Formation of NO, NO₂, and N₂O from Gardanne lignite and its char under pressurized conditions

The purpose of this investigation was to distinguish reactor-independent trends in the formation of N₂O, NO, and NO₂ from trends depending on the mode of combustion. The same char powder was studied with two different devices:  a pressurized thermobalance (PTGA) where the particles are in contact and the gas flows through the char bed, and a pressurized entrained flow reactor (PEFR) where the particles are far from each other, moving in the gas flow. In addition, the proportions of nitrogen oxides formed from volatile-N and char-N were investigated with the PEFR and a modified thermobalance (MTGA), where the sample was burned as single pellets. A French (Gardanne) lignite or its char was used in the experiments. An increase in total pressure decreased the emissions of NO and increased the emissions of NO₂ in both the PEFR and the PTGA. However, the effect of oxygen partial pressure was different. In PTGA, carbon monoxide emission was high, increased with P_O2, and participated in NO and N₂O reduction. Consequently, the partial pressure of oxygen strongly decreased the NO emissions. In PEFR, an increase in oxygen partial pressure increased only the homogeneous conversion of NO to NO₂ and has no important effect on NO emissions in the studied range. In the experiments performed to determine the contribution of volatile-N and char-N to nitrogen oxides formation, trends were similar in the PEFR and MTGA devices even though the char preparation was different. Volatile-N was responsible for at least 70% of N₂O formation. At atmospheric pressure, char-N and volatile-N participated roughly equally in the formation of NO, but an increase of pressure increased the importance of char-N. Char-N can produce even more NO₂ than volatile-N.