Laboratory Study of N₂O Formation from ¹⁵N-Isotope-Marked NO While Burning Char Particles at FBC Conditions

Abstract

This paper is a continuation of the discussion of N2O formation during char combustion started in a previous paper (Energy Fuels 1995, 9, 10−19). The effect of different NO and CO concentrations in the inlet gas, as well as the effect of changing gas residence time within the fixed bed and after the fixed bed on the emission of N2O from char particles burning at single particle conditions, is reported. In order to distinguish between char nitrogen and NO from the inlet gas a 15N-isotope-marked NO was used in the inlet gas. The experiments were carried out in a fixed bed reactor containing quartz sand (SiO2) to support the coal particles. The bituminous coal particles were devolatilized in situ prior to the actual combustion in an argon flow. The inlet gas mixture contained 15NO, CO, O2, and Ar or just 15NO, O2, and Ar. The 15NO concentration was varied between 500 and 1950 ppm, and the CO concentration was varied between 0 and 2500 ppm. The inlet gas flow was varied between 297 and 1200 mL/min NTP (273 K, 1 atm). The bed temperature was 1073 K. The off-gases were analyzed for N2O. The addition of 15NO to the inlet gas led to the formation of 15N14NO and 15N2O, and the formation of these species increased with increasing concentration of 15NO in the inlet gas, especially the formation of 15N2O. The formation of 15N14NO was greater than the formation of 15N2O under all circumstances examined, but with increasing 15NO concentration in the inlet gas the 15N2O formation increased, and was almost as great as the formation of 15N14NO. The influence of gas residence time within the fixed bed and after the fixed bed was confirmed, and the N2O formation and formation of 15N14NO and 15N2O increased substantially with increasing gas residence time. The addition of CO to the inlet gas had almost no effect on the N2O formation.

N2O Formation from Combustion of Char Particles with NO

“…The Tests. The char particles were prepared from a bituminous coal from Pennsylvania, USA, which had been used earlier in refs and . The considered parameters were gas residence time within and after the fixed bed, and NO concentration.…”

“…The formation of N 2 O from burning char particles has been an issue of discussion for a period of time, and in time two different formation mechanisms have become the kernel in the discussion, i.e., the heterogeneous formation mechanism (M2) versus the homogeneous ones (M3/M4). Both mechanisms involve the participation of gaseous NO and have been earlier described in detail. , In the heterogeneous formation mechanism, M2, NO reacts with a nitrogen atom fixed in the char matrix, or adsorbs onto the char surface, where it subsequently reacts with a second NO molecule and forms N 2 O. The N 2 O formation rate is independent of the gas residence time within and after the fixed bed.…”

“…Both mechanisms involve the participation of gaseous NO and have been earlier described in detail. 1,2 In the heterogeneous formation mechanism, M2, NO reacts with a nitrogen atom fixed in the char matrix, or adsorbs onto the char surface, where it subsequently reacts with a second NO molecule and forms N 2 O. The N 2 O formation rate is independent of the gas residence time within and after the fixed bed.…”

“…[3][4][5] In time, combustion experiments were performed with both O 2 and NO present in the combustion gas, and higher N 2 O yields were obtained with than without addition of NO to the combustion gases. 1,2,[6][7][8][9][10] Since no N 2 O was formed without O 2 present, it was concluded that O 2 is an essential tool in breaking up the aromatic rings in char and releasing or making available the nitrogen bound in the rings. 1,6,7,10,11 However, the molecular similarity between O 2 and NO makes it doubtful to limit this capability of breaking up the char structure just to O 2 .…”

“…At the beginning of the discussion of N 2 O formation from burning char particles the influence of NO in the combustion gas was ruled out because no N 2 O was formed when low concentrations of NO were reduced over char surfaces. − In time, combustion experiments were performed with both O 2 and NO present in the combustion gas, and higher N 2 O yields were obtained with than without addition of NO to the combustion gases. ,,− Since no N 2 O was formed without O 2 present, it was concluded that O 2 is an essential tool in breaking up the aromatic rings in char and releasing or making available the nitrogen bound in the rings. ,,,, However, the molecular similarity between O 2 and NO makes it doubtful to limit this capability of breaking up the char structure just to O 2 . Miettinen et al studied the formation of N 2 O from burning char particles under single particle conditions.…”