DETECTION OF REACTIVE INTERMEDIATE NITROGEN AND SULFUR SPECIES IN THE COMBUSTION OF CARBONS THAT ARE MODELS FOR COAL CHARS

The release of nitrogen and sulfur during the combustion of coal chars is a major environmental problem. Model coal chars, prepared by the high-pressure carbonisation of polynuclear aromatic precursors to 873 K and subsequent calcination at atmospheric pressure to 1273 K, were studied using temperature-programmed combustion (TPC) in 20% O-2/He. These carbons, which are derived from pure organic precursors with well-defined nitrogen and sulfur functionality, are highly reproducible and are virtually free of catalytic effects due to the absence of metallic species. The reactions were conducted using a thermogravimetric analyser coupled to a quadrupole mass spectrometer (TG-MS). The evolved gases were analysed throughout the course of the combustion. Sampling of the gases directly above the sample by means of a heated capillary line allowed the detection of reactive species including HCN, C2N2, and OCS. Sampling the gases at the exhaust of the TGA allowed the estimation of the gas composition at near-equilibrium conditions. In this case, the reactive species were no longer detected and gas-phase reactions, such as the conversion of HCN and C2N2 to NO and the conversion of CO to CO2, were apparently occurring in the gas phase. The results are discussed in terms of the nature of the surface nitrogen and sulfur species present during combustion.