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 PO2, 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.