gaseous N- or S-containing compounds in the carbonization process are discussed on the basis of the results of some thermodynamic calculations.