FT-i.r. investigation on char formation during the early stages of coal pyrolysis

The transformation of coal into char during low-temperature pyrolysis was studied by Fourier transform infrared (FT-i.r.) spectroscopy and solvent swelling measurements. Pyrolysis was carried out in a nitrogen-swept fixed-bed reactor at temperatures of 300–600 °C. Three Spanish coals varying in rank from lignite to bituminous were studied. The samples show very different cross-linking behaviour, related to CO2 and H2O evolution at low temperatures and to methane release at higher temperatures. The FT-i.r. examination shows that the aliphatic structures and the oxygen-containing functional groups (carboxyl and hydroxyl) are easily removed from chars, while aromatic hydrogen tends to increase with increasing pyrolysis temperature. Curve-fitting analyses for the 3000-2700 and 900-700 cm−1 regions permit the variations in the distribution of methyl and methylene groups and in the degree of aromatic substitution and condensation, respectively, to be evaluated as a function of temperature. Several structural parameters based on FT-i.r. data have been calculated for the chars. These structural parameters provide evidence on the loss of aromatic substituents and the process of hydrogen transfer to aromatic structures during pyrolysis.