Emission of oxygen, sulphur and nitrogen containing heterocyclic polyaromatic compounds from lignite combustion

Maritza–East Bulgarian lignite was burnt in an atmospheric fluidised bed pilot plant, laboratory scale, at conditions close to the real ones used in power generation (850 °C, 3% oxygen excess, limestone with ratio Ca/S=3.25) in order to analyse in a qualitative way the polycyclic aromatic compounds (PAC) emissions. The samples studied were collected by flowing an aliquot of the combustion gases through a sampling system, consisting of cyclones, nylon and teflon filters and an XAD-2 resin. Before sample analysis by gas chromatography with mass spectrometry detection (GC-MS), ligand exchange chromatography on PdCl2 impregnated silicagel was used to separate PAC with different heteroatoms, concretely, oxygen (O-PAC), sulphur (S-PAC) and nitrogen (N-PAC) of the polycyclic aromatic hydrocarbon (PAH) fraction. l samples studied, the highest PAC emissions were due to compounds containing three and four aromatic rings in their structure. The O-PAC constituted carbonyl-containing compounds Z(O)=−18 to −22, with carbon numbers n=13–17; anthraquinones, Z(O2)=−20, n=14 and 15, phenols, phenanthroaldehyde, and furans, Z(O)=−22, n=16, where concentration of quinones far predominated. A peculiar group of C19–C23 alkylbenzones, with a maximum at C20–C21, was found as well. Dibenzothiophenes, phenanthrothiophene and benzonaphthothiophenes, Z(S)=−16 to −20, were the major constituents of the S-PAC, while the N-PAC were determined at trace levels.